# Non-aqueous leaching of critical metals from black mass of spent lithium-ion batteries using a reusable acidic organophosphorus extractant

Kurniawan Kurniawan, Sookyung Kim, Mooki Bae, Jae-Chun Lee, Alexandre Chagnes

► To cite this version:

Kurniawan Kurniawan, Sookyung Kim, Mooki Bae, Jae-Chun Lee, Alexandre Chagnes. Non-aqueous leaching of critical metals from black mass of spent lithium-ion batteries using a reusable acidic organophosphorus extractant. Journal of Environmental Management, 2026, 397, pp.128220. 10.1016/j.jenvman.2025.128220. hal-05464068

HAL Id: hal-05464068

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Research article

# Non-aqueous leaching of critical metals from black mass of spent lithium-ion batteries using a reusable acidic organophosphorus extractant

Kurniawan Kurniawan<sup>a,b,c,*</sup>, Sookyung Kim<sup>a,b</sup>, Mooki Bae<sup>b</sup>, Jae-chun Lee<sup>a,b,** ID</sup>,
Alexandre Chagnes<sup>d ID</sup>

## ARTICLE INFO

### Keywords:

Non-aqueous leaching

Spent LIBs

NCM

D2EHPA

Critical metals

## ABSTRACT

Recycling critical metals from the black mass of spent lithium-ion batteries (LIBs) is essential for supply security and waste management. Conventional hydrometallurgical acid leaching is effective but generates large volumes of wastewater. In this study, a non-aqueous leaching system has been developed using an acidic organophosphorus extractant, di-(2-ethylhexyl)phosphoric acid (D2EHPA), as the lixiviant, consolidating leaching and solvent extraction (SX) into a single stage. Metallic copper and aluminum were evaluated as reductants; only copper enabled effective leaching in D2EHPA. Key parameters (stirring speed, copper dosage, D2EHPA concentration, temperature, time, and pulp density) were optimized. The optimum was 700 rpm, 0.15 g Cu per g black mass, 2.0 M D2EHPA, 90 °C, 4 h, and 1.0 g black mass per 20 mL D2EHPA. Under these conditions, leaching efficiencies were ~100 % for Li, 79.2 % for Ni, 94.6 % for Co, 47.3 % for Mn, and 96.4 % for Cu (reductant). D2EHPA was reusable over five leaching-stripping cycles with negligible performance loss. Metals were recovered from the loaded organic by (i) pH-controlled aqueous stripping and (ii) oxalic-acid precipitation stripping. The study also examined the potential of using copper-containing waste, such as waste printed circuit boards (WPCBs) and Cu-Al foil from spent LIBs as reducing agents, enabling an integrated waste-recycling scheme. This non-aqueous leaching process reduces unit operations and lowers chemical and water consumption while maintaining high recovery for key metals.

## 1. Introduction

Demand for lithium-ion batteries (LIBs) is rising rapidly, raising concerns about future supply security for battery materials (such as lithium (Li), nickel (Ni), cobalt (Co), and manganese) and the accelerating generation of spent LIBs (Chen et al., 2019). Reliance on primary ores, coupled with depletion of high-grade resources, challenges long-term supply for these battery materials (Costa et al., 2021). In parallel, global generation of spent LIBs is projected to reach ~11 million tons by 2030 (Garole et al., 2020; Su et al., 2025). Spent LIBs are hazardous due to their heavy metal content, toxic and potentially carcinogenic compounds, and organic electrolytes containing lithium hexafluorophosphate (LiPF₆) (Mrozik et al., 2021; Zhu et al., 2023). Yet they constitute an urban mine of recoverable metals and non-metals

(Choubey et al., 2017; Dalini et al., 2021), underscoring the need for robust collection, pre-processing, and recovery (recycling) systems. Among recycling options, extractive metallurgy, particularly targeting cathode materials that concentrate high-value metals, offers a promising economic and environmental pathway, with cathodes often representing ~80 % of recoverable value (Jena et al., 2021).

Hydrometallurgical acid leaching is widely applied due to moderate conditions, high recoveries, and scalability (Chagnes and Pospiech, 2013; Choubey et al., 2017; Dalini et al., 2021; Yao et al., 2018). Acid leaching typically uses sulfuric acid (H₂SO₄), which is accompanied by reductants such as hydrogen peroxide (H₂O₂) (Dalini et al., 2021; Kurniawan et al., 2025; Swain et al., 2007) or Cu/Al current collector (Chernyaev et al., 2021; Joulié et al., 2017), followed by solvent extraction (SX) for separation (Lei et al., 2022; Shuya et al., 2020).

<footnote><sup>a</sup> Resources Recycling, University of Science and Technology, Daejeon, 34113, Republic of Korea</footnote>

<footnote><sup>b</sup> Resources Utilization Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, 34132, Republic of Korea</footnote>

<footnote><sup>c</sup> Materials Recycling Group, Applied Materials Division, Argonne National Laboratory, Illinois, 60439, United States of America</footnote>

<footnote><sup>d</sup> Université de Lorraine, CNRS, GeoRessources, F-54000, Nancy, France</footnote>

<footnote>* Corresponding author. Resources Recycling, University of Science and Technology, Daejeon, 34113, Republic of Korea.</footnote>

<footnote>** Corresponding author. Resources Recycling, University of Science and Technology, Daejeon, 34113, Republic of Korea. E-mail addresses: kkurniawan@anl.gov (K. Kurniawan), jclee@kigam.sci.kr (J.-c. Lee).</footnote>
However, SX from acidic solutions require extensive pH conditioning: for example, organophosphorus extractants (e.g., D2EHPA, Cyanex 272) extract Mn(II) near pH ~2 and need pH > 4–5 for Co(II)/Ni(II) (Nguyen et al., 2014), often with saponification of the extractants and addition of phase modifiers to maintain the pH levels and avoid third-phase formation (Liu et al., 2015; Nguyen et al., 2014). These increase reagent use and generation of wastewater. Ammoniacal systems can improve selectivity but often show lower leaching efficiencies and require high ammonia or pre-reduction (Ku et al., 2016; Qi et al., 2020; Su et al., 2025; Yu et al., 2023).

