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Article

Sustainable Recovery of Critical Metals from Spent Lithium-Ion Batteries Using Deep Eutectic Solvents

by
Jafar Goudarzi
1,
Zhi Chen
1,2,*,
Gaixia Zhang
3,
Jinguang Hu
4,
Karim Zaghib
5,
Sixu Deng
5,
Afzal Ahmed Dar
1,
Xiaolei Wang
6,
Fariborz Haghighat
1,2,
Catherine N. Mulligan
1,2,
Chunjiang An
1,2 and
Antonio Avalos Ramirez
7
1
Department of Building, Civil, and Environmental Engineering, Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, QC H3G 1M8, Canada
2
Concordia Institute of Water, Energy and Sustainable Systems, Concordia University, Montreal, QC H3G 1M8, Canada
3
Department of Electrical Engineering, École de Technologie Supérieure (ÉTS), Montreal, QC H3C 1K3, Canada
4
Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive, NW, Calgary, AB T2N 1N4, Canada
5
Department of Chemical and Materials Engineering, Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, QC H3G 1M8, Canada
6
Department of Chemical and Materials Engineering, University of Alberta, 12-241 Donadeo Innovation Centre for Engineering, 9211-116 Street NW, Edmonton, AB T6G 1H9, Canada
7
Centre National en Électrochimie et en Technologies Environnementales, 2263 Av. du Collège, Shawinigan, QC G9N 6V8, Canada
*
Author to whom correspondence should be addressed.
Batteries 2025, 11(9), 340; https://doi.org/10.3390/batteries11090340
Submission received: 16 July 2025 / Revised: 15 August 2025 / Accepted: 11 September 2025 / Published: 14 September 2025
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Versions Notes

Abstract

The surging demand for lithium-ion batteries (LIBs) has intensified the need for sustainable recovery of critical metals such as lithium, manganese, cobalt, and nickel from spent cathodes. While conventional hydrometallurgical and pyrometallurgical methods are widely used, they involve high energy consumption, hazardous waste generation, and complex processing steps, underscoring the urgency of developing eco-friendly alternatives. This study presents a novel, water-enhanced deep eutectic solvent (DES) system composed of choline chloride and D-glucose for the efficient leaching of valuable metals from spent LiMn-based battery cathodes. The DES was synthesized under mild conditions and applied to dissolve cathode powder, with leaching performance optimized by varying temperature and duration. Under optimal conditions (100 °C, 24 h), exceptional recovery efficiencies were achieved: 98.9% for lithium, 98.4% for manganese, and 71.7% for nickel. Material characterization using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and inductively coupled plasma mass spectrometer (ICP-MS) confirm effective phase dissolution and metal release. Although this DES system requires relatively higher temperature and longer reaction time compared to traditional acid leaching, it offers clear advantages in terms of non-toxicity, biodegradability, and elimination of strong oxidizing agents. These results demonstrate the potential of water-enhanced choline chloride–glucose DES as a green alternative for future development in sustainable battery recycling, supporting circular economy objectives.

