Anion-type solvation structure enables stable zinc‑iodine flow batteries

Zinc-based flow batteries (ZFBs) have shown great promise as large-scale energy storage devices due to their high energy density, low cost and environmental friendliness. In order to prevent Zinc (Zn) dendrites growth and side-reactions which result in poor cell performance and short cycle life, low-cost potassium chloride is often utilized as the supporting electrolyte to enhance the Zn redox reversibility. In this work, the effect of this supporting electrolyte is studied. Experimental characterizations reveal that cation-type Zn(H 2 O) 5 Cl + solvation structure could transform to an anion-type water-free solvation structure ZnCl 4 2− by adding more chloride salt. The electrostatic shielding effect of potassium ion and anion-type water-free ZnCl 4 2− could homogenize Zn ion flux to achieve a dendrite-less morphology during the reduction process. The improved negolyte enables ZFBs to run for 300 h with a high energy efficiency (>83 %) at 40 mA cm −2 , 20 mAh cm −2 . Besides, the hydrogen-bond network in negolyte is reshaped, expanding the operating temperature range of the system. This work determines the effect of supporting electrolyte in Zn negolyte, which can guide the design of electrolyte and promote further research on Zn electrode in ZFBs.