Investigation of Manipulated Interface in Conventional Electrolytes for Rechargeable Magnesium Battery Application

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+ cation 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 high coulombic efficiency close to 100% is achievable, by decoupling the interaction between Mg2+ and TFSI- and enhanced reductive stability of free TFSI-. A critical adsorption step between the Mg0 atoms and active Mg cation clusters involving BH4 - anions is demonstrated on evolving electrified interface based upon 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 and provides an in-operando analysis kit of probing electrified interfaces with adsorption of active Mg cation clusters for rechargeable Mg batteries.