Efficient Ionic Liquid-Based Leaching and Extraction of Metals from NMC Cathodes

Abstract

The increasing demand for lithium-ion batteries (LIBs) and their limited lifespan emphasize the urgent need for sustainable recycling strategies. This study investigates the application of tetrabutylphosphonium-based ionic liquids (ILs) as alternative leaching agents for recovering critical metals, Li(I), Co(II), Ni(II), and Mn(II), from spent NMC cathode materials. Initial screening experiments evaluated the leaching efficiencies of nine tetrabutylphosphonium-based ILs for Co(II), Ni(II), Mn(II), and Li(I), revealing distinct metal dissolution behaviors. Three ILs containing HSO₄⁻, EDTA²⁻, and DTPA³⁻ anions exhibited the highest leaching performance and were selected for further optimization. Key leaching parameters, including IL and acid concentrations, temperature, time, and solid-to-liquid ratio, were systematically adjusted, achieving leaching efficiencies exceeding 90%. Among the tested systems, [TBP][HSO₄] enabled near-complete metal dissolution (~100%) even at room temperature. Furthermore, an aqueous biphasic system (ABS) was investigated utilizing [TBP][HSO₄] in combination with ammonium sulfate, enabling the complete extraction of all metals into the salt-rich phase while leaving the IL phase metal-free and potentially suitable for reuse, indicating the feasibility of integrating leaching and extraction into a continuous, interconnected process. This approach represents a promising step forward in LIB recycling, highlighting the potential for sustainable and efficient integration of leaching and extraction within established hydrometallurgical frameworks.

1. Introduction

In recent years, the rapid development of technology and its integration into modern society have significantly increased the global demand for lithium-ion batteries (LIBs). These batteries have become indispensable components of numerous everyday devices, such as smartphones and laptops, electric vehicles, and renewable energy storage systems [1,2,3]. Lithium-ion batteries are composed of four primary components: the anode, cathode, separator, and electrolyte [4,5]. Among all these components, the cathode material is especially important, as it contains valuable transition metals and largely defines the electrochemical performance and cost of the battery [6]. The cathode materials commonly used include lithium cobalt oxide (LiCoO₂, LCO), lithium manganese oxide (LiMn₂O₄, LMO), lithium iron phosphate (LiFePO₄, LFP), and lithium nickel manganese cobalt oxide (LiNi_xMn_yCo_zO₂, NMC), each with varying metal content and performance characteristics [7]. Lithium nickel manganese cobalt oxide (NMC, LiNi_xMn_yCo_zO₂) cathode materials exist in several formulations, classified by type, including NMC111 (LiNi_0.33Co_0.33Mn_0.33O₂), NMC532 (LiNi_0.5Co_0.2Mn_0.3O₂), NMC622 (LiNi_0.6Co_0.2Mn_0.2O₂), and NMC811 (LiNi_0.8Co_0.1Mn_0.1O₂) [8].

Given the growing demand for these batteries and the increase in their post-consumer disposal, the recovery of critical metals such as lithium, cobalt, nickel, and manganese from spent cathode materials has become an urgent environmental and economic priority [9,10,11,12]. Effective metal recovery not only mitigates the environmental risks associated with battery waste but also aligns with sustainability principles and complies with increasingly stringent environmental regulations [9]. The recycling of cathode material involves multiple steps, with leaching as a fundamental initial stage following the physical separation of battery components. The central goal of the leaching step is to dissolve metals released from the pretreated cathode materials into their soluble ionic forms, allowing for their subsequent separation and recovery from the aqueous leachates. Common leaching agents include mineral acids such as hydrochloric acid (HCl) [13,14,15], nitric acid (HNO₃) [16,17], and sulfuric acid (H₂SO₄) [18] at various concentrations, often used in combination with hydrogen peroxide (H₂O₂) to enhance metal dissolution. From an industrial perspective, these acids are favored as leaching agents primarily because of their rapid kinetics, lower operational temperatures, and ability to efficiently handle higher solid-to-liquid ratios [19], but depending on specific process conditions, they may exhibit lower selectivity, often requiring subsequent purification steps that increase wastewater production and overall process complexity. While mineral acids have been widely applied for cathode leaching, alternative approaches utilizing organic acids, such as lactic and citric acids, have also been explored, but although organic acids offer a more environmentally friendly alternative, they generally require longer processing times and elevated temperatures to achieve comparable leaching efficiencies [20,21]. Modern trends in hydrometallurgical metal recovery prioritize circular hydrometallurgy principles, emphasizing sustainable use of resources and minimal environmental impact. Another important step in this approach is the regeneration and reuse of reagents, significantly reducing waste generation and mitigating environmental risks [22]. Such circular practices require carefully designed process flowsheets to optimize reagent recycling and ensure lower energy consumption and operational costs.

In this context, ionic liquids (ILs) have emerged as versatile and highly promising reagents for sustainable metal recovery, owing to their unique combination of low vapor pressure, high thermal stability, and tunable physicochemical properties, such as polarity and solubility [23,24,25,26,27,28,29,30]. Composed of bulky asymmetric organic cations and a wide range of inorganic or organic anions, ILs are liquid salts with melting points typically below 100 °C [31,32,33]. These characteristics enable their application as both leaching agents and extraction solvents in hydrometallurgical processes, positioning them as efficient and environmentally friendly alternatives to conventional solvents used in metal leaching, separation, and electrodeposition [34,35,36,37,38]. Despite their pronounced advantages, practical applications are sometimes limited by their inherently high viscosities, which may hinder mass transfer and operational feasibility. Moreover, questions related to their toxicity and long-term environmental effects remain open, requiring further systematic evaluation [32,33]. While imidazolium- and pyridinium-based ILs have demonstrated significant toxicity and bioaccumulation potential, phosphonium-based ILs exhibit considerably different environmental behaviors [39], especially those paired with carboxylate, amino acid, and other bio-derived anions. These biocompatible anions contribute to reduced toxicity and enhanced biodegradability, making these ILs promising candidates for sustainable hydrometallurgical applications [40]. Recent studies have shown that phosphonium-based ILs possess low acute toxicity towards aquatic species and minimal bioaccumulation potential [39,41,42]. Moreover, their tunable structure offers unique advantages in hydrometallurgical applications. Their customizable selectivity and solubility enable efficient targeting of specific metal species, making them particularly valuable for the extraction and separation of critical metals from complex waste matrices, such as lithium-ion battery cathodes. To date, most IL-based leaching studies have focused on the recovery of different metals from electronic waste, particularly printed circuit boards, where acidic imidazolium-based ILs have shown excellent extraction performance under optimized conditions [23,24,25,26,27]. However, their application in lithium-ion battery recycling remains relatively limited. One recent study demonstrated that imidazolium ILs can destabilize the LCO lattice, promoting the reduction of Co(III) to Co(II) and enhancing metal solubilization via chlorocomplex formation [28]. This promising yet isolated example underscores the need for further research, particularly on NMC cathodes, which are widely used and contain multiple critical metals, making their recovery both valuable and challenging. Although the application of ILs for the direct leaching of LIB cathodes remains relatively underexplored, their role in the extraction of metals has been widely studied. Imidazolium-based ILs have shown high selectivity for metal recovery from cathode LIB waste, with notable extraction efficiencies achieved in optimized conditions [43,44,45,46]. Also, hydrophobic phosphonium-based ILs have demonstrated remarkable performance in the extraction and separation of cobalt, nickel, manganese, and lithium from real battery cathode leachates [47,48,49,50,51,52]. Recent studies have also highlighted the successful application of phosphonium-based aqueous biphasic systems for selective metal extraction from LCO leachates, achieving recovery efficiencies above 98% for cobalt while lithium was separated in another phase [36], or effective separation of iron and lithium from LFP cathodes, maintaining efficiency over multiple cycles [53].

