Insights into the electrochemical stability of ionic liquids from first principles calculations and...

Insights into the electrochemical stability of ionic liquids from first principles calculations and molecular dynamics simulations

Shyue Ping Ong, Oliviero Andreussi, Yabi Wu, Nicola Marzari, and Gerbrand Ceder

Aug 14 2014

ACS 248th National Meeting

Electrochemical applications of ILs

Aug 14 2014 ACS 248th National Meeting

High Thermal Stability, Low Volatility, Low Flammability

Wide Electrochemical

Window (~5-6V)

Abundance of Charge Carriers

Highly Customizable

Outline

Optimization of IL ions with Isolated-atom Quantum Chemistry Calculations

Accurate Electrochemical Windows with Molecular Dynamics and DFT

Hig

h-th

roug

hput

tre

nds

Detailed Insights


Electrochemical Windows of ILs


ECL (Cathodic Limit)

EAL (Anodic Limit)

EW = EAL – ECL

Potential (V) vs Reference

Cur

rent

den

sity

Ohno, H., (2005), Electrochemical Aspects of Ionic Liquids, Wiley-Interscience.

Same cation, different anion, slightly different

ECL

Different cations, same anion, very

different ECL

Different anions, similar cation, very different

EAL

Large Space of Ion Structures

Functional Groups

Base Ions


cationred

anionox

oxox

redred

Gq

Gq

VVEWeGV

eGV

eI

eIox

red

−=

Δ−=

Δ−=

→−

→+Δ

Δ

ProductsOxidation :Reaction Oxidation

ProductsReduction :ReactionReduction

Predicting Electrochemical Stability


???

Kroon, M. C.; Buijs, W.; Peters, C. J. & Witkamp, G. J. (2006), Green Chemistry 8(3), 241—245.

Proxy Measures for True Redox Stability

Hypothesis: Vred & Vox correlated

with electron affinity (EA) and

ionization energy (IE) respectively

EAs and IEs can be computed efficiently and accurately using simple computational methods at relatively low cost


Koch, V. R.; Dominey, L. A.; Nanjundiah, C.; Ondrechen, M. J. J. Electrochem. Soc.1996, 143, 798–803.

€

Iq + e→−EA

Iq−1

Iq − e→IEIq+1

Ohno, H. (2005), Electrochemical Aspects of Ionic Liquids, Wiley-Interscience.

Relative Redox Stability of Cation Types


S. P. Ong and G. Ceder, 2010. Investigation of the Effect of Functional Group Substitutions on the Gas-Phase Electron Affinities and Ionization Energies of Room-Temperature Ionic Liquids Ions using Density Functional Theory. Electrochimica Acta, 55(11), pp.3804-3811.

Effect of Alkylation on Cations

Appetecchi, G. B.; Montanino, M.; Zane, D.; Carewska, M.; Alessandrini, F. & Passerini, S. (2009), ELECTROCHIMICA ACTA 54(4), 1325-1332.

PYR1n3 -> PYR1n7 : −3.73V -> −3.89V


Effect of Fluoroalkylation on Anions

No monotonic decreasing trend in IE with fluoroalkylation observed Fluorine is the most electronegative element => great inductive stabilization effect Initial substitution do not result in significantly increased stabilization. Relative oxidative stability of common anions agrees with recent work by Ue et al. •  PF6 > BF4 > TFSI


0 100 200 300 4006.2

6.4

6.6

6.8

7

7.2

7.4

7.6

7.8

8

Molecular Weight / mol gm−1

Vert

ical

IP /

eV

BorateSulfonylimidePhosphate

TFSI

BF4

PF6

PF(CF3)5

B(CF3)3(CF(CF3)2)

Func. Group Substitutions on ��1,2,3-trimethylimidazolium


EW by Ind.

EW by Res.

EW by Ind. ED by Res.

ED by Ind.

EW by Ind. ED by Res.

Effect of substitution position

ED Resonance

Effect Dominates Over EW Inductive

Effect

EW Inductive

Effect Dominates Over ED

Resonance Effect


Func. Group Substitutions on PF5CF3 anion


Designing ILs for electrochemical applications

Electron-donating functional groups stabilize cations and electro-withdrawing groups stabilize anions •  Unfortunately, scope for increase in anodic limits appear to be limited

given that current anions seems to be near optimal.

Effect due to combination of inductive and resonance effects, relative strength of which depends on substitution position

Efficient computational methods can be used to quickly screen candidate IL structures to maximize electrochemical windows


Outline

Optimization of IL ions with Isolated-atom Quantum Chemistry Calculations

Accurate Electrochemical Windows with Molecular Dynamics and DFT

Hig

h-th

roug

hput

tre

nds

Detailed Insights


Electrochemical Windows of ILs


ECL (Cathodic Limit)

EAL (Anodic Limit)

EW = EAL – ECL

Potential (V) vs Reference

Cur

rent

den

sity

Ohno, H., (2005), Electrochemical Aspects of Ionic Liquids, Wiley-Interscience.

Same cation, different anion, slightly different

ECL

Different cations, same anion, very

different ECL

Different anions, similar cation, very different

EAL

Same EMI cation, different anions,

similar EAL!

Electrochemistry of TFSI at negative potentials

TFSI anion decomposes at less negative potentials than P13 cation.

Polarizable Continuum Model (PCM) calculations showed similar qualitative results, but unable to provide quantitative accuracy.

