Lithium secondary batteries that use a room-temperature ionic liquid containing a lithium salt as an electrolyte are prepared (cathode: LiCoO2, anode: lithium metal). The prepared batteries showed values near the theoretical charge-discharge capacity in the first cycle and excellent reversibility (initial discharge capacity: 145 mAh g(-1), 100th discharge capacity: 118 mAh g(-1), 4.2-3.0 V vs Li/Li+, C/8) at room temperature. (c) 2005 The Electrochemical Society.