High Ni‐content layered transition metal oxides cathodes exhibit high theoretical energy density, but their structure undergoes multiple phase transitions, which lead to reversible and even irreversible structural changes. Mechanisms leading to this degradation affect lithium transport and diffusivity, and promote oxygen loss, ultimately resulting in rock‐salt formation. Here, we investigate the dynamics of the degradation mechanism of lithium nickel oxide (LNO) with the O3 stacking sequence as the starting structure, which is the most dominant structure before further delithiation. We use in situ transmission electron microscopy (TEM) to study high temporal and spatial resolution in real time of dynamic structural changes of the LNO cathode during cycling, which allows us to identify and map different phase distribution within the LNO layers during and after cycling. Combined findings from high‐resolution (scanning) TEM, 4D‐STEM and electron energy‐loss spectroscopy reveal that the Li/Ni cation mixing layer on the pristine structure irreversibly affects the cycling efficiency by blocking the Li + diffusion paths. Additionally, the structural and compositional changes are accompanied by an expansion along the c‐axis when the LNO is charged versus Li metal at a constant potential of 4.3 V.
Siachos et al. (Sun,) studied this question.