Development of metal–ligand ion-exchange membranes functionalized with crown ether–ionic liquids for selective Li+/Mg2+ separation†

The effective and sustainable separation and recovery of monovalent and divalent metal ions from spent lithium-ion batteries after acid/alkali leaching still remains a big challenge to hydrometallurgical technologies. In this work, we prepared a novel crown ether–ionic liquid functionalized membrane for the selective separation of mono/divalent ions using electrodialysis (ED). A novel membrane was designed by incorporating 15-crown ether-5 (15C5), a crown ether with specific recognition for alkali metal ions, through combining interfacial polymerization and self-assembly techniques into the tunable ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim][NTf₂]). The resultant hybrid coating forms a channel structure for the selective transport of monovalent ions. Systematic characterisation of the membrane structure, physicochemical properties, and electrochemical performance was performed before and after modification. The ion transport mechanism was performed using ion diffusion mathematical modeling and density functional theory. The designed crown ether ionic liquid functionalized modified membrane 15C5–IL(2)/Cu–MPD showed excellent selectivity for various monovalent/divalent ion pairs of Li⁺/Mg²⁺, Na⁺/Mg²⁺, and K⁺/Mg²⁺ with values of 5.33, 5.45, and 13.39, respectively. Besides, the membrane was pH-responsive, and it showed a Li⁺/Mg²⁺ selectivity of 10.45 in the case of pH 7.5. This may be due to the synergistic effect of pore size screening brought about by the distribution of modified components and specific interactions, for instance, 15C5/O⁻–Li⁺. This simple approach opens a new avenue in the quest for selective ion-exchange membranes (IEMs) for separations of certain cations by ED.