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

By adding 42 parts of triethyl phosphate (TEP: produced by Daihachi Chemical Industries Co., Ltd.) to the methanol solution of 46 parts of 1,2,3,4-tetramethylimidazolinium methyl carbonate salt obtained in Production Example 1, followed by addition of 5.4 parts of water and stirring at 100° C. for 20 hours, triethyl phosphate was hydrolyzed and a salt exchange reaction was carried out to obtain a methanol solution of 1,2,3,4-tetramethylimidazolinium diethyl 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 diethyl phosphate monoanion, electrolyte (C-2), 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-2) was 280.