Artificial Cation–Chloride Co‐Transporters for Chloride‐Facilitated Lithium/Magnesium Separation

Abstract Inspired by nature, many artificial ion sieving materials have been developed, shedding light on the next‐generation ion, e.g., Li + , extraction applications. Artificial co‐transporters remain difficult to construct since they have a much more complex ion‐sieving property. For example, the cation–chloride co‐transporters have both alkaline ion and chloride ion selectivity but no alkaline ion/chloride ion selectivity. We here demonstrate a method to construct artificial co‐transporters, using a porous organic framework membrane which has a relatively disordered stacking structure and rich quaternary ammonium groups paired with counter‐ions. This imparts the membrane with extremely narrow pores (∼0.3 nm) and almost no surface charge, enabling size‐based high alkaline ion selectivity against other cations, high Cl − selectivity against other anions, but almost no alkaline ion/Cl − selectivity. Such synchronized sieving property allows us to enhance the extraction of high‐value cations (Li + ) by simply feeding excessive low‐value anions (Cl − ). As a demonstration, we realized high‐flux (0.44 mol m −2 h −1 ) and highly selective (selectivity: 185) Li + /Mg 2+ separation by reversing the current industrial brine‐based lithium extraction process, i.e., sieving Li + before removing NaCl.