Science of the Total Environment 902 (2023) 166095

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Science of the Total Environment

journal homepage: www.elsevier.com/locate/scitotenv

Recycling of waste lithium-ion batteries via a one-step process using a novel

deep eutectic solvent
Yi Luo , Chengzhe Yin , Leming Ou *
School of Minerals Processing and Bioengineering, Central South University, China

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• This work avoids using volatile acids

and achieves recovery at low
temperature.
• This recycling process employs eco-
friendly solvent.
• In-situ regeneration of precursors from
spent lithium-ion batteries

A R T I C L E I N F O A B S T R A C T

Editor: Yifeng Zhang Deep eutectic solvents (DESs) possess excellent solubility and selectivity, making them suitable for extracting
valuable metals and serving as a green alternative in the recycling process. This work introduces a low-viscosity
Keywords: DES consisting of dimethylthetin, oxalic acid, and water for the comprehensive recovery of cathode materials
Recycling process from LIBs. Leaching parameters such as ratio (1:1), leaching temperature (60 ◦ C), and reaction time (15 min) for
Deep eutectic solvent
were systematically optimized, resulting in a selective separation efficiency of 99.98 % for lithium ions.
Spent LIB
Furthermore, in-situ regeneration of the precursor can be achieved during the leaching process. Charge-discharge
Precursor
tests indicate that the initial charge and discharge capacities of the regenerated battery are 166.8 mAh/g and
138.4 mAh/g, respectively. The DES demonstrates stability and can be easily recycled by replenishing the
consumed components. This proposed strategy facilitates the reintroduction of nonrenewable resources into the
supply chain and reduces the environmental impact of heavy metals, aligning with the principles of a circular
economy.

1. Introduction compact size, lightweight construction, and high-power output (W.

Chen et al., 2021; Huang et al., 2022; Lei et al., 2021; Luo et al., 2023b).
Lithium-ion batteries (LIBs) have gained widespread popularity due The increasing global demand for sustainable energy sources has led to a
to their excellent electrochemical performance, including high stability, substantial rise in LIB production. However, the disposal of spent LIBs

* Corresponding author.
E-mail address: olmpaper@csu.edu.cn (L. Ou).

https://doi.org/10.1016/j.scitotenv.2023.166095
Received 12 June 2023; Received in revised form 12 July 2023; Accepted 4 August 2023
Available online 7 August 2023
0048-9697/© 2023 Published by Elsevier B.V.
Y. Luo et al. Science of the Total Environment 902 (2023) 166095