1. Introduction

Over the past decade, LIBs have become indispensable in modern technology due to their widespread application in portable electronics, electric vehicles (EVs), and grid-scale renewable energy storage systems [1,2,3,4]. Their superior energy density, lightweight design, and high charge–discharge efficiency have positioned LIBs as a cornerstone of clean energy transitions. However, the rising demand for LIBs raises environmental and economic concerns, particularly at their end-of-life (EoL) stage [2,3,4,5,6,7,8,9,10].
A typical LIB is composed of several key components: a cathode, anode, separator, electrolyte, and polymer binder [11,12]. Among these, the cathode is the most valuable and resource-intensive component, contributing approximately 40–50% of the total battery manufacturing cost [13,14]. Typical chemistries include lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium iron phosphate (LFP), nickel–manganese–cobalt oxide (NMC), and nickel–cobalt–aluminum oxide (NCA), each offering specific advantages [11,13,15,16]. These active materials are coated on aluminum foil using a polyvinylidene fluoride (PVDF) binder, while graphite anodes are typically adhered to copper foil. Consequently, spent LIBs are rich in recoverable metals such as Li, Co, Ni, Mn, Al, and Cu [7,17,18,19,20].
Recycling of LIBs is vital for environmental protection and resource conservation. Currently, pyrometallurgy and hydrometallurgy are the dominant recycling methods. Pyrometallurgy employs high temperatures for smelting and reduction, enabling recovery of valuable metals but at the cost of significant energy consumption and emission of toxic gases [21,22,23,24,25]. Hydrometallurgy dissolves metals into solutions using strong inorganic acids such as sulfuric or nitric acid, providing higher selectivity and recovery rates but resulting in corrosive wastewater and secondary pollution [26,27,28]. Although organic acids like citric, oxalic, and ascorbic acids have been explored as greener alternatives, they often require additional steps—such as the use of reducing agents or ultrasound—to achieve effective metal recovery, which can increase costs [29,30,31,32].
In recent years, deep eutectic solvents (DESs) have gained attention as promising alternatives due to their green chemistry profile. Formed by mixing a hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD), DESs are biodegradable, exhibit low toxicity, and possess negligible vapor pressure [33,34,35,36]. Choline chloride (ChCl), a common HBA, is often paired with HBDs like urea, ethylene glycol, or carboxylic acids to form effective metal-leaching systems [1,14,37]. Among them, D-glucose stands out as a naturally abundant, biodegradable sugar that offers multiple hydroxyl groups for hydrogen bonding, making it an excellent and underexplored HBD for DES-based leaching applications. This study leverages ChCl and D-glucose to formulate a novel eutectic mixture, aimed at enhancing both metal dissolution and system sustainability. The leaching mechanism typically involves metal–ligand complexation and redox reactions, facilitated by the DES’s acidity and coordination ability, while its inherently higher viscosity compared to aqueous solutions can impede mass transfer unless mitigated by dilution or co-solvent addition [38,39,40].
Ebrahimi et al. (2023) [17] demonstrated that a ternary DES system could recover metals from NMC cathodes at 100 °C within 72 h, illustrating the potential of DESs to compete with traditional hydrometallurgical processes under milder conditions. However, high viscosity remains a critical challenge, especially when processing complex residues like black mass. Strategies such as increasing temperature or adding reducing agents can improve leaching performance but may offset DESs’ environmental advantages by increasing energy consumption and operational costs [41].
Despite their promise, DESs still face challenges for industrial-scale implementation. One of the main obstacles is high viscosity, which reduces mass transfer efficiency and often requires the addition of co-solvents like dimethyl sulfoxide (DMSO) or elevated temperatures to improve leaching kinetics [41]. Moreover, many studies focus on pure cathode powders or pre-treated samples, whereas real spent batteries often yield a complex residue known as black mass (BM)—a heterogeneous mixture containing active materials, binders, carbon black, residual electrolyte, and Al/Cu foil particles [5,42,43]. Efficient recovery from BM requires DES systems that can overcome interference from these impurities while maintaining high selectivity for valuable metals.
Recent reviews have highlighted the potential of DESs in solvometallurgical recycling pathways that bridge the benefits of hydrometallurgy and solvent extraction [44]. These approaches reduce reliance on strong acids and minimize secondary waste generation. However, they require further optimization in terms of leaching time, temperature, solid-to-liquid ratio, and reagent regeneration. Moreover, the mechanism of metal complexation and precipitation from DES leachates remains an area of active investigation [14].
This study aims to develop and evaluate a deep eutectic solvent (DES) based on choline chloride and D-glucose as a potentially greener and effective medium for the recovery of critical metals, e.g., lithium, manganese, and nickel, from spent lithium-ion batteries. While the preparation and sourcing of choline chloride entail certain economic and environmental costs, the DES system investigated here offers operational advantages compared to conventional mineral acid leaching, including lower volatility, reduced corrosivity, and potential for solvent reuse. The primary objective is to explore the potential of this DES system to achieve high metal leaching efficiencies under mild operational conditions, such as lower temperatures and shorter reaction times, thereby reducing environmental and energy-related impacts. A further objective is to investigate the role of D-glucose as the hydrogen bond donor in enhancing leaching performance through improved hydrogen bonding and metal complexation. Additionally, the study seeks to address the challenge of high viscosity inherent to DESs by incorporating water into the formulation, with the goal of improving mass transfer, reducing energy input, and accelerating leaching kinetics while maintaining the overall environmental advantages of the solvent system.

2. Materials and Experimental

2.1. Materials

Spent lithium-ion batteries (LIBs) were used as the primary source of cathode material in this study. Specifically, MNKE IMR 18650 30A cells (model MH4669, MNKE Battery, Jiangsu Eastdye Battery Co., Ltd., Changzhou, Jiangsu, China) were selected. According to the manufacturer, these cells employ an LMO-type (LiMn2O4) spinel cathode. Consistent with this, our XRD identifies a LiMn2O4-type spinel with partial Ni/Co substitution, i.e., Li (Mn,Ni,Co)2O4, together with minor CoO. To recover the active cathode powder, the cells were dismantled, and the aluminum current collector was removed by immersing the cathode in a 7 M sodium hydroxide (NaOH) solution. The extracted powder was then subjected to acid digestion using a mixture of concentrated hydrochloric acid (HCl) and nitric acid (HNO3) to prepare the leachate for elemental analysis via inductively coupled plasma (ICP).
Deep eutectic solvent (DES) was synthesized using choline chloride (ChCl) and D-glucose. All chemicals, including NaOH, HCl, H2SO4, ChCl, and D-glucose, were purchased from Thermo Fisher Scientific (Waltham, MA, USA). Deionized (DI) water, obtained from an in-lab water purification system, was used for all experimental procedures, including sample preparation, washing, and dilution.

