Imidazolium-functionalized liquid ferrocene derivative positive material enables robust cycling stability of non-aqueous redox flow battery

The non-aqueous redox flow battery (NARFB) is a very promising technology for the grid-scale energy storage; however, its wide application is seriously limited by the cycling stability. Here, we designed a ferrocene derivative by imidazole-functionalization and grafting an oligoether chain, 1-ferrocenylmethyl-3-(2-(2-(2-methoxyethoxy)ethoxy)ethyl) imidazolium-bis(trifluoromethylsulfonyl)imide (FcMITEGTFSI). The imidazolium cationic group induces a positive shift in the redox potential of the ferrocene core whilst ensuring the stability of the active species; the flexible oligoether side chain renders the molecule a liquid at room temperature (25 °C) and miscible with acetonitrile solvent. The flow battery shows robust cycling stability, with an overall discharge capacity retention of ≈95.5% after 200 cycles, corresponding to a capacity retention of ≈99.98% per cycle at 25 mA cm −2 with 0.1 M electrolyte.