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 [ E 0 = −0.51 V vs standard hydrogen electrode (SHE)] of low valent sulfur redox (S 0 /S 2– ) and low discharge capacity (∼300 mA h g –1 ) of high valent sulfur redox (S 2 O 3 2– /S 4 O 6 2– or S 4+ /S 0 ). Herein, we develop a reversible alkaline sulfur cathode via introducing Cu 2+ and Zn 2+ 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 + Cu 2 O ↔ 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.