Insights into the electrochemical stability of ionic liquids from first principles calculations and molecular dynamics simulations
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The electrochemical stability of room-temperature ionic liquids (RTILs) is a critical design consideration for electrochemical applications. An electrochemical solvent, such as the electrolyte in a lithium-ion battery or supercapacitor, must support the voltage in which the device operates. In this talk, we present the insights into the electrochemical stability of RTILs obtained using a novel combination of first principles density functional theory calculations and classical molecular dynamics simulations. We show that while simple gas phase models can be used to reveal broad qualitative trends in electrochemical stability, quantitative accuracy can be achieved only by explicitly modeling all inter-ion interactions in the liquid. Additionally, detailed investigations into the six room-temperature ionic liquids (ILs) formed from a combination of two common cations, 1-butyl-3-methylimidazolium (BMIM) and N ,N -propylmethylpyrrolidinium (P13), and three common anions, PF6 , BF4 , and bis(trifl uoromethylsulfonyl)imide (TFSI) provide surprising evidence of possible cation anodic instability, particularly in BMIM-based ILs.
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Insights into the electrochemical stability of ionic liquids from first principles calculations and molecular dynamics simulations
- 1. 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
- 2. 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
- 3. Outline Optimization of IL ions with Isolated-atom Quantum Chemistry Calculations Accurate Electrochemical Windows with Molecular Dynamics and DFT High-throughputtrends DetailedInsights Aug 14 2014 ACS 248th National Meeting
- 4. ElectrochemicalWindows of ILs Aug 14 2014 ACS 248th National Meeting ECL (Cathodic Limit) EAL (Anodic Limit) EW = EAL – ECL Potential (V) vs Reference Currentdensity 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
- 5. Large Space of Ion Structures Functional Groups Base Ions Aug 14 2014 ACS 248th National Meeting
- 6. cation red anion ox ox ox red red G q G q VVEW e G V e G V eI eI ox red −= Δ −= Δ −= →− →+ Δ Δ ProductsOxidation:ReactionOxidation ProductsReduction:ReactionReduction Predicting Electrochemical Stability Aug 14 2014 ACS 248th National Meeting ??? Kroon, M. C.; Buijs,W.; Peters, C. J. & Witkamp, G. J. (2006), Green Chemistry 8(3), 241—245.
- 7. Proxy Measures forTrue 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 Aug 14 2014 ACS 248th National Meeting 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→ IE Iq+1
- 8. Ohno, H. (2005), Electrochemical Aspects of Ionic Liquids,Wiley-Interscience. Relative Redox Stability of CationTypes Aug 14 2014 ACS 248th National Meeting 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.
- 9. Effect ofAlkylation 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 Aug 14 2014 ACS 248th National Meeting
- 10. Effect of Fluoroalkylation onAnions 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 Aug 14 2014 ACS 248th National Meeting 0 100 200 300 400 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 Molecular Weight / mol gm −1 VerticalIP/eV Borate Sulfonylimide Phosphate TFSI BF4 PF 6 PF(CF 3 ) 5 B(CF3 )3 (CF(CF3 )2 )
- 11. Func.Group Substitutions on 1,2,3-trimethylimidazolium Aug 14 2014 ACS 248th National Meeting EW by Ind. EW by Res. EW by Ind. ED by Res. ED by Ind. EW by Ind. ED by Res.
- 12. Effect of substitution position ED Resonance Effect Dominates Over EW Inductive Effect EW Inductive Effect Dominates Over ED Resonance Effect Aug 14 2014 ACS 248th National Meeting
- 13. Func.Group Substitutions on PF5CF3 anion Aug 14 2014 ACS 248th National Meeting
- 14. 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 Aug 14 2014 ACS 248th National Meeting
- 15. Outline Optimization of IL ions with Isolated-atom Quantum Chemistry Calculations Accurate Electrochemical Windows with Molecular Dynamics and DFT High-throughputtrends DetailedInsights Aug 14 2014 ACS 248th National Meeting
- 16. ElectrochemicalWindows of ILs Aug 14 2014 ACS 248th National Meeting ECL (Cathodic Limit) EAL (Anodic Limit) EW = EAL – ECL Potential (V) vs Reference Currentdensity 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!
