Chloride self-supply electrolyte: Mitigating concentration-stability conflict in vanadium batteries

Vanadium redox flow batteries (VRFBs) are regarded as a core technology for large-scale energy storage, as the concentration and stability of the electrolyte directly influence the system's energy density and operating costs. However, traditional electrolyte systems generally suffer from low concentration, poor stability, and complex preparation processes. In this study, VOCl₃ was used as both the vanadium source and an endogenous chlorine donor, combined with sulfuric acid to construct a VOCl₃–H₂SO₄ electrolyte system (2.7 M V), achieving significant improvements over existing technologies. This strategy enables the simultaneous release of vanadium and chloride ions from VOCl₃, greatly enhancing the dissolution rate of vanadium ions and optimizing the stability of the solvation structure, while also eliminating the need for externally added hydrochloric acid. The synergistic coordination of Cl⁻ and SO₄²⁻ effectively suppresses the formation of precipitation. Experimental results show that this electrolyte remains stable and precipitation-free over a wide temperature range from −5 to 50 °C. The VRFB assembled based on this electrolyte achieves an energy efficiency of over 80 % at 50 °C and 80 mA cm⁻², with the coulombic efficiency remaining stable above 96 %. This study provides an important reference for the fundamental research and engineering application of high-concentration, high-performance electrolytes.