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 Sn⁴⁺/Sn²⁺ couple have limited high-capacity anode development. Here we achieve the reversible Sn⁴⁺/Sn²⁺ conversion, enabling a Sn⁴⁺/Sn⁰ anode for high-energy-density flow batteries. We accomplish this by adding iodide to highly soluble SnCl₄ solutions at ratios that form SnCl₆^2- and SnI₆^2- intermediate complexes. SnI₆^2- reduces reorganization energy during the electrochemical reaction of Sn⁴⁺/Sn²⁺, ensuring reversibility. SnCl₆^2- 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^-/Br₂, 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.