University of Southampton Research Repository ePrints Soton · interest in developing better...
Transcript of University of Southampton Research Repository ePrints Soton · interest in developing better...
University of Southampton Research Repository
ePrints Soton
Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders.
When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g.
AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination
http://eprints.soton.ac.uk
UNIVERSITY OF SOUTHAMPTON
FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES
SCHOOL OF CHEMISTRY
Selective Lithium Extraction from Salt Solutions by Chemical
Reaction with FePO4
by
Noramon Intaranont
Thesis for the degree of Doctor of Philosophy
September 2015
UNIVERSITY OF SOUTHAMPTON
ABSTRACT FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES
School of Chemistry
Thesis for the degree of Doctor of Philosophy
Selective Lithium Extraction from Salt Solutions by Chemical
Reaction with FePO4
Noramon Intaranont
The spectacular increase in lithium battery applications has raised the question
of whether global lithium resources will be enough in the future. Experts in
the field have estimated that the existing lithium resources will probably be
sufficient to support demands until the year 2100, assuming that lithium
batteries are recycled. Without lithium batteries being recycled, the resources
are expected to be depleted in 50 years’ time. Therefore, there is a great
interest in developing better methods of lithium recycling from batteries, and
also, better methods of lithium extraction from natural resources.
Currently, lithium is extracted from natural brines via the lime soda
evaporation process, i.e. a solar evaporation plus chemical plant process,
which takes between 12 and 24 months. The drawbacks of this process are
that it is complex, slow and inefficient. Also, the currently available methods of
lithium recycling from batteries are too complex and expensive. Thus, the
main objective of this work is to develop a novel, inexpensive and less time-
consuming approach to recover lithium chemically, from the lithium salts
(lithium sources) that contain other metal cations. The new process is also
based on environmental concerns.
A battery material, lithium iron phosphate (LiFePO4) has the olivine structure
and heterosite structure once it discharges to iron phosphate (FePO4). This
structure shows excellent properties of the charge/discharge reversibility. A
few studies on the heterosite FePO4 have reported that it is more selective for
lithium ions (Li+) over other cations. The main advantages of this structure are
the small potential differences of the redox couple, i.e. Fe(II)/Fe(III), and the
stability of LiFePO4 over a wide range of acid-based conditions in an aqueous
solution.
This work investigates a novel process that may be superior to the lime soda
evaporation process for extracting lithium. Heterosite FePO4 was employed to
selectively remove Li+ from lithium sources with the support of a reducing
agent, i.e. sodium thiosulphate (Na2S
2O
3). The resulting LiFePO
4 can be directly
sent not only to lithium battery industries, but also to other industrial uses. In
principle, the other cations could be retrieved back into their sources.
The novel process was examined and demonstrated lithium insertion into a
heterosite FePO4, working as a framework, in aqueous salt solutions. The
evaluation of this process is presented by the Li+ uptake value. The amount of
Li+ uptake can be up to 46 mgLi
+/gsolid
where other cations (i.e. sodium,
potassium, and magnesium) can take less than 3 mg/gsolid,
using this process.
Furthermore. This work could also be developed for future lithium recycling
processes.
i
Table of Contents
ABSTRACT .......................................................................................................................... i
Table of Contents ........................................................................................................... i
List of tables .................................................................................................................... v
List of figures ............................................................................................................... vii
DECLARATION OF AUTHORSHIP ......................................................................... xv
Acknowledgements ................................................................................................. xvii
Definitions and Abbreviations ............................................................................ xix
Chapter 1: Introduction ........................................................................................ 1
1.1 The Availability of Lithium-Present and Future ............................ 1
1.1.1 Consumption, Production and Price of Lithium ................ 1
1.1.2 Resources and Supply ..................................................... 4
1.2 Lithium Extraction from Brine ..................................................... 6
1.2.1 Lime Soda Evaporation .................................................... 6
1.2.2 Ion-sieve spinel type ........................................................ 6
1.2.3 LiFePO4 Electrochemical ................................................... 7
1.3 A Review of LiFePO4 .................................................................... 9
1.4 Thermodynamic Principles for the Selection of Suitable Oxidizing and Reducing Agents ................................................................ 12
1.4.1 Oxidizing Agent ............................................................ 12
1.4.2 Reducing Agent ............................................................. 13
1.5 Outline of the Thesis ................................................................ 18
1.6 References ............................................................................... 18
Chapter 2: Experimental Techniques ......................................................... 23
2.1 Powder X-ray Diffraction ........................................................... 23
2.2 Electrochemical Technique ....................................................... 28
2.2.1 Cyclic Voltammetry ....................................................... 28
2.2.2 Potentiometric Titration ................................................ 33
2.3 Inductively Coupled Plasma Mass Spectroscopy ........................ 35
ii
2.4 References ............................................................................... 38
Chapter 3: Physical and Electrochemical Characterization of the
LiFePO4 and FePO
4 system .................................................................... 41
3.1 Experimental Details ................................................................ 41
3.2 Results and discussion ............................................................. 44
3.3 References ............................................................................... 54
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to
FePO4 55
4.1 A Study of the Chemical Delithiation of LiFePO4 in Aqueous
Solutions .................................................................................. 55
4.1.1 Introduction .................................................................. 55
4.1.2 Experimental Details ..................................................... 55
4.1.3 Results and Discussion .................................................. 56
4.2 Analysis of the Delithiation rate using a Conductivity Measurement ........................................................................... 66
4.2.1 Introduction .................................................................. 66
4.2.2 Experimental Details ..................................................... 68
4.2.3 Results and Discussion .................................................. 69
4.3 References ............................................................................... 72
Chapter 5: Test of LiI and Na2S
2O
3 as Reducing Agent of FePO
4 to
LiFePO4 ............................................................................................................ 75
5.1 Introduction ............................................................................. 75
5.2 Experimental Details ................................................................ 76
5.3 Results and Discussion ............................................................. 79
5.4 References ............................................................................... 86
Chapter 6: Kinetic Studies of the Chemical Lithiation of FePO4 by
Na2S
2O
3 ............................................................................................................ 87
6.1 A preliminary study of the effect of varying the concentrations of both Li+ and S
2O
3
2- together ....................................................... 87
6.1.1 Introduction .................................................................. 87
6.1.2 Experimental Details ..................................................... 88
6.1.3 Results and Discussion .................................................. 90
iii
6.2 Kinetics of the Chemical Lithiation of FePO4 using Na
2S
2O
3,
varying the concentration of S2O
3
2- and Li+ independently ... 100
6.2.1 Introduction and Theory .............................................. 100
6.2.2 Results and Discussion ................................................ 101
6.3 References ............................................................................. 125
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
3
2-
/ FePO4 Reagents .................................................................................... 127
7.1 Chemical Lithiation of FePO4 from Aqueous Solutions Containing
an Excess of Na+ and Mg2+ ....................................................... 127
7.1.1 Experimental Details ................................................... 128
7.1.2 Results and Discussion ................................................ 129
7.2 Chemical Lithiation of FePO4 from Synthetic Brine Solutions.... 134
7.2.1 Experimental details .................................................... 135
7.2.2 Results and Discussion ................................................ 135
7.3 References ............................................................................. 140
Chapter 8: Alternative Reducing and Oxidising Agents ................ 141
8.1 Introduction ........................................................................... 141
8.2 Experimental details ............................................................... 142
8.3 Results and discussion ........................................................... 144
8.4 References ............................................................................. 149
Chapter 9: Conclusions and Future Work .............................................. 151
9.1 Conclusions ............................................................................ 151
9.2 Future work ............................................................................ 153
9.3 References ............................................................................. 155
Appendix A ................................................................................................................. 157
Appendices ................................................................................................................. 159
Bibliography ............................................................................................................... 161
v
List of tables
Table 1.1: Lithium consumption application changes between 2003 and 2011.1
Table 1.2: The comparison of cations between the concentration of cations in
brine and seawater ................................................................... 5
Table 1.3: The comparison of ionic fractions (Li+ vs. other cation from Table
1.2) between the concentration of cations in brine and seawater5
Table 1.4: Properties of possible lithium ion movements. .............................. 12
Table 1.5: Ionic radii of the potential cations that can be intercalated into
FePO449 .................................................................................... 13
Table 1.6: The unit cell parameters of FePO4, LiFePO
4, and NaFePO
4 products
based on XRD data52 ............................................................... 14
Table 2.1: The composition of an active material ink ..................................... 32
Table 2.2: The composition of an electrode ................................................... 34
Table 3.1: The composition of LiFePO4 and FePO
4 as an active material ......... 43
Table 3.2: the lattice parameter of the fitting pattern and the reference2 ....... 47
Table 3.3: Metal ions contained in LiFePO4 and the heterosite FePO
4 .............. 49
Table 3.4: A specific charge of each percentage Li content for validation ...... 52
Table 4.1: Chemical composition of the delithiation experiment by the use of
K2S
2O
8 as an oxidizing agent .................................................... 56
Table 4.2: Stoichiometric coefficient of x in LixFePO
4 samples obtained by
delithiation of LiFePO4 from each experiment at 30 m, 1 h, 2 h, 4
h, and 24 h ............................................................................. 64
Table 6.1: Concentration of Li2SO
4, Na
2S
2O
3, and FePO
4 in each sample .......... 88
Table 6.2: Li and Na concentrations of samples obtained by a 1:2 of [Li+]:[Na+]
solution with 1 g FePO4 at various times. (The data were
published in March 20141). ..................................................... 94
Table 6.3: Stoichiometric coefficient of x in LixFePO
4 samples obtained by
lithiation of FePO4 with a 4-fold excess of reagent (Li
2SO
4+Na
2S
2O
3
in molar ratio 1:2) for different times, using 3 techniques. ...... 95
Table 6.4: The reaction rate of each experiment when x = 1 (LixFePO
4) with
respects the reaction time (estimated data from Figure 6.9-
Figure 6.6) ............................................................................. 99
Table 6.5: Concentrations of the reagents ................................................... 100
vi
Table 6.6: Lithium molar content of LixFePO
4 samples obtained by lithiation
of FePO4 for different times, as estimated from (a) XRD and
(b) ICP measurements. ........................................................ 107
Table 6.7: The reaction rate of each experiment with respect to [Li+] ........... 112
Table 6.8: The reaction rate of each experiment with respect to [S2O
32-] ....... 115
Table 6.9: Values of calculated theoretical rates compares to the experimental
rates ..................................................................................... 118
Table 6.10: Li and Na concentrations of samples obtained by various ratios of
[Li+]:[S2O
3
2- ] solution with 1 g FePO4 at various time. ............... 123
Table 7.1: Ionic radii of the potential cations that can be intercalated into
FePO47 ................................................................................... 128
Table 7.2: Concentration of reagents used in the experiments [Li+]:[Na+] and
[Li+]:[Mg+2] ............................................................................. 129
Table 7.3: The XRD results of lithiated heterosite FePO4 for each experiment133
Table 7.4: Lithium and sodium concentrations found in samples ................. 134
Table 7.5: Chemical compositions of synthetic brine type A/B and the
heterosite FePO4 .................................................................... 135
Table 7.6: Concentration of metals; i.e. Li+, Na+, K+, and Mg2+, that contain the
LiFePO4 samples obtained from type A and type B synthetic
brines. .................................................................................. 137
Table 8.1: Composition of each solution ..................................................... 143
vii
List of figures
Figure 1.1: The world lithium production in 2012, categorised by lithium
mining companies, adapted from Maxwell, 2014.10 ................... 2
Figure 1.2: Estimated lithium carbonate equivalent prices from 1990 to 2013,
adapted from Maxwell, 2015.5 .................................................. 3
Figure 1.3: Cyclic voltammogram of an immobilized LiFePO4 on Pt electrode
(solid line) and bare Pt electrode (dotted line) in 1 M Li2SO
4.
Reproduced with permission from ref. 31. Copyright 2015,
Elsevier. (This figure was published in January 2007 by Mi et
al.31) ........................................................................................ 10
Figure 1.4: a)The structure of a unit-cell of LiFePO4, b) lithium ion migration
path and c) curved trajectories or wavelike path of lithium ion
migration. Reprinted with permission from Ref.34. Copyright
2015 American Chemical Society. (This figure was published in
July 2005 by Islam et al.34) ....................................................... 11
Figure 1.5: The useful range of reducing agent potential (vs. lithium) for FePO4
lithiation. ................................................................................ 15
Figure 2.1: Diagram shows the derivation of Bragg's law ............................... 24
Figure 2.2: The combination of the miller indices of LiFePO4 (blue)3 and FePO
4
(red)4 XRD patterns. ................................................................ 26
Figure 2.3: Binary phase diagram of FePO4-LiFePO
4, obtained from XRD data.
This figure is adapted from Kobayashi et al, 2009.8 ................ 28
Figure 2.4: One-phase CV a) Cyclic votammetry profile presents a peak height
(Ipred) and a peak position (E
pred) b) samples of CV profile
A)reversible, B) quasi-reversible, and C) irreversible electron
transfer. Reprinted with permission from Ref.10. Copyright 2015
Springer London. (This figure was published in July 2005 by
Brownson et al).10 .................................................................... 29
Figure 2.5: A schematic drawing of a two-phase cyclic voltammogram, adapted
from Roberts et. al.11 ............................................................... 30
Figure 2.6: a) before and b) after immersing a Pt mesh electrode into the ink 32
Figure 2.7: Basic diagram of 3 electrode system ............................................ 33
Figure 2.8: the assembled Swagelok cell A) negative and positive connectors,35
viii
Figure 3.1: SEM images of an initial carbon coated lithium iron phosphate
(Tatung), under magnification of a) 2,500x, b) 10,000x and c)
33,000x. The images were recorded with an acceleration voltage
of 15 kV. ................................................................................. 45
Figure 3.2: SEM images of a carbon coated heterosite iron phosphate, which
was obtained from a delithiation process of 5.8 g LiFePO4 + 0.1
M K2S
2O
8 (Chapter 4.1), under magnification of a) 2,500x, b)
10,000x and c) 35,000x. The images were recorded with an
acceleration voltage of 15 kV. ................................................. 46
Figure 3.3: Fit to XRD data of a) LiFePO4 (Tatung): R
wp 1.3%, R
p 1.0% and b)
FePO4:
R
wp 1.5%, R
p 1.1%.
Crosses mark the data points, red line
is the fit and blue line is the difference; a) blue and b) pink tick
marks show the allowed reflection positions for LiFePO4 and
FePO4, respectively. ................................................................. 48
Figure 3.4: Cyclic voltammogram of LiFePO4 in 1.5 M Li
2SO
4 aqueous electrolyte
with a scan rate of 10 mV s-1, where Epa and E
pc are anodic and
cathodic peak potentials, respectively. .................................... 51
Figure 3.5: Potentiometric titration of LixFePO
4 electrodes prepared with the
mixiture of LiFePO4 + FePO
4 = 100%, as indicated. Specific
current: 17 mAh g-1 (at C/10). ................................................. 52
Figure 3.6: A validating graph of lithium content (%) in LiFePO4 and specific
charge .................................................................................... 53
Figure 4.1: The XRD of a LFP sample treated with K2S
2O
8 with a molar ratio of
2:1 for 1 hour. The XRD refinement indicates that the sample
composition is Li0.09
FePO4 ......................................................... 57
Figure 4.2: As in Figure 4.1, but the reaction was left for 2 hours and the XRD
fitting indicates that the sample composition is Li0.03
FePO4 ....... 58
Figure 4.3: As in Figure 4.1, but the reaction was left for 4 hours and the XRD
fitting indicates that the sample composition is FePO4. ............ 58
Figure 4.4: As in Figure 4.1, but the reaction was left for 24 hours and the XRD
fitting indicates that the sample composition is FePO4 ............. 59
Figure 4.5: The XRD of a LFP sample treated with K2S
2O
8 with a molar ratio of
3:2 for 30 min. The XRD refinement indicates that the sample
composition is Li0.07
FePO4 ......................................................... 60
Figure 4.6: As in Figure 4.5, but the reaction was left for 1 hour and the XRD
fitting indicates that the sample composition is Li0.06
FePO4. ...... 60
ix
Figure 4.7: As in Figure 4.5, but the reaction was left for 2 hours and the XRD
fitting indicates that the sample composition is FePO4. ........... 61
Figure 4.8: As in Figure 4.5, but the reaction was left for 24 hours and the XRD
fitting indicates that the sample composition is FePO4. ........... 61
Figure 4.9: The XRD of a LFP sample treated with K2S
2O
8 with a molar ratio of
6:2 for 30 min. The XRD refinement indicates that the sample
composition is Li0.05
FePO4 ........................................................ 62
Figure 4.10: As in Figure 4.9, but the reaction was left for 1 hour and the XRD
fitting indicates that the sample composition is Li0.04
FePO4. ..... 63
Figure 4.11: As in Figure 4.9, but the reaction was left for 2 hours and the XRD
fitting indicates that the sample composition is FePO4. ........... 63
Figure 4.12: As in Figure 4.9, but the reaction was left for 24 hours and the
XRD fitting indicates that the sample composition is FePO4. .... 64
Figure 4.13: Delithiation of LiFePO4 during 24 hours, () 1:2 of K
2S
2O
8 to
LiFePO4, ( ) 3:2 of K
2S
2O
8 to LiFePO
4, ( ) 6:2 K
2S
2O
8 to LiFePO
4
experiments ........................................................................... 65
Figure 4.14: The kinetics delithiation of () 1:2 of K2S
2O
8 to LiFePO
4, ( ) 3:2 of
K2S
2O
8 to LiFePO
4, ( ) 6:2 K
2S
2O
8 to LiFePO
4 experiments ......... 66
Figure 4.15: The conductivity obtained from the mixture of 0.1 M K2S
2O
8 and 5
g LiFePO4 at 25°C with respect to time. .................................... 69
Figure 4.16: The conductivity and temperature obtained from the mixture of
0.2 M K2S
2O
8 and 5 g LiFePO
4 with respect to time. .................. 70
Figure 4.17: The exponential decay of 0.2 M K2S
2O
8 ...................................... 71
Figure 5.1: XRD patterns of 1 M LiI sample obtained after 30 m at room
temperature, compared to FePO4, and LiFePO
4. All diagrams were
indexed in the orthorhombic (Pnma (62)) crystallographic
system. ................................................................................... 79
Figure 5.2: XRD patterns of 2 M LiI obtained after 30 m at room temperature,
compared to FePO4, and LiFePO
4. All diagrams were indexed in
the orthorhombic (Pnma (62)) crystallographic system. ........... 80
Figure 5.3: XRD patterns of 2 M LiI +Zn sample obtained after 30 m at room
temperature, compared to FePO4, and LiFePO
4. All diagrams were
indexed in the orthorhombic (Pnma (62)) crystallographic
system. ................................................................................... 82
x
Figure 5.4: XRD patterns of 2Li2SO
4: 4Na
2S
2O
3: 1FePO
4 sample obtained after 1
h at room temperature, compared to FePO4, and LiFePO
4. All
diagrams were indexed in the orthorhombic (Pnma (62))
crystallographic system. .......................................................... 83
Figure 5.5: XRD patterns of 2LiCl: 2Na2S
2O
3: 0.5FePO
4 sample obtained 1 h at
room temperature, compared to FePO4, and LiFePO
4. All
diagrams were indexed in the orthorhombic (Pnma (62))
crystallographic system. .......................................................... 84
Figure 5.6: XRD patterns of 2:1 of Na2S
2O
3 :FePO4 ratio sample obtained 1 h at
room temperature, compared to FePO4, and LiFePO
4. All
diagrams were indexed in the orthorhombic (Pnma (62))
crystallographic system. .......................................................... 85
Figure 6.1: The XRD fitting obtained from a sample treated with 0.15 M Li2SO
4
+ 0.3 M Na2S
2O
3 (solution 1) for 300 seconds. The result shows
only heterosite, i.e. the FePO4 starting material. ....................... 90
Figure 6.2: The XRD fitting obtained from a sample treated with 0.35 M Li2SO
4
+ 0.7 M Na2S
2O
3 (solution 3) for 7200 seconds. The result shows
a mixed phase, i.e. partial conversion. Reproduced from Ref. 1
with permission from The Royal Society of Chemistry. (This
figure was published in March 20141) ...................................... 91
Figure 6.3: The XRD fitting obtained from a sample treated with 1.5 M Li2SO
4 +
3.0 M Na2S
2O
3 (solution 1) for 3600 seconds. The result shows
pure olivine, i.e. LiFePO4 .......................................................... 91
Figure 6.4: A validating graph of percentage lithium content and specific
charge. Reproduced from Ref. 1 with permission from The Royal
Society of Chemistry. (This figure was published in March 20141)92
Figure 6.5: Potentiometric titration of LixFePO4 electrodes prepared with the
reaction product of FePO4 in 0.35 M Li
2SO
4 + 0.7 M Na
2S
2O
3 for
different times, as indicated. Specific current: 17 mA g-1 at C/10
Reproduced from Ref. 1 with permission from The Royal Society
of Chemistry. (This figure was published in March 20141). ...... 93
Figure 6.6: The comparison of XRD, PT, and MS techniques in the extent of
lithiation which was obtained from 0.15 M Li2SO
4+0.3 M Na
2S
2O
3.96
Figure 6.7: The comparison of XRD, PT, and MS techniques in the extent of
lithiation which was obtained from 0.35 M Li2SO
4+0.7 M Na
2S
2O
3.97
xi
Figure 6.8: The comparison of XRD, PT, and MS techniques in the extent of
lithiation which was obtained from 0.75 M Li2SO
4+1.5 M Na
2S
2O
3.97
Figure 6.9: The comparison of XRD, PT, and MS techniques in the extent of
lithiation which was obtained from 1.5 M Li2SO
4+3 M Na
2S
2O
3. . 98
Figure 6.10: Phase fraction of olivine in the heterosite/ olivine composite. .... 98
Figure 6.11: Example of the XRD fitting data for 0.4:4:1 (0.03 M LiCl + 0.3 M
Na2S
2O
3 + 0.075 M FePO
4) for 172800 s (2 days) result in
Li0.25
FePO4 .............................................................................. 102
Figure 6.12: Example of the XRD fitting data for 0.8:4:1 (0.06 M LiCl + 3.0 M
Na2S
2O
3 + 0.075 M FePO
4) for 172800 s (2 days) result in Li
0.80
FePO4) ................................................................................... 103
Figure 6.13: Example of the XRD fitting data for 4:4:1(0.3M LiCl + 0.3 M
Na2S
2O
3 + 0.075 M FePO
4) for 36000 s (10 hours) result in
LiFePO4 ................................................................................. 103
Figure 6.14: Example of the XRD fitting data for 4:1.3:1 (0.3 M LiCl + 0.1 M
Na2S
2O
3 + 0.075 M FePO
4) for 172800 s (2 days) result in
Li0.35
FePO4 .............................................................................. 104
Figure 6.15: Example of the XRD fitting data for 4:8:1 (0.3 M LiCl + 0.6 M
Na2S
2O
3 + 0.075 M FePO
4) for 14400 s (4 hours) result in Li
0.80
FePO4) ................................................................................... 105
Figure 6.16: Rate of the lithiated reaction that was obtained from 0.03 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (0.4:4:1 ratio), with respect of
time. ..................................................................................... 108
Figure 6.17: Rate of the lithiated reaction that was obtained from 0.06 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (0.8:4:1 ratio), with respect of
time. ..................................................................................... 109
Figure 6.18: Rate of the lithiated reaction that was obtained from 0.1 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (1.3:4:1 ratio), with respect of
time. ..................................................................................... 109
Figure 6.19: Rate of the lithiated reaction that was obtained from 0.3 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (4:4:1 ratio), with respect of
time. ..................................................................................... 110
Figure 6.20: Rate of the lithiated reaction that was obtained from 0.6 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (8:4:1 ratio), with respect of
time. ..................................................................................... 110
xii
Figure 6.21: Rate of the lithiated reaction that was obtained from 0.9 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (12:4:1 ratio), with respect of
time. ..................................................................................... 111
Figure 6.22: Rate of the lithiated reaction that was obtained from 1.2 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (16:4:1 ratio), with respect of
time. ..................................................................................... 111
Figure 6.23: The reaction rates of the kinetic study with respect to the lithium
concentration. ....................................................................... 112
Figure 6.24: Rate of the lithiated reaction that was obtained from 0.3 M [Li+],
0.1 M [S2O
32-] and 0.075 [FePO
4] (4:1.3:1 ratio), with respect of
time. ..................................................................................... 113
Figure 6.25: Rate of the lithiated reaction that was obtained from 0.3 M [Li+],
0.3 M [S2O
32-] and 0.075 [FePO
4] (4:4:1 ratio), with respect of
time. ..................................................................................... 113
Figure 6.26: Rate of the lithiated reaction that was obtained from 0.3 M [Li+],
0.6 M [S2O
32-] and 0.075 [FePO
4] (4:8:1 ratio), with respect of
time. ..................................................................................... 114
Figure 6.27: Rate of the lithiated reaction that was obtained from 0.3 M [Li+],
0.9 M [S2O
32-] and 0.075 [FePO
4] (4:12:1 ratio), with respect of
time. ..................................................................................... 114
Figure 6.28: Rate of the lithiated reaction that was obtained from 0.3 M [Li+],
1.2 M [S2O
32-] and 0.075 [FePO
4] (4:16:1 ratio), with respect of
time. ..................................................................................... 115
Figure 6.29: The reaction rates of the kinetic study with respect to the
thiosulphate concentration. ................................................... 116
Figure 6.30: The comparison of the experimental rate and the theoretical rate
of the experiments were obtained from a fixed [S2O
32-] =0.3 M
and [Li+] = 0.03 to 1.2 M. ....................................................... 119
Figure 6.31: The comparison of the experimental rate and the theoretical rate
of the experiments were obtained from a fixed [Li+] =0.3 M and
[S2O
32-] = 0.1 to 1.2 M. ........................................................... 119
Figure 6.32: a schematic current density profile of S2O
32- and LiFePO
4 whenError! Bookmark not defin
Figure 7.1: The XRD pattern obtained from a sample 1:10 of [Li+]:[Na+] treated
with 0.5 M Na2S
2O
3 for 24 h is compared to the initial XRD
pattern of LiFePO4 and the XRD of the de-lithiated sample
(heterosite FePO4) pattern. The result shows only LiFePO
4. ..... 130
xiii
Figure 7.2: The XRD pattern obtained from a sample1:50 of [Li+]:[Na+] treated
with 0.3 M Na2S
2O
3 for 24 h is compared to the initial XRD
pattern of LiFePO4 and the XRD of the de-lithiated sample
(heterosite FePO4) pattern. The result shows only LiFePO
4. ..... 130
Figure 7.3: The XRD pattern obtained from a sample 1:100 of [Li+]:[Na+] treated
with 0.3 M Na2S
2O
3 for 24 h is compared to the initial XRD
pattern of LiFePO4 and the XRD of the de-lithiated sample
(heterosite FePO4) pattern. The result shows only LiFePO
4. ..... 131
Figure 7.4: The XRD pattern obtained from a sample 1:10 of [Li+]:[Mg+2] treated
with 0.5 M Na2S
2O
3 for 24 h is compared to the initial XRD
pattern of LiFePO4 and the XRD of the de-lithiated sample
(heterosite FePO4) pattern. The result shows only LiFePO
4. ..... 132
Figure 7.5: The XRD pattern obtained from a sample 1:20 of [Li+]:[Mg+2] treated
with 0.5 M Na2S
2O
3 for 24 h is compared to the initial XRD
pattern of LiFePO4 and the XRD of the de-lithiated sample
(heterosite FePO4) pattern. The result shows only LiFePO
4. ..... 132
Figure 7.6: The XRD pattern obtained from brine type A treated with 0.3 M
Na2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4
and the XRD de-lithiation (heterosite FePO4) sample pattern. The
result shows only LiFePO4. .................................................... 136
Figure 7.7: The XRD pattern obtained from brine type B treated with 0.3 M
Na2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4
and the XRD de-lithiation (heterosite FePO4) sample pattern. The
result shows only LiFePO4. .................................................... 137
Figure 7.8: Electrochemical data at rate 0.1C of LiFePO4 prepared by chemical
lithiation of FePO4 in synthetic brines; i.e. type A and B. The
results obtained with the initial LiFePO4 (Tantung) are also
included in the graph for a comparison. (This figure was
published in March 201413). .................................................. 138
Figure 7.9: Electrochemical cycling of LiFePO4 at different cycling rates, as
indicated (0.1C to C). ........................................................... 139
Figure 8.1: Cyclic voltammogram of a Pt working electrode in: ................... 144
Figure 8.2: Cyclic voltammogram of a) a Pt electrode coated with LiFePO
4 in 1.5
M Li2SO
4 (Solution 3*; black), Pt electrode
in 1.5 M Li
2SO
4 and 1.5
M Na2S
2O
3 (Solution 2*; blue), and a Pt electrode
coated with
xiv
LiFePO4 in 1.5 M Li
2SO
4 and 1.5 M Na
2S
2O
3 aqueous electrolyte
(Solution 4*; red). .................................................................. 145
Figure 8.3: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in ........................................................................... 146
Figure 8.4: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in ........................................................................... 147
Figure 8.5: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in ........................................................................... 148
Figure 8.6: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in ........................................................................... 148
Figure 1: The XRD fitting obtained from a sample treated with 0.3 M LiCl + 0.6
M Na2SO
3 for 24 hours. The result shows pure olivine, i.e.
LiFePO4 .................................................................................. 158
xv
DECLARATION OF AUTHORSHIP
I, ....................................................................................... [please print name]
declare that this thesis and the work presented in it are my own and has been
generated by me as the result of my own original research.
[title of thesis] ..................................................................................................
.........................................................................................................................
I confirm that:
1. This work was done wholly or mainly while in candidature for a research
degree at this University;
2. Where any part of this thesis has previously been submitted for a degree or
any other qualification at this University or any other institution, this has
been clearly stated;
3. Where I have consulted the published work of others, this is always clearly
attributed;
4. Where I have quoted from the work of others, the source is always given.
With the exception of such quotations, this thesis is entirely my own work;
5. I have acknowledged all main sources of help;
6. Where the thesis is based on work done by myself jointly with others, I have
made clear exactly what was done by others and what I have contributed
myself;
7. [Delete as appropriate] None of this work has been published before
submission [or] Parts of this work have been published as: [please list
references below]:
Signed: ..............................................................................................................
Date: .................................................................................................................
xvii
Acknowledgements
Firstly, I am grateful to have Professor John R. Owen and Dr. Nuria Garcia-Araez
as my supervisor and co-supervisor, respectively. I would like to thank them for
their supervision and support of the ideas for my work, also their
encouragement as I worked through my PhD life.
I owe several people a debt of gratitude for their help and knowledge.
Professor Andrew L. Hector for his support, especially on my XRD work; Dr. J.
Andy Milton from Ocean and Earth Science, National Oceanography Centre
Southampton, for his kindness and help with the ICP-MS analysis; Dr. Guy
Denuault for his expertise on microelectrodes; also, Heather Simpkins and
Andrew Fleming for their proofreading expertise.
I would like to acknowledge the Ministry of Science and Technology, the Royal
Thai Government for their financial support. I would also like to thank my
superiors and colleagues from FAMC group, National Metal and Materials
Technology Center (MTEC), National Science and Technology Development
Agency (NSTDA), and Office of Educational Affairs (The Royal Thai Embassy): in
particular Dr. Paritud B., Prof. Pramote D., Assoc. Prof.Siriluck N., Dr. Ekkarut
V., Dr. Samerkhae J., Chanida N., Somchai I., and Pakin J.
