이지현
(Ji Hyeon Lee)
1
김재형
(Jea Hyung Kim)
1
문병화
(Byeonghwa Moon)
2
홍현선
(Hyun Seon Hong)
3,*
-
성신여자대학교 미래응용과학학과
(Department of Next Generation Applied Sciences, Sungshin Women’s University, Seoul
01133, Republic of Korea)
-
연화신소재
(YEONWHA ADVANCED MATERIALS, Yesan 32433, Republic of Korea)
-
성신여자대학교 청정신소재공학과
(Department of Materials Science and Engineering, Sungshin Women’s University, Seoul
01133, Republic of Korea)
Copyright © The Korean Institute of Metals and Materials
Keywords
Complexation, Deep eutectic solvent, NCM black mass, Metal leaching, Reduction
1. INTRODUCTION
Lithium-ion batteries (LIBs) play a central role in modern energy storage applications,
particularly in portable electronics, electric mobility, and stationary storage systems[1]. As their applications continue to expand, the generation of spent LIBs has also
increased markedly[2,
3]. Spent LIBs contain valuable metals such as Ni, Co, Mn, and Li, and therefore the
development of efficient recovery technologies is important from the perspectives
of resource circulation and stable raw material supply[4-
6]. In addition, improper treatment of spent batteries can cause environmental contamination
due to the leakage of electrolytes and transition-metal-containing components, as
well as safety issues such as fires and explosions[7,
8].
Pyrometallurgical and hydrometallurgical processes have been widely employed as representative
methods for recovering valuable metals from spent batteries. Pyrometallurgical processing
involves the high-temperature treatment of spent batteries or black mass through melting,
reduction, or calcination, including smelting, roasting, and carbothermic reduction[8-
10]. Although this process is suitable for large-scale treatment, it has several limitations,
including high energy consumption, emission of harmful gases, and difficulty in recovering
Li[3]. In contrast, hydrometallurgical processing allows metals to be selectively dissolved
at relatively low temperatures in inorganic acid media, such as hydrochloric, nitric,
or sulfuric acid, as well as in organic acid solutions[11]. In particular, sulfuric acid–hydrogen peroxide leaching systems have been widely
used for the leaching of spent batteries[12]. However, the use of strong acids and reducing agents causes equipment corrosion,
wastewater treatment issues, and environmental burdens[13,
14].
Accordingly, environmentally benign leaching processes using organic acids, bioleaching,
and deep eutectic solvents (DESs) have recently attracted considerable attention[15-
17]. Organic acid leaching has lower toxicity and better biodegradability than inorganic
acid leaching; however, high acid concentrations, reducing agents, or prolonged reaction
times are often required for the effective dissolution of metal oxides[15]. Bioleaching has advantages in terms of low energy consumption and environmental
compatibility, but it is limited by slow reaction kinetics and the difficulty of controlling
process conditions to maintain microbial activity[16]. In contrast, DESs can control acidity, viscosity, reducing ability, and interactions
with metal ions by adjusting the combination and molar ratio of hydrogen bond acceptors
(HBAs) and hydrogen bond donors (HBDs). In addition, DESs can simultaneously induce
the dissolution and stabilization of metal ions through the combined effects of hydrogen-bonding
networks, organic acid ligands, and chloride ions. Therefore, DESs are promising alternative
solvent systems for recovering valuable transition metals from spent LIBs because
they offer greater freedom in solvent design than organic acid leaching and bioleaching
and can control both leaching efficiency and metal speciation in solution[18,
19].
Choline chloride (ChCl) is a representative hydrogen bond acceptor that can form DESs
when combined with hydrogen bond donors, such as citric acid (CA) and L-ascorbic acid
(LAA)[20,
21]. ChCl is widely used for DES preparation because of its low toxicity, biodegradability,
and ability to form stable hydrogen-bonding networks with various organic acid-based
HBDs. In addition, chloride ions (Cl-) derived from ChCl can contribute to the stabilization of dissolved metal species
through coordination interactions with metal ions[22]. Therefore, ChCl-based DESs have been applied to metal leaching in combination with
various organic acid HBDs[19].
