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Minakshi, M., Kandhasamy, S. and Meyrick, D. (2012) Synthetic strategies for better battery performance through advances in materials and chemistry: Olivine LiMn1/3Co1/3Ni1/3PO4. Journal of Alloys and Compounds, 544 . pp. 62-66.

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Accepted Manuscript

Synthetic strategies for better battery performance through advances in materials

and chemistry: Olivine LiMn1/3Co1/3Ni1/3PO4

Manickam Minakshi, Sathiyaraj Kandhasamy, Danielle Meyrick

PII: S0925-8388(12)01353-9

DOI: http://dx.doi.org/10.1016/j.jallcom.2012.07.136

Reference: JALCOM 26723

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Received Date: 10 April 2012

Revised Date: 27 July 2012

Accepted Date: 29 July 2012

Please cite this article as: M. Minakshi, S. Kandhasamy, D. Meyrick, Synthetic strategies for better battery

performance through advances in materials and chemistry: Olivine LiMn1/3Co1/3Ni1/3PO4, (2012), doi: http://

dx.doi.org/10.1016/j.jallcom.2012.07.136

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1

Synthetic strategies for better battery performance through advances in

materials and chemistry: Olivine LiMn1/3Co1/3Ni1/3PO4 Manickam Minakshi*, Sathiyaraj Kandhasamy and Danielle Meyrick

Faculty of Minerals and Energy, Murdoch University, Murdoch, WA 6150, Australia

Abstract

Olivine LiMn1/3Co1/3Ni1/3PO4 powder synthesized via wet chemistry route sol-gel

and combustion showed desirable features like reduced particles size with conductive

coatings on the surface and well cation ordering with respect to the traditional solid state

reaction. The morphological homogeneity is achieved through complexing agents

employed (chelating agent/fuel) resulting in high pure material even for substitution of

multi divalent cations (Co and Mn) for Ni in LiNiPO4. The low sintering temperature

prevents the particle agglomeration without any compromise on the degree of

crystallinity. The experimental conditions are very similar for both sol-gel and

combustion, except for the reaction inducers (chelating agent and fuel) that influenced the

composition and the particle size between them. The mixed transition-metal olivine

(LiMn1/3Co1/3Ni1/3PO4) cathodes synthesized by sol-gel and combustion methods are

compared and their electrochemical properties are discussed. Structural analyses of the

synthesized samples showed substitution of mixed cations crystallizes in the olivine

structure. The improved morphology achieved for combustion synthesis resulted in better

specific capacity with good capacity retention than for the sol-gel counterpart.

Keywords: LiMn1/3Co1/3Ni1/3PO4; sol-gel; combustion; LiOH; aqueous battery;

synthesis.

M. Minakshi ( )

E-mail: m.minakshi-sundaram@murdoch.edu.au; lithiumbattery@hotmail.com

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Introduction

Electrochemically active olivine phosphate was first demonstrated by Padhi et al.,

by employing LiFePO4 as a cathode in rechargeable battery system [1]. Later, the olivine

phosphate (LiMPO4) was substituted with a variety of transition metal cations M = Fe,

Mn, Co and Ni and established its suitability as cathode for energy storage applications

[2-5]. Olivine crystal structure is made with the building blocks of hexagonally closed

packed (hcp) oxygen atoms, corner-shared MO6 octahedra, edge-shared LiO6 and

tetrahedrally coordinated PO4 polyanions [2, 6]. This highly stabilized crystal structure

rewarded the olivine phosphates to explore as high voltage cathode material with

relatively high energy density and excellent capacity retention. However, the low

electrical and ionic conductivity of the olivine phosphates confines the utilisation of this

material as a host compound for lithium intercalation [7].

(a) Conventional solid-state reaction vs. wet chemistry route

Plenty of efforts were made to increase the conductivity of olivine phosphates by

reducing the particle size, wrapping the particle with uniform conductive coatings and

doping [8-11] with trivalent or divalent cations. These factors are proved to be useful in

increasing active material utilisation with better cyclability and capacity retention

characteristics. The stringent reaction conditions of the conventional solid state reaction

make it more difficult in synthesizing the materials having above said desired features.

