Reconstructed Polyamide Nanolayers via Two‐Stage Interfacial Polymerization Engineering for Precise Ion Sieving

Efficient lithium extraction from brines remains challenging due to the comparable hydrated radii of Li⁺ and Mg²⁺ and the extreme ionic strength of the feed solutions. In this work, a reconstructed polyamide (PA) nanofiltration membrane was developed via a two-stage interfacial polymerization strategy, achieving simultaneous structural and electrostatic engineering of the PA selective layer. In the first stage, the relationship between substrate physicochemical properties and the resulting density and stability of the nascent polyamide layer was established through regulation of piperazine (PIP) adsorption-diffusion behavior. In the second stage, careful selection of a non-aqueous solvent effectively suppressed acyl chloride hydrolysis and preserved abundant active sites, allowing the successful incorporation of a bidentate quaternary ammonium monomer into the newly formed PA network. This reconstruction generated a confined sub-nanometer selective layer with a tunable mild positive charge, enabling synergistic steric and electrostatic discrimination between Li⁺ and Mg²⁺. The optimized membrane exhibited excellent Li⁺/Mg²⁺ separation factors that exceeded 60 under diverse operating conditions, while the integrated nanofiltration process achieved nearly 60-fold lithium enrichment, demonstrating a practical applicability in complex brine matrices. This study establishes a generalizable molecular-level design reference for co-ion selective membranes capable of lithium extraction under chemically demanding, high-ionic-strength conditions.