"Innocent" Hexafluorophosphate Salts Induce Capacity Fade in Nonaqueous 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 stability of nonaqueous organic RFB electrolytes is generally limited by degradation reactions that cause capacity fade. These reactions are commonly thought to convert redox-active organics to products that are no longer electrochemically active. Here we uncover an additional pathway leading to capacity fade that involves the supporting electrolyte salt. Capacity fade in nonaqueous RFBs is studied in detail for the 1,2,4-benzotriazin-4-yl radical (1) as a model compound and extended to several other classes of representative redox-active organics. By using symmetrical batteries (1^0/–∥1^0/+), we delineate that capacity fade occurs in a nonlinear (autocatalytic) fashion via acid-induced decomposition of the supporting salt anion PF₆^– in the posolyte solution. This is shown to be a universal degradation reaction in the posolyte of nonaqueous RFBs. Although the acidic degradation products are not detrimental to the posolyte, the crossover of acid to the opposite compartment leads to capacity-limiting protonation of the negolyte active material. Replacement of PF₆^– with other anions substantially improves the stability of these nonaqueous electrolytes, as demonstrated with a symmetrical RFB based on 0.38 M active material 1 that can be cycled for >69 days with very high capacity retention (fade rate of ≤0.1% per day). The improved understanding of factors determining the lifetime of nonaqueous electrolytes unlocks rational strategies to develop more durable electrochemical energy storage systems.