The current synthetic methods for cathode materials are all related to solid state reactions,

co-precipitation method or the sol-gel route. In the present paper, our studies have shown

that the proposed combustion method can achieve the desired stoichiometric and particle

size distribution. The carbon coating obtained from this method not only overcomes the

electronic conductivity limitations but also enhance the ionic conductivity through an

3

easy percolation of the electrolyte. The mixed transition-metal cations

(LiMn1/3Co1/3Ni1/3PO4) of olivine type were synthesized by two techniques such as sol-

gel and combustion method and their results are compared in this work. The novel

solution (wet chemistry) synthesis like sol-gel and combustion has the ability to control

the composition and particle size, providing uniform conductive coatings (or) composites

and morphological homogeneity with a single phase compound. Both sol-gel and

combustion methods ensures uniform metal ion distribution, due to the well tuned atomic

level mixing of reactants through complexing agents. However, the particles obtained via

sol-gel method shows low crystallinity. The crystallinity can be improved by calcining

the product at a high temperature but that will results in severe agglomeration. Whereas,

in combustion synthesis (explained in the next section), the high temperature provided

internally by exothermic heat from sucrose as a fuel environment led to a quick reaction

time [12, 13]. The fracture surface of the combustion reacted product was shown to be

intra granular without any voids on the surface.

(b) Combustion synthesis

Combustion synthesis is an oxidation–reduction reaction between precursor’s

metal salts (oxidizers) and carbonaceous combustion fuel [14, 15]. The heating of a

saturated aqueous solution containing metal ions and a suitable fuel is continued until the

whole mixture ignites and a fast combustion reaction takes place due to exothermic

reaction. The unique characteristic of this method is a self sustained redox type chemical

reaction driving by itself. Usually, this fast reaction occurs well below the phase

formation of the material, hence it aids the product to be uniform in particle size and

phase pure. In the combustion reacted product, the particles morphology and size can be

easily controlled by varying the composition of the starting mixture (fuels to oxidizer

4

rates) and concentration of the precursor solution [16, 17]. Moreover, the well tuned

metal ion mixing and controlled reaction conditions led to nanostructured materials, such

as nano-rods, nano-sheets with uniform porosity [14]. The smaller particle size increases

the surface to volume ratio and brings more Li+ to take part in the redox reaction. It is

quite well know that metal acetates and nitrates are preferred precursors for sol-gel and

combustion synthesis [2, 16]. To make a systematic comparison on the synthetic

methodology, in this study, metal acetate as a precursor was chosen for both the methods.

Widely available and less expensive citric acid and sucrose are used as the chelating

agent for metal cations and a fuel for combustion respectively.

(c) Objectives of this work

On these grounds, the primary aim of this work is to synthesize and compare the

sol-gel and combustion synthetic methods. Another aim is to fabricate these materials as

a cathode for potential application in aqueous battery system.

Many efforts were made to improve the olivine material but all the studies are

mainly constrained to organic electrolytes. There is very limited information available on

aqueous solutions. The safety issues, cost effect, eco-friendly nature and high ionic

conductivity of aqueous lithium salts initiated the field of aqueous rechargeable battery

[18-20]. In this paper, we also compared the effect of synthesis methods in

electrochemical behaviour of the cathode in aqueous LiOH electrolyte.

Experimental

(a) Sol-gel Synthesis

Stoichiometric amount of Lithium acetate, Cobalt acetate, Manganese acetate,

Nickel acetate and Ammonium dihydrogen phosphate was dissolved homogeneously in

5

double distilled water with effective stirring at 80 °C. Then, citric acid was added to the

solution in 1:1 weight ratio to the metal ions and the pH was adjusted to ~ 3.5 by using

nitric acid. The process of stirring and heating were continued until thick transparent gel

state is achieved and then the product was further dried at 110°C in hot air oven for 12 h

to evaporate all the water molecules. Finally, furnace heated at 300oC for 8 h and 550oC

for 5 h for required phase formation, with an intermediate grindings.

(b) Combustion Synthesis

The starting metal acetate precursors are identical to that of sol-gel method except

for the chelating agent and their flow chart is schematically represented in Fig. 1.

Required amount of precursors was dissolved homogeneously in double distilled water.

The combustion fuel (sucrose) was added to the solution in the molecular ratio of 2:1 to

the metal ion and pH was adjusted below 2 by using nitric acid. By continuous stirring

and heating process, the entire water molecules are evaporated to initiate the combustion

reaction. The foam of precursor mixer was obtained after complete ignition of the

combustion fuel. Then the foam was dried at 110°C in hot air oven for 12 h and furnace

heated at 300oC for 8 h, subsequently at 550oC for 5 h with an intermediate grinding.

