Understanding the Structural and Electrochemical Properties of Anion‐Redox O3‐Na[Li _1/3 Mn _2/3 ]O ₂ Cathode for Sodium‐Ion Batteries

Abstract Activating anionic redox is an effective strategy for enhancing the capacity of cathode materials in both lithium‐ion and sodium‐ion batteries. However, the fundamental mechanisms of O3‐type sodium anionic redox cathodes remain poorly understood due to the limited availability of such materials, despite their inherent advantages such as high initial sodium content and high reversible capacity. Herein, we employ multiple characterization methods to elucidate the structural and electrochemical properties of O3‐Na[Li 1/3 Mn 2/3 ]O 2 , a promising sodium anionic redox cathode developed recently, with a particular focus on the effect of cutoff voltages during electrochemical aging. Our synchrotron powder X‐ray diffraction results indicate that charging the material to voltages above 4.1 V induces substantial structural changes, which compromise the structural and electrochemical reversibility of the material. This is corroborated by more rapid capacity decay and increased impedance, attributed to severe degradation on both the surface and in the bulk, as evidenced by the formation of lower valence manganese compounds. Furthermore, increasing the lower cutoff voltage for the discharge process effectively suppresses surface degradation, thereby improving cycling stability. This work offers a comprehensive understanding of a pivotal sodium anionic redox cathode material, providing valuable insights that can drive the development of advanced materials for sodium‐ion batteries.