My acknowledgement section would not be complete without thanking Phra Aj
Professor Khammai D., Phra Aj Uthai Y., Phra Aj Bhikkhu G., Phra Aj. Khoon T.
for teaching me mindfulness and providing me with hospitality.
I would like to thank the members of the Owen Group, past and present, for
their support and friendship (James F, Andy, Jake, Roy, Mike, Saddam, Will, P’
Pan, Kanjiro, Alex, Matt L, Matt R, James D and Louisa). My friends from the UK
and Thailand, i.e.Wai, Nut, Cho, Fon, P’ Toey, R’Tank, P’Ying, P’Pun, N’Eve, the
Fleming Family, P’Luck-P’Pong, Danni, Ashley, P’Khawn, P’Pan, P’Aoot, Dad-
Prasert, P’Nuch, P’Dee, N’Pai, and Luck.
Last, this part of my life could not have happened without my family’s support
in every way, trusting and encouraging me. I am very much obliged to my
family, the late Professor Kitti (Dad), Pilaiwan (Mom), and Manuswi (Sis)-
Intaranont. I would like to send a message to my Dad if only he could
acknowledge me from above. “Daddy, I did it! Hope that you are proud of me.
Love you maxxxx”
xviii
xix
Definitions and Abbreviations
ɑ An activity
c Concentration/ mol per decimetre (mol dm-1)
CE Counter electrode
CV Cyclic voltammogram
d density g cm-3
E Potential/ V
Ee0 The standard potential/ Volt
EV Electric vehicle
F Faraday constant (96485/ C mol-1)
GSAS General Structure Analysis System
∆G Gibbs free energy change/ Joule per mol (J mol-1)
I Current/ Ampere
ICP-MS Inductively coupled plasma mass spectrometer
ICSD Inorganic Crystal Structure Database
and anodic and cathodic rate constants
and standard rate constant of anodic and cathodic
κ conductivity/ Siemens per meter (S m-1)
LCE lithium carbonate equivalent
LFP lithium iron phosphate
LIB Lithium-ion battery
n moles of electrons
NMP N-Methyl-2-pyrrolidone
Mw molecular weight/ gram per mol (g mol-1)
xx
O, R Oxidised, reduced species
Ppb Part per billion
ppm Part per million
PT Potentiometric titration
PTFE Polytetrafluoroethylene
PVDF Polyvinylidene fluoride
R The ideal gas constant (8.314 /J mol-1K-1)
REF Reference electrode
SEM Scanning electron microscopy
SHE Standard hydrogen electrode
SHE Standard hydrogen electrode
T The absolute temperature (Kelvin)
VMP Versatile multichannel potentiostat
WE Working electrode
XRD X-ray diffraction
ᴧ molar conductivity/ Siemens meter square per mol (Sm2 mol-1)
∝ and ∝ anodic and cathodic transfer coefficient (∝ ∝ 1
Chapter 1: Introduction
1
Chapter 1: Introduction
1
Introduction Chapter 1:
The Availability of Lithium-Present and Future 1.1
1.1.1 Consumption, Production and Price of Lithium
Lithium (Li) is one of the most essential elements that we have been using in
everyday life, for example lithium-ion batteries (LIB), glass, ceramics, lubricant
greases, etc. In terms of lithium production and consumption worldwide, in
Table 1.1, the US geological survey reported that the production and
consumption were estimated to have increased 5% and 37%, respectively, from
2011-2014.1-5 The rate of lithium consumption seems to be an exponential
growth due to the expansion of the market for lithium-ion batteries. The
average of 2011-2014 annual growth rate is approximately 11%.
Table 1.1: Lithium consumption application changes between 2003 and 2011.
Year
Worldwide estimated/ KTonnes
Lithium consumption/ % b
Production a Consumption Ceramics and Glass
Batteries Other
20111 34 24 29 27 44 20122,3 35 28 30 22 48 20133 35 30 35 29 36 20144,5 36 33 35 31 34 a Excludes U.S. production b Percent of the worldwide consumption
In 2010, the LIB dominated the worldwide rechargeable battery marketplace,
with around 132 Ktonnes of battery weight sold per year.6 The majority of LIB
are used for laptops and tablets (60.75 Ktonnes of battery weight per year).6
Automotive Li-ion batteries only accounted for 10% of total electric vehicle (EV)
batteries which were produced and used for EV’s in 2008.7 However, Frost &
Sullivan’s research and consulting company predicted that the usage of Li-ion
for EV would be increased to 80% of the total EV batteries in 2015.7 This
information suggests that LIB will continue to grow in the future with greater
Chapter 1: Introduction
2
demand year after year. This indicates that the rising use of LIB will create a
problem of lithium scarcity.
In 2011, the four largest lithium producers were Australia (36%), Chile (35.6%),
China (17%) and Argentina (7.4%).8 However, Chile has switched to a leading
position with Australia since 2012. The top five lithium supplier companies in
2012 are Talison (36%; in Australia), SQM (26%; in Chile), Qinghai CITIC (18%;
in China), Chemetall or Rockwood (12%; in Chile and USA) and FMC Chemical
(7%; in Argentina), as shown in Figure 1.1. 9,10
Figure 1.1: The world lithium production in 2012, categorised by lithium mining companies, adapted from Maxwell, 2014.10
Li2CO
3 has the advantage of a very cheap extraction process which reacts
directly with an acid to produce a suitable salt e.g. LiPF6 for battery production.
Other products of lithium extraction such as lithium hydroxide (LiOH) and
lithium chloride (LiCl) are costed according to their lithium carbonate
equivalent (LCE) which is the mass of Li2CO
3 that would contain the same
amount of lithium as the product.
Talison36%
SQM26%
Qinghai CITIC18%
Rockwood12%
FMC7%
Others1%
Chapter 1: Introduction
3
Figure 1.2 shows prices of lithium in raw materials for many applications
consuming lithium, from 1990-2013. In approximately 1996, the price per LCE
showed a dramatic fall when SQM in Chile, joined the market10 and increased
its full operation capacity9 until it became one of the biggest lithium mining
companies in the world. Moreover, the production in South America was
cheaper than the US, causing many lithium mining companies in the US closing
down. From 2005, the price then increased significantly because of a drop in
supply due to climate impacts on the lithium production in Argentina. Later,
the price started to drop slightly as Chinese producers began to enter the
market (~2010), giving more supply. 5,9 After 2010, there have been two more
producers -Galaxy Resources (Australia) and Canada Lithium (Canada).
Although, some more lithium mining companies started to operate during the
past 10 years, the price of LCE is not as low as it was before 2005. This is
because the demand of lithium is growing whereas supply of lithium resources
is limiting and decreasing.
Figure 1.2: Estimated lithium carbonate equivalent prices from 1990 to 2013, adapted from Maxwell, 2015.5
0
1
2
3
4
5
6
7
8
1990 1995 2000 2005 2010 2015
US Dollars per kg LCE
Year
Chapter 1: Introduction
4
1.1.2 Resources and Supply
Resources and reserves are terminology to classify elements/ minerals.
According to the U.S. Geological survey, resources mean “a concentration of
naturally occurring materials in such form that economic extraction of a
commodity is regarded as feasible, either currently or at some future time”.3
Reserves mean “the resources that could be economically extracted or
produced at the time of determination”.3 Two questions arise from this
definition:
What is the total lithium content on Earth?
How soon will it be before lithium is scarce?
In 2014, the U.S. Geological survey reported that the world’s lithium resources
and reserves are approximately 34 million tonnes and 13 million tonnes,
respectively3. In 2009, 70% of the world’s lithium resources were in South
America; Argentina, Bolivia, Brazil and Chile.7 By 2013, America, Australia, and
China has become one of the main global lithium deposits countries.11 These
resources are expected to be depleted in 65 years’ time assuming a 11%
annual increase in lithium consumption as observed during 2011-2014. The
effect of the scarcity of lithium has promoted lithium recycling technology as
has been commercialised by companies such as Toxco (US), Umicore (Belgium),
and Sumitomo mining and metals (Japan).
Considering the reactivity of lithium with oxygen and water, it is not surprising
that lithium is not found in nature in metallic form. Various compounds of
lithium can be found in the four main deposit types, i.e. salar brines, minerals,
sedimentary rocks and seawater. The most recent economical way to extract
lithium is from salar brines- the second tier being from minerals, i.e.
sedimentary rocks (pegmatite) or clay.8,11,12
As already mentioned, the most concentrated lithium resources are mainly
found in brine lake deposits, or salar, containing lithium ion up to 5x103 ppm,
Chapter 1: Introduction
5
largely located in South America.11,13-17 One of the top three lithium resources is
in Uyuni, Bolivia which has about 27% of the world’s lithium resources.13,18
In Bolivia’s Salar de Uyuni, sodium ion (Na+) is known to have one of the most
concentrated cations, at a concentration of up to 113x103 ppm.13 Lithium (Li+),
magnesium (Mg2+) and potassium (K+) ion concentrations are up to 4.7x103,
75x103 ppm and 30x103 ppm, respectively, as shown in Table 1.2.13 The ratios
of values of lithium ion to some interesting cations such as sodium ion (Li+:Na+)
and magnesium ion (Li+:Mg2+).
The lithium concentration in seawater is comparatively very low approximately
at 0.15 ppm. Other cations such as Na+, K+ and Mg2+ are approximately 1.05 x
104, 4.53 x 102 and 3.08 x 104 ppm, respectively. 11,15-17,19-21
Table 1.3 illustrates how difficult it is to extract Li+ from the sources. The
higher ratios correspond to an easy Li+ extraction. Therefore, Li+ extraction from
brine is easier than extracting from seawater. Lithium from brine can be
extracted by an ion exchange, a solvent extraction, adsorption, and
electrochemistry. 11,14-16
Table 1.2: The comparison of cations between the concentration of cations in brine and seawater
Cations Brine13/ ppm Seawater11,15-17,19-21/ppm Li+ 4.70 x 103 1.50 x 10-1 Na+ 1.13 x 105 1.05 x 104 K+ 3.00 x 104 4.53 x 102
Mg2+ 7.50 x 104 3.08 x 104
Table 1.3: The comparison of ionic fractions (Li+ vs. other cation from Table 1.2) between the concentration of cations in brine and seawater
Cations Brine Seawater Li+/Na+ 4.16 x 10-2 9.43 x 10-6 Li+/K+ 1.57 x 10-1 2.63 x 10-4
Li+/Mg2+ 6.27 x 10-2 7.87 x 10-5
Chapter 1: Introduction
6
Total ratio* 2.61 x 10-1 3.51 x 10-4 *Total ratio: sum up of Li+/Na+, Li+/K+ and Li+/Mg2+
Lithium Extraction from Brine 1.2
1.2.1 Lime Soda Evaporation
Lithium chloride (LiCl) contained in brine is used to make Li2CO
3 as an end
product. The cheapest method to extract lithium from brine is by lime soda
evaporation. This process is done chemically. The most inefficient step is to
evaporate brine water using solar energy for 1-2 years.11,12 Many ponds are built
using salts as a wall and each pond is used to precipitate crystallised salts.
Basically, the purpose of the evaporation process is to crystallise out unwanted
salts and concentrate the more soluble lithium chloride in the brine. The
resultant solution is transported to another pond and lime or calcium
hydroxide (CaOH) is added to the concentrated pond to precipitate out such
compounds as magnesium hydroxide (Mg(OH)2), calcium sulphate (CaSO
4),
calcium borate (Ca3(BO
3)
2), etc. A further liquid transfer takes place and soda
ash or sodium carbonate (Na2CO
3) is added to precipitate the less soluble
Li2CO
3 as the raw material.
Approximately 50% of lithium is extracted from the brine using this process
and the rest of the lithium flows back to the first pond. The Li2CO
3 product is
usually 99.0% purity, which is sufficiently pure for glass and ceramic
applications.12 Therefore, further purification processes are needed to make
lithium acceptable for the grade needed for batteries, i.e. 99.9%.12
1.2.2 Ion-sieve spinel type
There are many spinel types of material that absorb cations from aqueous
solution with a high selectivity of lithium. Examples are λ-MnO2, H
1.33Mn
1.67O
4
H1.6
Mn1.6
O4, which are delithiated from LiMnO
2, and Li
1.33Mn
1.67O
4, Li
1.6Mn
1.6O
4
Chapter 1: Introduction
7
respectively.14,22-24 The recovery of lithium, using a manganese spinel type, can
be done chemically.
The following general scheme illustrates the chemical approach of ion
exchange, where the material is delithiated by an acid treatment. The lithium
ion is then removed from the structure and replaced by a proton, as shown in
Equation 1.1.
Equation 1.1 →
The delithiated material is then used to extract mostly lithium ions from a
lithium salt solution containing several other cations. The reaction should be a
simple ion-exchange, facilitated by an alkaline solution to help remove H+ from
the structure, as shown in Equation 1.2.
Equation 1.2 →
However, this reaction, as reported, can involve complications due to redox
reactions including oxygen evolution (as shown in Equation 1.3) and the
formation of soluble Mn(II) or Mn(VII) compounds which would contaminate the
effluent. Also, another disadvantage is the need to control the pH and redox
conditions simultaneously to preserve the structure.
Equation 1.3 →
1.2.3 LiFePO4 Electrochemical
An electrochemical method to recover lithium from brine using LiFePO4 was
introduced in 2012 by Pasta et al.25 There are four steps in the process, using a
3-electrode system. Both working and counter electrodes are a carbon cloth
covered by drop castinga of LiFePO4 and silver (Ag) particles slurry, respectively.
A silver/silver chloride electrode is used as the reference.
a Drop casting is an electrode preparation by dropping a mixture of active material and solvent solution on the electrode and evaporating the solvent.
Chapter 1: Introduction
8
Step 1, the 3-electrode system is in brine containing lithium as an electrolyte. A
charged working electrode, i.e. heterosite FePO4, is intercalated with lithium
ion (Li+) from the brine by applying a negative current to the WE (cathode), the
chloride ion (Cl-) from brine is captured at the Ag CE (anode). Step 2, the
electrodes are transferred and submerged into a recovery solution, where the
Li+ and Cl- are released by applying a reverse current to the cathode in step 3.
In step 4, the recovered solution is replaced with the new brine and the cycle is
repeated.
This process can recover lithium chloride solution selectively in a sodium-rich
brine, i.e. 1: 100 of Li+: Na+ while consuming only ~1 W h mol-1.25 This is better
than the ion exchange process with the spinel manganese oxide, which
consumes apparently approximately 33 W h mol-1.25 Nevertheless, there is a
drawback due to the high price of silver.
1.2.4 Aims and Objectives of this work
The aim is to investigate and evaluate a new, low cost method for lithium
extraction from lithium solutions contaminated with other metals as described
above.
The objectives are:
1. To demonstrate the effectiveness of the process using different
reagents
2. To assess the quality of the lithium product that can be obtained.
3. To investigate the cost and environmental acceptability of the method,
resource consumption and process time.
The principle of the process is to use FePO4 in conjunction with a reducing
agent as a lithium ion absorbent which can subsequently release lithium ions
Chapter 1: Introduction
9
upon addition of an oxidising agent as shown in Equation 1.4- Equation 1.5,
respectively.
Equation 1.4 → 3.45 . / or0.15 .
Equation 1.5 ←
A Review of LiFePO4 1.3
Since 1997 after the first reports on LiFePO4 by Padhi et al.26-28, there have been
a large number of research studies on this cathode, generating approximately
4200 reports discussing various aspects including its remarkable surface
stability and efforts to overcome its main disadvantage of poor electronic
conductivity that necessitates the application of a carbon coating to aid
electron transport.
Lithium iron phosphate (LiFePO4) is a well-known cathode material for
secondary lithium-ion batteries due to its safety, performance and cost. It has
been used commercially for some time. Heterosite iron phosphate (FePO4)
undergoes a reversible reaction to intercalate lithium at 3.45 V vs. Li/Li+ or
0.15 V vs. SCE, with a redox couple of Fe3+/Fe2+, as shown in Equation 1.4-
Equation 1.5 and Figure 1.3. The theoretical capacity of LiFePO4 is 170 mAh g-1
but in practical use is approximately ~150 mAh g-1.29
The surface stability of LiFePO4 is generally attributed to the strong P-O
covalent bonds in the orthorhombic olivine structure which restrain oxygen
from release, resulting in a high thermal safety battery.27 The strong covalent
bond of PO43- also decreases the iron ion covalent bond, giving a lower Fe3+/Fe2+
redox potential than that of simple hydrated ions in aqueous solution.30
Figure 1.3 shows a cyclic voltammogram (CV) of a two phase material, i.e.
LiFePO4 to FePO
4. The electron transfer occurs in a solid reagent (FePO
4), giving
symmetrical redox peaks on both oxidation and reduction. This characteristic
is distinctive to solid electrochemistry which is described more in Chapter 2.
Chapter 1: Introduction
10
Figure 1.3: Cyclic voltammogram of an immobilized LiFePO4 on Pt electrode
(solid line) and bare Pt electrode (dotted line) in 1 M Li2SO
4. Reproduced with
permission from ref. 31. Copyright 2015, Elsevier. (This figure was published in January 2007 by Mi et al.31)
LiFePO4
adopts an olivine structure type (known as triphylite), orthorhombic
lattice system, space group Pnma with a lattice parameter of a = 10.332(4) Å,
b = 6.010(5) Å and c = 4.787 Å, as shown in Figure 1.4.32 LiFePO4 consists of
PO4
tetrahedra, FeO6 octahedra and a distorted hexagonal close-packed
framework filled with lithium ion. The connections between PO4 tetrahedra and
FeO6
octahedra are presented as sharing oxygen (red circle) and sharing an
edge (blue circle), as shown in Figure 1.4a. When LiFePO4 is charged, the
topotactic phase transformation from olivine to heterosite FePO4 structure will
be performed, giving a volume in unit-cell changes of approximately 6.5%.27,33
The unusually small volume changes during the transformation again
contribute to excellent stability which provides the advantages of a high safety
battery and a suitable performance for the electric vehicle battery.27,30
LiFePO4
Bare Pt
electrode
Chapter 1: Introduction
11
Figure 1.4: a)The structure of a unit-cell of LiFePO4, b) lithium ion migration
path and c) curved trajectories or wavelike path of lithium ion migration. Reprinted with permission from Ref.34. Copyright 2015 American Chemical Society. (This figure was published in July 2005 by Islam et al.34)
Generally in LiFePO4, the lithium ion could migrate in three ways, which are
[010], [001], and [101] along the b, c and a-c axis, respectively, as shown in
Figure 1.4b. Nonetheless, the lithium ion in LiFePO4 is known for migrating
through one-dimension (1D) via the b axis or [010] direction, which indicates
the Miller index of the lattice plain, resulting in low conductivity compared to
structures having 2D or 3D migration.30,35-37 This point was confirmed by the
reports of Morgan et al.36 and Islam et.al.34, as illustrated in Table 1.4. The
average atom positions between lithium ions in each direction were reported,
i.e. 3.00 Å, 4.68 Å and 5.69 Å for the [010], [001] and [101] paths,
respectively.30,34,36,38 The diffusion coefficient (D) of each pathway was calculated
by Morgan et al., resulting in D[010]
> D[101]
> D[001]
. The high diffusion coefficient
implies a faster movement of lithium ion in that direction. The activation
c b
a
Chapter 1: Introduction
12
energy (Ea) indicates the possible diffusion pathway, and lower E
a represents the
favourable pathway. The report showed that Ea [010]
< Ea [101]
< Ea [001]
. The high Ea is
caused by accumulated cation where the FeO6 octahedral face-shared with two
PO4 tetrahedra.35,36 Islam et al. reported the migration energy (E
m) of each
pathway, resulting in Em[010]
< E
m[001] <
E
m[101]. 30,34 The lower E
m of lithium suggests
that lithium ion movement is more preferable. Therefore, these results agree
and confirm that lithium ion in the LiFePO4
[010] pathway is more desirable
than the others.
Table 1.4: Properties of possible lithium ion movements.
Pathway [010] [001] [101] Axis b c a
Atom position Li+-Li+ 34,36/Å 3.00 4.68 5.69 Diffusion coefficient36/cm2 s-1 10-8 10-45 10-19
Activation energy36/ eV 0.27 >2.5 1 Migration energy34/ eV 0.55 2.89 3.36
Thermodynamic Principles for the Selection of 1.4
Suitable Oxidizing and Reducing Agents
1.4.1 Oxidizing Agent
The preliminary research on the subject of an oxidizing agent to delithiate
LiFePO4
was undertaken. There are commonly known oxidizing agents to
extract lithium from LiFePO4,
namely bromine26,39 in acetonitrile and
nitronium40,41 in acetonitrile.42 However, acetonitrile or methyl cyanide is well
known for being hazardous to the human body and environmentally unsafe.
Thus, an aqueous based reagent is considered environmental preferable. At
the start of this project, only two oxidising agents had been reported to give
lithium extraction from LiFePO4, namely hydrogen peroxide42 in acid and
potassium persulphate43-47 in water. Potassium persulphate (K2S
2O
8) was chosen
here because of its neutral pH.
Chapter 1: Introduction
13
Equation 1.6, the redox couple of S2O
82-
/ SO
42- has a standard potential of
+2.05 V vs. SHE48 or +5.10 V vs. Li/Li+, where the hydrogen reference at E = 0 is
+ 3.05 vs. Li/Li+ (2.05+3.05 =5.10). The potential of S2O
82- is higher than the
potential of LiFePO4 which is 3.04 V vs. Li/Li+ and therefore lithium can be
extracted from LiFePO4 using S
2O
82-.46
Equation 1.6 2 2 5.10 . /
The lithium extraction was presented by Ramana et al. (2009). The report
shows a 1:2 molar ratio of K2S
2O
8 to LiFePO
4 for which Equation 1.7 is shown:43
Equation 1.7 2 → 2
1.4.2 Reducing Agent
The following discussion shows the importance of testing the reducing agent
to give the best discrimination between for the absorption of lithium and
rejecting other metals. Among the main small cations in brine, lithium has the
smallest ionic radius which is 59 pm, as shown in Table 1.5.49 Although the
sodium ion has a radius which is 43 pm higher than lithium ion, it can be
inserted into the heterosite FePO4 to form sodium iron phosphate; NaFePO
4.
The lattice parameters of NaFePO4 and LiFePO
4 are very similar34,50-52, as shown
in Table 1.6. The volume of the NaFePO4 unit cell is only slightly bigger than
that of LiFePO4. Although the c parameter of NaFePO
4 is significantly bigger
than the others, the relative sizes of the ionic radii of lithium and sodium are
not considered to be significant factors that could give an enhanced selectivity
for lithium insertion. Selectivity for sodium is particularly desirable because it
is present in larger concentrations than lithium itself.
Table 1.5: Ionic radii of the potential cations that can be intercalated into FePO
449
Ions Lithium
(Li+) Magnesium
(Mg2+) Sodium
(Na+ )
Potassium (K+)
Ionic radii, r/Å 0.59 0.72 1.02 1.38
Chapter 1: Introduction
14
Table 1.6: The unit cell parameters of FePO4, LiFePO
4, and NaFePO
4 products
based on XRD data52
Cathode material
Lattice Parameter/ Å Volume/Å3
a b c FePO
4 9.8152(5) 5.7885(3) 4.7803(3) 271.593(4)
LiFePO4
10.3202(6) 6.0035(4) 4.6928(4) 291.020(8) NaFePO
4 10.4051(4) 6.2216(2) 4.9486(2) 319.933(9)
Given the weakness of size selectivity, an investigation into a new principle for
selectivity was made, based on the different redox potentials at which the
insertion takes place. The operating voltage of LiFePO4 is 3.45 V vs. Li/Li+, as
shown in Equation 1.8. 30,46,50,53,54 Galvanostatic studies by Zaghib et al. reported
an onset potential for sodium insertion at ~ 2.7 V vs. Na/Na+.53 This value can
be calculated to the standard reduction hydrogen reference electrode potential:
SHE, using Equation 1.9 and Equation 1.10.
Equation 1.8 3.45 . /
Equation 1.9 → 2.71 .
Equation 1.10 → 3.05 .
For a meaningful comparison, the above potential for sodium insertion needs
to be converted to the Li/Li+ scale. This may be done by assuming that the
positive shift for sodium ion reduction (Equation 1.9) on conversion from SHE
to the lithium scale is the same for lithium plating (Equation 1.10), i.e. +3.05 V
vs. Li/Li+. Thus, Na/Na+ should occur at E = (-2.71+3.05) = 0.34 V so that E0 for
sodium insertion in FePO4 should be at ~3 V vs. Li/Li+ (2.71+0.34).
Regarding the potential voltage of NaFePO4
and LiFePO4, a useful reducing
agent potential has to be between 3.04 and 3.45 V vs. Li/Li+, respectively, to
give a thermodynamic driving force for Li+ insertion but not for Na+ insertion, as
shown in Figure 1.5. Thiosulfate (S2O
32-) is a soluble reducing agent (Equation
1.11)55, which has a potential of 3.13 V vs. Li/Li+. The fact that thiosulfate is
0.32 V lower than the insertion potential of Li+, means that it should be a
suitable reducing agent, as shown in Figure 1.5. Furthermore, this potential is
Chapter 1: Introduction
15
0.09 V higher than the insertion of Na+; therefore, thiosulfate should exhibit
the lithium selectivity.
Equation 1.11 2 2 3.13 . /
Figure 1.5: The useful range of reducing agent potential (vs. lithium) for FePO4
lithiation.
As an alternative to the standard reduction potentials, the Gibbs free energy
change (∆G/ Joule mol-1) can determine whether a reaction is spontaneous or
not, under a constant pressure and temperature. If the ∆G of the reaction is
more than zero (∆G > 0), this means the chemical reaction is non-spontaneous
or unfavourable. On the other hand, if the ∆G of the reaction is less than zero
(∆G < 0), the reaction is spontaneous or favourable. In other words, the
reaction should go forwards not backwards.
The free energy change (∆G) is related to the standard state free energy (∆G0)
and the activities of reactants and products according to Equation 1.12.56
Equation 1.12 ∆ ∆ ln
Where R = the ideal gas constant (8.314 /J mol-1K-1) T = the absolute temperature (Kelvin) ɑ = the activity of →
The overall reaction of FePO4
and S2O
32- can be written as shown in Equation
1.13. The free energy change of this reaction is shown in Equation 1.14.
Equation 1.13 2 2 2 → 2
0 3.0 3.25 3.50 3.75
LiFePO4
S2O
3
2- NaFePO4
Useful reducing agent potential
Li+ can be inserted
Li+ can be extracted
Na+ can be inserted
Li/ Li+
Chapter 1: Introduction
16
Therefore,
Equation 1.14 ∆ ∆ ln
The activity of a pure solid of LiFePO4 and FePO
4 is considered to be unity. The
activity of the soluble reagents and products, i.e. Li+, S2O
32-, and S
4O
62-are
approximated to the molar concentration56. Accordingly, the overall ∆G can be
written as shown in Equation 1.15. Notably, the driving force (∆G) is
proportional to the log .
Equation 1.15 ∆ ∆ ln
In this study, therefore, the direction of the reaction driving force (i.e.
Li++FePO4 + S
2O
32-) can be determined from Equation 1.16
Equation 1.16, using the equilibrium potential.56-58 E0 is defined for the case
when all reagents are in the standard state (1 M). Also, E0 is expressing the
activity of the electron.
Equation 1.16 ∆
Where n = moles of electrons F = Faraday’s constant (96485 C mol-1) E0 = the equilibrium potential.
The ∆G0 of the reaction from Equation 1.8 and Equation 1.11 is calculated as
shown below:
∆G0 of Equation 1.8 3.45 . ⁄
∆
∆ 1 96485 3.45
∆ 333
∆G0 of Equation 1.11 2 2 3.13 . /
∆
∆ 2 96485 3.13
Chapter 1: Introduction
17
∆ 604
The overall reaction from Equation 1.8 -Equation 1.9 can be written as Equation
1.13, i.e. (2 x Equation 1.8) - Equation 1.9.
2 2 2 → 2
Therefore, the ∆G0 of Equation 1.13 shows a negative value, as a favourable
reaction :
∆ 2∆ ∆
∆ 2 333 604
∆ 62
The calculation below is to determine whether ∆G0 overall between FePO4
(Equation 1.17) and sodium thiosulphate (Na2S
2O
3) (
Equation 1.18) is positive or negative.
Equation 1.17 3.04 . ⁄
∆
∆ 1 96485 3.04
∆ 293
Equation 1.18 2 2 2 → 2
Therefore,
∆ 2∆ ∆
∆ 2 293 604
∆ 18
Chapter 1: Introduction
18
The result is positive which can be interpreted as an unfavourable chemical
reaction.
Outline of the Thesis 1.5
There are nine chapters in this thesis. Chapter 2 describes techniques which
have been used in each experiment. Chapter 3 describes the electrochemical
and physical characterisation of LiFePO4 and FePO
4. Chapter 4 shows chemical
delithiation experiments using LiFePO4 as a framework and K
2S
2O
8 as an
oxidizing agent. Chapter 5 demonstrates chemical lithiation experiments using
reducing agents, lithium iodide (LiI) and Na2S
2O
3 in aqueous solutions. Once
Na2S
2O
3 was found as the more suitable reducing agent, the kinetics of the
lithiation of FePO4 by Na
2S
2O
3 were studied, as described in Chapter 6. In
Chapter 7, the selectivity of the ion insertion reaction towards Li is assessed
from the composition of the products, using ICP-MS to determine the content
of sodium, magnesium and potassium. Chapter 8 illustrates how to find an
alternative reducing agent by using cyclic voltammetry and all conclusions are
provided in Chapter 9.
