Eco-Friendly Leaching of Spent Lithium-Ion Battery Black Mass Using a Ternary Deep Eutectic Solvent System Based on Choline Chloride, Glycolic Acid, and Ascorbic Acid

Abstract

Lithium-ion batteries (LiBs) are utilized in numerous applications due to advancements in technology, and the recovery of end-of-life (EoL) LiBs is imperative for environmental and economic reasons. Pyrometallurgical and hydrometallurgical methods have been used in the recovery of metals such as Li, Co, and Ni in the EoL LiBs. Hydrometallurgical methods, which have been demonstrated to exhibit higher recovery efficiency and reduced energy consumption, have garnered increased attention in recent research. Inorganic acids, including HCl, HNO₃, and H₂SO₄, as well as organic acids such as acetic acid and citric acid, are employed in the hydrometallurgical recovery of these metals. It is imperative to acknowledge the environmental hazards posed by these acids. Consequently, solvometallurgical processes, which involve the use of organic solvents with minimal or no water, are gaining increasing attention as alternative or complementary techniques to conventional hydrometallurgical processes. In the context of solvent systems that have been examined for a range of solvometallurgical methods, deep eutectic solvents (DESs) have garnered particular interest due to their low toxicity, biodegradable nature, tunable properties, and efficient metal recovery potential. In this study, the leaching process of black mass containing graphite, LCO, NMC, and LMO was carried out in a short time using the ternary DES system. The ternary DES system consists of choline chloride (ChCl), glycolic acid (GLY), and ascorbic acid (AA). As a result of the leaching process of cathode powders in the black mass without any pre-enrichment process, Li, Co, Ni, and Mn elements passed into solution with an efficiency of over 95% at 60 °C and within 1 h. Moreover, the kinetics of the leaching process was investigated, and Density Functional Theory (DFT) calculations were used to explain the leaching mechanism.

1. Introduction

Lithium-ion batteries (LIBs) are advanced energy storage devices characterized by a wide operational temperature range, high voltage, and elevated energy and power density. These advantages have led to the widespread adoption of LIBs in electric vehicles (EVs), hybrid electric vehicles (HEVs), laptops, mobile phones, hard drives, solar panels, and wind turbines [1]. In smaller electronic devices (e.g., smartphones, laptops, cameras), the average lifespan of LIBs ranges from 3 to 5 years, whereas in larger systems such as EVs, their operational lifetime is estimated to be around 10 years [2]. With increasing demand each year and the growing number of applications, end-of-life LIBs are accumulating as millions of tons of waste [3]. By 2030, it is projected that approximately 2 million EVs will reach end-of-life annually, and this number is expected to increase exponentially over the following decade [4]. Furthermore, the rising demand for LIBs has led to significant price increases in critical elements such as lithium (Li), nickel (Ni), and cobalt (Co). Additionally, the large-scale accumulation of metals like Co and Li poses environmental and biological hazards due to their toxic nature. In particular, the fact that LIB cathodes typically consist of approximately 20% Co and 7% Li by weight highlights the critical importance of battery recycling. The presence of toxic and flammable electrolytes, such as lithium hexafluorophosphate (LiPF₆), further intensifies these risks. Therefore, it is crucial to recover raw materials from used LIBs in an environmentally friendly and sustainable manner [5,6].

Two of the most widely adopted techniques for recycling LIBs are pyrometallurgy and hydrometallurgy [7]. However, hydrometallurgical methods are often chosen as an alternative because of variables including high energy consumption, high prices, and the release of hazardous gases during the pyrometallurgical process [8,9]. Hydrometallurgical methods are notable for their lower energy needs, higher purity of recovered materials, and reduced gas emissions when compared to pyrometallurgical methods [10]. Moreover, the cumulative energy demand value of hydrometallurgical recycling processes was reported as 198.2 MJ kWh⁻¹, while that of pyro- + hydro- processes was reported as 228.2 MJ kWh⁻¹, and it was observed that hydrometallurgical processes exhibited less energy consumption than pyro- + hydro- combinations [11]. Nevertheless, the use of strong and toxic mineral acids, such as HNO₃, HCl, and H₂SO₄, during hydrometallurgical recycling presents a significant threat to environmental safety [12].

To address these environmental concerns and improve metal recovery rates, deep eutectic solvents (DESs) were introduced in 2003 by Abbott et al. as a combination of quaternary ammonium salts and hydrogen bond donors, resulting in low-melting, biodegradable, and environmentally friendly solvents [13,14]. Although the environmental and economic impacts associated with the production of raw materials for DES synthesis require further investigation, the use of biodegradable and less corrosive organic compounds in DES-based leaching processes may offer a more environmentally friendly alternative compared to commercial mineral acids. A number of studies in the literature have investigated the use of DESs with different solvent compositions for metal recovery from batteries. For instance, Peeters et al. formulated several DESs in varying ratios and reported the Co leaching efficiencies [15]. One of these investigations achieved a recovery rate of over 98% by leaching Co from lithium cobalt oxide (LCO) batteries using a DES made of ChCl and citric acid in a 2:1 molar ratio. The leaching procedure was carried out at 40 °C for 60 min. However, the study noted that citric acid exhibited high viscosity and therefore had to be diluted with 35% water to reduce the solvent’s resistance to mass transfer.