In particular, CA can enhance metal solubility and suppress re-precipitation through
complex formation with metal ions[23,
24]. In contrast, LAA can act as a reducing agent and promote oxide dissolution by reducing
high-valence transition metals to lower oxidation states[25,
26].
Recently, studies on the leaching of valuable metals from spent LIB black mass and
cathode materials using DESs have been continuously reported. Sitorus et al. investigated
the leaching behavior of Li, Co, Ni, and Mn from black mass using a DES composed of
citric acid and ChCl, and evaluated the effects of DES composition, leaching temperature,
and solid-to-liquid ratio on metal leaching efficiency[27]. Biniaz et al. demonstrated the possibility of selective leaching and recovery of
Co, Ni, and Mn from spent LIBs using a maleic acid and ChCl-based DES, and reported
that the DES composition could affect metal dissolution and subsequent recovery behavior[28]. In addition, Batkal et al. extracted Li, Co, and Ni from NMC111 cathode materials
using a DES composed of ChCl and pyrogallol, and suggested that the reducing properties
and complexing ability of the DES could contribute to the promotion of metal dissolution[29].
However, previous studies have mainly focused on improving leaching efficiency under
specific DES compositions, optimizing leaching conditions, or demonstrating the feasibility
of metal recovery. Therefore, the roles of DES components and the resulting changes
in dissolved metal species have not yet been sufficiently interpreted. In particular,
it remains unclear how complex formation by organic acid ligands and oxide reduction
by reducing HBDs vary depending on the DES composition, and how these changes affect
the leaching efficiencies of Ni, Co, and Mn and the coordination environment in solution.
Therefore, in this study, the leaching behavior of Li, Ni, Co, and Mn from NCM black
mass was systematically investigated by varying the composition of ChCl-CA-LAA-based
DESs. In addition, the optical properties of the solutions and the speciation of metal
ions were examined using UV-Vis spectroscopy. This study aims to elucidate the effects
of DES composition on both metal recovery efficiency and the chemical state of dissolved
metal species in solution.
2. EXPERIMENTAL
2.1 Materials and Characterization
NCM black mass derived from spent lithium-ion batteries was supplied by Company A
and used without any pretreatment. The morphology, particle size distribution and
crystalline phases of the sample were characterized using field-emission scanning
electron microscopy (FE-SEM; JSM-7500F, JEOL, Japan), particle size analysis (PSA;
Nano Plus HD, Micromeritics, USA) and X-ray diffraction (XRD; D8 Focus, Bruker, Germany),
respectively. The elemental composition of the black mass was analyzed by inductively
coupled plasma-optical emission spectrometry (ICP-OES; iCAP 7400, Thermo Fisher Scientific,
USA).
2.2 Reagents
Choline chloride (ChCl, 99%, Daejung) was used as the hydrogen bond acceptor (HBA),
whereas citric acid anhydrous (CA, 99.5%, Daejung) and L-ascorbic acid (LAA, 99%,
Sigma-Aldrich) were used as hydrogen bond donors (HBDs). Deionized water (DIW) was
prepared using a water purification system (Aqua MAXTM-Basic, Younglin, Korea). All reagents were used as received without further purification.
2.3 Preparation of DESs
DESs were prepared by combining ChCl with HBDs at predetermined molar ratios. Depending
on the HBD composition, the prepared DESs were classified into binary systems based
on a single HBD and ternary systems based on mixed HBDs. The detailed compositions
and sample designations are summarized in Table 1. ChCl, HBDs, and DIW (15 wt% relative to the DES) were mixed and stirred at 60 °C
until a homogeneous liquid phase was formed. The prepared DESs were stored in a desiccator
until use to prevent moisture absorption. The formation of the DESs was confirmed
by Fourier-transform infrared spectroscopy (FT-IR; Nicolet iS50, Thermo Scientific,
USA).