To improve leaching efficiency while minimizing the use of strong mineral acids, this study investigates the potential of tetrabutylphosphonium-based ILs with different anions as alternative leaching agents for metal leaching from NMC cathode materials. These ILs not only enable selective metal dissolution under milder conditions but also contribute to a more sustainable and environmentally friendly process. In addition to leaching, downstream separation and purification of dissolved metals represent crucial steps in the overall process. To address this, aqueous biphasic systems (ABSs) based on ILs have emerged as attractive alternatives to conventional organic solvent extraction systems [54]. In an IL-based ABS, the ionic liquid is combined with a salting-out agent, typically forming two immiscible aqueous phases [55]. These systems retain the selective extraction properties of ILs while reducing viscosity and improving mass transfer. The application of IL-based ABS offers an environmentally benign platform for metal recovery, wherein ILs can be reused, and phase separation is achieved without hazardous organic solvents [38].

In this study, we investigated the potential of tetrabutylphosphonium-based ionic liquids as both leaching agents and extraction media, aiming to develop an efficient and sustainable approach for the leaching and extraction of Li(I), Co(II), Ni(II), and Mn(II) from spent NMC cathode materials. The direct incorporation of ILs into the leaching system significantly reduced the required concentration of sulfuric acid while maintaining high metal dissolution efficiency. Among the nine synthesized ILs, tetrabutylphosphonium ethylenediaminetetraacetate [TBP][EDTA], tetrabutylphosphonium diethylenetriaminepentaacetate [TBP][DTPA], and tetrabutylphosphonium hydrogen sulfate [TBP][HSO₄] exhibited superior leaching performance and were selected for further process optimization. Following the leaching step, an [TBP][HSO₄] IL-based aqueous biphasic system was integrated using ammonium sulfate as the salting-out agent, enabling selective extraction of metals into the salt-rich phase while leaving the IL-rich phase free of metals and suitable for potential reuse. This combined leaching–extraction strategy underscores the versatility of ILs in metal dissolution and separation, offering a sustainable pathway for the efficient processing of lithium-ion battery waste.

2. Materials and Methods

2.1. Materials

Tetrabutylphosphonium hydroxide (40 wt.% in H₂O), salicylic acid (≥99% purity), glycolic acid (≥99% purity), ethylenediaminetetraacetic acid (EDTA, ≥99.5% purity), ammonium sulfate (≥99% purity), sodium chloride (≥99% purity), and acetone (≥99.5% purity) were obtained from Sigma-Aldrich (St. Louis, MO, USA). DL-lactic acid (≥90% purity), diethylene triaminepentaacetic acid (≥99% purity), and hydrochloric acid (37 wt.% in H₂O) were sourced from Fluka Chemie (Buchs, Switzerland). Nicotinic acid (≥99% purity) was supplied by ThermoFisher (Dreieich, Germany), while acetic acid (≥99% purity) came from MSK Kikinda (Kikinda, Serbia). Sulfuric acid (95–97 wt.% in H₂O) was provided by Zorka Pharma (Šabac, Serbia), while nitric acid (69 wt.% in H₂O) was provided by LOBA Chemie (Mumbai, India) by Fisher Scientific (Loughborough, UK). Hydrogen peroxide solution (30 wt.% in H₂O) was purchased from Alkaloid Skopje (Skopje, North Macedonia), and a 1000 mg L⁻¹ multi-element ICP standard solution was obtained from Chem-Lab (Zedelgem, Belgium). All experiments were conducted with ultrapure deionized water, prepared using a Purite Select Fusion water purification system.

A spent lithium-ion battery from a power tool was used as the real sample in this study, providing the cathode material for analysis.

2.2. Synthesis of Tetrabutylphosphonium-Based Ionic Liquids

Tetrabutylphosphonium-based ionic liquids, including tetrabutylphosphonium salicylate [TBP][Sal], tetrabutylphosphonium acetate [TBP][Ac], tetrabutylphosphonium glycolate [TBP][Gly], tetrabutylphosphonium lactate [TBP][Lac], tetrabutylphosphonium diethylenetriaminepentaacetate [TBP]₃[DTPA], tetrabutylphosphonium chloride [TBP]Cl, and tetrabutylphosphonium nicotinate [TBP][Nic], were synthesized in a previous study [36], and their chemical structures were confirmed using Fourier-transform infrared spectroscopy (FTIR). Additionally, two novel ionic liquids, tetrabutylphosphonium hydrogen sulfate [TBP][HSO₄] and tetrabutylphosphonium ethylenediaminetetraacetate [TBP]₂[EDTA], were synthesized via a neutralization reaction between tetrabutylphosphonium hydroxide and the respective acids, following established literature protocols [36,56]. The reaction mixtures, prepared with equimolar amounts of tetrabutylphosphonium hydroxide and the corresponding acid, were stirred for 24 h at room temperature with a constant stirring speed of 400 rpm. Water was removed through a 4-h vacuum treatment using a rotary evaporator, followed by further drying at 70 °C under vacuum conditions for 36 h. The dried ILs were stored in a desiccator containing P₂O₅. To assess water content, Karl Fischer titration was performed using an 831 Karl Fischer coulometer (Metrohm, Herisau, Switzerland), revealing values ≤ 250 ppm in all ILs.

The chemical structures of two newly synthesized ILs, [TBP][HSO₄] and [TBP]₂[EDTA], were confirmed through FTIR analysis in the 4000–500 cm⁻¹ range, conducted using a Nicolet iS5 spectrometer equipped with an iD7 ATR accessory (Thermo Fisher Scientific, Madison, WI, USA). Detailed spectra and peak assignments are provided in the Supplementary Materials (Figures S1 and S2). Ionic liquids [TBP]₃[DTPA] and [TBP]₂[EDTA] have the molar ratios between cations and anions of 3:1 and 2:1, respectively; however, for simplicity, they will be referred to as [TBP][DTPA] and [TBP][EDTA] throughout this manuscript.

2.3. Sample Preparation

The initial discharging of the lithium-ion battery from a power tool was performed by fully submerging it in a 10% (w/v) aqueous sodium chloride (NaCl) solution and leaving it overnight to ensure safe handling and complete discharging through enhanced ionic conductivity [57]. After discharging, a spent lithium-ion battery (NMC type) was carefully and manually disassembled. First, the plastic casing was removed using a precision knife, after which individual battery cells were separated and cut open using a grinder. The anode and cathode layers were then carefully unrolled and separated. The obtained materials were dried under ambient conditions for 24 h.

To isolate the cathode material from the aluminum foil, an ultrasonic separation process was combined with mechanical stirring. Cathode strips, approximately 1 cm × 2 cm, were immersed in 100 mL of deionized water and placed in an ultrasonic bath (SB-3LD, Belgrade, Serbia) operating at 40 kHz and 350 W for 2 h [58]. This process effectively detached the cathode material, dispersing it into the solution. The resulting suspension was then subjected to water evaporation at 100 °C using a laboratory magnetic stirrer (MAGD19, Belgrade, Serbia), leaving behind a fine black powder. The recovered material was filtered, thoroughly washed with deionized water, and dried at 120 °C for 5 h in a drying oven (Drying Oven Natural Air Convection DRYSA30, Belgrade, Serbia). The prepared cathode material was stored for subsequent leaching experiments. All pretreatment steps and the leaching process of the NMC battery are illustrated in Figure 1.

