Lithium-ion batteries safety is an issue that should be solved. The thermal stability of Li0.5CoO2 in the presence of solvents/electrolyte and LiPF6 salt were studied by using C80 micro-calorimeter in this paper. It was found that the thermal stability of Li0.5CoO2 either in the presence of cyclic carbonate or linear carbonate is decreased greatly, especially for the linear carbonate-Li0.5CoO2. coexisting system, and the thermal stability of dimethyl carbonate-Li0.5CoO2 coexisting system is the least stable one. LiPF6; can prevent oxygen releasing from Li0.5CoO2 at low temperature, but the delayed reaction will generate more heat at higher temperature. At last, thermal stability of two kinds of electrolytes with Li0.5CoO2 was explored. The LiPF6/ethylene carbonate + dimethyl carbonate + ethylmethyl carbonate-Li0.5CoO2 system shows less thermal stability, which is attributed to the activity between dimethyl carbonate and Li0.5CoO2, as well as agree with the thermal experiment on dimethyl carbonate-Li0.5CoO2 coexisting system. (C) 2008 Elsevier Ltd. All rights reserved.