Capacity Decay Mitigation by Asymmetric Positive/Negative Electrolyte Volumes in Vanadium Redox Flow Batteries

Capacity decay in vanadium redox flow batteries during charge–discharge cycling has become an important issue because it lowers the practical energy density of the battery. The battery capacity tends to drop rapidly within the first tens of cycles and then drops more gradually over subsequent cycles during long‐term operation. This paper analyzes and discusses the reasons for this early capacity decay. The imbalanced crossover rate of vanadium species was found to remain high until the total difference in vanadium concentration between the positive and negative electrolytes reached almost 1 mol dm−3. To minimize the initial crossover imbalance, we introduced an asymmetric volume ratio between the positive and negative electrolytes during cell operation. Changing this ratio significantly reduced the capacity fading rate of the battery during the early cycles and improved its capacity retention at steady state. As an example, the practical energy density of the battery increased from 15.5 to 25.2 Wh L−1 simply after reduction of the positive volume by 25 %. Pluses and minuses: The capacity of vanadium redox flow batteries tends to decrease very rapidly within the first tens of cycles, significantly lowering their practical energy density. The imbalanced crossover of vanadium species was found to remain very high until the total vanadium concentration difference between the positive and negative electrolytes reached ∼1 mol dm−3. To minimize this early imbalanced crossover, operation using an asymmetric positive/negative volume ratio is suggested.