Tuning the ferrous coordination structure enables a highly reversible Fe anode for long-life all-iron flow batteries

An aqueous all-iron flow battery is a promising alternative for large-scale energy storage applications due to its low cost and high safety. However, the inferior Fe plating/stripping reversibility and hydrolysis of Fe2+ at the anode significantly limit its capacity retention and lifespan. Herein, we propose a coordination strategy to delicately tune the coordination structure of Fe2+, enabling effective suppression of Fe2+ hydrolysis and a highly reversible Fe plating/stripping reaction. Firstly, citrate is screened to feature a strong ligand field with the largest splitting energy among various ligand anions. Subsequently, sodium citrate bearing a high LUMO and large binding energy is identified to be the most suitable additive for the anolyte. By adding sodium citrate into FeCl2, the formation of a highly stable Fe2+-citrate coordination structure is confirmed via carboxyl groups. This effectively alters the intrinsic [Fe(H2O)(6)](2+) structure and yields remarkably improved Fe deposition during charging, allowing a highly reversible Fe plating/stripping reaction at the anode. Finally, the all-iron flow cell adopting Fe2+-citrate anolyte delivers an averaged 100% CE for 300 charge-discharge cycles without capacity decay, which is the longest cycle-life reported in the open literature.