Recently, lithium ion battery is strongly requested for storage electric energy by electric vehicles and hybrid vehicles. Because, it has characteristics such as high potential, high energy density, and light weight. However, combustion reaction occur inside a cell by exothermic reaction of electrolyte and oxygen from cathode electrodes. Therefore, it is effective to control the production of the oxygen for improve safety of the battery. Since the thermal stability of the cathode electrode material has an important influence on safety improvement, it is widely studied by thermogravimetric analysis (TGA) and XRD. However, the research about the thermal stability fluctuation associated with SOC and the mechanism related to generated gases due to temperature is not satisfactory. Therefore, in this study, the thermal behavior of widely used cathode materials such as Li (Ni_{1 / 3}Mn_{1 / 3}Co_{1 / 3}) O₂ (NMC), LiMn₂O₄ (LMO), LiFePO₄ (LFPO) at different states of charge was investigated by thermogravimetric analysis mass spectrometer (TGA-MS). Furthermore, the surface of the cathode materials was analyzed by means of X-ray photoelectron spectroscopy(XPS).

The SOC of each sample was adjusted before measuring the thermal stability. First, a laminate cell was prepared and aging and initial characteristics were measured. Next, discharge was performed in accordance with each SOC (30, 50, 80, 100%). finally, the cell was disassembled and washed several times using DMC (Dimethyl carbonate).The thermal stability was measured using thermogravimetric analysis (TGA ;Rigaku). The measurement temperature range is 30 to 800 °C, and the temperature rise rate is 5 °C/min. In addition, the generated gas was analysis was using the mass spectrometer. Then the valence change of the surface of cathode materials was measured using X-ray Photoelectron Spectroscopy (shimazu). The power of X-ray was 225 kW and the measurement mode was measured with Al monochromatic.

Li (Ni_{1 / 3}Mn_{1 / 3}Co_{1 / 3}) O₂ was a difference due to SOC. Because, the mechanism of oxygen generation changes at around SOC = 100%. The main mechanism of oxygen generation is oxide decomposition reaction accompanying the phase change in SOC = 100%. As a result, the weight of cathode decreases at 250 °C in SOC = 100%, and 340 °C in other SOC. LiMn₂O₄ and LiFePO₄ had weight reduction at 380, 420 °C regardless of SOC, since the main mechanism of oxygen generation unchanged by SOC. The result of the gas generation temperature increased in the order of Li (Ni_{1 / 3}Mn_{1 / 3}Co_{1 / 3}) O₂ , LiMn₂O₄ , LiFePO₄ . Oxygen was generated at a temperature higher than that of Li (Ni_{1 / 3}Mn_{1 / 3}Co_{1 / 3}) O₂ because tetravalent MnO₂ has high thermal stability in LiMn₂O₄. In LiFePO₄, oxygen was generated at the highest temperature, because strong covalent bonds in the oxygen acid ions determine the oxygen generation temperature. We confirmed that these mechanisms are correct from determination of valence change of transition metal of cathode by XPS. Based on this oxygen generation mechanism, we try to develop high safety and high capacity density cathode materials.