Dual-skin layer nanofiltration membranes for highly selective Li+/Mg2+ separation

The wide application of lithium batteries in consumer electronics and electric vehicles bring about ever-rising demand of lithium resources globally. Nanofiltration (NF) membranes with high Mg2+/Li+ selectivity could achieve highly efficient separation of lithium from natural brines, which provides a good alternative for steady lithium supply. In this work, we developed a dual-skin layer nanofiltration membrane comprised of a polyethylene-imine (PEI) layer grafted on a polyamide layer. The polyamide layer was prepared via interfacial polymerization to endow the NF membrane with sub-nm pores, while the PEI layer was grafted on the polyamide surface via amidation reaction to further equip the membrane with enhanced positive charge. With the synergistic effect of size sieving from the polyamide layer and electrostatic exclusion from the PEI layer, the dual-skin layer NF membrane exhibits a high rejection of 98.5% to MgCl2 and a low rejection of 46.2% to LiCl, suggesting a high Mg2+/Li+ selectivity. As using this dual-skin layer NF membrane for separating Li+ from a simulated brine, a high Mg2+/Li+ separation factor (SMg,Li) up to 33.4 was achieved, which is the highest value among the pressure-driven polymeric nanofiltration membranes reported so far.