A 10 nm Polyamide Membrane with an Asymmetric Charge Structure for Efficient Lithium Extraction

Polyamide membranes are essential for lithium extraction from salt-lake brines, yet optimizing their ion-sieving efficiency remains difficult. Herein, we report an ultrathin polyamide membrane (∼10 nm thick) with a low surface roughness (R_a = 1.18 nm) and an engineered asymmetric charge distribution. The membrane combines a positively charged upper layer, a negatively charged lower layer, and an interfacial region with narrower pores formed via interlayer interpenetration. This architecture promotes differential dehydration of Mg²⁺ and Li⁺ at the upper layer and amplifies selectivity through the interfacial region. Passage through the positive layer increases ion charge exposure, enhancing diffusion selectivity within the negatively charged interior and raising the transmembrane energy barrier for Mg²⁺ relative to Li⁺. The membrane achieves a Li⁺/Mg²⁺ selectivity of 69 and a water permeation flux of 12 L m^-2 h^-1 bar^-1, surpassing commercial benchmarks. Its ultrasmooth surface and high hydrophilicity minimize fouling, maintaining a flux recovery above 90% under high contaminant loads. Scaled-up modules using a three-stage nanofiltration process reduced the Mg²⁺/Li⁺ ratio in a concentrated brine simulant (25 g L^-1) from 50 to 0.2, highlighting strong potential for industrial lithium recovery.