Regulating the Crystalline Structure and Ion Affinity of Covalent Organic Frameworks for Enhanced Lithium/Magnesium Separation

Selective ion transport is essential for many applications of membrane separation, such as rare metal and high-value element extraction from complex ionic sources. However, efficient regulation of permeability-selectivity remains a major challenge for advanced ionic transport membranes. Herein, we demonstrate that supercritical CO₂ (ScCO₂) drying combined with crown ether functionalization enables precise modulation of crystallinity and ion-specific affinity in covalent organic framework (COF) membranes. The pristine COF membrane prepared by solution casting was amorphous. Owing to its positively charged framework and sub-nanometer pores, the membrane exhibited a high Li⁺ transport rate over Mg²⁺ via a synergistic effect of size exclusion and electrostatic repulsion, resulting in a selectivity of 204. After ScCO₂ drying, the crystallinity and structural ordering of the COF membrane were significantly enhanced, leading to a 1.5-fold increase in Li⁺ flux, accompanied by a moderate decrease in selectivity to 147. To compensate for this trade-off, 12-crown-4 (12C4) was introduced as a Li⁺ recognition agent into the ScCO₂-treated membrane, restoring Li⁺/Mg²⁺ selectivity to 187 without compromising Li⁺ flux. Importantly, the selective Li⁺ transport performance was maintained in real salt lake brines. This structural-chemical co-regulation strategy provides a versatile approach for optimizing ion transport membranes in complex separation applications.