Redox flow batteries (RFBs) are one of the most attractive electroche-mical storage systems, which can be feasibly sizeable and need low maintenance. We demonstrated triiodide/iodide (I3^-/I^-) redox couple in aqueous catholyte, delivering high storage capacity (~98% of the theoretical capacity), Coulombic efficiency (＞99.5%) and cycling performance (＞99.5% capacity retention for 100 cycles) when linked with Li metal as the anode (with nonaqueous electrolyte and imper-meable separator). The high performance for aqueous Li-I battery is contributed to a suitable redox potential of ~3.5 V vs. Li+/Li in addition to high solubility, which offers a high energy density of ~0.33 kWh kg^-1. In addition, we also present a new research approach to design redox-couple molecules using coordination compounds, which allows us to feasibly tune redox potentials and guide design of promising redox-couple molecules.