As one of the fascinating high capacity cathodes, O3-type layered oxides usually suffer from their intrinsic air sensitivity and sluggish kinetics originating from the spontaneous lattice Na extraction during air exposure and high tetrahedral site energy of Na + diffusion transition state. What is worse, the improvement on the two handicaps is hard to simultaneously realize because of the contradiction between Na containment suggested in air stability mechanism and enhanced Na diffusion mentioned in kinetics strategy. Herein, it is shown that a simple strategy of introducing proper Na vacancies into lattice can simultaneously realize a dual performance improvement. Na vacancies decrease the charge density on transitional metal ions and enhance the antioxidative capability of material, ensuring a stable lattice Na containment for Na 0.93 Li 0.12 Ni 0.25 Fe 0.15 Mn 0.48 O 2 when exposed to air. Additionally, more Na + diffusional sites and enlarged Na layer spacing are obtained and result in a significantly decreased energy barrier from ≈1000 to 300 meV and a high rate capability of 70.8% retention at 2000 mA g −1 .Remarkably, such a strategy can be easily realized by either pre-or posttreating, which exhibits excellent universality for various O3 materials, implying its enormous potential to promote the commercial application of O3-type cathodes.

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Layered oxides are the most prevalent cathodes for sodium-ion batteries (SIBs), but their poor air stability significantly limits their practical application owing to the rapid performance degradation of aged materials and the cost increase for material storage and transportation. Here, an effective strategy of constructing stable transition metal (TM) layers with a highly symmetrical six-TM ring is suggested to enhance structure stability, thus hindering ambient air corrosion. The density functional theory calculations reveal that the higher symmetry ensures a higher thermodynamic energy for H 2 O insertion into Na layer. The combined analyses of selected area electron diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and chemical titration indicate that the six-TM ring structure can effectively suppress the series of aging processes including water insertion, the spontaneous loss of lattice sodium, TM valence increment and residual alkali formation. Benefiting from the overall suppression of aging process, the strategy results in an excellent improvement in capacity retention after air exposure from 13.57% to 95.59%, and exhibits a good universality for both P2-and O3-cathodes, which are the two most common structures of Nabased layered oxides with different aging mechanism. These findings provide new insight to design high-performance cathodes for SIBs.

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Two kinds of crystal orderings in layered oxides typically exhibit opposite influences on performances: Na+/vacancy ordering in alkali metal layers with an unfavorable effect on electrochemical performance and the cation ordering in transition metal layers with a positive effect on air stability. However, because the two kinds of orderings are associated with each other and often occur at the same time, it is difficult to achieve an excellent comprehensive performance. Herein, we propose a strategy of introducing a new cation ordering to construct the coexistence of Na+ disordering and transition metal ordering. An absolute solid‐solution reaction mechanism is realized in the Na+ disordered system, resulting in a superior cycling stability of 90.4% retention after 150 cycles and a rate performance of 82.7 mAh g−1 capacity at 10C, much higher than the original 81.3% and 66.4 mAh g−1. Simultaneously, the cation ordering strengthens the interlayer interaction and inhibits the insertion of water molecules from the air, ensuring stable lattice stability and thermostability after air exposure. The synergy of dis‐/ordering configuration provides new insights to design high‐performance layered oxide cathode materials for secondary‐ion batteries.

Manganese-based layered oxides are one of the most promising cathodes for Na-ion batteries, but the prospect of their practical application is challenged by high sensitivity to ambient air. The stacking structure of materials is critical to the aging mechanism between layered oxides and air, but there remains a lack of systematic study. Herein, comprehensive research on model materials P-type Na 0.50 MnO 2 and O-type Na 0.85 MnO 2 reveals that the O-phase displays a much higher dynamic affinity toward moisture air compared to P-type compounds. For air-exposed Otype material, Na + ions are extracted from the crystal lattice to form alkaline species at the surface in contact with air, accompanying by the increase of the valence state of transition metals. The series of undesired reactions result in an increase of interfacial resistance and huge capacity loss. Comparatively, the insertion of H 2 O into the Na layer is the main reaction during air-exposure of P-type material, and the inserted H 2 O can be extracted by high-temperature treatment. The H 2 O de/insertion process not only causes no performance degradation but also can enlarge the interlayer distance. With these understandings, we further propose a washing−resintering strategy to recover the performance of aged O-type materials and an aging strategy to build high-performance P-type materials.

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