Lithium secondary batteries that use a room-temperature ionic liquid as an electrolyte were investigated for the purpose of realizing high-safe batteries. For the improvement of stability under charge/discharge operation with electrodes, we focused attention on a series of 1-alkyl-3-methyl-imidazolium bis (trifluoromethane sulfonyl) imide. The temperature dependence of ionic conductivity and battery charge-discharge performance were examined by changing the alkyl chain lengths: -methyl/-ethyl/-butyl/hexyl/-octyl. According to the results, the effects of extending the alkyl chain were confirmed in, for example, the increase in carrier ion number, and the improvement of battery charge-discharge performance characteristics. (C) 2007 The Electrochemical Society.