Molecular Tailoring of Iron Chelates for Long-Cycling and High-Efficiency All-Iron Redox Flow Batteries.

All-soluble all-iron redox flow batteries (AS-AIRFBs) represent a highly promising next-generation technology for long-duration energy storage, leveraging their low cost, decoupled energy/power characteristics, and controllable side reactions. Achieving long-term cycling stability necessitates a stable Fe2+/3+-ligand chelate to mitigate the electrolyte crossover, which is the main bottleneck for the flow batteries, in particular for AIRFBs. Herein, we designed a novel ligand─2-hydroxy-1,3-propanediamine tetra-sulfonate sodium salt (HPDTS)─through tailoring both ligand structure and chelate configuration. Benefiting from the macromolecular and flexible ligand framework, HPDTS dramatically accelerates the electron transfer kinetics of Fe2+/Fe3+, leading to the highest energy efficiency (EE: 71.79% at 200 mA cm-2) and power density (446.9 mW cm-2 at 623.39 mA cm-2) reported so far. In particular, unlike the traditional penta-coordinate chelate, HPDTS forms a tightly bonded symmetric hexa-coordinate Fe chelate with thermodynamic stability, which not only inhibits the side reaction (e.g., decomposition and HER) but also suppresses the electrolyte crossover when paired with a nonfluorinated proton exchange membrane. Electrochemical testing reveals the record-long lifespan of 15,000 cycles with an ultralow fading rate of 0.000741% per cycle and an ultrahigh Coulombic efficiency of 99.25%. Furthermore, practical AIRFB was also demonstrated to stably cycle for over 6500 cycles without any decay. The new molecular configuration tailoring strategies presented here provide a new paradigm for the development of high-efficiency and long-life AIRFBs.