References 1.6
(1) United States Geological Survey, Mineral Commodity Summaries 2012. In Lithium; U.S. Government Printing Office: Washington, DC, 2012; pp 94-95. (2) United States Geological Survey, Mineral Commodity Summaries 2013. In Lithium; U.S. Government Printing Office: Washington, DC, 2013; pp 94-95. (3) United States Geological Survey, Mineral Commodity Summaries 2014. In Lithium; U.S. Government Printing Office: Washington, DC, 2014; pp 94-95. (4) United States Geological Survey, Mineral Commodity Summaries 2015. In Lithium; U.S. Government Printing Office: Washington, DC, 2015; pp 94-95. (5) Maxwell, P. Transparent and opaque pricing: The interesting case of lithium. Resources Policy 2015, 45, 92-97. (6) Wiaux, J.-P.: The Impact of New Applications on the European Rechargeable Battery Market. The 16th International Congress for Battery Recycling ICBR: Venice, Italy, 2011. (7) Jawad, I.: Reuse and Recycling to Ensure the Completion of the "Green Car", Analysis of the Global Market for Automotive Lithium-ion Battery
Chapter 1: Introduction
19
Recycling and Second Life (Frost and Sullivan). The 16th International Congress for Battery Recycling ICBR: Venice, Italy, 2011. (8) Speirs, J.; Contestabile, M.; Houari, Y.; Gross, R. The future of lithium availability for electric vehicle batteries. 2014, 35, 183-193. (9) Bauer, D.; Diamond, D.; Li, J.; Sandalow, D.; Telleen, P.; Wanner, B.: Critical Materials Strategy. In Chapter 3. Historical Supply, Demand and Prices for the Key Materials; U.S. Department of Energy: California, USA., 2010; pp 27-52. (10) Maxwell, P. Analysing the lithium industry: Demand, supply, and emerging developments. Mineral Economics 2014, 26, 97-106. (11) Talens Peiró, L.; Villalba Méndez, G.; Ayres, R. Lithium: Sources, Production, Uses, and Recovery Outlook. JOM 2013, 65, 986-996. (12) Moreno, L.: Lithium Industry: A Strategie Energy Metal. In Mineralogy and Resources; Euro Pacific Canada: Toronto, Canada, 2013; pp 3-9. (13) Risacher, F.; Fritz, B. Quaternary geochemical evolution of the salars of Uyuni and Coipasa, Central Altiplano, Bolivia. Chemical Geology 1991, 90, 211-231. (14) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H1.6Mn1.6O4) Derived from Li1.6Mn1.6O4. Industrial & Engineering Chemistry Research 2001, 40, 2054-2058. (15) Riley, J. P.; Tongudai, M. The lithium content of sea water. 1964, 11, 563-568. (16) Angino, E. E.; Billings, G. K. Lithium content of sea water by atomic absorption spectrometry. 1966, 30, 153-158. (17) Lindal, B. The production of chemicals from brine and seawater using geothermal energy. 1970, 2, Part 1, 910-917. (18) Gruber, P. W.; Medina, P. A.; Keoleian, G. A.; Kesler, S. E.; Everson, M. P.; Wallington, T. J. Global Lithium Availability. Journal of Industrial Ecology 2011, 15, 760-775. (19) Wang, T.; Kee Lee, H.; Yau Li, S. F. Determination of Sodium, Potassium, Magnesium, and Calcium in Seawater by Capillary Electrophoresis with Indirect Photometric Detection. Journal of Liquid Chromatography & Related Technologies 1998, 21, 2485-2496. (20) Tangen, A.; Lund, W.; Frederiksen, R. B. Determination of Na+, K+, Mg2+ and Ca2+ in mixtures of seawater and formation water by capillary electrophoresis. 1997, 767, 311-317. (21) Kang, K. C.; Linga, P.; Park, K.-n.; Choi, S.-J.; Lee, J. D. Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg2+, Ca2+, B3+, Cl−, SO
42−). 2014, 353, 84-90.
(22) Ooi, K.; Miyai, Y.; Katoh, S.; Maeda, H.; Abe, M. Topotactic lithium(1+) insertion to .lambda.-manganese dioxide in the aqueous phase. Langmuir 1989, 5, 150-157. (23) Ooi, K.; Miyai, Y.; Sakakihara, J. Mechanism of lithium(1+) insertion in spinel-type manganese oxide. Redox and ion-exchange reactions. Langmuir 1991, 7, 1167-1171. (24) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H
1.33Mn
1.67O
4 and H
1.6Mn
1.6O
4. Chemistry Letters 2012,
41, 1647-1649. (25) Pasta, M.; Battistel, A.; La Mantia, F. Batteries for lithium recovery from brines. Energy & Environmental Science 2012, 5, 9487-9491.
Chapter 1: Introduction
20
(26) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. Journal of the Electrochemical Society 1997, 144, 1188-1194. (27) Wang, J.; Sun, X. Olivine LiFePO
4: the remaining challenges for future
energy storage. Energy & Environmental Science 2015, 8, 1110-1138. (28) Tang, P.; Holzwarth, N. A. W. Electronic Structure of FePO
4, LiFePO
4,
and Related Materials. Physical Review B 2003, 68, 1-9. (29) Rangappa, D.; Honma, I.: Designing Nanocrystal Electrodes by Supercritical Fluid Process and Their Electrochemical Properties. In Nanocrystal; 1st ed.; Masuda, Y., Ed., 2011; pp 293-313. (30) Wu, B.; Ren, Y.; Li, N.: LiFePO
4 Cathode Material. In Electric Vehicles -
The Benefits and Barriers, 2011; pp 199-216. (31) Mi, C.; Zhang, X.; Li, H. Electrochemical behaviors of solid LiFePO
4
and Li0.99
Nb0.01
FePO4 in Li
2SO
4 aqueous electrolyte. Journal of Electroanalytical
Chemistry 2007, 602, 245-254. (32) Yakubovich, O. V.; Belokoneva, E. L.; Tsirel'son, V. G.; Urusov, V. S. Electron density distribution in synthetic triphylite LiFePO
4 1990, 45, 93-99.
(33) Okubo, M.; Asakura, D.; Mizuno, Y.; Kim, J. D.; Mizokawa, T.; Kudo, T.; Honma, I. Switching Redox-Active Sites by Valence Tautomerism in Prussian Blue Analogues A
xMn
y[Fe(CN)
6]∙nH
2O (A: K, Rb): Robust Frameworks for
Reversible Li Storage. J. Phys. Chem. Lett. 2010, 1, 2063-2071. (34) Islam, M. S.; Driscoll, D. J.; Fisher, C. A. J.; Slater, P. R. Atomic-Scale Investigation of Defects, Dopants, and Lithium Transport in the LiFePO
4 Olivine-
Type Battery Material. Chemistry of Materials 2005, 17, 5085-5092. (35) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Positive Electrode Materials for Li-Ion and Li-Batteries. Chemistry of Materials 2010, 22, 691-714. (36) Morgan, D.; Van der Ven, A.; Ceder, G. Li Conductivity in Li
x MPO
4 ( M = Mn , Fe , Co , Ni ) Olivine Materials. Electrochemical and Solid-
State Letters 2004, 7, A30-A32. (37) Allen, J. L.; Jow, T. R.; Wolfenstine, J. Kinetic Study of the Electrochemical FePO
4 to LiFePO
4 Phase Transition. Chemistry of Materials
2007, 19, 2108-2111. (38) Rissouli, K.; Benkhouja, K.; Bettach, M.; Sadel, A.; Zahir, M.; Derrory, A.; Drillon, M. Crystallochemical and magnetic studies of LiMi
1−xM
x'PO
4 (M, M' =
Mn, Co, Ni; O ≤ x ≤ 1). 1998, 23, 85-88. (39) Andersson, A. S.; Kalska, B.; Häggström, L.; Thomas, J. O. Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mössbauer spectroscopy study. Solid State Ionics 2000, 130, 41-52. (40) Rousse, G.; Rodriguez-Carvajal, J.; Patoux, S.; Masquelier, C. Magnetic Structures of the Triphylite LiFePO4 and of Its Delithiated Form FePO4. Chemistry of Materials 2003, 15, 4082-4090. (41) Yu, X.; Wang, Q.; Zhou, Y.; Li, H.; Yang, X.-Q.; Nam, K.-W.; Ehrlich, S. N.; Khalid, S.; Meng, Y. S. High rate delithiation behaviour of LiFePO
4 studied by
quick X-ray absorption spectroscopy. Chemical Communications 2012, 48, 11537-11539. (42) Lepage, D.; Michot, C.; Liang, G.; Gauthier, M.; Schougaard, S. B. A Soft Chemistry Approach to Coating of LiFePO4 with a Conducting Polymer. Angewandte Chemie International Edition 2011, 50, 6884-6887. (43) Ramana, C. V.; Mauger, A.; Gendron, F.; Julien, C. M.; Zaghib, K. Study of the Li-insertion/extraction process in LiFePO4/FePO4. Journal of Power Sources 2009, 187, 555-564.
Chapter 1: Introduction
21
(44) Zaghib, K.; Mauger, A.; Goodenough, J. B.; Gendron, F.; Julien, C. M. Electronic, Optical, and Magnetic Properties of LiFePO4: Small Magnetic Polaron Effects. Chemistry of Materials 2007, 19, 3740-3747. (45) Ait-Salah, A.; Dodd, J.; Mauger, A.; Yazami, R.; Gendron, F.; Julien, C. M. Structural and Magnetic Properties of LiFePO4 and Lithium Extraction Effects. Zeitschrift für anorganische und allgemeine Chemie 2006, 632, 1598-1605. (46) Dodd, J. L.; Yazami, R.; Fultz, B. Phase Diagram of Li
xFePO
4.
Electrochemical and Solid-State Letters 2006, 9, A151-A155. (47) Miao, S.; Kocher, M.; Rez, P.; Fultz, B.; Yazami, R.; Ahn, C. C. Local Electronic Structure of Olivine Phases of LixFePO4. The Journal of Physical Chemistry A 2007, 111, 4242-4247. (48) Atkins, P. W.; De Paula, J.: Standard Potentials. In The elements of physical chemistry; 4th, Ed.; Oxford University Press: Oxford, 2005; pp 612-613. (49) Atkins, P. W.; De Paula, J.: Mettallic, Ionic, and Covalent solids. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford, 2005; pp 376-400. (50) Whiteside, A.; Fisher, C. A. J.; Parker, S. C.; Saiful Islam, M. Particle shapes and surface structures of olivine NaFePO4 in comparison to LiFePO4. Phys. Chem. Chem. Phys. 2014, 16, 21788-21794. (51) Tang, P.; Holzwarth, N. A. W.; Du, Y. A. Comparison of the electronic structures of four crystalline phases of FePO
4. Physical Review B 2007, 76, 1-9.
(52) Avdeev, M.; Mohamed, Z.; Ling, C. D.; Lu, J.; Tamaru, M.; Yamada, A.; Barpanda, P. Magnetic Structures of NaFePO4 Maricite and Triphylite Polymorphs for Sodium-Ion Batteries. Inorganic Chemistry 2013, 52, 8685-8693. (53) Zaghib, K.; Trottier, J.; Hovington, P.; Brochu, F.; Guerfi, A.; Mauger, A.; Julien, C. M. Characterization of Na-based phosphate as electrode materials for electrochemical cells. Journal of Power Sources 2011, 196, 9612-9617. (54) Reale, P.; Panero, S.; Scrosati, B.; Garche, J.; Wohlfahrt-Mehrens, M.; Wachtler, M. A Safe, Low-Cost, and Sustainable Lithium-Ion Polymer Battery. Journal of The Electrochemical Society 2004, 151, A2138-A2142. (55) Center, U. o. R. I. C. R.; Company, C. R.: Handbook of chemistry and physics 0363-3055. 70th ed.; Weast, R. C., Lide, D. R., Eds.; CRC Press: Cleveland, Ohio, 1989; pp D155-D158. (56) Atkins, P. W.; Julio, D. P.: Electrochemistry. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford 2005; pp 200-224. (57) Pletcher, D.: An introduction to Electrode Reactions. In First Course in Electrode Processes; 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 1-47. (58) Pletcher, D.; Royal Society of, C.: A first course in electrode processes. 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 187-189.
Chapter 1: Introduction
22
Chapter 2: Experimental Techniques
23
Experimental Techniques Chapter 2:
Powder X-ray Diffraction 2.1
The X-ray diffraction (XRD) is one of the most time-efficient and reliable
techniques in identifying the crystal structure of LixFePO
4 sample. This chapter
describes the XRD principles to analyse powders in general as well as to
measure the relative proportions of LiFePO4 and FePO
4.
In principle, the XRD is a non-destructive analytical technique commonly used
to analyse structures of crystalline powder or solids which resulted in a
diffraction pattern. This technique can identify the crystal lattice type and the
separation of planes of lattice points. The XRD starts when the X-ray beams
with wavelength λ are incident onto lattice layers of a crystal sample at an
angle θ, the diffraction occurs once the reflected beams are scattered from the
lattice layers. The diffraction pattern is caused by constructive interference,
meaning the waves are in phase, as shown in Figure 2.1. On the other hand,
destructive interference occurs when the waves are out of phase.1,2 The angle θ
where constructive interference occurs can be calculated by the Bragg
equation, as shown in Equation 2.1.2 The distance that the waves travelled after
the reflection depends on the distance between the atoms. The distances xy
and yz, which travelled by the lower plane, are the extra distances to that
reflected from the upper plane, which is 2dsin θ, as shown in Equation 2.2 and
Equation 2.3.2 To obtain constructive interference, the distance must be equal
to nλ, which corresponds to the diffraction pattern.2 This leads to characterise
the sample.
Chapter 2: Experimental Techniques
24
Equation 2.1 2
Equation 2.2
Equation 2.3 2
Where n= an integer λ = a wavelength of X-ray d = a space between lattice planes. h, k, and l = miller indices
For the interpretation, the wavelength of the incidence beam, lattice parameter,
lattice types, and crystal system of the sample can be defined from the
position and the number of reflections on the sample. The position and types
of atom correspond to the intensity of the reflections which are needed to
characterize the crystal structure.
Figure 2.2 shows Miller indices (peaks; hkl) of LiFePO4 and FePO
4 XRD patterns.
For example, the Miller index of LiFePO4 peak at ~17 degree 2 is 200, i.e. h =
2, k = 0, and l = 0 as calculated by Equation 2.1and Equation 2.4.1 (Crystals
exist in a number of different orientations and each crystal has its own specific
orientation; The Rietveld fit assumes that crystal orientations are random. A
Figure 2.1: Diagram shows the derivation of Bragg's law
θ
θ θ
D hkl
x
y
z
Chapter 2: Experimental Techniques
25
preferred orientation, e.g. horizontally placed needles, would distort the peak
intensities. )
Equation 2.4
Where h, k, and l are Miller indices
ɑ,b, and c are lattice parameters
XRD software calculates d-spacing from λ and θ which are given and detected,
respectively, by the XRD machine. The θ positions of the diffraction peaks give
a characteristic pattern to a crystalline material. For example, a lattice
parameter of LiFePO4 sample, i.e. a, b and c can be calculated if the angle (θ =
10.218 degree), wavelength (standard Cu radiation λ=1.5406 Å) and miller
indices (hkl, eg. 101) are known from the machine. The calculation can be
carried out as shown below:
2
1.5406 2 10.218
1.54062 0.1774
4.34
The known miller indices are 101,
1
14.34
1 0 1
Therefore, the lattice parameters are 10.332Å, 6.010Å, 4.787Å
Chapter 2: Experimental Techniques
26
Figure 2.2: The combination of the miller indices of LiFePO4 (blue)3 and FePO
4
(red)4 XRD patterns.
The verification of the diffraction pattern from known structure patterns was
obtained via the Inorganic Crystal Structure Database (ICSD; Royal Society of
Chemistry). Then, one or more of the known diffraction patterns were fitted
onto the experimental data pattern, using Diffrac.eva, Celref and Rietveld
refinement programs. Diffrac.eva is a simple phase matching program, and
was done prior to each method. The Celref program was used to get an
accurate lattice parameter. The Rietveld refinement was used to analyse and
calculate, by adjusting the parameter of the known structure. This process was
run repeatedly until the calculated diffraction pattern achieved a good fit with
the results of the experimental pattern.
Rietveld refinement was done, using GSAS (General Structure Analysis System)
software, which analyses powder diffraction data obtained from X-rays.5 Three
files were prepared to employ the software, i.e. the XRD diffraction data, a
crystallographic information file (.CIF file) that was obtained from ICSD, and an
instrument parameter that was obtained from the XRD machine (D2 Phaser,
Bruker; .prm file). CIF files should be used which are equal to the known
compound type, and these were observed using the Diffrac.eva program,
giving a space group and lattice parameters of each crystal structure to fit onto
the experimental XRD pattern.
Chapter 2: Experimental Techniques
27
Rietveld refinement is a fitting method between the sample XRD pattern, which
was diffracted from Bragg reflections, and a model reference XRD pattern. The
software, then minimises the differences between those patterns. In the fitting,
peak positions, peak intensities and peak widths are important messages that
can characterise and identify a crystal structure. Peak positions are constrained
by the unit cell, corresponding to the size and shape of the unit cell.6 Peak
intensities and peak areas are determined by atom positions in the structure.
The shape of the peak identifies the crystallite size (Lx) and microstrain (Ly).
Thus, a broadened peak corresponds to a small crystallite size/strain and is
determined as a defective crystal structure.
Diffraction peak shape refinement is one of the key parameters in obtaining a
satisfactory agreement between the experiment and the reference intensity,
combining Gaussian and Lorentzian functions. The Gaussian function
corresponds to peak asymmetry, related to the instrumental effects, whereas,
the Lorentzian function corresponds to microstrain and crystallite size which
can be solved from peak broadening problems.
For this case, LiFePO4 and FePO
4 is a two-phase material which is easy to refine.
This is because the GSAS software package calculates the space fraction by
comparing integral characteristics between two patterns. A miscibility gap is
where two patterns (i.e. LiFePO4 and FePO
4) are superimposed, as shown in
Figure 2.3. However, this gap is not as complicated as in a solid solution
region. The software can only calculate the solid-solution region with one
pattern. Nevertheless, the LiFePO4 and FePO
4 solid solution can be determined
by Vegard’s law, by identifying how the lattice parameter changes with
composition within a single phase.7 By changing the value of x in LixFePO
4, the
lattice parameters will shift linearly with the x value.7
Chapter 2: Experimental Techniques
28
Figure 2.3: Binary phase diagram of FePO4-LiFePO
4, obtained from XRD data.
This figure is adapted from Kobayashi et al, 2009.8
For its sample preparation, LixFePO
4 was dried at 80°C for ~12 hours to get a
powder product, and ~ 1 g of sample was put on top of an XRD sample holder.
Then, the sample was levelled down to equalize between the surface of the
powder and the sample holder. The holder in the XRD machine was placed and
made ready to analyse. The XRD parameters are provided in each experiment
section.
Electrochemical Technique 2.2
The electrochemical technique is also one of the most efficient methods to
interpret the stoichiometric coefficient. This experiment employed a technique
called cyclic voltammetry (CV) and potentiometric titration (PT). Although, CV
will be shown in Chapter 8 and PT will be shown in Chapter 6. Here, basic
principle of CV and PT should be described before going into details in the
subsequent discussions.
2.2.1 Cyclic Voltammetry
The purpose of CV experiment was to study an alternative of reducing reagent.
The parameters and experimental details that applied with CV will be described
in detail in Chapter 8. CV is a versatile technique used to observe preliminary
electrochemical processes in a system. This section describes a principle of CV,
an electrode preparation and a cell construction for this particular experiment.
0 X in LixFePO
4 1
Solid-solution Solid-solution Miscibility Gap
Chapter 2: Experimental Techniques
29
CV is a reversal technique which scans the potential range of an electrode
linearly with time and measures the current, forwards and backwards. The
potential can be swept positive which is the direction of the oxidation reactions
and vice versa, when the negative sweep potential refers to the direction of the
reduction reactions. Normally, the potential scan rate (ν) of CV is in the range
of 25-1000 mV s-1.9 However, the higher potential scan rate results in IR drop
(the voltage drop due to energy losses in a resistor) and charging currents.
When increasing the scan rate, oxidation peaks shift up. In contrast, the
oxidation peaks shift down when decreasing the scan rate. Those peaks
indicates whether the reaction is electrochemical reversible, quasi reversible,
and irreversible, where the electron transfer take place. For electrochemical
reversibility, the electron transfer occurs very fast. Whereas, intermediate rates
and slow rates transfer of electron indicate quasi reversible and irreversible
reaction, respectively.10 As shown in Figure 2.4, Epox and E
pred are a position of an
oxidation peak and a reduction peak, respectively. If the different of these two
peaks (∆Ep= E
pox- E
pred) is less than 59 mV (at 298 K and n = 1), this corresponds
to a reversible reaction. While equal and higher ∆Ep than 59 mV will correspond
to quasi reversible and irreversible reactions. This value obeys the Nernst
equation as shown in Equation 2.5 and Equation 2.6. Plug in the standard
values that are given into Equation 2.6, getting the result of 59 mV as shown in
Equation 2.7.
Figure 2.4: One-phase CV a) Cyclic votammetry profile presents a peak height (I
pred) and a peak position (E
pred) b) samples of CV profile A)reversible, B)
quasi-reversible, and C) irreversible electron transfer. Reprinted with permission from Ref.10. Copyright 2015 Springer London. (This figure was published in July 2005 by Brownson et al).10
Oxidation peak
Reduction peak
Chapter 2: Experimental Techniques
30
Equation 2.5
Where O = oxidised species n = number of electrons (e-) R = reduced species
Equation 2.6 .
When E = the potential/ V E
e0 = the standard potential/ V
R = the ideal gas constant (8.314 /J mol-1K-1) T = 25 °C (298.15 /°K)
n = number of e-1 F = Faraday constant (96485/ C mol-1) [ ] = concentration of oxidised species and reduced species/ mol L-1
Equation 2.7
Equation 2.8
Two-phase reaction is different than one-phase reaction as described above.
With the two-phase system, as LiFePO4/FePO
4 material, the CV behaviour shows
a pair of antisymmetrical peaks of a redox couple, as shown in Figure 2.5.
Both anodic and cathodic currents increase and decrease sharply with respect
to the potential.11 Broadening of peaks can indicate resistance and/or diffusion
limiting effects. Chapter 3 describes the two-phase reaction in more details.
Figure 2.5: A schematic drawing of a two-phase cyclic voltammogram, adapted from Roberts et. al.11
+
+
- -
Chapter 2: Experimental Techniques
31
In the case of LiFePO4, as presented in Equation 2.8, the Nernst equation can
be written as Equation 2.9 and Equation 2.10 when an activity of solid, i.e. ɑFePO4
and ɑLiFePO4
is equal to one.
Equation 2.9
Equation 2.10 log
Where ɑ = an activity
For an electrode preparation, a lithium ion positive electrode generally consists
of a mixture of an active material (i.e. LiFePO4 and FePO
4), an acetylene black
and a binder. The binder, such as polytetrafluoroethylene (PTFE) or
polyvinylidene fluoride (PVDF), assists the electrode to obtain a plastic–like
material.12
In this experiment, the electrode was produced, using ink deposition
technique. A noble metal mesh such as platinum (Pt) was used as a conductive
based material. The composition of an ink deposition is shown in Table 2.1.
First, the PVDF binder was dissolved in ~4 ml of N-Methyl-2-pyrrolidone (NMP)
and stirred at ~50°C until the binder was dissolved. Then, an active material
and carbon black were mixed in and the ink stirred at ~50°C for ~30 minutes.
The mixture was then sonicated for ~1 hour. Finally, the mixture was
continuously stirred at ~50-60 °C to evaporate all the NMP and allow the ink to
thicken.
Before immersing in the ink, platinum mesh was prepared by cleaning with de-
ionized water and burning residue with a torch. A clean mesh Pt was immersed
into the prepared ink to a depth of ¾ of the electrode, as shown in Figure 2.6.
The electrode was then dried for 24 hours in an oven at ~80°C. The mass of
the Pt electrode was subtracted from the combined mass of the coated
electrode which was weighed before use to obtain the active material weight.
Chapter 2: Experimental Techniques
32
Figure 2.6: a) before and b) after immersing a Pt mesh electrode into the ink
Table 2.1: The composition of an active material ink
Ink of active materials/g
Active material (sample) (75%wt) 0.60
Carbon black (15%wt) 0.12
PVDF (10%wt) 0.08
Total (100%wt) 0.80
Once the electrode was ready, a three-electrode cell system: working electrode
(WE), counter electrode (CE), and reference electrode (RE) was constructed as
shown in Figure 2.7. The cell was controlled by the VMP (Various Multi-channel
potentiostat). The working electrode was a Pt mesh coated with an active
material ink. A Pt mesh was used as a counter electrode. Saturated calomel
electrode (SCE; accumet®, Fisher Scientific) was used as the reference
electrode. Electrolytes will be described in Chapter 8.
a) b)
¾ of the electrode
Chapter 2: Experimental Techniques
33
2.2.2 Potentiometric Titration
Potentiometric titration (PT) is a common electrochemical technique to
characterise battery material and to measure its performance. In this section
the principle of PT, an electrode preparation, and a cell construction are
described.
A battery cell’s capacity by PT is measured by a constant current which is
repeatedly charged and discharged with a potential limitation. The current
relates to the specific C-rate of the cell. The C-rate means the current is
required to charge and discharge the battery within one hour. Normally, charge
and discharge rates are applied in fractions of the C-rate. Thus, a 0.1 C-rate
means the current is required to charge and discharge completely in 10 hours.
The applied current can be calculated by Equation 2.11 and Equation 2.12.
Equation 2.11
Equation 2.12
Where Q
A = Capacity of the active material in the electrode/ mAh
mA = Mass of the active material in the electrode/ g
QT = Theoretical capacity of the active material/ mA h g-1
I = Applied current/ A C
r = C-rate
Figure 2.7: Basic diagram of 3 electrode system
Chapter 2: Experimental Techniques
34
The theoretical capacity can be calculated by using Equation 2.13.13
Equation 2.13 1000
11
3600
.
Where n = number of electrons F = Faradic number (96485)/ C mol-1
M = molecular mass / g For example, calculating the theoretic capacity of LiFePO
4 is done as follows:
. . 169.887 170
An electrode pellet is used for this type of experiment. The composition of a
pellet is shown in Table 2.2. The constituents were mixed well with a pestle
and mortar for ~20 minutes and pressed to a thickness ~200-100 microns.
Then, the mixed material was punched out into a 1 cm diameter disc. The
pellets were dried under vacuum (Buchi® tube) at 120°C overnight to remove
water from the material and to be ready to assemble into a cell on the next
day.
The active material (LixFePO
4) is coated with approximately 1-3% carbon.
However, carbon black is added into LiFePO4 as a conductive additive,
providing electronic pathways for lithium intercalation to take place. PTFE
(DuPont®) is used as a binder to provide mechanical strength.
Table 2.2: The composition of an electrode
Cathode-working electrode/g
Active material (sample) (75%wt) 0.225
Carbon black (15%wt) 0.045
PTFE (10%wt) 0.030
Total (100%wt) 0.300
A sample pellet was assembled, using the Swagelok® cell which is worked as a
2-electrode cell, as shown in Figure 2.8. The sample pellet was placed at the
cathode as a positive electrode and is followed by a 1.4 cm diameter separator
Chapter 2: Experimental Techniques
35
(0.67 mm glass microfiber filters; Whatman®). Eight drops of 1 M LiPF6 were
added as an electrolyte. A 1 cm diameter disc of Lithium foil was used at the
anode as a negative and reference electrode. Finally, a current collector, a
spring, a connector, and perfluoroalkoxy (PFA) tube were placed and tightened.
The cell was assembled within the glove box and wrapped with Parafilm® to
prevent oxygen penetration. The cell was then connected the VMP (varied
multi-potentiostat; Bio-Logic instrument) ready for an experiment.
Inductively Coupled Plasma Mass Spectroscopy 2.3
An inductively coupled plasma mass spectrometer (ICP-MS) is one of efficient
analytical techniques to determine trace of metal in concentration of liquid,
solid, or gas sample. The ICP-MS is a combination of an extremely high
temperature ICP source equipped with MS. The advantages of ICP-MS such as
low detection limits for most elements, high selectivity, and high accuracy. In
A A B
Negative and reference electrode
B C D B
Positive electrode Separator
Figure 2.8: the assembled Swagelok cell A) negative and positive connectors,
B) perfluoroalkoxy (PFA) tube fittings, C) spring, D) current collector
Chapter 2: Experimental Techniques
36
this section, a principle, a digestion procedure, and calculations to interpret
the sample should be described.
The principle is that each isotope has a unique mass-to-charge ⁄ ratio. This
ratio is analysed by mass spectrometry. Inductive coupled plasma (ICP) is argon
plasma at extremely high temperature (~10,000° K). ICP is used to convert the
sample into ions. The mass spectrometer then separates the ions by ⁄ ratio.
The ions that match the selected ⁄ ratio will be delivered to a detector. The
detector will determine the presence of a number of ions proportional to the
concentration.14,15
Here, a digestion acid solution and a procedure were carried out, as described
in Delacourt et al. (2005) and Dean et al. (2002)15,16. A stock solution of 20% wt
HCl (23 ml.), 20% wt HNO3
(10 ml.), and De-ionized (DI) water (17 ml.) was
mixed to give a volume of 50 ml of a yellow colour solution. Firstly,
approximately 0.1 g of each sample was weighed to an accuracy of 10 % in a
sample vial. Then, 2 ml of the stock solution was added into the vail and
covered the vial with a watch glass. The sample was heated to approximately
50-60°C and stirred for two hours on a hot plate. During this time, the solution
turned more yellow except for a black precipitate, assuming to be the carbon
coating. After that, the vial was removed from the hot plate and added DI water
to a total volume of 10 ml. Then, the digested sample was filtered out the
black precipitate with a Gooch funnel, rinse the sample vial with 5 ml of DI
water, and filter with the funnel. Lastly, the filtered sample was collected (15
ml. in total).The sample were sent for trace elements analysis by ICP-MS at
Ocean and Earth Science, University of Southampton, (NOC) for results of the
ICP analysis.
An X-SERIES 2 ICP-MS (Thermo Fisher Scientific, Bremen, Germany) was setup in
standard mode using an impact bead/cyclonic spray chamber and concentric
nebuliser. The instrument was tuned for optimum sensitivity, stability and low
oxide formation using a 1 ppb multi-element tuning solution. Data was then
acquired for all isotopes of interest in peak-jumping mode (4 x 30 second
Chapter 2: Experimental Techniques
37
repeats per sample). After each sample analysis, a wash solution containing 3%
HNO3 was run until background levels were achieved (typically 3 minutes). All
samples, standards and blanks were spiked with internal standard elements
beryllium (Be), indium (In) and rhenium (Re).
The data quality was monitored throughout the run by examination of the
statistics produced after each analysis. Within the run the reproducibility was
typically better than 1% relative standard deviation (RSD) for the 4 repeats. The
data processing was carried out using the Plasmalab software. Raw data were
blank sample and internally corrected and then calibrated against matrix
matched synthetic standards
The X-Series 2 ICP-MS instrument configuration and settings are listed below: Radio frequency (RF) Power: 1.40kW Forward, <1W Reflected. Sample Introduction System: Concentric nebulizer with low-volume impact
bead spray chamber Torch: Standard one-piece quartz torch with Plasma Screen Interface: Standard Xt (Ni sample/Ni Skimmer) Cool Gas Flow (L/min) 13
Auxiliary Gas Flow (L/min) 0.8 Nebuliser Gas Flow (L/min) 0.90 Sample Uptake
Rate (mL/min) 0.4 approx. Detector Simultaneous pulse/analogue Wash Time Monitored, minimum 60 seconds, maximum 300 seconds Detection limits are element and run specific, typically <0.1ppb.
The results were returned in part per million (ppm) units and needed to
calculate further for the interpretation. Percentages of lithium were desired to
be reported for comparison with the other two methods. The ppm units were
converted into the percentage of element content: i.e. lithium, using Equation
2.14.
Equation 2.14
%
Chapter 2: Experimental Techniques
38
.
.
Some calculations were done using results from ICP analysis to study the ratio
of lithium ion concentration to other cation: i.e. Na+, K+, and Mg2+,
concentration uptake. For example, these are determined as follows: Li+ and
Na+ uptake, the ratio of lithium ion concentration to sodium ion concentration
as in solid ([Li+]:[Na+] solid), and lithium selectivity are shown in Equation 2.15-
Equation 2.17.
Equation 2.15
Equation 2.16 :
Equation 2.17 :
:
References 2.4
(1) Atkins, P. W.; De Paula, J.: The elements of physical chemistry. 4th ed.; Oxford University Press: Oxford, 2005; pp 394-395. (2) Zumdahl, S.; DeCoste, D. J.: Chemical Principles. 7th ed.; Cengage Learning, 2013; pp 791-793. (3) Yakubovich, O. V.; Belokoneva, E. L.; Tsirel'son, V. G.; Urusov, V. S. Electron density distribution in synthetic triphylite LiFePO
4 1990, 45, 93-99.
