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 VS4 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 Mg2+ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS4 .
Keywords: VS4; anions; magnesium-ion batteries; multi-electron reactions; rechargeable calcium batteries.
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.