Ma et al. investigated a DES system made of ChCl and L-tartaric acid for the removal of metals from the cathodes of NMC and LCO batteries [16]. The DES was prepared in a 1:1 molar ratio, and varying amounts of water were added to the mixture. The optimal leaching temperature was found to be 70 °C, resulting in a metal recovery efficiency of 98.5%. Furthermore, it was observed that increasing the water content improved the leaching performance. However, higher water content was also found to reduce the rate of lithium recovery. Additionally, excessive water can disrupt the hydrogen bonding network of the DES, which may change the coordination and behavior of metal ions, ultimately decreasing leaching efficiency [17]. For this reason, water-free DES systems have also been explored.

Ternary DES systems have recently gained significant attention as an alternative to the binary systems frequently used in the literature. These systems offer several advantages over binary ones in the recovery of LCO from spent LIBs. Jafari et al. investigated the influence of a ternary DES system composed of ChCl:urea: ethylene glycol (EG) on the leaching process [18]. Multiple DES systems, such as the ternary ChCl:urea: EG system, ChCl: urea, and ChCl: EG system, were prepared in different mole ratios. Following preparation, 5 mL of DES and 0.1 g of battery powder were used in the leaching experiments. For every DES system, leaching tests were carried out at 50 °C, 75 °C, and 100 °C. When combined with urea and ChCl, the EG structure in the ternary DES remained chemically stable and unaltered, according to FT-IR measurements. By increasing its efficacy throughout the procedure, EG’s stability in the ternary system made a substantial contribution to the leaching performance. The findings of the characterization showed that metal recovery rose in direct proportion to temperature. With a high Li recovery rate of 97%, it was found that 100 °C was the ideal temperature for DES-based recovery. Furthermore, recovery efficiencies for Mn, Ni, and Co were 34%, 40%, and 41%, respectively. The ternary DES exhibited better selectivity for Co and Ni, whereas the binary DES system made of ChCl and urea showed great selectivity for Li and Mn. The literature also identifies important challenges in DES-based metal recovery procedures, such as high viscosity, the requirement for high temperatures, and extended leaching times.

This study aims to design and optimize novel DES systems to address persistent challenges. One major issue with DESs is their inherently high viscosity, which frequently requires water dilution, which runs counter to their ecologically friendly nature. To overcome this limitation, the present work focuses on developing water-free DES systems composed solely of organic acids and ammonium salts. In many DES formulations, glycolic acid (GLY) is a favored HBD, which makes it very useful for metal recovery operations. Van der Waals forces and electrostatic interactions, in addition to the strong hydrogen bonding interactions between HBDs and HBAs, are responsible for the high viscosity characteristic of DESs [19]. This type of viscosity hinders mass transport, which creates obstacles to effective leaching kinetics [20]. GLY is used as a viscosity-reducing HBD in anhydrous DES formulations to lessen these disadvantages. GLY (C₂H₄O₃), a member of the alpha-hydroxy acid (AHA) family [21], has a molecular weight of 76.05 g/mol, a melting point of 75–80 °C, and a decomposition temperature of 100 °C. Glycolic acid was selected in this study due to its carboxylic functional group, ability to reduce viscosity, and favorable coordination with transition metal ions, all of which contribute to enhanced leaching kinetics in DES systems. Moreover, it has no color or smell, is a weak acid, and is very water soluble [21]. Due to its diverse chemical characteristics, it is widely used in the food industry, textiles, dermatology, cosmetics, and metal extraction [22].

In the study by Kaur et al., ChCl was used as the HBA and GLY as the HBD. The GLY content was adjusted at molar ratios of 1:1, 1:2, and 1:3. Results indicated that increasing GLY content reduced viscosity [23]. However, a higher GLY proportion led to decreased dye extraction efficiency. At lower mole ratios, DESs with a 1:1 GLY: ChCl ratio exhibited the highest performance, followed by 2:1 and 3:1 ratios, while at higher DES dosages, extraction efficiencies among all ratios were comparable.