2.4. Leaching Procedures

The primary focus of this study was the investigation of ionic liquid-based leaching systems for metals from the black mass of spent NMC battery cathodes. In addition to IL leaching, aqua regia was employed as a reference method to determine the total metal content in the NMC cathode material, while sulfuric acid leaching was conducted under selected experimental conditions to serve as a benchmark for evaluating the performance of the IL-based systems. All leaching experiments were performed in triplicate to ensure reproducibility and statistical reliability of the obtained data.

The aqua regia method was used as a standard technique to determine the total metal content in the black mass, and this method was used as a benchmark for total metal content quantification. For this experiment, 1 g of the black mass was mixed with 20 mL of aqua regia (HCl:HNO₃ = 3:1). The reaction was conducted in a 100 mL round-bottom flask equipped with a reflux condenser to minimize evaporative losses. The mixture was stirred at 80 °C for 2 h using a magnetic stirrer set to 400 rpm, with the temperature maintained by a water bath. After this initial period, hydrogen peroxide was added to the leaching solution to a final volume fraction of 11 wt.%, and the reaction was allowed to proceed for an additional hour under the same conditions to enhance metal dissolution. After cooling to room temperature, the mixture was centrifuged using a Hermle Z287-A high-speed benchtop centrifuge (Hermle Labortechnik, Wehingen, Germany) at 5000 rpm for 5 min to separate the leachate from the residue. The supernatant was collected, diluted with ultrapure water, and analyzed for metal content using the inductively coupled plasma optical emission spectroscopy (ICP-OES) method (Thermo Scientific iCap 7400 duo ICP-OES instrument, Thermo Fisher Scientific, Waltham, MA, USA).

In parallel, sulfuric acid leaching was performed using 1.5 mol L⁻¹ H₂SO₄ with 4 wt.% hydrogen peroxide. The solid-to-liquid ratio was maintained at 50 g L⁻¹, and the reaction was carried out at 65 °C for 1 h under continuous stirring at 400 rpm, with temperature control provided by a water bath, following the same setup used for the aqua regia method. In addition to the standard leaching experiment, the effect of acid concentration on metal dissolution efficiency was systematically investigated. Leaching experiments were also conducted without the addition of sulfuric acid (deionized water and 4 wt.% hydrogen peroxide), as well as with 0.375 mol L⁻¹ and 0.75 mol L⁻¹ H₂SO₄, to assess the influence of acidity on the leaching performance. After each leaching experiment, the mixtures were centrifuged and analyzed for metal content by ICP-OES, following the same procedure described for aqua regia leachates.

Leaching using synthesized ionic liquids was the primary focus of this study, and the process was systematically optimized to determine the most effective conditions for metal recovery. All experiments were carried out in sealed 5 mL glass flasks placed in a thermo-shaker incubator (ALEMADR-MSC, Colo Lab Experts, Belgrade, Serbia) operating at a constant oscillation frequency of 400 rpm. For each experiment, 1 mL of leaching solution was used with the addition of 0.05 g, 0.1 g, or 0.2 g of NMC cathode material, corresponding to a solid-to-liquid ratio of 50, 100, or 200 g L⁻¹. All experiments were performed in triplicate to ensure reproducibility and statistical reliability of the results. The ionic liquids, whose structures are presented in Figure 2, were directly incorporated into the leaching systems, enabling a comprehensive evaluation of their performance in extracting Li(I), Co(II), Ni(II), and Mn(II) from spent NMC cathode material.

Several parameters were systematically varied to evaluate their influence on the leaching efficiency, including sulfuric acid concentration (0.0–1.5 mol L⁻¹), ionic liquid concentration (10 wt.%, 20 wt.%, 30 wt.%, and 40 wt.%), temperature (25, 45, and 65 °C), reaction time (5, 30, 45, 60, and 120 min), and solid-to-liquid ratio (50, 100, and 200 g L⁻¹). Each parameter was varied individually while keeping the others constant to assess its isolated effect. All experiments were performed in triplicate to ensure reproducibility, and detailed conditions are provided in

Table 1. Experimental parameters and their levels applied in leaching optimization.

Investigated Parameter	Investigated Levels	Constant Parameters
Sulfuric acid concentration (mol L−1)	0.000, 0.375, 0.750, 1.500	temperature = 65 °C, reaction time = 60 min, S/L ratio = 50 g L−1, stirring speed = 400 rpm
Amount of IL (wt.%)	10, 20, 30, 40	Concentration H2SO4 = 0.000 mol L−1, temperature = 65 °C, reaction time = 60 min, S/L ratio = 50 g L−1, stirring speed = 400 rpm
Amount of IL (wt.%)	10, 20, 30	Concentration H2SO4 = 0.375 mol L−1, temperature = 65 °C, reaction time = 60 min, S/L ratio = 50 g L−1, stirring speed = 400 rpm
Temperature (°C)	25, 45, 65	Concentration H2SO4 = 0.375 mol L−1, % of IL = 20%, S/L ratio = 50 g L−1, stirring speed = 400 rpm
Reaction time (min)	5, 30, 45, 60, 120	Concentration H2SO4 = 0.375 mol L−1, % of IL = 20%, S/L ratio = 50 g L−1, stirring speed = 400 rpm
Solid-to-liquid ratio (g L−1)	50, 100, 200	Concentration H2SO4 = 0.375 mol L−1, % of IL = 20%, temperature = 45 °C for [TBP][EDTA] and [TBP][DTPA], or 25 °C for [TBP][HSO4], stirring speed = 400 rpm

.

The concentrations of metals in all leachates were analyzed using inductively coupled plasma optical emission spectroscopy (Thermo Scientific iCap 7400 duo ICP-OES instrument). Leaching efficiency for each experiment was calculated using Equation (1), where n_leachate represents the number of moles of metal in the leachate and n_{aqua regia} is the total number of moles of metal in the aqua regia.

Leaching efficiency (%) = (n_leachate/n_{aqua regia}) × 100

(1)

2.5. Extraction of Metals Using an IL-Based Aqueous Biphasic System

Based on the previously established [TBP][HSO₄]-based ABS phase diagram determined by the cloud point titration method [59] and fitted using the Merchuk equation [60] (Figure S9 and Table S2, Supplementary Materials), ternary mixtures were prepared by combining 0.78 g of leaching solution (containing 20 wt.% IL [TBP][HSO₄]) with 0.22 g of ammonium sulfate. This yielded a final composition of 15 wt.% [TBP][HSO₄] and 22 wt.% salt, located within the biphasic region, with the corresponding extraction point indicated in Figure S9. The components were weighed and combined in 1.5 mL microcentrifuge tubes, resulting in a total system mass of 1 g. The mixtures were homogenized using a vortex mixer (Reax Top, Heidolph, Schwabach, Germany) at 5000 rpm and subsequently equilibrated for 2 h in a thermo-shaker incubator (ALEMADR-MSC, Colo Lab Experts). After the separation of phases, the system was centrifuged at 5000 rpm for 5 min (LLG-uniCFUGE 5, Lab Logistics Group, Meckenheim, Germany). Following centrifugation, the top and bottom phases were carefully separated and analyzed to determine the concentrations of metals. Metal concentrations were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Scientific iCap 7400 Duo).

Extraction efficiency (EE, %) is defined concerning the total metal content initially present in the biphasic system and for each metal was calculated using the following equation:

EE (%) = (n_{Salt-rich phase/}n₀) × 100

(2)

where n_{Salt-rich phase} and n₀ were the number of moles of the metal in the salt-rich phase and the entire system.