(4) Andersson, A. S.; Kalska, B.; Häggström, L.; Thomas, J. O. Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mössbauer spectroscopy study. Solid State Ionics 2000, 130, 41-52. (5) Larson, A. C.; Dreele, R. B. V.: GSAS Manual. 26 September 2004 ed.; Los Alamos National Laboratory: New Mexico, 2004; pp 1. (6) Langford, J. I.; Louër, D. Powder diffraction. Reports on Progress in Physics 1996, 59, 131-234. (7) Jacob, K. T.; Raj, S.; Rannesh, L.: Vegard's Law: A Fundamental Relation or an Approximation? ; International Journal of Materials Research, 2007; pp 1-7. (8) Kobayashi, G.; Nishimura, S.-i.; Park, M.-S.; Kanno, R.; Yashima, M.; Ida, T.; Yamada, A. Isolation of Solid Solution Phases in Size-Controlled LixFePO4 at Room Temperature. 2009, 19, 395-403. (9) Pletcher, D.; Royal Society of, C.: A first course in electrode processes. 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 187-189.
Chapter 2: Experimental Techniques
39
(10) Brownson, D. A. C.; Banks, C. E.: Interpreting Electrochemistry. In The Handbook of Graphene Electrochemistry; 1st ed.; Springer-Verlag London, 2014; pp 23-77. (11) Roberts, M. R.; Vitins, G.; Denuault, G.; Owen, J. R. High Throughput Electrochemical Observation of Structural Phase Changes in LiFe1 − xMnxPO4 during Charge and Discharge. Journal of The Electrochemical Society 2010, 157, A381-A386. (12) Prosini, P. P.: Iron Phosphate Materials as Cathodes for Lithium Batteries: The Use of Environmentally Friendly Iron in Lithium Batteries. In Iron Phosphate Materials as Cathodes for Lithium Batteries: The Use of Environmentally Friendly Iron in Lithium Batteries; Springer, 2011; pp 57. (13) Vincent, C. A.; Scrosati, C. A. V.: 2 - Theoretical background. In Modern Batteries; 2 nd ed.; Butterworth-Heinemann: Oxford, 1997; pp 18-64. (14) de Hoffmann, E.; Stroobant, V.: Mass Spectrometry: Principles and Applications. In Mass Spectrometry: Principles and Applications; 3rd ed.; Wiley, 2007; pp 69-71. (15) Dean, J. R.; Jones, A. M.; Holmes, D.; Reed, R.; Weyers, J.; Jones, A.: Practical Skills in Chemistry. 1st ed.; Pearson Education Limited: Dorset, 2002; pp 175-179. (16) Delacourt, C.; Poizot, P.; Tarascon, J.-M.; Masquelier, C. The existence of a temperature-driven solid solution in LixFePO4 for 0 ≤ x ≤ 1. Nat Mater 2005, 4, 254-260.
Chapter 2: Experimental Techniques
40
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
41
Physical and Electrochemical Chapter 3:
Characterization of the LiFePO4 and FePO
4
system
In this chapter, LiFePO4 and FePO
4 were characterised via scanning electron
microscopy (SEM), X-ray diffraction (XRD), mass spectroscopy (MS) and
electrochemistry techniques.
Experimental Details 3.1
In this study, LiFePO4
and FePO4
were mainly examined by using three
techniques: scanning electron microscopy (SEM), X-ray diffraction (XRD), and
inductively coupled plasma-mass spectroscopy (ICP-MS). Cyclic voltammetry
(CV) was used to observe LiFePO4 cycling in lithium salt. Potentiometric titration
(PT) was employed to calibrate a curve of the stoichiometric coefficient x of
LixFePO
4 against specific charge. The overview of each technique was previously
described in Chapter 2. These measurements were performed in order to
calibrate results for kinetics experiments in Chapter 6.
Scanning Electron Microscopy (SEM)
For sample preparation, 0.001 g of 3 % carbon coated LiFePO4 (Tatung) was
transferred to a 10 ml sample vial, and acetone was added to cover the sample
(1-2 ml). The mixture was sonicated in an ultrasonic bath for approximately 1
hour. Then, 1-2 drops of suspension were transferred on an SEM specimen
stub using a disposable pipette. Once the sample had dried, it was ready to
examine using SEM. The same procedure was applied to the FePO4 sample,
which obtained from Chapter 4.1(1:2 of K2S
2O
8 to LiFePO
4 ratio).
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
42
X-ray Diffraction (XRD)
Samples of LiFePO4
and FePO4 were examined using XRD (Bruker D2 Phaser),
and scanned from 15 to 50 degrees of XRD pattern with an increment of
0.0081 degrees with a time per step of two seconds, for approximately three
hours for the total scan. The reference XRD patterns of LiFePO4 were obtained
via the Inorganic Crystal Structure Database (ICSD; RSC). The diffrac.eva
program was employed to observe, roughly, the result of the sample XRD data.
Then, the GSAS (General structure analysis system) program was applied to
complete the Rietveld refinement of the experimental XRD pattern.
Cyclic voltammetry (CV)
The purpose of CV experiment is to study the behaviour of LiFePO4 and FePO
4.
A standard three-electrode cell was used. LiFePO4 ink coated on platinum mesh
(Pt), Pt mesh and a saturated calomel electrode (SCE) were used as a working, a
counter, and a reference electrode, respectively. A solution of 1.5 M Li2SO
4 was
made by adding 12.37 g of Li2SO
4 (0.1125 mol) (Sigma-Aldrich, ≥98.5 %) to 75
ml of de-ionized water (DI water) which was added and used as an electrolyte.
The electrode preparation and cell setup were as described in the previous
chapter. The cell was argon purged for ~1 hour. The cell were cycled between
-1.2 V and 1.2 V with a scan rate of 10 mV s-1 for 20 cycles.
Potentiometric Titration
Another electrochemical technique is the validation method, using Pt to
identify the stoichiometric coefficient x of LixFePO
4 for a series of pellets,
containing various percentages of LiFePO4 mixed with FePO
4. (FePO
4 was
obtained by delithiating LiFePO4 which will be described in Chapter 4.1.) Table
3.1 shows the actual masses of LiFePO4 initially and FePO
4 used. Each sample
pellet was incorporated into the Swagelok® cell, adding the mixture pellet of
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
43
LiFePO4, separator, and lithium foil. Potentiometric cycling with potential
limitation was applied to the cells at a rate of 0.1C while monitoring the
potential in order to observe charge and discharge of the cells. More
information about the main pellet compositions, cell setup and the current
calculations were given in Chapter 2.
For example, %Li = 0
Equation 3.1
= (0.0173 g x 75%) x170 mA h g-1
= 2.205 mA h
Where active material is 75% mass of the pellet.
Equation 3.2
= 0.1 h-1 x 2.205 mA h
= 0.2205 mA
Table 3.1: The composition of LiFePO4 and FePO
4 as an active material
%Li
Active material mass/g Pellet
LiFePO4
(MW=157.76 g/mol) FePO
4
(MW=150.82 g/mol) Total
Average mass/g
I/mA
100 0.1578 0 0.1578 0.0234 0.2983
75 0.1578 x 0.75 =
0.1184 0.1508 x 0.25 =
0.0377 0.1561 0.0323 0.4120
50 0.1578 0.1508 0.3086 0.0252 0.3206
25 0.1578 x 0.25 =
0.0395 0.1508 x 0.75 =
0.1131 0.1526 0.0314 0.3998
0 0 0.1508 0.1508 0.0173 0.2205
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
44
Mass-spectrometry
Lithium content in LiFePO4 and delithiated sample of LiFePO
4 was determined
by mass-spectrometery. Approximately 0.1 g samples containing both LiFePO4
and FePO4
were dissolved in an aqueous solution containing 20% wt HCl and
20% wt HNO3, and then the solution samples were sent to the National
Oceanography Centre Southampton (NOCS) for analysis. The analysis and
preparation methods were described in details in Chapter 2.
Results and discussion 3.2
The results of SEM, XRD, ICP-MS, CV and GM are shown below in order.
SEM
Morphological image analyses of the LiFePO4 sample are shown in Figure 3.1a-
c. Large agglomerates of large irregular needle and potato shapes are found
with dimensions of approximately 2 μm long by 0.5 μm thick (blue arrows) and
1 μm long by 0.2 μm thick (red arrows), respectively. All small sphere
agglomerates are presumably carbon coating.
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
45
Figure 3.1: SEM images of an initial carbon coated lithium iron phosphate (Tatung), under magnification of a) 2,500x, b) 10,000x and c) 33,000x. The images were recorded with an acceleration voltage of 15 kV.
Figure 3.2 shows the morphology of the heterosite FePO4
obtained from the
delithiated LiFePO4 sample for 24 hours (see Chapter 4.1). The morphology of
heterosite FePO4
looks similar to LiFePO4, with large agglomerates of large
irregular needle and potato shapes. The agglomerates of the irregular needle
shape are approximately 2 μm long and 0.8 μm wide (blue arrows). For the
irregular potato shape, the sizes are approximately 1 μm long and 0.3 μm wide
(red arrows). All small sphere agglomerates are presumably carbon coating.
a) b)
c)
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
46
From the morphological studies, it can be concluded that there are no
significant changes in terms of shapes and dimensions of both samples, with
respect to a micron scale. This demonstrates that the extent of corrosion was
insignificant, despite the fact that the delithiation process (heterosite FePO4
sample) involved a strongly oxidizing solution with agitation for 24 hours.
Figure 3.2: SEM images of a carbon coated heterosite iron phosphate, which was obtained from a delithiation process of 5.8 g LiFePO
4 + 0.1 M K
2S
2O
8
(Chapter 4.1), under magnification of a) 2,500x, b) 10,000x and c) 35,000x. The images were recorded with an acceleration voltage of 15 kV.
a) b)
c)
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
47
XRD
Figure 3.3a-b shows the Rietveld refinement patterns obtained from the initial
LiFePO4 and FePO
4 samples, where the black cross marks represent the
experimental XRD pattern, red lines define the refined or calculated pattern,
blue-pink tick marks show the reflection position of LiFePO4- FePO
4 and below
the patterns is the difference plot shown in a dark blue line.
As shown in Figure 3.3a-b, the calculated patterns do not completely fit on the
experimental data. However, their lattice parameters are in agreement with the
references as shown in Table 3.2. Also, both X2 values are close to 1 which
indicates an ideal fitting.1
Table 3.2: the lattice parameter of the fitting pattern and the reference2
Sample Lattice Parameter/ Å X2
a b c
LiFePO4
Reference3 10.332(4) 6.010(5) 4.787 - Experiment 10.3154(2) 5.99938(12) 4.6889(1) 1.9
FePO4
Reference2 9.8142(2) 5.7893(2) 4.7820(2) - Experiment 9.8173(3) 5.7879(1) 4.7815(2) 2.8
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
48
Figure 3.3: Fit to XRD data of a) LiFePO
4 (Tatung): R
wp 1.3%, R
p 1.0% and b)
FePO4:
R
wp 1.5%, R
p 1.1%.
Crosses mark the data points, red line is the fit and
blue line is the difference; a) blue and b) pink tick marks show the allowed reflection positions for LiFePO
4 and FePO
4, respectively.
a)
b)
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
49
ICP-MS
Table 3.3 shows a list of metal ions, i.e. Li+, Na+, Mg2+ and K+, which are
contained in the initial sample of LiFePO4 and the heterosite FePO
4 , by using
ICP-MS. The results from LiFePO4
show approximately 300 ppm of lithium
content. For FePO4, a lithium contained of approximately 18 ppm was found,
which indicates that the sample was not completely delithiated. Due to the
solid solution as described in Chapter 2, XRD could not detect the small
amount of lithium in the heterosite FePO4 structure, as resulted in Figure 3.3b.
The report shows some contamination of Na+ and Mg2+ in both samples, but the
quantity is not significantly high. K+ was found in FePO4 to be approximately 15
ppm compared to LiFePO4, owing to K
2S
2O
8 working as an oxidizing agent for
the delithiation process.
Table 3.3: Metal ions contained in LiFePO4 and the heterosite FePO
4
Compound Metal ions/ ppm
Li+ Na+ Mg2+ K+
LiFePO4 306.65 0.99 2.16 *bd
FePO4 18.10 1.70 2.28 15.12
*bd = below detection
Cyclic voltammetry
A cyclic voltammogram of LiFePO4 with a Pt counter electrode versus SCE in 1.5
M Li2SO
4 at the scan rate of 10 mV s-1 is shown in Figure 3.4. The CV profile is
associated with lithium ion extraction and insertion towards the oxidation and
reduction of the Fe2+/Fe3+ redox couple. The red linear dashed line in the profile
represents a constant slope on both sides (i.e. oxidation and reduction) which
extrapolates to the same value of E0, i.e. 0.2 V. This characteristic of one pair
of symmetrical redox peaks is peculiar to a two-phase system such as LiFePO4
and heterosite FePO4.4
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
50
The profile starts at E0, scanning towards positive potential with a full
conversion to Fe3+. Position “a” shows a current that is limited by resistance
according to Equation 3.3-Equation 3.4.
Equation 3.3
Equation 3.4
Where i = current/ A E = the potential/ V E0 = the standard potential/ V
R = resistance/ Ω
The drop in the current after position “b” suggests a limitation by diffusion
within the LiFePO4 particles. Lithium extraction at the surface was almost
completed at “c” after starting a backward scan. Lithium insertion was then
started at E0 to “d”, with a gradient depending on the resistance, in the same
way as “a”. Position “e” indicates the accumulation of lithium in the surface
until saturation when the scan reaches the potential limit of -1.2 V. Then, the
forward scan starts with no reaction occurs at “f”, until the scan reaches E0 as
for a cycle.
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
51
`
Figure 3.4: Cyclic voltammogram of LiFePO4
in 1.5 M Li2SO
4 aqueous
electrolyte with a scan rate of 10 mV s-1, where Epa and E
pc are anodic and
cathodic peak potentials, respectively. Deviations due to uncompensated Ohmic drops can be observed by measuring potentials from the blue lines. At the top left corner shows the corrected potential value.
The CV voltammogram is characteristic of a phase-change material, because of
the very small difference between Epa and E
pc after IR compensation (R=0.088
Ω), so could be described as essentially reversible with fast kinetics at the
given scan rate of 10 mV s-1. However, the main purpose of the
electrochemistry was to determine E0 and the reaction free energy in the
forthcoming chapters. Cyclic voltammograms of a LiFePO4 electrode in the
presence of several reduction agents were also recorded in order to evaluate
the rate of chemical lithiation of FePO4 by those reducing agents, as described
in Chapter 8.
Potentiometric Titration
A calibration curve was made by galvanostatic charge-discharge of the
prepared cells at the rate of 0.1C to calibrate the LixFePO
4 samples in the
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-4
-2
0
2
4
6
Ec
p
E0
I/ m
A m
g-1
E/ V vs. SCE
E0
a
b
c
d
e
f
Li+ extraction
Li+ insertion
Ea
p
-1.0 -0.5 0.0 0.5 1.0
-4
-2
0
2
4
6
Ecorr/ V vs. SCE
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
52
kinetics study in Chapter 6. The first cycle of each cell extraction was used to
calculate a specific charge, as shown in Figure 3.5. Generally, a high specific
charge has a high percentage of Li content which corresponds to the value in
Table 3.4 and Figure 3.6.
Table 3.4: A specific charge of each percentage Li content for validation
%Li Specific Charge /mAh g-1
Initial extraction
100 154 75 120 50 82 25 42 0 4
Figure 3.5: Potentiometric titration of LixFePO
4 electrodes prepared with the
mixiture of LiFePO4 + FePO
4 = 100%, as indicated. Specific current: 17 mAh g-1
(at C/10).
0 20 40 60 80 100 120 140 1603.4
3.6
3.8
4.0
4.2
4.4100%75%25% 50%
E/ V
vs
Li/L
i+
Specific charge/ mAh g-1
0%
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
53
Figure 3.6: A validating graph of lithium content (%) in LiFePO4 and specific
charge
In summary, this chapter demonstrated as shown below;
The morphological images of LiFePO4 and FePO
4 showed a similar
agglomerates of irregular needle and potato shapes, with a dimension of
approximately 4 μm long and 0.5 μm wide.
The Rietveld refinements of LiFePO4 and FePO
4 were illustrated, showing
lattice parameters derived from the experimental results similar to those in
the references.
Contamination with metal ions was less than 1% in LiFePO4. FePO
4 showed
some traces of lithium approximately 6% with the other metal ions, due to
the delithiation process.
The cyclic voltammetry of LiFePO4 showed a pair of symmetrical redox
peaks.
A calibration curve determination of the percentage of lithium content in
LiFePO4 is shown for the kinetics study in Chapter 6.
0 20 40 60 80 100
0
20
40
60
80
100
120
140
160
Sp
ecif
ic c
har
ge/
mA
h g
-1
% Li LiFePO4 FePO
4
Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and
FePO4 system
54
References 3.3
(1) McCusker, L.; Von Dreele, R.; Cox, D.; Louer, D.; Scardi, P. Rietveld refinement guidelines. Journal of Applied Crystallography 1999, 32, 36-50. (2) Andersson, A. S.; Kalska, B.; Häggström, L.; Thomas, J. O. Lithium extraction/insertion in LiFePO
4: an X-ray diffraction and Mössbauer
spectroscopy study. Solid State Ionics 2000, 130, 41-52. (3) Yakubovich, O. V.; Belokoneva, E. L.; Tsirel'son, V. G.; Urusov, V. S. Electron density distribution in synthetic triphylite LiFePO
4 1990, 45, 93-99.
(4) Mi, C.; Zhang, X.; Li, H. Electrochemical behaviors of solid LiFePO4
and Li0.99
Nb0.01
FePO4 in Li
2SO
4 aqueous electrolyte. Journal of Electroanalytical
Chemistry 2007, 602, 245-254.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
55
Test of K2S
2O
8 as an Oxidising Chapter 4:
Agent of LiFePO4 to FePO
4
Chapter 3 studied the properties of LiFePO4
and heterosite FePO4, which are
selective to lithium. Heterosite FePO4 is used for lithium recovery and can be
obtained by extraction of lithium from LiFePO4. Therefore, the next step is to
find an oxidizing agent to act as an electron accepter, to remove lithium from
LiFePO4. In this chapter, we studied K
2S
2O
8 as an oxidising agent to obtain
FePO4 from LiFePO
4.
A Study of the Chemical Delithiation of LiFePO4 in 4.1
Aqueous Solutions
4.1.1 Introduction
The preliminary research of an oxidizing agent to delithiate LiFePO4 was
obtained by potassium persulphate (K2S
2O
8)1, using a 2:1 molar ratio of LiFePO
4
to K2S
2O
8 for which Equation 4.1is shown:1
Equation 4.1 2 → 2
This process stirred the aqueous mixture of K2S
2O
8 and LiFePO
4 at room
temperature for 24 hours. However, the literature did not indicate the
concentration of K2S
2O
8 used. Therefore, a small concentration of 0.1 M K
2S
2O
8
was used in this experiment. Three experiments were made, i.e. 1:2
(stoichiometric molar ratio), 3:2 and 6:2 of K2S
2O
8 to LiFePO
4 ratio.
4.1.2 Experimental Details
Five grams of K2S
2O
8 (Sigma-Aldrich, ACS reagent, ≥99.0%) was dissolved in
0.184 litres of de-ionized water (DI water; Purite) to obtain 0.1 M solution.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
56
Then, 5.805 grams LiFePO4 (Tatung, ~3% carbon coated) with a molar ratio of
1:2 K2S
2O
8 to LiFePO
4 was added to the 0.1 M solution. The mixture was stirred
for approximately 24 hours at room temperature (~25°C). The samples were
collected at 1 h, 2 h, 4 h and 24 h after the experiment started. Each sample
was filtered with qualitative grade No. 1 (110 mm diameter) filter paper,
washed with DI water, and dried at 80 °C in an oven for approximately 12 h.
The samples were examined using X-ray diffraction technique (XRD; D2
Pharser) to measure the extent of conversion to FePO4 heterosite. The sample
was scanned from 15 to 50 degrees of XRD pattern. The reference XRD
patterns of LiFePO4 and FePO
4 were obtained via the Inorganic Crystal Structure
Database (ICSD; RSC). Celref and Diffrac.eva programs were used to analyse
the XRD patterns.
For the second and the third experiments, 3:2 and 6:2 molar ratio of LiFePO4 to
K2S
2O
8 were employed. These ratios were chosen to enhance a conversion rate
of FePO4
heterosite, as compared to that in the literature.1-5 The chemical
compositions of each experiment are shown in Table 4.1. The process of
collecting samples and analysis was the same as for the previous experiment.
Table 4.1: Chemical composition of the delithiation experiment by the use of K
2S
2O
8 as an oxidizing agent
Molar ratio of
K2S
2O
8:LiFePO
4
Chemical composition/g DI
water/L
Molar
K2S
2O
8/ M K
2S
2O
8 LiFePO
4
1:2
(stoichiometric) 5 5.805 0.184 0.1
3:2 10 3.881 0.369 0.1
6:2 10 1.94 0.369 0.1
4.1.3 Results and Discussion
Three types of molar ratio of K2S
2O
8 to LiFePO
4, i.e. 1:2, 3:2 and 6:2 are shown
below.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
57
The K2S
2O
8 to LiFePO
4 ratio of 1:2 (stoichiometric molar ratio)
The samples were taken after 1-hour-long and 2-hour-long experiment, the
XRD patterns indicated a two-phase mixture of LiFePO4 and FePO
4 heterosite in
both samples, as shown in Figure 4.1 and Figure 4.2. The fitting to the XRD
patterns illustrated that these samples contain 9% and 3% of lithium.
The XRD fitting of the De-LiFePO4 at 4 hours (h) was not sufficient to identify
lithium content; however, the XRD pattern was found to show a very small
trace of LiFePO4 by visual examination as shown in Figure 4.3, whereas the
fully converted FePO4 result was indicated by XRD from the sample taken after
24 h. Figure 4.4 shows a single-phase FePO4 heterosite of the De-LiFePO
4 24 h
sample.
Figure 4.1: The XRD of a LFP sample treated with K2S
2O
8 with a molar ratio of
2:1 for 1 hour. The XRD refinement indicates that the sample composition is Li
0.09FePO
4
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO
4 and the lower pink tick marks are for heterosite FePO
4.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
58
Figure 4.2: As in Figure 4.1, but the reaction was left for 2 hours and the XRD fitting indicates that the sample composition is Li
0.03FePO
4
Figure 4.3: As in Figure 4.1, but the reaction was left for 4 hours and the XRD fitting indicates that the sample composition is FePO
4.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
59
Figure 4.4: As in Figure 4.1, but the reaction was left for 24 hours and the XRD fitting indicates that the sample composition is FePO
4
3:2 of K2S
2O
8 to LiFePO
4 ratio
These experiments were done by adding a three times higher concentration
than the stoichiometric molar ratio experiment (1:2 of K2S
2O
8 to LFP). The
results from the sample taken after 30 minutes and 1 hour were found to be a
two-phase mixture. The XRD fitting from De-LiFePO4 for 30 min and 1 h
indicated 7% and 6% of lithium content, respectively, as shown in Figure 4.5
and Figure 4.6. The sample collected after 2 h to 24 h resulted as fully
converted to FePO4 heterosite as shown in Figure 4.7 and Figure 4.8.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
60
Figure 4.5: The XRD of a LFP sample treated with K2S
2O
8 with a molar ratio of
3:2 for 30 min. The XRD refinement indicates that the sample composition is Li
0.07FePO
4
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO
4 and the lower pink tick marks are for heterosite FePO
4.
Figure 4.6: As in Figure 4.5, but the reaction was left for 1 hour and the XRD fitting indicates that the sample composition is Li
0.06FePO
4.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
61
Figure 4.7: As in Figure 4.5, but the reaction was left for 2 hours and the XRD fitting indicates that the sample composition is FePO
4.
Figure 4.8: As in Figure 4.5, but the reaction was left for 24 hours and the XRD fitting indicates that the sample composition is FePO
4.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
62
6:2 of K2S
2O
8 to LiFePO
4 ratio
To improve the technique with the possibility of a faster yet complete
conversion of FePO4, a six times higher K
2S
2O
8 was used. The results from 30
min and 1 h were found to be a two-phase mixture, which were similar to the
3:2 of K2S
2O
8 to LiFePO
4 ratio experiment. However, both of the results which
samples were taken at 30 m and 1 h (6:2 ratio), contained lower lithium
content than the 3:2 of K2S
2O
8 to LiFePO
4 ratio experiment, i.e. 5% and 4% Li,
respectively, as illustrated in Figure 4.9 and Figure 4.10. All samples taken
after 2 h resulted as fully delithiated LiFePO4, which was similar to the 3:2 of
K2S
2O
8 to LiFePO
4 ratio experiment, as reported in Figure 4.11and Figure 4.12.
The stoichiometric coefficient of lithium content from samples in each
experiment is shown in Table 4.2.
Figure 4.9: The XRD of a LFP sample treated with K2S
2O
8 with a molar ratio of
6:2 for 30 min. The XRD refinement indicates that the sample composition is Li
0.05FePO
4
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO
4 and the lower pink tick marks are for heterosite FePO
4.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
63
Figure 4.10: As in Figure 4.9, but the reaction was left for 1 hour and the XRD fitting indicates that the sample composition is Li
0.04FePO
4.
Figure 4.11: As in Figure 4.9, but the reaction was left for 2 hours and the XRD fitting indicates that the sample composition is FePO
4.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
64
Figure 4.12: As in Figure 4.9, but the reaction was left for 24 hours and the XRD fitting indicates that the sample composition is FePO
4.
Table 4.2: Stoichiometric coefficient of x in LixFePO
4 samples obtained by
delithiation of LiFePO4 from each experiment at 30 m, 1 h, 2 h, 4 h, and 24 h
K2S
2O
8: LiFePO
4
Time / h
0.5 1 2 4 24
1:2 (stoichiometric)
- Li0.09
FePO4 Li
0.03FePO
4 FePO
4 FePO
4
3:2 Li0.07
FePO4 Li
0.06FePO
4 FePO
4 FePO
4 FePO
4
6:2 Li0.05
FePO4 Li
0.04FePO
4 FePO
4 FePO
4 FePO
4
As shown in Table 4.2, in the 1:2 of K2S
2O
8 to LiFePO
4 ratio experiment, the
LiFePO4 sample was fully converted into FePO
4 heterosite after 4 h, whereas in
the other two experiments were fully converted after 2 h.
Figure 4.13 shows a preliminary analysis of the kinetic data. The graphs show
that the experiment using 1:2 of K2S
2O
8 to LiFePO
4 ratio completed the chemical
oxidation last, while the other experiments were faster. However, the kinetics
study cannot be defined due to the shortage of the collected data vs. time.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
65
As shown in Figure 4.14, not all data show a linear relationship. The graph of
1:2 of K2S
2O
8 to LiFePO
4 ratio experiment shows a rough estimation of first
order reaction in the concentration of the reactant, i.e. K2S
2O
8 where the plot at
2 h indicates a depletion of the reactant. For the other two experiments, the
reactions almost completed after 30 min and continue for another 30 min,
which might be due to the residual reaction, i.e. reaction with large particles.
The large particles react to the reactant slowly; whereas, the small particles
react fast. In fact, these graphs show a distribution of particle. The main
limitation to the reaction rate is the concentration of S2O
82-. Low concentration
of S2O
82- slows down the reaction rate, as shown in the graph of 1:2 of K
2S
2O
8 to
LiFePO4 ratio experiment in Figure 4.14. If the concentration of S
2O
82- is high,
the concentration of S2O
82- is not a limiting step but the depletion of FePO
4.
0 5 10 15 20 25
0.0
0.2
0.4
0.6
0.8
1.0
x in
Li (1
-x)F
eP
O4
Time/ h
1K2S
2O
8:2 LiFePO
4
3K2S
2O
8:2 LiFePO
4
6K2S
2O
8:2 LiFePO
4
Figure 4.13: Delithiation of LiFePO4 during 24 hours, () 1:2 of K
2S
2O
8 to
LiFePO4, ( ) 3:2 of K
2S
2O
8 to LiFePO
4, ( ) 6:2 K
2S
2O
8 to LiFePO
4 experiments
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
66
Figure 4.14: The kinetics delithiation of () 1:2 of K2S
2O
8 to LiFePO
4, ( ) 3:2 of
K2S
2O
8 to LiFePO
4, ( ) 6:2 K
2S
2O
8 to LiFePO
4 experiments
Analysis of the Delithiation rate using a Conductivity 4.2
Measurement
4.2.1 Introduction
The extent of the reaction from the previous section 4.1 can be monitored by
measuring the conductivity of the solution. Assuming the molar conductivity of
all ions are similar, the conductance is expected to be double in the reaction,
as presented in Equation 4.2.
Equation 4.2 2 2 → 2 2 2 2
3 ions 6 ions
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
67
Dilute solutions of salts usually have an ionic conductivity that is proportional
to the concentration. Conductivity can be used to monitor the ionic
concentration in the solution according to Equation 4.36;
Equation 4.3 ᴧ
Where κ = conductivity/ S m-1
ᴧ= molar conductivity/ Sm2 mol-1
c = concentration/ mol dm-1
The conductivity is calculated from measurements of the conductance and
dimensions of a material as shown in Equation 4.4.
Equation 4.4
Where G = conductance/ S (Siemen) l = length of the current path/ m A = area/ m-2
Conductance is a reciprocal of resistance and measured from Equation 4.5 as
follows;
Equation 4.5
Equation 4.6 is a result of substitution of Equation 4.3 and Equation 4.4 in
Equation 4.5. Therefore, the current is proportional to the concentration if all
the other terms remain constant as refer in Equation 4.6.
Equation 4.6 ᴧ
Where I = current/ A V= potential/ V
Here, two experiments are undertaken in order to study the rate of the reaction
depending on the concentration of K2S
2O
8. In each case, the conductivity and
temperature of the solution were measured versus time.
The reason for measuring the temperature is that the temperature indicates a
measure of the energy spent in heating or cooling the solution. Therefore, the
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
68
extent of the reaction is reflected in the temperature increase or decrease
according to the sign of the enthalpy change, ∆H, which is usually negative,
indicating heating. In either case, the change in temperature will reflect the
extent of the reaction.
4.2.2 Experimental Details
Two experiments were completed, using 0.1 M K2S
2O
8 and 0.2 M K
2S
2O
8 as
reactants. The first of these, 0.1 M K2S
2O
8 was used and measured the
conductivity with time, before and after the addition of LiFePO4. For the second
experiment, 0.2 M K2S
2O
8 was used, and both conductivity and the temperature
were measured as a function of time.
0.1 M K2S
2O
8 (measuring conductivity versus time)
A solution of 0.1 M K2S
2O
8 was prepared by adding 5.40 g of K
2S
2O
8 (0.02 mol)
to 200 ml of DI water in a 250 ml Erlenmeyer flask. Then, the solution was
stirred and the conductivity of the solution was measured by conductivity
meter (Hanna). 5 g or 0.03 mol LiFePO4 was then added to 0.02 mol K
2S
2O
8 to
give a small excess of K2S
2O
8 with respect to the stoichiometry described in
Equation 4.2. Under this condition the rise in conductance was defined by the
amount of LiFePO4.