2.2. Instrumentation and Analysis Methods

A comprehensive set of analytical techniques was employed to characterize the materials and assess the efficiency of the leaching process. Inductively Coupled Plasma Mass Spectrometer (ICP-MS, 7700 series, Agilent, Santa Clara, CA, USA) was used to determine the concentration of metal ions in the cathode material. Additionally, an Atomic Absorption Spectrometer (AAS, PerkinElmer, Waltham, MA, USA) was utilized to measure metal concentrations during the leaching process, enabling the calculation of leaching efficiency.
The characterization of the DES and its components was conducted using Fourier Transform Infrared Spectroscopy (FTIR, PerkinElmer, Waltham, MA, USA), while Differential Scanning Calorimetry (DSC, TA Universal, New Castle, DE, USA) was employed to investigate the thermal behavior and confirm DES formation.
The structural properties of the cathode material were analyzed using X-ray Diffraction (XRD, Rigaku, Tokyo, Japan). Particle size distribution was determined with a HORIBA Laser Scattering Particle Size Distribution Analyzer (LA-960, HORIBA Scientific, Kyoto, Japan). Additionally, a ball milling machine (Retsch, PM 200, Haan, Germany) was used to grind the cathode material with the aim of achieving a particle size below 75 µm for further processing.

2.3. Methodology

Spent LIBs were collected from the waste storage at Concordia University, Montreal, Canada. To mitigate the risk of fire during disassembly, the batteries were submerged in a 10% sodium chloride solution for 48 h to reduce their voltage to below 2 V, ensuring safe handling before the dismantling process [45,46]. The spent LIBs were manually disassembled using standard tools such as pliers and blades, to isolate the active cathode materials [47]. To minimize contamination, non-cathode components, including steel casings, anode sections, and plastic parts, were carefully removed. The cathode material, still adhered to aluminum foil, was then treated with a 7 M NaOH solution to selectively dissolve the aluminum according to the reaction:
A l + 2 N a O H + 2 H 2 O → 2 N a A l O 2 + 3 H 2
After the aluminum dissolution, the remaining cathode material was separated via filtration and thoroughly washed to eliminate any residual solution. To remove organic binders such as polyvinylidene fluoride (PVDF) and conductive additives like carbon black, the material was dried at 70 °C for 24 h. Subsequently, the spent cathode material underwent calcination at 600 °C for 6 h to eliminate residual graphite and other impurities. Finally, the purified cathode material was finely milled using a ball milling machine to reduce particle size and increase surface area for further processing [46,47]. The particle size distribution of the cathode powder was measured using a Laser Scattering Particle Size Distribution Analyzer (Horiba LA-960, HORIBA Scientific, Kyoto, Japan). The analysis was performed under agitation speed setting 3, with ultrasonic dispersion applied for 20 s to ensure homogenous dispersion. A refractive index of 1.44–0.10i was used. The output included frequency and cumulative distributions, as well as statistical values such as mean, median, and standard deviation. To analyze the metal content of the cathode powder, 0.5 g of the dried sample was weighed and dissolved in 20 mL of an acid mixture consisting of concentrated nitric acid (HNO3, 65%) and hydrochloric acid (HCl, 36%) in a 1:3 (v/v) ratio. The mixture was digested for 1.5 h at room temperature. After digestion, 15 mL of the solution was transferred to a 100 mL volumetric flask and diluted to the mark with demineralized water. From this solution, 1 mL was taken and further diluted to 25 mL using 1% HNO3 to prepare the final sample for ICP analysis.

2.3.1. Preparation for DES

Two types of deep eutectic solvents (DESs) were prepared: binary DES and ternary DES, both utilizing choline chloride (ChCl) ([(CH3)3NCH2CH2OH] Cl) as the hydrogen bond acceptor. For the binary DES, choline chloride and dextrose (D-glucose) (C6H12O6) were mixed in a 2:1 molar ratio and heated at 70 °C with continuous stirring at 400 rpm for 3 h until a homogeneous and transparent liquid was formed [46,48]. However, the binary DES exhibited excessively high viscosity, making handling difficult and potentially limiting metal ion diffusion during the leaching process.
To reduce viscosity and enhance leaching efficiency, a ternary DES was formulated by incorporating 10 wt.% water into the ChCl-D-glucose system. The primary advantage of the ternary DES lies in its enhanced leaching performance due to improved fluidity and mass transfer, rather than simply reducing the preparation time or mixing energy. This modification reduced the formation time, as the ternary DES was obtained within 30 min under the same temperature (70 °C) and stirring speed (400 rpm). The addition of water improved fluidity while maintaining solvent stability [48]. Both DES formulations were stored in sealed containers at room temperature and used for subsequent leaching experiments.