3. Results and Discussion

3.1. Leaching of Metals from NMC Cathode Material Using Mineral Acids

Aqua regia, one of the most powerful leaching agents available [61], was selected to determine the total metal content in the NMC cathode material. This aggressive acid mixture ensured complete dissolution of the sample, resulting in concentrations of 2.185 g L⁻¹ for Li(I), 4.865 g L⁻¹ for Co(II), 12.050 g L⁻¹ for Ni(II), and 7.225 g L⁻¹ for Mn(II). These values are considered benchmark concentrations, representing the maximum recoverable metal content and providing a critical reference point for evaluating the efficiency of alternative leaching systems explored in this study. Based on the relative proportions of the extracted transition metals, the elemental composition of the cathode suggests a material consistent with the NMC622 formulation (LiNi_0.6Co_0.2Mn_0.2O₂), characterized by a dominant nickel content. The observed elemental ratios are in good agreement with this identification, confirming the material type [8].

Also, leaching was performed using 1.5 mol L⁻¹ sulfuric acid and 4% hydrogen peroxide at 65 °C for 60 min, resulting in the following metal concentrations: Li(I) 2.31 g L⁻¹, Co(II) 4.51 g L⁻¹, Ni(II) 12.09 g L⁻¹, and Mn(II) 6.915 g L⁻¹. These values are comparable to those obtained with aqua regia, confirming that the combination of sulfuric acid and hydrogen peroxide serves as an effective leaching system for metal recovery. Optimal leaching conditions are generally acidic solutions (pH < 6), with maximum efficiency consistently observed in a narrower pH range between approximately 3.0 and 3.7, peaking particularly between 3.25 and 3.55 [62,63,64]. Under these conditions, transition metals readily dissolve, facilitated by the presence of H₂O_2, which significantly enhances metal dissolution by acting simultaneously as an oxidizing and reducing agent (Equations (3) and (4)). Specifically, H₂O₂ reduces Co(III) to Co(II) and Mn(IV) to Mn(II), improving their solubility and overall extraction efficiency [65,66,67]. Although lithium and nickel do not experience changes in oxidation state during leaching, their dissolution is promoted by the structural breakdown of the NMC lattice under acidic conditions.

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O  E₁⁰ = 1.77 V

(3)

O₂ + 2H⁺ + 2e⁻ → H₂O₂    E₁⁰ = 0.68 V

(4)

The chemical equation describing the leaching process of LiNi_0.6Co_0.2Mn_0.2O₂ cathode material in the presence of sulfuric acid and hydrogen peroxide can be represented as follows:

10LiNi_0.6Co_0.2Mn_0.2O₂(s) + 15H₂SO₄(aq) + 5H₂O₂→ 5Li₂SO₄(aq) + 6NiSO₄(aq) + 2CoSO₄(aq) + 2MnSO₄(aq) + 20H₂O(l) + 5O₂(g)

(5)

The influence of sulfuric acid concentration on the leaching efficiencies of Li(I), Co(II), Ni(II), and Mn(II) was assessed under controlled conditions at 65 °C with 400 rpm stirring speed, 50 g L⁻¹ pulp density, and a leaching time of 60 min with the presence of 4% H₂O₂ (Figure 3).

The leaching performance using sulfuric acid showed a strong dependence on acid concentration, particularly for cobalt, nickel, and manganese. In the absence of H₂SO₄, the leaching agent was deionized water and 4 wt.% hydrogen peroxide. Co(II) and Ni(II) exhibited minimal dissolution, with leaching efficiencies of only 0.3% and 0.2%, respectively, while Mn(II) showed slightly higher solubility at 27.3%, reflecting its relatively weaker bonding in the NMC lattice. The enhanced dissolution of Mn(II) in the presence of hydrogen peroxide, even under mild acidic conditions, can be attributed to its coordination environment within the NMC lattice. According to Koyama et al. [68], the Mn–O bonds are slightly shorter in the NMC structure compared to its pure oxide form, suggesting weaker lattice stability. This structural characteristic facilitates its oxidative dissolution, which exhibits stronger bonding and greater stability under similar conditions. Increasing the acid concentration to 0.375 mol L⁻¹ led to moderate improvements, and Co(II), Ni(II), and Mn(II) reached 44.0%, 40.1%, and 31.2%, respectively. However, a significant leap was observed at 0.75 mol L⁻¹, where leaching efficiencies exceeded 93% for all three metals, indicating nearly complete dissolution. Further increases in acid concentration (1.5 mol L⁻¹) resulted in only marginal gains, with efficiencies stabilizing around 99% for Co(II) and Ni(II) and 97% for Mn(II). Lithium exhibited consistently high leaching efficiencies under all tested conditions, with values slightly exceeding 100%, showing minimal sensitivity to acid concentration, and such elevated values can be attributed to minor analytical variations during ICP-OES calibration. These results align with expectations based on M–O bond strengths and binding energies within the LiNi_0.6Co_0.2Mn_0.2O₂ lattice [68]. The weaker Li–O bond facilitates lithium release into the solution even under mild conditions, whereas Co(II), Ni(II), and Mn(II), which are more strongly bonded in the oxide matrix, require sufficiently low pH and oxidative conditions for effective dissolution. The high leaching efficiencies with 0.75 mol L⁻¹ H₂SO₄ confirm that optimal acid concentration is essential for maximizing metal leaching while minimizing excess reagent consumption, thereby enhancing the environmental sustainability of the process.

3.2. Leaching Studies of Metals from NMC Cathode Material Using Ionic Liquids

To reduce sulfuric acid consumption in the leaching process, this study examined the potential of nine hydrophilic tetrabutylphosphonium-based ionic liquids ([TBP]-ILs) with various anions (Cl⁻, [Ac]⁻, [Gly]⁻, [Lac]⁻, [Nic]⁻, [Sal]⁻, [EDTA]²⁻, [DTPA]³⁻, and [HSO₄]⁻) as alternative agents for the leaching of Li(I), Co(II), Ni(II), and Mn(II) from NMC cathode material. All ILs were fully water-soluble and compatible with aqueous leaching, and their hydrophilicity contributed to reduced viscosity, facilitating mass transfer. Leaching efficiencies were investigated at three acid concentrations (0, 0.375, and 0.75 mol L⁻¹ H₂SO₄) with 20 wt.% IL, under controlled conditions (65 °C, 60 min, S/L = 50 g L⁻¹, 400 rpm), and the results are presented in Figure 4.

Leaching efficiency for Li(I) (Figure 4a) showed minimal sensitivity to sulfuric acid concentration across all tested ionic liquids. Even without acid (0 mol L⁻¹ H₂SO₄), lithium leaching remained high, with efficiencies exceeding 60% regardless of the IL anion. For instance, [TBP][HSO₄], [TBP][EDTA], [TBP][DTPA], [TBP[Lac], and [TBP][Nic] all achieved lithium recoveries of above 95% at 0.75 mol L⁻¹, 0.375 mol L⁻¹ H₂SO₄, and even without acid addition. This consistent behavior confirms lithium’s readiness to dissolve under mild conditions due to its weak interaction within the oxide lattice [68], making it less dependent on acid strength or IL structure. As such, lithium extraction efficiency was uniformly high in all systems, contrasting the behavior of transition metals, which exhibited stronger acid dependence.

