Multifunctional Water Additive Enabling Stable Cycling of Chloride‐Free Magnesium Metal Batteries Based on Carbonate Electrolyte

Abstract Rechargeable magnesium batteries (RMBs) face with the challenge of interphase passivation between electrolytes and Mg anodes. Compared with ether electrolytes, carbonate solvents possess the superior electrochemical stability at cathode side, but their incompatibility with Mg metal, high viscosity, and desolvation energy barrier restrict their practical utilization in RMBs. Herein, the “unwanted‐impurity” water with high concentration is revisited and employed as multifunctional additive in carbonate electrolyte to improve the reversibility of RMBs. Water additive enables the localized deep eutectic effect, reduces the viscosity of carbonate electrolyte, and improves the Mg ion conductivity. The water molecules also participate the solvation sheath of Mg ions, resulting in the reduction of Mg deposition overpotential and inhibition of parasitic reaction. Furthermore, the co‐intercalated water molecules in V₂O₅ cathode layers enable the stabilization of intercalation structure and supply of additional magnesiophilic sites. Cooperated with the binder‐decorated Mg powder anode, the propylene carbonate electrolyte with water additive endows Mg||Mg symmetric cells and Mg||V₂O₅ full cells with satisfactory cycling performance and high‐voltage stability. This work revisits the impact of impurity water and provides a practical strategy for the utilization of conventional low‐cost carbonate electrolyte family, broadening the design and formulation of electrolytes for chlorine‐free and high‐voltage RMBs.