Intercalation–Conversion and Displacement–Conversion Strategies in Magnesium–Sulfur Batteries

Magnesium–sulfur (Mg–S) batteries have attracted considerable attention because of their high volumetric energy density and safety as well as the earth abundance of Mg and S. However, the high porosity of the conventional cathode composed of porous carbon mixed with sulfur requires a large amount of liquid electrolyte to fill the pores, decreasing the practical energy density. In this study, Mo 6 S 8 or Cu 2 Mo 6 S 8 materials with high density and high electronic conductivity were used to largely replace the lightweight carbon materials. This results in substantially decreased porosity and improved specific capacity of the S–Mo 6 S 8 and S–CuMo 6 S 8 composite cathodes compared with those of the S–C cathode. Ex situ X-ray diffraction and X-ray photoelectron spectroscopy analyses revealed that the S–Mo 6 S 8 and S–CuMo 6 S 8 composite cathodes undergo intercalation–conversion and displacement–conversion reactions, respectively, with the changed extent of sulfur redox processes. Our work offers important insight into cathode design strategies for Mg–S batteries, which can lead to improved electrochemical performance.