‘Innocent’ Hexafluorophosphate Salts Induce Capacity Fade in Non-Aqueous Redox Flow Batteries

The use of organic active materials in redox flow batteries (RFBs) presents a promising approach to sustainable large-scale energy storage. However, the cycling stability of non-aqueous organic RFB electrolytes is generally limited by the occurrence of degradation reactions that cause capacity fade. These reactions are commonly thought to form products that are no longer electrochemically active. Here we study capacity fade in non-aqueous RFBs using the bipolar 1,2,4-benzotriazin-4-yl radical (1) in a symmetric cell (10/-║10/+). Active material crossover does not contribute to capacity fade in these symmetric RFBs, allowing the determination of intrinsic electrolyte stability. We show that the negative redox reaction (10/-) acts as a ‘reporter’ of electrolyte degradation, which reveals that capacity fade occurs in a non-linear (autocatalytic) fashion caused by acid-induced decomposition of the commonly used supporting salt anion PF6- in the posolyte solution. This is shown to be a universal degradation reaction in non-aqueous RFBs with PF6- supporting salts. Although the posolyte is not negatively affected by acidic degradation products, crossover of acid to the opposite compartment leads to capacity-limiting protonation of the negolyte active material. By replacing PF6- with other anions, the stability of these non-aqueous electrolytes was substantially improved. This allowed the construction of a symmetrical RFB based on 0.38M of active material 1 that can be cycled for >43 days with very high capacity retention.