The timer was started after adding LiFePO4. Readings of the conductivity of the
solution were taken at 2, 5, 10, 15, 20, 30, 60, 1080, 1140, and 1440 minutes
after adding LiFePO4. The solution was stirred continuously except when
collecting data at which times the stirrer was turned off.
0.2 M K2S
2O
8 (measuring conductivity versus time)
The procedure for this experiment was similar to the one for experiment 1.
However, a solution of 0.2 M K2S
2O
8 was made by adding 5.40 g (0.02 mol)
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
69
K2S
2O
8 to 100 ml of DI water in a 250 ml beaker (to add a thermometer to the
glass container). The temperature and conductivity of the solution were
measured then the solution was stirred while adding, 5 g (0.03 mol) LiFePO4
quickly while the timer was started. Readings of the conductivity and
temperature of the solution were taken at 2, 5, 10, 15, 20, 30, 60, 160, 180,
1080, 1140,1200, 1260 and 1440 minutes
4.2.3 Results and Discussion
0.1 M K2S
2O
8 (measuring conductivity versus time)
Figure 4.15 shows a rapid increase in the conductivity of 0.1 M K2S
2O
8 during
the first 10 minutes, from ~20 μS cm-1 to ~28 μS cm-1 and then it falls to a
plateau ~26 μS cm-1. The peak indicates a completed reaction between 10-30 m
possibly due to the residual reaction or the reduced temperature. While the
experiment of 0.2 M K2S
2O
8 started at ~30 μScm-1 as shown in Figure 4.16. It is
noted that the initial conductivity of 0.2 M K2S
2O
8 solution is ~10 μS cm-1 higher
than the 0.1 M K2S
2O
8 one, as expected due to the higher K
2S
2O
8 concentration.
Figure 4.15: The conductivity obtained from the mixture of 0.1 M K2S
2O
8 and 5
g LiFePO4 at 25°C with respect to time.
0 200 400 600 800 1000 1200 1400
0
10
20
Co
nd
uct
ivit
y/
S c
m-1
time/ min
0 10 20 30 40 50 60
20
22
24
26
28
time/ min
Co
nd
uct
ivit
y/
S c
m-1
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
70
Figure 4.16: The conductivity and temperature obtained from the mixture of 0.2 M K
2S
2O
8 and 5 g LiFePO
4 with respect to time.
Figure 4.16 shows a rapid rise in both conductivity and temperature followed
by a fall in the temperature according to an exponential decay at a rate of -
0.053 m-1, as shown in Figure 4.17. The initial rise in both before and after
adding LiFePO4 is attributed to the concentration and temperature effects as
shown in Equation 4.7 and Equation 4.8.
Equation 4.7 ᴧ
Equation 4.8 ᴧ
When Equation 4.8-Equation 4.7 = Equation 4.9
Equation 4.9 ∆ ᴧ
Where κ = conductivity/ S m-1
c = concentration/ mol dm-1
ᴧ25
= molar conductivity at room temperature/ Sm2 mol-1
α = temperature coefficient T = actual temperature/ °C T
25 = room temperature (25°C)
0 200 400 600 800 1000 1200 1400
0
10
20
30
40
50
time/ min
Co
nd
uct
ivit
y/
S c
m-1
0
5
10
15
20
25
30
Conductivity/ S cm-1
Temperature/ °C
Tem
per
atu
re/ °
C
0 20 40 60 80 100 120
30
35
40
45
50
55
Time/ min
Co
nd
uct
ivit
y/
S c
m-1
16
18
20
22
24
26
28
Te
mp
erat
ure
/ °C
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
71
The contribution due to ᴧ25
(c2-c
1) is shown by the change in conductivity at the
end of the experiment after the solution cooled, as shown in Equation 4.9. The
reason for the initial peak rises is due to the second term that reflect the effect
of a temporary temperature rises before the solution cool to room
temperature.
Figure 4.17: The exponential decay of 0.2 M K2S
2O
8
where T= temperature
Tinitial
= temperature at start
Tfinal
= temperature at final
The summary of Chapter 4 are shown as follows;
90% conversion to FePO4 was obtained within an hour for all reagent
concentrations.
Almost 99% conversion to FePO4 occurred after 30 minutes for the 3:2
and 6:2 of K2S
2O
8 to LFP ratio, after 2 hours for the 1:2 of K
2S
2O
8 to LFP
ratio.
The delithiation results showed that a depletion of FePO4 occurred with
an excessive amount of S2O
82- in the solution.
The conductivity result showed that the reaction completed in 10
minutes.
A high conductivity reflects a high concentration, and so does the
temperature.
0 10 20 30 40 50
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
ln (
T-T
fin
al)/
(Tin
itia
l-T
fin
al)
time/ min
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
72
References 4.3
(1) Ramana, C. V.; Mauger, A.; Gendron, F.; Julien, C. M.; Zaghib, K. Study of the Li-insertion/extraction process in LiFePO
4/FePO
4. Journal of Power
Sources 2009, 187, 555-564. (2) Zaghib, K.; Mauger, A.; Goodenough, J. B.; Gendron, F.; Julien, C. M. Electronic, Optical, and Magnetic Properties of LiFePO4: Small Magnetic Polaron Effects. Chemistry of Materials 2007, 19, 3740-3747. (3) Ait-Salah, A.; Dodd, J.; Mauger, A.; Yazami, R.; Gendron, F.; Julien, C. M. Structural and Magnetic Properties of LiFePO4 and Lithium Extraction Effects. Zeitschrift für anorganische und allgemeine Chemie 2006, 632, 1598-1605. (4) Dodd, J. L.; Yazami, R.; Fultz, B. Phase Diagram of Li
xFePO
4.
Electrochemical and Solid-State Letters 2006, 9, A151-A155. (5) Miao, S.; Kocher, M.; Rez, P.; Fultz, B.; Yazami, R.; Ahn, C. C. Local Electronic Structure of Olivine Phases of LixFePO4. The Journal of Physical Chemistry A 2007, 111, 4242-4247. (6) Atkins, P. W.; Julio, D. P.: Electrochemistry. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford 2005; pp 200-224.
Chapter 4: Test of K2S
2O
8 as an Oxidising Agent of LiFePO
4 to FePO
4
73
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
75
Test of LiI and Na2S
2O
3 as Chapter 5:
Reducing Agent of FePO4 to LiFePO
4
Introduction 5.1
The previous chapter, studied on the oxidizing agent to extract lithium to
obtain the heterosite FePO4
as a framework. Here, this section discusses the
choice of a suitable reducing agent to selectively insert lithium into the
heterosite FePO4 framework without inserting other cations, such as sodium,
calcium and magnesium. These cations are all present in brine in a larger
concentration than lithium.
There are several reports of lithiation of FePO4 from solution of lithium iodide
(LiI) in acetonitrile.1-4 In this work, an aqueous reducing agent was preferred for
environmental reasons and therefore in the first approach, LiI was studied in
aqueous solution instead, Equation 5.1
Equation 5.1 2 2 → 2
A high concentration of LiI was used to make the reaction faster, according to
Le Chatelier’s principle. Another way is to use an activating agent, i.e. zinc, to
aid an increase in the rate of the reaction, as shown in Equation 5.2. Zinc
reacts with iodine (I2) which removes I
2 from the reaction in Equation 5.1.
Equation 5.2 →
Here, this experiment is considered to examine LiI, LiI+Zn and S2O
32- in aqueous
solutions as a possible reducing agent. Nevertheless, lithium salt (e.g. LiCl or
LiSO4) would be added to the solution as a lithium source.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
76
Experimental Details 5.2
Lithium iodide was used twice with the aid of a higher concentration (1M LiI
and 2 M LiI). The last experiment with LiI also obtained Zn as an activating
agent. Then, thiosulphate was introduced, with and without the lithium salts in
order to investigate sodium behaviour from Na2S
2O
3.
The products from each experiment were examined using X-ray diffraction
(XRD; D2 Pharser) to characterize the LiFePO4
olivine structure or FePO4
heterosite structure. The sample was scanned from 15 to 45 degrees of XRD
pattern. The reference XRD patterns of LiFePO4 and FePO
4 were obtained via the
Inorganic Crystal Structure Database (ICSD; RSC). Celref and Diffrac.eva
programs were used to analyse the sample XRD patterns.
LiI as a reducing agent
a) 1 M LiI
A molar ratio of 3:1 of LiI to FePO4 was used. A solution of 1 M LiI was made by
adding 5 g of LiI (0.0373 mol) (Sigma-Aldrich, ≥99.0 %) to 37.3 ml of de-
ionized water (DI water). Then, 1.875 g of FePO4 (from the previous delithiation
LiFePO4 experiments) was added. While LiI and FePO
4 were mixed by stirring on
a hotplate for approximately for 30 minutes at room temperature, the reaction
was observed. The product was then filtered, washed with DI water, and dried
at 80°C for approximately 12 hours. The product was examined using XRD.
b) 2 M LiI, 2 M LiI +Zn
A molar ratio of 3:1 of LiI to FePO4 was used in the same way as the experiment
above. The same amount of LiI and FePO4 weight was used. A solution of 2 M
LiI was made in 18.65 ml of DI water. The mixing procedure for collecting and
analysing the product was done in the same way as previously.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
77
Zn was used as an activating agent in 2 M LiI. According to Equation 5.2,
stoichiometry molar ratio is 1:2:2 (Zn: FePO4: LiI). However, there were an
excess molar ratio of Zn to LiI which was adjusted to 3:1:3. A solution of 2 M
LiI was used by adding 5 g of LiI (0.0373mol) in 18.65 ml of DI water. One
2.443 g purified Zn granule (0.0373 mol) (BDH Chemicals, Ltd., 99.8 %) and
1.875 g of FePO4 (0.0124 mol from the previous delithiation LiFePO
4
experiments) were prepared.
The FePO4 and the Zn granules were added to the LiI solution. The mixture was
combined by stirring on a hotplate for approximately 30 minutes at room
temperature. The procedure for collecting and analysing the product was
similar to the previous experiment.
Using Na2S
2O
3 as a reducing agent with/without lithium salts
a) With lithium salts
According to Equation 5.3, lithium sulphate (Li2SO
4) and lithium chloride (LiCl)
were used as lithium salts as shown in Equation 5.4 and Equation 5.5.
Equation 5.3 2 2 2 → 2
Equation 5.4 2 2 → 2
Equation 5.5 2 2 2 → 2 2
Equation 5.4 shows a molar ratio of 1:2:2 of Li2SO
4: Na
2S
2O
3: FePO
4. However, a
molar ratio of 2:4:1 of Li2SO
4:Na
2S
2O
3:FePO
4 was used. The excess of reducing
agent, i.e. Na2S
2O
3, was applied. A solution of 1 M Li
2SO
4 was made by adding 4
g of Li2SO
4 (0.0363 mol) (Sigma-Aldrich, ≥98.5 %) in 36.38 ml of DI water. The
solution was stirred and heated to approximately 50°C on a hotplate to aid its
dissolution. The solution was allowed to cool down to room temperature.
Then, 18.021 g of Na2S
2O
3 (0.0728 mol) (Na
2S
2O
3·5H
2O, Timstar Laboratory
Supplier, Ltd.) was added, stirred, and heated. After Na2S
2O
3 was dissolved,
2.719 g of FePO4 (0.0182 mol) was then mixed into the solution and continued
to be stirred for approximately for 1 hour at room temperature. The process of
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
78
collecting and analysing the product was done in the same way as the
procedure above.
Equation 5.5 shows a molar ratio of 2:2:2 of LiCl: Na2S
2O
3: FePO
4. A molar ratio
of 2:2:0.5 of LiCl: Na2S
2O
3: FePO
4 was used. The excess of reducing agent, i.e.
Na2S
2O
3, was applied. A solution of 1 M LiCl was made by adding 5 g of LiCl
(0.1179mol) (Sigma-Aldrich, ≥ 99.9%) in 117.9 ml of DI water and stirred at
room temperature. After the LiCl was dissolved, 29.594 g of Na2S
2O
3 (0.1179
mol) was mixed into the solution and continued to be stirred at room
temperature. 4.44 g of FePO4 (0.0294 mol) was added to the solution when
Li2SO
4 and Na
2S
2O
3 were dissolved. The mixture was stirred on a hotplate for
approximately 1 hour at room temperature. The process of collecting and
analysing the product was done in the same way as the procedure above.
b) Without lithium salts
Equation 5.6 2 2 → 2
The purpose of this experiment is to observe whether sodium can be inserted
into FePO4 by S
2O
32- reagent. A possible reaction in the absence of lithium salt is
shown in Equation 5.6. A molar ratio of 2 Na2S
2O
3: 1 FePO
4 was used. A
solution of 1 M of Na2S
2O
3 was made by adding 10 g of Na
2S
2O
3 (0.0403 mol) in
40.29 ml of DI water. Heating (~50°C) and stirring were applied on a hotplate
to dissolve the Na2S
2O
3. Then, after the solution cooled down, 3.037 g of FePO
4
(0.020 mol) was added after Na2S
2O
3 had dissolved. The mixture was
continuously stirred for 1 hour then filtered, washed and dried at 80°C. The
product was analysed in the same as previously mentioned.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
79
Results and Discussion 5.3
The XRD pattern samples were intended to match approximately with the
indexed pattern of LiFePO4, FePO
4, and NaFePO
4, which were obtained via the
Inorganic Crystal Structure Database (ICSD; RSC).
LiI as a reducing agent
a) 1M LiI
During this experiment, a yellowish colour formed in the solution. This
indicates the colour of iodine (I2) which corresponds to Equation 5.1. However,
the XRD pattern of 1 M LiI sample showed no conversion from FePO4 to LiFePO
4
as shown in Figure 5.1. The figure shows the combination of the Miller indices
of the FePO4 XRD pattern and LiFePO
4 XRD pattern. The sample XRD pattern
matched the FePO4 XRD pattern. The conclusion was that 1 M LiI in aqueous
solution did not react with FePO4.
Figure 5.1: XRD patterns of 1 M LiI sample obtained after 30 m at room temperature, compared to FePO
4, and LiFePO
4. All diagrams were indexed in the
orthorhombic (Pnma (62)) crystallographic system.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
80
a) 2M LiI Throughout 2 M LiI experiment, a yellow-brownish colour formed in the
solution which indicates the colour of iodine (I2), again in the Equation 5.1. This
occurred the same way as in the 1 M LiI experiment. However, the XRD pattern
of 2 M LiI sample still showed no trace of olivine structure in the sample as
shown in Figure 5.2.
15 20 25 30 35 40 450
1
2
3
4
XR
D In
ten
sity
/CP
S X
103
2Theta/ Degree
FePO4 pattern
LiFePO4 pattern
40
0
41
1
112
31
1
10
1
21
0
01
1 111
020
112
121
10
1
21
0
01
1
20
2
00
2121
212
321
202
401
22141
0102
311
22030
1
020
211
201
111
011
101
210
200
200
10
1
21
0
01
1
201
21
10
20
30
1
220 102
41
0
221
401
212
15 20 25 30 35 40 450
1
2
3
4
5
6
7
112
2M LiI
200
101
210
011
111
201
211
020
301
311
121
102 410
221
401
202
41
1
212
Figure 5.2: XRD patterns of 2 M LiI obtained after 30 m at room temperature, compared to FePO
4, and LiFePO
4. All diagrams were indexed in the
orthorhombic (Pnma (62)) crystallographic system.
2M LiI + Zn
The last attempt was made using Zn as an activating agent in 2 M LiI solution.
The colour of the solution changed to a yellow- brownish colour indicating I2 as
shown in Equation 5.1. In the reaction, LiI was oxidized to form I2, giving the
brownish colour. Lithium ion might have remain as Li+ in the solution because
the potential from LiI is not enough to provide for Li+ insertion into FePO4. Zinc
weights before and after the experiment were 2.5815 g and 2.5742 g.
Therefore, approximately 0.0073 g of Zn was dissolved in the solution.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
81
2.443 g of Zn is expected to dissolve in the solution as shown in the
calculation below.
.
65.39 2.443
However, 0.0073 g of Zn was used in the solution which is equivalent to 1.116
x10-4 mole as shown below
0.0073 65.39
1.116 10
This corresponds to the product of LiFePO4, as shown in the calculation below.
2 1.116 10 157.76
0.0352
Therefore, the amount of Zn consumed in the experiment was little compared
to the amount expected from the reaction. Also, the XRD result did not show
any trace of LiFePO4, as expected in Figure 5.3.
However, the concentration of LiI may need to be more than 2 M in order to
convert FePO4 to LiFePO
4. Since 2 M LiI is quite high concentration, therefore
another reducing agent is used.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
82
15 20 25 30 35 40 450
1
2
3
4
LiFePO4 pattern
XR
D In
ten
sity
/CP
S X
103
2Theta/ Degree40
0
411
11
2
311
10
1
210
011
11
1
112
121
10
1
210
011
202
00
2
121
212
321
202
401
22141
0102
311
220301
020
211
201
111
011
101
210
200
20
0
10
1
210
011
20
1
211
020
301
22
0
10
24
10
22
14
01
21
2
FePO4 pattern
15 20 25 30 35 40 45
3
4
5
6
7 2M LiI +Zn
112
200 10
1
210
011
111
201
211
020
301
311
121
102
410
221
401 20
2
41
12
12
Figure 5.3: XRD patterns of 2 M LiI +Zn sample obtained after 30 m at room temperature, compared to FePO
4, and LiFePO
4. All diagrams were indexed in the
orthorhombic (Pnma (62)) crystallographic system.
Using Na2S
2O
3 as a reducing agent with/without lithium salts
Lithium salts, i.e. Li2SO
4 and LiCl, were used as a lithium source in the lithiation
experiment. Na2S
2O
3 acted as a reducing agent as shown in Equation 1.13. The
results showed a total conversion of FePO4 to LiFePO
4. A control experiment,
i.e. no lithium salts, was performed using Na2S
2O
3. The result found no trace of
NaFePO4 in its sample. The result of each experiment is shown in the
following.
a) With lithium salts
Li2SO
4
Approximately 30 minutes into the experiment, the optical observation of the
lithiated material suspension was shown to have darker colour. This evidence
indicated a change of the sample. According to Equation 5.4, the sample
molecule was expected to show a conversion of FePO4 to LiFePO
4. As the result
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
83
shown in Figure 5.4 indicates fully lithiated FePO4 was found in the XRD
sample pattern.
15 20 25 30 35 40 450
1
2
3
4
2Theta/ Degree
FePO4 pattern
LiFePO4 pattern
400
411
112
311
101
210
011 11
1
112
121
101
210
011
202
002
121
212
321
20240
122
141010
2
311
22030
1
020
211
201
111
011
101
210
200
200
101
210
011
201
211
020
301
220
102
410
221
401
212
15 20 25 30 35 40 450
1
2
3
4
112
202
32122
14
01
102410
002
121
220
311
301
201
111
0112
10
101
200
XR
D In
ten
sity
/CP
S X
103
2Li2SO
4: 4Na
2S
2O
3: 1FePO
4
211
020
Figure 5.4: XRD patterns of 2Li2SO
4: 4Na
2S
2O
3: 1FePO
4 sample obtained after
1 h at room temperature, compared to FePO4, and LiFePO
4. All diagrams
were indexed in the orthorhombic (Pnma (62)) crystallographic system.
LiCl
The reaction of this experiment (Equation 5.5) was similar to the experiment
using Li2SO
4. A black colour of the solution was found during the reaction. The
sample was analysed and showed the result given in Figure 5.5. A total
conversion LiFePO4 was found for the XRD sample pattern.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
84
15 20 25 30 35 40 450
1
2
3
4
101
2Theta/ Degree
FePO4 pattern
400
411
112
311
111
112
121
202
00212
1
212
3212 0
2401
22141
0
102
311
220301
020
211
2 01
111
0 11
101
210
2 00
200
101
210
011 2
01
211
020
301 220
102
410
221
401
21
2
LiFePO4 pattern
15 20 25 30 35 40 450
1
XR
D In
ten
sity
/CP
S X
103
002
102
112 20
2
21
2
121
220
311
301
020
21101
1210
200
2LiCl: 2Na2S
2O
3: 0.5FePO
4
111
201
401
221
Figure 5.5: XRD patterns of 2LiCl: 2Na2S
2O
3: 0.5FePO
4 sample obtained 1 h
at room temperature, compared to FePO4, and LiFePO
4. All diagrams were
indexed in the orthorhombic (Pnma (62)) crystallographic system.
b) Without lithium salts
The re-insertion without lithium salts (sodiation or sodium insertion) was done
using 2:1 of Na2S
2O
3 to FePO
4 ratio. The main reason was to see if sodium ion is
more difficult to be inserted into the heterosite structure than lithium ion, as in
Equation 5.6, and to see whether Na2S
2O
3 acted as a reducing agent for
NaFePO4 or not.
During the experiment, there was no evidence of a colour change in the
suspension. This suggests that Fe2+ compounds were not formed. In Figure 5.6,
the XRD sample pattern found no trace of NaFePO4. Therefore, the sodium ion
is harder to insert into the heterosite structure than lithium ion by stirring at
room temperature.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
85
15 20 25 30 35 40 450
1
2
3
4
5
6
401
4 10
400
220
2 20
301
0 20
211
1 01
2 00
2 02
2 12
321
411
2Theta/ Degree
NaFePO4 pattern
FePO4 pattern
101
011 22
140
1 1 12
212
410
1 0212
13 1
1
301
020
211
201
210
1 11
200
112
2 02
210
011
201
111
311 1 2
100
210
2
15 20 25 30 35 40 450
1
2
3
4
XR
D I
nte
nsi
ty /
CP
S X
103
101
011
221
401
112
212
410
10212
13 1
1
301
0 20
211
20121
0
1112 0
0
2Na2S
2O
3 : 1FePO
4
Figure 5.6: XRD patterns of 2:1 of Na2S
2O
3 :FePO4 ratio sample obtained 1 h at
room temperature, compared to FePO4, and LiFePO
4. All diagrams were indexed
in the orthorhombic (Pnma (62)) crystallographic system.
In summary, this chapter demonstrated as shown below;
Aqueous LiI, by contrast with LiI in acetonitrile, does not give any trace
of lithium insertion into FePO4, even when Zn is added to remove the I
2
product.
Na2S
2O
3 reduces FePO
4 completely in the presence of excess lithium salt,
resulting in fully lithiated FePO4. Therefore, Na
2S
2O
3 is considered to be a
suitable reducing agent for lithium insertion in heterosite FePO4.
Na2S
4O
6 was formed not to react with S
2O
32- in the absence of lithium salt.
This means that NaFePO4
will not be produced by S2O
32- as a reducing
agent.
Chapter 5: Test of LiI and Na2S
2O
3 as a Reducing Agent of FePO
4 to LiFePO
4
86
References 5.4
(1) Prosini, P. P.; Carewska, M.; Scaccia, S.; Wisniewski, P.; Passerini, S.; Pasquali, M. A New Synthetic Route for Preparing LiFePO
4 with Enhanced
Electrochemical Performance. Journal of The Electrochemical Society 2002, 149, A886-A890. (2) Shiratsuchi, T.; Okada, S.; Yamaki, J.-i.; Yamashita, S.; Nishida, T. Cathode performance of olivine-type LiFePO
4 synthesized by chemical
lithiation. Journal of Power Sources 2007, 173, 979-984. (3) Prosini, P. P.; Carewska, M.; Scaccia, S.; Wisniewski, P.; Pasquali, M. Long-term cyclability of nanostructured LiFePO
4. Electrochimica Acta 2003, 48,
4205-4211. (4) Galoustov, K.; Anthonisen, M.; Ryan, D. H.; MacNeil, D. D. Characterization of two lithiation reactions starting with an amorphous FePO
4
precursor. Journal of Power Sources 2011, 196, 6893-6897.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
87
Kinetic Studies of the Chemical Chapter 6:
Lithiation of FePO4 by Na
2S
2O
3
A preliminary study of the effect of varying the 6.1
concentrations of both Li+ and S2O
3
2- together
6.1.1 Introduction
As already mentioned, thiosulphate is a suitable reducing reagent for the
lithiation of FePO4. However, a kinetic study of the lithiation reaction is
important to understand the lithiation mechanism. It is also significant to
identify an optimal concentration of the reducing agent. That is because high
cost and waste management problems may arise if too much concentrated
reducing agent is consumed. On the other hand, a sufficiently high
concentration may be required to provide a rapid reaction and thus a clear
advantage over the original process, lime soda evaporation, which takes 1-2
years to produce Li2CO
3 as the raw product of lithium-ion battery industry.
Therefore, this section discusses the kinetic mechanism and the choice of an
optimal concentration of thiosulphate as the reducing agent to selectively
insert lithium into the heterosite FePO4 framework.
The following studies, the kinetics of chemical lithiation of FePO4
are
investigated with respect to variations in both lithium ion (Li+) and thiosulphate
ion (S2O
32-) concentrations together, and independently. All experiments were
performed using Na2S
2O
3 as the reductant.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
88
6.1.2 Experimental Details
The stoichiometry of the chemical reaction is shown in Equation 6.1. A molar
ratio of 2 Li2SO
4: 4 Na
2S
2O
3: 1FePO
4 was used, i.e. with a 4 excess of reducing
agent (Na2S
2O
3) and lithium salt to aid faster conversion. A solution preparation
and a sample collection procedure are described in detail below.
Equation 6.1 2 2 2
Solution preparation
Solutions of 0.15 M, 0.35 M, 0.75 M, and 1.5 M Li2SO
4 (Sigma-Aldrich, ≥98.5 %)
in de-ionized (DI) water were made. A solution of 0.15 M Li2SO
4 was made by
adding 1.50 g (0.0136 mol) of Li2SO
4 (Sigma-Aldrich, ≥98.5 %) in 90.93 ml of DI
water and the solution was stirred until all the salt dissolved. Then, 6.77 g of
Na2S
2O
3 (0.0273 mol) (Na
2S
2O
3•5H
2O, Timstar Laboratory Supplier, Ltd.) was
added, then the solution was stirred, and heated to approximately 50°C to
dissolve the Na2S
2O
3, then cooled to room temperature before adding 1.028 g
of FePO4 (0.00682 mol). The concentration of substances is shown in Table
6.1.
For 0.75 M, 0.35 M, and 0.15 M Li2SO
4, the amounts of Li
2SO
4, Na
2S
2O
3, and
FePO4 used were the same; however, the volumes of DI water were different.
The volumes of DI water used for 0.35 M, 0.75 M, and 1.5 M Li2SO
4 were 38.97
ml, 18.19 ml, and 9.10 ml, respectively.
Table 6.1: Concentration of Li2SO
4, Na
2S
2O
3, and FePO
4 in each sample
Solution Li2SO
4 /M Na
2S
2O
3 /M FePO
4 /M*
1 0.15 0.3 0.075
2 0.35 0.7 0.175
3 0.75 1.5 0.375
4 1.5 3 0.75
Note: *Average concentration in the suspension in mol/cm3
1 ml = 1 cm3
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
89
Sample collection procedure
Once the solutions were dissolved and cooled to room temperature, the
insoluble FePO4 was added to form a suspension, as the timer was started. The
suspensions were stirred and samples were taken for filtration at 300, 1200,
3600, 72000, 14400, 36000 and 86400 seconds (5 min, 20 min, 1 h, 2 h, 4 h,
10 h, and 24 h, respectively). The filtration was done using a vacuum pump
with grade 1 qualitative filter paper diameter 110 mm. Each sample was then
washed with DI water, and dried at 80°C for approximately 12 h. Samples were
analysed using X-ray diffraction (XRD), galvanostatic cycling, and inductive
coupled plasma-mass spectrometry (ICP-MS).
For each sample the extent of conversion from the LiFePO4 (olivine) structure to
FePO4 (heterosite) structure, was determined by XRD, using the Bruker D2
Phaser. Samples were scanned from 15 to 50 degrees of XRD pattern. The
reference XRD patterns of LiFePO4 and FePO
4 were obtained via the Inorganic
Crystal Structure Database (ICSD; RSC).
Samples were also fabricated into composite electrodes in lithium-ion cells to
determine the lithium content by potentiometric titration at a rate of 0.1 C and
the results were compared with the calibration curve from Chapter 3.
Some samples: i.e. 0.15 M Li2SO
4 at 300, 36000 and 86400 seconds, 0.35 M
Li2SO
4 at 3600 and 7200 seconds, 0.75 M Li
2SO
4 at 1200 seconds, 1.5 M Li
2SO
4
at 300, 1200 and 36000 seconds were digested and sent to the National
Oceanography Centre Southampton (NOCS) for ICP-MS analysis. The analysis
and preparation method are described more in Chapter 2.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
90
6.1.3 Results and Discussion
Examples of results from XRD, potentiometric titration and ICP-MS are listed
below. Table 6.2 shows preliminary results on lithium selectivity. Table 6.3
illustrates the stoichiometric coefficient of x in LixFePO
4 that was found by all
methods.
X-ray Diffraction
Figure 6.1-Figure 6.3 show examples of the XRD pattern of heterosite FePO4,
and fully and partially lithiated LiFePO4, that were collected from the
experiments. Rietveld refinement of the data provides an estimate of the
extent of lithiation, defined as the % phase of the olivine vs. the heterosite
structure. The refinement method is described in details in Chapter 2.
Figure 6.1: The XRD fitting obtained from a sample treated with 0.15 M Li
2SO
4 + 0.3 M Na
2S
2O
3 (solution 1) for 300 seconds. The result shows only
heterosite, i.e. the FePO4 starting material.
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The pink tick marks show the allowed reflection positions for heterosite FePO
4.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
91
Figure 6.2: The XRD fitting obtained from a sample treated with 0.35 M Li
2SO
4 + 0.7 M Na
2S
2O
3 (solution 3) for 7200 seconds. The result shows a
mixed phase, i.e. partial conversion. Reproduced from Ref. 1 with permission from The Royal Society of Chemistry. (This figure was published in March 20141)
Figure 6.3: The XRD fitting obtained from a sample treated with 1.5 M Li
2SO
4
+ 3.0 M Na2S
2O
3 (solution 1) for 3600 seconds. The result shows pure
olivine, i.e. LiFePO4
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for olivine LiFePO
4 and the lower pink tick marks are for heterosite
FePO4.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
92
Potentiometric Titration
As already mentioned, electrochemical potentiometric titration was used to
determine the redox capacity, and hence the molar lithium deficiency with
respect to LiFePO4 according to Faraday's Law. This could be done by either
oxidative removal of lithium or by the reductive insertion of lithium - both
methods providing consistent results. The specific charge results were
compared with the specific charge in the calibration graph, which was given in
Chapter 3 (reproduced here as Figure 6.4). The comparison gives a percentage
of lithium content to compare with the other methods, i.e. XRD and MS (in
Table 6.3). Figure 6.5 illustrates an example of the potentiometric removal of
lithium, where the higher values of the specific charge are associated with a
higher molar lithium content.
0 20 40 60 80 100
0
20
40
60
80
100
120
140
160
Sp
ecif
ic c
har
ge/
mA
h g
-1
% Li
Figure 6.4: A validating graph of percentage lithium content and specific charge. Reproduced from Ref. 1 with permission from The Royal Society of Chemistry. (This figure was published in March 20141)
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
93
Figure 6.5: Potentiometric titration of LixFePO4 electrodes prepared with the reaction product of FePO
4 in 0.35 M Li
2SO
4 + 0.7 M Na
2S
2O
3 for different times,
as indicated. Specific current: 17 mA g-1 at C/10 Reproduced from Ref. 1 with permission from The Royal Society of Chemistry. (This figure was published in March 20141).