In contrast, transition metals demonstrated a pronounced dependency on both the type of anion and the acid concentration (Figure 4b–d). Among all tested systems, [TBP][HSO₄], [TBP][DTPA], and [TBP][EDTA] showed superior leaching performance. At 0.75 mol L⁻¹ H₂SO₄, they achieved nearly complete dissolution of Co(II), Ni(II), and Mn(II), with efficiencies exceeding 98%. Notably, even at 0.375 mol L⁻¹ H₂SO₄, these ILs delivered high efficiencies (>93%), highlighting their capacity to reduce acid consumption without compromising performance. In contrast, ILs with non-chelating and weakly acidic anions such as [Ac]⁻, [Sal]⁻, [Gly]⁻, and Cl⁻ showed limited leaching even with acid, rarely surpassing 60% for any transition metal. This behavior is closely linked to the chemical properties of IL anions.

The influence of pH is particularly evident in systems containing ILs with weak organic acid-derived anions. Table S1 (Supplementary Materials) shows that the pH of leaching solutions with [Ac]⁻, [Gly]⁻, [Lac]⁻, [Sal]⁻, and [Nic]⁻ ranges from 7 to 8 without acid and decreases to between 3.0 and 4.5 upon the addition of H₂SO₄. Near-neutral or mildly basic conditions are insufficient to promote the dissolution of Co(II), Ni(II), and Mn(II), which are known to leach effectively only under acidic conditions, typically below pH 6 [62]. Also, pH values, with the addition of sulfuric acid, approach or fall below the respective pKa values of the organic acid anions (e.g., pKa[Ac] = 4.54, pKa[Sal] = 2.79, pKa[Nic] = 2.83; Figures S3–S7 in Supplementary Materials), indicating that the anions exist predominantly in their protonated form. Under such conditions, their ability to act as ligands is suppressed, as only deprotonated species are capable of coordinating metal ions. Consequently, the IL solutions lack effective chelating agents, which explains the observed poor leaching performance for transition metals despite acidification. Also, chloride-based IL ([TBP]Cl) showed particularly low leaching efficiency, with less than 1% Co and Ni leached without acid and only marginal improvement (from 20% to 60%) even with different concentrations of H₂SO₄. The limited leaching efficiency can be attributed to insufficient chloride concentration, pH of the leaching solutions (pH without H₂SO₄ about 7), and competition from sulfate ions in the system. Cobalt extraction via [CoCl₄]²⁻ complex formation requires high chloride and acidic concentrations (>4 mol L⁻¹ HCl); however, the leaching system in this work contained only 0.6782 mol L⁻¹ (20%) of IL, which was not sufficient to sustain complexation reactions [48,69,70,71]. Even after acidification with sulfuric acid, no significant improvement was observed, likely due to sulfate ions outcompeting chloride for metal coordination, which does not contribute to leaching [69].

On the other hand, ionic liquids containing polyaminocarboxylate anions, such as [DTPA]³⁻ and [EDTA]²⁻, demonstrated high leaching efficiencies due to their strong ability to form stable metal–ligand complexes. This is supported by their notably high stability constants, for example, Co-EDTA (logK = 16.31), Ni-EDTA (logK = 18.62), Mn-EDTA (logK = 13.70), and even higher values for DTPA: Co-DTPA (logK = 21.34), Ni-DTPA (logK = 23.98), and Mn-DTPA (logK = 16.92) [72,73,74,75]. These complexation abilities allow effective leaching of transition metals even under moderately acidic conditions, as evidenced by pH values around 5 in systems without added H₂SO₄. Moreover, both DTPA and EDTA possess multiple functional groups with distinct pKa values, DTPA (1.68, 2.10, 2.60, 4.15, 8.20, and 9.90) and EDTA (1.99, 2.67, 6.16, and 10.26), providing numerous coordination sites for metal binding, which enhances their chelating potential.

In the end, acidic hydrogen sulfate-based ILs promote proton-assisted dissolution through their strong acidity (pH = 2.54 without H₂SO₄, dropping to 1.73 with 0.75 mol L⁻¹ H₂SO₄), resulting in high leaching efficiencies even under mild conditions. Among all the tested ILs in this study, those containing HSO₄⁻ consistently outperformed others in terms of metal dissolution efficiency. Their excellent performance can be attributed not only to their inherent acidity but also to favorable physicochemical properties such as lower viscosity and enhanced mass transfer behavior [25,29]. Literature further supports the superior performance of HSO₄⁻-based ILs in metal leaching applications from various real samples, where they have been shown to achieve high extraction yields and selectivity across a broad range of metals and conditions [25,29].

In summary, the efficiency of metal leaching from NMC cathode materials using [TBP]-ILs is determined by several factors: the acidity of the system, the complexation strength of the anion, and the degree of anion protonation. Chelating anions such as [DTPA]³⁻ and [EDTA]²⁻ show excellent performance through ligand–metal complexation, while [HSO₄]⁻ promotes leaching via strong protonation. In contrast, ILs based on weak acids do not facilitate sufficient metal dissolution under the tested conditions.

3.3. Optimization Study of Metal Leaching

After the initial evaluation of ILs for metal leaching, as discussed in Section 3.2, the results revealed three ionic liquids, [TBP][EDTA], [TBP][DTPA], and [TBP][HSO₄], as the most effective candidates due to their high leaching efficiency. To evaluate their practical applicability, key operating parameters were optimized, including the percentage of IL in the leaching solution, temperature, time, and solid-to-liquid ratio. These factors have a significant impact on leaching performance, so it is important to understand their effects on efficient metal recovery from real samples. In addition, process optimization plays a crucial role in developing a resource-efficient process with lower costs, lower energy consumption, and lower environmental impact, which is crucial for sustainable applications.

3.3.1. Effect of Ionic Liquid Concentration

The influence of four ionic liquid concentrations (10 wt.%, 20 wt.%, 30 wt.%, and 40 wt.%) on the leaching efficiencies of Li(I), Co(II), Ni(II), and Mn(II) was assessed under controlled conditions at 65 °C with 400 rpm stirring speed, 50 g L⁻¹ pulp density, and a leaching time of 60 min with the presence of 4 wt.% H₂O₂ and without H₂SO₄, and the obtained results are presented in Figure 5.

The results presented in Figure 5 indicate that lithium achieved high leaching efficiencies across all IL concentrations, maintaining values above 90% at 10%, 20%, and 30%. However, at 40% IL concentration, lithium leaching efficiency noticeably decreased below 90%, suggesting that changes in solution density at higher IL concentrations may influence metal solubility and separation efficiency. For Co(II), Ni(II), and Mn(II), leaching efficiencies varied significantly depending on the IL type and concentration. At 10% IL, the leaching efficiencies of these metals were minimal, with [TBP][EDTA] achieving less than 15%, [TBP][DTPA] below 20%, and [TBP][HSO₄] reaching approximately 30% for all three metals. Increasing the IL concentration to 20% significantly improved metal leaching, with Co(II) and Ni(II) reaching around 40% and Mn(II) around 50% for both [TBP][EDTA] and [TBP][DTPA]. In the case of [TBP][HSO₄], the leaching efficiencies at 20% were notably higher, ranging between 60% and 65%, reflecting its stronger acidic nature and effective proton transfer capabilities. At 30% IL, the efficiencies remained relatively stable, closely matching those observed at 20%, suggesting that this concentration is near the optimal point for Co(II), Ni(II), and Mn(II) leaching in the absence of sulfuric acid. However, when the IL concentration was increased to 40%, a clear decline in leaching efficiencies was observed for all three metals, continuing the downward trend. This behavior is likely influenced by changes in solution density and phase behavior at elevated IL concentrations, which can inhibit mass transfer and reduce metal solubilization, similar to the trend observed for lithium.

