Elucidating the Mechanism of Lithium Ion Extraction in a Tributyl Phosphate-Ionic Liquid Mixed System from the Perspective of Water Coordination

Abstract

With the rapid development of the lithium-ion battery industry, the demand for lithium resources is becoming more and more urgent. Lithium extraction is a widely used process; especially, tributyl phosphate (TBP) systems have attracted much attention. During the extraction and purification process of lithium ions, the extractant TBP may encounter issues including aging, emulsification, precipitation, and low utilization rates. These problems arise due to an inadequate understanding of the extraction mechanism. In this work, the TBP-ionic liquid system was utilized to extract Li⁺, the variations of trace water in the organic phase were systematically and quantitatively investigated, and the extraction mechanism was further investigated. Experiments have confirmed that TBP forms a 1:1 complex with water. Various complex ratios of TBP, Li⁺, and water were determined, and the structures were verified by simulations. The complexes have three different ratios of TBP, Li⁺, and water: 2:1:2, 3:1:1, and 4:1:0. Water molecules are substituted during the complexation of Li⁺ with TBP, and meanwhile, the NTf₂^– anion equilibrates the charge of the Li⁺-TBP complex. This work provides experimental data and a theoretical basis for understanding Li⁺ extraction in the TBP-ionic liquid system, which is conducive to the sustainable development of the lithium industry.