Table 1. Molar ratio of ChCl-CA-LAA based DESs
|
No.
|
HBA
|
HBD
|
|
ChCl (mol)
|
CA (mol)
|
LAA (mol)
|
|
1
|
2
|
1
|
0
|
|
2
|
2
|
0.8
|
0.2
|
|
3
|
2
|
0.5
|
0.5
|
|
4
|
2
|
0.2
|
0.8
|
|
5
|
2
|
0
|
1
|
2.4 Leaching procedure
Leaching experiments were conducted by mixing NCM black mass with the prepared DESs
at an solid-to-liquid (S/L) ratio of 40 g/L. The reaction was carried out at 60 °C
and 300 rpm for 2 h. After leaching, the mixture was centrifuged to separate the leachate
from the insoluble residue. The collected supernatant was diluted according to the
purpose of analysis. For metal ion concentration analysis, the supernatant was diluted
with a 2 vol% HNO3 aqueous solution, and the concentrations of Ni, Co, and Mn were determined using
ICP-OES. The leaching efficiency of each metal was calculated using Eq. (1):
where L is the leaching efficiency (%), c is the metal concentration in the leachate (g/L), V is the volume of the leachate (L), m is the mass of black mass used for leaching (g), and ω is the mass fraction of the corresponding metal in the black mass.
For optical analysis, the collected supernatant was diluted with the DES of the same
composition to a concentration of approximately 4 wt% and homogenized. The absorption
spectra were then measured using a UV-Vis spectrophotometer (SPECORD PLUS, Analytik
Jena, Germany).
3. RESULTS AND DISCUSSION
3.1 Characterization of Black Mass
Fig. 1(a) shows the SEM image of the black mass. The sample consisted of agglomerated spherical
and irregular particles with sizes of several micrometers. The particle surfaces exhibited
rough features, which are likely associated with the mechanical crushing and thermal
treatment processes applied during the battery recycling. Consistent with the SEM
observation, the PSA results showed that the D10, D50, and D90 values of the black
mass were 6.1, 15.4, and 39.3 μm, respectively, indicating a broad particle size distribution.
The XRD pattern of the black mass is shown in Fig. 1(b). The diffraction peaks corresponding to LiNiCoO2 and LiNiMnO2 phases were observed, indicating that the sample contained layered lithium transition
metal oxides. In particular, the peaks located at approximately 2θ = 18.7°, 36.5°,
44.4°, and 64.4°, which correspond to the major crystallographic planes such as (003),
(101), and (104), represent the characteristic diffraction behavior of layered NCM-type
lithium transition metal oxides.
The quantitative elemental composition of the black mass, analyzed by ICP-OES, is
summarized in Table 2. The contents of the major transition metals Ni, Mn, and Co were 27.0 wt%, 10.0 wt%,
and 6.89 wt%, respectively, while the Li content was approximately 4.27 wt%. In addition,
small amounts of impurities, including Al (3.26 wt%), Fe (0.202 wt%), and Cu (0.149
wt%), were detected. The “Others” content was calculated by subtracting the sum of
the analyzed metallic elements from 100 wt% and may include carbon, sulfur, oxygen-containing
compounds, binder, electrolyte residues, and other minor components. This chemical
composition suggests that the black mass was derived from spent LIBs containing NCM-based
cathode materials.
Fig. 1. Characterization of the black mass sample: (a) SEM image showing particle
morphology and (b) XRD pattern showing layered transition metal oxide phases.
Table 2. Elemental composition of the black mass sample analyzed by ICP-OES.
|
Elemental composition (wt%)
|
|
Ni
|
Co
|
Mn
|
Al
|
Li
|
Cu
|
Fe
|
others
|
|
27.0
|
6.89
|
10.0
|
3.26
|
4.27
|
0.149
|
0.202
|
48.2
|
3.2 Characterization of DESs
Fig. 2 shows the FT-IR spectra of ChCl-CA-LAA-based DESs with different HBD compositions.
All samples exhibited a broad O-H stretching band in the region of approximately 3200-3500
cm-1
[30], indicating hydrogen-bonding interactions among ChCl, CA, LAA, and water molecules.
