Modular TEMPO Dimerization for Water-in-Catholyte Flow Batteries with Extreme Energy Density, Power, and Stability

Aqueous organic redox flow batteries (AORFBs) hold great promise for safe, sustainable, and cost-effective grid energy storage. However, developing catholyte redox molecules with desired energy density, power, and stability simultaneously has long been a critical challenge for AORFBs. Here, we report a novel class of ionic liquid mimicking TEMPO dimers (i-TEMPODs) that can be produced by our newly developed building block assembly synthetic platform. By systematically investigating 21 derivatives, we reveal i-TEMPODs have optimized size and charge that is compatible with highly conductive membrane and can form a “water-in-catholyte” (WiC) state. The tight coordination dynamics with water molecules deliver extreme solubility with promoted electrochemical stability at highly positive potentials. Leveraging these advances, we identify a champion molecule and demonstrate record overall AORFB performance in energy density (47.3 Wh/L), power density (0.325 W/cm2), and stability (no apparent capacity decay after 96 days) with low-cost and scalable chemistry.