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 I2(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 (Keq) 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 Keq 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.
Keywords: aqueous flow batteries; energy density; iodine film; penta-iodide; triethylene glycol; zinc-polyiodide.