P. Howlett, E. Izgorodina, M. Forsyth, & D. Macfarlane. Zeitschrift fur Physikalische Chemie, 2006, 220, 1483-1498. doi:10.1524/zpch.2006.220.10.1483


Investigated six ILs formed from two cations and three anions

N NH3C C4H9 B

F

F

FF

N

H3C C3H7

PFF

FFF

F

NSS

O

O

O

O

CF3F3C

TFSI

PF6 BF4

P13

BMIM

Cations Anions


Simpler Approximations – Isolated molecules and PCM

Treat ions as isolated molecules in vacuo or in a polarizable continuum model (PCM).1

Calculate electron affinity (EA) and ionization energy (IE) of each ion.

1J. Tomasi, B. Mennucci, B., & R. Cammi. Chemical Reviews, 2005, 105, 2999-3093.

ε= 12


−5 0 5 10 15

P13

BMIM

PF6

BF4

TFSI

Potential (V)

CathodicLimit

AnodicLimit

Predicted electrochemical windows from isolated molecule approximation

Predicts that cathodic limits are always set by the cation, and anodic limits are always set by the anion.

Reasonable electrochemical windows of ~3-5V are predicted.

S. P. Ong & G. Ceder, G. Electrochimica Acta, 2010, 55(11), 3804-3811. Aug 14 2014 ACS 248th National Meeting

0 5 10

P13

BMIM

PF6

BF4

TFSI

Potential (V)

CathodicLimit

AnodicLimit

Predicted electrochemical windows from PCM approximation

Predicts that both cathodic and anodic limits are always set by the cation, except P13TFSI. Severely overestimates the electrochemical windows; EWs of >6.5V are predicted. Dielectric medium overstabilizes highly-charged ions.

S. P. Ong, O. Andreussi, Y. Wu, N. Marzari, & G. Ceder, G. Chemistry of Materials, 2011, 23(11), 2979-2986. doi:10.1021/cm200679y


Explicit modeling of entire liquid structure

Molecular dynamics simulations of IL

−8 −6 −4 −20

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

Gap= 3.82 eV

BMIMBF4

DFT Calculations of the Density of States (DOS)



Calculated electrochemical windows

1 2 3 4 5 6 7 8 9

BMIM PF6

BMIM BF4

BMIM TFSI

P13 PF6

P13 BF4

P13 TFSI

− EFermiLi

CathodicLimit

AnodicLimit

Potential (V)

Clear difference in CL for P13 vs BMIM

Same AL for all BMIM-based ILs, despite different anions!

Expected behavior for P13-based ILs. PF6 > BF4 > TFSI



Are the cathodic and anodic limits set by the cation or anion?

−8 −6 −4 −20

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

GGA Gap= 3.82 eV

BMIMBF4

−8 −6 −4 −20

50

100

HSE06 Gap= 5.08 eVHOMO –

Determines anodic limit

LUMO – Determines cathodic limit


−8 −6 −4 −20

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

Gap= 3.82 eV

BMIMBF4

−8 −6 −4 −20

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

Gap= 3.74 eV

BMIMPF6

−8 −6 −4 −20

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

Gap= 3.87 eV

BMIMTFSI

Cathodic limit is always set by BMIM cation

Anodic limit also almost always set by BMIM cation

BMIM-based ILs


−8 −6 −4 −2 00

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

Gap= 5.36 eV

P13BF4

−8 −6 −4 −20

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

Gap= 5.38 eV

P13PF6

−8 −6 −4 −2 00

20

40

60

80

100

120

Energy (eV)

Den

sity

of S

tate

s

Gap= 4.46 eV

P13TFSI

Possible cathodic instability in P13 PF6 and P13 TFSI

Anodic limit generally set by anion, except for P13PF6.

P13-based ILs


Further refinements to MD + DFT method


DFT HOMO/LUMO

AIMD relaxation and quenching

Classical MD

Zhang, Y.; Shi, C.; Brennecke, J. F.; Maginn, E. J. Refined Method for Predicting Electrochemical Windows of Ionic Liquids and Experimental Validation Studies., J. Phys. Chem. B, 2014, doi:10.1021/jp5034257.

Conclusions

−8 −6 −4 −20

20

40

60

80

100

120

Energy (eV)D

ensi

ty o

f Sta

tes

GGA Gap= 3.82 eV

BMIMBF4

−8 −6 −4 −20

50

100

HSE06 Gap= 5.08 eV

MD + DFT

Explicitly modeled entire IL structure

Achieved semi-quantitative accuracy

in EW prediction

Surprising prediction of BMIM instability, consistent with exp.

evidence

Cathodic limits are not always set by cations and anodic limits are not always

set by anions


Acknowledgements and Publications

Funding from E. I. du Pont de Nemours & Co. via the DuPont-MIT Alliance program.

William L. Holstein and Steve R. Lustig from DuPont for useful insights and assistance.

TeraGrid resources provided by the Pittsburgh Supercomputing Center.


S. P. Ong, O. Andreussi, Y. Wu, N. Marzari, & G. Ceder. Electrochemical Windows of Room-Temperature Ionic Liquids from Molecular Dynamics and

Density Functional Theory Calculations. Chemistry of Materials, 2011, 23(11), 2979-2986.

S. P. Ong & G. Ceder, Investigation of the Effect of Functional Group

Substitutions on the Gas-Phase Electron Affinities and Ionization Energies of Room-Temperature Ionic Liquids Ions using Density Functional Theory.

Electrochimica Acta, 2010, 55(11), 3804-3811.

Thank you.

Aug 14 2014

ACS 248th National Meeting

Insights into the electrochemical stability of ionic liquids from first principles calculations and...

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