In response to these limitations, non-aqueous leaching, using non-aqueous/water-limited/organic solvents as lixivants, has emerged as a promising route (Binnemans and Jones, 2017). This approach can integrate leaching and SX into a single stage, potentially reducing unit operations and wastewater generation, enabling direct reuse of the organic phase, and enhancing selectivity. Realizing these benefits depends on the use of hydrophobic solvent systems that minimize partitioning to water and resist loss to aqueous phases. Candidate non-aqueous solvents include hydrophobic ionic liquids (ILs), deep eutectic solvents (DESs), and established organic extractants used in SX. ILs and DESs are attractive due to tunable physicochemical properties (Binnemans and Jones, 2017) but achieving the desired advantages of non-aqueous leaching require the use of hydrophobic ILs and DESs. Currently, many hydrophobic ILs and some DES formulations rely on components that can be persistent and hazardous, and their large-scale availability, cost, and environmental fate remain under active investigation. As summarized by Binnemans and Jones (2023), current knowledge gaps and tradeoffs necessitate careful selection when targeting sustainable non-aqueous leaching.

Commercial organic extractants represent a practical alternative for non-aqueous leaching. They are widely available and well-characterized, and can be dissolved in hydrophobic diluents to minimize losses and control their viscosity and concentration. The literature on non-aqueous leaching of spent LIBs using organic extractants is still nascent (Matsui et al., 2025; Peeters et al., 2022). For example, Matsui et al. (2025) used N-[N,N-di (2-ethylhexyl)aminocarbonylmethyl] glycine (D2EHAG) in n-dodecane with ascorbic acid as a reductant; under mild conditions (60 °C, 400 rpm, 24 h) with 0.6 M D2EHAG, 0.3 M ascorbic acid, and ~3.3 vol% water (near saturation), leaching efficiencies for Li, Co, Ni, and Mn from NCM were 77.2 %, 96.1 %, 96.5 %, and 76.6 %, respectively. Peeters et al. (2022) employed D2EHPA in kerosene with mixed Cu/Al as reductants to leach chemical-grade LCO and reported near-quantitative Li and Co leaching after 6 h at 80 °C using 75 vol% D2EHPA at a pulp density of 25 g/L. D2EHPA has also been applied in non-aqueous leaching of various chemical-grade metal oxides (Kurniawan et al., 2024), showing high efficiency for Li₂O, Cu₂O/CuO, CoO, and MnO; moderate leaching for Co₃O₄, NiO, and MnO₂; and negligible leaching for Al₂O₃. In these systems, D2EHPA-based organics did not exhibit problematic phase disengagement or third-phase formation.

Building on these promising results, this study investigates D2EHPA as a lixiviant for non-aqueous leaching of black mass from spent LIBs containing NCM cathode materials, representative of complex, multi-component matrices. Firstly, Cu, Al, and Cu/Al mixtures were assessed as reductants to simplify flowsheets by leveraging materials already present in LIBs. Consistent with expectations of aluminum passivation in this medium (Kurniawan et al., 2024), only copper effectively acted as a reductant for transition-metal dissolution in D2EHPA, in contrast to observations in acidic aqueous leaching. Hence, the non-aqueous leaching equilibria were proposed, which then supported via spectroscopic characterization of the loaded organic phase (Fourier-transform infrared (FTIR) and UV-Vis). Process variables (stirring, reductant loading, D2EHPA concentration, temperature, time, pulp density) were optimized. The effect of graphite, on non-aqueous leaching was examined. Metal recovery from the loaded organic phase was evaluated using two routes: (i) pH-controlled aqueous stripping and (ii)

precipitation-stripping using oxalic acid. The recyclability of D2EHPA was assessed over five leaching-stripping cycles. The study also explored copper-bearing waste, including waste printed circuit boards (WPCBs) and Cu-Al foils from spent LIBs, as reductants to enable integrated recycling of multiple waste streams. While non-aqueous leaching with D2EHPA intensifies the process by combining leaching and extraction together, limitations and operational considerations are identified and addressed in the article.

## 2. Experimental

### 2.1. Materials

Sample of NCM cathode material (+0.45 µm) was obtained from a local recycler. The metal compositions of the sample were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Model: iCAP6000 series, Thermo Scientific, USA) after acid digestion using aqua regia. The composition of the sample was: 11.52 % Ni, 7.71 % Co, 7.48 % Mn, 2.94 % Li, 0.70 % Al and 0.45 % Cu, while the remaining was graphite. The X-ray diffraction pattern recorded by using the D8 Advance X-ray diffractometer (Bruker, USA) showed the presence of two strong peaks at 26.5° and 44.5° attributed to graphite whereas the other peaks correspond to $LiNi_xCo_yMn_{1-x-y}O_2$ (NCM (Fig. S1). Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS, Jeol-JSM-6380 L A, Japan) showed the presence of smooth spherical particles composed of Ni, Co, Mn) and O, which were related paragenetically (Fig. S2).

D2EHPA (purity 97 %, Sigma Aldrich, USA), diluted in kerosene (Junsei, Japan) was used as the lixiviant. The organic phases, including fresh-, loaded- and stripped D2EHPA, were characterized by Fourier-transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Scientific, USA) and UV-Vis spectroscopy (Optizen POP, Korea). Sulfuric acid (95 %, Junsei, Japan) and oxalic acid (98 %, Sigma Aldrich, USA) diluted in deionized water were used as the striping solutions. Copper (99.5 %, ~400 µm, Sigma-Aldrich, USA) and aluminum (91 %, 10 µm, Sigma-Aldrich, USA) powders were used as reductants. Graphite powder (20 µm) was procured from Kanto Chemical (Japan); NiO (99 %) and $MnO_2$ (90 %) were from Junsei (Japan); and $Co_3O_4$ (99.5 %) and $LiCoO_2$ (99.8 %) were from Sigma-Aldrich (USA). Hydranal™-composite 5, hydranal water standard 10.0 and hydranal methanol rapid, all from Honeywell Fluka (Germany), were used for the Karl-Fisher volumetric titration (V10S, Mettler Toledo, USA) to determine the amount of water content in the organic phase.