Inductively Coupled Plasma-Mass Spectrometry
The third estimation of the lithium content in the LixFePO
4 reaction product was
found by ICP-MS. The lithium mole fraction in the solid samples were
calculated from the ion concentrations found by ICP-MS to determine lithium
and sodium uptake, and lithium selectivity using Equations 2.14-2.17 from
Chapter 2, as shown in Table 6.2.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
94
Table 6.2: Li and Na concentrations of samples obtained by a 1:2 of [Li+]:[Na+] solution with 1 g FePO
4 at various times. (The data were published in March
20141).
Solution
[Li2SO
4]
+[Na2S
2O
3]
/M
Result* Time/
s
Uptake /mg g-1 [Li+]:[Na+]soli
d
Lithium selectivit
y [Li+] [Na+
]
1 0.15 +
0.3
Li0.01
FePO
4
300 2.8 0.7 13.3 26.6
Li0.50
FePO
4
36000 25.8
1.4 59.5 118.9
LFP 86400 45.1
1.7 89.0 178.0
2 0.35 +
0.7
Li0.30
FePO
4
3600 15.5
1.5 34.3 68.6
Li0.47
FePO
4
7200 22.8
1.1 67.9 135.8
3 0.75 +
1.5 Li
0.25FeP
O4
1200 14.3
1.3 37.4 74.9
4 1.5 + 3.0
Li0.09
FePO
4
300 5.9 1.9 10.1 20.2
Li0.39
FePO
4
1200 20.7
3.3 21.0 42.0
LFP 36000 44.7
2.2 66.3 132.7
*average result from Table 6.3 *LFP =LiFePO
4
Table 6.3 summarizes the results for the lithium contents as determined by
XRD, potentiometric, and ICP measurements vs. the treatment time and
condition. The area between the linear orange trend lines in Table 6.3 shows
the range of conversion from FePO4 to LiFePO
4. The lowest concentration, i.e.
solution 1, showed a trace of about 50% Li and a fully insertion at 36000 s (10
h) and 86400 s (24 h), respectively. For solution 2, the sample results were
found to be about 30% Li after 3600 s (1 h) and 100% Li after 14400 s (4 h).
The sample from solution 3 at 1200 s (20 min) started to show the insertion of
Li in the FePO4 structure as an average of 25% of Li. The suspension from
solution 3 completed the reduction after 3600 s. The highest concentration,
i.e. solution 4, starts to convert into an average of 9% LFP at 300 seconds (5
min) and the samples converted almost completely to LiFePO4 after 3600
seconds, similarly to solution 3.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
95
Table 6.3: Stoichiometric coefficient of x in LixFePO
4 samples obtained by
lithiation of FePO4 with a 4-fold excess of reagent (Li
2SO
4+Na
2S
2O
3 in molar ratio
1:2) for different times, using 3 techniques.
Solution
[Li2SO
4]+
[Na2S
2O
3]/
M
Techniques
Time/ s
300 1200 3600 7200 14400 36000 86400
1 0.15 + 0.3
XRD 0.00 0.00 0.00 0.00 0.00 0.48 1.00
PT 0.02 0.01 0.05 0.04 0.12 0.50 1.01 ICP-MS 0.03 - - - - 0.54 0.98
2 0.35 + 0.7
XRD 0.00 0.00 0.31 0.44 1.00 1.00 1.00 PT 0.02 0.04 0.28 0.50 1.00 1.01 -
ICP-MS - - 0.32 0.47 - - -
3 0.75 + 1.5
XRD 0.00 0.22 1.00 1.00 1.00 1.00 1.00 PT 0.02 0.25 0.86 1.00 1.00 1.02 -
ICP-MS - 0.29 - - - - -
4 1.5 + 3.0
XRD 0.08 0.28 1.00 1.00 1.00 1.00 1.00 PT 0.09 0.41 0.82 1.02 1.02 1.10 -
ICP-MS 0.10 0.48 - - - 0.95 -
For the electrochemical and ICP-MS analysis, the lithium percentage of each
sample is similar to the results from the XRD analysis except for the sample of
solution 1 at 1200 s, which shows almost a 50 percentage difference.
Therefore, it was concluded that XRD was a less reliable measurement of the
conversion to LiFePO4
than the electrochemical and ICP methods. Fitting the
XRD pattern to a two-phase mixture was difficult when one of the phases was
present in a small proportion, e.g. in solution 2-3 at 300-1200 s, in which case
the XRD patterns were easier to fit with a single-phase model.
Figure 6.6-Figure 6.9 show estimations of the extent of lithiation according to
the three methods. In all cases the data show an approximately linear
dependence with time at a reaction extent up to about 0.8 and then a tapering
off towards 1.0 as the reaction nears completion. Also, the rates of lithium
uptake clearly increase from the lowest to the highest reagent concentrations.
However, there are some subtle differences in the data obtained according to
the method used and some subtle differences are discussed as follows.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
96
Mass spectrometry (MS) measures the mole fraction of lithium in the product,
and in most cases shows higher values of lithium than other measurements as
shown in Figure 6.6-Figure 6.9. This is due to the fact that MS is a very
sensitive method to analyse. Also, it is possibly enhanced by some lithium ion
absorption at surface sites without redox activity or structural change, similarly
to larger ionic radii ions (i.e. Na+, K+, Mg2+) behaviour.2 Potentiometric titration
(PT) measures the extent of reduction of Fe(III) to Fe(II) as facilitated by the
thiosulphate reducing agent. It does not distinguish between the cations that
may counterbalance the electron charge during the reduction process, which
are Li+, Na+ and H+ in this case. XRD simply measures the phase fraction of
olivine, which is only approximately related to the lithium uptake according to
the phase diagram shown schematically below. We note that during the course
of the reaction, the heterosite phase should still exist up to a limiting x value
corresponding to the maximum supersaturation before nucleation of olivine;
similarly the Olivine phase can fully consume the heterosite at x values less
than 1.0, as shown in Figure 6.10. Therefore the structural measurement using
Rietveld refinement based on the phase ratio is not an accurate measure of the
x value, particularly in the dilute heterosite and dilute Olivine regions.
Figure 6.6: The comparison of XRD, PT, and MS techniques in the extent of lithiation which was obtained from solution 1 (0.15 M Li
2SO
4+0.3 M Na
2S
2O
3).
0 20000 40000 60000 80000 100000
0.0
0.2
0.4
0.6
0.8
1.0
0.15 M Li2SO
4+ 0.3 M Na
2S
2O
3
XRD PT MS
x in
Li x
FeP
O4
Time/ s
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
97
0 2000 4000 6000 8000 10000 12000 14000 16000
0.0
0.2
0.4
0.6
0.8
1.0
0.35 M Li2SO
4+ 0.7 M Na
2S
2O
3
XRD PT ICP
x in
Li x
FeP
O4
Time/ s
Figure 6.7: The comparison of XRD, PT, and MS techniques in the extent of lithiation which was obtained from solution 2 (0.35 M Li
2SO
4+0.7 M Na
2S
2O
3).
0 2000 4000 6000 8000
0.0
0.2
0.4
0.6
0.8
1.0
0.75 M Li2SO
4+ 1.5 M Na
2S
2O
3
XRD PT MS
x in
Li x
Fe
PO
4
Time/ s
Figure 6.8: The comparison of XRD, PT, and MS techniques in the extent of lithiation which was obtained from solution 3 (0.75 M Li
2SO
4+1.5 M Na
2S
2O
3).
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
98
Figure 6.9: The comparison of XRD, PT, and MS techniques in the extent of lithiation which was obtained from solution 4 (1.5 M Li
2SO
4+3 M Na
2S
2O
3).
Figure 6.10: Phase fraction of olivine in the heterosite/ olivine composite.
0 2000 4000 6000 8000
0.0
0.2
0.4
0.6
0.8
1.0x
in L
i xF
ePO
4
Time/ s
XRD PT MS
1.5 M Li2SO
4+ 3.0 M Na
2S
2O
3
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
99
Discounting the XRD results, which errors in the determination of low degrees
of lithiation, the above data show an approximately constant rate during the
progress of the reaction until the FePO4 is almost fully lithiated. Therefore each
plot can be considered to represent pseudo zero order kinetics due to the fact
that both reagents are present in large excess, i.e. the reagent concentrations
are almost constant during each reaction and measured in M s-1 with respect to
the effective molarity of FePO4. We define the reaction rate as the change in the
molar lithium content in LixFePO
4 with time:
Equation 6.2
where x = the stoichiometric coefficient in LixFePO
4.
Since the present results show a pseudo-zero order behaviour, the reaction
rate can be calculated as the inverse of the reaction time, where the reaction
time corresponds to the time when all FePO4 has been converted into LiFePO
4.
The rates are determined in Table 6.4.
It is obvious that increasing the Li+ concentration and the S2O
32- concentration
rises the reaction rate. The following subchapter studies the effect of
increasing Li+ concentrations at a constant S2O
32- concentration and of
increasing S2O
32- concentrations at a constant Li+ concentration.
Table 6.4: The reaction rate of each experiment when x = 1 (LixFePO
4) with
respects the reaction time (estimated data from Figure 6.9-Figure 6.6)
[FePO4]
/M
[Li2SO
4]
/M [Na
2S
2O
3]
/M [Li
2SO
4]x[Na
2S
2O
3]
/M2
Reaction time/s (xrd)
Reaction rate/s-1
0.075 0.15 0.3 0.045 85000 1.2x10-5 0.175 0.35 0.7 0.245 14000 7.1x10-5
0.375 0.75 1.5 1.125 4200 2.4x10-4
0.75 1.5 3 4.5 3900 2.6x10-4
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
100
Kinetics of the Chemical Lithiation of FePO4 using 6.2
Na2S
2O
3, varying the concentration of S
2O
3
2- and Li+
independently
6.2.1 Introduction and Theory
The stoichiometry of this chemical reaction is shown in Equation 6.3. Lithium
chloride (LiCl) was used in this experiment instead of lithium sulphate (Li2SO
4),
as in the previous experiment. This was to show that there is no significant
difference between lithium salts.
Equation 6.3 2 2 2 → 2 2
The order of reaction and rate constant with respect to the concentration of Li+
were studied by fixing the concentration of S2O
32- and varying the concentration
of Li+; similarly the kinetics of the reaction with respect to S2O
32- concentration
were studied by fixing the Li+ concentration and varying the concentration of
S2O
32-. The FePO
4 itself was considered as a minority component so that the rate
of reaction was measured by the rate of conversion of FePO4 to LiFePO
4 rather
than the rate of consumption of the solution reagents, which was considered
to be negligible.
Solution preparation
The preparation process was done the same way as in Chapter 5.1. 0.03 M,
0.06 M, 0.1 M, 0.3 M, 0.6 M, 0.9 M, and 1.2 M of LiCl (Sigma-Aldrich, ≥ 99.9%)
were used for studying the kinetics of Li+ concentration by fixing the S2O
32-
concentration and fixed FePO4 concentration at 0.3 M and 0.075 M,
respectively. The kinetics study of S2O
32- concentration was done by using 0.1
M, 0.6 M, 0.9 M, and 1.2 M Na2S
2O
3 with the fixed concentration of 0.3 M LiCl
and 0.075 M FePO4 as shown in Table 6.5.
Table 6.5: Concentrations of the reagents
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
101
Solution Substrate / M
Volume/ L Ratio [Li+]:[S2O
3
2-]:[FePO4]
LiCl Na2S
2O
3
1 0.03
0.3
0.884
0.4 : 4 : 1 2 0.06 0.8 : 4 : 1 3 0.1 1.3 : 4 : 1 4 0.3 4 : 4 : 1 5 0.6 8 : 4 : 1 6 0.9 12 : 4 : 1 7 1.2 16 : 4 : 1 8
0.3
0.1 4 :1.3 : 1 9 0.6 4 : 8 : 1
10 0.9 4 :12 :1 11 1.2 4: 16 :1
Sample collection procedure
The processes of mixing the substrates were similar to experiment 5.1.
Nevertheless, for each solution, samples were made individually and collected
for filtration at 1200, 3600, 7200, 14400, 36000, 86400, and 172800 seconds
(20 m, 1 h, 2 h, 4 h, 10 h, 1 day and 2 days, respectively) after adding FePO4.
The samples were made separately due to the fact that a sufficient amount ~1
g of samples was needed for each analysis. Thus, 77 samples were made. Each
sample was filtered using grade No. 1 qualitative filter paper with 110 mm
diameter, washed with DI water, and dried at 80°C for approximately 12 hours.
Samples were analysed using XRD (scanned from 15 to 50 degrees) and
inductive coupled plasma-mass spectrometry (ICP-MS) as described in Chapter
2. The results are reported in terms of stoichiometric coefficient of x in
LixFePO
4.
6.2.2 Results and Discussion
Two studies are observed which are the effect of lithium ion concentration with
respect to thiosulphate ion concentration and the effect of thiosulphate ion
concentration with respect to lithium ion concentration. The results of LixFePO
4
are shown in Table 6.6. The reaction rates are shown in Table 6.7-Table 6.9.
The uptake of Li+ and Na+ of each sample was calculated and is shown in
Table 6.10.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
102
The effect of Li+ concentration
In this experiment, the various Li+ concentrations were mixed with a fixed
[S2O
32-] and a fixed [FePO
4]. The various concentrations of Li+ consisted of 0.03
M, 0.06 M, 0.1 M, 0.3 M, 0.6 M, 0.9 M, and 1.2 M LiCl. The fixed
concentrations were 0.3 M [Na2S
2O
3] and 0.075 M [FePO
4] (added as a
suspension). The samples were first examined by XRD which indicated the
amount of each phase by XRD fitting. Examples of XRD fitting profiles are
shown in Figure 6.11- Figure 6.13.
Figure 6.11: Example of the XRD fitting data for 0.4:4:1 (0.03 M LiCl + 0.3 M Na
2S
2O
3 + 0.075 M FePO
4) for 172800 s (2 days) result in Li
0.25FePO
4
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO
4 and the lower pink tick marks are for heterosite FePO
4.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
103
Figure 6.12: Example of the XRD fitting data for 0.8:4:1 (0.06 M LiCl + 3.0 M Na
2S
2O
3 + 0.075 M FePO
4) for 172800 s (2 days) result in Li
0.80 FePO
4)
Figure 6.13: Example of the XRD fitting data for 4:4:1(0.3M LiCl + 0.3 M Na
2S
2O
3 + 0.075 M FePO
4) for 36000 s (10 hours) result in LiFePO
4
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO
4 and the lower pink tick marks are for heterosite FePO
4
(in Figure 6.12).
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
104
The kinetics study of S2O
32-
In a similar way, the various [S2O
32-] were mixed with a fixed [Li+] and a fixed
[FePO4]. The various concentrations of S
2O
32- consisted of 0.1 M, 0.3 M, 0.6 M,
0.9 M, and 1.2 M. The fixed concentrations were 0.3 M [LiCl] and 0.075 M
[FePO4]. Samples were done individually for the times that were mentioned
earlier. The samples were first examined by XRD which indicated the amount of
each phase by XRD fitting. Examples of XRD fitting profiles are shown in
Figure 6.14 and Figure 6.15.
Figure 6.14: Example of the XRD fitting data for 4:1.3:1 (0.3 M LiCl + 0.1 M Na
2S
2O
3 + 0.075 M FePO
4) for 172800 s (2 days) result in Li
0.35FePO
4
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO
4 and the lower pink tick marks are for heterosite FePO
4.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
105
Figure 6.15: Example of the XRD fitting data for 4:8:1 (0.3 M LiCl + 0.6 M Na
2S
2O
3 + 0.075 M FePO
4) for 14400 s (4 hours) result in Li
0.80 FePO
4)
The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO
4 and the lower pink tick marks are for heterosite FePO
4.
Table 6.6 shows the conversion of FePO4
to LiFePO4 for both studies; i.e.
kinetics of Li+ and S2O
32- concentrations. The linear orange trend lines show a
range of starting points of the conversion. The starting point of the conversion
is more extended at the lowest lithium concentration ([Li+] = 0.03 M at 4 hours)
than the lowest concentration of thiosulphate ([S2O
32-] = 0.1 M at 2 hours). The
fully lithiated FePO4 result was not found at 0.4: 4: 1 and 0.8: 4: 1 after 2 days.
Presumably, this means the ratio of [Li+] was not enough for the full
conversion. However, 1.3-16: 4: 1 at 10 hours results indicated a 100% LiFePO4.
For the S2O
32- kinetics study, the starting point of the conversion at the least
S2O
32- concentration was indicated at 4 hours and the full conversion was found
after 2 days. The 4: 4 to 8: 1 ratio started to convert after ~ 2 hours and
completed its conversion at 10 hours. The last two ratios, i.e. 4: 12: 1 and 4:
16: 1, started to convert before 1 hour and finished the conversion at 4 hours.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
106
These results correspond to the kinetics models, i.e. diffusion limited model
and surface-reaction limited model.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
107
Table 6.6: Lithium molar content of LixFePO
4 samples obtained by lithiation of FePO
4 for different times, as estimated from
(a) XRD and (b) ICP measurements.
[Li+]:[S
2O
3
2-]:[FePO4]
Concentration/ M Time/s [LiCl] [Na
2S
2O
3 ] [FePO
4] 1200 3600 7200 14400 36000 86400 172800
0.4 : 4 : 1 0.03
0.3
0.075
0.00a 0.00a 0.00a 0.00a
0.04b
0.07a
0.09b
0.11a
0.14b
0.25a
0.23b
0.8 : 4 : 1 0.06 0.00a 0.00a 0.00a 0.00a
0.04b
0.09 a
0.13b
0.28 a 0.26b
0.80 a 0.70b
1.3 : 4 : 1 0.1 0.00a
0.00b
0.00a
0.07b 0.00a
0.07b 0.01 a
0.12b 0.30 a
0.40 b 0.91 a
0.79 b 1.00a
1.00 b
4 : 4 : 1 0.3 0.00a
0.05b 0.10a
0.10b 0.18a
0.25b 0.36a
0.44b 1.00a
1.00b - -
8 : 4 : 1 0.6 0.00a
0.05b
0.16a
0.15b
0.22a
0.20b
0.46a
0.43b
1.00a
1.00b - -
12 : 4 : 1 0.9 0.00a
0.10b 0.00a
0.17b 0.24a
0.30b 0.56a
0.64b 1.00a
1.10b - -
16 : 4 : 1 1.2 0.00a
0.10b 0.17a
0.23b 0.34a
0.44b 0.58a
0.78b 1.00a
1.18b - -
4 : 1.3 : 1
0.3
0.1 0.00a
0.06b 0.00a
0.07b 0.00a
0.06b 0.00a
0.06b 0.08a
0.13b 0.33a
0.47b
0.35a
0.32b
4 : 4 : 1 0.3 0.00a
0.05b 0.10a
0.10b 0.18a
0.25b 0.36a
0.44b 1.00a
1.00b - -
4 : 8 : 1 0.6 0.00a
0.10b 0.21a
0.23b 0.40a
0.46b 0.70a
1.08b 1.00a
1.26b 1.00a
1.04b -
4 : 12 : 1 0.9 0.00a
0.13b 0.29a
0.36b 0.87a
1.12b 1.00a
1.16b - - -
4 : 16 : 1 1.2 0.00a
0.13b 0.54a
0.68b 0.98a
1.13b
1.00a
1.23b - - -
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
108
Figure 6.16-Figure 6.22 show a graph of the stoichiometric coefficient x in
LixFePO
4 with respect to time. The graphs were obtained with various
concentrations of Li+ and fixed concentrations of S2O
32- and FePO
4. Each graph
presents a linear relationship of the x-y axis. Each slope indicates the reaction
rate. For the reaction rate evaluation, the XRD data that corresponds to nearly
fully lithiated or delithiated sample (i.e. x values were close to zero or one)
were discarded. The reaction rate can be defined as a derivative of x with
respect to time where x is a stoichiometric coefficient for lithium as in LixFePO
4.
Figure 6.23 and Table 6.7 show the relationship between the reaction rate and
the concentration of lithium. It is shown that the reaction rate varies with the
square root of the lithium concentration.
Figure 6.16: Rate of the lithiated reaction that was obtained from 0.03 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 1; 0.4:4:1 ratio), with respect of
time.
0 50000 100000 150000 200000
0.00
0.05
0.10
0.15
0.20
0.25
y=1.4310-6 x
XRD MS
x in
Li x
FeP
O4
Time/ s
0.03 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
109
Figure 6.17: Rate of the lithiated reaction that was obtained from 0.06 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 2; 0.8:4:1 ratio), with respect of
time.
Figure 6.18: Rate of the lithiated reaction that was obtained from 0.1 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 3; 1.3:4:1 ratio), with respect of
time.
0 40000 80000 120000 160000
0.0
0.2
0.4
0.6
0.8
y=4.0610-6 x
0.06 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
XRD MS
x in
Li x
FeP
O4
Time/ s
0 20000 40000 60000 80000 100000
0.0
0.2
0.4
0.6
0.8
1.0
y=9.6510-6 x
0.1 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
Time/ s
x in
Li x
FeP
O4
XRD MS
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
110
Figure 6.19: Rate of the lithiated reaction that was obtained from 0.3 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 4; 4:4:1 ratio), with respect of time.
Figure 6.20: Rate of the lithiated reaction that was obtained from 0.6 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 5; 8:4:1 ratio), with respect of time.
0 5000 10000 15000 20000 25000 30000 35000 40000
0.0
0.2
0.4
0.6
0.8
1.0
y=2.7810-5 x
0.3 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
Time/ s
x in
Li x
FeP
O4
XRD MS
0 5000 10000 15000 20000 25000 30000 35000 40000
0.0
0.2
0.4
0.6
0.8
1.0
y=2.8410-5 x
0.6 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
x in
Li x
FeP
O4
XRD MS
Time/ s
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
111
Figure 6.21: Rate of the lithiated reaction that was obtained from 0.9 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 6; 12:4:1 ratio), with respect of time.
Figure 6.22: Rate of the lithiated reaction that was obtained from 1.2 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 7; 16:4:1 ratio), with respect of time.
0 5000 10000 15000
0.0
0.2
0.4
0.6
y=4.1110-5 x
XRD MS
x in
Li x
FeP
O4
0.9 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
Time/ s
0 5000 10000 15000 20000
0.0
0.2
0.4
0.6
0.8
y=4.9010-5 x
1.2 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
x in
Li x
FeP
O4
XRD MS
Time/ s
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
112
Table 6.7: The reaction rate of each experiment with respect to [Li+]
Ratio [Li+]/M [S2O
3
2-]/M Reaction rate /s-1
0.4:4:1 0.03
0.3
1.43 x 10-6
0.8:4:1 0.06 4.06 x 10-6 1.3:4:1 0.1 9.65 x 10-6 4:4:1 0.3 2.78 x 10-5 8:4:1 0.6 2.84 x 10-5 12:4:1 0.9 4.11 x 10-5 16:4:1 1.2 4.90 x 10-5
Figure 6.23: The reaction rates of the kinetic study with respect to the lithium concentration.
For the kinetic study of S2O
32- concentration, Figure 6.24-Figure 6.28 show a
graph of the stoichiometric coefficient of x in LixFePO
4 with respect to time. The
graphs were obtained with various concentrations of S2O
32- and fixed
concentrations of Li+ and FePO4. Each graph also presents a linear relationship
between the x-y axis, similar to the kinetic study of Li+ concentration’s. Figure
6.29 and Table 6.8 show the relationship between the reaction rate and the
concentration of thiosulphate. This result indicates that the reaction rate is
proportional to the S2O
32- concentration.
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0
1x10-5
2x10-5
3x10-5
4x10-5
5x10-5
Rea
ctio
n r
ate/
s-1
[Li+]/ M
[S2O2-
3]=0.3 M
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
113
Figure 6.24: Rate of the lithiated reaction that was obtained from 0.3 M [Li+], 0.1 M [S
2O
32-] and 0.075 [FePO
4] (solution 8; 4:1.3:1 ratio), with respect of
time.
Figure 6.25: Rate of the lithiated reaction that was obtained from 0.3 M [Li+], 0.3 M [S
2O
32-] and 0.075 [FePO
4] (solution 4; 4:4:1 ratio), with respect of time.
0 20000 40000 60000 80000 100000
0.0
0.1
0.2
0.3
0.4
0.5
y=4.7910-6 x
x in
Li x
FeP
O4
XRD MS
Time/ s
0.3 M [Li+]+0.1 M [S2O2-
3]+0.075 M [FePO
4]
0 5000 10000 15000 20000 25000 30000 35000 40000
0.0
0.2
0.4
0.6
0.8
1.0
y=2.7810-5 x
0.3 M [Li+]+ 0.3 M [S2O2-
3]+0.075 M [FePO
4]
Time/ s
x in
Li x
FeP
O4
XRD MS
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
114
Figure 6.26: Rate of the lithiated reaction that was obtained from 0.3 M [Li+], 0.6 M [S
2O
32-] and 0.075 [FePO
4] (solution 9; 4:8:1 ratio), with respect of time.
Figure 6.27: Rate of the lithiated reaction that was obtained from 0.3 M [Li+], 0.9 M [S
2O
32-] and 0.075 [FePO
4] (solution 10; 4:12:1 ratio), with respect of
time.
0 5000 10000 15000 20000
0.0
0.2
0.4
0.6
0.8
y=5.3110-5 x
0.3 M [Li+]+0.6 M [S2O2-
3]+0.075 M [FePO
4]
Time/ s
x in
Li x
FeP
O4
XRD MS
0 2000 4000 6000 8000
0.0
0.2
0.4
0.6
0.8
1.0
y=1.1010-4 x
0.3 M [Li+]+0.9 M [S2O2-
3]+0.075 M [FePO
4]
Time/ s
x in
Li x
FeP
O4
XRD MS
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
115
Figure 6.28: Rate of the lithiated reaction that was obtained from 0.3 M [Li+], 1.2 M [S
2O
32-] and 0.075 [FePO
4] (solution 11; 4:16:1 ratio), with respect of
time.
Table 6.8: The reaction rate of each experiment with respect to [S2O
32-]
Ratio [Li+]/M [S2O
3
2-]/M Reaction rate/ s-1
4:1.3:1
0.3
0.1 4.79 x 10-6
4:4:1 0.3 2.78 x 10-5
4:8:1 0.6 5.31 x 10-5 4:12:1 0.9 1.10 x 10-4
4:16:1 1.2 1.48 x 10-4
0 2000 4000 6000 8000
0.0
0.2
0.4
0.6
0.8
1.0
y=1.4810-4 x
0.3 M [Li+]+1.2 M [S2O2-
3]+0.075 M [FePO
4]
Time/ s
x in
Li x
FeP
O4
XRD MS
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
116
Figure 6.29: The reaction rates of the kinetic study with respect to the thiosulphate concentration.
In order to analyse the effect of Li+ and S2O
32- concentration on the reaction
rate, we developed two models: diffusion limited model and surface-reaction
limited model, to interpret the kinetics of Li+ and S2O
32- concentrations, using
the data graphs from Figure 6.23- Figure 6.29.
Diffusion Limited Model
The FePO4 particle is considered as a microelectrode. So, we want to calculate
the rate at which ions can diffuse towards the surface of the FePO4 particle. The
rate can be defined as a derivative of x in respect of time where x is a
stoichiometric coefficient for lithium as in LixFePO
4, as shown in Equation 6.4.
The rate unit is s-1.
Equation 6.4
The flux of ions diffusing towards a FePO4 particle that behaves as a micro
electrode is3:
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.0
2.0x10-5
4.0x10-5
6.0x10-5
8.0x10-5
1.0x10-4
1.2x10-4
1.4x10-4
1.6x10-4
Rea
ctio
n r
ate/
s-1
[S2O2-
3]/ M
[Li+]=0.3 M
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
117
Equation 6.5 4
Where D = diffusion coefficient of the ion in solution (D of Li+ in solution 4~ 1.0
x 10-5/ cm2s-1, D of S2O
32- in solution 5 ~ 6.46 x 10-6/ cm2s-1)
c = the concentration of Li+ or S2O
32- / mol cm-3
r = the radius of agglomerate FePO4 ~ 1.0 x 10-4 /cm (data from Tatung
in Appendix) In order to obtain the reaction reate, the number of Li+ sites on the FePO
4
particle
Equation 6.6 43
3
Where d = density of FePO
4 ~ 3.4/ g cm-3
Mw = molecular weight of FePO4 ~ 151/ g mol-1
π = 3.1415, 1 mol L-1 = 1.0 x 10-3 mol cm-3
The reaction rate ( unit s-1) was obtained by dividing the flux by the number of
Li+ sites, since the rate has been defined as the derivative of the stoichiometric
coefficient x in LixFePO
4 with time, as shown in Equation 6.7.
Equation 6.7 43
3
The idea is to compare the theoretical reaction rate model to the experimental
reaction rate, considering a diffusion limited process.
The reaction rate of the experiment can calculate from a slope of a graph of x,
LixFePO
4 in respect of time (s). For example, from the kinetics of the Li+
concentration graph, the rate of experiment that obtained from 0.03 M Li+
concentration, 0.3 M S2O
32- concentration, and 0.075 M FePO
4 concentration is
1.43 x 10-6 s-1.
The theoretical model,
443
4 10 3 10 10
43 10 3.4 1
151
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
118
1.20 10
9.43 10
1.27
The calculation shows that the experimental rate is not limited by the diffusion
of Li+ ions in the solution. The experimental rate is 106 times lower, compared
to the theoretical rate.
Table 6.9: Values of calculated theoretical rates compares to the experimental rates
[Li+]/M [S2O
32-]/M Experimental rate (slope) /s-1 Theoretical rate/ s-1
0.03
0.30
1.43 x 10-6 1.27 0.06 4.06 x 10-6 2.54 0.10 9.65 x 10-6 4.24 0.30 2.78 x 10-5 1.27 x 101 0.60 2.84 x 10-5 2.54 x 101 0.90 4.11 x 10-5 3.82 x 101 1.20 4.90 x 10-5 5.09 x 101
0.30
0.10 4.79 x 10-6 2.74 0.30 2.78 x 10-5 8.22 0.60 5.13 x 10-5 1.64 x 101 0.90 1.10 x 10-4 2.47 x 101 1.20 1.48 x 10-4 3.29 x 101
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
119
Figure 6.30: The comparison of the experimental rate and the theoretical rate of the experiments were obtained from a fixed [S
2O
32-] =0.3 M and [Li+] = 0.03
to 1.2 M.
Figure 6.31: The comparison of the experimental rate and the theoretical rate of the experiments were obtained from a fixed [Li+] =0.3 M and [S
2O
32-] = 0.1 to
1.2 M.
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0
1x10-5
2x10-5
3x10-5
4x10-5
5x10-5
Reaction rate Theoretical rate
Re
acti
on
ra
te/ s
-1
[Li+]/ M
[S2O2-
3]=0.3 M
Th
eore
tica
l rat
e/ s
-1
0
10
20
30
40
50
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.0
2.0x10-5
4.0x10-5
6.0x10-5
8.0x10-5
1.0x10-4
1.2x10-4
1.4x10-4
1.6x10-4
Experimental rate Theoretical rate
[S2O2-
3]/ M
Exp
erim
enta
l rat
e/ s
-1
0
10
20
30
Th
eore
tica
l rat
e/ s
-1
[Li+]=0.3 M
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
120
Surface-reaction limited model
This model is based on a simple electron transfer redox couple (Equation 6.8)
which occurred at the FePO4 surface.