Ascorbic acid (AA), commonly known as vitamin C, has the chemical formula C₆H₈O₆ [24]. Its density is around 1.65 g/cm³, its melting and decomposition points are between 190 and 192 °C, and its molecular weight is 176.12 g/mol. It is highly soluble in both water and ethanol [24,25]. Owing to its carbon groups, AA exhibits strong reducing properties, and the proximity of its hydroxyl and carbonyl groups enables it to act as a hydrogen donor [26]. In DES systems, cobalt is reported to dissolve in its +2 oxidation state. Accordingly, Peeters et al. achieved successful Co recovery from LiCoO₂ cathodes by employing reductants such as Cu and Al to reduce Co³⁺ ions to Co²⁺ [15]. Similarly, Dixon et al. utilized L-ascorbic acid to reduce a pentaamine chromate cobalt (III) complex through a two-step process: first, Cr was reduced, and then Co³⁺ was reduced to Co²⁺ [27]. To minimize the introduction of additional metals into the system, we propose the use of AA as an alternative to conventional reductants such as Cu and Al.

One of the innovative aspects of this study is the use of a three-component DES system (ChCl:GLY:AA) instead of a conventional binary system. The high solubility of GLY, combined with the reduction potential of AA under +2 oxidation state conditions during the leaching process, contributes to the uniqueness of the system. Furthermore, this study aims to address existing gaps in the battery recycling sector by optimizing novel DES systems and providing solutions to the challenges identified in the current literature.

2. Materials and Methods

2.1. Materials

Choline chloride (ChCl, ≥98%, Sigma Aldrich, Shanghai, China), glycolic acid (GLY, ≥98%, Sigma-Aldrich, St. Louis, MO, USA), and L (+)-ascorbic acid (AA, ≥99%, Merck Ltd. Beijing, China) were used to prepare DESs. Moreover, HNO₃ (65%, Tekkim Kimya San. ve Tic. A.Ş. Istanbul, Türkiye) was used for sample preparation. Ethanol (Merck KGaA Darmstadt, Germany), acetone (99.5%, Tekkim Kimya San. ve Tic. A.Ş. Istanbul, Türkiye), and de-ionized water (produced by Milli-Q) were utilized at various stages.

2.2. Characterization of Black Mass

The black mass used in this study was obtained as industrial waste from a lithium-ion battery recycling facility located in Türkiye. It consisted primarily of shredded cathode materials and residual graphite collected from spent LIBs. The characterization of black mass was conducted in our previous study [28]. Briefly, the presence of LiCoO₂, LiNi_0.33Mn_0.33Co_0.33O₂, LiMn₂O_4, and graphite phases was observed in the black mass. Additionally, the composition of the black mass was ascertained to be 35.38% graphite by weight, 29.62 ppm Li, 195.86 ppm Co, 79.55 ppm Ni, and 59.48 ppm Mn. Furthermore, the particle morphology of the black mass was found to exhibit a heterogeneous structure, with an average particle size of 70 µm.

2.3. Preparation of DES Solutions

To prepare homogeneous binary and ternary DESs, ChCl was used as the HBA at a constant concentration of 1 mole, while GLY and AA served as HBDs in varying mole ratios. The mole ratios of the prepared solutions are given in

Table 1. Binary and ternary DES compositions.

DES Codes	Choline Chloride Ratio (Mole)	Glycolic Acid Ratio (Mole)	Ascorbic Acid Ratio (Mole)
1:2 ChCl:GLY	1	2	-
1:1.9:0.1 ChCl:GLY:AA	1	1.9	0.1
1:1.8:0.2 ChCl:GLY:AA	1	1.8	0.2
1:1.7:0.3 ChCl:GLY:AA	1	1.7	0.3
1:3 ChCl:GLY	1	3	-
1:2.9:0.1 ChCl:GLY:AA	1	2.9	0.1
1:2.8:0.2 ChCl:GLY:AA	1	2.8	0.2
1:2.7:0.3 ChCl:GLY:AA	1	2.7	0.3
1:4 ChCl:GLY	1	4	-
1:3.9:0.1 ChCl:GLY:AA	1	3.9	0.1
1:3.8:0.2 ChCl:GLY:AA	1	3.8	0.2
1:3.7:0.3 ChCl:GLY:AA	1	3.7	0.3

. The components were mixed and stirred at 700 rpm at 60 °C, via a magnetic heating stirrer (model: MS300HS-MTOPS), until a uniform, homogeneous solution was obtained. A 5 mL sample of the prepared DES was transferred to a glass container.

2.4. Leaching Experiments

Black mass containing graphite, lithium nickel-manganese-cobalt oxide (NMC), lithium cobalt oxide (LCO), and lithium manganese oxide (LMO) was accurately weighed and added to the 5 mL of DES system at different solid-to-liquid ratios. The leaching process was conducted on a magnetic stirrer at 700 rpm and 60 °C for 1 h. Experimental parameters, including leaching temperature and time, DES composition, solid-to-liquid ratio, and stirring rate, were systematically varied to optimize the leaching efficiencies. After the leaching process, the solution was subjected to filtration using a syringe filter with a pore size of 0.22 µm to facilitate the separation of solid and liquid phases.

