Electronanofiltration Membranes with a Bilayer Charged Structure Enable High Li⁺/Mg²⁺ Selectivity

Achieving separation of lithium and magnesium with similar radii is crucial for the current lithium extraction technology from salt lakes, which usually possess a high lithium-to-magnesium ratio. Herein, we proposed the facile sequential interfacial polymerization (SIP) approach to construct electronanofiltration membranes (ENFMs) with a bilayer charged structure consisting of a high positively charged surface and a negatively charged sublayer. The trimesoyl chloride (TMC) concentration was adjusted to enhance the -COOH content and negative charge of the polyamide sublayer to promote Li⁺ migration, and then the quaternized polyethylenimine was introduced to the membrane surface by the SIP process to increase the positive charge density on the surface of the ENFMs, which would block the migration of Mg²⁺ and enhance the Li⁺/Mg²⁺ selectivity of the ENFMs. The optimal quaternary-modified ENFMs achieved outstanding selectivity for Li⁺/Mg²⁺ (49.85) and high Li⁺ flux (4.10 × 10^-8 mol cm^-2 s^-1) at a current density of 10 mA cm^-2. Moreover, in simulated brines with low lithium concentration and high Mg²⁺/Li⁺ ratio, the optimal ENFMs also displayed elevated Li⁺/Mg²⁺ selectivity (>45), highlighting the substantial promise of the membranes for practical applications.