From cationic to anionic redox in layered oxide cathodes for Sodium-ion batteries

In this review, we systematically examine the evolution of redox reaction mechanisms in layered oxide cathodes for Sodium-ion batteries, from conventional cation redox to emerging anion redox. First, the electronic structure fundamentals of layered oxides are elucidated, including energy band formation, d-orbital splitting under an octahedral crystal field, and the intrinsic correlation between CFSE/OSSE and the Jahn-Teller effect. Subsequently, the nature of electron transfer, core reaction characteristics, and representative progress in cation redox are elaborated, along with its inherent limitations. For anion redox, the microscopic mechanism is analyzed, wherein the competitive interplay between the d-d Coulomb repulsion parameter U and the charge-transfer energyΔserves as the core criterion for feasibility; its dual nature of promise and challenge is discussed. Furthermore, recent strategies for regulating anion redox are comprehensively summarized, including transition-metal or alkali-metal site doping, biphasic composite design, high-entropy engineering, high‑sodium-content tuning, interface reconstruction, superstructure construction, and local electronic environment modulation. The underlying mechanism and remaining limitations of each strategy are illustrated through representative case studies. By clarifying the research landscape and core scientific issues in this field, this review provides a systematic reference for the rational design of sodium-ion battery cathode materials with high energy density and long cycle life. Finally, future directions are outlined, including in situ multiscale characterization, the integration of multiscale theoretical simulations with machine learning, full-cell compatibility studies, and industrialization-oriented material design.