Equation 6.8
Where O = oxidised species n = number of electrons (e-) R = reduced species
For kinetics, the rate of oxidation and reduction can be written as shown in
Equation 6.9 - Equation 6.106,7.
Equation 6.9
Equation 6.10
Where and = anodic and cathodic rate constants [R] and [O] = the concentration of the reduced species and the oxidised species.
Both and depends on the potential gradient at the surface of FePO4
as
shown in Equation 6.11 and Equation 6.12.
Equation 6.11 ∝
Equation 6.12 ∝
Where and = standard rate constant of anodic and cathodic
∝ and ∝ = anodic and cathodic transfer coefficient (∝ ∝ 1 n = number of e-1 F = Faraday constant 96485/ C mol-1 E0 = the standard potential of the redox reaction/ V E = the applied potential/ V R = the gas constant (8.314 / J K-1 mol-1) T = the temperature/ K
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
121
Thus, the rate of oxidation and reduction can also be defined as in Equation
6.13 and Equation 6.14.
Equation 6.13 ∝
Equation 6.14 ∝
At equilibrium, when the steady state is reached, the potential and the rates
are equal. According to the Butler-Volmer equation, the potential value will
adjust itself until the rate is equal to another.6,7 If there is no mass transport
limitation, the concentration at the surface is proportional to the concentration
at the bulk. For example, when the concentration at the bulk is doubled, the
concentration at the surface is expected to be twice as much.
In this experiment, the thiosulphate reaction (oxidation reaction) is assumed as
the rate determination step (Equation 6.15), meaning that once this equation
occurs, the rest of the reaction will happen.
Equation 6.15 → 1
The rate of thiosulphate reaction can be defined as shown in Equation 6.16. At
the equilibrium, is equal to zero, due to the fact that any
number to the zero power is one. Hence, in this case, the rate of the
thiosulphate reaction is proportional to S2O
32- concentration. Likewise, the
lithium concentration is proportional to the rate reaction with respect to the Li+
concentration. The rate will increase exponentially by increasing the potential
if the potential depends on the concentration. In other words, if the potential
does not depend on the concentration, the rate is proportional to the
concentration, which corresponds to this case. However, this theory only
applies when S2O
32-> Li+ concentrations.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
122
Equation 6.16 ∝
Where E0 = the standard potential of the redox reaction/ V E = the applied potential/ V
R = the gas constant (8.314 J/ K mol) F = the Faraday constant (96485/ C mol-1) T = the temperature/ K n = the number of electrons transferred k = the rate constant α = transfer coefficient T = the temperature/ K.
In the case of Li+>S2O
32- concentrations, the experimental rate of Li+
concentration is limited by the diffusion of Li+ ion in the solution. The potential
does not depend on S2O
32 concentration but Li+ concentration. The reaction of
thiosulphate oxidation is very slow, whereas, the reaction of lithium reduction
is not. This means that the insertion of lithium ion into FePO4 framework is
assumed to be fast. Therefore, the electrode potential will vary with Li+
concentration in a Nernstian way (Equation 6.18). Plug Equation 6.18 into
Equation 6.16 and simplify the equation.
Equation 6.17 →
Equation 6.18 /
ln
If ∝ =0.5, the model is shown to predict that the reaction will be first order
with respect to the S2O
32- concentration. The reaction will also vary with [Li+]0.5,
as observed experimentally.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
123
Table 6.10: Li and Na concentrations of samples obtained by various ratios of
[Li+]:[S2O
3
2- ] solution with 1 g FePO4 at various time.
ratio [LiCl] [Na
2S
2O
3] time/s
uptake/ mg g-1 [Li+]:[Na+] solid
lithium selectivity [Li+]:[S
2O
32-]: [FePO
4] Li+ Na+
0.4:4:1 0.03
0.3
14400 0.5 1.6 1.0 20.4
36000 2.6 2.2 3.9 77.4
86400 5.2 2.5 6.9 137.3
172800 9.3 2.7 11.7 233.0 259200 8.6 2.3 12.3 246.7
0.8:4:1 0.06
14400 0.9 1.8 1.5 15.5 36000 4.9 2.3 6.9 69.2 86400 10.7 2.8 12.6 126.3
172800 30.9 3.1 32.8 327.5
1.3:4:1 0.1
3600 2.0 3.2 2.1 12.5 7200 1.9 5.0 1.3 7.8
14400 4.4 3.2 4.6 27.3 36000 17.1 3.7 15.3 91.8 86400 34.7 2.7 43.6 261.3
4:4:1 0.3
1200 1.0 1.8 2.0 3.9 3600 3.1 1.9 5.5 11.1 7200 10.2 3.5 9.8 19.6
14400 19.0 2.9 22.1 44.1 36000 44.6 4.4 33.8 67.7
8:4:1 0.6
1200 1.1 2.1 1.7 1.7 3600 5.5 2.0 8.9 8.9 7200 7.9 2.1 12.8 12.8
14400 18.3 2.1 29.2 29.2 36000 39.4 3.1 42.7 42.7
12:4:1 0.9
1200 3.2 2.5 4.1 2.8 3600 6.3 2.6 7.9 5.3 7200 12.4 2.3 18.0 12.0
14400 27.9 3.1 30.4 20.3 36000 49.2 3.5 47.0 31.4
16:4:1 1.2
1200 3.1 1.4 7.4 3.7 3600 9.1 1.8 16.8 8.4 7200 18.7 1.9 33.5 16.8
14400 34.6 2.2 52.7 26.4 36000 52.6 2.5 69.4 34.7
4:1.3:1 0.3 0.1 3600 1.6 1.7 3.1 2.1 7200 1.8 2.1 2.9 1.9
14400 1.3 1.2 3.6 2.4
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
124
36000 4.8 1.2 13.3 8.8 86400 20.3 2.5 27.1 18.1
172800 13.4 1.8 24.5 16.4
4:8:1 0.6
1200 3.0 3.8 2.7 10.6 3600 9.3 4.0 7.8 31.4 7200 19.8 4.2 15.9 63.5
14400 48.2 5.9 27.3 109.4 36000 56.6 6.2 30.3 121.2 86400 46.5 5.2 29.7 118.8
4:12:1 0.9
1200 4.6 4.4 3.6 21.3 3600 14.9 4.1 12.2 72.9 7200 50.2 5.9 28.4 170.3
14400 51.9 6.0 28.7 172.1
4:16: 1 1.2
1200 4.5 4.0 3.7 30.0 3600 29.9 5.1 19.7 157.8 7200 50.6 5.3 32.0 256.4
14400 55.2 6.2 29.8 238.7
Table 6.10 shows values of Li+ or Na+ in milligrams that absorbed into 1 g of
FePO4 (uptake) and selectivity of lithium which obtained from MS analysis. The
maximum Li+ uptake value is approximately 56 mg g-1 which is higher than
some advanced manganese oxide ion sieves; i.e. they absorbed Li up to 38-46
mg g-1. 1,8-11
The maximum Na+ uptake value is approximately 6 mg g-1, with respect to the
maximum of [Na2S
2O
3] (1.2 M). A high lithium selectivity value indicates that the
ratio of lithium to sodium in solution is lower than the ratio in solid. Therefore,
a fully converted product does not have to be the same value.
In summary, this chapter demonstrated as shown below;
The kinetic studies of chemical lithiation of FePO4 with respect to both
Li+ and S2O
32- concentrations demonstrates a pseudo-zero order reaction,
calculated by using the inverse of the reaction time.
The fully lithiated FePO4 can be obtained from as low as 0.3 M Na
2S
2O
3
with 0.15 M Li2SO
4 for 24 h.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
125
Potentiometric titration and mass spectrometry are considered to be the
most reliable to find the coefficient x in LixFePO
4 techniques. However,
XRD technique is very beneficial for rapid crystal structure identification.
Among all experiments, the thiosulphate reaction is the rate
determining step. The surface-reaction limited model is considered to
be a suitable model to interpret the kinetics with respect to Li+ and S2O
32-
concentrations, defining a first order rate reaction when S2O
32-> Li+
concentrations. Inversely, S2O
32-< Li+ concentrations, the rate of the
experiment is limited by Li+ diffusion. This agrees to the reaction rate
with respect to Li+ concentration, which the rate increases with the
square root of the lithium concentration.
References 6.3
(1) Intaranont, N.; Garcia-Araez, N.; Hector, A. L.; Milton, J. A.; Owen, J. R. Selective lithium extraction from brines by chemical reaction with battery materials. Journal of Materials Chemistry A 2014, 2, 6374-6377. (2) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Hydrothermal Synthesis of Lithium and Sodium Manganese Oxides and Their Metal Ion Extraction/Insertion Reactions. Chemistry of Materials 1995, 7, 1226-1232. (3) Denuault, G.; Mirkin, M. V.; Bard, A. J. Direct determination of diffusion coefficients by chronoamperometry at microdisk electrodes. 1991, 308, 27-38. (4) Lepage, D.; Sobh, F.; Kuss, C.; Liang, G.; Schougaard, S. B. Delithiation kinetics study of carbon coated and carbon free LiFePO4. Journal of Power Sources 2014, 256, 61-65. (5) Sabzi, R. E. Electrocatalytic oxidation of thiosulfate at glassy carbon electrode chemically modified with cobalt pentacyanonitrosylferrate. Journal of the Brazilian Chemical Society 2005, 16, 1262-1268. (6) Spiro, M. Polyelectrodes: the behaviour and applications of mixed redox systems. Chemical Society Reviews 1986, 15, 141-165. (7) Pletcher, D.: An introduction to Electrode Reactions. In First Course in Electrode Processes; 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 1-47. (8) Zhang, Q.-H.; Li, S.-P.; Sun, S.-Y.; Yin, X.-S.; Yu, J.-G. Lithium selective adsorption on 1-D MnO
2 nanostructure ion-sieve. Advanced Powder Technology
2009, 20, 432-437. (9) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H
1.6Mn
1.6O
4) Derived from
Li1.6
Mn1.6
O4. Industrial & Engineering Chemistry Research 2001, 40, 2054-2058.
(10) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H
1.33Mn
1.67O
4 and H
1.6Mn
1.6O
4. Chemistry Letters 2012,
41, 1647-1649. (11) Yu, Q.; Sasaki, K.; Hirajima, T. Bio-templated synthesis of lithium manganese oxide microtubes and their application in Li+ recovery. Journal of Hazardous Materials 2013, 262, 38-47.
Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na
2S
2O
3
126
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
127
Selectivity for Li+ versus Other Chapter 7:
Cations Using the S2O
3
2-/ FePO4 Reagents
The studies of lithium absorption into a framework, such as manganese oxide,
manganese dioxide and lithium iron phosphate, as an adsorbent have been
reported in many research studies.1-6 Most were reported using lithium uptake
to interpret how much lithium can be inserted or absorbed into a particular
framework. Other metal ions were reported to be absorbed, along with lithium
ions.
The aims in this chapter are to demonstrate thiosulphate/ FePO4 reagent for Li+
sequestering and testing the selectivity for Li versus other metals. Therefore,
the study of metal uptake, namely lithium, sodium, magnesium and potassium,
into the framework, i.e. FePO4, using thiosulphate as a reducing agent are
reported. Also, the selectivity of lithium ions are calculated to show the
efficiency of the framework.
Chemical Lithiation of FePO4 from Aqueous Solutions 7.1
Containing an Excess of Na+ and Mg2+
Lithium (Li+), sodium (Na+), and magnesium (Mg2+) ions are considered in this
section. Owing to sodium being known as one of dominant cations in brine,
though Na+ is almost double size to Li+, as shown in Table 7.1. The selectivity
between lithium and sodium is thus interesting. For magnesium, although the
concentration of magnesium is lower than sodium in brine, Mg2+ has a smaller
size to Na+.7 Therefore, Na+ and Mg2+ are considered to have a high probability
for insertion and contamination in an FePO4 lithiation process.
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
128
Table 7.1: Ionic radii of the potential cations that can be intercalated into FePO
47
Ions Lithium
(Li+) Magnesium
(Mg2+) Sodium
(Na+ )
Potassium (K+)
Ionic radii, r/pm 59 72 102 138
Here, experiments compare Li+ to Na+ insertion ratios ([Li+]:[Na+]) in response to
a reducing agent and the FePO4 in aqueous solutions containing both lithium
and sodium salts. Three different ratio of concentrations of lithium to sodium
were employed; i.e. 1:10, 1:50, and 1:100 of [Li+]:[Na+]. Likewise, for
magnesium, experiments compared the lithium ion to magnesium ion
concentration ratios ([Li+]:[Mg+2]). In the case of magnesium, the ratios of the
interferent ion to lithium varied from 1:10 to 1:20.
7.1.1 Experimental Details
Equation 7.1 2 2 2 → 2 2
The stoichiometry of this chemical reaction is shown in Equation 7.1. Table 7.2
shows the concentrations of chemical compositions in each experiment. The
concentration of 0.3 M Na2S
2O
3 was mainly used because this concentration
was found to be optimal for the 24 hour experiment, as described in Chapter
6. Another Na2S
2O
3 solution of 0.5 M was also included in the study. An excess
of lithium in solution compared with the amount of FePO4 solid with respect to
Equation 7.1 was used in all cases to ensure full lithiation.
LiCl (Sigma-Aldrich, ≥99.0 %), Na2S
2O
3 (Sigma-Aldrich, ≥99.5 %), and other salts,
i.e. NaCl (Sigma-Aldrich, ≥99.0 %), MgCl2 · 6H
2O (Fisher Scientific, ≥99.0 %)
were dissolved in deionized water. The salts, which were added for each
experiment, are shown in Table 7.2. FePO4, which was prepared by delithiation
of LiFePO4, using 0.1 M K
2S
2O
8 as mentioned in Chapter 4, was then added to
the solution and stirred at room temperature for 24 hours. Each of the
products was filtered, washed and dried at 80°C for 24 hours. All powder
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
129
samples were examined by X-ray Diffraction (XRD). The lithiated samples from
experiments [Li+]:[Na+] and [Li+]:[Mg2+] were examined further by inductively
coupled plasma mass spectrometry analysis (ICP-MS) to check for
contaminations and to measure the lithium- sodium uptake (Equations in
Chapter 2) into a solid structure (FePO4).
Table 7.2: Concentration of reagents used in the experiments [Li+]:[Na+] and [Li+]:[Mg+2]
Experiment Ratio [Chemical composition]/M
DI/L Na
2S
2O
3 LiCl NaCl MgCl
2·6H
2O FePO
4
[Li+]:[Na+]
1:10 0.30 0.06 - - 0.03 0.201
1:10 0.50 0.10 - - 0.05 0.121
1:50 0.30 0.06 2.4 - 0.03 0.201
1:100 0.30 0.06 5.4 - 0.03 0.201
[Li+]:[Mg+2]
1:10 0.30 0.06 - 0.60 0.03 0.201 1:10 0.50 0.10 - 1.00 0.05 0.121
1:20 0.30 0.06 - 1.20 0.03 0.201
1:20 0.50 0.10 - 2.00 0.05 0.121
7.1.2 Results and Discussion
The XRD patterns results are presented under the lithium to sodium ratio and
lithium to magnesium ratio experiment headings. ICP results are shown at the
end.
Lithium to Sodium Ratio Experiment
The XRD sample patterns of the totally converted LiFePO4, obtained from the
1:10 (at 0.5M Na2S
2O
3), 1:50, 1:100 of [Li+]:[Na+] experiments, are shown in
Figure 7.1 - Figure 7.3. The XRD sample patterns were roughly compared with
the LiFePO4 and FePO
4 initial patterns using the position of peaks that indicate
the Miller indices (numbers in blue and red). These imply that with the effect of
high sodium ion concentration in the solution, the results show no sodium
contamination in the crystal structure.
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
130
15 20 25 30 35 40 45
15 20 25 30 35 40 45
15 20 25 30 35 40 45
2/ degree
De-lithiation
Inte
nsi
ty/ A
.U.
LiFePO4 initial
1:10 defect FePO4 2nd run
200
200
200
101
101
101
210
011
111
201
210
011
111
201
210
111
201
211
020
020
211
211
020
301
301
301
311
311
311
121
410
121
410
121
102
102
102
221
401
112 20
2
212
221
401
112 20
2
212
410
221
112
212
401
Figure 7.1: The XRD pattern obtained from a sample 1:10 of [Li+]:[Na+] treated with 0.5 M Na
2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4
and the XRD of the de-lithiated sample (heterosite FePO4) pattern. The result
shows only LiFePO4.
15 20 25 30 35 40 45
15 20 25 30 35 40 45
15 20 25 30 35 40 45
2/ degree
De-lithiation
Inte
nsi
ty/ A
.U.
LiFePO4 initial
011
210
101
200
1:50 defect FePO4
200
200
101
101
210
011
111
201
210
111
201
020
211
211
020
301
301
311
311
121
410
121
102
102
221
401
112 20
2
212
410
221
112
212
401
111
201
020
211
301
311
121
410
102 22
14
01
112 20
2
212
Figure 7.2: The XRD pattern obtained from a sample1:50 of [Li+]:[Na+] treated with 0.3 M Na
2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4
and the XRD of the de-lithiated sample (heterosite FePO4) pattern. The result
shows only LiFePO4.
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
131
15 20 25 30 35 40 45
15 20 25 30 35 40 45
15 20 25 30 35 40 45
2/ degree
De-lithiation
Inte
nsi
ty/ A
.U.
LiFePO4 initial
1:100 defect FePO4
200
200
200 10
110
11
01
210
011
11
1
201
210
011
111
201
210
111
201
211
020
020
211
211
020
301
301
301
311
311
311
121
410
121
410
121
102
102
102
221
401
112
202 2
12
221
401
112
202
212
410
221 11
2
212
401
Figure 7.3: The XRD pattern obtained from a sample 1:100 of [Li+]:[Na+] treated with 0.3 M Na
2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4
and the XRD of the de-lithiated sample (heterosite FePO4) pattern. The result
shows only LiFePO4.
Lithium to Magnesium Ratio Experiment
As shown in Figure 7.4 - Figure 7.5, results showing fully converted LiFePO4
were obtained from 1:10 and 1:20 of lithium ion to magnesium ion ratio with
0.5 M Na2S
2O
3 for 24 hours. This suggests that there is no major effect in the
LiFePO4 results with the presence of an excess of magnesium. Besides, full
intercalation of magnesium into FePO4 has not been reported in any literature
yet. This might be due to the size of magnesium ion, which is slightly bigger
than lithium ion.
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
132
15 20 25 30 35 40 45
15 20 25 30 35 40 45
15 20 25 30 35 40 45
2/ degree
De-lithiation
Inte
nsi
ty/ A
.U.
LiFePO4 initial
1Li:10Mg defect FePO4
011
210
101
200
200
200 10
110
1
210
011
111
201
210
111
201
020
211
211
020
301
301
311
311
121
410
121
102
102
221
401
112 20
2
212
410
221
112
212
401
111
201
020
211
301
311
121
410
102 22
140
1
112 20
2
212
Figure 7.4: The XRD pattern obtained from a sample 1:10 of [Li+]:[Mg+2] treated with 0.5 M Na
2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4
and the XRD of the de-lithiated sample (heterosite FePO4) pattern. The result
shows only LiFePO4.
15 20 25 30 35 40 45
15 20 25 30 35 40 45
15 20 25 30 35 40 45
2/ degree
De-lithiation
Inte
nsi
ty/ A
.U.
LiFePO4 initial
1Li:20Mg defect FePO4
011
210
101
200
200
200 10
110
1
210
011
111
201
210
111
201
020
211
211
020
301
301
311
311
121
410
121
102
102
221
401
112
202
212
410
221
112
21240
1
111
201
020
211
301
311
121
410
102 22
14
01
112 20
2
212
Figure 7.5: The XRD pattern obtained from a sample 1:20 of [Li+]:[Mg+2] treated with 0.5 M Na
2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4
and the XRD of the de-lithiated sample (heterosite FePO4) pattern. The result
shows only LiFePO4.
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
133
The majority of the results show only LiFePO4, as shown in Table 7.3. These
confirm that the full conversion to LiFePO4 occurred with 0.5 M Na
2S
2O
3 and
that partial lithiation sometimes occurred with 0.3 M Na2S
2O
3. Most
significantly, no crystalline NaFePO4 is shown and there is no evidence of
magnesium intercalation.
Table 7.3: The XRD results of lithiated heterosite FePO4 for each experiment
Name [Na2S
2O
3]/M ratio XRD Result
[Li+]:[Na+]
0.3 1:10 LFP+FePO4
0.5 1:10 LFP 0.3 1:50 LFP 0.3 1:100 LFP
[Li+]:[Mg+2]
0.3 1:10 LFP+FePO4
0.5 1:10 LFP 0.3 1:20 LFP+FePO
4
0.5 1:20 LFP *LFP=LiFePO
4
ICP results for the dissolved products are shown in Table 7.4. The lithium
uptake of 40-46 mg g-1 for the samples found by XRD to be fully lithiated
agrees well with the theoretical value of ~46 mg g-1. Comparing to other
literatures, the heterosite FePO4 structure absorbs lithium higher than other
types of absorbent, except for some advanced manganese oxide ion-sieves
which absorb up to 38-40 mg g-1. 1,3,8-11
The maximum sodium uptake is 3.8 mg of sodium per gram of FePO4 solid,
obtained from 1:100 of lithium to sodium ratio, resulting in the fully lithiated
sample. For the Li+ to Mg2+ concentrations ratio experiment, the results suggest
that magnesium uptake is very small compared to the lithium uptake, about
1/40 of Mg2+ uptake to Li+ uptake, as shown in Table 7.4.
It is surprising that the contaminant levels in the lithiated samples were
relatively constant, and did not increase significantly with the concentrations of
contaminant in the initial solution. This finding suggested a more useful
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
134
measure of the efficiency, in terms of a selectivity coefficient defined as the
lithium/ contaminant ratio for the product divided by the same ratio for the
original solution. The final column in the table shows a dramatic selectivity
increase as the concentration of lithium in solution is made much smaller than
that of other ions. The high values suggest that S2O
32-/FePO
4 reagent for Li+
sequestering could be used in brine solutions.
Table 7.4: Lithium and sodium concentrations found in samples
Solution Ratio [Na
2S
2O
3]/
M Result
Uptake/mg g-1 Solid
[Li+]:[metal] Li selectivity
Li Na Mg
[Li+]:[Na+]
1/10 0.3 LFP+ FePO
4 35.1 3.0 - 39 390
1/10 0.5 LFP 40.4 2.8 - 47 470 1/50 0.3 LFP 44.8 3.0 - 48 2400
1/100 0.3 LFP 45.3 3.8 - 40 4000
[Li+]:[Mg2+] 1/10 0.5 LFP 46.9 1.5 1.8 112 1123 1/20 0.5 LFP 44.5 0.8 1.4 111 2221
Note: The formulas of the metal uptake, the solid ratio and lithium selectivity are in Chapter 2.3
Chemical Lithiation of FePO4 from Synthetic Brine 7.2
Solutions
The purpose of the present study was to confirm the reaction of lithium
insertion into FePO4 by S
2O
32- in a synthetic brine environment and to study the
selectivity of the interested metal ions, i.e. Li+, Na+, Mg2+ and K+, where Na+,
Mg2+ and K+ are highly concentrated in brine, with their ion sizes being in the
same range compared to Li+.
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
135
7.2.1 Experimental details
Two types of brine were made mimicking those in the Bolivia’s Salar de Uyuni
and called type A and type B.12 The chemical compositions of both brine types
are shown in Table 7.5.
Table 7.5: Chemical compositions of synthetic brine type A/B and the heterosite FePO
4
Synthetic
Brine
[Chemical composition]/ M DI/L
Na2S
2O
3 LiCl NaCl MgCl
2 ∙ 6H
2O KCl K
2SO
4 FePO
4
Type A 0.3 0.06 4 0.3 0.2 - 0.03 0.134
Type B 0.3 0.20 2.4 1.3 - 0.3 0.10 0.067
The procedure was similar to experiment 7.1. LiCl (Sigma-Aldrich, ≥99.0 %),
Na2S
2O
3 (Sigma-Aldrich, ≥99.5 %), and other salts, i.e. NaCl (Sigma-Aldrich,
≥99.0 %), MgCl2 ∙ 6H
2O (Fisher Scientific, ≥99.0 %) and KCl (Sigma-Aldrich,
≥99.5 %) / K2SO
4 (Fisher Scientific, ≥99.0 %) were dissolved in deionized water.
The salts, which were added for each experiment, are shown in Table 7.5.
FePO4, which was prepared by delithiated LiFePO
4, using 0.1 M K
2S
2O
8 as
mentioned in Chapter 4, was then added to the solution and stirred at room
temperature for 24 hours. Each of the products was filtered, washed and dried
at 80°C for 24 hours. All powder samples were examined by XRD to confirm
the crystal structure, ICP-MS analysis to check for contaminations and to
measure the lithium-sodium uptake into a solid structure (FePO4), and
galvanostatic measurement to measure the capacities and cycles of the
products.
7.2.2 Results and Discussion
Figure 7.6 - Figure 7.7 show the XRD data of the samples that were obtained
from synthetic brine type A and B with 0.3 M Na2S
2O
3 after 24 hours of reaction
time. It is observed that the reaction results in full lithiation of FePO4, with no
major presence of any contaminant. This confirms that Na2S
2O
3 is a suitable
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
136
reducing agent to extract Li+ from salt based solutions into the heterosite
FePO4. The samples were examined by ICP and found to have a very small
contamination of metal ions compared to Li+, as shown in Table 7.6. Again,
this method confirms that the heterosite FePO4 can absorb up to 45-46 mg of
lithium per gram of absorbent with the full insertion of lithium, and with a
similar value to experiment 7.1. This process has a very high selectivity
towards lithium, leading to an enrichment in lithium concentration vs. other
ions of more than 500 under the conditions relevant to lithium extraction from
brines.
15 20 25 30 35 40 45
15 20 25 30 35 40 45
15 20 25 30 35 40 45
2/ degree
De-lithiation
Inte
nsi
ty/ A
.U.
LiFePO4 initial
Brine Normal 1 Day
200
200
200 10
110
110
1
210
011
111
201
210
011
111
201
210
111
201
211
020
020
211
211
020
301
301
301
311
311
311
121
410
121
410
121
102
102
102
221
401
112 20
2
21
2
221
401
112 20
2
21
2
410
221
112
212
401
Figure 7.6: The XRD pattern obtained from brine type A treated with 0.3 M Na
2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4 and the
XRD de-lithiation (heterosite FePO4) sample pattern. The result shows only
LiFePO4.
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
137
15 20 25 30 35 40 45
15 20 25 30 35 40 45
15 20 25 30 35 40 45
2/ degree
De-lithiation
Inte
nsi
ty/
A.U
.
LiFePO4 initial
Brine with K2SO
4 1 Day
200
200
200 10
110
110
1
210
011
111
201
210
011
111
201
210
111
201
211
020
020
211
211
020
301
301
301
311
311
311
121
410
121
410
121
102
102
102
221
401
112 20
2
212
221
401
112 20
2
212
410
221
112
212
401
Figure 7.7: The XRD pattern obtained from brine type B treated with 0.3 M Na
2S
2O
3 for 24 h is compared to the initial XRD pattern of LiFePO
4 and the
XRD de-lithiation (heterosite FePO4) sample pattern. The result shows only
LiFePO4.
Table 7.6: Concentration of metals; i.e. Li+, Na+, K+, and Mg2+, that contain the LiFePO
4 samples obtained from type A and type B synthetic brines.
Metal
Uptake/mg g-1 [Li+]:[Me]solution
[Li+]:[Me]solid
Lithium
selectivity
Type A Type B Type
A
Type
B
Type
A
Type
B Type A Type B
Li+ 45.7 46.4 - - - - - -
Na+ 2.5 0.4 1/77 1/15 61 370 4700 5550
K+ 1.5 1.5 1/3 1/3 170 180 510 540
Mg2+ 0.8 0.2 1/5 1/6.5 200 870 1000 5600
Note: Me = metal The formulas for the calculations are in Chapter 2.3
Galvanostatic measurements were taken to compare the character of the fully
lithiated FePO4 samples that were obtained from both synthetic brines, type A
and B, with the commercial LiFePO4
from Tatung. The results are shown in
Figure 7.8. The first extraction of the samples (LiFePO4) at a rate of 0.1C
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
138
demonstrates a similar specific charge (electrochemical performance) of type B
and the commercial LFP.
Figure 7.8: Electrochemical data at rate 0.1C of LiFePO4 prepared by chemical
lithiation of FePO4 in synthetic brines; i.e. type A and B. The results obtained
with the initial LiFePO4 (Tantung) are also included in the graph for a
comparison. (This figure was published in March 201413).
The results of the electrochemical cycling of LiFePO4, obtained from
commercial, synthetic brine type A and B, at different rates and cycles are
shown in Figure 7.9.
At a slow cycling rate, all electrodes showed no significant changes in the
charge delivered. However, the LiFePO4 obtained from brine type B delivered
smaller charges at fast cycling rates. The second 0.1C and 1C runs shows no
major significant difference in the specific charge than the first run.
-20 0 20 40 60 80 100 120 140 1603.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
Po
ten
tial
/ V v
s. L
i/Li+
Specific Charge/ mAh g-1
Brine type A Brine type B LiFePO
4
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
139
0 10 20 30 40 500
20
40
60
80
100
120
140
160
180
Sp
ecif
ic c
har
ge
/mA
h g
-1
Cycle number
Figure 7.9: Electrochemical cycling of LiFePO4 at different cycling rates, as
indicated (0.1C to C).
: Electrodes prepared with the initial LiFePO4 (Tatung)
: The synthetic brine type A : The synthetic brine type B Closed symbols: charge; open symbols: discharge. (This figure was published in March 201413).
The summary of Chapter 7 is given below:
These results confirm that the complete conversion of LiFePO4
can be
achieved using Na2S
2O
3 under synthetic brine conditions.
The lithium uptake can be as high as 46 milligram per gram of FePO4
and lithium selectivity can be as high as 5600, depending on lithium ion
to metal ion ratio in solution.
The electrochemical performance of both brine type samples were very
satisfactory, compared to the commercial LiFePO4.
C/10
C
2C
4C
6C 8C
10C
C/10
C
Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O
32-/ FePO
4
Reagents
140
References 7.3
(1) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H
1.6Mn
1.6O
4) Derived from
Li1.6
Mn1.6
O4. Industrial & Engineering Chemistry Research 2001, 40, 2054-2058.
