Today, the development of new generation extraction systems is critically important for the environmentally friendly and efficient recycling of spent Li-ion batteries. Traditional organic solvents are toxic, volatile, and flammable. Hydrophobic deep eutectic solvents (HDESs) offer a green alternative: they have low volatility, incombustibility, efficiency, and can be obtained from their available raw materials. A large number of eutectic solvents (e.g. based on TOPO, D2EHPA, etc.) have already been studied in the literature and have proven themselves to be promising extraction systems. Their key advantage is the ability to fine-tune the composition for the selective and highly efficient extraction of valuable metals (Li, Co, Ni, Mn, etc.).

The purpose of this work is to develop a hydrophobic deep eutectic solvent based on triisobutylphosphine sulfide (TBPS) and thymol and to study its extraction properties.

The HES was prepared from the hydrogen bond acceptor (TBPS) and donor (thymol) at a molar ratio of 1:1. All extraction experiments were carried out at a temperature of 25 °C and an atmospheric pressure of ~100 kPa in graduated centrifuge tubes with a thermostatically controlled shaker.

The phase equilibrium of the "solid–liquid" system was investigated, and a detailed characterization was conducted. The viscosity of the proposed eutectic solvent is below 30 mPa∙s, indicating their technological suitability. The hydrophobicity and chemical stability of the deep eutectic solvent were also examined. An extraction method was developed for the recovery of major metal ions present in the active material of spent LFP batteries (Li, Cu, Al, and Fe). It was found that it is possible to selectively isolate Cu(II) and Fe(III) from 2 to 5 M HCl with high separation factors of 18 and 350, respectively.

Thus, the prospects of using TBPS/thymol HDES for metal ion separation contained in the active materials of spent LFP batteries are shown.