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.