Ultrathin Microporous Poly(quaternary ammonium) Membrane for Lithium Extraction and Recovery

Efficient lithium recovery from alkaline salt lake brines and acidic battery leachates demands nanofiltration (NF) membranes with high pH-resistance. Poly(quaternary ammonium) (PQA) has arisen as an ideal alternative. However, low monomer reactivity and uncontrolled interfacial monomer diffusion usually lead to thick PQA layer with broad pore size distribution, restricting the separation performance. Herein, a branched tertiary amine monomer, tris(2-dimethylaminoethyl)amine, which possesses active sites with higher N-exposure factor and homogeneous spatial distribution, was designed to accelerate the monomer reaction. Simultaneously, sodium dodecyl sulfate (SDS) was introduced to endow homogeneous monomer distribution and retarded monomer diffusion. Accordingly, the resultant PQA-T membrane exhibits ultralow thickness (∼15 nm) and narrow pore size distribution, exhibiting exceptional water permeance of 30.8 L m^-2 h^-1 bar^-1 and a high MgCl₂ rejection of 99.2 ± 0.4% (S_Li/Mg up to 136), overperforming all reported acid/alkali-resistant membranes. Notably, it shows high separation factors under acidic and alkaline conditions (S_Li/Mg = 81.8 at pH 10; S_Li/Co = 28.7 at pH 2), showing great potential for lithium extraction under harsh pH conditions, which is further confirmed by two-stage NF for both real salt lake brine and battery leachates. Additionally, we demonstrate large-area fabrication of the PQA membrane (∼625 cm²) while maintaining consistent separation performance.