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.