Towards High Energy Density Transition Metal Oxide Cathode for Sodium – Ion Batteries: A Review on Oxygen Anionic Redox Strategy

Sodium-ion batteries (SIBs) offer significant promise for grid-scale energy storage due to resource abundance, low cost, and enhanced safety. However, the capacity limitations inherent in traditional transition metal (TM) cationic redox necessitate exploring oxygen anionic redox (OAR) to surmount energy density limitations. This review systematically summarizes recent research progress on this mechanism in transition metal oxide (TMO) cathodes, examining its efficacy in both sodium-deficient and sodium-rich systems, and elucidates OAR activation/deactivation mechanisms from a multiscale perspective. Substantial additional capacity arises via anionic redox (O^2-→(O₂)^n-→O₂) or metal-ligand charge transfer. Nevertheless, challenges including irreversible capacity loss, structural distortion, and voltage hysteresis induced by anionic redox require optimization. To address these, four modification strategies are highlighted: ionic doping, phase boundary engineering, crystal facet engineering, and surface coating. These approaches enhance reversibility by introducing foreign ions to modulate electronic band structure, stabilizing the oxygen sublattice through strengthened bonds, controlling phase structure via cation potential, and optimizing interfacial stability. Although current research has achieved progress in balancing capacity and stability, further exploration of the collaborative regulation mechanisms of OAR and its efficient application in practical battery systems remain essential for future development.