2.3.2. Leaching Process

A specific amount of cathode powder (50 mg) was added to 5 mL of the produced DES in a glass vial. Due to sample availability and instrument access limitations, each leaching condition was tested once as a representative measurement. The results are intended to indicate the general trends observed under the tested conditions. To examine their impact on metal leaching and extraction, the mixture was stirred at 400 rpm on a hotplate stirrer under various temperature and time conditions. Temperatures between 60 and 110 °C and extraction durations between 2 and 24 h were among the parameters under investigation. Following the extraction procedure, each sample was filtered using a 0.45 μm syringe filter, and the filtrate was then analyzed by atomic absorption spectroscopy (AAS) to determine the concentrations of dissolved metals. The following equation was used to calculate the metal extraction efficiency (η):
η =   ( C M e × V ) / ( m × X ) × 100
where CMe is the metal ion concentration of Li, Co, Ni, and Mn (mg·L−1); V is the volume of the leaching solution (L); m is the mass of the NCM powder (cathode active material, mg); and X is the mass fraction of Li, Co, Ni, and Mn in the cathode active material.
Each leaching condition was performed once as a single batch due to sample and instrument constraints. For analytical reliability, metal concentrations in each filtrate were measured in triplicate (three aliquots of the same solution) by AAS, and the mean value is reported. These are instrumental/analytical replicates, not independent experimental replicates. Consequently, error bars representing experimental variability are not shown, and the results are intended to indicate the observed trends under the tested conditions.

