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

•An artificial interphase composed of Zn/MgZn2/MgCl2 was pre-constructed by replacement reaction of ZnCl2 and Mg.•The Mg plating/stripping overpotential of the modified Mg electrode can be reduced 0.2 V at 0.1 mA/cm2.•The reversibility of cathode can be improved by serving Cu powders as additive in cathode.•The change of redox couple from S/S2− to Cu2S/Cu0 is completed during the cycle. 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.