Based on the ambipolar characteristics and high solubility of ZnI 2 , zincpolyiodide 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 (I 2 ) solid precipitates at the positive electrode, the limiting state-ofcharge (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) , I 3 − (aq) , I 5 − (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 − , I 3− , and I 5 − 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 I 3 − can raise the availability of I − for electrooxidation by suppressing I 2 precipitation, and (2) promoting the production of higher-order polyiodides such as I 5− 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 I 5 − ions in the form of the I 5 − /tri-EG complex, thereby converting the main oxidation product from I 3 − to I 5 − . The preferred electrochemical production of I 5 − 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 − , I 3 − , and I 5 − 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.