present remarkable challenges, as they contain toxic substances and direction in the realm of green hydrometallurgy.
electrolytes that can release harmful gases and vapors into the envi­ This work proposes a new strategy for comprehensive recycling
ronment, posing risks to human health and air quality (Zhang et al., through the utilization of a multifunctional DES to address the chal­
2020; Zhong et al., 2020; Zhu et al., 2023). Additionally, the cathode lenges of waste accumulation and resource depletion. A novel DES
materials of LIBs often contain heavy metals, which can cause soil and formulation was designed, comprising dimethylthetin (DMT), oxalic
water pollution, leading to environmental concerns and ecological acid dihydrate (H2C2O4⋅2H2O), and water, serving as chelating,
degradation (Zante et al., 2020; Zhang et al., 2018; Zheng et al., 2021). reducing, and leaching agents, respectively. Oxalic acid (OA) is a
Waste LIBs also contain valuable metallic components, including man­ naturally occurring organic acid widely found in nature. Dimethylthetin
ganese (Mn), cobalt (Co), lithium (Li), and nickel (Ni) (Yuan et al., is a betaine derivative that provides abundant chloride ions as com­
2022). Recycling these materials, especially those found in the battery plexing agents. Compared with conventional DES leaching methods, this
cathode, is crucial for addressing metal resource shortages and reducing proposed DES enables selective leaching of spent cathode material at
the manufacturing costs of LIBs (Yang et al., 2022; Z. Xu et al., 2021; Yao low temperature (60 ◦ C) and within a short period (15 min), making it
et al., 2018; Yang et al., 2021). energy-efficient and cost-effective. The in situ regenerated precursors
Currently, the primary industrial approach for LIB recycling relies on obtained from the process were used to prepare button cells, and their
pyrometallurgical processes (Tao et al., 2022; Wu et al., 2020; Xiao electrochemical performance was evaluated. Solvent recovery can be
et al., 2017; Xiao et al., 2020; P. Xu et al., 2021; Wang et al., 2018). easily achieved by replenishing the consumed components, demon­
However, this method consumes high amounts of power and results in strating the stability of this DES. The mechanism of in situ regeneration
the emission of hazardous and corrosive gases. Alternatively, the hy­ of DES precursors was discussed and revealed through integrated
drometallurgical process, involving organic acid and inorganic acid characterization. This recycling approach shows promise in advancing
leaching, has garnered substantial research attention for its potential to the circular economy of LIBs and supporting their sustainable
extract and separate high-purity metal elements at lower temperatures utilization.
(Lei et al., 2020; Roy et al., 2021; Sun et al., 2018; B. Wang et al., 2019;
Vieceli et al., 2018; W. Wang et al., 2019). Inorganic acid leaching 2. Experimental
typically employs strong acids, such as H2SO4, HCl, and HNO3, to ach­
ieve high leaching rates, but this can lead to substantial secondary 2.1. Materials and reagents
pollution (Neumann et al., 2022; Ou et al., 2023). By contrast, organic
acid leaching is considered an environmentally friendly process (Gol­ DMT (C4H9O2SCl, 98 %) and H2C2O4⋅2H2O with analytical purity
mohammadzadeh et al., 2018). However, it has limited capacity, re­ were obtained from Aladdin Pharmaceutical Co. Equimolar amounts of
quires operation at higher temperatures and high reagent DMT and H2C2O4⋅2H2O were mixed, and 30 wt% water was added to
concentrations, and poses challenges in reusing expensive organic synthesize the DES used in the experiment. The resulting mixture was
leaching agents challenging (Chen et al., 2015; Bai et al., 2020). stirred until a clear liquid was obtained. Spent LIBs were discharged
Therefore, an efficient and green cyclic leaching system that operates using a 10 wt% sodium chloride solution. The discharged batteries were
under mild conditions is urgently needed. dried and disassembled to retrieve the cathode electrodes and anode.
Deep eutectic solvents (DESs) have gained popularity as a promising The cathode electrodes were further roasted at 450 ◦ C for 2 h to elimi­
option for leaching value metals from used LIBs (Schiavi et al., 2021; Luo nate the binder and electrolyte, and pure cathode powder was obtained
et al., 2022; Tang et al., 2022a; Wang et al., 2021; Thompson et al., through water leaching for use in the experiment.
2022). DESs are unique eutectic mixtures formed through hydrogen
bonding or halogen bonding between components, demonstrating
excellent dissolution capabilities for positive materials (Roldán-Ruiz 2.2. Selective leaching and in situ regenerable precursor
et al., 2020; Peeters et al., 2020; Yu et al., 2021; Shi et al., 2022). In
comparison to traditional solvents, DESs possess remarkable advan­ For the DES leaching test, NCM positive powder and DES (1:1 + 30
tages, such as high biodegradability, thermal and chemical stability, wt% water) were combined in a 50 mL conical flask. The flask was
design flexibility, and ease of preparation, which can reduce water placed in an oil bath and stirred at 200 rpm. The temperature was
consumption, minimize wastewater discharge, and prevent the release carefully controlled within the range of 50 ◦ C–80 ◦ C during the exper­
of environmentally hazardous gases (Li et al., 2022; Lai et al., 2023; Ma iment. Various leaching times and temperatures were evaluated to
et al., 2022). Abbott et al. demonstrated the ability of hydrogen bond optimize the process. After leaching, the leachate was filtered to selec­
donors in DES to act as oxygen acceptors, promoting the breakdown of tively separate the precipitates, which were identified as regenerated
metal oxide bonds (Abbott et al., 2003). Tran et al. (2019) pioneered the precursors and subjected to detailed analysis. The metal ion leaching
use of ChCl/glycol DES for leaching waste LIBs. Subsequent studies rate (η) was calculated using Eq. (1)
investigated the efficacy of various DES types, including ChCl/urea, LA/
η (%) = [(Ci × Vi )/m ] × 100 (1)
GHC, ChCl/TA, PEG200/thiourea, ChCl/OA, and others, in solubilizing
LIB cathode materials, including lithium cobalt oxide (LCO) and ternary where Ci represents the concentration of metal ion i in the leachate (mg/
lithium batteries (NCM) (Chang et al., 2022; L. Chen et al., 2021; Du L), Vi represents the volume of the leachate (L), and m represents the
et al., 2022; Fan et al., 2023; Hanada and Goto, 2022; Luo et al., 2023a). mass of the spent cathode material (g).
However, previous studies showed that optimal results require pro­
longed durations (10–48 h) and high temperatures (100 ◦ C–200 ◦ C)
when using DES. Additionally, the high viscosity of DES can hinder the 2.3. New battery regeneration
recovery process, posing practical challenges for implementation.
Furthermore, recovering metals from DES leachate usually involves the The precipitates were heated to 500 ◦ C at a rate of 5 ◦ C per minute
addition of precipitants for chemical precipitation or multistep solvent and calcined for 2 h to form (Ni0.5Co0.2Mn0.3)3O4. Subsequently, the
extraction, introducing impurities and complex separation procedures intermediate was blended with Li2CO3 in water (Li/(Ni + Co + Mn)
that make the overall process inefficient and costly (Tian et al., 2022). molar ratio = 1.05), and the mixture was heated and stirred until
Given that research on DESs is still in its nascent stages, exploring novel complete evaporation of water. The resulting mixture was roasted for 10
DES formulations with selective separation capabilities and low vis­ h at 850 ◦ C to form Li(Ni0.5Co0.2Mn0.3)O2 (X. Chen et al., 2020; Y. Chen
cosity is crucial in facilitating efficient metal recovery under mild et al., 2020; Liu et al., 2021). Electrochemical tests were performed on
operating conditions. This exploration represents an important research the newly generated materials.

