Conditioning–Free Electrolytes for Magnesium Batteries Using Sulfone–Ether Mixtures with Increased Thermal Stability

We report an investigation of electrochemical properties and speciation of electrolytes composed of magnesium bis(hexamethyldisilazide) (Mg(HMDS)₂) and magnesium chloride in sulfone-ether mixtures. The inclusion of sulfones dramatically increased the thermal stability of the electrolytes to 80 °C. The addition of ether was necessary to form an electrochemically active species. Electrochemical reversibility was maintained above 90% for 50 cycles for equivolume mixtures of butyl sulfone and THF. Sulfolane based solutions demonstrated low reversibilities and mass spectrometry measurements showed the preferential formation of an MgCl⁺ cation over the Mg₂Cl₃⁺ cation. Greater amounts of Mg₂Cl₃⁺ compared to MgCl⁺ is linked with higher performing electrolytes. Spectroscopic measurements showed co-solvation of the active magnesium cation with both solvents.