Unlocking Capacity and Stability in Halide Flow Batteries with Zwitterionic Trappers

Aqueous redox flow batteries are promising grid energy storage technology due to their safety and ability to decouple power and capacity. Particularly, halide-based catholytes have been used due to their high solubility and redox potential. However, the performance of halide catholytes has been hindered by their intrinsic properties such as crossover, free dihalogen release, and low capacity utilization due to the formation of polyhalides. To overcome those issues, polyhalide complexing cations (PCCs), which consist of an organic cationic motif such as quaternary ammonium, imidazolium, and pyridinium, have been used as additives to trap charged polyahlide species. Although these PCCs effectively reduced the crossover and free dihalogen release, the hydrophobic PCC-polyhalide complex requires complex flow engineering and hindered discharge kinetics. Here, we designed organic soft-hard zwitterionic trappers (SH-ZITs) as novel polyhalide complexing agents composed of soft cationic and water-soluble hard anionic motifs. The addition of SH-ZITs not only prevented polyhalide crossover and dihalogen release but also maintained a homogeneous solution at a high state-of-charge, unlocking the capacity and stability of the halide catholyte. This results in stable operation with an average coulombic efficiency of over 99.9% and no significant decay after more than 1000 cycles over two months.