3D electrospray printing dual-charged Janus nanofiltration membrane for precise and efficient separation of Li+ and Mg2+ ions

The efficient separation of lithium (Li + ) and magnesium (Mg 2+ ) from salt-lake brines remains a critical challenge due to their similar hydrated radii. Herein, we report a novel dual-charged Janus nanofiltration (NF) membrane fabricated via 3D electrospray printing for precise Li + /Mg 2+ separation. The membrane features a negatively charged bottom layer and a positively charged top layer. Crucially, an amphiphilic ionic liquid ([Bmim][BF 4 ]) was introduced into the polyethyleneimine (PEI) aqueous phase of the top layer as a modulator. This ionic liquid enhanced PEI diffusion across the liquid-liquid interface, significantly increasing the density of residual amino groups. Consequently, the optimized membrane (EP-P 30 ) exhibited a highly positive surface charge (22.63 mV at pH 7), increased hydrophilicity, and a narrow pore size distribution. These structural and chemical optimizations translated to exceptional separation performance. The EP-P 30 membrane achieved a high pure water permeance of 13.88 L m −2 h −1 bar −1 and an outstanding Li + /Mg 2+ separation factor of 85.64 in an equimolar mixed solution. Mechanistic analysis revealed that this precise fractionation is governed by the synergistic effects of strong electrostatic repulsion against divalent Mg 2+ and the lower hydration energy of monovalent Li + , which facilitates its rapid dehydration and permeation. This study provides an innovative and scalable strategy for designing high-performance NF membranes for lithium extraction.