Reversible electrochemical oxidation of sulfur in ionic liquid for high-voltage Al-S batteries.

Sulfur is an important electrode material in metal−sulfur batteries. It is usually coupled with metal anodes and undergoes electrochemical reduction to form metal sulfides. Herein, we demonstrate, for the first time, the reversible sulfur oxidation process in AlCl3/carbamide ionic liquid, where sulfur is electrochemically oxidized by AlCl4− to form AlSCl7. The sulfur oxidation is: 1) highly reversible with an efficiency of ~94%; and 2) workable within a wide range of high potentials. As a result, the Al−S battery based on sulfur oxidation can be cycled steadily around ~1.8 V, which is the highest operation voltage in Al−S batteries. The study of sulfur oxidation process benefits the understanding of sulfur chemistry and provides a valuable inspiration for the design of other high-voltage metal−sulfur batteries, not limited to Al−S configurations. The sulfur cathode in metal-sulfur batteries normally undergoes electrochemical reduction to form metal sulfides. Here, the authors demonstrate the electrochemical oxidation of sulfur in ionic liquid for high-voltage aluminium-sulfur batteries.