Electrochemically Driven Phase Transition in LiCoO Cathode.

Lithium cobalt oxide (LiCoO), which has been successfully applied in commercial lithium-ion batteries for portable devices, possesses a theoretical specific capacity of 274 mAh g. 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 LiCoO during delithiation would occur, resulting in severe capacity fading. Therefore, it is essential to investigate the electrochemically driven phase transition of LiCoO cathode material to approach its theoretical capacity. In this work, it was observed that LiCoO partially degraded to CoO after 150 charging-discharging cycles. From the perspective of crystallography, the conventional cell of LiCoO was rebuilt to an orthonormal coordinate, and the transition path from layered LiCoO to cubic CoO proposed. The theoretical analysis indicated that the electrochemically driven phase transition from LiCoO to CoO underwent several stages. Based on this, an experimental verification was made by doping LiCoO 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 LiCoO, and the phase transition can be eliminated to improve the capacity of LiCoO to its theoretical value.