Randomly oriented covalent organic framework membrane for selective Li⁺ sieving from other ions

Certain biological channels exhibit remarkable selectivity, effectively distinguishing between competing cations. If artificial membranes could achieve similar precision in differentiating competing ions from Li⁺, it could advance sustainable technologies in lithium extraction. In this study, we present a covalent organic framework (COF) membrane featuring a randomly oriented structure that enables selective separation of major competing ions from Li⁺. The random orientation results in narrow pores, which impart size-based selectivity among alkaline ions. Additionally, the COF incorporates sulfonic groups that preferentially bind to Na⁺ and K⁺, facilitating their transport while retaining Li⁺. These synergistic mechanisms endow the membrane with a selectivity beyond detection limit for K⁺ and Na⁺ over Li⁺. When driven by an electrical potential, the ion flux through the membrane is enhanced by over an order of magnitude. Notably, the membrane also permits the transport of Mg²⁺ and Ca²⁺ while still rejecting Li⁺, leveraging differences in their ion mobility. This work should advance the design and construction of biomimetic materials for the extraction of valuable species from seawater and other aqueous sources.