Nanoconfinement Engineering of Covalent Organic Frameworks in Polyamide Membranes for High‐Perselectivity Li ⁺ /Mg ²⁺ Separation

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Efficiently separating Li + over Mg 2+ from brines remains a significant challenge due to their minimal size difference of only several angstroms, posing a substantial difficulty for traditional nanofiltration (NF) membranes. Therefore, a nanoconfinement regulation strategy utilizing a porous covalent organic framework (COF) layer is proposed to precisely control the interfacial polymerization (IP) process, thereby obtaining a thin, uniform polyamide (PA) membrane for high‐efficiency Li + /Mg 2+ separation. The nanoconfinement microenvironment fundamentally modulates both monomer spatial distribution and reaction kinetics through porous confined space and abundant interaction sites provided by the COF layer. Consequently, the resulting TpPa‐S/PA membrane exhibits a narrower pore size distribution, achieving an ion sieving precision of 0.46 Å. Due to the strict size sieving effect, the Li + /Mg 2+ separation factor exceeded 120, which is one to two orders of magnitude higher than all the currently reported NF membranes. This study addressed the typical limitations inherent in conventional NF membranes, establishing a promising foundation for advancing lithium extraction technologies through nanoconfinement strategy‐regulated IP processes.