Thermal and structural stabilities of Li _x CoO ₂ cathode for li secondary battery studied by a temperature programmed reduction

Temperature programmed reduction (TPR) method was introduced to analyze the structural change and thermal stability of Li _x CoO ₂ (LCO) cathode material. The reduction peaks of delithiated LCO clearly represented the different phases of LCO. The reduction peak at a temperature below 250 °C can be attributed to the transformation of CoO ₂ –like to Co ₃ O ₄ –like phase which is similar reduction patterns of CoO ₂ phase resulting from delithiation of LCO structure. The 2 ^nd reduction peak at 300~375 °C corresponds to the reduction of Co ₃ O ₄ –like phase to CoO–like phase. TPR results indicate the thermal instability of delithiated LCO driven by CoO ₂ –like phase on the surface of the delithiated LCO. In the TPR kinetics, the activation energies (E _a ) obtained for as-synthesized LCO were 105.6 and 82.7 kJ mol ^-1 for T _{m_H1} and T _{m_H2} , respectively, whereas E _a for the delithiated LCO were 93.2, 124.1 and 216.3 kJ mol ^-1 for T _{m_L1} , T _{m_L2} and T _{m_L3} , respectively. As a result, the TPR method enables to identify the structural changes and thermal stability of each phase and effectively characterize the distinctive thermal behavior between as-synthesized and delithiated LCO.