The electrochemical behavior of trialkylimidazolium imide based ionic liquids and their polymer gel electrolytes

The physical and electrochemical properties of 1,2-dimethyl-3-n-R-imidazolium ionic liquids [R=propyl (MMPI) and butyl (MMBI)] were investigated. The anions studied were bis(trifluoromethanesulfonyl)imide (TFSI) or bis(perfluoroethanesulfonyl)imide (PFESI). In addition, polymer gel electrolytes were prepared using polyethylene oxide of poly(vinylidenefluoride-co-hexafluoropropene) (PVDF-HFP) and these ionic liquids with and without 1.0 M Li+. Electrochemical measurements showed similar behavior for the TFSI based ionic liquids, although MMBITFSI exhibited lower ionic conductivity than MMPITFSI. The MMPIPFESI is an extremely viscous liquid, which solidifies if cooled slightly below room temperature. MMBIPFESI is a wax at room temperature. For both MMPIPFESI and MMBIPFESI, addition of 1.0 M lithium (as LiPFESI) resulted in crystallization of the pure ionic liquid at room temperature. Electrochemical studies indicated that in all cases, the PVDF-HFP polymer gel systems exhibited higher charge/discharge efficiencies and higher ionic conductivity than the PEO based polymer gel systems. (c) 2007 The Electrochemical Society.