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2.4. Electrochemical tests 3.2. Selective leaching of NCM via DES

An electrode slurry was prepared by combining acetylene black (10 3.2.1. Effect of water addition
wt%), regenerated NCM (80 wt%), and polyvinylidene fluoride (10 wt DESs often exhibit slow mass transfer rates due to their high vis­
%) in N-methyl-2-pyrollidone solvent. The slurry was applied onto an cosity, which can limit their leaching efficiency and require high tem­
aluminum foil using a squeegee and then dried in a vacuum oven at peratures for effective use. The addition of water is crucial for enhancing
80 ◦ C for 10 h. The dried coated foil was stamped into 1.2 cm disks. A the leaching efficiency of DESs. Water effectively reduces the viscosity of
new battery was fabricated within an argon-filled glove box, utilizing DESs, thereby lowering the required reaction temperature and
lithium metal as the anode and LiPF6 electrolyte dissolved in a mixture improving the reaction efficiency. The addition of water has distinct
of ethylene diethyl carbonate (DEC) and carbonate (EC). The perfor­ effects on the macroscopic properties and microscopic structure of DESs.
mance of the regenerated cells was assessed via charge–discharge tests As the water content increases, the microstructure of the DES gradually
conducted using a Land CT2001A battery test system at a temperature of transforms toward that of an aqueous solution, resulting in a reduction
25 ◦ C. in solubility performance.
To achieve a balance between the structure of the DES and the water
2.5. Characterization content, we investigated the effects of different water amounts on
leaching efficiency at low temperatures. The findings from Fig. 2a reveal
Fourier transformed infrared spectroscopy (FTIR) patterns were ob­ that the leaching efficiency of lithium ions can be enhanced by adding an
tained using a Bruker Tensor 27 FTIR spectrometer. The concentrations appropriate amount of water. Increasing the water content from 0 % to
of metal ions were measured using an Optima 8000 instrument equipped 30 % showed a clear improvement in the leaching rate, eventually
with inductively coupled plasma–atomic emission spectroscopy. X-ray achieving complete leaching. This improvement can be attributed to the
diffraction (XRD) analyses were conducted using a Bruker D8 diffrac­ reduction in DES viscosity caused by water, facilitating faster mass
tometer with a scan rate of 10◦ /min. Proton nuclear magnetic resonance transfer. However, the leaching efficiency of lithium ions decreased
(1H NMR) spectra were obtained using a Bruker Avance 400 MHz in­ when the water addition increased from 50 % to 100 %. This indicates
strument, with dimethyl sulfoxide serving as the solvent. that the DES structure was disrupted, resulting in the gradual transition
of the solvent into an aqueous solution and weakening the DES’s solu­
3. Results and discussion bility. This observation aligns with the findings of Hammond et al., who
reported compromised performance and structure of low eutectic sol­
3.1. Characterization of DES vents when the water content exceeds 42 % (Hammond et al., 2017).

