This paper presents the results of applying hydrophobic deep eutectic solvents (HDESs) for the extraction of metal ions from a real hydrochloric acid solution after leaching the cathodes of three different types of Li-ion batteries. Aliquat 336-, D2EHPA- and menthol-based HDESs developed by us were used in this study. The optimal HCl leaching conditions chosen are 80 °C, 2 M HCl, 6 h, solid:liquid ratio = 1:25. The results of stepwise separation of the major elements using extraction with HDESs are presented. The HDESs used in the cross-current extraction made it possible to extract all elements with extraction ratios above 98%. It was shown that the suggested method could potentially be used in the process of recycling end-of-life Li-ion batteries.

Hydrophobic deep eutectic solvents (HDES) are widely used as extractants. Usually, when preparing HDES, only the extraction ability of one component is taken into account, with the second serving as an “inert” component, whose effect on the extraction process is not taken into account. The present study demonstrates the possibility of controlling the selectivity of a hydrophobic deep eutectic solvent based on trioctylphosphine oxide (TOPO) by varying the substance that acts as a hydrogen bond donor, but which does not have an extractive ability. In the course of the work, the influence of the “inert” component on the physicochemical and extraction properties of HDES was confirmed by experimental, spectroscopic, and also calculation methods. A number of phenols with different structural features were chosen as the HDES’ hydrogen bond donors to modify: phenol (Ph), para-tert-butylphenol (PTBP) and thymol (Th). Using the example of separation of the Sm/Co pair, the influence of the structure of a hydrogen bond donor on the extraction ability of a hydrophobic deep eutectic solvent was established, where the degree of extraction of Sm (III) increased in the series Th:TOPO < PTBP:TOPO < Ph:TOPO. HDES based on TOPO and phenols can potentially be used to separate Sm and Co from the process leach solutions generated during the hydrometallurgical processing of waste SmCo magnets.

A new approach to managing the extraction properties of eutectic solvents based on aliphatic alcohols is proposed. Aliphatic alcohols, when functioning as hydrogen bond donors within a eutectic solvent, significantly enhance the solvent’s efficiency in extracting metal ions. Conversely, when the alcohol acts as a hydrogen bond acceptor, its extraction properties diminish. Molecular modelling reveals that the extraction efficiency of these alcohols is directly proportional to the intermolecular interaction energy between the components of the eutectic solvent.

The recycling of lithium-ion batteries is increasingly important for both resource recovery and environmental protection. However, the complex composition of cathode and anode materials in these batteries makes the efficient separation of metal mixtures challenging. Hydrometallurgical methods, particularly liquid extraction, provide an effective means of separating metal ions, though they require periodic updates to their extraction systems. This study introduces a hydrophobic deep eutectic solvent composed of trioctylphosphine oxide, di(2-ethylhexyl)phosphoric acid, and menthol, which is effective for separating Ti(IV), Co(II), Mn(II), Ni(II), and Li⁺ ions from hydrochloric acid leachates of NMC (LiNi_xMn_yCo_1−x−yO₂) batteries with LTO (Li₄Ti₅O₁₂) anodes. By optimising the molar composition of the trioctylphosphine oxide/di(2-ethylhexyl)phosphoric acid/menthol mixture to a 4:1:5 ratio, high extraction efficiency was achieved. The solvent demonstrated stability over 10 cycles, and conditions for its regeneration were successfully established. At room temperature, the DES exhibited a density of 0.89 g/mL and a viscosity of 56 mPa·s, which are suitable for laboratory-scale extraction processes. Experimental results from a laboratory setup with mixer-settlers confirmed the efficiency of separating Ti(IV) and Co(II) ions in the context of their extraction kinetics.

Hydrophobic eutectic solvents (HES) show significant promise as extractants for metal ions. At their current stage of development, however, they have many disadvantages, such as high costs, limited data on reusability and, often, lower extraction efficiency when compared with traditional extraction systems. This study investigates the physico-chemical properties of five HES formulations based on 1-octanol in combination with camphor, 2′-hydroxypropiophenone, menthol, 1-octanoic acid, and thymol. The 1-octanol/camphor HES exhibited substantially higher extraction efficiency for Fe(III) ions than a solution of 1-octanol in toluene at the same concentration. Furthermore, it showed stability when used in a mixer-settler type extractor. The 1-octanol/camphor HES achieved a rapid extraction and re-extraction rate, with phase contact time reduced to just 2 min, without loss of extraction efficiency. Using the supported liquid membrane method, the proposed Oct/Cam HES enabled a threefold concentration of iron ions in the raffinate phase under continuous operation, confirming its potential for reusability.

In this work, a new hydrophobic eutectic solvent (HES) based on bis(2,4,4-trimethylpentyl)phosphinic acid and menthol was synthesized and characterized for the first time. Important physical properties of the prepared HES, such as the density, viscosity, refractive index, as a function of temperature in the range 15–60 °C, were obtained. The new HES was applied for the extraction of Co, Ni, Cu, Mn, Al, and Fe from chloride solution. The extraction rate of Fe(III) reached 44%, while for other metals the extraction rate was not more than 3%. The potential possibility of the selective extraction of iron ions from an aqueous solution of a mixture of metals using the proposed HES was shown.