Lithium cobalt oxide (LiCoO2), which has been successfully applied in commercial lithium-ion batteries for portable devices, possesses a theoretical specific capacity of 274 mAh g-1. However, its actual capacity is only half of the theoretical specific capacity, because the charging voltage is restricted below 4.2 V. If a higher charging voltage is applied, an irreversible phase transition of LiCoO2 during delithiation would occur, resulting in severe capacity fading. Therefore, it is essential to investigate the electrochemically driven phase transition of LiCoO2 cathode material to approach its theoretical capacity. In this work, it was observed that LiCoO2 partially degraded to Co3O4 after 150 charging-discharging cycles. From the perspective of crystallography, the conventional cell of LiCoO2 was rebuilt to an orthonormal coordinate, and the transition path from layered LiCoO2 to cubic Co3O4 proposed. The theoretical analysis indicated that the electrochemically driven phase transition from LiCoO2 to Co3O4 underwent several stages. Based on this, an experimental verification was made by doping LiCoO2 with Al, In, Mg, and Zr, respectively. The doped samples theoretically predicted behavior. The findings in this study provide insights into the electrochemically driven phase transition in LiCoO2, and the phase transition can be eliminated to improve the capacity of LiCoO2 to its theoretical value.
Keywords: Co3O4; LiCoO2; degradation; phase transition.