Shearing Liquid-Crystalline MXene into Lamellar Membranes with Super-Aligned Nanochannels for Ion Sieving

Ion-selective membranes are crucial in various chemical and physiological processes. Numerous studies have demonstrated progress in separating monovalent/multivalent ions, but efficient monovalent/monovalent ion sieving remains a great challenge due to their same valence and similar radii. Here, this work reports a two-dimensional (2D) MXene membrane with super-aligned slit-shaped nanochannels with ultrahigh monovalent ion selectivity. The MXene membrane is prepared by applying shear forces to a liquid-crystalline (LC) MXene dispersion, which is conducive to the highly-ordered stacking of the MXene nanosheets. The obtained LC MXene membrane (LCMM) exhibits ultrahigh selectivities toward Li⁺ /Na⁺ , Li⁺ /K⁺ , and Li⁺ /Rb⁺ separation (≈45, ≈49, and ≈59), combined with a fast Li⁺ transport with a permeation rate of ≈0.35 mol m^-2  h^-1 , outperforming the state-of-the-art membranes. Theoretical calculations indicate that in MXene nanochannels, the hydrated Li⁺ with a tetrahedral shape has the smallest diameter among the monovalent ions, contributing to the highest mobility. Besides, the weakest interaction is found between hydrated Li⁺ and MXene channels which also contributes to the ultrafast permeation of Li⁺ through the super-aligned MXene channels. This work demonstrates the capability of MXene membranes in monovalent ion separation, which also provides a facile and general strategy to fabricate lamellar membranes in a large scale.