3. Results and Discussion

3.1. Characterizations of the Spent LIB Sample and DES

The particle size distribution of the cathode powder is presented in Figure 1. The red frequency distribution curve indicates that most particles fall within the clay-size range. The green cumulative curve shows that approximately 90% of the particles are smaller than 3 µm, with a sharp rise suggesting a uniform and fine particle distribution. The measured median particle size was 1.513 µm, and the mean size was 1.802 µm, with a standard deviation of 1.093 µm.
After the acid digestion process, an ICP (Inductively Coupled Plasma) analysis was performed to measure the concentrations of various metals in the cathode powder. As shown in Table 1. Elemental composition of cathode powder analyzed by ICP, very small trace amounts of aluminum and copper were detected in the powder. This indicates that the aluminum removal and impurity elimination processes, carried out through dissolution in sodium hydroxide, were effectively executed. The low presence of these metals (0.17 wt.% Al and 0.017 wt.% Cu) suggests that the purification process was successful, leading to a relatively clean cathode material suitable for further analysis and recovery processes. The use of ICP-MS was crucial in accurately characterizing the metal composition of the cathode powder.
In this study we focus on introducing a water-enhanced choline chloride–D-glucose DES that lowers viscosity and melting point, accelerating mass transfer and enabling greener leaching of Li-ion cathodes. It is expected that this environmentally friendly, non-toxic, and biodegradable alternative also offers tunable metal selectivity via DES composition. Li, Mn, and Ni will be examined using the enhanced DES based on prioritization. Building on the results, we will next apply the same DES platform to cobalt and other precious metals.
The X-ray diffraction (XRD) pattern of the cathode material after heat treatment at 600 °C for 6 h (Figure 2) is dominated by reflections characteristic of a spinel Li (Mn,Ni,Co)2O4 structure, confirming partial substitution of Mn by Ni and Co as indicated by the elemental composition in Table 1. The primary diffraction peaks at 2θ ≈ 18.7° (111), 36.3° (311), 44.2° (400), 58.3° (511), and 64.0° (440) correspond well to the LiMn2O4-type spinel phase (ICDD PDF 35-0782). The pronounced (111) reflection indicates high crystallinity, while the close match of other reflections shows that the overall spinel framework is preserved. The slight shifts in peak positions are consistent with lattice distortion caused by Ni/Co substitution at Mn sites. In addition to the Li-containing spinel phase, weak reflections near ~42.4° and ~73.7° match those expected for a minor CoO phase. Furthermore, weak peaks near ~18.9° and ~44.5° may coincide with LiCoO2 (003, 104) reflections, but their low intensity and the compositional data suggest LiCoO2 is a minor or impurity phase rather than a primary constituent.
The Differential Scanning Calorimetry (DSC) analysis confirms the successful formation of deep eutectic solvents (DESs) through a significant reduction in melting points compared to their individual components (Figure 3). The binary DES (ChCl + D-glucose) exhibits a melting point of 81.06 °C, while the ternary DES (ChCl + D-glucose + 10 wt.% water) melts at 50.34 °C. This decreases in melting temperature, relative to pure choline chloride (302 °C) and D-glucose (146 °C), confirms eutectic formation. While strong hydrogen bonding between ChCl and glucose can restrict molecular mobility and thus raise the melting point, the incorporation of water disrupts these interactions, lowering the melting temperature. The very small molecular size of water, compared to ChCl and glucose, likely further enhances the entropy of mixing and stabilizes the liquid phase at lower temperatures [33,49].
The addition of 10 wt.% water in the ternary DES significantly influences its thermal properties. Water acts as a hydrogen bond donor and plasticizer, weakening intermolecular interactions and further lowering the melting point [50,51,52,53]. This effect is well-documented in previous studies, where water incorporation enhances fluidity, reduces viscosity, and expands the liquid phase stability of DESs [53,54]. The lower melting point of the ternary DES compared to the binary system highlights the critical role of water in modifying the physicochemical properties of eutectic solvents, making them more adaptable for various applications.
In addition to the shift in melting points, the DSC thermograms reveal a distinct difference in peak shapes between the binary and ternary DESs. The binary DES shows a sharp, well-defined endothermic peak, indicating a relatively ordered hydrogen bonding network and a clear phase transition. In contrast, the ternary DES exhibits a broader and smoother melting peak. This can be attributed to the structural disruption caused by the presence of water, which introduces molecular disorder and increases configurational entropy. Water interferes with the organized hydrogen bonding between ChCl and glucose, resulting in a distribution of melting events over a broader temperature range. Similar broad thermal transitions in water-containing DESs have been reported in previous studies [49], supporting the view that the broadened peak reflects increased heterogeneity in the system.
Figure 4 illustrates the viscosity behavior of binary and ternary DES systems evaluated at room temperature (25 °C) across various spindle speeds ranging from 50 to 500 RPM. The binary DES, composed of choline chloride and glucose, exhibited significantly higher viscosities, starting at 6.6 Pa·s at 50 RPM and gradually decreasing to 5.31 Pa·s at 500 RPM. In contrast, the ternary DES—prepared by incorporating 10 wt.% water—showed a much lower viscosity range, from 2.1 Pa·s to 1.065 Pa·s. The consistent decrease in viscosity with increasing speed indicates pseudoplastic (shear-thinning) behavior, which is typical of DES systems. The considerable reduction in viscosity upon water addition is attributed to the disruption of the strong hydrogen bonding network between DES components. This not only reduces internal resistance and improves fluidity, but also enhances mass transport and diffusion [48,51,55]. Such improved dynamic properties are particularly beneficial in leaching applications, where efficient metal recovery from spent batteries depends heavily on effective solvent mobility.
Figure 5 presents the UV-Vis absorption spectra of both binary and ternary DES systems, as well as a reference blank, over the wavelength range of 200 to 350 nm. The binary DES (Choline Chloride: Glucose) and the ternary DES (with 10 wt.% water added) exhibit remarkably similar spectral profiles, with a strong absorbance peak below 220 nm followed by a rapid decline toward baseline levels above 250 nm. The pronounced absorption in the deep UV region (<220 nm) is attributed to the n→σ* and π→π* electronic transitions associated with functional groups such as hydroxyls and amines, which are abundant in both DES constituents. The similarity between the two spectra suggests that the addition of 10 wt.% water does not significantly alter the electronic environment or the primary functional group interactions within the DES system. This implies that while water affects physical properties like viscosity, it does not disrupt the core molecular structure or bonding configurations responsible for UV absorbance. Therefore, comparable spectral behavior supports the chemical stability of the DES system upon water incorporation, which is favorable for maintaining leaching functionality during hydrometallurgical processing.
The FTIR spectra (Figure 6) confirms formation of the DES (ChCl + D-glucose + water) through shifts in key vibrational bands. The broad O–H stretching band, observed at ~3220 cm−1 (ChCl) and ~3198 cm−1 (D-glucose), shifts to ~3265 cm−1 in the DES, indicating a modified (weaker) hydrogen-bonding network. A C–H stretching band appears at ~2922 cm−1 in the DES. The glucose C–O stretching band shifts from ~990 to ~1028 cm−1 in the DES, consistent with hydrogen-bond-affected bond environments. A band near ~1471 cm−1 is assigned to C–H scissoring, and the ~954 cm−1 band associated with the quaternary ammonium of ChCl remains, indicating retention of ionic character. Taken together, these features confirm DES formation with an overall weaker hydrogen-bonding network, consistent with the DSC evidence that added water disrupts strong interactions and lowers the melting point.

