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

• New ionic liquids (ILs) with a super-delocalized sulfonimide anion ([CF3SO(NSO2CF3)2]−, [sTFSI]−) have been introduced.
• Physical and electrochemical properties of the [sTFSI]-based ILs are measured.
• The [sTFSI]-based ILs show high ionic conductivities and wide electrochemical windows.
• The Li/LiFePO4 cell using the [sTFSI]-based IL electrolyte shows good capacity retention.

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)2N]−, [TFSI]−), in terms of thermal properties, density, viscosity, ionic conductivity, and electrochemical stability. These new [sTFSI]-based ILs show low glass transitions (between −95 and −81 °C), relatively low viscosities (36–120 cP at 25 °C), good thermal stabilities (Td > 400 °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 [sTFSI]− 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.