The term was introduced by Abbott et al. (2003) for mixtures of choline chloride and urea, which at a 1:2 molar ratio are liquid at room temperature although both components are high-melting solids. Mixtures of choline chloride with carboxylic acids behave similarly. The general picture is of a hydrogen bond acceptor and a hydrogen bond donor whose interaction lowers the melting point, and reviews describe the range of compositions and uses (Smith et al., 2014). The definition has been tightened by Martins et al. (2019), who argue that the name should be kept for mixtures whose melting point lies well below that predicted for an ideal mixture, a test that many so-called deep eutectic solvents have never been put to.

In battery recycling these liquids have been used in two distinct ways. Hydrophilic mixtures such as choline chloride with ethylene glycol can dissolve cathode oxides directly, as Tran et al. (2019) showed. That is leaching. The present article concerns the second use, liquid-liquid extraction from an aqueous leachate, which requires a solvent immiscible with water. Hydrophobic deep eutectic solvents of this kind were introduced by van Osch et al. (2015), who combined decanoic acid with quaternary ammonium salts, and were soon applied to the removal of alkali and transition metal ions from water (van Osch et al., 2016). Mixtures designed for lithium, in which a β-diketone is paired with a neutral organophosphorus donor, have also been described (Hanada and Goto, 2021).

Extraction performance is expressed through a few standard quantities (Rydberg et al., 2004). The distribution ratio D of a metal is its total concentration in the organic phase divided by that in the aqueous phase at equilibrium. The separation factor between two metals is the ratio of their distribution ratios. The percentage extracted depends on D and on the phase volume ratio, and should never be quoted without the latter. For lithium there are two possible strategies: extract the transition metals and leave lithium in the raffinate, or extract lithium away from everything else. The lithium ion is small, strongly hydrated and a hard acid in the sense of Pearson (1963), so that it is extracted only by hard oxygen donors and usually from neutral or alkaline solution. Any screening scheme has to state which strategy a candidate is meant to serve.

The composition of the feed determines everything that follows. The user records the cathode chemistry or mixture assumed, the leaching acid and its residual concentration, the pH, and the expected concentrations of lithium, cobalt, nickel and manganese together with the impurities aluminium, copper and iron and any sodium introduced by neutralisation. The point in the flowsheet at which the solvent would act is stated, as is the strategy, lithium-selective or lithium-rejecting.

Candidates are chosen for reasons that can be written down: an acidic component able to exchange protons for metal ions, a neutral oxygen donor to complete the coordination of lithium, a long alkyl chain for hydrophobicity. Where resources allow, a computational prescreen with a model such as COSMO-RS (Klamt, 1995) can rank mixtures by predicted mutual solubility with water. Each candidate is prepared at a defined molar ratio, and its melting behaviour is checked so that it is known to be a liquid mixture and, if the name is to be used, a deep eutectic in the stricter sense.

A solvent that cannot be handled is rejected before any metal is added. Viscosity and density are measured at the working temperature, since a viscous phase of density close to that of water will not disengage. The candidate is then contacted with metal-free aqueous solution at the pH of the feed, and both phases are analysed for the solvent components. This test is essential. The components of a hydrophobic eutectic mixture partition into water to different extents, so that the organic phase after contact may no longer have the composition that was prepared (Florindo et al., 2019). Loss of a component also contaminates the raffinate, and a mixture that sheds one of its constituents at this stage is set aside whatever its promise as an extractant.

Each metal of interest is extracted separately from a solution matching the ionic background of the feed. Equal volumes are shaken at controlled temperature until equilibrium, the time to equilibrium having been established in a preliminary run. Both phases are analysed, for example by inductively coupled plasma optical emission spectrometry after digestion or stripping of the organic phase, so that the mass balance can be closed and not inferred by difference. The equilibrium pH is measured, not the initial one. Plotting the distribution ratio against equilibrium pH for each metal shows at once whether a window exists in which lithium is separated from the rest.

Candidates that pass are tested on a synthetic solution containing all the metals at the concentrations defined in Step 1. Competition for the extractant, changes in ionic strength and co-extraction often erode the selectivity seen with single metals. The variables that matter most, usually pH, phase ratio, temperature and the molar ratio of the solvent components, are best explored with a factorial design instead of one factor at a time, so that interactions are detected with a manageable number of runs (Montgomery, 2017). Loading capacity is determined by repeated contact of the same solvent with fresh feed, with attention to the appearance of a third phase or a precipitate.

An extractant that will not release the metal is of no use. The loaded solvent is contacted with a strip solution, commonly dilute acid, and the fraction of each metal returned to the aqueous phase is measured. The solvent is then analysed for its own components and returned to extraction for several cycles. A steady fall in distribution ratio, a drift in component ratio or a rise in viscosity is a ground for rejection.

The few remaining candidates are tested on leachate prepared from actual spent cathode material, which contains fluoride and phosphate from the electrolyte salt, organic residues from the binder and trace metals absent from synthetic solutions. Results are reported in a fixed form: feed composition, solvent composition and water content, phase ratio, temperature, equilibrium pH, distribution ratios with their replicate scatter, separation factors and mass balance closure.