On the Much-Improved High-Voltage Cycling Performance of LiCoO by Phase Alteration from O to O Structure.

Lithium cobalt oxide (LiCoO) is an irreplaceable cathode material for lithium-ion batteries with high volumetric energy density. The prevailing O phase LiCoO adopts the ABCABC (A, B, and C stand for lattice sites in the close-packed plane) stacking modes of close-packed oxygen atoms. Currently, the focus of LiCoO development is application at high voltage (>4.55 V versus Li/Li) to achieve a high specific capacity (>190 mAh g). However, cycled with a high cutoff voltage, O-LiCoO suffers from rapid capacity decay. The causes of failure are mostly attributed to the irreversible transitions to H1-3/O phase after deep delithiation and severe interfacial reactions with electrolytes. In addition to O, LiCoO is also known to crystalize in an O phase with ABAC stacking. Since its discovery, little is known about the high-voltage behavior of O-LiCoO. Herein, through systematic comparison between electrochemical performances of O and O LiCoO at high voltage, the significantly better stability of O-LiCoO (>4.5 V) than that of O-LiCoO in the same micro-sized particle morphology is revealed. Combining various characterization techniques and multiscale simulation, the outstanding high-voltage stability of O-LiCoO is attributed to the high Li diffusivity and ideal mechanical properties. Uniform Li distribution and balanced internal stress loading may hold the key to improving the high-voltage performance of LiCoO.