Systematic Designs of Dicationic Heteroarylpyridiniums as Negolytes for Nonaqueous Redox Flow Batteries

Many organic redox materials are chemically unstable and sparingly soluble in nonaqueous media. Additionally, the crossover of redox materials and the availability of limited membranes have restricted the examination of the long-term cyclability of these materials in nonaqueous redox flow batteries (RFBs). To overcome these limitations, we developed a new class of pyridinium-based negolytes. The π-conjugation structure of the pyridinium molecules was extended by introducing benzothiazole into the C4-position of pyridinium, which improved the stability of these molecules. Cationic ammonium functional groups at the N-substituent suppressed the crossover of the pyridinium negolytes through an anion exchange membrane. Furthermore, the solubility of the negolyte was increased up to ∼1 M in acetonitrile and 0.3–0.5 M with tetrabutylammonium bis(trifluoromethanesulfonyl)imide (TBATFSI) and acetonitrile. A 0.1 M solution of the dicationic benzothiazolylpyridinium exhibited 0.0083% capacity-fading rate per cycle in symmetric RFBs for 250 cycles and 0.08% in full RFBs comprising the ammonium-substituted ferrocene as a posolyte for 500 cycles.