Thermal behavior of Li x CoO2 cathode material from cells charged to different voltages has been analyzed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The mass loss appearing between 60 and 125°C in TGA and the exothermic peaks with 4.9 and 7.0Jg−1 in DSC around 75 and 85°C for the Li x CoO2 cathodes of 4.20 and 4.35V cells has been explained based on solid electrolyte interface (SEI) film-break down. The SEI film-break down for the highly charged cathode at low temperature region has been attributed to the conversion of lithium fluoride into hydrofluoric acid in concomitant with the reaction, Li2CO3 +2HF→2LiF+CO2 +H2O. Presence of ionic carbonate in the positive electrode has been identified by ion chromatography (IC). The thermal peaks appearing in different temperature regions have been explained based on decomposition reaction of Li x CoO2 cathodes and the SEI film.