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. Low temperatures trigger phase changes and slow kinetic process in zinc–bromine flow batteries. Here, the authors show NH4+ as the supporting cation stabilizes the electrolyte and improves ion transport, enabling long-life cycling from room temperature down to −20 °C.