Although the tested ILs demonstrated notable leaching efficiencies for Li(I), Co(II), Ni(II), and Mn(II) in the absence of sulfuric acid, the leaching efficiencies were not sufficient for practical application, particularly for transition metals. To enhance metal dissolution, a minimal concentration of 0.375 mol L⁻¹ H₂SO₄ was introduced into the leaching system with 20% IL, as higher IL concentrations did not yield significant improvements. The influence of this optimized system on leaching efficiencies was assessed under controlled conditions: 65 °C, 400 rpm stirring speed, 50 g L⁻¹ pulp density, and a leaching time of 60 min with the presence of 4 wt.% H₂O₂. The results of this enhanced process are presented in Figure 6, clearly indicating improved leaching for all targeted metals.

Increasing the concentration of [TBP][EDTA] and [TBP][DTPA] led to a notable enhancement in metal leaching, particularly for transition metals (Figure 6a,b). At 10% IL, the dissolution of Co(II), Ni(II), and Mn(II) was limited, with leaching efficiencies mostly below 20%, indicating that such low IL content was insufficient to drive effective complexation. However, at 20%, leaching efficiencies for Co(II), Ni(II), and Mn(II) increased substantially; Co(II) and Ni(II) exceeded 93%, while Mn(II) reached over 100% in some cases, demonstrating that moderate IL concentrations significantly improve metal solubilization. Interestingly, further increasing the IL concentration to 30% did not lead to additional benefits and, in the case of [TBP][EDTA] and [TBP][DTPA], even resulted in reduced efficiencies for Co(II), Ni(II), and Mn(II), which dropped to around 60%. This decline can be attributed to the increased viscosity of the leaching solution due to the higher IL content. Although direct viscosity measurements for the [TBP]-based ILs were not conducted, previous studies involving similar systems provide valuable insights into their expected rheological properties. TBP-based ionic liquids are generally characterized by high viscosity, which can influence their performance in hydrometallurgical processes [76]. However, the addition of polar solvents, such as water, has been reported to significantly reduce the viscosity of these ILs, thereby enhancing their flow properties and facilitating chemical reactions [76]. The study also highlights that the viscosity of TBP-based ILs is influenced by the type of anion present, indicating that the selection of anion can be strategically utilized to optimize the rheological behavior of these ILs for specific applications [76].

[TBP][HSO₄] exhibited superior leaching behavior across all tested concentrations (Figure 6c). Even at 20%, the leaching of Co(II), Ni(II), and Mn(II) exceeded 90% in most cases, and at 30%, all three metals were extracted with efficiencies over 100%, indicating nearly complete dissolution. This consistent performance is attributed to the favorable physicochemical profile of [TBP][HSO₄], including its strong acidity and full water miscibility, which facilitates proton-assisted metal leaching without significant mass transfer limitations. As observed throughout the study, lithium exhibited uniformly high leaching efficiencies across all IL types and concentrations, typically near or slightly above 100%, confirming its rapid and near-complete leaching regardless of experimental conditions.

3.3.2. Effect of Temperature and Time

Temperature plays a key role in leaching kinetics by accelerating reaction rates, enhancing metal ion mobility, and lowering viscosity, thereby improving dissolution efficiency. To investigate the kinetics and the effect of leaching time and temperature on the leaching efficiencies, experiments were conducted at 25, 45, and 65 °C using three selected ionic liquids with a fixed composition (20% IL, 4 wt.% H₂O₂, and 0.375 mol L⁻¹ H₂SO₄). The leaching efficiencies for each metal at various reaction times (5, 30, 45, 60, and 120 min) and temperatures are presented in Figure 7.

Also, for the kinetic analysis of the leaching process, the reaction kinetics were investigated using the shrinking-core models typically applied in hydrometallurgical studies. The kinetic models used to interpret experimental data are expressed by the following equations [77]:

1 − 2/3x − (1 − x)^2/3 = k₁ × t

(6)

1 − (1 − x)^1/3 = k₂ × t

(7)

where x is the fraction of the leached metal extracted, k₁ and k₂ are the kinetic reaction rate constants, and t is the leaching time.

Also, Arrhenius’ equation was used for the determination of the relationship between the kinetic reaction rate constant and temperature:

k = A × e^−Ea/RT

(8)

where k (min⁻¹) represents the reaction rate constant, A is the frequency factor, Ea is the activation energy, R (8.3145 J K⁻¹ mol⁻¹) is the gas constant, and T (K) is the temperature.

Activation energies for the leaching reactions can be determined by applying the Arrhenius equation in its linearized form:

lnk = lnA − Ea/RT

(9)

Based on the obtained reaction rate constants, the relationship between ln k and 1/T was graphically presented in Figure S8 in Supporting Materials and

Table 2. Activation energy (Ea, kJ mol⁻¹) for the leaching of Li(I), Co(II), Ni(II), and Mn(II).

	[TBP][EDTA]	[TBP][DTPA]	[TBP][HSO4]
Li(I)	29.66	35.75	−0.15
Co(II)	37.20	33.31	6.26
Ni(II)	36.62	33.35	4.58
Mn(II)	50.09	46.82	6.89

.

In general, the results indicate a positive correlation between temperature, contact time, and leaching efficiency, with pronounced differences between the systems. Lithium showed fast and complete dissolution across all three IL systems. At 45 °C and 65 °C for all ILs, leaching exceeded 100% within 45 min, but at 25 °C after 30 min, significant leaching was observed only with [TBP][HSO₄], while [TBP][EDTA] and [TBP][DTPA] required elevated temperatures to achieve comparable efficiencies (Figure 7). This behavior aligns with the calculated activation energies (Table 2), where [TBP][EDTA] and [TBP][DTPA] showed moderate Ea values around 30 kJ mol⁻¹, suggesting some temperature dependence. In contrast, [TBP][HSO₄] displayed an exceptionally low Ea of −0.15 kJ mol⁻¹. A negative value of activation energy suggests a diffusion-controlled or spontaneous dissolution pathway rather than a kinetically limited reaction.

The leaching behavior of Co(II) and Ni(II) was strongly influenced by temperature, particularly in systems employing [TBP][DTPA] and [TBP][EDTA] (Figure 7). At 25 °C, metal dissolution remained below 50%, indicating insufficient kinetic activation under these conditions. However, a marked increase was observed at 45 °C, where leaching efficiencies exceeded 94% and further improved to values above 98% at 65 °C. This temperature-dependent enhancement is supported by the calculated activation energies of approximately 30 kJ mol⁻¹ for both metals (Table 2), confirming that the process is chemically controlled and governed by thermally facilitated ligand–metal complexation. Manganese exhibited a comparable temperature dependency but with slightly higher Ea values (ranging from 40 to 50 kJ mol⁻¹; Table 2), which can be attributed to its multivalent redox behavior and relatively lower complexation affinity compared to Co(II) and Ni(II). [TBP][HSO₄] maintained high leaching efficiencies across all temperatures, with over 97% dissolution of all transition metals even at 25 °C. This superior performance highlights the effectiveness of its acid-promoted leaching mechanism, which proceeds efficiently with minimal thermal input. In terms of time dependency, most systems achieved >95% metal leaching within 45 min at elevated temperatures. However, at 25 °C, leaching with [TBP][EDTA] and [TBP][DTPA] remained slower, reflecting the reduced reactivity of these chelating systems under mild thermal conditions.

