Aqueous Zn-I flow batteries using low-cost porous membranes are promising for large-scale energy storage. However, capacity loss and low Coulombic efficiency due to polyiodide crossover hinder performance. This study develops colloidal chemistry for iodine-starch catholytes, creating enlarged active materials via chemisorption-induced aggregation. This size-sieving effect suppresses crossover, enabling high ionic conductivity porous membranes. The resulting flow battery achieves a high power density of 42 mW cm² at 37.5 mA cm², >98% Coulombic efficiency, and prolonged cycling (200 cycles at 32.4 Ah L⁻¹ posolyte, 50% state of charge), even at 50 °C. A scaled-up module with photovoltaic packs demonstrates practical renewable energy storage, and cost analysis shows a 14.3-fold reduction in installed cost due to cheap membranes.
