Mixed-Anion Contact Ion-Pair Formation Enabling Improved Performance of Halide-Free Mg-Ion Electrolytes

Authors

Abstract

Discovery of stable and efficient electrolytes that are compatible with magnesium metal anodes and high-voltage cathodes is crucial to enabling energy storage technologies that can move beyond existing Li-ion systems. Many promising electrolytes for magnesium anodes have been proposed, with chloride-based systems at the forefront; however, Cl-containing electrolytes lack the oxidative stability required by high-voltage cathodes. In this work, we report magnesium trifluoromethanesulfonate (triflate) as a viable co-anion for Cl-free, mixed anion magnesium electrolytes. The addition of triflate to electrolytes containing bis(trifluoromethane sulfonyl) imide (TFSI ) anions yields significantly improved Coulombic efficiency, up to a 100 mV decrease in the plating/stripping overpotential, improved tolerance to trace H2O, and improved oxidative stability (0.35 V improvement compared to hybrid TFSI-Cl electrolytes). Based on 19F nuclear magnetic resonance (NMR) and Raman spectroscopy measurements, we propose that these improvements in performance are driven by the formation of mixed anion contact ion pairs, where both triflate and TFSI are coordinated to Mg2+ in the electrolyte bulk. The formation of this mixed-anion magnesium complex is further predicted by density functional theory to be thermodynamically driven. Collectively, this work outlines guiding principles for the improved design of next generation electrolytes for magnesium batteries.