2.5. Characterization

FTIR analysis was conducted in the wavenumber range of 400–4000 cm⁻¹ using a Bruker FTIR spectrometer. The DES samples were analyzed after complete homogenization in the liquid phase to ensure uniform hydrogen bonding interaction across the ternary system. After leaching, the samples were filtered using a 0.22 µm syringe filter. The resulting filtrates were diluted to appropriate levels depending on metal concentration—typically 50×, 100×, 250×, 500×, or 1000× dilutions were prepared with a 2% HNO₃ solution (v/v). The Atomic Absorption Spectroscopy (AAS) instrument (Agilent Technologies, Santa Clara, CA, USA) was calibrated using certified standard solutions of Li, Co, Ni, and Mn. The metal ion concentrations in the leachates were calculated based on the corresponding calibration curves. Leaching efficiency was calculated with Equation (1) [29].

$$\begin{array}{l}The leaching efficiencies \left(\%\right)\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}={\displaystyle \frac{Metal ion concentration \left(\frac{g}{L}\right)\ast Leach volume\left(L\right)}{Metal amount of specific metal \left(\%\right)\ast Initial metal amount \left(g\right)}}\ast 100\end{array}$$

(1)

The complex formation energies between Co²⁺ and selected ligands—ascorbic acid, glycolic acid, and choline chloride—were calculated using the ORCA 6.0.1 quantum chemistry package [30]. All calculations were carried out using the density functional theory (DFT) hybrid functional B3LYP, in conjunction with Grimme’s D3BJ dispersion correction to improve the accuracy of non-covalent interactions. The def2-TZVP basis set was employed for all atoms, providing a balance between computational cost and chemical accuracy. Geometry optimizations were performed using tight self-consistent field (SCF) convergence criteria and without imposing any symmetry constraints. To simulate solvation conditions more realistically, the conductor-like polarizable continuum model (CPCM) was applied during all geometry optimizations and energy evaluations. Binding energies (ΔE) were computed to evaluate the relative stability of the complexes, using the following Equation (2)

$$\Delta E=E_{\mathit{complex}}-\left(E_{{\mathit{Co}}^{2+}}+\sum E_{\mathit{ligand}}\right)$$

(2)

In this equation $E_{\mathit{complex}}$ refers to the total energy of the optimized Co–ligand complex, while $E_{{\mathit{Co}}^{2+}}$ and $E_{\mathit{ligand}}$ denote the energies of the isolated cobalt ion and individual ligands, respectively. All computed energies are expressed in Hartree units. This methodology enables a theoretical assessment of the coordination behavior and binding preferences of cobalt in the presence of environmentally benign ligands under solvated conditions.

3. Results and Discussion

3.1. Characterization of DES

Figure 1A illustrates the chemical structures of GLY, AA, and ChCl. Both GLY and AA act as HBD owing to the presence of hydroxyl (-OH) groups in their molecular structures. In contrast, ChCl behaves as an HBA, primarily due to its Cl⁻ anion [31]. These compounds can form a DES through hydrogen bonding interactions between the HBD and HBA components. Moreover, this interaction ensures that the binary and ternary DESs prepared at different molar ratios remain in liquid form at room temperature, lowering their eutectic points below room temperature. Figure 1B shows the FTIR pattern of ChCl:GLY binary DESs prepared at 1:2 and 1:3 molar ratios. Strong absorption peaks were observed at 1085 cm⁻¹, 1422 cm⁻¹, 1732 cm⁻¹, and 3290 cm⁻¹ in the 1:2 ChCl:GLY DES spectra. These peaks correspond to the stretching of the C-O, the stretching of the –COO⁻, the stretching of the C=O, and the stretching of the O-H group from pure GLY, respectively [32]. Moreover, the ChCl presence led to the appearance of N–H stretching peaks at 2918 and 3032 cm⁻¹ [33]. Furthermore, the characteristic peak derived from the ChCl (CH₃)₃N⁺ group was observed at 1477 cm⁻¹ [33]. Increasing the GLY ratio from 2 to 3 does not significantly alter the FTIR pattern; however, a decrease in the intensity of the characteristic ChCl peak at 1477 cm⁻¹ is observed, which can be attributed to the reduced ChCl mole ratio. The FTIR pattern of the DESs containing AA is presented in Figure 1C. Upon the addition of AA, a vibration band corresponding to the C=C bond in its structure appears at around 1600 cm⁻¹ [34], and the intensity of this peak increases with higher amounts of AA. The peak observed in the 3000–3600 cm⁻¹ range for all DESs appears broad, with no distinct sharp peak in this region, indicating that the OH groups are involved in hydrogen bonding interactions with ChCl [33,34]. The addition of ascorbic acid to the DES system led to a notable increase in the absorbance intensity within the 1000–1100 cm⁻¹ region, as highlighted in Figure 1C. This spectral region is primarily associated with C–O stretching vibrations, which are characteristic of the multiple hydroxyl groups present in ascorbic acid molecules. The emergence and intensification of these peaks upon ascorbic acid incorporation indicate its successful integration into the DES matrix. This observation suggests the formation of additional hydrogen bonding interactions between ascorbic acid and other DES components, potentially enhancing the overall hydrogen bond network density. Consequently, the presence of ascorbic acid is expected to influence the physicochemical properties of the DES, such as polarity, viscosity, and its capacity for metal chelation or redox reactions.

