Interface regulation of Mg anode and redox couple conversion in cathode by copper for high-performance Mg-S battery

The Mg-S battery has great development potential benefiting from its high volume energy density and high-safety. However, it also confronts with two key issues especially when using the Mg(TFSI)₂-based electrolyte. One is the poor compatibility between electrolyte and Mg anode, and the other is the poor reversibility of cathode. In this work, an artificial interphase including Zn/ZnCl₂/MgZn₂/MgCl₂ was firstly pre-constructed by replacement reaction of ZnCl₂ and Mg, which can reduce Mg plating/stripping overpotential to 0.2 V at 0.1 mA/cm². Meanwhile, the reversibility of cathode can be improved by using copper powders as additive in sulfur/carbon composite cathode, and the improvement effect is directly related to the addition amount and particle size of copper powders. In addition, it is also revealed that the chemical reaction can occur between copper and intermediates MgS₈ at the initial stage of discharge process. The reaction product of Cu₂S continues to participate in the electrochemical reduction reaction to regenerate copper, which can be reoxidized to Cu₂S during the charging process. The conversion of redox couple from S/S²⁻ to Cu₂S/Cu⁰ in cathode improves the cycling stability of the battery in comparison with the conversional Mg-S battery.