### 2.2. Methods

#### 2.2.1. Aqueous leaching procedure

A preliminary experiment was carried out to investigate the applicability of copper/aluminum/their mixtures as reductants in the aqueous acid leaching of NCM using $H_2SO_4$. The experiments were carried out in a glass vial of 50 mL placed in a water bath to control the temperature and equipped with a magnetic stirring bar under the following conditions: $H_2SO_4$ concentration = 1.0 M, reducing agent/ NCM mass ratio (g/g) = 0.2, Cu/Al mass ratio = 1/0–0/1, temperature = 80 °C, pulp density = 50 g/L and time = 3 h.

#### 2.2.2. Non-aqueous leaching procedure

Non-aqueous leaching experiments were performed using the same equipment as those applied in aqueous leaching, following the procedure presented in Fig. S3. In all experiments, 20 mL of D2EHPA at desired concentrations was transferred to the glass vial, and then heated. After attaining the temperature, a weighed amount of NCM sample + reductant was put in the vessel and stirring was started. Firstly, the applicability of copper/aluminum/their mixtures as reductant in the non-aqueous leaching of NCM using D2EHPA was investigated; experiments were carried out under the same conditions as that of aqueous
leaching while using 1.0 M D2EHPA. The non-aqueous leaching parameters were then optimized by varying one factor at a time in the range: stirring speed: 300–800 rpm; reductant:NCM mass ratio: 0–0.25:1 (g/g); D2EHPA concentration: 0.5–2.3 M; temperature: 25–90 °C; time: 1–6 h; and solid/liquid ratio: 0.5–2.0 g NCM/20 mL of D2EHPA (25–100 g/L pulp density). The metal concentrations in leaching solutions (loaded D2EHPA) were stripped with 4.0 M H₂SO₄ solution at an organic/aqueous (O/A) phase ratio of 3/5 (initial assessment showed that this stripping was adequate to ensure the complete metal stripping) and analyzed by ICP-AES after a proper dilution using 5 % HNO₃. The leaching percentage was calculated with Eq. (1). All the leaching experiments were carried out three times while reporting the mean values.

$$ \text{Leaching (\%)} = \frac{[M_{\text{org}}] \times V_{\text{org}}}{m_{\text{NMC}} \times wt\%_{\text{metal}}} \times 100 \quad (1) $$

where $[M_{org}]$ is metal concentration in loaded D2EHPA, $V_{org}$ is volume of D2EHPA solution, $m_{NMC}$ is mass of NCM sample and $wt\%_{metal}$ is the weight percent of metal in NCM samples.

The applicability of two rich-materials, namely metal concentrate of WPCBs (75.4 % Cu, 300–125 µm; obtained after processing reported by Kurniawan et al. (2023)) and Cu-Al foil scrap from spent LIBs (21.9 % Cu, 11.2 % Al, 11.0 % Co, 5.9 % Ni, 5.6 % Mn and 2.1 % Li; original size +2.0 mm and ground to < 75 µm) as reductant in NCM leaching was then investigated under optimal leaching conditions (2.0 M D2EHPA, 90 °C, 700 rpm, 4 h).

### 2.2.3. Stripping experiments of loaded D2EHPA

Loaded D2EHPA was subjected to stripping experiments using aqueous solutions at different pH values (adjusted by using H₂SO₄/NaOH solutions). The loaded D2EHPA was contacted with pH-controlled aqueous solutions at an O/A = 1/1 for 5 min. After finishing the contact, equilibrium pH was measured. Metal concentrations in the stripping solutions were analyzed, and the stripping efficiency was calculated via Eq. (2).

$$ \text{Stripping (\%)} = \frac{M_{aq} \times V_{aq}}{[M_{org}] \times V_{org}} \times 100 \quad (2) $$

where $M_{aq}$ is metal concentration in stripping solution.

### 2.2.4. Precipitation-stripping experiments of loaded D2EHPA

Precipitation-stripping of metals (Li, Co, Ni, Mn and Cu) from the loaded D2EHPA was investigated using 0.2–1.0 M oxalic acid solutions. The loaded D2EHPA was contacted with the solutions at O/A = 1/1 for 10 min at 25 °C. At the end of experiments, the flask was allowed to stand for 1 h, and three distinguished phases (stripped D2EHPA, aqueous solution and solid precipitate) were obtained. The stripped D2EHPA was washed with water and re-stripped with a 4.0 M H₂SO₄ solution at an O/A = 3/5 to quantify the remaining metals in the organic phase and determine the amounts of metal stripped through mass balance. Accordingly, stripping efficiency was calculated. The stripped solutions were diluted properly to determine the amounts of metal remaining in the solution, and the precipitation efficiency was calculated according to Eq. (3).

$$ \text{Precipitation (\%)} = \frac{m_{\text{stripped}} - m_{\text{final}}}{m_{\text{stripped}}} \times 100 \quad (3) $$

### 2.2.5. Reusability test

Reusability of D2EHPA was tested for several leaching-stripping cycles. In the first cycle, a 25 mL of 2.0 M D2EHPA in the glass vial was heated to 90 °C, followed by charging 0.5 g of NCM sample +0.075 g of Cu powder (Cu/NCM mass ratio of 0.15). After leaching for 4 h and filtration, the loaded D2EHPA was stripped with 4.0 M H₂SO₄ solution at O/A = 3/5; the stripped solution was analyzed for calculation of leaching yields (Eq. (1)). A 21 mL stripped D2EHPA was used in the

second leaching while maintaining other conditions same as in the first one. This procedure was repeated until the fifth leaching, whereby 12 mL of stripped D2EHPA was used.

