Transition Metal Vacancy in Layered Cathode Materials for Sodium-Ion Batteries

Anionic redox has been considered as a promising strategy to break the capacity limitation of cathode materials that solely relies on the intrinsic cationic redox in secondary batteries. Vacancy, as a kind of defect, can be introduced into transition metal layer to trigger oxygen redox, thus enhancing the energy density of layer-structured cathode materials for sodium-ion batteries. Herein, the formation process, recent progress in working mechanisms of triggering oxygen redox, as well as advanced characterization techniques for transition metal (TM) vacancy were overviewed and discussed. Strategies applied to stabilize the vacancy contained structures and harness the reversible oxygen redox were summarized. Furthermore, the challenges and prospects for further understanding TM vacancy were particularly emphasized.