Cryo‐Sampling Enables Atomistic Insights Into Bulk Degradation of High‐Voltage LiCoO 2

Abstract Conventional disassembly of a lithium–ion battery leads to a cathode self‐discharge issue, which decreases the original state of charge (SOC) and significantly changes its original lattice structure and phase distribution. Herein, to prevent the self‐discharge issue, a cryo‐sampling method is introduced, which enables the obtaining of high SOC layered LiCoO 2 (LCO) and to directly visualize the H1‐3 and O1 phases by aberration‐corrected transmission electron microscopy. The most critical discovery is that Co migration into the vacant Li‐layers is an energy‐favored process in highly delithiated H1‐3 and O1 phases, which is the root‐cause of interlayer mixing degradation of layered structure. It is first clarified that phase transitions between O3 and H1‐3/O1 phases not only involves (003) in‐plane gliding but also out‐of‐plane Co ion migration, and the latter one has poor reversibility, leading to inevitable degradation of the layered structure after repeated cycles. The atomistic process of microcrack nucleation and spinel domain formation are revealed based on Co migration, which well explains the fast performance decay of a commercial LCO after 4.65 V cycling. The unique crystal structures of H1‐3/O1 phases along with their atomistic phase transition mechanisms revealed by this work deeply refresh the fundamental understanding of the layer structured cathode materials.