## 3. Results and discussion

### 3.1. Applicability of copper/aluminum as reductants in non-aqueous leaching

The leaching of NCM with Cu/Al as the reductant may occur via reactions (4–5)

$$ \text{LiNi}_x\text{Co}_y\text{Mn}_{1-x-y}\text{O}_2 + \text{H}^+ \rightarrow \text{Li}_{1-m}\text{Ni}_x\text{Co}_y\text{Mn}_{1-x-y}\text{O}_2 + \text{mLi}^+ \quad (4) $$

$$ \text{Li}_{1-m}\text{Ni}_x\text{Co}_y\text{Mn}_{1-x-y}\text{O}_2 + \text{pM} + (3 - m + np)\text{H}^+ \rightarrow (1 - m)\text{Li}^+ + x\text{Ni}^{2+} + y\text{Co}^{2+} + (1 - x - y)\text{Mn}^{2+} + \text{pM}^{n+} + \text{H}_2\text{O} \quad (5) $$

The applicability of Cu and Al as reductants for non-aqueous leaching of NCM with D2EHPA was evaluated and compared with classical aqueous H₂SO₄ leaching (Table S1). In the non-aqueous system, only Cu acted as an effective reductant: leaching with D2EHPA alone yielded 52.0 % Li, 22.0 % Ni, 21.7 % Co, and 12.2 % Mn; with Cu, leaching efficiencies increased to 75.9 % (Li), 41.9 % (Ni), 45.0 % (Co), and 23.1 % (Mn), with 41.1 % Cu dissolution. By contrast, in aqueous H₂SO₄ leaching, both Cu and Al functioned as reductants, agreeing with the thermodynamics in Fig. S4. Detailed results and the non-aqueous versus aqueous comparison are provided in Table S1.

The results here indicate that the NCM non-aqueous leaching using D2EHPA occurs through a distinct mechanism compared to aqueous leaching. Then, the equilibria depicted in Fig. 1 was suggested to describe the non-aqueous leaching.

#### - Step 1: D2EHPA dissociation and H⁺ adsorption onto solid metal sources

In nonpolar diluents (e.g., kerosene), D2EHPA behaves as a Brønsted acid and coordinating ligand. Despite dimerization in hydrophobic media, it releases protons that diffuse through the organic phase and adsorb at solid surfaces (NCM particles, metallic reductants), initiating acid-driven de-lithiation and surface protonation. Interfacial wetting is aided by the polar phosphoric headgroup, while hydrophobic tails ensure solubility. Aluminum rapidly develops an Al₂O₃-passivation layer in water-lean organics, limiting electron transfer/proton access and explaining Al's negligible reductant role (Peeters et al., 2022). Copper, on the other hand, is less strongly passivated and participates in redox reactions (Eqs. (4–5)).

#### - Step 2: chemical reactions of NCM, copper and H⁺

Adsorbed H⁺ drives leaching with/without copper (Eqs. (4) and (5)). Without Cu, partial leaching occurs via proton attack and ligand-assisted dissolution; spectral changes (Fig. S5) and color changes (Fig. S6) indicate oxidative degradation of the organic by high-valent Co(III)/Mn (IV). With Cu, electrons from Cu oxidation reduce Co(III)/Mn(IV) to Co (II)/Mn(II), enhancing leaching and protecting the organic. This protective effect is reflected in less pronounced color changes and FTIR shifts consistent with formation of metal-D2EHPA complexes rather than ligand oxidation (Fig. S5–S6). UV–Vis spectra of the loaded organic in Fig. S7 show features consistent with Cu²⁺ coordination (e.g., a broad d–d band near ~780–800 nm), corroborating copper dissolution into the organic phase as Cu(D2EHPA)₂.

The non-aqueous leaching of chemical-grade metal oxides (MOs) corroborate these results (Table S2). Without Cu, leaching yields were 35.0 % (Ni from NiO), 66.7 % Li and 33.4 % Co (from LCO), 23.5 % Co (Co₃O₄), and 20.3 % Mn (MnO₂); with Cu, Li approached ~100 % and
<figure><img src="image_3.png" /><figcaption>Fig. 1. Equilibria involved during non-aqueous leaching by D2EHPA in the presence of copper as reductant. (1) D2EHPA dissociation and H⁺ adsorption onto solid metal sources, (2) chemical reactions, and (3) proton exchange and metal-D2EHPA complexation.</figcaption></figure>

Co/Mn rose to 81.3 % (Co from LCO), 69.0 % (Co from Co₃O₄), and 32.6 % (Mn from MnO₂).

Water is formed stoichiometrically during the acid leaching of MOs; however, the amounts generated under our conditions are small and not visually apparent. FTIR spectra of loaded organics do not show discernible water bands, and Karl-Fischer titration indicates low water content in the organic phase, with evaporation likely at 90 °C in sealed vials, consistent with the observed pressure buildup and prior reports (Peeters et al., 2022).

### - Step 3: proton exchange and metal-D2EHPA complexation

Following metal leaching, metal ions are captured by D2EHPA through a proton-exchange extraction mechanism according to the following reaction:

$$n[H]^{+} + M^{n+} \rightleftharpoons nH^{+} + [M]^{n+} \quad (6)$$

where [] represents the organic phase.

Coupled acid dissociation and exchange reactions allow released protons to sustain interfacial reactions (Eqs. (4–5)) while deprotonated ligan coordinates the metals. The strength of metal complexation depends on metal properties and speciation. Qualitatively, hard-soft acid-base (HSAB) considerations are useful to explain such behavior: D2EHPA provides hard oxygen donor sites (P=O, P–O–H), favoring coordination of hard and borderline cations. Li⁺ and Mn²⁺ (hard) and Co²⁺, Ni²⁺, Cu²⁺ (borderline) all form extractable complexes with D2EHPA. FTIR spectra (Fig. S5) show shifts in v(P=O) and v(P–O) bands consistent with formation of M-D2EHPA complexes. Strong complexation, together with aluminum passivation, suppresses Al-driven cementation, unlike in aqueous leaching where active Al dissolution can lead to cementation of Ni²⁺/Cu²⁺.

