Ferrocene/naphthalimide bi-redox molecule for enhancing the cycling stability of symmetric nonaqueous redox flow battery

Derived from N-(ferrocenylmethyl)-N-(butylphthalimide)-N,N-dimethylammonium bis(trifluoromethane-sulfonyl)imide (FcPI-TFSI), ferrocene/naphthalimide (FcNI-TFSI) based ionic bipolar redox-active organic molecule is constructed via nucleophilic substitution between amines and halogenated hydrocarbons. Due to the elevated delocalized negative charge density in the radical anion, the FcNI-TFSI based nonaqueous redox flow batteries (NARFBs) exhibits substantially improved cycling stability; 764 and 233 cycles are successfully performed before 50% of the initial discharge capacity is lost at 0.01 M and 0.1 M, respectively, whereas 82 cycles for FcPI-TFSI at 0.01 M. Similar to capacity diving phenomenon in lithium-ion batteries, sudden accelerated capacity loss is observed in the high-concentration battery. The possible degradation mechanism is discussed, involving crossover issue and parasitic reactions.