Successive Redox Mediation Supported by Pnictogen Chalcohalide Enables High-Performance Zinc-Sulfur Batteries

Sulfur is a promising high-capacity cathode but suffers from sluggish redox kinetics and severe side reactions in aqueous electrolytes. We introduce pnictogen chalcohalide Bi_12.67S₁₈Br₂ nanorods (abbreviated as Bi-S-Br) as a multifunctional cathode material that functions as a sulfur/bismuth reservoir and a catalytic redox mediator (RM). By integrating a trace amount of iodide electrolyte additive, we demonstrate a potential pathway, featuring several redox couples with gradient potentials: I⁰/I^- (0.530 V, RM1) > Bi³⁺/Bi⁰ (0.308 V, RM2) > S⁰/S^2- (0.144 V vs. standard hydrogen electrode), whose stepwise dual mediation accelerates sulfur redox kinetics. The sulfurphilic Bi-S-Br nanorods also strengthen surface Bi-S interfacial interactions, thus localizing active species and suppressing sulfur disproportionation. As a result, Zn||Bi-S-Br cells exhibit synergistic discharge plateaus at 0.66 and 0.59 V and deliver a high specific capacity of 792 mAh g_Bi-S-Br ^-1 at 0.4 A g^-1, with an energy density of 488.2 Wh kg^-1 and a power density of 244.1 W kg^-1, normalized to the Bi-S-Br mass. The practical Zn||Bi-S-Br pouch cell configuration sustains 250 cycles at 0.8 mA cm^-2 with capacity retention of 70.1%. This study elucidated the successive redox mediation strategy for enhanced sulfur redox kinetics, offering a new design principle for high-performance aqueous energy storage systems.