A Long‐Life Zinc‐Bromine Single‐Flow Battery Utilizing Trimethylsulfoxonium Bromide as Complexing Agent

Abstract Aqueous zinc‐bromine single‐flow batteries (ZBSFBs) are highly promising for distributed energy storage systems due to their safety, low cost, and relatively high energy density. However, the limited operational lifespan of ZBSFBs poses a significant barrier to their large‐scale commercial viability. Here, trimethylsulfoxonium bromide (TMSO), a nonquaternary ammonium salt, is introduced as a bromine complexing agent to extend the cycle life of ZBSFBs by reducing the imbalance of active substances. Benefiting from the strong interaction between TMSO and H 2 O, the hydrogen evolution reaction is notably suppressed compared with the traditional N‐ethyl‐N‐methyl‐pyrrolidinium bromide (MEP) complexing agent, resulting in reduced bromine accumulation at the cathode. The resultant solid polybromide‐TMSO complex, featuring rapid electrochemical redox reaction of Br 2 /Br − , further contributes to reduce the residual bromine. Consequently, the ZBSFB with TMSO demonstrates a longer lifespan of 1500 cycles with a higher average energy efficiency (EE) of ≈81.6% than that with MEP (less than 300 cycles with an average EE of ≈80.2%). This research explores a sulfonium complexing agent and provides a feasible strategy to effectively extend the cycle life of ZBSFBs.