Anionic Redox Reactions in Cathodes for Sodium-Ion Batteries

Intercalation-based cathodes typically rely on the cationic redox activity of transition metals to deliver capacity, but, recently, anionic redox chemistry has emerged as a way to increase the energy density of rechargeable batteries. However, the irreversible structural disorder and voltage fading accompanying oxygen release are major problems preventing commercial use. To overcome these limitations, the connection between structural stability and anionic redox activity must be understood. Here, we present a review of theoretical and experimental progress in anionic redox in sodium intercalation cathodes. First, the effects of structural factors including stacking sequences and cationic vacancies on the reversible capacity originating from anionic redox are discussed. Second, the effects on anionic redox activity of cationic substitution with alkaline earth metals (Li or Na) and the coordination environment are highlighted. Third, the progress and challenges facing materials based on 3d/4d/5d metals are reviewed. Finally, research directions for the development of anionic redox active materials are outlined.