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

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. Benefited from the synergistic regulation effect of interfacial chemistry and ion‐solvent interactions, the chloride‐free Mg(TFSI)2‐DME+PrIm electrolyte ensures a 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 battery.