Water → 1,2,3,4-Tetramethylimidazolinium dibutyl phosphate

By adding 62 parts of tributyl phosphate to the above-described methanol solution of 46 parts of 1,2,3,4-tetramethylimidazolinium methylcarbonate salt, followed by addition of 5.4 parts of water and stirring at 100° C. for 20 hours, tributyl phosphate was hydrolyzed and a salt exchange reaction was carried out to obtain a methanol solution of 1,2,3,4-tetramethylimidazolinium dibutyl phosphate monoanion. The above-described solution was heated under reduced pressure of 1.0 kPa or less at 50° C. to distill methanol until no distillate of methanol was observed. Thereafter, the temperature was raised from 50° C. to 100° C. and heating was continued for 30 minutes to distill and remove monomethyl carbonate (HOCO2CH3), methanol, and carbon dioxide (slight amounts of methanol and carbon dioxide are formed by thermal decomposition of monomethyl carbonate. Hereinafter, these are abbreviated as by-products). Thus, 1,2,3,4-tetramethylimidazolinium dibutyl phosphate monoanion, electrolyte (C-1), was obtained. The yield was 99 parts (yield based on the weight of 46 parts of 1,2,3,4-tetramethylimidazolinium methyl carbonate salt; hereinafter the same shall apply). The molecular weight of the electrolyte (C-1) was 280.