Enhanced Lithium Recovery from Salt-Lake Brines via Advanced Nanofiltration Membranes: Polymeric Structure–Sieving Performance Relationships

Lithium and its compounds have become crucial energy metals and industrial necessities. Driven by technological advancements and expanding applications in energy storage and portable electronics, ensuring sustainable lithium supply chains is highly important. Thus, the development of efficient extraction methods from salt-lake brines, particularly those with high Mg2+/Li+ ratios, has become a priority. Nanofiltration (NF) separation technology has recently emerged as a key process for selective lithium recovery, presenting remarkable advantages over conventional methods. This review systematically assesses the relationships between the polymeric structure and sieving performance of NF membranes for lithium extraction. This research emphasizes the influence of the membrane architecture on ionic selectivity and permeability. Advanced modification strategies for positively charged NF membranes are meticulously analyzed. These strategies include surface functionalization, copolymer design, and hybrid nanocomposite engineering, all of which are aimed at increasing the Mg2+/Li+ separation efficiency. Moreover, the review delves into innovative membrane module configurations and coupling processes (such as the integration of NF-electrodialysis) to satisfy the requirements of industrial scalability. Finally, the critical challenges and future research directions are highlighted. Our focus lies on cost-effective membrane fabrication, the optimization of long-term stability, and system-level process intensification. This comprehensive analysis not only provides an in-depth mechanistic understanding of high-selectivity lithium extraction from complex brines but also stimulates the rational design of next-generation membranes with precisely tailored ion-transport properties.