Enhanced cycle life of vanadium redox flow battery via a capacity and energy efficiency recovery method

In this work, the cycle life of vanadium redox flow batteries (VRFBs) is extended by resolving the inevitable loss of capacity and energy efficiency after long-term cycle operation. The electrolyte concentration, volume, and valence are rebalanced by mixing the electrolyte as well as adding a quantitative amount of a reducing agent. Without disassembling the battery, the energy efficiency decay induced by electrode degradations is almost completely restored by interchanging the positive and negative electrodes. By adopting this method, the capacity and energy efficiency after 500 cycles can be restored to 473 mAh and 90.8%, almost reaching the previous highest level at the beginning of the cycle test (478 mAh and 91.0%). In addition, the rate performance tests also demonstrate that even at a current density of as high as 300 mA cm−2, the energy efficiency of the recovered battery is only 0.8% lower than the new battery. All the results show that the recovery method reported here allows the cycle life of VRFBs to be greatly prolonged with no need to replace electrolytes and electrodes.