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

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 Mg2+ cation facilitates the dehydrogenation of the NH2 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 Mg2+ salt. This electrolyte composition delivers long-term Mg metal anode cycling with 99.6% Coulombic efficiency for 800 cycles.