Ferrocene/Phthalimide Ionic Bipolar Redox-Active Molecule for Symmetric Nonaqueous Redox Flow Batteries

Symmetric nonaqueous redox flow batteries (NARFBs) that utilize bipolar redox-active organic molecules (BROMs) provide a facile strategy to mitigate the crossover issue. However, their performance has lagged behind due to the low solubility of organic redox species and poor high-current operations. To address these technical hurdles, a series of ionic BROMs based on ferrocene (Fc) and phthalimide (Ph) moieties with fast mass and charge-transfer kinetic are synthesized, which show high solubility and ionic conductivity. Both computational and experimental results show that the extended chain length between phthalimide moiety and quaternary nitrogen atom and the acidity of the solvent play a pivotal part in determining the stability of active materials and thus the cycling stability of NARFB. The assembled symmetric NARFB shows an open-circuit voltage of 2.04 V, cycling capacity retention of 99.8% per cycle, and energy efficiency of 77.0% over 50 cycles at 20 mA cm–2. Furthermore, the battery yields a peak power density of 110 mW cm–2 at 90 mA cm–2, which outperforms most NARFBs. This work demonstrates a promising molecular engineering strategy to improve the cycling stability of BROMs and to enable the high-current operation of symmetric NARFB.