The presence of this broad band suggests that the hydrogen-bonding network was retained
even after the addition of 15 wt% DIW. Therefore, the ChCl-CA-LAA systems containing
15 wt% DIW can be regarded as homogeneous hydrated DES systems with a hydrogen-bonding
structure, rather than simple aqueous organic-acid solutions.
Fig. 2. FT-IR spectra of ChCl-CA-LAA-based DESs with different HBD compositions.
Fig. 3 shows the color changes of ChCl-CA-LAA-based DESs depending on the HBD composition.
The relative contents of CA and LAA in the HBDs affected the intrinsic optical characteristics
of the prepared DESs. The composition using CA as the sole HBD (2:1:0) remained a
colorless and transparent liquid upon visual observation. In contrast, the binary
and ternary compositions containing LAA retained transparency but exhibited yellowish
to brownish colors. These color differences in the DESs can be regarded as qualitative
indicators reflecting differences in intermolecular interactions and the chemical
stability of LAA depending on the HBD composition. In particular, the yellowish or
brownish colors observed in LAA-containing compositions suggest the possible partial
oxidation or structural change of LAA under the DES preparation conditions[31-
33]. This indirectly supports the potential involvement of LAA in redox reactions during
the subsequent leaching process. Indeed, LAA can donate electrons during its conversion
to DHA, thereby reducing high-valence transition metals in the oxide structure to
lower oxidation states[31], which can promote the dissociation of metal-oxygen bonds and the dissolution of
transition metals. However, the color of the DES before leaching should not be considered
a direct determinant of leaching efficiency. Thus, these color differences can provide
basic information for interpreting the chemical reactivity of LAA and the subsequent
color changes and UV-Vis absorption characteristics of the leachates.
Fig. 3. Photographs of DESs with different compositions
3.3 Transition Metal Leaching Behavior Depending on DES Composition
Fig. 4 displays the leaching efficiencies of Li, Ni, Co, and Mn obtained using DESs with
various compositions. In this study, the DES compositions are expressed based on the
molar ratio of ChCl:CA:LAA. The binary systems, 2:1:0 and 2:0:1, exhibited different
leaching behaviors. Under the 2:1:0 condition, the leaching efficiencies of Li, Ni,
Co, and Mn were 34%, 25%, 16%, and 31%, respectively, and the total Li-NCM leaching
efficiency was limited to 24.6%. In contrast, under the 2:0:1 condition, Mn and Li
showed relatively high leaching efficiencies of 77% and 51%, respectively, whereas
Ni and Co exhibited lower leaching efficiencies of approximately 29% and 30%, respectively.
As a result, the total Li-NCM leaching efficiency was 38.7%.
Fig. 4. (a) Leaching efficiencies of Li, Ni, Co, and Mn depending on the DES composition
and (b) overall leaching efficiency of Li, Ni, Co, and Mn transition metals.
In the ternary DES systems, a clear composition-dependent synergistic effect was observed.
In particular, the 2:0.5:0.5 condition exhibited high leaching efficiencies of 72%
for Li, 78% for Ni, 58% for Co, and over 99% for Mn, resulting in the highest total
Li-NCM leaching efficiency of 79.5%. In contrast, the leaching efficiencies decreased
to 69.5% and 58.2% under the CA-rich and LAA-rich conditions, respectively, corresponding
to the 2:0.8:0.2 and 2:0.2:0.8 compositions.
This leaching behavior is considered to result from the complementary roles of CA
complexation and the reducing ability of LAA in the DES system. Fig. 5 schematically illustrates the roles of CA and LAA in transition metal leaching using
ChCl-CA-LAA-based DESs. CA, as a polycarboxylic acid, can coordinate with dissolved
metal ions and contribute to the formation of M-CA complexes. In general, when metal
ions are stabilized in solution as complexes, the concentration of free metal ions
decreases, thereby providing a driving force for the continuous dissolution of metal
ions from solid oxides. In addition, complexation can suppress the re-precipitation
or hydrolysis of dissolved metal ions, thereby improving their stability in the leachate[22,
23].
