Structural Distortion-Induced Charge Gradient Distribution of Co Ions in Delithiated LiCoO₂ Cathode

Layered LiCoO₂ has drawn tremendous attention as a modeling cathode for Li-ion batteries, while its structural instability, especially in the high delithiation region, remains unsolved. With the aim of revealing the structural fundamentals, LiCoO₂ electrodes are investigated at a long delithiation range using both in situ and ex situ techniques. In the highly delithiated LiCoO₂ electrode, the unique charge compensation process leads to a spatial charge gradient of Co²⁺/Co³⁺/Co⁴⁺ ions from surface to bulk, which can be further manipulated by structural distortion, Li extraction, and surface side reactions. The coordinated surface oxygen is shown to be electrochemically active and fully reversible in participating in the charge compensation during cycling. Moreover, the active lattice O can be significantly stabilized by introducing the undesired surface Li-Co antisites, which also play an effective role in accommodating the internal stress induced by volume changes. These findings effectively bridge the structural changes with the Li⁺/e migration kinetics to elucidate the degradation of LiCoO₂ cathode upon delithiation, demonstrating a rewarding avenue for improving the electrochemical performance of LiCoO₂ itself and developing high energy density cathodes for the battery community as well.