Enhancing the Reversibility of Phase Transition and Anionic Redox Chemistry in O3‐Type Sodium‐Ion Oxide Cathodes via Precise Alkali‐Metal Layer Spacing Modulation

ABSTRACT Sodium‐ion batteries (SIBs) with O3‐type cathodes are among the most promising alternatives to lithium‐ion batteries. However, their application is limited by the severe irreversible anionic redox reactions and phase transitions under high‐voltage conditions. In this study, we found that these irreversible processes could be effectively alleviated by precise alkali‐metal layer spacing modulation via tuning initial sodium content, which significantly enhances the high‐voltage performance of O3‐type layered materials. Our results reveal that irreversible anionic redox reactions are triggered prematurely in cathodes with a high initial sodium content. It is noteworthy that these irreversible processes always lead to detrimental phase transitions, incomplete transition‐metal redox, and exacerbated cation migration in layered cathodes. Through precise regulation of the initial Na content from unity to 0.8, the aforementioned issues were effectively alleviated, enabling a high initial discharge capacity of 174 mAh g −1 at 0.2 C, with 161 mAh g −1 retention after 100 cycles. Moreover, the optimized sample delivered outstanding full‐cell performance with a specific capacity of 154 mAh g −1 at 1 C and a capacity retention of 81.8% after 200 cycles. These findings highlight the critical role of sodium stoichiometry in regulating anionic redox and stabilizing layered structures, offering insights into the high‐performance SIB cathode design.