Ionic liquids with low melting points and their application to double-layer capacitor electrolytes

Ionic liquids, 1-ethyl-3-methylimidazolium hexafluorotantalate (EMITaF6) and hexafluoroniobate (EMINbF6) were synthesized by a new method. They showed lower melting points and higher decomposition temperatures than popular EMIBF4 due to larger anion size. Electrolytic conductivities of these ionic liquids were about 6 mS cm(-1) at 25degreesC. Lowering the melting point of ionic liquids did not result in the improvement of low-temperature characteristics of carbon double-layer capacitors, however, the new ionic liquids showed long life compared with EMICF3SO3, EMI(CF3SO2)(2)N, and EMI(C2F5SO2)(2)N at 3.0 V, 70degreesC. The derivatives of the new ionic liquids having a methyl group in 2-position (1-ethyl-2,3-dimethylimidazolium salts) were not ionic liquids at room temperature. (C) 2002 The Electrochemical Society.