A Janus nanofiltration membrane synergistically constructed by a CNC-COOH interlayer and crown–ether channels for high-efficiency lithium extraction

The increasing demand for lithium resources highlights the need for nanofiltration (NF) membranes with high permeability and selectivity for processing salt-lake brines. In this study, an ultrathin Janus NF membrane featuring Li + -selective nanochannels was fabricated via interfacial polymerization for Li + /Mg 2+ separation. Carboxylated cellulose nanocrystals (CNC-COOH) were deposited onto a polyvinylidene fluoride (PVDF) substrate to form an interlayer, and 15-crown-5 (15-C-5) was incorporated into the polyamide (PA) layer, yielding the PVDF/CCNC-15C5-PA membrane. The CNC-COOH interlayer increased membrane hydrophilicity and surface negative charge, and synergistically interacted with 15-C-5 to enhance the interaction with polyethyleneimine, resulting in a thin, defect-free PA layer of approximately 63 nm. Meanwhile, 15-C-5 formed host–guest complexes with Li + , establishing Li + -selective nanochannels that promoted Li + transport. The combined modifications yielded a PVDF/CCNC-15C5-PA membrane with a water permeance of 15.4 L·m -2 ·h -1 ·bar -1 and a Li + /Mg 2+ separation factor of 17.8. This work presents a dual-functional design strategy that integrates interfacial engineering with molecular recognition for advanced ion-selective separation.