Crown ether-based Tröger's base membranes for efficient Li+/Mg2+ separation

Lithium extraction via membrane-based technologies from salt-lake brines has become an exciting research direction. However, the lithium extraction efficiency is drastically affected by Mg2+ ions, which have similar chemical and physical properties, and separating Li+ from Mg2+ is currently the biggest obstacle. Here we designed a Li+ selective membrane, namely PIM-DB18C6-TB, by incorporating dibenzo-18-crown-6 as Li+ selective functionality and Tröger's Base units to induce free volume for Li+ transport. They combine to render the PIM-DB18C6-TB membrane satisfactory performance in separating Li+ from Mg2+ in both static diffusion and electrodialysis. The PIM-DB18C6-TB membrane demonstrates a Li+/Mg2+ selectivity of 35.80 in binary Li+/Mg2+ solutions under static diffusion and a Li+/Mg2+ selectivity of 24.35 during electrodialysis. We found the permeation rate of Li+ is almost proportional to the size of free volume elements in PIM-DB18C6-TB, while the selective Li+ transport is mainly owing to the selective and appropriate binding of Li+ to the crown ether units. The current work proposes a feasible strategy for designing highly selective membranes for Li+/Mg2+ separation.