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 $V{S}_4$ 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 $>$300 mAh g${}^{-1}$ at a current density of 100 mA g${}^{-1}$ in both RMBs and RCBs, resulting in a high energy density of $>$300 Wh kg${}^{-1}$ for RMBs and $>$500 Wh kg${}^{-1}$ for RCBs. Mechanistic studies reveal a unique redox activity mainly at anionic sulfides moieties and fast $M{g}^{2+}$ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of $V{S}_4$. (© 2020 Wiley‐VCH Verlag GmbH and Co. KGaA, Weinheim)