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.