Synergistic Ligand Engineering and Redox-Mediated Pathways for Stable Alkaline Tin-Based Redox Flow Batteries

Redox flow batteries (RFBs) are emerging as a promising technology for large-scale energy storage applications due to their low cost, high safety, and environmentally benign nature. Among them, aqueous tin-based redox flow batteries (TRFBs) have attracted increasing attention for their dendrite-free plating and suppression of the hydrogen evolution reaction. However, their development has been hindered by the low solubility of Sn(II) electrolytes, sluggish Sn(II)/Sn reaction kinetics, and the irreversible formation of inactive “dead Sn.” Herein, we present a dual strategy combining ligand engineering and redox-mediated reactions to overcome these limitations. By introducing ethylenediaminetetraacetic acid (EDTA), a stable EDTA–Sn(OH)3– complex is formed, which enhances the Sn(II) solubility up to 0.3 M and modulates the Sn deposition morphology, thereby improving diffusion behavior and redox kinetics. Furthermore, 7,8-dihydroxyphenazine-2-sulfonic acid (DHPS) is employed as a redox mediator, which spontaneously reacts with inactive Sn deposits, effectively dissolving “dead Sn” and significantly improving capacity retention. When paired with potassium ferrocyanide, the EDTA-Sn-DHPS/Fe RFB exhibits excellent rate capabilities, prolonged cycling stability (300 cycles at 80 mA cm–2 and 20 mAh cm–2), a high energy efficiency of 84.5%, and a peak power density of 228.6 mW cm–2. This strategy offers a promising route for the development of high-performance TRFBs.