Nanofiltration Membranes for Li⁺/Mg²⁺ Separation: Materials and Mechanisms

Nanofiltration (NF) membranes have garnered significant interest for Li⁺/Mg²⁺ separation, a crucial step in lithium extraction from brines. The state-of-the-art commercial LiNE-XD membrane exhibits a Li⁺/Mg²⁺ separation factor (SF_Li/Mg) of 42 at pH 3.3, due to the positive charges on the membrane surface and its strong size-sieving ability. Membranes with higher separation factors at a broad pH range are being pursued to improve separation efficiency. This review aims to provide a timely and comprehensive assessment of advanced NF membranes with superior Li⁺/Mg²⁺ separation properties, as well as their structure and property relationships for designing next-generation membranes. We describe transport mechanisms for ions in NF membranes and discuss governing parameters and models employed to quantify the Li⁺/Mg²⁺ separation. High-performance membrane materials are exhaustively introduced, including commercial and modified polyamides, two-dimensional materials, metal–organic frameworks, crown ethers, and their blends. Finally, we critically compare these membranes in an upper bound plot and highlight the opportunities and challenges of NF membranes for practical Li⁺/Mg²⁺ separations.