The formation of DMT/OA DESs is attributed to hydrogen bond in­ 3.2.2. Effect of DMP: OA ratio
teractions, as confirmed by FTIR and 1H NMR analyses. The FTIR spectra Under the condition of ensuring a water content of 30 wt%, we
(Fig. 1a) exhibit characteristic peaks of each component, indicating the investigated the effect of the DMT: OA ratio on the leaching rate of NCM
presence of DES. The –OH vibrational peak of OA at 3431.4 cm− 1 red­ cathode materials (Fig. 2b).
shifts to 3396.6 cm− 1 in DES, suggesting a decrease in absorption fre­ As the DMT:OA ratio gradually decreased from 2:1, there was a
quency due to the formation of hydrogen bonds. The broadened significant increase in lithium ion leaching rate. When the ratio reached
vibration absorption peak further indicates enhanced intermolecular 1:1, the leaching rate of Li increased from an initial 90 % to 99.98 %,
hydrogen bonding between the components. The 1H NMR results indicating complete leaching reaction. Simultaneously, with the
(Fig. 1b) demonstrate substantial changes in the peaks corresponding to decrease in the DMT: OA ratio, the content of manganese ions also de­
DMT after the formation of DES (green region), indicating that hydrogen creases. This is attributed to the higher concentration of oxalate ions,
bonding alters the electron cloud density of DMT, leading to a shift to­ which leads to increased precipitation of manganese ions, achieving the
ward higher fields. goal of selective separation. However, it is important to note that an
excessive amount of oxalate can impact the formation of DES. Therefore,
based on the above, we selected DMT:OA = 1:1 as the optimal condition.

Fig. 1. FTIR spectra (a) and 1H NMR spectra of DES, OA, and DMT.

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Fig. 2. Effect of (a) water addition and (b) DMP: OA ratio on the metal leaching efficiency of different metal ions. Effect of time and temperature on the leaching
efficiency of different metal ions, with leaching temperatures of (c) 50 ◦ C, (d) 60 ◦ C, (e) 70 ◦ C, and (f) 80 ◦ C.

