Synergistic Effects of Interfacial Chemistry and Ion-Solvent Interactions to Enable Reversible Magnesium Metal Anode in Chloride-Free Mg(TFSI)2 Electrolytes

Abstract

Passivation of magnesium (Mg) anode in the chloride-free magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) electrolyte is a key challenge for Mg metal batteries. Tailoring solvation structure and solid electrolyte interphase (SEI) has been considered an effective strategy. Herein, a series of imidazole co-solvents with different branched-chain structures (methyl, ethyl, and propyl) are introduced into the Mg(TFSI)₂-ether electrolyte to address the passivation issue. The ion-solvent interaction, interfacial adsorption effect, and SEI formation are comprehensively studied by theoretical calculations and experimental characterizations. Through molecular structure analysis, the long-chain 1-propylimidazole (PrIm) exhibits a strong coordination ability to Mg²⁺ and a favorable parallel adsorption configuration on the Mg surface. As a result, PrIm co-solvent can not only restructure the solvation sheath of Mg²⁺, but also act as a dynamic protective shield to repel a part of TFSI⁻ and 1,2-dimethoxyethane (DME) away from the Mg surface. Benefiting from the synergistic regulation effect of interfacial chemistry and ion-solvent interactions, the chloride-free Mg(TFSI)₂-DME + PrIm electrolyte ensures minimal interface passivation and achieves highly reversible Mg plating/stripping. This work provides a guiding strategy for solvation structure regulation and interface engineering for rechargeable Mg metal batteries.