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 (∼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.