Study of the cross-transportation of V(II)/V(III) in vanadium flow batteries based on online monitoring of nonlinear absorption spectra

Abstract

In vanadium flow batteries (VFBs), the concentration and volume imbalances cause continual capacity loss during the long charge/discharge cycles due to the accumulation of vanadium cross-transportation. To detect these imbalances, the total vanadium concentration (TVC) and state of charge (SOC) need to be simultaneously monitored online. In this work, the absorption spectra of the negative electrolyte are calibrated to further obtain empirical formulas for the absorbance at 605 nm and 640 nm versus the concentrations of V(II) and V(III). Real-time TVC and SOC can be obtained by solving the equations formed by the empirical equations during online monitoring. Compared with the potentiometric titration, this method demonstrates high accuracy and resolution. Based on this, the specific behavior of vanadium cross-transportation during 200 charge/discharge cycles is studied and confirmed that the vanadium is transported from the positive electrolyte to the negative electrolyte during charging and vice versa during discharging. Vanadium cross-transportation amount and its contribution to the capacity loss are further quantified. The absorption spectra online monitoring method proves to be an excellent way to detect the imbalances of negative electrolyte and is expected to effectively guide the design of VFBs with higher capacity and longer cycle life.