Ethylenediamine Derivative Ligands and Design Principles for High-Performance All-Iron Flow Batteries

Ligand design is crucial for enhancing all-iron flow batteries' (AIFB) electrochemical performance. This work proposes design principles of ligands for AIFB based on coordination chemistry theory, and designs a class of ethylenediamine derivative ligands. These principles integrate three key functions: (1) regulation of Fe^II/Fe^III potential through coordination functional groups, (2) enhancement of iron-complex solubility via hydrophilic modification and molecular asymmetry design, (3) improvement of cycling stability through hexadentate chelation and size exclusion effect. Herein, N,N'-dimethyl-N,N'-(2-(2'-pyridyl)-6-pyridylmethyl) ethylenediamine (EDMB) and N,N'-di(2-hydroxybenzyl) ethylenediamine-N,N'-diacetic Acid (HBED) are designed as catholyte and anolyte ligands of AIFB. An AIFB with a voltage of 1.32 V is designed, which can cycle stably for 13 000 times. To further enhance the battery's energy density, a more hydrophilic anolyte ligand, SHBED, is designed. Ultimately, the solubilities of Fe-EDMB and Fe-SHBED reach 1.85 and 1.97 mol L^-1, respectively. The AIFB can cycle normally with catholyte and anolyte concentrations of 1.5 and 1.25 mol L^-1. It shows record specific capacity and energy density of 18.27 Ah L^-1 and 21.01 Wh L^-1, almost doubled compared to the currently reported highest energy density of AIFB. Although this design principle is only validated on ethylenediamine derivative ligands, its underlying logic can be extended to other iron-based ligands.