Hydration parameters (hydration energy (ΔhydG°₂₉₈K), radius (r), width of hydration shell (Δr) and number of water molecules (n) (Table S3)) further rationalize the results. For example, Al³⁺ has the

most negative ΔhydG°₂₉₈K (−4525 kJ/mol) and a large n value (20.4) (Marcus, 1991), making transfer to the organic least favorable and contributing to negligible Al leaching.

### 3.1.1. Effect of graphite on non-aqueous leaching of NCM using D2EHPA

Graphite is present in black mass of spent LIBs, as shown in Fig. S1. Hence, the influence of graphite on the leaching of MOs (LCO, Co₃O₄, and MnO₂) during non-aqueous leaching was examined, and results are presented in Table S2. Adding graphite to MOs (MOs + graphite) produced only a marginal increase in leaching efficiencies. In contrast, when copper was also present (MOs + Cu + graphite), leaching yields increased substantially: Li and Co from LCO reached ~100 % and 84.7 %, respectively; Co from Co₃O₄ reached 77.3 %; and Mn from MnO₂ reached 43.4 %. These values were indeed higher than those obtained from the corresponding MOs + Cu systems without graphite.

The enhancement is consistent with galvanically coupled redox pathways (Eqs. (4) and (5)). Graphite provides an electronically conductive and low-resistance network that accelerates electron transfers between particles (MOs and Cu). In acidic media, graphite can also act as a cathodic surface for hydrogen evolution, sustaining galvanic currents and indirectly promoting reduction of higher-valent metal centers within the MO matrices.

## 3.2. Effects of parameters on non-aqueous leaching of NCM using D2EHPA

### 3.2.1. Effect of stirring speed

Non-aqueous leaching tests were conducted at different stirring speeds, ranging from 300 to 800 rpm (Fig. 2(a)). Results demonstrate that the metal leaching yield increased as the stirring speed increased. At 300 rpm, the leaching yields of Li, Ni, Co, Mn and Cu were 60.3 %, 15.4 %, 16.0 %, 12.7 % and 25.4 %, respectively, and then increased to 89.8 %, 49.9 %, 54.6 %, 24.5 % and 59.2 % at 700 rpm; the change was negligible at still higher stirring speed (800 rpm). Clearly higher stirring
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a) Effect of stirring speed (conditions: 300–800 rpm, 0.15 (g/g) Cu/NCM, 1.0 M D2EHPA, 80 °C, 3 h, 50 g/L pulp density), (b) effect of Cu (reductant) amount (0.05–0.25 (g/g) Cu/NCM, 1.0 M D2EHPA, 700 rpm, 80 °C, 3 h, 50 g/L pulp density), (c) effect of D2EHPA concentration (0.5–2.3 M D2EHPA, 0.15 (g/g) Cu/NCM, 700 rpm, 80 °C, 3 h, 50 g/L pulp density), (d) effect of temperature (25–90 °C, 0.15 (g/g) Cu/NCM, 700 rpm, 2.0 M D2EHPA, 3 h, 50 g/L pulp density), (e) effect of time (30 min–6 h, 0.15 (g/g) Cu/NCM, 700 rpm, 2.0 M D2EHPA, 90 °C, 50 g/L pulp density), and (f) effect of pulp density (25–100 g/L, 0.15 (g/g) Cu/NCM, 700 rpm, 2.0 M D2EHPA, 90 °C, 4 h), on the leaching of NCM from black mass of spent LIBs using D2EHPA as the lixiviant.</figcaption></figure>

speed accelerates the mass transfer and diffusion of H⁺ from the bulk onto the surface of NCM/reductant and reaction products (Mⁿ⁺) to D2EHPA. Removing bulk density is particularly important as D2EHPA has high viscosity. A 700-rpm stirring speed was chosen for further experiments.

### 3.2.2. Effect of copper amount

As depicted in Fig. 2(b), an increase in the Cu/NCM mass ratio led to a rise in metal leaching yield due to more copper available for reaction with NCM. The optimal ratio was determined to be 0.15/1 (g/g) Cu/NCM, resulting in leaching yields of 89.3 % for Li, 49.2 % for Ni, 54.8 % for Co, 22.7 % for Mn, and 59.0 % for Cu. However, higher copper amounts decreased the metal leaching yield. This can be ascribed to the elevated consumption of D2EHPA by copper, consequently diminishing its availability for reacting with NCM. Consequently, a Cu/NCM ratio of 0.15 (g/g) was chosen for subsequent experiments.

### 3.2.3. Effect of D2EHPA concentration

The effect of D2EHPA concentration (0.5–2.3 M, equal to 17.5 %–80 % D2EHPA) on NCM leaching yield is depicted in Fig. 2(c). Leaching yields for Li, Ni, Co, and Mn increased with D2EHPA up to 2.0 M and declined at higher concentrations. At 0.5 M, yields were 70.6 % (Li), 22.7 % (Ni), 25.5 % (Co), and 12.9 % (Mn); at 2.0 M they reached ~100 %, 63.9 %, 85.0 %, and 40.3 %, respectively. Cu showed the same trend.

In non-aqueous media, D2EHPA reacts with metals to form [(MA)n]• (HA)x] complexes (M = metal, HA = D2EHPA; n = 1–2, x = 1–4) (Jang et al., 2017; Mohammadzadeh et al., 2022; Preston, 1982). The stoichiometric D2EHPA concentration required for quantitative dissolution of NCM and Cu under our conditions was calculated as 1.11 M, slightly

below the experimental optimum (~2.0 M), consistent with incomplete D2EHPA dissociation in hydrophobic diluents (Bennett et al., 2018; Bhandari and Dhawan, 2022). The drop in leaching at 2.3 M is attributed to increased lixiviant viscosity, evidenced by more difficult filtration. Additional tests at higher temperature could further isolate viscosity effects. Based on Figs. 2(c), 2.0 M D2EHPA was selected for subsequent experiments.

