Toward High-Performance Nonaqueous Redox Flow Batteries through Electrolyte Design

Redox flow batteries (RFBs) have emerged as a promising solution for large-scale stationary energy storage. However, nonaqueous flow batteries, despite having promising potential, are lagging behind aqueous flow batteries due to the lack of suitable redox pairs that can deliver high energy density and long cycle life. In this study, we implemented a counterion modification strategy to greatly enhance the solubility of both catholyte and anolyte active materials. Specifically, we increased the solubility of anthraquinone-2-sulfonic acid sodium salt (AQS) by three orders of magnitude in acetonitrile by replacing a sodium countercation with tetra-n-butylammonium. We present the first report of the flow cell cycling of all anionic active materials with a tetra-n-butyl countercation in supporting-salt-free conditions. We investigated the electrochemical behavior of each individual active material in a symmetric flow cell and then paired the AQS anolyte with the bio-inspired catholyte, tetra-n-butylammonium vanadium-bis-hydroxyiminodiacetate (TBA2VBH), in a full cell. The significant crossover observed in a full cell was mitigated by using a compositionally symmetric, mixed electrolyte as both the catholyte and anolyte. Additionally, because AQS and VBH coexist stably in the mixed electrolyte, even at high concentrations, we demonstrate that the cell capacity can be fully restored by rebalancing the electrolyte leading to long cycle life. This strategy, which has been employed in aqueous, acidic, all-vanadium flow battery systems, could be a promising pathway toward robust, high-performance nonaqueous flow batteries