The observed temperature dependence can be attributed to the distinct leaching mechanisms of the investigated ionic liquids. [TBP][HSO₄] operates through a protonation-driven pathway, which allows for highly efficient metal dissolution even at room temperature. In contrast, [TBP][DTPA] and [TBP][EDTA] depend on ligand–metal complexation, a mechanism that is kinetically favored at elevated temperatures due to enhanced molecular interactions and reaction rates. From both environmental and economic standpoints, systems that maintain high performance at lower temperatures are more appropriate. While [TBP][DTPA] and [TBP][EDTA] require thermal input to achieve optimal efficiency, [TBP][HSO₄] demonstrates the ability to facilitate effective leaching under ambient conditions, thereby reducing energy consumption. Although low heating requirements represent just one aspect of sustainability, this characteristic contributes to decreased operational costs and enhances industrial scalability.

3.3.3. Effect of Solid-to-Liquid (S/L) Ratio

The solid-to-liquid (S/L) ratio is a key factor in hydrometallurgical leaching, affecting metal leaching efficiency, reagent consumption, and process economics. Higher S/L ratios (100–200 g L⁻¹) reduce reagent usage and wastewater generation but can limit mass transfer and mixing efficiency, lowering metal dissolution [77,78]. Conversely, lower S/L ratios improve leaching by ensuring better reagent availability but increase operating costs due to higher reagent consumption. Industrially, inorganic acids like sulfuric acid (1.5–3 mol L⁻¹) with S/L ratios of 100–200 g L⁻¹ have been identified as optimal, enabling effective leaching even at elevated solid loadings [79].

To determine the optimal S/L ratio for metal extraction from NMC cathode material, leaching experiments were conducted at S/L ratios of 50 g L⁻¹, 100 g L⁻¹, and 200 g L⁻¹ under previously optimized conditions: [TBP][DTPA] and [TBP][EDTA]: 45°C, 45 min, 0.375 mol L⁻¹ H₂SO₄, and 4% H₂O₂ and [TBP][HSO₄]: 25°C, 45 min, 0.375 mol L⁻¹ H₂SO₄, and 4 wt.% H₂O₂ (Figure 8).

Increasing the solid-to-liquid (S/L) ratio from 50 to 200 g L⁻¹ significantly influenced leaching performance, with the magnitude of this effect depending on the ionic liquid used (Figure 8). For [TBP][DTPA] and [TBP][EDTA] (Figure 8a,b), Li(I) extraction slightly decreased but remained above 90%, while the efficiencies for Co(II), Ni(II), and Mn(II) progressively declined. At an S/L ratio of 100 g L⁻¹, leaching efficiencies for transition metals dropped to approximately 60–70% and further decreased to below 50% at 200 g L⁻¹. These reductions can be attributed to mass transfer limitations, decreased reagent accessibility, and possible saturation of reactive sites on the solid surface [77]. Unlike [TBP][DTPA] and [TBP][EDTA], [TBP][HSO₄] (Figure 8c) maintained consistently high leaching efficiencies (>97%) for all metals across the entire S/L range. This resilience is likely due to the high acidity of [TBP][HSO₄], which ensures efficient protonation and sustained dissolution even at higher solid loadings. Additionally, its physicochemical properties facilitate better mass transfer compared to bulkier, less reactive ILs.

At the end of the optimization studies, the best leaching conditions for each IL were determined and are summarized in

Table 3. Final optimized conditions for IL-based leaching process.

	[TBP][EDTA]	[TBP][DTPA]	[TBP][HSO4]
IL composition (wt.%)	20	20	20
Acid concentration (mol L−1)	0.375	0.375	0.375
Temperature (°C)	45	45	25
Reaction time (min)	45	45	45
Solid-to-liquid ratio (g L−1)	50	50	200
Leaching efficiency (%)	>85% for Co(II), Ni(II), Mn(II) ≈100% for Li(I)	>94% for Co(II), Ni(II), Mn(II) ≈100% for Li(I)	>97% for Co(II), Ni(II), Mn(II) ≈100% for Li(I)

. These optimal parameters were selected based on the maximum leaching efficiencies achieved for Co(II), Ni(II), Mn(II), and Li(I) under different experimental conditions, including temperature, reaction time, solid-to-liquid (S/L) ratio, and acid concentration. The results reflect the influence of each IL’s structure on metal extraction performance, with notable differences observed between [TBP][HSO4], [TBP][EDTA], and [TBP][DTPA].

As illustrated in Table 3, the optimized leaching conditions vary significantly depending on the type of IL used. [TBP][HSO₄] demonstrated the highest efficiency for all target metals at room temperature (25 °C) and a high S/L ratio of 200 g L⁻¹, indicating its suitability for processing larger amounts of cathode material. On the other hand, [TBP][EDTA] and [TBP][DTPA] required slightly elevated temperatures (45 °C) and lower S/L ratios (50 g L⁻¹) to achieve optimal extraction. These differences underscore the importance of IL selection in tailoring the process for specific metal leaching goals.

3.4. Proposed Ionic Liquid-Based Leaching and Extraction Process for NMC Cathode Material

Among the tested ionic liquids, [TBP][HSO₄] demonstrated the highest leaching efficiency for Li(I), Co(II), Ni(II), and Mn(II), maintaining excellent dissolution performance even at room temperature. This represents a significant advantage over [TBP][EDTA] and [TBP][DTPA], which require elevated temperatures (45–65 °C) to reach comparable efficiencies. Furthermore, [TBP][HSO₄] enabled effective metal leaching at an S/L ratio of 200 g L⁻¹, four times higher than the optimal ratios for [TBP][EDTA] and [TBP][DTPA], which are limited to 50 g L⁻¹. This enhanced capacity reduces the volume of leachate produced per unit mass of processed material, directly contributing to more sustainable waste management. Additionally, the higher S/L ratio minimizes the consumption of extraction solvents and reduces the energy required for downstream separation and purification processes, further lowering the overall operational costs. This results in not only improved process efficiency but also enhanced potential economic feasibility in large-scale applications.

In addition to its high leaching performance, [TBP][HSO₄] was shown to form a stable aqueous biphasic system when combined with ammonium sulfate, enabling efficient metal extraction from the leachate. Before the extraction studies, a ternary phase diagram for the [TBP][HSO₄] + (NH₄)₂SO₄ + H₂O system was established at 25 ± 1 °C and atmospheric pressure using the cloud point titration method, with details provided in the Supplementary Materials (Table S2 and Figure S9). The leaching solution, obtained after metal dissolution, which contains 20 wt.% [TBP][HSO₄], was used directly for ABS preparation. To maintain compatibility with the selected ABS composition, 0.22 g of solid ammonium sulfate was added to 0.78 g of the leaching solution, resulting in a final system containing 15 wt.% IL and 22 wt.% salt. According to the phase diagram, the extraction point corresponding to this composition is located within the biphasic region above the binodal curve (Figure S9). After mixing, the system was left to equilibrate; then, the phases were separated and analyzed.

Extraction results showed excellent partitioning of all target metals into the salt-rich phase, with extraction efficiencies of 92.3 ± 2.1% for Li(I), 97.0 ± 1.9% for Co(II), 96.6 ± 2.1% for Ni(II), and 97.0 ± 2.3% for Mn(II). Significantly, the IL-rich phase remained nearly free of metals, indicating that [TBP][HSO₄] could be potentially recycled and reused in subsequent leaching cycles. This behavior can be attributed to the strong affinity of the sulfate anions (SO₄²⁻) present in the salt-rich phase, which form more stable aqueous complexes with metal ions compared to the hydrogen sulfate anions (HSO₄⁻) in the IL-rich phase. Moreover, the higher ionic strength and favorable pH conditions in the sulfate-rich region further stabilize the metal ions, preventing their transfer into the IL rich phase [80].

