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

Lithium cobalt oxide (LiCoO₂) is an irreplaceable cathode material for lithium-ion batteries with high volumetric energy density. The prevailing O₃ phase LiCoO₂ 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 LiCoO₂ 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, O₃-LiCoO₂ suffers from rapid capacity decay. The causes of failure are mostly attributed to the irreversible transitions to H1-3/O₁ phase after deep delithiation and severe interfacial reactions with electrolytes. In addition to O₃, LiCoO₂ is also known to crystalize in an O₂ phase with ABAC stacking. Since its discovery, little is known about the high-voltage behavior of O₂-LiCoO₂. Herein, through systematic comparison between electrochemical performances of O₃ and O₂ LiCoO₂ at high voltage, the significantly better stability of O₂-LiCoO₂ (>4.5 V) than that of O₃-LiCoO₂ in the same micro-sized particle morphology is revealed. Combining various characterization techniques and multiscale simulation, the outstanding high-voltage stability of O₂-LiCoO₂ 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 LiCoO₂.