The new vanadium battery stack is generally configured using a mixed electrolyte with a concentration ratio of V(III) to V(IV) of 1:1, that is, the average valence of vanadium ions in the electrolyte is 3.5. Such electrolyte can be directly added to the positive and negative electrode chambers for use, which is easy to operate. The purity of the vanadium electrolyte plays a crucial role in performances of the battery, and high concentration of impurities in the electrolyte will bring about the following problems: (1) there is a competitive reaction between impurity ions and vanadium ions, which reduces the efficiency of the battery. (2) In the positive electrode chamber, impurity ions are deposited on the graphite felt electrode, which results in the blockage of the gap in the graphite felt and reduction of the specific surface area of the graphite felt, thus affecting charge and discharge efficiencies. (3) In the negative electrode chamber, impurity ions will affect the hydrogen evolution over-potential, and the production of the gas will affect the pressure balance inside the battery. (4) Impurity ions reduce the lifetime of the proton exchange membrane. (5) Impurity ions affect the stability of vanadium ions, leading to premature aging of the electrolyte.

The activity of the vanadium electrolyte refers to the effective concentration of the vanadium ions in the electrolyte that can be used for charge and discharge. The vanadium ions in the electrolyte are affected by the temperature, impurities, etc., and an oxygen-bridge bond will be formed, which results in the production of polycondensation, and the reduction of the electrochemical activity. Therefore, increasing the activity of the vanadium electrolyte can effectively improve the utilization efficiency of the vanadium resources, thus reducing the cost of the vanadium battery.