The furnace cooled LiMn1/3Co1/3Ni1/3PO4 powders obtained by both the methods

were ground for homogeneity using mortar and pestle. The synthesized

LiMn1/3Co1/3Ni1/3PO4 powders were subjected to systematic physical and electrochemical

studies. Phase of the synthesized material was analyzed by Siemens D500 X-ray

diffractometer 5635 with Cu-Ka source in scan speed of 1 degree per minute with voltage

(30 kV) and current (28 mA). Philips Analytical XL series 20 scanning electron

microscope (SEM) instrument was used to image the surface morphology of the

materials. Cyclic voltammogram (CV) analysis for the synthesized materials was

6

performed in EG&G Princeton Applied Research Versa Stat III model with Hg/HgO as

reference electrode (Koslow Scientific) at scan rate of 25µV/s. An 8 channeled battery

analyzer by MTI Corp., USA, operating with battery testing system (BTS) was used to

test the charge/discharge cyclability of the materials. The electrode fabrication, cell

design and experimental procedure for the CV and charge/discharge studies were made as

similar to our earlier studies [20].

Results and Discussion

To understand the effect of synthetic procedures, we have carried out synthesis on

two identical precursors (metal acetates) both having same reaction parameters but one

containing citric acid as chelating agent (for sol-gel) and the other sucrose as fuel (for

combustion). The as-synthesized product was analyzed for structural determination using

X-ray diffraction (XRD). Figure 2 compares the XRD patterns of the

LiMn1/3Co1/3Ni1/3PO4 powders obtained through sol-gel and combustion method. The

major diffraction peaks for both the powder samples are identified in the orthorhombic

Pmna space group (JCPDS: 01-085-0002) and the peaks are indexed with olivine phase

[3, 21]. The presence of high intensity reflections evidenced the synthesized products,

irrespective of the methods, are crystalline. This implies the mechanism on metal ion

complex formation is similar for both the chelating agent and a fuel [22]. However, the

sucrose induces the combustion synthesis by the oxidation–reduction reaction with the

starting mixture (oxidizer to fuel ratio). As explained in the previous section

(introduction), this reaction arises well before the formation of end product and acquires

an abrupt increase in internal heat [14, 22]. This self-propagating and non-explosive

exothermic reaction led the product to highly crystalline and homogeneity in nature for

combustion reacted product. The appearance of narrow peaks (well crystalline) along

7

with the absence of second phase (high purity) in Fig. 2b confirms the novel role of fuel

in combustion reacted product. While, the sol-gel product (in Fig. 2a) showed slightly

broader peaks and small shoulder peaks (open circles “o” denotes Li3PO4 as the second

phase) given the reaction conditions are identical for both the samples as shown in Fig. 1.

Figure 3 shows the SEM images of the as-synthesized LiMn1/3Co1/3Ni1/3PO4

powders derived from the sol-gel and combustion methods. All the samples presented a

relatively dense structure (free from voids) though some micro-structural differences can

be identified. It is reported earlier [23] that the solid state synthesized sample is not

homogenous with irregular particle sizes and well separated grains compared to that of

wet chemistry route (presented here). The sol-gel product (Fig. 3a) contains relatively big

particles in size with agglomerated particle distribution. For the combustion reacted

sample evenly distributed smaller particle size without any agglomeration is seen in Fig.

3b. Also, the morphology of the grain boundaries is not easily distinguishable. The

observed difference could be due to the mixture ignition and thereon the self-combustion

effect. During this reaction, the fuel burns while evolving out large volume of thermolytic

gases, creating pores on the particle and fractures the particle into fragments [14]. This

tends the particle size to be smaller and more porous than the sol-gel counterpart. Porous

structure in the range of 20-40 nm range was observed by TEM analysis (Fig. 4b-c) for

the combustion product whereas such porous morphology is less visible for the sol-gel

reacted sample (in Fig. 4a). TEM images in Fig. 4 also evidence the difference in particle

size between the sol-gel and combustion synthesized product in the range of 500 nm -

1µm and 100 - 300 nm respectively. In combustion reaction, post heat treatment of the

product led to divide the particles finer, but in sol-gel results to agglomerated product.