The metal-loaded, salt-rich phase can be further processed using standard precipitation techniques under controlled pH conditions to isolate and recover individual metals, as reported in existing literature [81,82]. Additionally, the ammonium sulfate-rich phase offers potential for recycling and reuse after metal precipitation, aligning with sustainable hydrometallurgical practices.

The synthesis of [TBP][HSO₄], [TBP][EDTA], and [TBP][DTPA] involves significantly higher costs compared to conventional sulfuric acid leaching solutions. The cost disparity is particularly pronounced for [TBP][EDTA] and [TBP][DTPA] due to the cation-to-anion ratios (2:1 for [TBP][EDTA] and 3:1 for [TBP][DTPA]), which inherently increase the overall synthesis cost. However, it is important to note that cost estimation is based on laboratory-scale production, where reagent prices are considerably higher. In large-scale industrial applications, bulk production and optimized synthesis routes could potentially reduce costs, making the process more economically feasible. Furthermore, the recyclability of ILs presents a viable path for cost mitigation. Studies suggest that for the economic feasibility of ionic liquids, it is necessary to achieve between 10 and 50 reuse cycles to offset the initial synthesis costs [83]. This level of recyclability would significantly contribute to reducing the overall cost per cycle, making the process more competitive compared to conventional leaching methods. Recent studies have demonstrated the successful reuse of phosphonium-based ILs in multiple extraction-stripping cycles without significant loss of performance. For example, phosphonium-based IL Cyphos IL 102 was effectively recycled over ten cycles [48], maintaining its efficiency through a stripping process with diluted HCl, while tetraoctylphosphonium oleate [TOP][oleate] demonstrated chemical stability across several months of continuous use [84]. Similarly, [TBP][DTPA] maintained its cobalt extraction efficiency over four cycles, with only a slight decline attributed to minor IL solubility in the aqueous phase [36].

Despite promising demonstrations of IL reusability, large-scale application presents significant challenges [85]. During hydrometallurgical processes, IL phases can accumulate trace metals and organic residues, impairing their leaching efficiency and selectivity. This contamination becomes particularly problematic in industrial operations, where gradual impurity buildup destabilizes phase equilibrium and hinders metal recovery [85]. Several regeneration strategies have been explored to address these limitations, including selective precipitation to remove metal ions, ABS-based regeneration with inorganic salts to extract impurities, and ion-exchange techniques to restore IL integrity [85]. However, surface adsorption and ion exchange with solid residues can lead to irreversible IL losses, impacting both economic viability and process sustainability [85]. Although ILs are non-volatile, which prevents evaporation, their purification remains challenging due to the need for complex washing and phase separation techniques [85]. While the discussed regeneration techniques show promise, further investigation into the accumulation of organic impurities and changes in the physicochemical properties of ILs over multiple cycles is necessary. This issue represents a critical limitation for validating the “closed-loop” concept on an industrial scale, as it directly affects both economic feasibility and long-term process sustainability. Addressing these challenges in future studies is essential for optimizing regeneration efficiency and ensuring the applicability of IL-based hydrometallurgical processes in large-scale operations.

Based on the findings of this study, a schematic representation with a flowchart of a hypothetical hydrometallurgical route for metal leaching and extraction from spent NMC batteries using an IL-based process has been developed (Figure 9). This conceptual approach outlines the key steps involved, demonstrating the feasibility of integrating IL-based leaching with subsequent metal extraction and potential recovery. In the flowchart, solid lines represent the processes successfully conducted in this study, while dotted lines indicate the proposed recovery and purification steps that remain to be optimized in future research. This visual representation underscores the scalability and modularity of the process, allowing for further integration of metal recovery strategies in line with sustainable hydrometallurgical practices.

The successful implementation of IL-based leaching and ABS technology in this study demonstrates its potential for large-scale application in lithium-ion battery cathode material recycling. Our findings confirm that integrating hydrophilic ILs with sulfate-based leaching and aqueous biphasic extraction enables efficient leaching and extraction of Co(II), Ni(II), Mn(II), and Li(I) while minimizing acid consumption and waste generation. Notably, [TBP][HSO₄] exhibited exceptional leaching efficiency under mild conditions, highlighting its suitability as a sustainable alternative to conventional leaching agents. Furthermore, the application of IL-ABS allowed for the selective extraction of transition metals into the salt-rich phase, leaving the IL-rich phase metal-free and potentially suitable for reuse, supporting the concept of a more sustainable and efficient leaching process.

This approach aligns closely with the several key principles of circular hydrometallurgy proposed by Binnemans and Jones [22]. It demonstrates strong adherence to principles such as high efficiency, selectivity, and minimized reagent consumption. By operating under low temperatures and avoiding organic solvents, the process contributes to energy efficiency and reduced environmental impact. The dual functionality of IL as leaching agents and ABS-forming components addresses key aspects of process circularity and resource efficiency. Some of the remaining challenges pertain to the economic feasibility of scaling IL synthesis and recovery, particularly for structurally complex ILs. While the process already replaces traditional solvents and minimizes acid usage, an economic evaluation would be necessary to confirm its competitiveness with established technologies. Although some principles remain areas for future improvement, this study lays the groundwork for a greener hydrometallurgical platform.

The demonstrated versatility of this approach positions it as a valuable strategy not only for NMC cathode materials but also for systems such as LCO, LMO, or NCA cathode materials while offering a robust platform for the selective leaching and extraction of critical metals from more complex real samples. By combining mild operating conditions and reduced chemical input, this process represents a potentially sustainable, scalable, and environmentally conscious route for lithium-ion battery cathode material recycling.

4. Conclusions

In this study, an integrated IL-based process was developed and optimized for the leaching and extraction of Li(I), Co(II), Ni(II), and Mn(II) from spent NMC cathode materials. Leaching experiments were conducted using nine synthesized tetrabutylphosphonium-based ILs, with [TBP][HSO₄], [TBP][DTPA], and [TBP][EDTA] identified as the most effective, achieving metal dissolution efficiencies exceeding 90% under optimized conditions. This innovative approach represents a significant advance in sustainable leaching, as it considerably reduces the required sulfuric acid concentration while ensuring high metal leaching efficiency. In contrast to conventional methods, the integration of ionic liquids not only minimizes acid consumption but also enables effective metal recovery at room temperature, which significantly reduces energy requirements. Among the evaluated ILs, [TBP][HSO₄] demonstrated superior performance, maintaining high extraction efficiencies even at room temperature due to its strong acidity, which facilitated proton-assisted metal dissolution. Following the optimized leaching step, an IL-based aqueous biphasic system (IL-ABS) with ammonium sulfate was employed, efficiently extracting all target metals into the salt-rich phase while leaving the IL-rich phase metal-free. The proposed IL-based process represents a promising, energy-efficient alternative to conventional methods, demonstrating reduced reagent consumption, minimized waste generation, and efficient metal recovery at room temperatures with low acid requirements. These advantages suggest strong potential for industrial applications, particularly in lithium-ion battery recycling. The recyclability of [TBP][HSO₄], if further investigated and optimized, could significantly enhance the economic feasibility of the process, aligning with principles of green chemistry and circular hydrometallurgy. With further optimization and pilot-scale studies, this method could be seamlessly integrated into existing hydrometallurgical operations, contributing to more sustainable and cost-effective recycling processes. To fully realize its potential, future work should focus on scaling up the process; evaluating its applicability to other LIB cathode materials such as LCO, LMO, and NCA; and conducting comprehensive economic and lifecycle assessments to validate its competitiveness with established technologies. Additionally, long-term stability studies and improved IL recovery techniques are essential for ensuring process sustainability and economic viability at an industrial scale.