The vanadium flow battery is a promising electrochemical technology for large-scale energy storage; however, its operational temperature is limited by the low solubility and stability of vanadium ions in sulfuric acid solution. To broaden the operational temperature of the vanadium flow battery while maintaining the non-cross-contamination property of the electrolytes, the complexing agent 1-hydroxyethane-1,1-diphosphonic acid is added to the electrolytes. This compound interacts with the vanadium ions to help stabilize the electrolyte solutions. The results of this study show that the stable temperature range of the electrolyte with 2.0 M vanadium and 2.0 M H2SO4 is increased from 0-25 degrees C to 0-40 degrees C by adding 1 wt% 1-hydroxyethane-1, 1-diphosphonic acid. Both the Job plot and the Benesi-Hildebrand plot methods confirm that 1-hydroxyethane-1, 1-diphosphonic acid interacts with VO2+ in a 1:1 binding stoichiometry. Battery cell tests demonstrate that cells with the modified electrolyte can steadily operate without decreased efficiencies. These results indicate that the thermal stability of the electrolyte can be improved by the interaction of 1-hydroxyethane-1,1-diphosphonic acid and vanadium ions without decreasing the battery efficiency. This improvement is beneficial for further applications of vanadium flow batteries in all-climate conditions.