(2) Zhang, Q.-H.; Li, S.-P.; Sun, S.-Y.; Yin, X.-S.; Yu, J.-G. Lithium selective adsorption on 1-D MnO
2 nanostructure ion-sieve. Advanced Powder Technology
2009, 20, 432-437. (3) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H
1.33Mn
1.67O
4 and H
1.6Mn
1.6O
4. Chemistry Letters 2012,
41, 1647-1649. (4) Ooi, K.; Miyai, Y.; Katoh, S.; Maeda, H.; Abe, M. Topotactic lithium(1+) insertion to .lambda.-manganese dioxide in the aqueous phase. Langmuir 1989, 5, 150-157. (5) Ooi, K.; Miyai, Y.; Sakakihara, J. Mechanism of lithium(1+) insertion in spinel-type manganese oxide. Redox and ion-exchange reactions. Langmuir 1991, 7, 1167-1171. (6) Pasta, M.; Battistel, A.; La Mantia, F. Batteries for lithium recovery from brines. Energy & Environmental Science 2012, 5, 9487-9491. (7) Atkins, P. W.; De Paula, J.: Mettallic, Ionic, and Covalent solids. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford, 2005; pp 376-400. (8) Yu, Q.; Sasaki, K.; Hirajima, T. Bio-templated synthesis of lithium manganese oxide microtubes and their application in Li+ recovery. Journal of Hazardous Materials 2013, 262, 38-47. (9) Wang, L.; Meng, C. G.; Ma, W. Study on Li+ uptake by lithium ion-sieve via the pH technique. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2009, 334, 34-39. (10) Tian, L.; Ma, W.; Han, M. Adsorption behavior of Li+ onto nano-lithium ion sieve from hybrid magnesium/lithium manganese oxide. Chemical Engineering Journal 2010, 156, 134-140. (11) Wang, L.; Meng, C. G.; Han, M.; Ma, W. Lithium uptake in fixed-pH solution by ion sieves. 2008, 325, 31-40. (12) Risacher, F.; Fritz, B. Quaternary geochemical evolution of the salars of Uyuni and Coipasa, Central Altiplano, Bolivia. Chemical Geology 1991, 90, 211-231. (13) Intaranont, N.; Garcia-Araez, N.; Hector, A. L.; Milton, J. A.; Owen, J. R. Selective lithium extraction from brines by chemical reaction with battery materials. Journal of Materials Chemistry A 2014, 2, 6374-6377.
Chapter 8: Alternative Reducing and Oxidising Agents
141
Alternative Reducing and Chapter 8:
Oxidising Agents
Introduction 8.1
As revealed in Chapter 1 and 5, the potential range for looking for useful
reducing agents for FePO4 lithiation is between 3.04 and 3.45 V vs. Li/Li+. These
numbers correspond to the onset potential of NaFePO4 and LiFePO
4 formation,
respectively.1-5 Sodium thiosulphate (Na
2S
2O
3) has a potential of 3.13 V vs. Li/Li+
6 (Equation 8.1). It was successfully examined as the useful reducing agent,
although identifying an alternative reducing agent is also useful because of
environmental concerns and price of the reagent.
Equation 8.1 2 2
0.08 . , 0.17 . , 3.13 /
The above potentials are based on thermodynamic potentials whereas in
practice kinetic considerations could extend the range to reducing reagents
having potentials lower than 3.04 V vs. Li/Li+. Also, the conversion of E0 values
between aqueous and lithium scales is not very accurate. Therefore, possible
reducing agents could include sodium nitrite (NaNO2)6, formaldehyde (CH
2O)6,
formic acid (HCOOH)6, and sodium sulphite (Na2SO
3)6, as shown in Equation 8.2-
Equation 8.5.
Equation 8.2 2 2
14, 0.01 . , 0.24 . , 3.06 . /
7, 0.42 . , 0.18 . , 3.47 . /
Equation 8.3 4 4
0, 0.07 . , 0.32 . , 2.98 /
7, 0.48 . , 0.73 . ,2.57 . /
Chapter 8: Alternative Reducing and Oxidising Agents
142
Equation 8.4 2 2
0, 0.20 . , 0.45 . , 2.85 /
7, 0.61 . , 0.86 . ,2.44 . /
Equation 8.5 2 2
14, 0.93 . , 1.18 . , 2.12 /
7, 0.52 . , 0.76 . , 2.53 . /
The following experiments were undertaken to test the four reducing agents by
cyclic voltammetry to determine their ability to provide electrons at a rate that
would be useful for lithiation of FePO4. The experiments were first carried out
with thiosulphate as a control experiment to confirm the validity of the test
procedures.
Experimental details 8.2
A standard three-electrode cell was used in all experiments, using a Pt mesh
counter and a saturated calomel electrode (SCE) reference electrode. Four types
of CV experiments were conducted in order to study Na2S
2O
3 as the reducing
agent:
1) Measuring the CV of a Pt electrode, assumed to be inert, in 1.5 M Li2SO
4.
2) Measuring the CV of a Pt electrode in a mixed solution of 1.5 M Li2SO
4
and 1.5 M Na2S
2O
3.
3) Measuring the CV of a Pt electrode coated with LiFePO4 (LFP) in a
solution of 1.5 M Li2SO
4.
4) Measuring the CV of a LFP coated Pt electrode in a mixed solution of 1.5
M Li2SO
4 and 1.5 M Na
2S
2O
3.
Chapter 8: Alternative Reducing and Oxidising Agents
143
For the other alternative reagents, four types of CV experiment were measured
using a mixture of solutions as follows:
1) 1.5 M Li2SO
4 and 1.5 M NaNO
2
2) 1.5 M Li2SO
4 and 1.5 M formaldehyde
3) 1.5 M Li2SO
4 and 1.5 M formic acid
4) 1.5 M Li2SO
4 and 1.5 M Na
2SO
3
An electrolyte solution was made, for example, a solution of 1.5 M Li2SO
4 and
1.5 M Na2S
2O
3 by adding 12.37 g of Li
2SO
4 (0.1125 mol) (Sigma-Aldrich, ≥98.5
%) and 27.91 g of Na2S
2O
3 (0.1125 mol) (Na
2S
2O
3·5H
2O, Timstar Laboratory
Suppliers Ltd.) to 75 ml of de-ionized water (DI water) and mixed well. For the
other solutions, the solution was set up in a similar way. The same weight of
Li2SO
4 was used with the addition of a reducing agent, as shown in Table 8.1,
in 75 ml of DI water.
The electrode preparation and cell setup were mentioned in the previous
chapter, i.e. Chapter 2. The cell was purged with argon for ~1 hour. These
experiments were observed for any reduction or oxidation between the
potential limits of -1.2 to 1.2 V vs. SCE with a scan rate of 10 mV s-1, towards
positive potential, for 20 cycles.
Table 8.1: Composition of each solution
Solution Electrolyte Electrode
Reagent Amount Active
material Weight/ mg
1* Li2SO
4 12.37 g Pt N/A
2* Li2SO
4+Na
2S
2O
3 12.37 g+27.91 g Pt N/A
3* Li2SO
4 12.37 g LFP 12.32
4* Li2SO
4+Na
2S
2O
3 12.37 g+27.91 g LFP 10.62
1 Li2SO
4+NaNO
2 12.37 g+7.76 g LFP 9.46
2 Li2SO
4+CH
2O 12.37 g+4.14 ml LFP 21.44
3 Li2SO
4+HCOOH 12.37 g+4.24 ml LFP 23.85
4 Li2SO
4+Na
2SO
3 12.37 g+14.18 g LFP 27.72
* Control experiment
Chapter 8: Alternative Reducing and Oxidising Agents
144
Results and discussion 8.3
All the CV experiments were started at the open circuit potential, indicated by
the green arrow, as shown in Figure 8.1. The voltammogram were measured at
10 mV s-1 scan rate in a potential window of 1.2 V to -1.2 V vs. SCE.
Figure 8.1: Cyclic voltammogram of a Pt working electrode in: a) 1.5 M Li
2SO
4 (Solution 1*; black)
b) 1.5 M Li2SO
4 and 1.5 M Na
2S
2O
3 (Solution 2*; blue)
The CV profiles were recorded at the rate of 10 mV s-1 with the potential range of 1.2 and -1.2 V vs. SCE.
Figure 8.1 shows the CV profiles of a Pt electrode in 1.5 Li2SO
4 (black CV with
the onset reduction potential of -0.80 V) and in a mixed solution of 1.5 M
Li2SO
4 and 1.5 M Na
2S
2O
3 (blue CV with the onset oxidation and reduction
potential of 0.23 V and -0.32 V, respectively). The oxidation current in the
presence of Na2S
2O
3 is ascribed to the oxidation of S
2O
32- to tetrathionate (S
4O
62-).
The reduction current is ascribed to the reduction of S4O
62- back to S
2O
32-, as
shown in Equation 8.1. The reduction peak on the CV (black curve in Figure
8.1) indicates hydrogen evolution7, as shown in Equation 8.6.
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1.8 2.3 2.8 3.3 3.8 4.3 4.8
-40
-20
0
20
40
60
80
Pt electrode in 1.5 M Li2SO
4
Pt electrode in 1.5 M Li2SO
4+1.5 M Na
2S
2O
3
Potential/ V vs. Li/ Li+
Cu
rren
t/ m
A
Potential/ V vs. SCE
2S2O
3
2- S4O
6
2-+2 e-
S4O
6
2-+ 2e- 2S2O
3
2-
2H2O++2e- H
2+2OH-
SO4
2- S2O
8
2-+ +2e-
Chapter 8: Alternative Reducing and Oxidising Agents
145
Equation 8.6 2 2 → 2
0.00 . , 0.25 . , 3.05 /
7, 0.41 . , 0.66 . , 2.64 . /
Figure 8.2 shows a comparison of three CV profiles, i.e. the LFP electrode in
1.5 M Li2SO
4 (black CV), the Pt working electrode in a mixed solution of 1.5 M
Li2SO
4 + 1.5 M Na
2S
2O
3 (blue CV), and the LFP electrode in a mixed solution of
1.5 M Li2SO
4 + 1.5 M Na
2S
2O
3 (red CV). The black CV profile indicates an
oxidation reaction of LFP to FePO4 on the positive scan. The backward reaction
shows a reduction of FePO4 to LFP. The blue CV profile in Figure 8.2 is the
same as in Figure 8.1. Last, the red CV profile indicates an oxidation of LiFePO4
to FePO4, plus a conversion of FePO
4 to LiFePO
4 which was produced by S
2O
32- in
the solution.
Figure 8.2: Cyclic voltammogram of a) a Pt electrode coated with LiFePO
4 in
1.5 M Li2SO
4 (Solution 3*; black: onset oxidation potential = 0.26 V, Onset
reduction potential = 0.16 V), Pt electrode in 1.5 M Li
2SO
4 and 1.5 M Na
2S
2O
3
(Solution 2*; blue), and a Pt electrode coated with LiFePO
4 in 1.5 M Li
2SO
4 and
1.5 M Na2S
2O
3 aqueous electrolyte (Solution 4*; red: onset oxidation potential
= 0.39 V, onset reduction potential = 0.16 V). The CV profiles were recorded at the rate of 10 mV s-1 with the potential range of 1.2 and -1.2 V vs. SCE.
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1.8 2.3 2.8 3.3 3.8 4.3 4.8
-50
0
50
100
Potential/ V vs. Li/ Li+
Potential/ V vs. SCE
Cu
rren
t/ m
A
LFP electrode in 1.5 M Li2SO
4
Pt electrode in 1.5 M Li2SO
4+1.5 M Na
2S
2O
3
LFP electrode in 1.5 M Li2SO
4+1.5 M Na
2S
2O
3
LiFePO4FePO
4+Li++e-
FePO4+Li++e- LiFePO
4
LiFePO4 FePO
4+Li++e-
2FePO4+2Li++2S
2O
3
22LiFePO4+S
4O
6
2-
2S2O
3
2- S4O
6
2-+2 e-
S4O
6
2-+ 2e- 2S2O
3
2-
Chapter 8: Alternative Reducing and Oxidising Agents
146
Two features indicate that S2O
32- is the suitable reducing agent for FePO
4. The
increased oxidation current due to two reactions occurred, i.e. an oxidation of
LiFePO4 to FePO
4 and a conversion of FePO
4 to LiFePO
4 which was produced by
S2O
32-. Another feature is the decreased reduction current measured with the
LFP electrode in the presence of S2O
32-. This is due to the previous conversion
by S2O
32- that left a small amount of FePO
4 to reduce and form LiFePO
4.
Figure 8.3-Figure 8.6 demonstrate the CV profiles of a lithium electrode in the
other alternative reducing reagents, i.e. NaNO2, CH
2O, HCOOH and Na
2SO
3,
which are represented by the red graph. All of them were compared to the
profile of the LFP electrode in 1.5 M Li2SO
4 which is represented in black.
Figure 8.3: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in
a) 1.5 M Li2SO
4 (Solution 3*; black: onset oxidation potential = 0.26 V,
Onset reduction potential = 0.16 V) b) 1.5 M Li
2SO
4 and 1.5 M NaNO
2 (Solution 1; red: onset oxidation
potential = 0.26 V, Onset reduction potential = 0.14 V)
All the CV profiles of the alternative reagents show an increased oxidation
current, compared to the black profile, except for the CH2O one in Figure 8.4.
In the case of Na2SO
3 (Figure 8.6), a decreased reduction current was also
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1.8 2.3 2.8 3.3 3.8 4.3 4.8
-50
0
50
100 LFP electrode in 1.5 M Li
2SO
4
LFP electrode in 1.5 M Li2SO
4+1.5 M NaNO
2
Cu
rren
t/ m
A
Potential/ V vs. Li/ Li+
Potential/ V vs. SCE
Chapter 8: Alternative Reducing and Oxidising Agents
147
observed, similar to the case of Na2S
2O
3 (Figure 8.2). This suggests that
probably Na2SO
3 will be an efficient reducing agent. All other reagents might
also work; however, further studies are required. The least promising reagent
is CH2O because the CV is almost identical to the black CV, i.e. measured
without CH2O.
Beside this CV experiment of Na2SO
3, the actual experiment of FePO
4 lithiation
using Na2SO
3 as a reducing agent was performed and illustrated a successful
result of a fully converted FePO4, as shown in the appendix. Thus, SO
32- is
shown to be an alternative reducing agent, as expected from the CV profile.
Figure 8.4: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in
a) 1.5 M Li2SO
4 (Solution 3*; black: onset oxidation potential = 0.26 V,
Onset reduction potential = 0.16 V) b) 1.5 M Li
2SO
4 and 1.5 M formaldehyde
(Solution 2; red: onset oxidation
potential = 0.34 V, Onset reduction potential = 0.12 V)
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1.8 2.3 2.8 3.3 3.8 4.3 4.8
-60
-40
-20
0
20
40
60
Potential/ V vs. Li/ Li+
Cu
rren
t/ m
A
Potential/ V vs. SCE
LFP electrode in 1.5 M Li2SO
4
LFP electrode in 1.5 M Li2SO
4+1.5 M CH
2O
Chapter 8: Alternative Reducing and Oxidising Agents
148
Figure 8.5: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in a) 1.5 M Li
2SO
4 (Solution 3*; black: onset oxidation potential =
0.26 V, Onset reduction potential = 0.16 V) b) 1.5 M Li
2SO
4 and 1.5 M formic acid
(Solution 3; red: onset
oxidation potential = 0.25 V, Onset reduction potential = 0.12 V)
Figure 8.6: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt
electrode in a) 1.5 M Li
2SO
4 (Solution 3*; black: onset oxidation potential =
0.26 V, Onset reduction potential = 0.16 V) b) 1.5 M Li
2SO
4 and 1.5 M Na
2SO
3 (Solution 4; red: onset
oxidation potential = 0.36 V, Onset reduction potential = 0.14 V)
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1.8 2.3 2.8 3.3 3.8 4.3 4.8
-100
-50
0
50
LFP electrode in 1.5 M Li2SO
4
LFP electrode in 1.5 M Li2SO
4+1.5 M HCOOH
Potential/ V vs. Li/ Li+
Cu
rren
t/ m
A
Potential/ V vs. SCE
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1.8 2.3 2.8 3.3 3.8 4.3 4.8
-50
0
50
100 LFP electrode in 1.5 M Li
2SO
4
LFP electrode in 1.5 M Li2SO
4+1.5 M Na
2SO
3
Potential/ V vs. Li/ Li+
Cu
rren
t/ m
A
Potential/ V vs. SCE
LiFePO4 FePO
4+Li++e-
2FePO4+2Li++SO
3
2-+2OH- 2LiFePO
4+ SO
4
2-+ H2O
FePO4+Li++e- LiFePO
4
Chapter 8: Alternative Reducing and Oxidising Agents
149
The summary of Chapter 8 is given below:
The CH2O reagent found to be the least promising reducing agent.
NaNO2 and HCOOH might be the suitable reducing agents; however,
they further study is required.
Na2SO
3 was found to be the most suitable reducing agent for inserting
lithium ion into FePO4.
References 8.4
(1) Whiteside, A.; Fisher, C. A. J.; Parker, S. C.; Saiful Islam, M. Particle shapes and surface structures of olivine NaFePO4 in comparison to LiFePO4. Physical Chemistry Chemical Physics 2014, 16, 21788-21794. (2) Wu, B.; Ren, Y.; Li, N.: LiFePO
4 Cathode Material. In Electric Vehicles -
The Benefits and Barriers, 2011; pp 199-216. (3) Zaghib, K.; Trottier, J.; Hovington, P.; Brochu, F.; Guerfi, A.; Mauger, A.; Julien, C. M. Characterization of Na-based phosphate as electrode materials for electrochemical cells. Journal of Power Sources 2011, 196, 9612-9617. (4) Reale, P.; Panero, S.; Scrosati, B.; Garche, J.; Wohlfahrt-Mehrens, M.; Wachtler, M. A Safe, Low-Cost, and Sustainable Lithium-Ion Polymer Battery. Journal of The Electrochemical Society 2004, 151, A2138-A2142. (5) Dodd, J. L.; Yazami, R.; Fultz, B. Phase Diagram of Li
xFePO
4.
Electrochemical and Solid-State Letters 2006, 9, A151-A155. (6) Center, U. o. R. I. C. R.; Company, C. R.: Handbook of chemistry and physics 0363-3055; 70th ed.; CRC Press: Cleveland, Ohio, 1989. (7) Mi, C.; Zhang, X.; Li, H. Electrochemical behaviors of solid LiFePO
4
and Li0.99
Nb0.01
FePO4 in Li
2SO
4 aqueous electrolyte. Journal of Electroanalytical
Chemistry 2007, 602, 245-254.
Chapter 8: Alternative Reducing and Oxidising Agents
150
Chapter 9: Conclusions and Future Work
151
Conclusions and Future Work Chapter 9:
Conclusions 9.1
It is true that lithium is abundant in nature. In terms of world economics,
lithium has not yet needed to be recycled. The demand of lithium is obviously
dramatic and has been raised due to the increased use of electronic devices
and vehicles.
Lithium sources are mainly brine in the form of lithium chloride (LiCl) which is
used to make lithium carbonate (Li2CO
3) for Li-ion battery manufacture. The
leading lithium mineral producers are in the continent of South America, i.e.
Argentina, Bolivia, and Chile, due to salt lake deposits. Tahil reported that
Li2CO
3 is chemically obtained from LiCl by a known process called lime soda
evaporation.1 It is the most efficient method employed so far. However, this
process takes almost 1.5 years to obtain LiCl by the evaporation of the salt
water. Therefore, the aim of this project is to search for any method that can
reduce the length of production in sequestering lithium in an environmentally
friendly way.
In this thesis, a chemical method of lithium sequestration from brine using
heterosite FePO4 was investigated and evaluated for effectiveness. The key
substances of the lithiation process are lithium sources/brine (Li2SO
4, LiCl), a
reducing agent (S2O
32-), and a frame work to trap lithium (heterosite FePO
4).
The heterosite FePO4 was chosen due to its structure.2 This structure can be
fitted with Li+ as a small cation, producing LiFePO4 with olivine structure. The
lithiation of FePO4 is achieved with a suitable reducing agent.
Chapter 9: Conclusions and Future Work
152
Heterosite FePO4 was formed by the delithiation of LiFePO
4 with the use of 0.1
M potassium persulphate (K2S
2O
8) as an oxidizing agent.3 It was revealed that a
higher concentration of K2S
2O
8 extracts Li+ from LiFePO
4 at a quicker rate.
Almost 99% completion was shown after 30 minutes in the higher molar ratio
of K2S
2O
8:LiFePO
4 (3:2 and 6:2) and after 1 h in the lower molar ratio (1:2). As
demonstrated in the experiments conducted in Chapter 4.1, a depletion of
FePO4 could occur while K
2S
2O
8 was still in the mixed solution. However, the
conductivity measurement demonstrated that the reaction completed in 10
minutes.
Once the method of FePO4 delithiation was confirmed, the next phase was to
search for a reducing agent to extract Li+ from brine and insert it into the
heterosite FePO4. Various reducing agents were studied such as lithium iodide
(LiI) at high concentration levels and with an activating agent (Zinc; Zn) to aid
the reactions. The results however showed no insertion to FePO4. Nevertheless,
a reducing agent, Na2S
2O
3, was examined, giving positive results to complete
the conversion of LiFePO4 but not to NaFePO
4 conversion. After the processes
of insertion and deinsertion of FePO4 were completed, the final LiFePO
4 was
tested in a battery, showing a satisfactory performance of LiFePO4.
Two models were developed to explain the kinetic study, i.e. a diffusion limited
model and surface-reaction limited model. The experimental data could be
explained with a surface-reaction model, where it is assumed that the
oxidation of S2O
32- concentration was the rate determining step. The results
from the collated data showed that this reaction was a first-order rate reaction
with respect to S2O
32- concentration (S
2O
32->Li+) and the reaction rate was found
to increase with the square root of the lithium concentration (S2O
32-<Li+).
The chemical lithiation of FePO4 into LiFePO
4 could be achieved in the presence
of an excess of Na+, Mg2+, and in an artificial brines experiment. For the fully
lithiated samples, the amount of lithium absorbed into heterosite FePO4 can be
as high as ~46 milligrams Li per gram of solid, whereas the insertion of other
Chapter 9: Conclusions and Future Work
153
cations; i.e. sodium, potassium, and magnesium, were smaller than ~ 4
milligrams per gram of solid. The heterosite FePO4 structure absorbs more
lithium than other types of absorbents, with the exception of some advanced
manganese oxide ion-sieves which absorb up to 38-40 mg g-1.4-7
An alternative reducing agent was found to aid lithiate the FePO4, i.e. sulphite
(SO32-). It works the same way as S
2O
32-. Both acted as a mediator for the redox
couple Fe3+/Fe2+, as shown clearly in the cyclic voltammograms (CV) in Chapter
8. In fact, a chemical lithiation of heterosite FePO4 by Na
2SO
3 experiment was
done to provide confirmation, resulting in a full conversion to LiFePO4
, as
shown in Appendix.
Future work 9.2
Further kinetic studies with respect to Li+ and S2O
32- concentrations would be
useful, for example, the concentrations could be reduced below 0.03 M of Li+
and 0.1 M of S2O
32- to achieve more accurate reaction rate data.
The lithiation reaction of FePO4 in synthetic brine solutions has been studied.
The next step is to try with actual brine including sea water to observe the
selectivity of lithium ion into the framework. Therefore, this method can be
attempted anywhere even in Thailand. This method cannot just be done in
brine, but also in all sorts of lithium solutions. It is thus suitable for recycling
lithium ion batteries.
As mentioned in Chapter 8, one of the alternative reducing agents is sulphite
(SO32-). The kinetics of SO
32- should be studied to compare with S
2O
32-. This will
lead to the superior choice of a suitable and economical reducing agent. A
conductivity measurement would also be interesting to pursue with both
reagents owing to the comparison with the oxidizing agent, i.e. K2S
2O
8.
Chapter 9: Conclusions and Future Work
154
For practical applications, the framework (FePO4 heterosite) will be used for
many cycles of lithiation and delithiation. According to Talebi-Esfandrani’s
research, platinum (Pt) was doped into LiFePO4 compound which gave an
outstanding performance on charge and discharge. Pt works as pillars for the
structure which create a strong and stable framework.8 Regardless of the price
of platinum, research into Pt doped LiFePO4 as a framework for lithium
recycling/recovering could be interesting. Thus, a study of suitable doped
material other than Pt is worth considering for further work.
In order to make this study more useful and economical in the lithium industry,
all the main raw material/reagents costs, i.e. LiFePO4/ K
2S
2O
8 /Na
2S
2O
3, and the
selling price of Li+ products in the market should be calculated. Another
question that should be considered is the amount of times the lithiation/
delithiation cycle could be done with the framework, i.e. heterosite FePO4,
before it loses its effectiveness. These results will give an estimated turnover
of this process.
Chapter 9: Conclusions and Future Work
155
References 9.3
(1) Tahil, W. The Trouble with Lithium. Implications of Future PHEV Production for Lithium Demand. Martainville: Meridian International Research 2007, 5-6. (2) Chen, J.; Vacchio, M. J.; Wang, S.; Chernova, N.; Zavalij, P. Y.; Whittingham, M. S. The hydrothermal synthesis and characterization of olivines and related compounds for electrochemical applications. 2008, 178, 1676-1693. (3) Ramana, C. V.; Mauger, A.; Gendron, F.; Julien, C. M.; Zaghib, K. Study of the Li-insertion/extraction process in LiFePO4/FePO4. Journal of Power Sources 2009, 187, 555-564. (4) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H
1.6Mn
1.6O
4) Derived from
Li1.6
Mn1.6
O4. Industrial & Engineering Chemistry Research 2001, 40, 2054-2058.
(5) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H
1.33Mn
1.67O
4 and H
1.6Mn
1.6O
4. Chemistry Letters 2012,
41, 1647-1649. (6) Yu, Q.; Sasaki, K.; Hirajima, T. Bio-templated synthesis of lithium manganese oxide microtubes and their application in Li+ recovery. Journal of Hazardous Materials 2013, 262, 38-47. (7) Zhang, Q.-H.; Li, S.-P.; Sun, S.-Y.; Yin, X.-S.; Yu, J.-G. Lithium selective adsorption on 1-D MnO
2 nanostructure ion-sieve. Advanced Powder Technology
2009, 20, 432-437. (8) Talebi-Esfandarani, M.; Savadogo, O. Synthesis and characterization of Pt-doped LiFePO4/C composites using the sol–gel method as the cathode material in lithium-ion batteries. Journal of Applied Electrochemistry 2014, 44, 555-562.
Chapter 9: Conclusions and Future Work
156
Appendix
157
Appendix A
From Chapter 8, the actual lithiation experiment using sodium sulphite
(Na2SO
3) as a reducing agent was undertaken. The stoichiometry of the
chemical reaction is shown in Equation 7. A molar ratio of 1Li+: 2S2O
32-: 1FePO
4
was used, i.e. with an excess of reducing agent (S2O
3) to aid faster conversion.
A solution preparation and a sample collection procedure are described in
detail below.
Equation 7 2 2 2 2
Solution preparation
Solutions of 0.3 M LiCl in de-ionized (DI) water were made. The solution was
made by adding 0.56 g of LiCl (Sigma-Aldrich, ≥ 99.9%) in 44.2 cm3 of DI water
and the solution was stirred until all the salt dissolved. Then, 3.34 g of Na2SO
3
(0.0273 mol; Sigma-Aldrich, ≥ 98%) was added, then the solution was stirred,
and heated to approximately 50°C to dissolve the Na2S
2O
3, then cooled to room
temperature before adding 2 g of FePO4 (0.0133 mol).
Sample collection procedure
Once the solutions were dissolved and cooled to room temperature, the
insoluble FePO4 was added to form a suspension, as the timer was started. The
suspensions were stirred and samples were taken for filtration at 24 hours.
The filtration was done using a vacuum pump with grade 1 qualitative filter
paper diameter 110 mm. Each sample was then washed with DI water, and
dried at 80°C for approximately 12 h.
Samples were analysed using X-ray diffraction (XRD; the Bruker D2 Phaser).
Samples were scanned from 15 to 50 degrees of XRD pattern. The reference
XRD patterns of LiFePO4 and FePO
4 were obtained via the Inorganic Crystal
Structure Database (ICSD; RSC).
Appendix A
158
Result
Figure 7 shows a fully lithiated LiFePO4 as expected, according the cyclic
voltammogram experiment in Chapter 8.
Figure 7: The XRD fitting obtained from a sample treated with 0.3 M LiCl + 0.6 M Na
2SO
3 for 24 hours. The result shows pure olivine, i.e. LiFePO
4
Appendices
159
Appendices
List of paper
1. Intaranont, N.; Garcia-Araez, N.; Hector, A. L.; Milton, J. A.; Owen, J. R.
Selective lithium extraction from brines by chemical reaction with
battery materials. Journal of Materials Chemistry A 2014, 2, 6374-6377.
(in Biliography)
List of conferences
1. Attending- The 16th International Congress for Battery Recycling ICBR
2011 (September 21-23, 2011), Venice, Italy.
2. Attending- Resources That Don’t Cost the Earth (December 1-2, 2011),
Berlin, Germany.
3. Attending- The Advances in Li-Battery Research (April 10-11, 2013) at
University of Huddersfield, Huddersfield, United Kingdom.
4. Poster presentation- University of Southampton Chemistry Poster Day
(February 18, 2014), in Southampton, United Kingdom.
5. Poster presentation- The 2nd Workshop in the Advances of Li-Battery
Research (April 10-11, 2014) at University of Liverpool, Liverpool, United
Kingdom.
6. Oral presentation- Electrochemistry final year postgraduate
presentations at University of Southampton (May 28, 2014),
Southampton, United Kingdom
7. Oral presentation-The 2014 ECS and SMEQ Joint International Meeting
(October 5-10, 2014) in Cancun, Mexico.
In Preparation
An article of the second phase from the first paper is in preparation for
publication in the Journal of Energy and Environmental Science. The content is
focused on the kinetics of the chemical lithiation of FePO4 using Na
2S
2O
3 , by
varying the concentration of S2O
32- and Li+ independently (in Chapter 6).
Bibliography
161
Bibliography
Reproduced by permission of The Royal Society of Chemistry:
http://pubs.rsc.org/en/content/articlelanding/2014/ta/c4ta01101e#!divAbstra
ct
Bibliography
162
Bibliography
163
Bibliography
164
Bibliography
165
26/06/2015 Rightslink® by Copy
Title: Atomic-Scale Investigation of Logged in as:
Defects, Dopants, and Lithium Noramon Intaranont Transport in the LiFePO4
OlivineType Battery Material
Account #: 3000930549
Author: M. Saiful Islam, Daniel J. Driscoll,
Craig A. J. Fisher, et al
Publication: Chemistry of Materials Publisher: American Chemical Society Date: Oct 1, 2005
Copyright © 2005, American Chemical Society
PERMISSION/LICENSE IS GRANTED FOR YOUR
ORDER AT NO CHARGE
This type of permission/license, instead of the standard Terms & Conditions, is sent to you because no fee is being charged for your order. Please note the
following:
Permission is granted for your request in both print and electronic formats, and translations.
If figures and/or tables were requested, they may be adapted or used in part. Please print this page for your records and send a copy of it to your publisher/graduate school. Appropriate credit for the requested material should be given as follows: "Reprinted
(adapted) with permission from (COMPLETE REFERENCE CITATION). Copyright (YEAR) American Chemical Society." Insert appropriate information in place of the capitalized words.
One-time permission is granted only for the use specified in your request. No additional uses are granted (such as derivative works or other editions). For any other uses, please submit a new request.
If credit is given to another source for the material you requested, permission must be obtained from that source.
Bibliography
166
Copyright © 2015 Copyright Clearance Center, Inc. All Rights Reserved. Privacy statement. Terms and Conditions. Comments? We would like to hear from you. E-mail us at [email protected]
https://s100.copyright.com/AppDispatchServlet
Bibliography
167
Bibliography
168
Bibliography
169