Contact Ion-Pair-Dominated Electrolyte Enabling Inorganic-Rich Solid–Electrolyte Interphase for Long-Cycling Magnesium Metal Anodes

Mg anodes are hindered by a huge overpotential and limited cycling life, stemming primarily from the unstable interphase between Mg anodes and the electrolyte. An effective approach lies in establishing an anion-derived, inorganic-rich solid–electrolyte interphase (SEI) that mitigates the continuous reduction of the electrolyte. Nevertheless, the high charge density of divalent cations poses a significant challenge in balancing the coordination and dissociation of anions within the Mg2+ solvation sheath. Herein, by selecting small-sized OTf–, diglyme solvent, and trimethyl phosphate (TMP) as cosolvents with similar donor number (DN) values, an electrolyte-dominated by Mg2+–OTf– contact ion-pair configuration is achieved, further deriving a stable inorganic SEI containing fluoride and phosphide components. Among them, TMP can break the high lattice energy of magnesium salts, while OTf– with low electron delocalization can ensure a high degree of coordination with Mg2+, jointly realizing anion dissociation chemistry. MgF2 and MgS, dominated by OTf– decomposition at a potential of 0.6 V (vs Mg/Mg2+), enhance the electronic insulation of the interphase. Consequently, Mg anodes exhibit superior cycling performance of over 3200 h with low polarization (<0.1 V) and excellent Mg plating/stripping with a Coulombic efficiency over 1000 cycles at 0.1 mA cm–2.