Alkali Cations Mediated Subnanochannels in MXene Membranes for Enhanced Selective Ion Transport.

Artificial ion nanochannels enabling precise discrimination between monovalent and multivalent cations are essential for resource recovery and ion separation. However, due to the inadequate differentiation of their transmembrane energy barriers, these nanochannels still face challenges in achieving high permeability alongside high selectivity. Herein, we report that a trisodium citrate-mediated MXene laminar membranes (MLM-CA-3Na) possess highly permeable and selective Li+/Ca2+ separation. As confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy, the uniform stacking of the citrate ligand/Na+ synergistically modified MXene monolayer nanosheets has achieved highly ordered confined subnanochannels of MLM-CA-3Na membranes. Molecular dynamics simulations, potential of mean force calculations, and transport energy barrier analysis revealed that the confined subnanochannels of MLM-CA-3Na membranes efficiently regulate the dehydration and transport behaviors of Li+ and Ca2+, leading to a pronounced differentiation in their transmembrane energy barriers. The obtained MLM-CA-3Na membranes exhibited a Li+ permeation rate of 0.0725 mol m-2 h-1, a Li+/ Ca2+ selectivity of 84, and long-term durability over 100 h. This work identifies ligand-cation interactions as key regulators for ion separation, providing a design paradigm for sustainable lithium extraction.