A new zero-dimensional dynamic model to study the capacity loss mechanism of vanadium redox flow batteries

The study of the capacity loss mechanisms of vanadium redox flow batteries (VRFBs) is important for optimising battery design and performance. To facilitate this, a new zero-dimensional (0-D) dynamic model is proposed in this study that considers different electrolyte transfer (osmosis and electro-osmosis) and vanadium species crossover (convection, electro-migration and diffusion) mechanisms based on the configuration of a 5 kW/3 kWh VRFB system with cation membranes (Nafion 115). The proposed model is validated under three constant current regimes and achieves a mean absolute error (MAE) of less than 2%. Furthermore, its accuracy in estimating capacity over 100 cycles is evaluated using the experimental results of a single-cell VRFB system, which achieves a low MAE of 1.9%. Most importantly, an in-depth analysis of the capacity loss mechanism, including the electrolyte volume transfer, electrolyte imbalance, and electrolyte flow rate, is conducted under different constant current and flow rate regimes. The influence of all electrolyte transfer and crossover mechanisms mentioned above are carefully examined and discussed. This work offers practical recommendations to mitigate capacity loss. Furthermore, the proposed model facilitates the development of electrolyte re-balancing techniques and advanced optimisation methods for optimal battery operation with low computational requirements for battery management systems (BMS).