The high internal temperature in the combustion effect prevents the particles from grain

8

boundary effect (as seen in Fig. 3b) giving a homogeneity. The XRD pattern observed in

Fig. 2b didn’t show any significant peak splitting relating to the grain boundary defect

[23]. Hence the physical characterization of the olivine LiMn1/3Co1/3Ni1/3PO4 cathode

synthesized through these two methods suggests that the material differs from their

particle size, morphology and crystallinity. Combustion product showed smaller and

homogeneous particles than that of the powder obtained by sol-gel [14].

The electrochemical activity of the synthesized LiMn1/3Co1/3Ni1/3PO4 powders

prepared by sol-gel and combustion methods was investigated by potential controlled

cyclic voltammetric technique and their results are shown in Fig. 5. The cyclic

voltammetry was performed with synthesized LiMn1/3Co1/3Ni1/3PO4 powder as working

(disc like) electrode, Zn foil as counter and Hg/HgO as standard electrode at slow scan

rate of 25µV/s. Initial scan was made in anodic direction to extract the Li+ from cathode

and reversed back in cathodic direction to re-insert the extracted Li+ in to the cathode.

This extraction/re-insertion of Li+ was performed by suitable oxidation and subsequent

reduction reaction of the metal ion in the cathode, which was represented as redox peaks

A1 and C1 respectively. The redox peaks (A1 and C1) observed for both the sol-gel (Fig.

5a) and combustion (Fig. 5b) derived product shows they are electrochemically active in

the aqueous LiOH electrolyte. However, the finer and porous combustion reacted product

offers larger surface to be exposed to the electrolyte and initiates more Li+ to take part in

the redox reaction than the sol-gel reacted product [24, 25]. This reflects in the presence

of well defined reduction peak (C1) for combustion reacted product implying nicely

carbon wrapped and highly crystalline in nature enhances the lithium de-intercalation and

re-insertion behavior. Whereas, the large agglomerated particles with low crystallinity

9

obtained via sol-gel results in tiny amount of Li+ re-insertion than Li+ extraction during

oxidation [26-28] with the presence of poorly defined redox peaks.

The battery characterization was carried out by galvanostatic charge and

discharge cycling studies between 1.75 and 0.5 V at a constant charge/discharge current

rate of C/4. Figure 6 shows the charge-discharge curves for the synthesized cathodes vs

Zn anode in LiOH aqueous electrolyte for multiple cycles. It is observed that the

combustion product shows better performance of 95 mAh/g (Fig. 6b) than the sol-gel

reacted sample resulted in 63 mAh/g (Fig. 6a). This higher discharge capacity for

combustion material than the sol-gel was in agreement with the redox behavior observed

by cyclic voltammetry analysis (shown in Fig. 5). The first cycle discharge capacity

shows more Li+ extracted and reinserted in the finer particles of the combustion product

[29] (95 mAh/g) product than the agglomerated sol-gel product (63 mAh/g). The finer

combustion product not only resulted in higher discharge capacity for initial cycles, but

also prevents the cathode from structural dissolution and maintains the amount of Li+

exertion/re-insertion for longer cycling. Thus the combustion sample shows just 15%

capacity loss while sol-gel sample shows 43% capacity loss, decreased to 20 mAh/g after

the initial 5 cycles The galvanostatic (charge-discharge) studies showed the evenly

distributed smaller particles resulted in better specific capacity with good capacity

retention than their counter-part i.e. sol-gel technique. It is been well reported by Gao et

al that the electrochemical performance of the electrode strongly depends on their

physico-chemical properties like particle size, pore structure etc. [30]. The advantages of

having smaller particles can be explained through very large ‘surface area to mass ratios’,

short path lengths for solid state diffusion, improved ionic and electronic conductivity

observed in combustion technique [11, 27, 31].

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Conclusion

The olivine phase LiMn1/3Co1/3Ni1/3PO4 was successfully synthesized by citric

acid assisted sol-gel and sucrose assisted combustion synthesis. Effect of the synthesis

procedure in terms of phase purity, crystallinity and particle morphology are compared

by the XRD and SEM analysis. Our studies have shown that the combustion method can

achieve the desired stoichiometric and particle size distribution. The carbon coating

obtained from this method not only overcomes the electronic conductivity limitations but

also enhance the ionic conductivity through an easy percolation of the electrolyte.

