Polyethylene glycol modified tetrathiafulvalene for high energy density non-aqueous catholyte of hybrid redox flow batteries

•Bulky and polar polyethylene glycol tails were attached to tetrathiafulvalene.•Viscous oils with infinite miscibility with electrolyte solvents were obtained.•Excellent electrochemical stability was achieved with high redox potentials.•Cross-over of active species within flow batteries was addressed by membrane design.•Excellent flow battery performance was achieved and supported by DFT calculations. Redox flow batteries (RFBs) are highly promising electrochemical systems for large-scale energy storage. To develop high energy density non-aqueous catholyte for RFBs, herein, two polar and bulky polyethylene glycol tails have been attached to the tetrathiafulvalene (TTF) core by ester exchange reactions. Such modification turns scarcely soluble TTF particles into viscous oils, which are totally miscible with the conventional organic electrolytes. The 4,4′(5′)-bis(2-(2-methoxyethoxy)ethyl)carboxylate -tetrathiafulvalene (TTF-BMEEC) with a mass-based specific capacity of 107.84 mAh g−1 is systematically evaluated in a hybrid RFB. When paired with a Li foil electrode and a permselective separator, the catholyte consisting of 1.0 M of TTF-BMEEC delivers two high discharge voltage plateaus of 3.55 and 3.88 V, a large volumetric capacity of 49.6 Ah L−1, an outstanding energy density of 178.0 Wh L−1, and a high cycling capacity retention of 91.5% after 100 cycles at the current density of 10 mA cm−2. The excellent electrochemical stability of TTF-BMEEC is confirmed by UV–vis and 1H NMR investigations and supported by density functional theory (DFT) calculations. These results demonstrate that molecular engineering has great potential for the development of high performance organic redox-active materials for RFBs.