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Abstract Redox flow batteries (RFBs) with high energy densities are essential for efficient and sustainable long‐term energy storage on a grid scale. To advance the development of nonaqueous RFBs with high energy densities, a new organic RFB system employing a molecularly engineered tetrathiafulvalene derivative ((PEG3/PerF)‐TTF) as a high energy density catholyte was developed. A synergistic approach to the molecular design of tetrathiafulvalene (TTF) was applied, with the incorporation of polyethylene glycol (PEG) chains, which enhance its solubility in organic carbonate electrolytes, and a perfluoro (PerF) group to increase its redox potential. When paired with a lithium metal anode, the two‐electron‐active(PEG3/PerF)‐TTFcatholyte produced a cell voltage of 3.56 V for the first redox process and 3.92 V for the second redox process. In cyclic voltammetry and flow cell tests, the redox chemistry exhibited excellent cycling stability. The Li|(PEG3/PerF)‐TTFbatteries, with concentrations of 0.1 M and 0.5 M, demonstrated capacity retention rates of ~94 % (99.87 % per cycle, 97.52 % per day) and 90 % (99.93 % per cycle, 99.16 % per day), and the average Coulombic efficiencies of 99.38 % and 98.35 %, respectively. The flow cell achieved a high power density of 129 mW/cm². Furthermore, owing to the high redox potential and solubility of(PEG3/PerF)‐TTF, the flow cell attained a high operational energy density of 72 Wh/L (100 Wh/L theoretical). A 0.75 M flow cell exhibited an even higher operational energy density of 96 Wh/L (150 Wh/L theoretical). 

Abstract Redox flow batteries (RFB) have emerged as one of the most promising technologies for large‐scale energy storage owing to their high safety, long operation life, and decoupled design of energy and power. However, the problems of high cost and low energy density restrict their further development. The cost merit and tunable structure of organic redox‐active materials have prompted the development of organic RFBs. The solubility of the redoxmer is recognized as a parameter that contributes directly to the energy density. Herein, we focus on strategies for enhancing the solubility of organic redoxmers in aqueous RFBs. The effects of incorporating different hydrophilic functional groups on the solubility of the redoxmer and its effect on the performance of other batteries are systematically and exhaustively described. Other strategies, such as molecular symmetry tuning and employing more soluble counterions and cosolvents, are also summarized. The development trends and prospects for organic RFBs are also discussed.