Unraveling Atomic-Level Mechanisms of Structural Transitions in Lithium Cobalt Oxide under High-Voltage Conditions.

High-voltage cycling of layered cathode materials in lithium-ion batteries presents challenges related to structural instability. Deciphering atomic-scale structural degradation mechanisms is essential for improving their electrochemical performance at high voltages. This study utilized advanced electron microscopy and principal component analysis to detect subtle spinel-like structure induced by the migration of cobalt atoms within LiCoO subjected to high-voltage cycling at charge voltages of 4.6 and 4.8 V. The formation of the spinel-like configuration was accompanied by the emergence of a densified O1 phase beneath thin spinel-like layer on the (003) facets during charging, along with an intriguing local O3- to P3-type oxygen stacking transition observed in LiCoO charged to 4.8 V. Upon discharge, an enlarged and defective spinel phase preferentially formed on the non-(003) facets, and the migrated cobalt atoms cannot fully return to their original lattice sites, leading to the irreversible structural changes in LiCoO. Long-term cycling revealed that the initial extended spinel phase underwent voltage-dependent evolution pathways, which contributed to accelerated capacity fading observed at the cutoff voltage of 4.8 V. Our findings provide new insights into the atomic-level structural transitions in LiCoO under high-voltage conditions, offering guidance for the development of more structurally robust LiCoO for high-voltage applications.