Atomic-Scale Observation of O1 Faulted Phase-Induced Deactivation of LiNiO2 at High Voltage.

LiNiO2 and cobalt-free ultrahigh-Ni content cathodes suffer from rapid capacity loss and severe chemomechanical degradation, especially when operated at high voltages. Here, by cycling LiNiO2 up to 4.7 V, we report the atomic-scale observation of O1 faulted phase-induced deactivation of LiNiO2. We find that, although a thin layer of the O3 phase forms on the particle surface by reversible O3 → O1 transformation during discharge, the bulk interior still maintains the O1 faulted phase, leading to rapid capacity loss of LiNiO2. Moreover, the atomic configuration of the O1/O3 interface is investigated comprehensively. We reveal that the misfit along the c axes of the O1 and O3 phases results in the formation of misfit dislocations, whereby cation mixing is promoted at the dislocation cores. A transition zone with continuous shear along the a-b plane is uncovered between the O1 and O3 phases for the first time. Besides, severe oxygen loss-induced pore formation and concurrent rock salt transformation are also identified.