Reversible Electrochemical Interface of Mg Metal and Conventional Electrolyte Enabled by Intermediate Adsorption

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 that 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 cosalt, highly reversible Mg plating/stripping with a high Coulombic efficiency is achieved by neutralizing the first solvation shell of Mg cationic clusters 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 the distribution of relaxation times (DRT) from operando electrochemical impedance spectroscopy (EIS), operando electrochemical Xray absorption spectroscopy (XAS), nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations