Thermal behavior of LixCoO2 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 degrees C in TGA and the exothermic peaks with 4.9 and 7.0 J g(-1) in DSC around 75 and 85 degrees C for the LixCoO2 cathodes of 4.20 and 4.35 V 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 LixCoO2 cathodes and the SEI film. (C) 2008 Elsevier B.V. All rights reserved.