3.2.3. Effect of time and temperature agents for high-valence metals. To provide a more intuitive explanation
We investigated the leaching conditions for NCM523 using the DES of the leaching mechanism, we conducted a visual comparative analysis
under various leaching times and temperatures, as shown in Fig. 2c–f. between DMT/OA/H2O DES and DMT/OA DES.
Higher temperatures promote the leaching reaction by increasing the In a typical experiment, 0.5 g of NCM523 powder was added to 20 g
kinetic energy of the molecules. This leads to more frequent and ener­ of DMT/OA DES. A series of NCM523-DES mixtures were prepared and
getic collisions, thereby accelerating the leaching process. In our study, maintained at a constant temperature of 60 ◦ C for different durations,
we observed a substantial increase in the leaching efficiency of lithium ranging from 0 min to 20 min. The solution exhibited rapid color
ions as the temperature was raised from 50 ◦ C to 60 ◦ C. Moreover, the changes and substantial bubble formation (Fig. 3a). Over time, the
time required for complete leaching of lithium ions was only 2.5 min at mixture transitioned from black to a clear green color. This color change
temperatures of 70 ◦ C and 80 ◦ C, indicating the strong solvency of the is attributed to the formation of chloride complexes resulting from the
DES. The leaching behavior of Co and Ni exhibits similar trends, with interaction of metal ions and chloride ions during the leaching reaction.
initial increases in the leaching rate followed by rapid decreases. The Ultraviolet–visible spectroscopy (UV–Vis) analysis of the DES filtrate,
destruction of the metal–oxygen bonds in the NCM cathode material diluted with pure DES, revealed a triple peak at approximately 700 nm,
enables the transition metal ions to enter the solution. These ions then with peaks at 630, 666, and 695 nm (as shown in Fig. 3b), indicating the
form oxalate salt precipitates with OA anions, and the process is accel­ formation of a tetrahedral Co(II) complex [CoCl4]2− . In NCM523,
erated with increasing temperature. Notably, the precipitation behavior manganese, nickel, and cobalt exist in the form of Mn4+, Ni2+, and Co3+,
of manganese ions differed from that of nickel and cobalt. Specifically, at respectively. This suggests that oxidation–reduction reactions occur
50 ◦ C–60 ◦ C, the amount of manganese ions gradually decreased with during the leaching process, with high-valence transition metal ions are
increasing leaching time, whereas at higher temperatures of reduced to lower valence states. During the leaching process, a sub­
70 ◦ C–80 ◦ C, the amount of manganese ions increased over time. This stantial amount of bubbles is observed before the reaction is completed.
observation suggests that the solubility of manganese oxalate is This observation suggests that during the reduction of nickel, cobalt, and
temperature-dependent, and excessively high temperatures may hinder manganese, OA, acting as a reducing agent, produces a substantial
the extraction of manganese ions. Therefore, the optimized conditions of amount of carbon dioxide (Eq. (2)). The leaching mechanism of car­
60 ◦ C and 15 min were chosen as a compromise between leaching effi­ boxylic acid-based DESs, as summarized by Padwal et al. and Luo et al.,
ciency and selective separation. involves three key factors: the chloride anion used to form chlor­
ometalate complexes, the reducing agent (hydrogen bond donor) that
reduces high-valence metals, and the reaction of acidic protons with
3.3. Visualization analysis oxygen atoms in metal oxides, generating water molecules (Padwal
et al., 2022; Luo et al., 2022). These findings align with the results ob­
Previous studies investigated the application of DESs in battery tained in our study.
recycling; however, different DESs exhibit distinct leaching mecha­ During the leaching of NCM cathode materials using DMT/OA/H2O
nisms. Tran et al. (2019) found that ChCl/EG-based DES showed high DES, the color of the mixture changes from black to gray-white due to
solubility for LCO at 180 ◦ C, attributing it to the hydrogen bond donor the direct formation of oxalate salts upon the reduction of metal ions
ethanol acting as an oxygen acceptor, leading to cobalt reduction and (Fig. 3a). Chen et al. reported that H2O, as a ligand, exhibits stronger
the formation of tetrachlorocobaltate complexes. Other studies have binding ability than chloride ions, enabling the formation of octahedral
also emphasized the importance of hydrogen bond donors as reducing

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Fig. 3. (a) Visualization images of positive powder in DMT/OA/H2O DES and DMT/OA DES at various time points. UV–Vis spectra of the (b) DMT/OA DES and (c)
DMT/OA/H2O DES leaching solution. (d) The structure of transition metal ions with different ligands.

