Unlocking Molecular Interactions of Biredox Eutectic Electrolyte for Non-Aqueous Symmetrical Organic Redox Flow Batteries

Biredox deep-eutectic solvents (DESs)-based electrolytes have shown unique features in non-aqueous asymmetrical redox flow batteries (RFBs) that can potentially mitigate the crossover issue. However, strong intermolecular interactions among the electrolyte constituents in DESs bring in practical challenges such as limited mass transfer rates, slow redox kinetics, and degraded cyclability. Here, by using a novel biredox-type DES based on 4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy (4-MeO-TEMPO) and 3,3′-dimethylazobenzene (3,3′-Azo) as a model electrolyte system, the intermolecular interaction involving in the biredox DESs is unlocked by first-principles calculations, and their influence on the electrochemical performance of biredox DESs couples is systematically investigated in comprehensive consideration of the solvents and the supporting salts. It demonstrates that employing tetrabutylammonium bis-trifluoromethane sulfonimidate-acetonitrile as the supporting electrolyte synergistically weakens the intermolecular interactions within the DESs, thereby significantly promoting the redox kinetics and electrochemical reversibility in non-aqueous symmetrical RBFs. With such an optimized electrolyte, a prototype symmetrical cell under static mode is capable of delivering a high output voltage of ∼2.15 V, a decent cyclability, and the exceptional rate capability with a current density of 25 mA cm −2 . This study provides a simple yet effective way to develop advanced RFBs with dependable electrolyte regulation.