Recent advances of thin film composite nanofiltration membranes for Mg2+/Li+ separation

The prevalent adoption of lithium-ion batteries (LIBs) has sparked a surge in interest regarding lithium extraction, particularly from lithium-rich brines. As some brine sources contain a higher ratio of Mg²⁺ ions to Li ​⁺ ​ions, Mg²⁺/Li⁺ separation becomes essential to improve extraction efficiency. Multiple membrane technologies were utilized in this application, including electrodialysis, membrane capacitive deionization, and nanofiltration (NF). Among the different technologies, NF membranes fabricated through interfacial polymerization have gained interdisciplinary attention due to their ease of modification, relative simplicity, and cost-effectiveness. Despite that, there are still multiple challenges in Mg²⁺/Li⁺ ​separation such as high Mg²⁺/Li⁺ ratio (MLR), trade-off between Mg²⁺/Li⁺ separation factor and pure water permeance (PWP), membrane fouling, and optimal working pH. To address these challenges, this review summarizes different nanofillers used to enhance the NF membrane performance, including carbon-based nanofillers, and polyhedral oligomeric silsesquioxane (POSS). Additionally, different NF membranes were categorized based on the modification to the interfacial polymerization, such as types of aqueous monomer, addition of nanofillers in aqueous phase, addition of nanofillers in substrate, addition of an extra layer within the membrane, and other modifications. Lastly, perspectives on the factors that affect the separation performance of the NF membranes including surface zeta potential, feed pH, pore size, hydrophilicity, and MLR will be discussed. It is anticipated that this comprehensive review can provide insights into the current progress of various modification strategies on NF membranes to drive future research and development of Mg²⁺/Li⁺ separation using this technology among the community.