Stable Cycling of Mg Metal Anodes by Regulating the Reactivity of Mg2+ Solvation Species

Abstract Rationally designing stable nonaqueous electrolytes for Mg metal anodes demands a thorough understanding of their interfacial behaviors. Here, the critical role of cation–anion pairing in improving the cathodic stabilities of amine‐based electrolytes against solvent reduction and H 2 evolution is identified. It is demonstrated that strong coordination between solvating amine groups and the Mg 2+ cation facilitates the dehydrogenation of the ─NH 2 group, which is mainly responsible for low reversibility during Mg metal plating and stripping. Introducing ion‐pairing into the primary solvation shell can effectively weaken the amine coordination such that its reduction is suppressed. A novel interfacial behavior regarding parasitic reaction product dissolution is also identified, which is responsible for the failure of interfacial passivation. An ion‐pairing electrolyte is developed based on a weakly‐solvated amine molecule and strongly coordinating Mg 2+ salt. This electrolyte composition delivers long‐term Mg metal anode cycling with 99.6% Coulombic efficiency for 800 cycles.