Electrochemical studies reveal that the well crystallined, nano-structured combustion

reacted product showed high discharge capacity of 93 mAh/g with excellent capacity

retention than the sol-gel reacted product. It is concluded that the particle size effect, role

of chelating agent and carbon have determined the electrochemical properties.

Acknowledgement

The author (M. M) wishes to acknowledge the Australian Research Council

(ARC). This research was supported under Australian Research Council (ARC)

Discovery Project funding scheme (DP1092543). The views expressed herein are those of

the authors and are not necessarily those of the ARC.

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Figure Captions

Fig. 1 Flow chart for the preparation of cathode olivine LiMn1/3Co1/3Ni1/3PO4 synthesized

by (a) sol-gel and (b) combustion methods.

Fig. 2 X-ray diffraction patterns of olivine LiMn1/3Co1/3Ni1/3PO4 synthesized by (a)

sol-gel and (b) combustion methods. Open circle “o” denotes Li3PO4 as the second

phase.

Fig. 3 SEM images of LiMn1/3Co1/3Ni1/3PO4 synthesized by (a) sol-gel and (b)

combustion method.

Fig. 4 TEM image of LiMn1/3Co1/3Ni1/3PO4 synthesized by (a) sol-gel and (b-c)

combustion method in different magnifications. Particle size is smaller and pores are

clearly visible in the combustion reacted product.

Fig. 5 Cyclic voltammogram of LiMn1/3Co1/3Ni1/3PO4 synthesized cathodes by (a)

sol-gel and (b) combustion method, in aqueous LiOH electrolyte.

Fig. 6 Charge – discharge behavior of synthesized LiMn1/3Co1/3Ni1/3PO4 cathode by

(a) sol-gel and (b) combustion method, in LiOH electrolyte.

14

Fig. 1 Flow chart for the preparation of olivine LiMn1/3Co1/3Ni1/3PO4 cathode synthesized

by (a) sol-gel and (b) combustion methods.

15

15 20 25 30 35 40

oo

ooo

o (102

)(0

02)

2 theta / degrees

(a)

(1

21)

(311

)

(301

)

(020

)(111

)(0

11)(210

)

(101

)

(200

)

(b)

Fig. 2 X-ray diffraction patterns of olivine LiMn1/3Co1/3Ni1/3PO4 synthesized by (a) sol-

gel and (b) combustion methods. Open circle “o” denotes Li3PO4 as the second phase.

16

Fig. 3 SEM images of LiMn1/3Co1/3Ni1/3PO4 synthesized by (a) sol-gel and (b)

combustion method.

17

(a)

(b)

(c)

Fig. 4 TEM image of LiMn1/3Co1/3Ni1/3PO4 synthesized by (a) sol-gel and (b-c)

combustion method in different magnifications. Particle size is smaller and pores are

clearly visible in the combustion reacted product.

18

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

C1

A1(a)

Cur

rent

/ A

Potential vs. Hg/HgO / V

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

C1

A1

(b)

Cur

rent

/ A

Potential vs. Hg/HgO / V

Fig. 5 Cyclic voltammogram of LiMn1/3Co1/3Ni1/3PO4 synthesized cathodes by (a)

sol-gel and (b) combustion method, in aqueous LiOH electrolyte.

19

0 20 40 60 80 1000.0

0.5

1.0

1.5

2.0

5thcycle

discharge

(a)

1st charge-dischargecurve

Cel

l Pot

entia

l / V

Cell Capacity / mAh g-1

0 20 40 60 80 1000.0

0.5

1.0

1.5

2.0

5th cycle

discharge

(b)

Cel

l Pot

entia

l / V

Cell Capacity / mAh g-1

1st charge-dischargecurve

Fig. 6 Charge – discharge behavior of synthesized LiMn1/3Co1/3Ni1/3PO4 cathode by

(a) sol-gel and (b) combustion method, in aqueous LiOH electrolyte.

20

Highlights Metal ion doped olivine LiMn1/3Co1/3Ni1/3 PO4 was successfully synthesized.

Combustion method achieved the desired phase and particle size distribution.

Carbon coating enhanced the ionic conductivity of the olivine cathode.

Improved morphology resulted in better battery performance.

Role of chelating agent and fuel have determined the cell capacity.