3.2. Leaching of Black Mass

3.2.1. Effect of GLY Mole Ratio on Leaching Efficiency

To investigate the effect of the amount of carboxylic groups on the leaching efficiency, three different Na-DES with different mole ratios were prepared. ChCl and GLY solutions were prepared at a molar ratio of 1:2, 1:3, and 1:4, and leaching experiments were carried out at a solid-liquid ratio of 10 g/L for 1 h at 60 °C. Leaching efficiencies obtained for the three different Na-DES are presented in Figure 2. Increasing the GLY molar ratio from 2 to 4 resulted in an enhancement of leaching efficiency, with increases observed from 20.24% to 47.47%, from 4.31% to 12.07%, from 17.50% to 43.92%, and from 19.28% to 28.48%, respectively, for Li, Co, Ni, and Mn. Lu et al. (2022) reported that the leaching efficiency of Malonic acid (MA) (containing a carboxyl group) with ChCl was higher than EG: ChCl and EG: urea [35]. DESs containing carboxyl groups provide higher leaching efficiency than other DESs due to the large number of free protons and the strong coordination ability between the carboxylate and metals [36]. Consequently, the increase in the GLY molar ratio exhibited a significant effect on the leaching efficiency.

3.2.2. Effect of Ternary System Mole Ratios on Leaching Efficiency

The leaching efficiency could not be increased above 48% for four metals by the use of ChCl and GLY. Leaching studies with DESs prepared with oxalic acid (OxA) or MA (containing a carboxyl group): ChCl have been conducted in the literature, and leaching efficiencies are high [35,37]. Among the three acids (oxalic, malonic, and glycolic), oxalic acid demonstrates the highest degree of acidity due to its pronounced resonance and inductive effects. MA exhibits weaker interactions due to the presence of a leaving methylene group. GLY demonstrates the least acidic properties due to the absence of multiple carboxylic groups and the reliance on weaker inductive effects from its hydroxyl group. Nevertheless, the viscosity order of the DESs prepared separately with these three acids and ChCl is as follows: ChCl:GLY < ChCl: MA < ChCl:OxA [38]. The high viscosity values of DES pose a significant challenge in the utilization of these solvents in industrial recycling processes [39]. Therefore, the leaching efficiency of the mixture of GLY (containing a carboxyl group) and ChCl was tried to be increased by adding a third organic substance in DES. AA was added to the system to maintain the ChCl:acid ratio due to its reducing and solvent properties. As seen in Figure 3A, the addition of AA into the ChCl:GLY system significantly improved the leaching efficiencies of Li, Co, Ni, and Mn. In the reaction where 1:2 ChCl:GLY DES solution was used, 20.24%, 4.31%, 17.50%, and 19.28% leaching efficiency was obtained for Li, Co, Ni, and Mn, respectively. Moreover, in the reaction where 1:1.9:0.1 ChCl:GLY:AA was used, 99.68%, 98.95%, 98.58%, and 95.56% leaching efficiency was obtained for Li, Co, Ni, and Mn, respectively. However, increasing the amount of AA (1:1.8:0.2 and 1:1.7:0.3 ChCl:Gly:AA) caused a significant decrease in leaching efficiency. In their study, Liu et al. (2018) reported that an increase in the amount of AA in the ChCl:AA DES solution increased solution viscosity [40]. Additionally, Kaur et al. (2018) reported in their study that increasing the GLY ratio in the ChCl:GLY system decreased the viscosity [23]. It is thought that the decrease in the amount of GLY and the increase in the amount of AA in the ternary DES system increased viscosity. It has been demonstrated that elevated viscosity leads to increased solvent flow, as well as greater resistance to heat and mass transfer [41]. Consequently, this results in a reduction in leaching efficiency.

In the leaching reactions where 1:3 ChCl:Acid and 1:4 ChCl:Acid systems were used, it was observed that the leaching efficiency did not exceed 70% (see Figure 3B,C). While 70.52% Co leaching efficiency was obtained in the leaching reaction using 1:2.9:0.1 ChCl:GLY:AA system, 56.01% Co leaching efficiency was obtained in the leaching reaction using 1:3.9:0.1 ChCl:GLY:AA system. The dissolution mechanism of LCO powders in DESs prepared with MA containing a carboxyl group and ChCl was explained [35]. It was shown that after the reduction from Co³⁺ to Co²⁺ by HBD, Cl⁻ anions in HBA formed complexes with Co²⁺, and the formed complexes were tetrachloro complexes ([Co(II)Cl₄]^2–). This study observed that the leaching efficiency decreased with increasing acid-mole ratio in DES systems. The reason for this was thought to be the decrease in the amount of Cl⁻ anions that complex with metal ions. Due to the reducing nature of AA in ternary DES in this study, it easily reduces metal cations and then forms a complex with Cl⁻ in ChCl. It was thought that the decrease in the amount of ChCl decreased the leaching efficiency. Furthermore, an increase in AA content was observed in 1:3 and 1:4 ChCl:Acid systems, which exhibited a decrease in leaching efficiency, which was hypothesized to be attributable to an increase in viscosity. Additionally, leaching efficiencies varying with DES composition are summarized in

