Heteropolyacid Ligands in Two-Dimensional Channels Enable Lithium Separation from Monovalent Cations

Extracting lithium from salt lakes requires ion-selective membranes with customizable nanochannels. However, it remains a major challenge to separate alkali cations due to their same valences and similar ionic radius. Inspired by the K+ channel of KcsA K+, significant progress has been made in adjusting nanochannel size to control the ion selectivity dominated by alkali cations dehydration. Besides, several works involved incorporating ligands, such as crown ether, into nanochannels based on coordination chemistry to try to promote alkali cation selectivity; nevertheless, only the separation between mono-/bivalent cations has been achieved. Herein, a series of heteropolyacid (HPA) ligands are designed to functionalize two-dimensional (2D) nanochannels, achieving superior lithium perm-selectivity over other alkali cations (16 for Li+/K+), with the Li+ permeation rate increased to four times that of the pristine 2D membrane. We discover that the switching of an ion between its hydration and ion-HPA coordination states elucidates ion-selective transport, and the relatively lower depth of energy well for the exchange from Li+ hydration to Li+-HPA coordination results in the separation of Li+ from other alkali cations. This work demonstrates a principle for exploring novel ligands to develop alkali cation-selective membranes, expanding the potential applications of ion separation membranes in lithium extraction from aquatic sources.