Aqueous Zinc‐Bromine Battery with Highly Reversible Bromine Conversion Chemistry

Br₂/Br^- conversion reaction with a high operating potential (1.85 V vs. Zn²⁺/Zn) is promising for designing high-energy cathodes in aqueous Zn batteries. However, the ultrahigh solubility of polybromides causes significant shuttle effects, capacity deterioration, and self-discharge, rendering the study of static zinc-bromine batteries still in its infancy. Here, various aqueous zinc salt electrolytes are first screened, showing that, compared to other salts, ZnSO₄ is more suitable for Br-based cathodes benefiting from its higher negative charge density and lower cost. Nevertheless, the significant shuttle effect of polybromides remains in such an electrolyte. We further develop a targeted sequestration strategy to fundamentally confine polybromide migration from KBr cathode into electrolyte. In high-mass-loading (22 mg_KBr cm^-2) pouch cells, the average Coulombic efficiency enhances from 92.3 % to 99.8 %, and self-discharge performance dramatically improves from 17.4 % capacity retention to 85.2 % after 72 h of resting, indicating the effectiveness of our strategy in confining the shuttle effects. Furthermore, an Ah-scale pouch cell delivers an average Coulombic efficiency of 99.88 % and a zinc utilization of 22 % at a high rate of 3 C. Our findings also pave the way for the design of advanced Br-based cathodes.