Covalent organic framework membranes achieving Mg/Li separation by permeating Mg²⁺ while retaining Li⁺

Due to the growing demand for lithium in the new energy industry, significant attention has been focused on developing lithium extraction technologies from salt-lake brine. However, the high Mg/Li ratio in salt-lake brine presents challenges for membrane separation technology. If a membrane can allow Mg²⁺ and water molecules to pass through while retaining Li⁺, the retained brine will have concentrated Li⁺ with a reduced Mg/Li ratio, creating the facilitation of further lithium extraction. In this study, we discovered through non-equilibrium molecular dynamics simulations that strongly hydrophilic covalent organic frameworks membranes capture Li⁺ in their pores, preventing additional Li⁺ from entering the nanopores. Meanwhile, Mg²⁺ can freely penetrate these nanopores along with water molecules. This adsorption of Li⁺ and the free permeation of Mg²⁺ with water molecules result in the effective separation of Li⁺ and Mg²⁺. Consequently, the retained brine becomes lithium-rich with reduced Mg/Li ratio. The findings of this work provide valuable guidance for designing nanofiltration membranes for extracting lithium from salt lakes with high Mg/Li ratio.