Constructing a selective blocked-nanolayer on nanofiltration membrane via surface-charge inversion for promoting Li+ permselectivity over Mg2+

The separation of Li+ and Mg2+ is a decisive step for achieving high-grade lithium extraction from salt-lake brines to circumvent the lithium supply shortage and sustainability. However, polymer nanofiltration membranes suffer from low lithium selectivity and are hindered by complicated preparation and miniaturized scales. Here, we report a facile surface charge-inversion strategy via a unique secondary non-aqueous medium interfacial reaction of polyethyleneimine to construct a selective positive-nanolayer on nanofiltration membranes surface for selective separation of lithium ions from concentrated magnesium/lithium mixtures. The introduction of amine moieties not only increases the Donnan potential but also improves the compactness of the membrane, which simultaneously enhances the Donnan exclusion and size sieving for the Mg2+ ion with higher valence and larger hydration ion size. The surface interaction theory (DLVO) verified that the high positive-charge density provided an excellent selectivity to allow passage of Li + but reject Mg2+, and the separation factor (SLi/Mg) achieved 12.37 at a high Mg2+/Li+ mass ratio of 150. Such highly positive transmembrane potential barrier simultaneously endowed the membrane surface with the anti-fouling performance against Ca2+. We anticipate that our facile and scalable surface charge-inversion will greatly advance lithium ions permselectivity membranes design and application in lithium extraction.