Method of Reflow and Online Electrolysis in the Vanadium Redox Battery: Benefits and Limitations

Capacity decay during the long-time cycles is one of the drawbacks of an all-vanadium redox flow battery which limits its application in the large-scale energy storage. Based on the convenient and effective reflow method, we add online electrolysis to restore the capacity by reducing the accumulated VO2+ (pentavalence) in the positive electrolyte during long-time charge and discharge cycles. In this work, the capacity restore effects of online electrolysis with three reflow volumes are tested; the mechanism of the special discharge capacity retention behavior is discussed, and the benefits and limitations of the capacity recovery method are explored. In this work, the capacity is restored apparently, and the all-vanadium redox flow battery runs 4680 cycles stably through eight online electrolysis processes, which is owing to the controlled volume imbalance by the reflow and the compensated valence imbalance by the online electrolysis. Although the concentration imbalance accumulated during the long-time cycles limits the capacity recovery which needs to be addressed by combining with mixing the positive and negative electrolytes in the future, considering the convenience and the low cost of this method, it has promising application in large-scale energy storage. Hence, this method is free from replacing electrolyte, lowers the maintenance cost, and improves the competitiveness and further popularization of the all-vanadium redox flow battery as a smart grid energy storage module.