3.2. Leaching of Spent LIB Sample

3.2.1. Effect of Temperature on Leaching Efficiency Using Binary and Ternary DES

The leaching experiments were conducted at various temperatures with a fixed leaching time of 24 h. Figure 7 and Figure 8 present the comparative leaching efficiency of Li, Mn, and Ni at different temperatures (60–110 °C) using binary and ternary deep eutectic solvents (DESs). The results show that the ternary DES system exhibits significantly higher leaching efficiencies for all three metals across the tested temperature range compared to the binary DES system.
In the case of the binary DES system (Figure 8), the leaching efficiencies of Li, Mn, and Ni increased with temperature, reaching maximum values of 88.1%, 84.2%, and 54.8% at 110 °C, respectively. Although the melting point of the binary ChCl–glucose DES is approximately 81 °C (as confirmed by DSC analysis), leaching tests were also conducted at 60 °C to assess the threshold of leaching performance. At this lower temperature, the DES exists in a semi-solid or highly viscous state, but partial liquefaction and limited molecular mobility occur under continuous heating and stirring, permitting partial metal leaching with reduced efficiency. These results illustrate the onset of leaching capability below the melting point and support the identification of 100 °C as the optimal temperature for efficient metal extraction with this DES system. At lower temperatures (e.g., 60 °C), extraction efficiencies were substantially lower, particularly for Ni (7.6%) likely due to the stronger crystal-field stabilization and ionic bonding of Ni within the host spinel lattice (with Ni/Co substitution). Similar trends have been reported in previous studies, where Ni exhibited lower leaching rates because of its higher lattice energy and lower mobility compared to Li and Mn [30,56]. The consistent improvement in Li and Mn leaching with increasing temperature aligns with kinetic models indicating that higher temperatures enhance diffusion and surface reaction rates in DES systems.
In contrast, the ternary DES system (Figure 7), modified with 10 wt.% water showed higher performance under experimental conditions at all temperature levels. The addition of water has been shown to significantly reduce the viscosity of DESs, improving ion mobility and enhancing metal dissolution kinetics [48,51,55,57]. At 60 °C, Li, Mn, and Ni showed higher leaching efficiencies (23.5%, 24.5%, and 7.9%, respectively) compared to the binary DES. The efficiencies increased with temperature and reached 98.9%, 98.4%, and 71.4% for Li, Mn, and Ni, respectively, at 110 °C. However, leaching efficiency plateaued beyond 100 °C, indicating that increased temperature no longer yields proportional improvement due to the saturation of metal-DES complexation and changes in solvent structure at elevated temperatures, consistent with observations by [46].
Importantly, the ternary DES achieved >90% recovery for Li and Mn at just 90 °C, whereas the binary DES required 100 °C to reach similar extraction levels. This result underscores the higher reactivity and lower energy demand of the ternary DES, which aligns with recent findings that show aqueous-modified DESs can significantly enhance leaching rates while reducing thermal requirements [58,59,60].
The values reported in Figure 7 are means of triplicate analytical measurements; as each condition was tested once, the results are presented without error bars and interpreted as observed trends. Overall, these findings demonstrate the superiority of the ternary DES system for metal recovery from spent LIBs, attributable to its lower viscosity, enhanced mass transfer, and improved complexation capacity. Such systems offer a promising low-energy, environmentally friendly alternative to conventional leaching methods.

3.2.2. Effect of Leaching Time on Metal Extraction Efficiency Using Ternary DES

Figure 8 illustrates the effect of leaching time at 100 °C on the extraction efficiency of Li, Mn, and Ni from spent LIB cathode material using the ternary DES system. The results demonstrate a clear time-dependent improvement in metal dissolution, with substantial increases observed as the reaction time progressed from 2 to 24 h.
At the initial stage (2 h), the leaching efficiencies were relatively low, with Li, Mn, and Ni reaching only 29.3%, 3.4%, and 2.0%, respectively. This suggests that the dissolution of active cathode components requires an initial activation period, during which DES components penetrate the solid matrix and begin breaking down the metal-ligand bonds [34,40].
With increasing leaching duration, metal extraction efficiency showed a steady enhancement. After 12 h, Li, Mn, and Ni reached 53.7%, 24.5%, and 12.0%, respectively, indicating that prolonged contact time significantly facilitates the diffusion and solubilization of metal ions into the DES phase.
A substantial improvement was observed after 18 h, where Li and Mn leaching efficiencies reached 87.9% and 42.5%, while Ni extraction remained comparatively lower at 33.1%, likely due to the stronger interaction of Ni within the cathode material lattice. This trend aligns with previous studies reporting that lithium is more readily leached due to its higher mobility and weaker bonding in layered oxide structures, while Ni shows stronger bonding within the crystal lattice, making it less accessible under similar conditions [57,61]. The moderate recovery of Mn is consistent with its intermediate bonding strength and partial reduction during leaching processes. These observations suggest that extended leaching time favors the dissolution of loosely bound metals, while more tightly bound elements like Ni may require harsher conditions or stronger complexing agents for higher recovery.
At 24 h, the dissolution process reached its peak, with Li, Mn, and Ni achieving 98.9%, 98.4%, and 71.2% leaching efficiency, respectively. This indicates that near-complete metal recovery is attainable with prolonged leaching, confirming the high capability of ternary DES in facilitating metal dissolution over extended durations.
These findings indicate that shorter reaction times do not yield optimal recovery, whereas extending the leaching duration beyond 18 h markedly improves efficiency, particularly for Li and Mn. The notable increase in Ni dissolution at longer durations underscores the importance of prolonged leaching for enhanced metal recovery; however, the incomplete Ni dissolution (~71%) suggests the presence of an undissolved solid residue likely enriched in Ni. Due to the small experimental scale (50 mg in 5 mL), the recovered residue was insufficient for phase or elemental analysis. Therefore, no further speculation is made regarding its exact composition.

