A Molecular Model for Understanding the Membrane Separation of Li+/Mg2+

High-purity lithium chloride is used in energy storage applications. A possible process to obtain it is through the separation of Li+ and Mg2+ ions from salt-lake brines with a high mass ratio of Mg2+/Li+. This separation can be achieved by nanofiltration and is not only important but also environmentally friendly. It requires a deep understanding of the competitive transport of Li+/Mg2+ ions in the nanoscale. It is expected to be achieved by theoretical approaches since it remains a challenge for experimental observation. Here, a theoretical model combining the classical density functional theory (CDFT) with the Navier–Stokes (NS) equations was developed to study the selective separation of Li+/Mg2+ ions in charged nanochannels. We found nanochannels have selectivity for the co-ions with a smaller radius and lower valence state, and the relationship between ion selectivity and surface potential is approximately exponential. Finally, we proposed an accurate empirical formula to characterize the influence of the pore structure, surface properties in the Li+/Mg2+ separation performance, indicating that our work provides an effective approach to studying selective ionic transport.