Asymmetric auto-rebalancing of electrolyte for high capacity retention and high energy efficiency of vanadium redox flow batteries

Capacity fade and performance degradation under long-term operation are critical concerns in the application of vanadium redox flow batteries (VRFBs) in large-scale energy-storage systems. This study introduces an innovative electrolyte-rebalancing technique named asymmetric auto-rebalancing (AAR) to achieve high capacity retention and high efficiency of VRFBs during long-term operation. Three VRFBs—one each without rebalancing (NR), with auto-rebalancing (AR), and with AAR—were prepared for a performance comparison. Also, the capacity and performance degradation mechanisms of VRFBs with three different electrolyte rebalancing methods were theoretically analyzed. Long-term charge–discharge cycling tests were conducted on the VRFBs to verify the effectiveness of AAR according to the current, temperature, and electrolyte flow rate. AAR showed the most consistent performance under long-term operation, with negligible electrolyte imbalance. Further, AAR achieved the most stable capacity retention and highest energy efficiency (84.66 %) compared to NR (82.77 %) and AR (82.98 %). Finally, the electrolyte volume change was predicted by a theoretical analysis, which was consistent with the experimental results.