3.2.3. Visual Indicators of Leaching Efficiency

The color change of the deep eutectic solvent (ternary DES) solution serves as a qualitative marker of metal dissolution, providing visual confirmation of leaching efficiency under varying experimental conditions. As shown in Figure 9, higher temperatures (a) and longer reaction times (b) result in progressively darker solutions, indicating increased dissolution of metal ions into the solvent. This phenomenon aligns with quantitative leaching results, demonstrating that elevated temperatures and extended durations enhance the extraction of lithium and manganese, while nickel exhibits relatively lower solubility under the same conditions. The correlation between color intensity and metal dissolution underscores the potential of DES-based leaching as an efficient and environmentally friendly approach for LIB recycling.
The differences in liquid volume observed in Figure 10b are due to aliquot withdrawal and subsequent dilution for AAS analysis; all leaching efficiencies were calculated based on an initial DES volume of 5.0 mL, and reported concentrations were corrected for the applied dilution factors. In the ChCl–D-glucose–H2O DES, Li dissolves via rapid de-intercalation and solvation of Li+ without a redox step [1]. Surface Mn (III/IV) is reduced to Mn (II) by glucose and subsequently stabilized through chloride/oxygen coordination [1]. Ni dissolution is ligand-promoted but proceeds more slowly, consistent with its stronger crystal-field stabilization and M–O bonding within the spinel lattice [62]. Overall, the observed selectivity reflects coordination-environment control in the DES, favoring Li and Mn over Ni under the present conditions [62].

4. Conclusions

This study successfully introduced and evaluated the performance of deep eutectic solvents (DESs), with a particular focus on a ternary choline chloride–D-glucose–water formulation, for the selective leaching of Li, Mn, and Ni from spent lithium-ion battery cathodes. By examining the effects of temperature and leaching duration, it provides clear evidence that DES-based hydrometallurgical systems can be optimized for high-efficiency, environmentally sustainable metal recovery.
The ternary DES demonstrated markedly superior performance over its binary counterpart, owing to its reduced viscosity, enhanced mass transfer, and improved metal ion solubility. Incorporating 10 wt.% water significantly accelerated leaching kinetics, enabling near-complete extraction of Li (98.9%) and Mn (98.4%) under optimized conditions, while Ni reached a recovery of 71.7% despite its stronger lattice binding. Efficiency gains plateaued above 100 °C, indicating an optimal temperature threshold, and time-dependent experiments showed that the most substantial improvements occurred within the first 18–24 h. The observed correlation between solution color intensity and metal dissolution further suggests a practical visual indicator for leaching progress.
From a broader perspective, these results confirm the potential of water-enhanced DES systems as a low-energy, non-toxic, and biodegradable alternative to conventional acid-based leaching methods, eliminating the need for strong acids and oxidants. Their tunable coordination environment offers opportunities to selectively target different metals, supporting more sustainable battery recycling pathways. Future studies could focus on tailoring DES compositions to further enhance Ni dissolution, exploring synergistic effects with other green additives, and assessing solvent recyclability and regeneration. Process scale-up with independent replicates and robust statistical analysis will help establish industrial applicability, while economic and life-cycle assessments can guide commercial adoption. By building on the promising outcomes of this work, continued research can accelerate the transition of DES-based recycling into practical, scalable solutions that advance the circular economy for lithium-ion batteries.

