Aqueous Zinc‐Bromine Battery with Highly Reversible Bromine Conversion Chemistry

Abstract Br 2 /Br − conversion reaction with a high operating potential (1.85 V vs . Zn 2+ /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, ZnSO 4 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.