A Highly Reversible Aqueous Sulfur-Dual-Halogen Battery Enabled by a Water-in-Bisalt Electrolyte

The chlorine-based redox reaction applied in aqueous rechargeable batteries (ARBs) has attracted extensive attention owing to the high theoretical capacity and redox potential. However, it generally suffers from low reversibility and poor Coulombic efficiency due to the evolution of toxic Cl₂ gas and the decomposition of aqueous electrolytes. Herein, an aqueous sulfur-dual halogen chemistry is demonstrated by employing highly-concentrated water-in-bisalt (WiBS) electrolyte, sulfur anode, and iodine composite electrodes. The freestanding iodine/carbon cloth cathode and Cl^--containing WiBS electrolyte not only enable the continuous I⁺/I⁰ reaction by forming [ICl_x]^1-x interhalogens but also achieve the oxidation of Cl^- in [ICl_x]^1-x at higher redox potential and immobilize Cl⁰ species via I⁺─Cl⁰ chemical bonds. Therefore, the as-assembled aqueous sulfur-dual halogen batteries (ASHBs) based on the dual-halogen conversion on the cathode and the S/S_x ^2- redox reaction on the anode deliver a high energy density of 304 Wh kg^-1 with an average output voltage of 1.32 V. These key findings open an avenue for the development of low-cost and high-performance ARBs for energy storage applications.