Polyethylene Glycol‐Regulated Finger‐Sponge Substrate Enabling High‐Performance Nanofiltration Membranes for Efficient Mg 2+ /Li + Separation

Efficient lithium recovery from salt‐lake brines relies on the precise discrimination and lithium ions separation from magnesium ions. In this study, polyethersulfone (PES) ultrafiltration (UF) substrate layer featuring finger‐like and optimized finger‐sponge transitional structures were fabricated by varying polyethylene glycol (PEG) concentrations via phase inversion. These substrate layers served as platforms for the growth of polyamide (PA) active layers through interfacial polymerization (IP). The performance of the synthesized nanofiltration (NF) membranes was assessed via the fractionation of Mg 2+ and Li + from high Mg/Li ratio brines. These results indicate the substrate layer with a finger‐sponge transitional structure exhibits higher surface hydrophilicity. The PA NF membranes constructed on this substrate layer exhibit significantly enhanced divalent cations (Mg 2+ ) rejection, while maintaining effective Mg 2+ /Li + separation performance under high Mg/Li ratio and high salinity conditions. When the Mg/Li ratio of the feed solution is 20:1, the PA NF membrane achieves 99.6% ± 0.1% rejection of divalent cation Mg 2+ , with an Mg/Li separation factor (SF) of up to 50 ± 7. In conclusion, this study demonstrates that optimizing substrate layer transitions is an effective approach yet previously overlooked to fine‐tune PA layer properties, thereby significantly advancing the Mg 2+ /Li + separation performance of PA NF membranes.