Reversible Sn4+/Sn0 anode for high-energy-density flow batteries

Aqueous batteries offer a relatively safe, energy-dense means to store electrochemical energy. Tin is a promising anode material, due to its potential four-electron redox-active states and lack of dendrite formation. However, the irreversible electrochemical behaviour of the Sn 4+ /Sn 2+ couple have limited high-capacity anode development. Here we achieve the reversible Sn 4+ /Sn 2+ conversion, enabling a Sn 4+ /Sn 0 anode for high-energy-density flow batteries. We accomplish this by adding iodide to highly soluble SnCl 4 solutions at ratios that form SnCl 6 2− and SnI 6 2− intermediate complexes. SnI 6 2− reduces reorganization energy during the electrochemical reaction of Sn 4+ /Sn 2+ , ensuring reversibility. SnCl 6 2− imparts anolyte solubility. The redox peak potential difference decreased from 978 mV (Cl − ) to 97 mV (I − ), with a substantial decrease to the electron transfer impedance decrease. Coupled with Br − /Br 2 , the assembled full battery showed 89% energy efficiency at 40 mA cm −2 , 227 Ah l −1 anolyte capacity and 205 Wh l −1 anolyte energy density. Based on these studies, we developed a kilowatt-scale stack that can operate stably for over 1,000 h.