### 3.2.4. Effect of temperature

The effect of temperature (range 25–90 °C) on the leaching yield is displayed in Fig. 2(d). An increase of the temperature from 25 to 70 °C was responsible for a significant increase of the leaching efficiency of Li, Ni, Co, Mn and Cu from 65.3 % to ~99.9 %, 26.2 %–61.9 %, 31.1 %–81.1 %, 18.8 %–37.6 % and 33.1 %–75.2 %, respectively. At 90 °C, the leaching efficiency further increased to 73.9 % (Ni), 89.5 % (Co), 45.4 % (Mn) and 90.6 % (Cu). Higher temperatures can increase the diffusion in solution by reducing the bulk viscosity, increase the dissociation of D2EHPA and provide more energy for chemical reactions (Samuelsen et al., 2019).

### 3.2.5. Effect of time

After 4 h, the leaching efficiencies were approximately 100 %, 79.7 %, 95.5 %, 48.4 %, and 95.5 % for Li, Ni, Co, Mn, and Cu, respectively (Fig. 2(e)). Extending the duration beyond this duration, however, resulted in an insignificant increase in metal leaching. An increasing water content was noted during the first 30 min of leaching, from 0.2 mg/L (initial water content in 2.0 M D2EHPA) to 0.62 mg/L (Fig. S8). Despite the continuous metal dissolution as the non-aqueous leaching progressed, the water content dropped to 0.58 mg/L after 1 h and
continuously decreased thereafter. Such decrease might indicate that the formed water was evaporated with the longer leaching time.

### 3.2.6. Effect of pulp density

Experiments were performed at different pulp densities, ranging from 25 to 100 g/L. The results presented in Fig. 2(f) show that the NCM leaching efficiency decreased with increasing pulp density. At a pulp density of 25 g/L, almost quantitative leaching yields of Li, Ni, Co and Cu could be achieved whereas only 57.2 % Mn were dissolved. Li leaching could still be quantitative at higher pulp density, but the leaching efficiency of the other metals (Ni, Co, Mn and Cu in an order manner) decreased to 79.2 %, 94.6 %, 47.3 % and 98.4 % at 50 g/L pulp density, and to 51.4 %, 58.9 %, 31.3 % and 68.7 % at 100 g/L. At higher pulp density, not enough D2EHPA molecules were available to react with the solid phase. Meanwhile, the concentration of Li, Ni, Co, Mn and Cu in organic phase increased from 0.75, 2.79, 1.99, 1.65 and 3.88 g/L to 1.73, 4.56, 3.65, 2.72 and 7.6 g/L when the pulp density increased from 25 to 50 g/L, and it could increase further at higher pulp density. Considering the leaching efficiency and metal concentrations in the organic phase, a 50 g/L pulp density was chosen for subsequent experiments.

### 3.2.7. Analysis of leaching residue

The leaching residues after 1–4 h leaching (following washing and drying) were analyzed by SEM (Fig. S9) and XRD (Fig. S10). A significant difference in the surface morphology of NCM before and after the non-aqueous leaching can be seen. The NCM surface became porous and rough after leaching, as compared to that of the initial particles which had smooth surface (Fig. S2). Longer the leaching progressed, more porous and corroded surface was formed. The EDS depicts decreased metal intensities while increase in concentration (intensities) of aluminum was noticed, which is in accordance with the leaching behavior of concerned metals. The surface of graphite remained smooth even after 4 h of leaching. Consistent is in the XRD spectra of the residues, which showed the gradual decrease in intensities of LiNiₓCoᵧMn₁₋ₓ yO₂ phases while the intensity of Al-phase increased as the leaching progressed. No new phases are formed during the leaching as the reactions only form metal cations and H₂O. The Cu leaching is relatively fast, therefore no corresponding peaks are detected by XRD.

## 3.3. Metal recovery from loaded D2EHPA

### 3.3.1. Stripping using aqueous solutions

The reaction for metal stripping from loaded D2EHPA ([HA]) can be written as:

$$[MA_n \cdot (HA)_2] + nH^+ \rightleftharpoons M^{n+} + \frac{1}{2}(n+2)[HA]_2 \quad (7)$$

Based on Eq. (7), pH critically governs stripping from loaded D2EHPA. Stripping of Li, Co, Cu, Ni, and Mn was evaluated by controlling the equilibrium pH (Fig. 3). Li is stripped at higher pH, followed by Ni-Co, whereas Cu and Mn require lower pH (stronger acid). The pH₅₀ was 3.40 (Li), 2.71 (Ni), 2.41 (Co), 1.58 (Cu), and 1.11 (Mn), indicating progressively greater complex stability from Li to Mn/Cu. These trends are consistent with coordination chemistry and HSAB considerations: the monovalent Li⁺ forms weaker D2EHPA complexes that dissociate more readily at higher pH, while Cu(II) and Mn(II) form more stable complexes that require stronger acidity to protonate the ligand and displace the metal.

### 3.3.2. Precipitation-stripping using oxalic acid

Precipitation-stripping offers a significant benefit by combining the aqueous stripping and precipitation processes into a single stage (process intensification), which allows for the direct recovery of final products. As only one chemical is utilized, it means reduced chemical and aqueous consumptions. To some degree, it can be utilized to

<figure><img src="image_5.png" /><figcaption>Fig. 3. Stripping efficiencies of metals (Li, Co, Ni, Mn, Cu) from loaded D2EHPA as a function of equilibrium pH.</figcaption></figure>

enhance the selectivity. Potential precipitation-stripping of metals (Li, Co, Ni, Cu, Mn) from loaded D2EHPA was investigated using 0.2–1.0 M oxalic acid at the O/A phase ratio = 1/1 and 25 °C for 10 min of shaking time.

