Multi-Electron Reactions Enabled by Anion-Based Redox Chemistry for High-Energy Multivalent Rechargeable Batteries.

The development of multivalent metal (such as Mg and Ca) based battery systems is hindered by lack of suitable cathode chemistry that shows reversible multi-electron redox reactions. Cationic redox centres in the classical cathodes can only afford stepwise single-electron transfer, which are not ideal for multivalent-ion storage. The charge imbalance during multivalent ion insertion might lead to an additional kinetic barrier for ion mobility. Therefore, multivalent battery cathodes only exhibit slope-like voltage profiles with insertion/extraction redox of less than one electron. Taking VS as a model material, reversible two-electron redox with cationic-anionic contributions is verified in both rechargeable Mg batteries (RMBs) and rechargeable Ca batteries (RCBs). The corresponding cells exhibit high capacities of >300 mAh g at a current density of 100 mA g in both RMBs and RCBs, resulting in a high energy density of >300 Wh kg for RMBs and >500 Wh kg for RCBs. Mechanistic studies reveal a unique redox activity mainly at anionic sulfides moieties and fast Mg ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS .