Stabilization of 3.5-valent vanadium electrolytes via thermodynamic phase-boundary mapping

Despite the potential of vanadium flow batteries (VFBs) for large-scale energy storage, their commercialization is still hindered by the poor thermal stability of VO 2 + in the positive electrolyte. Additives have improved the electrolyte to some extent, yet they are usually introduced after solution preparation. Therefore, we still lack a design of the initial electrolyte composition based on the thermodynamic properties of vanadium electrolyte. This paper systematically elucidates the stability boundaries of vanadium electrolytes, including the operating temperature range and sulfate concentration window, by investigating phase equilibria in a multicomponent system. Combined with actual electrochemical performance tests, the thermodynamic stability window of the vanadium electrolyte was identified as 20-40 °C and 3-6 mol L −1 total sulfate concentration. Under the favorable operating condition of 25 °C and 4.5 mol L −1 total sulfate concentration, the vanadium electrolyte delivered a capacity retention of 88.19% after 100 cycles at 200 mA cm −2 , with an average capacity decay rate of only 0.118% per cycle. The discharge capacity reached 28.9 Ah·L −1 , and coulombic efficiency was as high as 99.69%. This work reveals the correlation between phase-equilibrium stability boundaries and practical electrochemical responses, providing a thermodynamic reference for electrolyte design.