Multi-electron reactions enabled by anion-participated redox chemistry for high-energy multivalent rechargeable batteries
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Intense research efforts in electrochemical energy storage are being devoted to multivalent ion technologies in order to meet the growing demands for high energy and low-cost energy storage systems. However, the development of multivalent metal (such as Mg and Ca) based battery systems is hindered by lack of suitable cathode chemistries that show well reversible multi-electron redox reactions. Cationic redox centers in the classical cathodes could only afford stepwise single electron transfer, which we believe are not ideal for multivalent ion storage. The possible local charge balance issue would set additional kinetic barrier for ion mobility. Therefore, most of the multivalent battery cathodes only exhibit slope-like voltage profiles with insertion/extraction redox of less than one electron. To address this issue, we propose to activate anionic redox chemistry enabling multi-electron transfer in insertion cathodes for high-energy multivalent batteries. Taking VS 4 as a model material, reversible two-electron redox with synergetic cationic-anionic contribution has been 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 the unique redox activity at anionic sulphides moieties and demonstrate fast Mg 2+ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS 4 . The concept of coupling a cathode based on anionic redox reactions with a multivalent metal anode provides a general approach towards high performance multivalent batteries.