PNNL

Abstract

Rationally designing stable nonaqueous electrolytes for Mg metal anodes demands the thorough understanding of their interfacial behaviors. Here, we identify the critical role of cation-anion pairing in improving the cathodic stabilities of amine-based electrolytes against solvent reduction and H2 evolution. 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 for the first time, 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 composition realizes long-term Mg metal anode cycling with 99.6 % coulombic efficiency for 800 cycles.