Mechanistic insights into electric field-assisted nanofiltration for efficient lithium-magnesium separation

Lithium, a pivotal resource in the new energy sector, demands the development of efficient and energy-saving lithium-magnesium separation technologies. This study employed electric field-assisted nanofiltration (E-NF) technology to achieve efficient lithium-magnesium separation. Compared with conventional nanofiltration, at a current density of 2 mA·cm^-2, the rejection rate of Li⁺ decreased from -27.4 % to -32 %, while that of Mg²⁺ increased from 95.8 % to 99 %. The underlying molecular mechanisms were explored using operando Raman spectroscopy, X-ray scattering (XRS), and molecular dynamics (MD) simulations. Results indicate that the electric field disrupts hydrogen bonds in LiCl solutions more significantly than in MgCl₂ solutions, leading to a looser bulk water structure around lithium ions. XRS and Raman data show reduced peak intensities for Ow-Ow and ion-water interactions, with more pronounced effects in LiCl solutions. The decrease in Li-O and Mg-O characteristic peak intensities indicates a weakening interaction between ions and water molecules. MD simulations reveal that the electric field enhances Li⁺ dehydration and membrane permeation by reducing its coordination shells, while only slightly affecting Mg²⁺_. Overall, the electric field accelerates Li⁺ dehydration, facilitating its rapid passage through the membrane, while Mg²⁺ is retained via Donnan effects and dielectric exclusion, effectively separating Li⁺ from Mg²⁺.