N-ferrocenylphthalimide; A single redox couple formed by attaching a ferrocene moiety to phthalimide for non-aqueous flow batteries

N-ferrocenylphthalimide (FcPI) is prepared by replacing the hydrogen atom at the N-site of phthalimide with a ferrocene moiety. Phthalimide shows irreversible redox reaction due to H atom at the N-site of phthalimide. By replacing H atom of phthalimide with non-detachable functional group, such as alkyl group, redox reaction becomes reversible. Reversibility of redox reaction is requirement for redox couple due to cycling performance in battery system. In this work, electroactive group, not the simple functional group, is introduced to substitute H atom of phthalimide. As an electroactive group, ferrocenyl group has two main benefits. First, the redox reaction of the initial phthalimide with H atom at the N-site becomes reversible at E1/2 = −1.87 V (vs. Fc/Fc+) with an excellent stability. Second, FcPI is a single redox couple with Ecell = 1.94 V in a non-aqueous electrolyte because the ferrocene moiety in the molecule is also redox-active at E1/2 = 0.07 V (vs. Fc/Fc+) with stability. A non-flowing coin-type cell employing FcPI as a single redox couple exhibits an excellent Coulombic efficiency of 97.3% and stable cycle performance during the 50th cycle. Furthermore, rate test realizes fast charge transfer kinetics as voltage efficiency of 96.5-90.1% at 1.0-3.0 C.