Results are summarized in Table 1. By using 0.2 M oxalic acid, the precipitation-stripping yields could not be determined as the precipitates were distributed in both organic phase and at the bottom of the stripping (aqueous) solution. Analysis of the stripping solution showed that about 0.46 g/L Li, 1.58 mg/L Co, 0.93 mg/L Ni, 1.67 mg/L Mn and 11.87 mg/L Cu remained in the solution. This problem did not happen while using a higher oxalic acid concentration. In fact, quantitative stripping of all the metals was achieved when 0.4 M oxalic acid was used. At this concentration, almost all of the stripped Co, Ni, Cu and Mn were transferred to the precipitates (refer the precipitation yield in Table 2) while all the stripped Li remained in the solution, indicating a potential for their separation. Further increasing the oxalic acid increased the Li precipitation. These results are consistent with the results in Fig. 3 and follows the solubility product constants (Kₛₚ) of metal oxalates (Kₛₚ CuC₂O₄ = 4.43·10⁻¹⁰, Kₛₚ CoC₂O₄ = 6·10⁻⁸, MnC₂O₄, Ksp MnC₂O₄·2H₂O = 1.7·10⁻⁷, Kₛₚ NiC₂O₄·2H₂O = 3.98·10⁻¹⁰) (Verma et al., 2019). Lithium oxalate has a relatively high solubility in water; this explains its persistence in solution at moderate oxalate concentrations, while for transition metals (Cu, Co, Mn, and Ni), the lower Ksp values favor quantitative precipitation once stripped.

## 3.4. Reusability of D2EHPA

The leaching properties of D2EHPA after being used for several leaching-stripping cycles were evaluated, and the results are presented in Table 2. Results show that D2EHPA has an excellent reusability without loss of its extraction performance for the non-aqueous leaching of NCM. This also agreed to the FTIR spectra of stripped D2EHPA in Fig. S5. Indeed, this is beneficial in economic and sustainability perspectives.

## 3.5. Integrated non-aqueous leaching of black mass from spent LIBs and other waste

Non-aqueous leaching of NCM using D2EHPA and copper is effective, and under ideal condition, copper can be co-recovered (after selective separation). Hence, virgin Cu is required only for the initial leach; subsequent cycles can use reclaimed Cu, enhancing the appeal of Cu as a
<table><caption>**Table 1**
Precipitation-striping of NCM-loaded D2EHPA using oxalic acid (O/A = 1/1, 25 °C, 10 min).</caption><thead><tr><th rowspan="2">Oxalic acid conc. (M)</th><th colspan="2">Li</th><th colspan="2">Co</th><th colspan="2">Ni</th><th colspan="2">Mn</th><th colspan="2">Cu</th></tr><tr><th>Stripping (%)</th><th>Precipitation (%)</th><th>Stripping (%)</th><th>Precipitation (%)</th><th>Stripping (%)</th><th>Precipitation (%)</th><th>Stripping (%)</th><th>Precipitation (%)</th><th>Stripping (%)</th><th>Precipitation (%)</th></tr></thead><tbody><tr><td>0.2</td><td>NA</td><td>NA (0.46 g/L)</td><td>NA</td><td>NA (1.58 mg/L)</td><td>NA</td><td>NA (0.93 mg/L)</td><td>NA</td><td>NA (1.67 mg/L)</td><td>NA</td><td>NA (11.87 mg/L)</td></tr><tr><td>0.4</td><td>99.9</td><td>0</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.6</td></tr><tr><td>0.6</td><td>99.9</td><td>39.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.8</td></tr><tr><td>0.8</td><td>99.9</td><td>42.7</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td></tr><tr><td>1.0</td><td>99.9</td><td>43.3</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td><td>99.9</td></tr></tbody></table>

NA = not available; () = indicates the metal concentrations remained in the stripping solutions.

<table><caption>**Table 2**
Results on the reusability test of D2EHPA in the non-aqueous leaching of NCM-chemistry black mass.</caption><thead><tr><th rowspan="2">Leaching no.</th><th colspan="5">Leaching (%)</th></tr><tr><th>Li</th><th>Co</th><th>Ni</th><th>Mn</th><th>Cu</th></tr></thead><tbody><tr><td>1st</td><td>99.9</td><td>98.3</td><td>83.5</td><td>58.1</td><td>96.5</td></tr><tr><td>2nd</td><td>99.9</td><td>98.3</td><td>83.4</td><td>57.9</td><td>96.5</td></tr><tr><td>3rd</td><td>99.9</td><td>97.9</td><td>83.4</td><td>57.8</td><td>96.3</td></tr><tr><td>4th</td><td>99.9</td><td>97.7</td><td>83.0</td><td>57.4</td><td>96.2</td></tr><tr><td>5th</td><td>99.9</td><td>97.0</td><td>82.6</td><td>57.3</td><td>96.2</td></tr></tbody></table>

reductant. However, reliance on chemical-grade Cu may raise cost and CO₂ footprint concerns. Hence, utilizing Cu-rich secondary resource, including WPCBs and Cu-Al foil scraps from spent LIBs, is approached. This, at the same time, facilitates the establishment of an integrated waste recycling process.

Hence, a study was conducted to examine the feasibility of WPCBs or Cu-Al foil scraps as a reductant in the NCM non-aqueous with D2EHPA. Results presented in Fig. S11 demonstrate that Adding WPCBs increased leaching substantially: Li rose from 72.3 % (no WPCBs) to near-quantitative at all tested WPCBs/NCM mass ratios; Ni, Co, and Mn increased from 26.0 %, 32.7 %, and 16.7 %–50.0 %, 74.8 %, and 42.6 % at 0.15/1 (g/g), and to 62.9 %, ~99.9 %, and 55.8 % at 0.25/1, as [Cu] increased from 5.13 to 6.97 g/L. Cu-Al foil scraps showed the same trend, with negligible Al leaching, but lower yields due to lower Cu content. Efficient operation requires appropriate WPCB/foil loading and particle size.

