Metal‐Organic Frameworks Facilitating Complexation for Long‐Cycle Zinc‐Bromine Flow Batteries

Abstract Aqueous zinc‐bromine flow batteries (ZBFBs) are one of the most attractive candidates for large‐scale stationary energy storage due to their high energy density, intrinsic safety, and low cost. However, the low efficiency and restricted lifespan caused by the bromine shuttling and slow reaction kinetics severely limit their future development. Herein, the zeolitic imidazolate framework (ZIF) is proposed to be used with a novel strategy to improve the Br 2 complexing ability of the mature complexing agent (i.e., N‐ methyl‐ N ‐ethyl pyrrolidinium bromide (MEP) ionic liquid). The organic ligands (imidazole and benzimidazole) from the decomposed zinc ZIF‐62 facilitate the complexation of MEP. Both advanced experimental characterizations and theoretical calculations confirm that the optimal mass ratio of MEP/ZIF‐62 is 5%, the synergistic effect between imidazole/benzimidazole of the decomposed ZIF‐62 toward bromine species and MEP toward bromine species, effectively inhibits the bromine shuttle effect and improves the transformation kinetics of bromine species. Notably, the assembled in situ optical characterization directly confirmed that ZIF‐62/5% MEP enhances the complex ability toward bromine species, promotes the formation of more oily phase products, and makes the electrolyte more uniform to form well‐distributed floating droplets. The obtained ZBFBs deliver superior long‐cycle performance up to 1500 cycles without obvious performance decay, and, notably, even at a high areal capacity of 80 mAh cm −2 , they also stably operate for 50 cycles. The auxiliary complexation strategy based on metal‐organic frameworks is beneficial in improving the comprehensive performance of ZBFBs.