- 17. Electrochemistry ofTFSI 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 Aug 14 2014 ACS 248th National Meeting
- 18. Investigated six ILs formed from two cations and three anions N N H3C C4H9 B F F F F N H3C C3H7 P FF F FF F N SS O O O O CF3F3C TFSI PF6 BF4 P13 BMIM Cations Anions Aug 14 2014 ACS 248th National Meeting
- 19. SimplerApproximations – 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 Aug 14 2014 ACS 248th National Meeting
- 20. −5 0 5 10 15 P13 BMIM PF6 BF4 TFSI Potential (V) Cathodic Limit Anodic Limit 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
- 21. 0 5 10 P13 BMIM PF6 BF4 TFSI Potential (V) Cathodic Limit Anodic Limit 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 Aug 14 2014 ACS 248th National Meeting
- 22. Explicit modeling of entire liquid structure Molecular dynamics simulations of IL −8 −6 −4 −2 0 20 40 60 80 100 120 Energy (eV) DensityofStates Gap = 3.82 eV BMIM BF 4 DFT Calculations of the Density of States (DOS) S. P. Ong, O.Andreussi,Y.Wu, N. Marzari, G. Ceder, G. Chemistry of Materials, 2011, 23(11), 2979-2986. doi:10.1021/cm200679y Aug 14 2014 ACS 248th National Meeting
- 23. Calculated electrochemical windows 1 2 3 4 5 6 7 8 9 BMIM PF6 BMIM BF4 BMIM TFSI P13 PF6 P13 BF4 P13 TFSI − EFermi Li Cathodic Limit Anodic Limit 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 S. P. Ong, O.Andreussi,Y.Wu, N. Marzari, G. Ceder, G. Chemistry of Materials, 2011, 23(11), 2979-2986. doi:10.1021/cm200679y Aug 14 2014 ACS 248th National Meeting
- 24. Are the cathodic and anodic limits set by the cation or anion? −8 −6 −4 −2 0 20 40 60 80 100 120 Energy (eV) DensityofStates GGA Gap = 3.82 eV BMIM BF4 −8 −6 −4 −2 0 50 100 HSE06 Gap = 5.08 eVHOMO – Determines anodic limit LUMO – Determines cathodic limit Aug 14 2014 ACS 248th National Meeting
- 25. −8 −6 −4 −2 0 20 40 60 80 100 120 Energy (eV) DensityofStates Gap = 3.82 eV BMIM BF4 −8 −6 −4 −2 0 20 40 60 80 100 120 Energy (eV) DensityofStates Gap = 3.74 eV BMIM PF 6 −8 −6 −4 −2 0 20 40 60 80 100 120 Energy (eV) DensityofStates Gap = 3.87 eV BMIM TFSI Cathodic limit is always set by BMIM cation Anodic limit also almost always set by BMIM cation BMIM-based ILs Aug 14 2014 ACS 248th National Meeting
- 26. −8 −6 −4 −2 0 0 20 40 60 80 100 120 Energy (eV) DensityofStates Gap = 5.36 eV P13 BF 4 −8 −6 −4 −2 0 20 40 60 80 100 120 Energy (eV) DensityofStates Gap = 5.38 eV P13 PF 6 −8 −6 −4 −2 0 0 20 40 60 80 100 120 Energy (eV) DensityofStates Gap = 4.46 eV P13 TFSI Possible cathodic instability in P13 PF6 and P13 TFSI Anodic limit generally set by anion, except for P13PF6. P13-based ILs Aug 14 2014 ACS 248th National Meeting
- 27. Further refinements to MD + DFT method Aug 14 2014 ACS 248th National Meeting 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 ExperimentalValidation Studies., J. Phys. Chem. B, 2014, doi:10.1021/jp5034257.
- 28. Conclusions −8 −6 −4 −2 0 20 40 60 80 100 120 Energy (eV) DensityofStates GGA Gap = 3.82 eV BMIM BF4 −8 −6 −4 −2 0 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 Aug 14 2014 ACS 248th National Meeting
- 29. 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. Aug 14 2014 ACS 248th National Meeting S. P. Ong, O.Andreussi,Y. Wu, N. Marzari, G. Ceder. Electrochemical Windows of Room-Temperature Ionic Liquids from Molecular Dynamics and Density FunctionalTheory 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 FunctionalTheory. Electrochimica Acta, 2010, 55(11), 3804-3811.
- 30. Thank you. Aug 14 2014 ACS 248th National Meeting