A trifunctional electrolyte for high-performance zinc-iodine flow batteries

Zinc-iodine flow battery (ZIFB) holds great potential for grid-scale energy storage because of its high energy density, good safety and inexpensiveness. However, the performance of ZIFB is hindered by conventional electrolyte that offers low ionic conductivity, suffers from iodine precipitation and triggers severe Zn dendrite growth. Here, we report an NH4Br improved electrolyte to address the issues simultaneously. The addition of NH4Br enhances the ionic conductivity of electrolyte from 120 to 180 mS cm−1. Meanwhile, Br− ions complex with I2 forming soluble I2Br−, which frees up I− ions and thus unlocks the battery capacity, while NH4+ ions complex with Zn2+ ions to mitigate Zn dendrite formation with the electrostatic shielding effect. Experimental results reveal that the improved electrolyte promotes the kinetics and reversibility of both positive and negative redox couples. A ZIFB with the improved electrolyte delivers a high energy efficiency (85%), a high charge capacity (35 A h L−1) and a long cycle life (100 cycles without degradation) at 40 mA cm−2, while conventional ZIFB shows an energy efficiency of 80%, a charge capacity of 25 A h L−1, and a short cycle life of only around 15 cycles. These encouraging results indicate that NH4Br, a cost-effective salt, offers great prospects for high-performance ZIFB applications.