A Highly Concentrated Catholyte Enabled by a Low-Melting-Point Ferrocene Derivative

Nonaqueous redox flow batteries (NRFBs) exhibit a wide potential window (>3.0 V) but have been limited by the low solubility of the active materials. Here, we propose and demonstrate a high-energy-density nonaqueous redox flow battery based on a low-melting-point (37–40 °C) ferrocene derivative, 1,1-dimethylferrocene (DMFc), operated at its liquid state. The liquid redox-active DMFc not only contributes to high capacity but also acts as a solvating medium to the ion-conducting salts. Taking advantage of DMFc’s high concentration (3 M), superior stability, and fast kinetics, the lithium/DMFc battery achieves a high volumetric density (∼68 Ah L –1 catholyte ) with a high Coulombic efficiency (>95%) and high cycling stability. Our work demonstrates that exploiting a low-melting-point redox-active species at its melting state is a promising direction for developing high-energy-density NRFBs for next-generation energy storage technologies.