The stability of P2-layered sodium transition metal oxides in ambient atmospheres.

Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable batteries. The promising P2-layered sodium transition metal oxides (P2-NaTmO) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-NaTmO have not yet been clearly elucidated. Herein, taking P2-NaMnO and P2-NaNiMnO as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-NaTmO in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-NaTmO is determined by the amount of extracted Na, which is mainly compensated by Na/H exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-NaTmO is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable NaTmO cathodes.