Homogeneous Positive Surface Charge via Interfacial Amino‐yne Polymerization for Efficient Li + /Mg 2+ Separation

ABSTRACT Efficient Li + /Mg 2+ separation is essential for lithium recovery from salt‐lake brines, yet remains highly challenging because of the similar physicochemical properties of these ions and the large excess of Mg 2+ in practical feed streams. Polyamide (PA) membranes offer a promising solution, representing the most widely used platform. However, their intrinsically negative surface of conventional PA membranes limits selectivity under Mg 2 + ‐rich conditions. Although introducing positive charges can enhance selectivity through Donnan exclusion, current approaches rarely achieve a homogeneous distribution. Such interfacial charge heterogeneity prevents the formation of a uniformly selective transport pathway and thus limits the membrane separation performance. Herein, we introduce an interfacial amino‐yne click polymerization strategy that simultaneously incorporates positive charges and promotes their homogeneous distribution, yielding a defect‐suppressed membrane with a uniformly positive surface. By avoiding hydrolysis‐prone acyl chloride chemistry, this strategy eliminates hydrolysis‐induced negatively defects and forms a continuous, amine‐rich selective layer. Multiscale characterization reveals that high‐molecular‐weight polyethyleneimine (PEI) suppresses local structural defects and bridges nanoscale gaps in charge distribution, thereby establishing a homogeneous positive surface. Owing to the synergistic effects of these surface properties, the optimized membrane exhibits markedly improved Li + /Mg 2+ separation, enhanced tolerance to Mg 2+ ‐rich feeds, and stable long‐term performance.