Origin of crack propagation in lithium cobalt oxide positive electrode for lithium-ion batteries.

Although the anionic activity in layered oxide positive electrodes is partially understood, the mechanism by which it induces internal crack propagation under high-voltage conditions remains unclear, severely limiting stable cycle life and available capacity. In this study, the mechanism underlying internal crack propagation is elucidated in LiCoO under harsh float-charge conditions at 60 °C and a potential of 4.6 V vs. Li/Li. We reveal that sequential phase transitions from O3 to O1 and then to a cation-mixed phase during deep Li extraction generate significant lattice-mismatch stresses that initiate internal cracks. Crucially, the accompanying anionic activity drives the migration of the oxidized lattice O (0 < n < 2) towards these cracks, where it even accumulates as molecular O that can be released upon crack propagation to the surface. It is identified that the sustained O1tocation-mixed phase transition and the bulk lattice O migration synergistically drive the interior crack propagation, ultimately leading to LiCoO disintegration and electrochemical degradation. This work not only deepens our understanding of the anionic activity in layered positive electrodes but also provides mechanistic insights for the future design of stable LiCoO positive electrodes.