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.

Automated summary

Anionic redox is a promising strategy to overcome the capacity limitation of cathode materials in secondary batteries. The introduction of vacancies in the transition metal layer enables the triggering of oxygen redox, thus improving the energy density of sodium-ion battery cathode materials. This review discusses the formation process, recent progress in understanding the working mechanisms of oxygen redox, and advanced characterization techniques for transition metal vacancies. Strategies to stabilize vacancy-containing structures and utilize reversible oxygen redox are summarized, while the challenges and prospects for further understanding transition metal vacancies are emphasized.