A rationally designed scalable thin film nanocomposite cation exchange membrane for precise lithium extraction

Precise separation of ions of the same polarity and similar valence and size remains a critical need in resource recovery from waste streams. Here, we report the rational design and scalable fabrication of a thin film nanocomposite (TFN) cation exchange membrane to achieve precise selectivity for lithium over competing cations. The precise selectivity is realized by an ultrathin polyamide (PA) layer incorporated with amine functionalized β-monoclinic lithium titanium oxide (N-LTO) nanoparticles using a scalable interfacial polymerization process that allows high N-LTO loading while minimizing interfacial defects. The TFN membrane demonstrates superior Li⁺ permeability, with Li⁺/Ca²⁺ and Li⁺/Na⁺ selectivity reaching 173.90 and 13.58, respectively. The Li⁺/Na⁺ selectivity is attributed to the Li⁺-exclusive transport pathway in the layered structure of the N-LTO, while size exclusion by the highly cross-linked N-LTO-PA also contrubutes to the Li⁺/Ca²⁺ selectivity. Molecular dynamics simulation shows that the electrical field drives Li⁺ dehydration and accelerates the migration of the dehydrated Li⁺ while Na⁺ is blocked due to its larger size than the Li⁺ cavity. The high Li⁺ selectivity and permeability enable energy-efficient, precise, and chemical-free lithium extraction using the electrodialysis process. The TFN membrane architecture also allows simple and scalable fabrication of a multi-functional polymer-inorganic nanocomposite membrane.