Table 2. Summarized leaching efficiencies obtained with varying DES composition (60 °C, 10 g/L, 1 h).

DES Compositions	Leaching Efficiencies (%)
	Li	Co	Ni	Mn
(1:2)	20.24	4.31	17.50	19.28
(1:1.9:0.1)	99.68	98.95	98.59	95.56
(1:1.8:0.2)	69.62	68.92	66.05	62.45
(1:1.7:0.3)	47.47	47.09	44.09	41.49
(1:3)	28.78	6.51	24.12	20.03
(1:2.9:0.1)	70.09	70.52	68.25	58.63
(1:2.8:0.2)	64.08	66.88	66.67	58.01
(1:2.7:0.3)	60.63	59.85	57.48	48.13
(1:4)	47.47	12.07	43.92	28.48
(1:3.9:0.1)	69.62	56.01	72.31	57.64
(1:3.8:0.2)	63.77	48.98	71.43	50.43
(1:3.7:0.3)	50.32	38.06	60.14	47.78

.

3.2.3. Effect of Time on Leaching Efficiency

The time-dependent leaching efficiency of the leaching reaction (60 °C, and 10 g/L) using the 1:1.9:0.1 ChCl:GLY:AA ternary DES system is given in Figure 4. It was determined that the leaching reaction achieved maximum efficiency at the 60th minute. The leaching of cathode powders utilizing binary DES, composed of an HBD and an HBA species, typically requires tens of hours or even days [42]. In the ternary system where GLY contains a carboxyl group, which has a high reducing capacity, such as AA, was used, complete dissolution of the cathode powder in the black mass was achieved in 60 min. Furthermore, in this study, black mass containing pure cathode powder was not utilized; however, the leaching process of cathode powders containing ~40% graphite by weight and exhibiting varying chemical compositions was conducted. Although the presence of graphite may reduce the leaching efficiency because it creates a physical barrier, a leaching efficiency of over 95% was achieved for all metals in 60 min.

3.2.4. Effect of Temperature on Leaching Efficiency

Temperature is a crucial parameter in leaching processes involving DES. Due to the high viscosity of DES, higher temperatures are required compared to leaching reactions using inorganic acids [43]. Moreover, the viscosity of DES can be reduced either by adding water to the solution or by increasing the temperature; however, adding water may decrease leaching efficiency, as it can weaken the hydrogen bonding network within the DES [17]. In literature studies, ChCl–GLY DESs have been reported to decrease viscosities by 20-fold as the temperature increases from 20 °C to 80 °C [12]. Therefore, in this study, instead of adding water to DES, the effect of temperature on leaching efficiency was investigated. Additionally, the potential esterification of carboxyl groups in DES with the HBD must be considered when determining the operating temperature range [38]. Esterification reactions for DESs that use carboxylic acids as HBD and ChCl as HBA occur at temperatures above 60 °C [39,44]. Rodriguez et al. (2019) reported that esterification of 1%, 5%, and 9% (molar ratio) occurred in the system with the preparation of 1:2 ChCl:GLY at 60 °C, 80 °C, and 100 °C, respectively [45]. Due to these considerations, the leaching experiments in this study were limited to a temperature range between 20 °C and 60 °C, with no experiments conducted above 60 °C. The dependency of leaching efficiency on temperature is illustrated in Figure 5 (1:1.9:0.1 ChChl:GLY:AA, 60 min, and 10 g/L). Increasing the temperature from 20 °C to 60 °C caused an increase in leaching efficiencies from 24.63% to 95.56%, from 12.11% to 98.35%, from 12.9% to 98.59%, and from 12.95% to 99.68% for Li, Co, Ni, and Mn, respectively.

3.2.5. Effect of Solid to Liquid Ratio on Leaching Efficiency

The effect of the solid-to-liquid ratio on leaching efficiency was investigated. The effect of 10–40 g/L solid-to-liquid ratio on leaching efficiency is given in Figure 6 (1:1.9:0.1 ChCl:GLY:AA, 60 min, 60 °C). Increasing the solid-to-liquid ratio from 10 g/L to 40 g/L resulted in a decrease in metal leaching efficiency from above 98% to 75.27%, 16.26%, 66.35%, and 39.06% for Li, Co, Ni, and Mn, respectively. At a certain solid-liquid ratio, the solution becomes saturated, and the amount of insoluble particles increases [46]. The optimum solid-liquid ratio was therefore maintained at 10 g/L.

