High-Selectivity Separation of Lithium from Spent Lithium-Ion Battery Leachates Using Surface-Modified Commercial Nanofiltration Membranes

Achieving high-efficiency and environmentally friendly metal recovery from spent lithium-ion batteries (LIBs) is essential to address the growing demand for valuable metals consumed in LIB cathode materials, particularly lithium (Li). As an advanced separation method, nanofiltration (NF) offers significant advantages over traditional methods for Li recovery. In this study, a simply surface-modified commercial NF membrane demonstrates outstanding Li⁺/M^x+ selectivity, which is a magnitude higher than those in all current research and achieves values of 645.9 for Li⁺/Ni²⁺, 508.8 for Li⁺/Co²⁺, and 307.4 for Li⁺/Mn^x+ by short-term filtration, respectively. The membrane successfully produces battery-grade Li₂CO₃ with a purity of 99.52% through a two-pass nanofiltration process without the use of any hazardous chemicals. Additionally, the investigation into the effects of the leachate pH and operating pressure on selectivity reveals that optimal NF separation is achieved at a lower leachate pH and higher operating pressure. The positively charged surface of NF membranes significantly enhances Li⁺/M^x+ selectivity; meanwhile, a smaller pore radius and narrower pore radius distributions further contribute to this improvement. The accurate prediction of ion flux and Li⁺/M^x+ selectivity confirms that ion transport in LIB leachates during NF separation follows the Donnan-Steric Pore Model with Dielectric Exclusion (DSPM-DE). The above results demonstrate the feasibility of extracting lithium effectively from spent LIB leachates and improve the efficiency and environmental friendliness of LIB recycling. Moreover, the study of ion transport in LIB leachates provides a mechanistic foundation for developing NF membranes capable of producing battery-grade Li₂CO₃ in a single-pass NF process.