In summary, accessible Cu-containing materials are viable reductants for non-aqueous NCM leaching. Selection should consider compositional similarity to NCM to enrich the loaded D2EHPA with target metals and the impurity profile of the Cu source.

### 3.6. Further considerations

Non-aqueous leaching of NCM-containing black mass from spent lithium-ion batteries using D2EHPA offers process intensification by combining leaching and SX in a single stage. The operation generates a metal-loaded organic phase analogous to classical SX, from which dissolved metals can be recovered by aqueous stripping or direct precipitation (e.g., with oxalic acid). The approach avoids mineral acids (e.g., H₂SO₄) during leaching and requires only modest acid for stripping; with direct precipitation, downstream H₂SO₄ use can be eliminated or substantially reduced. The organic extractant is recyclable, lowering organic waste.

Optimized conditions for non-aqueous leaching of NCM black mass were: 700 rpm stirring; Cu/NCM = 0.15 g/g (copper as reductant); 2.0 M D2EHPA; 4 h at 90 °C; solids loading 1.0 g NCM per 20 mL D2EHPA (≈50 g/L). Leaching yields were ≈100 % Li, 79.2 % Ni, 94.6 % Co, 47.3 % Mn, and 96.4 % Cu. Although lower than typical aqueous H₂SO₄ leaching, these results are comparable to organic-acid systems. Quantitative dissolution, beneficial for metal recovery and residue handling,

could be pursued by lowering pulp density and implementing two-stage, counter-current non-aqueous leaching; the second stage may require fresh copper to maintain reductant levels. Residual organic on solid residues could be leveraged during graphite recovery (e.g., flotation; Verdugo et al., 2022; Zhou et al., 2018).

Two practical issues merit attention. The first is viscosity; high viscosities are a known limitation. Mitigation includes dilution in water-immiscible diluents and elevated temperatures, with careful safety management (flash points and vapor pressures). Beyond kerosene, candidates include n-dodecane (Matsui et al., 2025) and synthetic paraffins (e.g., Shell GTL Fluid G80) (Li et al., 2025). Prior work suggests that equilibrating the organic with water can improve leaching (Matsui et al., 2025) and warrant evaluation. The second one is selectivity: D2EHPA displays broad extraction under the tested conditions, reflected in co-leaching. Stripping pH₅₀ values for Li, Ni, Co, Cu, and Mn are relatively close (Fig. 3), limiting separation; oxalate precipitation similarly provided limited selectivity among transition metals. Enhanced separations could employ alternative organophosphorus extractants with known selectivity (e.g., Cyanex 272 for Co/Ni), synergistic mixtures as in SX, and tailored stripping agents.

This work is at the proof-of-concept stage, consistent with the limited literature on non-aqueous leaching using acidic organic extractants (Kurniawan et al., 2024; Matsui et al., 2025; Peeters et al., 2022). The promising results, combined with one-stage leaching-extraction and distinctive equilibria, support further development and scale-up. As the process is optimized and broader datasets (kinetics, phase behavior, solvent recycle, mass balances) become available, comprehensive techno-economic and environmental assessments can benchmark non-aqueous leaching against classical aqueous routes.

### 4. Conclusions

This study proposes a sustainable recycling method for nickel-rich NCM cathode of spent LIBs via non-aqueous leaching using D2EHPA as the lixiviant and copper as the reductant. The non-aqueous leaching occurs through the mechanisms: (1) interaction between D2EHPA and metal sources (NCM and copper), (2) leaching reactions between H⁺, copper and NCM, (3) proton exchange and coordination between exchanged metal cations and D2EHPA. These mechanisms determine the efficiency and advanced selectivity of non-aqueous leaching process. Leaching parameters (stirring speed, Cu dosage, D2EHPA concentration, temperature, time, pulp density) significantly affected yields. Conditions of 700 rpm stirring speed, 0.15 (g/g) Cu/NCM, 2.0 M D2EHPA, 4 h, 90 °C and 1.0 g NCM/20 mL D2EHPA were determined as the optimum, achieving the leaching efficiency of ~100 % Li, 79.2 % Ni, 94.6 % Co, 47.3 % Mn and 96.4 % Cu. Recovery of metals from loaded D2EHPA was investigated by stripping with pH-controlled solutions and precipitation-stripping using oxalic acid. Results of the stripping experiments showed the pH₅₀ for Li, Ni, Co, Cu and Mn as: 3.4, 2.71, 2.41, 1.58 and 1.11, which also indicate that these metals can be separated by controlling the equilibrium pH. Precipitation-stripping could also be used for a simpler metal recovery process; in fact, Li could be totally
separated from NCM and Cu by using 0.4 M oxalic acid. D2EHPA has proven to have good reusability without loss of extraction performance even after five leaching-stripping cycles. Copper-containing scraps/wastes (e.g., Cu-Al foils, WPCBs) can function as reductants, supporting an integrated recycling approach and advancing process circularity.

## CRediT authorship contribution statement

**Kurniawan Kurniawan:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Sookyung Kim:** Supervision, Project administration, Funding acquisition. **Mooki Bae:** Formal analysis. **Jae-chun Lee:** Writing – review & editing, Conceptualization. **Alexandre Chagnes:** Writing – review & editing.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Sookyung Kim reports financial support was provided by The National Research Foundation of Korea (NRF). Sookyung Kim reports financial support was provided by The Korea Institute of Energy Technology Evaluation and Planning (KETEP). If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2023-00254424) and supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20229A10100070, Development of Pt catalysts precursor with low carbon emission recycling process from waste PEMFC).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jenvman.2025.128220.

## Data availability

Data will be made available on request.

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