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 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 Mg 2+ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS 4 .

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.

Magnesium (Mg) anode-electrolyte interaction is not trivial and investigation of the interfacial process can be helpful for the development of Mg batteries. In this work, we studied the Mg metal anode cycled in a chloride (Cl)-free magnesium tetrakis (hexafluoroisopropyloxy) borate electrolyte using a full-cell configuration with TiS 2 model cathode. Electrochemical measurements and structural analysis of the cathode showed reversible de-/magnesiation of TiS 2 with some entrapment of irreversibly bound Mg 2 + . Electrochemical impedance spectroscopy (EIS) was applied to analyze the Mg-electrolyte interaction in a three-electrode system. The results showed a rapid increase in charge transfer resistance on the anode side with increasing resting time. In contrast, we observed a significant drop in the charge transfer impedance upon cycling along with the appearance of an additional semi-circle, which suggested to the development of a solid interphase. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) corroborated the EIS results and confirmed the solid interphase layer formation, in which MgF 2 was identified as the primary species contributing to its formation. The current study provides fundamental insights into the interfacial phenomena between the metallic Mg anode and Cl-free electrolyte by highlighting the role played by the formed interphase on the reversible Mg stripping and plating in a Mg full-cell.

Calcium (Ca) batteries represent an attractive option for electrochemical energy storage due to physicochemical and economic reasons. The standard reduction potential of Ca (−2.87 V) is close to Li and promises a wide voltage window for Ca full batteries, while the high abundance of Ca in the earth's crust implicates low material costs. However, the development of Ca batteries is currently hindered by technical issues such as the lack of compatible electrolytes for reversible Ca 2+ plating/stripping and high-capacity cathodes with fast kinetics. Herein, we employed FeS 2 as a conversion cathode material and combined it with a Li + /Ca 2+ hybrid electrolyte for Ca batteries. We demonstrate that Li + ions ensured reversible Ca 2+ plating/ stripping on the Ca metal anode with a small overpotential. At the same time, they enable the conversion of FeS 2 , offering high discharge capacity. As a result, the Ca/FeS 2 cell demonstrated an excellent long-term cycling performance with a high discharge capacity of 303 mAh g −1 over 200 cycles. Even though the practical application of such an approach is questionable due to the high quantity of electrolytes, we believe that our scientific findings still provide new directions for studying Ca batteries with long-term cycling.