Vanadium redox flow battery capacity loss mitigation strategy based on a comprehensive analysis of electrolyte imbalance effects

Electrolyte imbalance is the main cause of capacity loss in vanadium redox flow batteries. It has been widely reported that imbalance caused by vanadium crossover can be readily recovered by remixing the electrolytes, while imbalance caused by a net oxidation of the electrolyte can only be reverted by means of more complex chemical or electrochemical methods. At the moment, however, the joint effect of both types of imbalances on the battery capacity is still not well understood. To overcome this limitation, generalised State of Charge and State of Health indicators that consider both types of imbalances are derived in this work. Subsequently, a thorough analysis on how the battery capacity depends on electrolyte imbalance is performed. As a result of this analysis, two specific outcomes are highlighted. Firstly, it is shown that standard electrolyte remixing may be counterproductive under certain imbalance conditions, further reducing the battery capacity instead of augmenting it. Secondly, it is demonstrated that most of the capacity loss caused by oxidation can be mitigated by inducing an optimal mass imbalance in the system. Consequently, a systematic procedure to track this optimum is proposed and validated through computer simulation.