Ultrathin Poly(Ionic Liquid) Nanomembranes for High Performance Mg ²⁺ /Li ⁺ Separation

Abstract The fragility and brittleness of glassy poly(ionic liquid)s (PILs) hinders the processing of ultrathin PILs membranes. This work is reported the interfacial polymerization of PILs, which enables the first preparation of defect‐free PIL nanomembranes for high flux separation of lithium (Li + , 3.8 Å) and magnesium (Mg 2+ , 4.3 Å) ions mixture. Aqueous solution of an amine‐functionalized PIL is brought in contact with trimesoyl chloride (TMC) hexane solution. The “amine ≈ acyl chloride” condensation reaction occurs at the “water ≈ hexane” interface to form ≈50 nm thin PILNH 2 ‐TMC nanomembranes consisting of water‐stable nanoaggregates (≈20 nm in size). Water permeance of the PIL nanomembrane is 3 times as high as the state‐of‐the‐art polyamide nanofiltration membranes, while its MgCl 2 rejection is as high as 95%. Both the structures and separation performance of PIL nanomembranes are stable versus operation time and pressure. This work opens a new avenue to process fragile PILs, such as into functional nanomembranes for ion separation.