Dual‐functional regulation of nanofiltration membranes using <scp>covalent organic framework</scp> nanosheets for efficient Li ⁺ /Mg ²⁺ separation

Abstract With growing demand for lithium resources, developing high‐performance Li + /Mg 2+ separation methods for salt‐lake brines has become crucial, especially the highly efficient energy‐saving membrane technology. For the traditional nanofiltration (NF) membranes for mono−/multivalent ion separation, achieving simultaneously high permeance and selectivity for lithium extraction from brine remained a critical challenge. Herein, we proposed a kind of cationic covalent organic frameworks (COF), TpEBr nanosheets with positive charge and structural regulation capability as dual‐functional regulator for polyamide (PA) NF membranes. The introduction of TpEBr as additive reduced the diffusion rate of polyethyleneimine (PEI), forming a thin and loose membrane. Additionally, quaternary ammonium groups on TpEBr increased the membrane's surface Zeta potential, significantly strengthening the Donnan exclusion effect. The optimized membrane exhibited a significant Li + /Mg 2+ selectivity of 46.4 with a 25% increase in permeance. Such dual‐functional regulation strategy for constructing high‐performance NF membranes proposed in this work is promising for future highly efficient lithium extraction from brines.