Ionic Liquids with Low Melting Points and Their Application to Double-Layer Capacitor Electrolytes

Ionic liquids, 1-ethyl-3-methylimidazolium hexafluorotantalate ( EMITaF 6 ) and hexafluoroniobate ( EMINbF 6 ) were synthesized by a new method. They showed lower melting points and higher decomposition temperatures than popular EMIBF 4 due to larger anion size. Electrolytic conductivities of these ionic liquids were about 6 mS cm −1 at 25°C. 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 EMICF 3 SO 3 , EMI ( CF 3 SO 2 ) 2 N , and EMI ( C 2 F 5 SO 2 ) 2 N at 3.0 V, 70°C. 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. © 2002 The Electrochemical Society. All rights reserved.