A High-Capacity Alkaline Tin–Iron Aqueous Redox Flow Battery with Stable Cycling Performance

High-capacity, low-cost alkaline metal aqueous redox flow batteries (ARFBs) are of great significance for large-scale energy storage. Among them, tin-based flow batteries have attracted increasing interest in recent years due to their high solubility of active materials and the advantages of less dendrite formation. However, the stability and reaction mechanism of Sn-based ARFBs still need to be further investigated. This study presents the design and demonstration of an alkaline Sn–Fe ARFB with K4[Fe(CN)6] and K2Sn(OH)6 in the catholyte and anolyte respectively, achieving a high-capacity and low-cost electrochemical energy storage system. The active material K2Sn(OH)6 exhibits a solubility above 3 M in an alkaline electrolyte at a temperature of 60 °C, resulting in an anolyte volume capacity of 321.6 Ah L–1. It is determined using density functional theory computation that the binding energy between the surface of Sn and copper is higher than that of carbon-based materials, which leads to the formation of uniform small-particle crystal nuclei on the surface of the copper. Furthermore, the operando electrochemical tests prove that the solubility of SnO22– is still one of the reasons that the energy efficiency cannot increase steadily with increasing concentration. A capacity retention of 74% is achieved after 5000 cycles with a stable voltage >1.3 V for the Sn–Fe ARFB. The demonstrated high-capacity and low-cost alkaline Sn–Fe ARFB shows superior performance in cycle life by alleviating the dendrite issue compared with Zn-based ARFBs, providing a promising Sn-based anolyte for high-energy metal-based ARFBs.