Sulfonated-Ligand Engineering Enables a Stable Alkaline All-Iron Ion Redox Flow Battery

Alkaline all-iron ion redox flow batteries (RFBs) are considered promising devices for large-scale energy storage due to their remarkable resistance to dendrite formation and the hydrogen evolution reaction. However, the decomposition of negative complexes and ligand crossover issues have limited their stable operation. Herein, we have developed a tetra-sulfonated ethylenediamine derivative ligand (EDTS) to chelate with iron ions to serve as the negative active species (Fe(EDTS)). Benefiting from its stable hexa-coordinated structure, the Fe(EDTS) chelate exhibits unprecedented stability during long-term cycling. Additionally, the EDTS ligand with high charge density and large volume effectively mitigates crossover through size exclusion and Donnan effects. Paired with the Fe(CN)6 electrolyte, the Fe(CN)6/Fe(EDTS) RFB demonstrates a formal cell voltage of 1.25 V, a high energy density of 25.04 Ah L–1, and an unprecedented durability among reported capacity-symmetric all-iron RFBs, achieving a Coulombic efficiency of 99.93% and capacity retention of 96.08% after 3000 cycles.