Theory-guided design of high-efficiency lithium extraction system for high Mg/Li ratio brines

Abstract

Global demand for lithium necessitates efficient extraction technologies, especially from low-quality brines with high magnesium-to‑lithium (Mg/Li) ratios, where conventional methods falter. This study presents a novel, quantum chemistry-guided methodology for the rational design of solvent extraction systems to address this challenge. By evaluating lithium binding affinities, triisopropylphenyl phosphate (TiPPP) was identified as a superior extractant. When formulated into an TiPPP-FeCl₃ system, it demonstrated exceptional performance under optimized conditions (90 % vol extractant, Fe/Li molar ratio of 1.2, O/A ratio of 1). In a single stage, the system achieved a lithium extraction efficiency of 92.8 %, corresponding to a remarkable Li/Mg separation factor of 1699. After scrubbing and stripping, the Mg/Li mass ratio in the final product was reduced to 0.0199, a 4156-fold decrease from the initial brine (82.7), outperforming the conventional tributyl phosphate (TBP)-FeCl₃ system. A combination of DFT calculations and spectroscopic analyses (Infrared spectroscopy, Ultraviolet spectroscopy, Raman spectroscopy, and electrospray ionization mass spectrometry) confirmed that the high selectivity is governed by both the electronic environment of the phosphoryl (P=O) group and the steric hindrance effects. This study not only presents a high-performance system for direct lithium extraction but also establishes a rational design paradigm for developing advanced extractants for challenging separation tasks.