Layered Oxide Cathodes for Li-Ion Batteries: Oxygen Loss and Vacancy Evolution

Loss of oxygen in layered transition-metal oxides is a major reason for the structural degradation and thus the fade of electrochemical performance in the cathodes for Li-ion batteries. Via in situ transmission electron microscopy observations of LiNi_0.80Co_0.15Al_0.05O₂ (NCA), we found that the oxygen loss in the layered cathode is a two-stage process with distinct release rates. The initial rapid oxygen loss generates a high concentration of oxygen vacancies, which results in the formation of an amorphized, vacancy-containing rock-salt layer in the surface. In the second stage, the slower rate of oxygen loss allows recrystallization of this defective phase via coalescing of atomic oxygen vacancies, which results in the formation of a cavity-containing surface layer with a crystalline rock-salt structure over the layered phase in the bulk. Comparison of the in situ results with electrochemically cycled NCA cathodes confirms this two-stage process of oxygen loss. These results provide unprecedented microscopic details regarding the structural degradation of layered oxides arising from oxygen loss and have broader implications in manipulating the oxygen activity in the electrode.