The charge-discharge performance of LiCoO2 Positive electrode was observed in a mixed electrolyte system consisting of two ionic liquids: cyano-substituted quaternary ammonium bis(trifluoromethane)sulfoneimide (TFSI) and a same-anion salt of 1-ethyl-3-methyl imidazolium (EMI). The positive electrode exhibited a discharge capacity rather close to the theoretical one when NNNN-cyanoethyl trimethyl ammonium salt was applied. Differential scanning calorimetry (DSC) studies revealed that these electrolytes exhibited exotherm only around 260 degrees C, 50 degrees C higher than conventional carbonate-based electrolytes. This is the first attempt to reveal the thermal stability of ionic liquid electrolyte under a practical situation. (c) 2006 Elsevier B.V. All rights reserved.