A Choline‐Based Antifreezing Complexing Agent with Selective Compatibility for Zn–Br2 Flow Batteries

Abstract The high freezing point of polybromides, charging products, is a significant obstacle to the rapid development of zinc–bromine flow batteries (Zn–Br 2 FBs). Here, a choline‐based complexing agent (CCA) is constructed to liquefy the polybromides at low temperatures. Depending on quaternary ammonium group, choline can effectively complex with polybromide anions and form dense oil‐phase that has excellent antifreezing property. Benefiting from indispensable strong ion‐ion interaction, the highly selectively compatible CCA, consisting of choline and N‐methyl‐N‐ethyl‐morpholinium salts (CCA‐M), can be achieved to further enhance bromine fixing ability. Interestingly, the formed polybromides with CCA‐M are able to keep liquid even at −40 °C. The CCA‐M endows Zn–Br 2 FBs at 40 mA cm −2 with unprecedented long cycle life (over 150 cycles) and high Coulombic efficiency (CE, average ≈98.8%) at −20 °C, but also at room temperature (over 1200 cycles, average CE: ≈94.7%). The CCA shows a promising prospect of application and should be extended to other antifreezing bromine‐based energy storage systems.