Acid-base pairs engineering enables ultra-selective lithium-magnesium separation via sulfonated polybenzimidazole membranes
The escalating demand for lithium, driven by electric vehicles and renewable energy storage, necessitates efficient extraction from salt-lake brines where high Mg2+/Li+ ratios and similar physicochemical properties of Li+ and Mg2+ pose a technical bottleneck. Conventional precipitation and solvent extraction are inefficient and energy-intensive. Electrodialysis (ED) offers lower energy consumption but is limited by the selectivity of ion exchange membranes. This study introduces acid-base pairs within a polybenzimidazole matrix via controlled sulfonation to enhance Li+/Mg2+ selectivity. The optimal SP45 membrane, with a sulfonation degree of 45%, forms a cross-linked structure with contracted ionic clusters and discrete hydrophilic domains, imposing higher energy barriers for Mg2+ transport. The SP45 membrane achieves a perm-selectivity of 48.1 at 2 mA cm−2, with less than 10% selectivity degradation over multiple cycles in mixed-salt systems. In a 4-stage ion-distillation device, a separation factor exceeding 60,000 between Li+ and Mg2+ is attained. This work provides fundamental insights into ion transport regulation through molecular-level acid-base pairs engineering, offering a pathway for advanced ion-selective separation membranes.