Electric Field-Driven Ultraefficient Li+/Mg2+ Separation through Graphyne Membrane

The exploration of a new membrane material to achieve ultraefficient separation and recovery of Li+ from salt lake brine is of tremendous importance in consideration of the soaring demand of the lithium resource. Here, we, by utilizing classical molecular dynamics simulations, investigated the selective separations of monolayer graphyne-3 and graphyne-4 membranes to Li+ and Mg2+ under applied electric fields. It is observed that the graphyne-3 membrane displays excellent Li+ selectivity and permeability under the electric field of 0.4 V/nm, with Mg2+ being completely rejected by the triangular nanopores of the graphyne-3 membrane. However, there is no preference for the graphyne-4 membrane to Li+ and Mg2+ since both ions can travel through the triangular nanopores of the graphyne-4 membrane, regardless of external electric field intensities. Our in-depth analysis uncovered that the thorough separation of the graphyne-3 membrane for Li+ and Mg2+ at an electric field of 0.4 V/nm is largely due to its dehydration difference in the second hydration shell. It is found that the dehydration numbers of water molecules around Mg2+ are remarkedly less than the values needed for the ions to cross the nanopores, thus impeding Mg2+ permeation. Besides, we noted that Li+ tends to transport through the graphyne-3 nanopores via random positions, while the migration of Mg2+ is concentrated in the central position. Our findings provide inspiration for the application of the graphyne-3 membrane in high-efficiency extraction and recovery of Li+ from salt lakes.