PNNL

Abstract

Conventional electrolytes made by mixing simple Mg2+ salts and aprotic solvents, analogous to those in Li-ion batteries, are incompatible with Mg anodes because Mg metal readily reacts with such electrolytes, producing a passivation layer which blocks Mg2+ transport. Here, we report that, through tuning a conventional electrolyte—Mg(TFSI)2 (TFSI- is N(SO2CF3)2-) with an Mg(BH4)2 additive, highly reversible Mg plating/stripping with a high coulombic efficiency is achieved, by decoupling the interaction between Mg2+ and TFSI- and enhanced reductive stability of free TFSI-. A critical adsorption step between Mg0 atoms and active Mg cation clusters involving BH4- anions is identified to be the key enabler for reversible Mg plating/stripping through analysis of distribution of relaxation times (DRT) from operando electrochemical impedance spectroscopy (EIS), operando electrochemical X-ray absorption spectroscopy (XAS), nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations. This study suggests a new approach for developing advanced electrolytes for Mg batteries and provides a set of in-operando analysis tools for probing electrified Mg/electrolyte interfaces.