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 ([CF 3 SO( NSO 2 CF 3 ) 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 ([(CF 3 SO 2 ) 2 N] − , [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 ( T d   >   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 CF 3 SO 2 N   =   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/LiFePO 4 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.