Modular dimerization of organic radicals for stable and dense flow battery catholyte

Aqueous organic redox flow batteries (AORFBs) hold promise for safe, sustainable and cost-effective grid energy storage. However, developing catholyte redox molecules with the desired stability, power and energy density remains challenging. In this study, we synthesized a class of ionic liquid-mimicking (2,2,6,6-tetramethylpiperidin-1-yl)oxyl dimers (i-TEMPODs) through a building-block assembly platform. By systematically investigating 21 i-TEMPOD derivatives, we uncovered the optimal size and charge properties that prevent membrane crossover and allow formation of a water-in-salt state. Leveraging these advances, we realized substantial improvements in AORFB performance using the optimum i-TEMPOD catholyte at 2 M concentration. These enhancements encompass several crucial metrics showcased across multiple experiments, including robust cycling stability without apparent capacity decay during 96 days of cycling, facile electrochemical kinetics with a high maximum power density of 0.325 W m−2 and a high full-cell energy density of 47.3 Wh l−1 in a capacity-balanced configuration. These molecular designs pave the way towards low-cost and scalable AORFBs.