Surface-engineered cation exchange membranes with asymmetric structural modification layers: Toward high-efficient Li+ recovery from salt lakes via electrodialysis

Selective cation exchange membranes (SCEMs) are crucial for renewable energy storage and conversion, seawater softening and wastewater treatment, and ionic resource extraction and purification. However, precisely controlling the pore microstructure of SCEMs to efficiently separate mono-/divalent cations with similar hydrated radii remains a critical challenge in electrodialysis (ED) membrane research. Herein, we report a surface-programmed cross-linking strategy to fabricate an ultrathin, asymmetric modification layer on a commercial CEM (CTG) using diaminoethyl imidazole bromide (DAIB) monomers and polyethyleneimine (PEI) macromolecules. A hydrophilic, loose cross-linked DAIB-rich sub-layer was first formed on the CTG surface via the Debus–Radziszowski reaction, followed by the construction of a densely cross-linked PEI-rich upper layer through secondary surface cross-linking. The asymmetric architecture and chemical design of the PEI-DAIB composite modification layer facilitate rapid monovalent ion transport while efficiently rejecting divalent ions, thus overcoming the permeability-selectivity trade-off. The optimized PEI-DAIB composite membrane exhibits a high Li + permeation rate (0.75 mol m −2  h −1 ) and exceptional Li + /Mg 2+ permselectivity (72), outperforming most state-of-the-art ED membranes reported in the literature. Furthermore, the membrane demonstrates excellent stability and efficient Li + extraction from simulated salt-lake brines via two-stage ED, ultimately achieving 99.9% Li + purity with low energy consumption (∼0.2 kWh mol −1 Li + ). This work provides a promising strategy for developing high-performance SCEMs to achieve sustainable ionic resource extraction and purification.