Meanwhile, LAA can donate electrons during its oxidation to dehydroascorbic acid (DHA),
enabling the reduction of high-valence transition metals in the oxide structure, such
as Co3+ and Mn4+, to lower oxidation states, such as Co2+ and Mn2+
[24,
25]. This reductive reaction weakens metal-oxygen bonds and promotes the formation of
lower-valence metal ion species that are more readily dissolved, thereby facilitating
the dissolution of transition metal oxides. Therefore, LAA is considered to promote
the initial release of metal ions from the solid oxide lattice, whereas CA stabilizes
the dissolved metal ions in the solution[34].
Fig. 5. Proposed roles of CA and LAA in the ChCl-CA-LAA-based DES leaching system:
(a) CA-assisted complexation of dissolved metal ions and (b) LAA-assisted reductive
dissolution of transition metal oxides.
ChCl, as the HBA, contributes to the formation of the DES hydrogen-bonding network
with CA and LAA. In addition, chloride ions derived from ChCl can act as coordinating
ligands for dissolved metal ions, thereby contributing to the formation of metal-chloride
species and the stabilization of dissolved metal species in the leachate[22]. Consequently, in the ChCl-CA-LAA ternary DES system, the enhanced leaching efficiency
compared with the binary DES systems can be attributed to the simultaneous contribution
of reductive dissolution by LAA and complex stabilization by CA and metal species
stabilization by chloride ions. In other words, LAA promotes the dissolution of metal
oxides, while CA and chloride ions stabilize the dissolved metal ions, enabling a
synergistic reduction-dissolution-stabilization process to be continuously maintained.
In the case of Li, its dissolution is considered to be less dependent on reductive
reactions because Li is already present as Li+ in the layered cathode structure. Therefore, Li leaching may be mainly associated
with acid-assisted Li+ extraction and stabilization of Li+ in the DES phase[35]. However, when a specific component is present in excess, either the reductive reaction
or complexation may become relatively dominant, disturbing the balance between overall
metal dissolution and stabilization. Therefore, the highest leaching efficiency observed
at the ChCl:CA:LAA molar ratio of 2:0.5:0.5 is considered to result from the appropriate
balance between CA and LAA. Indeed, the 2:0.5:0.5 composition exhibited leaching efficiencies
approximately 1.14 and 1.37 times higher than those of the 2:0.8:0.2 (69.5%) and 2:0.2:0.8
(58.2%) compositions, respectively, quantitatively confirming that the balance of
DES composition significantly affects the reaction efficiency. In addition, the visual
changes in the leachates appear to reflect differences in composition-dependent reactivity
and the coordination environment of metal ions, which are qualitatively discussed
in the following section.
3.4 Optical Properties of DES and Leachates
The visual color and UV-Vis absorption spectra of the leachates obtained using DESs
with different compositions are shown in Fig. 6(a) and (b), respectively. In the CA-only DES (2:1:0) and LAA-only DES (2:0:1), the Ni leaching
efficiencies were similar, at 25% and 29%, respectively; however, their UV-Vis absorption
behaviors were clearly different. This indicates that not only the total amount of
dissolved metal ions but also the ligand type and coordination environment determined
by the DES composition are major factors governing the optical properties of metal
ions in the leachate[36,
37].
In the LAA-only DES (2:0:1), a strong absorption peak was observed at approximately
394 nm. This peak can be interpreted as an absorption associated with Ni-chloride
complexes formed by the coordination of ChCl-derived Cl- ions with Ni2+, particularly tetrahedral [NiCl4]2- species. In contrast, no distinct peak near 394 nm was observed in the CA-only DES
(2:1:0) or the ternary DES compositions. This suggests that the carboxylate groups
of CA may coordinate with Ni2+ and induce the formation of Ni-carboxylate coordination species[36].
