Electrochemical stability of low viscosity ion-pair electrolytes: Structure and interaction in quaternary ammonium-based Ionic liquid containing bis(trifluoromethylsulfonyl)imide anion and analogue

Abstract

Quaternary ammonium-based ionic liquids (QAIL) consist of an aliphatic cation and possess characteristic properties different from those of aromatic (like imidazolium) with extensive cited applications. The redox stability of QA cations has attractively nominated them to achieve static and dynamic electrochemical phenomena. Herein, we investigated the influence of various anions (bis(fluorosulfonyl)imide ${\left[\text{FSI}\right]}^{-}$ and bis(trifluoromethylsulfonyl)imide ${\left[\text{NT}{f}_2\right]}^{-}$) on the structure, electronic, and electrochemical properties of low-viscosity ILs comprising QA cation (triethylpentyl ammonium [N₂₂₂₅]⁺) by employing highly versatile empirical exchange-correlation functional M06–2X in the context of density functional theory (DFT). The electronic structure, cation-anion interaction energy, dipole moment, cation-anion charge transfer, cathodic and anodic potential boundaries, electrochemical window (ECW), and density of state (DOS) were calculated and discussed. Liquid state studied by polarizable continuum model (PCM). The thermodynamics cycle method, an alternative to HOMO/LUMO method, was used to predict ECW's more realistic. ECWs for [N₂₂₂₅][FSI] $\ge$ [N₂₂₂₅][NTf₂] were found to be wider than the experimental imidazolium counterpart by about two volts. ${\left[\text{FSI}\right]}^{-}$ anion is overwhelmingly involved in charge transfer with the cation compared to ${\left[\text{NT}{f}_2\right]}^{-}$. The more redox-stable [N₂₂₂₅][FSI] IL is expected to have less corrosive effects with metal electrodes because the anion (1) cis conformer takes a particular orientation relative to the cation, (2) has a higher charge transfer, and (3) F's atoms have a higher occupation number than [N₂₂₂₅][NTf₂] IL. Therefore, these results provide insight for further investigation to expand the electrochemical application of redox-stable aliphatic QAILs involving the two targeted anions.