New ionic liquids based on a super-delocalized perfluorinated sulfonimide anion: physical and electrochemical properties

A new family of hydrophobic ionic liquids (ILs) based on a super-delocalized perfluorinated sulfonimide anion, namely (trifluoromethyl(S-trifluoromethylsulfonylimino)sulfonyl)(trifluoromethylsulfonyl)imide ([CF3SO(=NSO2CF3)(2)], [sTFSI]), with various oniums, including imidazolium, quaternary ammonium, pyrrolidinium and piperidinium, have been prepared and characterized. Their physical and electrochemical properties are extensively characterized, and comparatively studied with those based on a lesser delocalized analogue, bis(trifluoromethanesulfonyl) imide ([(CF3SO2)(2)N], [TFSI]), in terms of thermal properties, density, viscosity, ionic conductivity, and electrochemical stability. These new IsTFSII-based ILs show low glass transitions (between -95 and -81 degrees C), relatively low viscosities (36-120 cP at 25 degrees C), good thermal stabilities (T-d > 400 degrees C), and wide electrochemical windows. Particularly, the viscosities are generally lower for the [sTFSI]-based ILs than for the corresponding [TFSI]-based ones, due to better charge delocalization and higher degrees of freedoms for IsTFSII vs. [TFSI]. It is found that the [sTFSI] anion is more resistant toward oxidation, but less resistant toward reduction than the [TFSI] one, suggesting that replacement of a = O group in [TFSI] with a strong electron-withdrawning CF3SO2N = group increase both the reductive and oxidative potential. The coulombic efficiencies of Li deposition/stripping on Ni electrode are comparable in the [sTFSI]- and [TFSI]-based ionic liquid electrolytes; however, Li/LiFePO4 cell using the [sTFSI]-based ionic liquid electrolyte shows more stable cycling performance than that using the [TFSI]-based one, which would be attributable to the improved anodic stability of the [sTFSI] anion and stable electrode/electrolyte interphases formed on Li anode. (C) 2016 Elsevier Ltd. All rights reserved.