Biological nanofiltration membrane: Electrostatic steering for breaking ion selectivity-permeability trade-off in Mg²⁺/Li⁺ separation

Lithium (Li) is indispensable for low-carbon energy systems, yet its large-scale extraction from high-Mg salt-lake brines is fundamentally constrained by the insufficient selectivity of existing separation technologies. Traditional nanofiltration (NF) membranes suffer from an intrinsic selectivity-permeability trade-off, arising from the strong coupling between electrostatic interactions and ion transport within uniformly charged nanopores. Here, we present a biological ion channel–inspired nanofiltration (BICNF) membrane with an asymmetric charge heterostructure, fabricated by grafting positively charged quaternary ammonium salts onto a negatively charged polyamide nanopore. The as‑prepared membrane effectively breaks the ion selectivity‑permeability trade‑off in Mg²⁺/Li⁺ separation. Unlike uniformly charged NF membranes, the asymmetric charge heterostructure of the BICNF membrane spatially decouples ion pre-screening from transmembrane transport. Moreover, molecular dynamics simulations reveal that the bioinspired charge configuration establishes a directionally biased electrostatic steering effect, which actively guides Li⁺ ions through the nanopores while effectively repelling Mg²⁺. As a result, the BICNF membrane achieves a high Mg²⁺/Li⁺ selectivity (~75) with a high Li⁺ permeability rate of ~0.65 mol m^-2 h^-1. Using a simulated salt-lake brine, the BICNF membrane enables crystallization of Li₂CO₃ with a high purity of 99.1%. Thus, this work provides a bioinspired strategy to advance highly efficient ion separation for lithium extraction.