Suppressing Bromine Crossover via Bispyrrolidinyl Complexation toward High-Capacity Zinc-Bromine Flow Batteries

Bromine complexing agents are essential for suppressing bromine volatilization and polybromide crossover in zinc-bromine flow batteries (ZBFBs). However, under the high bromine concentrations required for high-capacity operation, inadequate complexation and phase separation of conventional agents severely compromise battery performance. Herein, we report a dual-site molecular design strategy to overcome this limitation. A bispyrrolidinyl complexing agent (N 2 EDP) featuring two quaternary ammonium centers and hydroxyl functionalization was synthesized. Through electrostatic interactions, the N 2 EDP captures polybromide anions to form bulky N 2 EDP(Br 2 n +1 ) 2 complexes, increasing molecular size and thereby suppressing crossover. Density functional theory calculations confirm that this dual-site agent exhibits stronger polybromide binding and more negative solvation free energies than the conventional mono-site agent MEP, enhancing both complexation stability and aqueous compatibility. Electrochemical measurements show that the N 2 EDP reduces the diffusion coefficient of Br 3 – by 61% and decreases bromine species permeability by 87.9%. Consequently, the N 2 EDP-containing cell achieves a discharge capacity of 74.1 Ah L –1 at 50% SOC with 100% capacity retention over 50 cycles, and stable operation up to 60% SOC delivers a maximum capacity of 86.1 Ah L –1 . This dual-site molecular design offers an effective strategy for high-capacity ZBFBs.