Boosting Nanofiltration Membrane Selectivity via Amine‐Polymer Additive Engineering for Efficient Lithium Extraction From Brine

Nanofiltration (NF) membranes used in direct lithium extraction (DLE) from brine offer a more efficient and environmentally friendly alternative to traditional evaporation-based methods, and are promising for lithium enrichment owing to their ability to selectively reject multivalent cations. However, conventional polyamide (PA) NF membranes prepared using piperazine (PIP) suffer from low Li⁺/Mg²⁺ selectivity due to excessive negative surface charge and a trade-off between membrane permeance and selectivity. In this study, a scalable and facile strategy to enhance lithium separation performance by incorporating poly(allylamine) (PAA), an amine-rich polymer, into the aqueous PIP solution as an additive during the interfacial polymerization synthesis of PA is presented. PAA not only introduces additional positive charges into the PA network, improving Mg²⁺ rejection via the Donnan effect, but also alters monomer diffusion, leading to a crumpled PA morphology that contributes to water permeance. The resulting optimum membrane exhibited a high water permeance of 12.1 L m^-2 h^-1 bar^-1, a low LiCl rejection of 12.6 %, and significantly enhanced the rejection of MgCl₂ from 18.8 % to 94.7 %. Furthermore, a two-stage NF process using the optimized membrane effectively purified lithium from simulated salt lake brine, highlighting the potential of this strategy.