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-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.