Toward High-Performance Nonaqueous Redox Flow Batteries through Electrolyte Design

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