Positively charged nanofiltration membranes via ion-track technology for high-efficiency Mg2+/Li+ separation

The escalating demand for lithium-ion batteries necessitates efficient extraction technologies for lithium-rich brines, where the formidable challenge lies in selectively separating Li + from ubiquitous Mg 2+ ions due to their similar sizes. This work addresses this critical separation bottleneck by engineering a high-performance positively charged nanofiltration membrane. We leverage ion-track technology to fabricate a polyethylene terephthalate (PET) substrate featuring vertically aligned, uniformly distributed nanochannels and a hydrophilic, carboxyl-functionalized surface. This optimized substrate facilitates the controlled interfacial polymerization of polyethyleneimine (PEI) and 1,3,5-benzenetricarbonyl chloride (TMC), resulting in a defect-free, robust polyamide selective layer with strong interfacial adhesion. The resultant PEI-TMC/PET-5 membrane exhibits exceptional separation performance, achieving a remarkably high water permeance of 7.6 L m −2  h −1 bar −1 coupled with an outstanding Mg 2+ /Li + separation factor of 29.6. Comprehensive analysis reveals that this superior selectivity stems from the synergistic interplay of the Donnan effect and dielectric exclusion: the membrane's strong positive surface charge electrostatically repels divalent Mg 2+ more effectively than monovalent Li + , while the dielectric discontinuity at the membrane-solution interface imposes significantly higher Born solvation energy and image charge forces on Mg 2+ due to its greater charge density. Critically, the membrane demonstrates notable long-term stability, maintaining 94.5 % Mg 2+ rejection and a stable permeance of 5.1 L m −2  h −1 bar −1 over 720 h of continuous filtration. Under simulated brine conditions (Mg 2+ /Li +  = 80:1, 2000 ppm), it further sustains stable performance over two weeks of operation, with a S Li / Mg of 28.8. This study presents a rational substrate-to-active-layer design strategy, underpinned by fundamental insights into ion transport mechanisms, paving the way for scalable and energy-efficient membrane technologies for sustainable lithium extraction from high-magnesium brines.