Under the ternary DES conditions, broad absorption behavior was observed in the 350-450
nm region. This absorption feature is interpreted not as a peak originating from a
specific single complex species, but rather as a result of changes in the coordination
environment of Ni in the solvent environment where CA and LAA coexist. Under the ternary
DES conditions, aqua ligands derived from water present in the DES may also participate
in the coordination sphere of Ni, together with chloride and carboxylate ligands,
leading to the formation of mixed-ligand Ni species. This result supports the interpretation
proposed in Fig. 5, in which CA contributes to the complexation and stabilization of dissolved metal
ions, while LAA promotes the reductive dissolution of transition metal oxides.
The multiple absorption bands observed in the visible region of 600-700 nm are considered
to be associated with the d-d transitions of Co-chloride complexes formed through
the coordination of Co ions with chloride ligands, particularly [CoCl4]2-
[38]. In general aqueous solutions, Co2+ mainly exists as octahedral aqua complexes; however, in Cl--rich environments such as ChCl-based DESs, tetrahedral Co-chloride complexes can
be formed[36,
39]. The blue color of the leachate obtained using the CA-only DES, as shown in Fig. 6(a), can be attributed to the significant optical contribution of these Co-based complexes.
The greenish colors observed in the LAA-only and ternary DES leachates are considered
to result from the combined contributions of Ni-based absorption features and Co-based
absorption bands in the 600-700 nm region. In particular, the LAA-only DES showed
a distinct peak related to Ni-chloride complexes near 394 nm, whereas the ternary
DES exhibited broad absorption behavior in the 350-450 nm region. This indicates that
the absorption characteristics of Ni-based coordination species significantly influenced
the color changes of the leachates under LAA-containing conditions.
Consequently, changes in the HBD composition of the DESs affected not only the amount
of dissolved metal ions but also their coordination stability and species distribution.
These differences were reflected in the distinct UV-Vis absorption characteristics
and color changes of the resulting leachates.
Fig. 6. Optical properties of DES leachates: (a) photographs and (b) UV-Vis absorption
spectra.
4. CONCLUSIONS
In this study, the effects of the composition of ChCl-CA-LAA-based DESs on the transition
metal leaching efficiency from NCM black mass and the chemical characteristics of
the leachates were investigated. In the binary DESs using a single HBD, the leaching
behavior of each metal was limited or unbalanced, whereas the ternary DESs containing
both CA and LAA significantly enhanced the dissolution of transition metals. In particular,
at the ChCl:CA:LAA molar ratio of 2:0.5:0.5, the leaching efficiencies of Li, Ni,
Co, and Mn reached approximately 72%, 78%, 58%, and over 99%, respectively, and the
total Li-NCM leaching efficiency was the highest at 79.5%.
This improved performance can be attributed to the combined effects of reductive dissolution
of high-valence transition metals by LAA and complexation and stabilization of dissolved
metal ions by CA. In addition, ChCl contributes to the formation of the hydrogen-bonding
network of the DES, and chloride ions derived from ChCl may partly participate in
the stabilization of dissolved metal species. In other words, LAA promotes the dissolution
of metal oxides, while CA enhances the stability of metal ions in solution, thereby
forming a complementary leaching environment in the ternary DES system.
UV-Vis analysis also revealed that even under conditions with similar metal leaching
efficiencies, the absorption behavior and leachate color clearly varied depending
on the DES composition. This indicates that the characteristics of the leachates are
not determined solely by the total concentration of dissolved metals, but are strongly
affected by the metal coordination environment and species distribution involving
chloride, carboxylate, and aqua ligands.
Overall, this study demonstrates that HBD composition design in ChCl-based DESs is
an important factor for controlling not only metal leaching efficiency but also the
coordination state and optical properties of metal ions in solution. Therefore, the
design of ternary DESs using a combination of CA and LAA can serve as a promising
strategy for the efficient recovery of transition metals from spent LIB-derived black
mass.
ACKNOWLEDGEMENT
This work was supported by the Technology Innovation Program (RS-2024-00432186, Development
of electrolyte recovery and materials manufacturing technology in battery recycling
process) funded By the Ministry of Trade Industry & Energy(MOTIE, Korea).
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