Cation design in complex aqueous electrolytes for low-temperature zinc-bromine flow batteries

The cation design of the multi-component dynamic concentration electrolytes that simultaneously achieve effective phase transition prevention and rapid kinetic process is challenging but vital for enhancing the performance of low-temperature zinc-bromine flow batteries. Herein, we employ NH₄⁺ as supporting electrolyte cation to break inherent trade-off between salting-out at low temperatures and ion transport kinetic from the alkali cations (K⁺, Na⁺ and Li⁺). Compared to commonly used K⁺ in the traditional electrolyte, NH₄⁺ achieves an improved ability to prevent the phase transition of the electrolyte and obtains better ion transport property, which is a characteristic that Na⁺ and Li⁺ do not possess. Additionally, NH₄⁺ can significantly enhance the cycling stability by appropriately increasing the solubility of polybromides. Consequently, the zinc-bromine flow batteries supported by NH₄⁺ not only demonstrate sustained lifespan (>2,300 cycles at 40 mA cm^-2, 40 mAh cm^-2, over half a year) and consistent high-rate cyclic stability (charging at 200 mA cm^-2, discharging at 80 mA cm^-2, >3,300 h) at room temperature; they also exhibit stable cyclic stability (over 1,600 cycles at 40 mA cm^-2) at -20 °C. This work provides an effective path for the design of complex electrolytes for low-temperature aqueous zinc-based flow batteries.