Lithium-ion (abbreviated as Li-ion) cells are widely used as power sources for portable electric devices. Recently, the study of the utilization of the high-performance Li-ion cells as power sources of electric vehicles (EV) and other large-sized equipments has been undertaken by many corporations and laboratories. However large-sized Li-ion cells are not used practically, primarily due to safety considerations. Organic compounds are used as electrolytes of Li-ion cells, and these compounds are flammable. Therefore the generation of heat by chemical decomposition and the chemical reactions in the cells is an important factor to be considered. It is not easy to measure the thermal stability of electrolytes because of their low boiling points. Although, it would be quite interesting to evaluate the stability in a closed sample case such as a cell case, there has been no previous report of such investigations. Therefore, we used DSC to measure the thermal stability of electrolytes as well as the influence of Li metal, C₆Li and cathode (Li_0.49CoO₂) addition on the thermal behavior of these electrolytes under air tight conditions. This report reviews recent studies on thermal stability of materials in Li-Ion cells including our study.