Dehydration-enhanced Ion Recognition of Triazine Covalent Organic Frameworks for High-resolution Li+/Mg2+ Separation

Abstract

The precise and rapid extraction of lithium from salt-lake brines is critical to meeting the global demand for lithium resources. However, it remains a major challenge to design ion-transport membranes with accurate recognition and fast transport path for the target ion. Here, we report a triazine covalent organic framework (COF) membrane with high resolution for Li⁺ and Mg²⁺ that enables fast Li⁺ transport while almost completely inhibiting Mg²⁺ permeation. The remarkably high rejection of Mg²⁺ by the COF membrane is achieved via imposed ion dehydration and the construction of the energy well. The proper hydrophilic environment of the COF channel promotes the dissociation of Li⁺ from the negatively charged functional groups, allowing Li⁺ for hopping transport supported by the sulfonate side-chains to shorten the diffusion path of Li⁺. Under high-salinity electrodialysis conditions, the COF membrane demonstrates robust Li⁺/Mg²⁺ separation performance (No Mg²⁺ were detected in the collected solution), achieving efficient lithium recovery and high product purity (Li₂CO₃: 99.3 %). This membrane design strategy enables high energy efficiency and powerful lithium extraction in the electrodialysis lithium extraction process, and can be generalized to other energy and separation related membranes.