Ultrathin Membrane With Ion‐Specific Gating Enables Interfacial Decoupling of Conductivity–Selectivity for Aqueous Flow Batteries

Ion‐exchange membranes (IEMs) are essential for efficient and durable flow batteries, yet their inherent conductivity–selectivity trade‐off severely limits flow battery performance. Most conventional strategies to improve selectivity rely on regulating ion diffusion within ion‐exchange channels through size exclusion and electrostatic repulsion, inevitably compromising ionic conductivity. Here we shift the paradigm to ion partitioning at the membrane–electrolyte interface by establishing ion‐specific gating that selectively intercepts active species while preserving rapid charge carrier transport. The developed ultrathin (29 ± 2 µm) polar oxygen‐vacancy‐enhanced (POVE) membrane suppresses polysulfide crossover through strong chemisorption and a strengthened hydration layer, delivering over 3.4 times the selectivity of the base membrane at comparable conductivity and over 24.8 times the selectivity of commercial Nafion membranes. In the polysulfide–ferrocyanide flow battery, it achieved 82% energy efficiency at 40 mA cm −2 and sustained over 2000 cycles with negligible capacity decay, far exceeding 286 cycles for the base membrane and < 100 cycles for Nafion. This work decouples conductivity from selectivity via a scalable interfacial ion‐specific gating strategy, offering a transformative framework for developing affordable, high‐performance IEMs.