Structure-Dependent Degradation Mechanism of Layered Sodium Oxides upon Air Exposure

Cation disorder often occurs in layered sodium oxides when severe Na deficiency is induced by thermodynamic Na extraction in ambient air, resulting in serious electrochemical degradation. Herein, two commercialized layered oxides, P2-type Na_0.75(Ni_0.25Fe_0.25Mn_0.5)O₂ (P2-NNFM) and O3-type Na(Ni_0.33Fe_0.33Mn_0.33)O₂ (O3-NNFM), are adopted to systematically investigate the reversibility of cation disorder caused by air corrosion. Experiments and theoretical calculations show that cation disorder is detected at the near surface of both layered oxides after exposure to air. For the O3 layered oxide, cation disorder caused by Na⁺/H⁺ exchange occurs during air storage, and the disorder is irreversible by reinjecting Na⁺, similar to the behavior observed in analogous layered lithium oxides. Interestingly, for P2 layered oxide, the degradation mechanism during air storage mainly involves H₂O exchanging with Na⁺, and the charge loss caused by Na⁺/H₂O exchange is compensated for by the transition metal ions. The inserted H₂O could function as intense electrostatic shielding, leading to a P2-to-OP4 phase transition within the cation-disordered region at the particle surface. Importantly, this cation disorder is reversible, and the structure degradation can be effectively repaired by electrochemically reinjecting Na⁺ back into the air-stored P2-NNFM. As a result, the repaired P2-NNFM exhibits a discharge capacity of 120 mAh g^-1 at 0.1 C, accompanied by a decent capacity retention of 80.2% after 700 cycles, which is slightly better than that of the pristine state. This insightful understanding of the degradation mechanism of layered sodium oxides during air storage is important for the development of high-energy-density sodium-ion batteries. We herein demonstrate that the reversibility of cation disorder in layered SIB cathode materials caused by air corrosion highly depends on the layered structure. Particularly, the reversible migration of disordered cations occurs by electrochemically reinjecting Na⁺ back into the air-stored P2 layered oxides, which significantly addresses air-sensitive issues of structure distortion and capacity fading for P2 layered oxides.