3.2.6. Leaching Kinetics

It is imperative to comprehend the kinetics of the leaching process to achieve a more profound comprehension of solid-liquid interactions and to ascertain the rate-limiting step of the reaction. A prevalent model employed for this purpose is the shrinking core model, which analyzes the interaction between solid particles and the leaching solution [44]. This model emphasizes the gradual reduction of the unreacted core within solid particles over time. This model is particularly useful in determining which stage governs the overall reaction rate during leaching. In instances where the rate-limiting step is a chemical reaction, Equation (3) is employed [47];

$$1-(1-x{)}^{{\displaystyle \frac{1}{3}}}=k_{1}t$$

(3)

In this equation, ‘x’ denotes the metal leaching rate, ‘t’ represents time (in minutes), and ‘k₁’ is the reaction rate constant. Equation (4) provides a comprehensive description of the leaching process governed by chemical reaction control in dense shrinking spherical particles [47].

$$1-(1-x{)}^{{\displaystyle \frac{1}{2}}}=k_{2}t$$

(4)

The constants in this equation are identical to those in the previous one. Equation (5) signifies the scenario in which the reaction rate is governed by diffusion through the product layer surrounding the particles [15].

$$1 + 2(1-x)-3(1-x{)}^{{\displaystyle \frac{2}{3}}}=k_{3}t$$

(5)

The leaching process was carried out with the parameters of 1:1.9:0.1 ChCl:GLY:AA, 60 min, 60 °C, 10 g/L, and the compatibility of 3 different models with the obtained leaching efficiencies was examined. The leaching kinetics of Li, Co, Ni, and Mn were evaluated using surface chemical reaction control (see Figure 7A), diffusion control (see Figure 7B), and shrinking dense particles (see Figure 7C). Additionally, kinetic model constants are presented in

Table 3. Kinetic model fitting parameters for the leaching of Li⁺, Co²⁺, Ni²⁺, and Mn²⁺.

Metals	Surface Chemical Reaction		Diffusion		Shrinking Dense Particle
	Equation	R2	Equation	R2	Equation	R2
Li	y = 0.0116x − 0.0067	0.9917	y = 0.0141x − 0.0716	0.9625	y = 0.0154x + 0.0602	0.9854
Co	y = 0.012x − 0.0282	0.9806	y = 0.0142x − 0.0859	0.9416	y = 0.016x + 0.0234	0.9914
Ni	y = 0.0124x − 0.0459	0.9231	y = 0.0143x − 0.1032	0.8676	y = 0.0158x + 0.0099	0.9935
Mn	y = 0.0095x + 0.0215	0.9907	y = 0.0116x − 0.0423	0.9735	y = 0.0138x + 0.0907	0.966

. However, it is difficult to conclude that any of these models accurately describes the leaching behavior of all four metals. The R values of four metals in all three models were found to be greater than 0.9 (except Ni, Diffusion), and it was thought that the kinetics of dissolution of the four metals in model 3 could be explained. The reason for this was thought to be the use of cathode powders with different phases and chemical compositions. However, the R values for Li and Mn (>0.99) indicate a strong correlation with the surface chemical reaction model, whereas those for Co and Ni (>0.99) are in good agreement with the dense shrinking particle model. Furthermore, the reaction rate constants (k₁) for the dissolution of Li and Mn were calculated as 0.0116 and 0.0095 min⁻¹, respectively. The rate constants (k₂) for Co and Ni obtained from the diffusion-controlled model were found to be 0.016 and 0.0158 min⁻¹, respectively. Based on the obtained rate constants, the metal dissolution rates can be ranked as follows: Co > Li > Ni > Mn.

The Arrhenius equation, which is presented in Equation (6), is employed to calculate the dissolution activation energies of Li, Co, Ni, and Mn [48];

$$K_d=A \mathrm{e}\mathrm{x}\mathrm{p}({\displaystyle \frac{{-E}_a}{RT}})$$

(6)

Here, the activation energy is denoted by E_a, the rate constant by K_d, the temperature in K by T, and the gas constant by R (8.314472 J k⁻¹mol⁻¹). The Arrhenius plots of ln (K_d) versus 1000/T for the leaching reactions of Li, Co, Ni, and Mn are presented in Figure 8. Moreover, the Arrhenius model constants are presented in

Table 4. Arrhenius model fitting parameters for the leaching of Li⁺, Co²⁺, Ni²⁺, and Mn²⁺.

