Effective Enhancement of Energy Density of Zinc-Polyiodide Flow Batteries by Organic/Penta-iodide Complexation

Based on the ambipolar characteristics and high solubility of ZnI₂, zinc-polyiodide flow batteries (ZIFB) have attracted attention as high-energy density flow batteries. However, due to the various oxidation products of iodide (I^-) and the formation of iodine (I₂) solid precipitates at the positive electrode, the limiting state-of-charge (SoC) of ZIFB has not been clearly defined. Herein, a clear definition of SoC in ZIFBs is given based on the thermodynamic relationship among I^-_(aq), I₃^-_(aq), I₅^-_(aq), and I_2(aq) in the electrolyte. Conventional ZIFBs are limited by their maximum attainable SoC of 87%, at which the fully charged catholyte includes I^-, I₃^-, and I₅^- ions at molar ratios of 49.6, 32.2, and 18.1%, respectively. Furthermore, two effective strategies to extend the maximum SoC are suggested: (1) increasing the formation constant (K_eq) of I₃^- can raise the availability of I^- for electrooxidation by suppressing I₂ precipitation, and (2) promoting the production of higher-order polyiodides such as I₅^- can increase the oxidation state of the charged electrolyte. The addition of 5 vol % triethylene glycol (tri-EG) to the electrolyte increased K_eq from 710 to 1123 L mol^-1; this increase was confirmed spectrophotometrically. Tri-EG stabilized I₅^- ions in the form of the I₅^-/tri-EG complex, thereby converting the main oxidation product from I₃^- to I₅^-. The preferred electrochemical production of I₅^- in the tri-EG electrolyte was observed by electrochemical and computational analyses. As a result, the maximum attainable SoC was enhanced remarkably to 116%, yielding molar ratios of I^-, I₃^-, and I₅^- ions of 9.1, 11.2, and 79.7%, respectively. This SoC extension effect was confirmed in the ZIFB flow cell with stable charge-discharge cycling at the SoC 120% limit, demonstrating the highest energy density, 249.9 Wh L^-1, among all reported ZIFBs.