Homogeneous Positive Surface Charge via Interfacial Amino-yne Polymerization for Efficient Li⁺/Mg²⁺ Separation

Efficient Li⁺/Mg²⁺ 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 Mg²⁺ 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⁺/Mg²⁺ separation, enhanced tolerance to Mg²⁺-rich feeds, and stable long-term performance.