Highly Stable Alkaline All-Iron Redox Flow Batteries Enabled by Disulfonated Ligands Chelation

Abstract

Alkaline all-iron flow batteries possess intrinsic safety and low cost, demonstrating great potential for large-scale and long-duration energy storage. However, their commercial application is hindered by the issue of capacity decay resulting from the decomposition of iron complexes and ligand crossovers. In this paper, an robust anolyte Fe(TEA-2S) is reported, which is formed by chelating iron ions with the sulfonate-enriched ligand (TEA-2S) in alkaline environment. By skillfully designing the ligands, the binding energy of the iron complexes increases and ligand crossovers are suppressed, making the iron complexes highly stable in the charge–discharge cycles. Alkaline all-iron flow batteries coupling with Fe(TEA-2S) and the typical iron-cyanide catholyte perform a minimal capacity decay rate (0.17% per day and 0.0014% per cycle), maintaining an average coulombic efficiency of close to 99.93% over 2000 cycles along with a high energy efficiency of 83.5% at a current density of 80 mA cm⁻². In addition, Fe(TEA-2S) exhibits high solubility of up to 1.85 м (with a theoretical capacity of up to 49.58 Ah L⁻¹), even at low temperatures as extreme as −30 °C. This work demonstrates a promising pathway toward achieving long-duration and large-scale energy storage.