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

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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  • 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

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