On the Much‐Improved High‐Voltage Cycling Performance of LiCoO2 by Phase Alteration from O3 to O2 Structure

Lithium cobalt oxide (LiCoO 2 ) is an irreplaceable cathode material for lithium‐ion batteries with high volumetric energy density. The prevailing O 3 phase LiCoO 2 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 2 development is application at high voltage (>4.55 V versus Li + /Li) to achieve a high specific capacity (>190 mAh g −1 ). However, cycled with a high cutoff voltage, O 3 –LiCoO 2 suffers from rapid capacity decay. The causes of failure are mostly attributed to the irreversible transitions to H1‐3/O 1 phase after deep delithiation and severe interfacial reactions with electrolytes. In addition to O 3 , LiCoO 2 is also known to crystalize in an O 2 phase with ABAC stacking. Since its discovery, little is known about the high‐voltage behavior of O 2 –LiCoO 2 . Herein, through systematic comparison between electrochemical performances of O 3 and O 2 LiCoO 2 at high voltage, the significantly better stability of O 2 –LiCoO 2 (>4.5 V) than that of O 3 –LiCoO 2 in the same micro‐sized particle morphology is revealed. Combining various characterization techniques and multiscale simulation, the outstanding high‐voltage stability of O 2 –LiCoO 2 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 2 .