Development of a chloride-free dual-salt electrolyte based on Mg(NO₃)₂ and Mg(TFSI)₂ for magnesium-sulfur batteries in a binary solvent system

Magnesium-sulfur (Mg-S) batteries represent a highly promising next generation energy storage system, combining high theoretical energy density, intrinsic safety, and low cost. However, their progress is severely hindered by the lack of compatible, noncorrosive electrolytes that enable reversible Mg^{2 +} plating/stripping and stable sulfur redox chemistry. Conventional chloride-based electrolytes, while effective for Mg deposition, suffer from corrosivity, limited oxidative stability, and poor compatibility with sulfur cathodes. Herein, a novel chloride free (Cl-free) electrolyte is presented based on magnesium nitrate hexahydrate (Mg(NO₃)₂·6 H₂O) dissolved in a binary solvent mixture of acetonitrile (ACN, 5.7 mL) and tetraethylene glycol dimethyl ether (G4, 2.7 mL), with magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) as a co-salt. Structural and spectroscopic analyses confirm strong Mg²⁺ coordination with both ACN and G4 molecules. On the other hand, NO₃⁻ and TFSI⁻ anions stabilize the ion pairing environment. Electrochemical impedance spectroscopy and linear sweep voltammetry reveal high ionic conductivity, a wide electrochemical stability window, and efficient Mg²⁺ transport kinetics. This study presents systematic investigations of nitrate-based Cl free electrolytes for Mg-S batteries, providing new insights and a viable pathway toward safe, high performance magnesium energy storage systems.