Geometric-Topology-Driven Membrane Design for Suppressing Polysulfide Crossover in Aqueous Redox Flow Batteries

Developing highly selective ion-exchange membranes to suppress active-species crossover remains a critical challenge in aqueous redox flow batteries (ARFBs). Unfortunately, their practical performance is often compromised by hydration-induced membrane swelling, giving rise to non-selective water channels that facilitate catholyte-anolyte crossover. Herein, we show that geometric topology is a previously overlooked but decisive parameter governing membrane microstructure, hydration behavior, and ion selectivity. Using a coarse-grained molecular dynamics framework, we reveal a geometry-driven phase-separation mechanism under membrane hydration. Isotropic 0D geometric motifs remain uniformly dispersed and promote the formation of highly interconnected yet spatially confined hydration networks, whereas anisotropic one- and 2D geometric motifs exhibit a strong propensity to bundle and aggregate, inducing phase separation and interfacial voids that promote non-selective transport. Guided by this principle, a membrane incorporating 0D geometric motifs simultaneously achieves high cationic conductivity and strong polysulfide rejection, enabling stable operation of a polysulfide-based aqueous redox flow battery for over 700 h with Coulombic efficiencies exceeding ∼99.5% and peak power densities of ∼138 mW cm^{- 2}, dramatically outperforming conventional commercial membranes and demonstrating the effectiveness of the geometry-guided membrane design principle in advancing future membrane engineering.