Rationally engineered metal-organic framework membrane structure for effective Mg2+/Li+ separation

Developing high-precision ion-selective membranes remains critical for sustainable lithium extraction, especially from brines with high Mg 2+ /Li + ratios. Metal-organic framework (MOF) membranes with tunable pore architectures hold great promise for Mg 2+ /Li + separation, but combining selective adsorption with ion sieving effects for efficient ion separation remains challenging. Here, we fabricate a defect-free UiO-66-4F polycrystalline membranes on polyethylene terephthalate (PET) substrates by in-situ growth. The fluorine-functionalized sub-nanochannels synergistically combine selective Li + adsorption and size exclusion mechanisms, where electronegative F group enhances Li + affinity while the angstrom-scale pores impose steric hindrance on hydrated Mg 2+ . The dual mechanism amplifies the permeation difference between Li + and Mg 2+ transport, achieving an exceptional Mg 2+ /Li + separation factor of 283 alongside high monovalent ion permeation rates (0.16 mol·m −2 ·h −1 ) under an applied electric potential difference. This study presents a flexible approach for the design and fabrication of ion-separation membranes that are both highly permeable and selectively efficient, achieved through the integration of size sieving and binding affinity interactions.