Abstract 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-Na x TmO 2 ) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-Na x TmO 2 have not yet been clearly elucidated. Herein, taking P2-Na 0.67 MnO 2 and P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-Na x TmO 2 in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-Na x TmO 2 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-Na x TmO 2 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 Na x TmO 2 cathodes.

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