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.　©　2025　Wiley-VCH　GmbH.