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.