The demand for lithium resources in China has been increasing year by year, and their reserves are mainly concentrated in the brine of salt lakes. However, traditional lithium extraction technologies have problems such as high energy consumption and low selectivity. Metal-organic frameworks (MOFs) have shown great potential in the field of lithium extraction from salt lakes due to their designable sub-nanometer channels. However, the dispersed MOFs particles still tend to agglomerate during the membrane-making process, which makes processing difficult and prone to intergranular defects, seriously affecting the separation performance. In this study, a seed-induced in-situ growth strategy was adopted to construct a continuous and defect-free MOFs membrane on the substrate surface. Firstly, Nano zinc oxide (ZnO) particles were synthesized and uniformly distributed on the surface of polyvinylidene fluoride (PVDF) substrates as metal sources. Subsequently, the ZIF-90 selective layer was grown in situ on the PVDF substrate by the hydrothermal method, and the intergranular defects of the MOFs layer were rapidly repaired by the spin coating process. Finally, the defect-free ZIF-90 composite membrane was successfully prepared. ZIF-90 composite membrane exhibits excellent separation performance in the process of separating lithium and magnesium ions (S_Li⁺_/Mg²⁺=20), with a lithium-ion permeation as high as 0.225 mol·h^-1·m^-2. Simulation calculations verified that the selectivity of the composite membrane is attributed to the higher binding energy (-6.5 eV) of ZIF-90 channels toward Mg²⁺, which inhibits Mg²⁺ transport. Characterization confirmed the successful synthesis of ZIF-90. The synthesized composite membrane exhibits suitable pore sizes in ZIF-90, enabling precise separation of lithium and magnesium ions despite their small size difference. This study further investigated the effects of epoxy dosage towards ion selectivity. The optimal selectivity between monovalent and divalent ions was achieved at an epoxy dosage. Temperature-dependent experiments revealed that an increase in temperature led to enhanced ion flux but reduced selectivity. The activation energies for the transport of K⁺, Na⁺, Li⁺, and Mg²⁺ through the composite membrane, as derived from Arrhenius equation fitting, followed the order: E_Mg > E_Li > E_Na > E_K. The rapid membrane defect repair strategy proposed in this study provides a new idea for the preparation of high-performance ion-screening membranes and has important theoretical reference and practical application value for promoting the efficient separation and recovery of lithium resources in salt lake brine.

Key words: MOF membrane, Ion separation, Defect repair method, In-situ growth, Spin-coating