Lithium selective membranes for direct lithium extraction from complex brine

Membrane separation technology is an effective method for lithium extraction, boasting advantages such as environmental sustainability and continuous production feasibility. As high-grade lithium resources become increasingly depleted, the extraction paradigm must shift toward processing complex brines and seawater, which are characterized by ultra-low Li⁺ concentrations and a high content of competing ions, particularly Na⁺, K⁺, and Mg²⁺. Lithium selective membranes (LSMs) have emerged as critical enablers for sustainable lithium extraction from unconventional resources. Despite various LSMs have been proposed, there is a lack of systematic summarization and analysis of their lithium selective extraction mechanism and performance. This review systematically classifies state-of-the-art LSMs based on the lithium selective mechanisms of size sieving effect, binding affinity difference and hybrid mechanisms. The relationships of composition-structure–property in LSMs are analyzed in detail. The characteristics of various functional materials (including inorganic solid-state electrolytes, ionic liquids, phosphate esters, crown ethers, lithium ion-sieves, and metal–organic frameworks) used in the fabrication of LSMs are analyzed. Additionally, this review discussed the key technical challenges of the LSMs, and presented the potential future research directions to provide viable recommendations for the design, fabrication, and application of high-performance LSMs.