Applications of crown ether-based materials for enhancing lithium recovery from brines

With the increasing demand for lithium, efficient Li+ recovery from brines has become a key research focus. Traditional materials such as adsorbents, extractants, nanofiltration membranes, and ion exchange membranes (IEMs) often exhibit limited selectivity for Li+. Recent studies indicate that crown ethers (CEs) can significantly enhance Li+ selectivity due to their specific cavity sizes. However, there is lack of comprehensive reviews on CEbased materials for Li+ recovery. This critical review provides an overview of standard Li+ recovery techniques, including adsorption, nanofiltration, and electrodialysis. It further explores the role of CEs-12 crown-4 ether (12CE), 14 crown-4 ether (14CE), 15 crown-5 ether (15CE), and 18 crown-6 ether (18CE)-in modifying materials to enhance Li+ selectivity. The separation mechanisms of CE-modified materials are discussed in detail. Comparisons reveal that 12CE and 14CE are primarily used in adsorbents, while 15CE and 18CE are more effective in nanofiltration membranes and IEMs, owing to their compatibility with Li+. Finally, we discuss challenges and future research directions for optimizing CE-based materials to improve Li+ recovery efficiency and scalability. This review aims to guide further advancements in selective lithium extraction technologies.