Reversible Alkaline Sulfur Cathode Based on Six-Electron Electrochemistry for Advanced Aqueous Sulfur Batteries

Aqueous sulfur batteries are promising for high-performance and low-cost energy storage. However, their energy density is limited by low battery voltages due to the negative potential [<i>E</i>⁰ = -0.51 V vs standard hydrogen electrode (SHE)] of low valent sulfur redox (S⁰/S^2-) and low discharge capacity (∼300 mA h g^-1) of high valent sulfur redox (S₂O₃^2-/S₄O₆^2- or S⁴⁺/S⁰). Herein, we develop a reversible alkaline sulfur cathode via introducing Cu²⁺ and Zn²⁺ ion mediators, exhibiting a redox potential above 0 V vs SHE, which is higher than low valent sulfur redox and high specific capacity of 1340 mA h g^-1. Furthermore, the proposed rechargeable alkaline sulfur batteries achieve a high operating battery voltage of approximately 1.1 V and rapid reaction kinetics, sustained even at high current densities of up to 10 A g^-1. In-depth characterization and DFT calculations reveal that the alkaline sulfur electrochemistry follows a six-electron conversion reaction S ↔ CuS ↔ ZnS + Cu₂O ↔ Cu, delivering an energy density of 1168 W h kg^-1 and a power density of 8110 W kg^-1. This work offers insights into aqueous sulfur electrochemistry and shows an alternative to achieving high energy density aqueous batteries.