Toward Highly Reversible Magnesium-Sulfur Batteries with Efficient and Practical Mg[B(hfip)(4)](2) Electrolyte

The rechargeable magnesium (Mg) battery has been considered a promising candidate for future battery generations due to unique advantages of the Mg metal anode. The combination of Mg with a sulfur cathode is one of the attractive electrochemical energy storage systems that use safe, low-cost, and sustainable materials and could potentially provide a high energy density. To develop a suitable electrolyte remains the key challenge for realization of a magnesium sulfur (Mg-S) battery. Herein, we demonstrate that magnesium tetrakis(hexafluoroisopropyloxy) borate Mg-[B(hfip)(4)](2) (hfip = OC(H)(CF3)(2)) satisfies a multitude of requirements for an efficient and practical electrolyte, including high anodic stability (>4.5 V), high ionic conductivity (similar to 11 mS cm(-1)), and excellent long-term Mg cycling stability with a low polarization. Insightful mechanistic studies verify the reversible redox processes of Mg-S chemistry by utilizing Mg[B(hfip)(4)](2) electroylte and also unveil the origin of the voltage hysteresis in Mg-S batteries.