Untapped Potential of Fluoride Ions in Maximizing the Electrochemical Stability of Deep Eutectic Solvents

Advancing batteries is pivotal to propelling our society toward a sustainable, electrified future. The stability of the electrolytes forms the backbone of energy storage systems. This is particularly the case for redox flow batteries (RFBs). Their deployability depends on their longevity and dependability. The presence of unstable electrolytes can trigger undesirable reactions, degrade performance, and lead to battery failure. Aqueous electrolytes, with limited electrochemical stability window (E_SW), are prone to hydrogen and oxygen evolution. Conversely, nonaqueous electrolytes offer enhanced stability. In this study, we unveil the E_SW of a nonaqueous eutectic solvent, comprising choline fluoride and ethylene glycol─a composition that has eluded experimental investigation until now. Our findings show that the stability window, reduction, and oxidation potential limits of deep eutectic solvents are sensitive to variations in the halide component of the ammonium salt. This work not only highlights the benefits of novel deep eutectic solvents but also sets the stage for their strategic use in future battery electrolytes.