Author Contributions

Conceptualization, J.H. and K.Z.; Methodology, J.G. and Z.C.; Formal analysis, G.Z.; Writing—original draft, J.G.; Writing—review & editing, G.Z., J.H., K.Z., S.D., A.A.D., X.W., F.H., C.N.M., C.A. and A.A.R.; Supervision, Z.C.; Funding acquisition, Z.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Volt-Age Seed Grant at Concordia University.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Results of particle size distribution analysis for the ball-milled cathode powder obtained using the LA-950 analyzer. The green line represents the cumulative particle size distribution (%), while the red line indicates the frequency distribution (%).
Figure 1. Results of particle size distribution analysis for the ball-milled cathode powder obtained using the LA-950 analyzer. The green line represents the cumulative particle size distribution (%), while the red line indicates the frequency distribution (%).
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Figure 2. XRD pattern of the active cathode material after heat treatment at 600 °C for 6 h. Major peaks are indexed as (hkl) and correspond to the spinel Li (Mn,Ni,Co)2O4 phase; weak features are consistent with minor CoO (e.g., weak features near ~42.4° and ~73.7° are consistent with CoO).
Figure 2. XRD pattern of the active cathode material after heat treatment at 600 °C for 6 h. Major peaks are indexed as (hkl) and correspond to the spinel Li (Mn,Ni,Co)2O4 phase; weak features are consistent with minor CoO (e.g., weak features near ~42.4° and ~73.7° are consistent with CoO).
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Figure 3. The DSC of the ternary and binary DES from −50 °C to 180 °C.
Figure 3. The DSC of the ternary and binary DES from −50 °C to 180 °C.
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Figure 4. Viscosity comparison of binary and ternary DESs at room temperature.
Figure 4. Viscosity comparison of binary and ternary DESs at room temperature.
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Figure 5. UV-Vis spectra of binary and ternary DESs between 200 and 400 nm.
Figure 5. UV-Vis spectra of binary and ternary DESs between 200 and 400 nm.
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Figure 6. FTIR spectra comparing the DES with its individual components (choline chloride and glucose).
Figure 6. FTIR spectra comparing the DES with its individual components (choline chloride and glucose).
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Figure 7. Effect of temperature on leaching efficiency (ternary DES) after 24 h leaching (each condition was measured once; triplicate values refer to analytical replicates of the same filtrate).
Figure 7. Effect of temperature on leaching efficiency (ternary DES) after 24 h leaching (each condition was measured once; triplicate values refer to analytical replicates of the same filtrate).
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Figure 8. Effect of temperature on leaching efficiency (binary DES) after 24 h leaching (each condition was measured once; triplicate values refer to analytical replicates of the same filtrate).
Figure 8. Effect of temperature on leaching efficiency (binary DES) after 24 h leaching (each condition was measured once; triplicate values refer to analytical replicates of the same filtrate).
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Figure 9. Effect of time on leaching efficiency (ternary DES).
Figure 9. Effect of time on leaching efficiency (ternary DES).
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Figure 10. (a) Change in color of leaching solution for different temperatures over 24 h. (b) Change in color of leaching solution for different times at 100 °C.
Figure 10. (a) Change in color of leaching solution for different temperatures over 24 h. (b) Change in color of leaching solution for different times at 100 °C.
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Table 1. Elemental composition of cathode powder analyzed by ICP-MS.
Table 1. Elemental composition of cathode powder analyzed by ICP-MS.
ElementLithium (Li)Manganese (Mn)Nickel (Ni)Cobalt (Co)Aluminum (Al)Copper (Cu)
Weight Percentage (%)2.533.788.747.900.170.017
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Goudarzi, J.; Chen, Z.; Zhang, G.; Hu, J.; Zaghib, K.; Deng, S.; Dar, A.A.; Wang, X.; Haghighat, F.; Mulligan, C.N.; et al. Sustainable Recovery of Critical Metals from Spent Lithium-Ion Batteries Using Deep Eutectic Solvents. Batteries 2025, 11, 340. https://doi.org/10.3390/batteries11090340

AMA Style

Goudarzi J, Chen Z, Zhang G, Hu J, Zaghib K, Deng S, Dar AA, Wang X, Haghighat F, Mulligan CN, et al. Sustainable Recovery of Critical Metals from Spent Lithium-Ion Batteries Using Deep Eutectic Solvents. Batteries. 2025; 11(9):340. https://doi.org/10.3390/batteries11090340

Chicago/Turabian Style

Goudarzi, Jafar, Zhi Chen, Gaixia Zhang, Jinguang Hu, Karim Zaghib, Sixu Deng, Afzal Ahmed Dar, Xiaolei Wang, Fariborz Haghighat, Catherine N. Mulligan, and et al. 2025. "Sustainable Recovery of Critical Metals from Spent Lithium-Ion Batteries Using Deep Eutectic Solvents" Batteries 11, no. 9: 340. https://doi.org/10.3390/batteries11090340

APA Style

Goudarzi, J., Chen, Z., Zhang, G., Hu, J., Zaghib, K., Deng, S., Dar, A. A., Wang, X., Haghighat, F., Mulligan, C. N., An, C., & Avalos Ramirez, A. (2025). Sustainable Recovery of Critical Metals from Spent Lithium-Ion Batteries Using Deep Eutectic Solvents. Batteries, 11(9), 340. https://doi.org/10.3390/batteries11090340

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