Figure 2.

Separation mechanisms, modification strategies, and separation performances for nanofiltration (NF) membranes in Li⁺/Mg²⁺separation. (A) Schematic illustrations of solute–solute separation for Li⁺/Mg²⁺ separation. (B) Separation mechanisms of NF membranes, including (i) steric hindrance, (ii) Donnan exclusion, and (iii) dielectric effects. (C) Typical interfacial polymerization reaction for the fabrication of conventional NF membranes. Modification strategies for enhancing NF membranes to differentiate between Mg²⁺ and Li⁺ ions: (D) Monomer diffusion manipulation by controlling monomer diffusion rate during the IP process to optimize the physicochemical properties of the polyamide rejection layer; (E) Novel monomers by the incorporation of functional monomers to enhance Li⁺ selectivity and/or permeability; (F) Additive incorporation with the integration of molecular additives to create preferential pathways for Li⁺ transport; (G) Post modification using surface treatment or grafting to fine-tune membrane surface properties and charge; (H) Interlayer construction by the introduction of an intermediate layer between the substrate and the rejection layer to enhance separation performance; (I) Substrate modification by altering the properties of the support layer to improve overall membrane performance; (J) Layer-by-layer assembly with the precise control of membrane structure and composition through sequential deposition of polyelectrolytes or other materials; and (K) Nonpolyamide membrane by the exploration of alternative membrane chemistries to overcome limitations of traditional polyamide-based membranes. (L) Critical analysis of Li⁺/Mg²⁺ separation factors achieved by NF membranes synthesized using the modification strategies (C–K). The data demonstrate the significant improvements in selectivity achieved through these modification techniques, with some strategies yielding separation factors >4000,³⁷ compared to <5 for commercial NF membranes.³² These strategies aim to overcome the trade-off between water permeance and Li⁺/Mg²⁺ selectivity, enabling the development of high-performance NF membranes for efficient Li⁺ extraction.