Tuning Ferrous Coordination Structure Enables a Highly Reversible Fe Anode for Long-life All-iron Flow Batteries

Aqueous all-iron flow battery is a promising alternative for large-scale energy storage applications due to low cost and high safety. However, 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 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 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 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 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 open literature.