Long-life aqueous zinc-iodine flow batteries enabled by selectively intercepting hydrated ions

Aqueous Zn-I flow batteries are attractive for grid storage owing to their inherent safety, high energy density, and cost-effectiveness. However, Zn anode deposition/dissolution reactions cause severe water migration owing to ionic imbalance, especially under harsh conditions with high state-of-charge and high areal/volumetric capacities, further exacerbating intrinsic challenges for practical Zn-I systems. Herein, we develop a tailored ionic-molecular sieve membrane to regulate the transport behaviors of water/hydrated ion clusters, enabling the electrolyte balance by precise size sieving effects. Systematic investigations of different subnanometer pore sizes reveal that the optimal range (0.55-0.65 nm) can selectively intercept large hydrated ion clusters and reduce polyiodide shuttling. In this way, Zn-I flow batteries with this membrane exhibit a stable cycling over 2000 h (500 cycles) under harsh conditions (50% state-of-charge), achieving 66.4 mAh cm^-2/53.2 Ah L^-1_posolyte/27.66 Wh L^-1_system. This systems also deliver a low self-discharging rate, retaining a high Coulombic efficiency of 98.5% after 3 days static flow. Furthermore, techno-economic cost analysis reveals a competitive levelized cost of long-term energy storage for systems incorporating this membrane (551.98 USD MWh^-1 at an energy-to-power ratio of 18 h). This work offers insights into controlling water transport behaviors for realizing long-life flow batteries.