Crown‐Ether‐Based Covalent Organic Network Membranes for Hydration‐Governed Li + /Mg 2+ Separation

ABSTRACT Selective Li + /Mg 2+ separation remains a central challenge for sustainable lithium extraction because ion transport in confined aqueous environments is governed by coupled steric and hydration effects rather than ionic size alone. Here, we report a crown‐ether‐based covalent organic network membrane for hydration‐governed mono/divalent ion sieving. Unlike conventional crown‐ether materials that rely on cavity‐specific host‐guest recognition, the conformational flexibility and steric hindrance of dibenzo‐18‐crown‐6 induce locally ordered yet globally disordered packing, transforming the nominal ∼3 nm intrinsic cavity into sub‐nanometer transport channels with an effective aperture of ∼0.9 nm. Structural analyses reveal that the exposed ether oxygen atoms of crown‐ethers reconstruct oxygen‐rich confined hydration interfaces that regulate hydrated‐ion partitioning and migration. Thus, the membrane exhibits high mono/divalent ion discrimination, delivering a Li + /Mg 2+ selectivity of ∼3900 together with high Li + flux, and also demonstrating efficient K + /Mg 2+ and Cs + /Sr 2+ separation with selectivities of ∼3140 and ∼840, respectively. Spectroscopic studies, density functional theory calculations, and molecular dynamics simulations collectively reveal a dual physical‐thermodynamic blockade mechanism, where steric confinement suppresses hydrated divalent ions and hydration‐mediated thermodynamic compensation facilitates monovalent ion transport. The membrane maintains robust performance under concentrated and Mg‐rich conditions, highlighting the potential of engineered transport architectures for lithium extraction and precise ionic separations.