Robust positively charged polyurea nanofiltration membranes with acid resistance for efficient lithium extraction and recovery

Given the growing demand for lithium in energy storage and electric vehicle industries, the development of acid-resistant membranes for efficient lithium extraction from brine and recycling of spent lithium-ion batteries is crucial for advancing sustainable and scalable resource recovery technologies. Herein, a strong acid-tolerant and positively charged polyurea (PU) nanofiltration (NF) membrane was fabricated via the interfacial polymerization of toluene-2, 4-diisocyanate (TDI) monomers with poly(allylamine) (PAA) monomers with a polyethersulfone ultrafiltration membrane as the substrate. The newly-developed typical PU NF membrane performed high cation-cation separation selectivity (mixed-salt separation factor: 16.6 for Li⁺/Mg²⁺, 19.3 for Li⁺/Ni²⁺, 11.3 for Li⁺/Co²⁺, and 15.7 for Li⁺/Mn²⁺) even if exposed to 10 ​wt% H₂SO₄ solution for 96 ​h. The high cation separation accuracy is attributed to the narrow positively-charged ion sieving channels constructed with TDI and PAA as building blocks. The urea units containing abundant bidentate hydrogen bonds and electron-rich dinitrogen atoms is responsible for the excellent acid tolerance of the PU membranes. This work has the potential to contribute to more sustainable and cost-effective lithium recovery from both brine and discarded cathode materials, making it a crucial step toward scaling up these technologies for industrial applications.