Metals	Arrhenius Model Constant
	Equation	R2
Li	y = −5.7804x + 14.154	0.8975
Co	y = −8.5464x + 22.587	0.8222
Ni	y = −8.2663x + 21.844	0.9061
Mn	y = −7.8235x + 20.77	0.8708

. Based on the Arrhenius plots, activation energies for the leaching of Li, Co, Mn, and Ni were determined as 48.06, 71.06, 68.73, and 65.05 kJ/mol, respectively. Activation energy above 42 kJ/mol indicates a chemically controlled reaction, whereas values between 4.2 kJ/mol and 13 kJ/mol suggest diffusion control, and those ranging from 13 kJ/mol to 42 kJ/mol imply mixed control [18]. The calculated activation energies for the four metals indicate that the dissolution reactions are chemically controlled.

3.2.7. Leaching Mechanism

As demonstrated in the extant literature, AA is employed in studies of the aqueous leaching of waste LIBs to reduce transition metals from +3 to +2 [49,50]. The oxidation reaction of AA is given in Figure 9 [48]. Moreover, the dissolution reactions of LCO in an aqueous system are presented in Equation (7) [51]. AA acts as a reducing agent for transition metal ions, facilitating their reduction while being oxidized to dehydroascorbic acid, thereby enhancing the leaching efficiency. In aqueous systems, AA functions not only as a reducing agent but also contributes to solvation, potentially enhancing the dissolution and mobility of metal ions [52]. However, in DES, this process may become more complex due to altered solvation dynamics and redox behavior. Carboxylic group-containing acids employed in DESs contribute to the system’s acidity, functioning as leaching agents and, in certain cases, also exhibiting reducing properties [39]. Wen et al. (2023) investigated the leaching behavior of LCO cathode powders using a DES composed of ChCl and MA, demonstrating that the carboxyl groups in MA facilitated the reduction of Co³⁺ to Co²⁺, which subsequently formed a [CoCl₄]²⁻ complex with chloride anions [53]. Lu et al. (2022) reported comparable findings in a system utilizing MA and ChCl, further supporting the role of carboxyl groups in facilitating Co reduction and complexation in DES media [35]. The system utilized in this study combined AA, GLY, and ChCl, wherein AA exhibited reductive capabilities, while GLY contributed to increased solution acidity, effectively accelerating the leaching process.

$$LiCo{O}_{2\left(s\right)}+{4H}_{\left(aq\right)}^{+}+e_{\left(aq\right)}^{-}\to {Li}_{\left(aq\right)}^{+}+ {Co}_{\left(aq\right)}^{2+}+{2H}_2{O}_{\left(l\right)}$$

(7)

To identify the complexes that Co can potentially form following the leaching process, the bond energies between Co and various ligands were calculated. The complexes that Co can form and their bond energies are given in Figure 10. The presence of Co in tetrahedral form in DES solutions has been reported in another study [53]. The tetrahedral Co-glycolate (ΔE = 1.454 Hartree) complex exhibits a lower bonding energy compared to the corresponding Co-ascorbate (ΔE = 1.462 Hartree) complex. Following the leaching process, Co appears to exhibit a higher affinity for complexation with GLY rather than AA in the ternary DES system. The introduction of Cl⁻ anions into the tetrahedral coordination led to a further decrease in the bonding energies of the Co-ascorbate and Co-glycolate complexes, respectively. The [CoCl₄]²⁻ complex, representing the complete occupation of tetrahedral coordination sites by Cl⁻ anions, exhibits the lowest bonding energy difference (ΔE = −0.413 Hartree). Based on the calculated bond energies, it is inferred that Co preferentially forms the [CoCl₄]²⁻ complex with chloride anions rather than coordinating with AA or GLY. Comparable findings were reported in a separate study employing a similar methodological approach [53].

4. Conclusions

In this study, an environmentally friendly leaching method was developed using a ternary DES system based on ChCl, GLY, and AA for the recovery of precious metals such as Li, Co, Ni, and Mn from the black mass fraction of spent lithium-ion batteries. The results showed that with the proposed 1:1.9:0.1 molar ratio ChCl:GLY:AA system, leaching efficiencies of over 95% for all metals could be achieved at 60 °C within only 1 h without applying a pre-enrichment process. Due to the reducing property of AA and the synergistic effect of the low viscosity structure of GLY, leaching kinetics were accelerated and metal solubility was optimized in DES medium. Also, as a forward-looking approach, the dissolved metal ions can be co-crystallized by antisolvent addition after adjusting their stoichiometry, enabling the regeneration of cathode precursors and closing the material recovery loop. In addition, kinetic analyses and DFT calculations showed that the leaching mechanism was controlled by chemical reactions, and Co ions predominantly formed [CoCl₄]²⁻ complexes after dissolution. This study demonstrates that ternary DES systems based on environmentally friendly components such as glycolic acid and ascorbic acid can provide an effective leaching medium for the recovery of metals such as Li, Co, Ni, and Mn from lithium-ion battery waste. However, further research is needed regarding the industrial scalability, recyclability, and process integration of the method.