Space‐Confined Synthesis of Thinner Ether‐Functionalized Nanofiltration Membranes with Coffee Ring Structure for Li+/Mg2+ Separation

Abstract Positively charged nanofiltration membranes have attracted much attention in the field of lithium extraction from salt lakes due to their excellent ability to separate mono‐ and multi‐valent cations. However, the thicker selective layer and the lower affinity for Li + result in lower separation efficiency of the membranes. Here, PEI‐P membranes with highly efficient Li + /Mg 2+ separation performance are prepared by introducing highly lithophilic 4,7,10‐Trioxygen‐1,13‐tridecanediamine (DCA) on the surface of PEI‐TMC membranes using a post‐modification method. Characterization and experimental results show that the utilization of the DCA‐TMC crosslinked structure as a space‐confined layer to inhibit the diffusion of the monomer not only increases the positive charge density of the membrane but also reduces its thickness by ≈35% and presents a unique coffee‐ring structure, which ensures excellent water permeability and rejection of Mg 2+ . The ion–dipole interaction of the ether chains with Li + facilitates Li + transport and improves the Li + /Mg 2+ selectivity ( S Li,Mg = 23.3). In a three‐stage nanofiltration process for treating simulated salt lake water, the PEI‐P membrane can reduce the Mg 2+ /Li + ratio of the salt lake by 400‐fold and produce Li 2 CO 3 with a purity of more than 99.5%, demonstrating its potential application in lithium extraction from salt lakes.