The use of a nonflammable electrolyte in a lithium secondary battery will greatly improve battery safety. Therefore, we focused on the nonflammability of ionic liquids and investigated these liquids as possible electrolyte solvents. In this study, we synthesized 1,2-diethyl-3,4(5)-dimethyl imidazolium (DEDMI) cations with imido anions. DEDMI cations with bis(trifluoromethylsulfonyl)imido anions (DEDMI-TFSI) showed a lower solidification point of −35°C, and higher specific conductivity than DEDMI cations with BF4−. We prepared an electrolyte consisting of DEDMI-TFSI with LiTFSI, and applied it to a Li/LiCoO2 cell. The cell was capable of charge–discharge cycling with LiCoO2 cathode material showing a specific capacity of about 100mAhg−1.