structures (Me(H2O)6) with transition metals in solution, which even­ [ ]2−

Me(Cl)4 + 2H2 O + C2 O4 2− = MeC2 O4 ⋅2H2 O↓ + 4Cl− (3)
tually form MeC2O4⋅2H2O metal oxalate precipitates (Fig. 3d, Eq. (3))
(Chang et al., 2022). This viewpoint was further confirmed by UV–Vis
spectroscopy, which showed the disappearance of the characteristic 3.4. Comparison of the in situ recovery process of DMT/OA/H2O DES
peak for tetrachloride ions (Fig. 3c). The oxalate precipitate can be with previously reported DESs
selectively separated from lithium ions by filtration and directly used as
a precursor material for battery fabrication, achieving in situ regener­ With the initial application of ChCl/EG for extracting valuable
ation from the leaching solution (implementation details can be found in metals from spent LIBs, research on utilizing DES for LIB recycling has
Section 3.5). gained increasing attention. These DESs can be primarily classified into
[ ]2− two categories, as shown in Table 1. The first type is based on ChCl as a
2LiMeO2 + C2 O4 2− + 8Cl− + 8H+ = 2Li+ + 2 Me(Cl)4 + 4H2 O + 2CO2 ↑(Me
hydrogen bond acceptor, such as ChCl/EG, ChCl/Urea, and ChCl-LA.
= Ni, Co, Mn) ChCl-based DESs have a high viscosity, resulting in a longer leaching
(2) process. Additionally, these DESs show limited performance in subse­
quent metal recovery, often requiring extraction or electrochemical

Table 1
Comparison of the metal leaching efficiency using DMT/OA/H2O DES and other reported DESs.
DES Conditions Efficiency (%) Ref.

T (◦ C) T (min) Li Co Ni Mn

ChCl/EG 180 1440 70.8 31.71 6.65 60.16 (Tran et al., 2019)
ChCl/Urea 180 720 94.7 97.9 – – (Wang et al., 2020)
ChCl/FA 90 720 99.8 99.1 – – (L. Chen et al., 2021)
ChCl-LA 105 1440 100 100 – – (Morina et al., 2022)
EG/OAD 90 620 94.4 94.1 – – (Tang et al., 2022a)
EG/SAD 110 360 100 94.8 99.1 94.8 (Tang et al., 2022b)
PEG/PSA 100 1440 100 100 – – (Y. Chen et al., 2021)
PEG-thiourea 160 1440 – 71.5 – – (X. Chen et al., 2020; Y. Chen et al., 2020)
DMT/OA/H2O 60 15 100 – – – This work

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methods, thereby increasing the difficulty of recycling. Another type of 854.3, and 779.6 eV, respectively. Following leaching, substantial
DES utilizes ethylene glycol as the hydrogen bond acceptor, such as EG/ changes were observed in the fitting curves for cobalt and manganese,
OAD, PEG/PSA, and PEG-thiourea. However, the leaching temperature with main peaks detected at 781.8 eV (Co2+) and 641.7 eV (Mn2+). This
of these DESs is high, which remarkably increases the cost of recovery suggests that a redox reaction occurred during the leaching process,
(Table 1). By contrast to most DESs reported in the literature, the DMT/ resulting in the reduction of transition metals to divalent metals, which
OA/H2O DES investigated in this study achieved a similar leaching rate is consistent with the visualization analysis presented in Section 3.3.
under mild conditions. As a low-viscosity solvent, DES enables in situ Previous studies commonly used OA to achieve the coprecipitation of
precursor regeneration during the leaching and separation process, precursors from inorganic acid leaching solutions. However, our study
remarkably shortening the recovery process and providing favorable demonstrates the in situ regeneration of precursors through the DES
conditions for industrial production. leaching process, simplifying the recycling process. This innovative
method offers a green and efficient solution for the comprehensive
recycling of used LIBs.
3.5. In situ regeneration and characterization of precursor

Extracting valuable metals from the DES leachate is an essential step 3.6. Cathode regeneration and electrochemical performance
in the recycling process. This work proposes a novel in situ regeneration characterization
process of precursors based on DES for the comprehensive recovery of
waste LIBs. In this study, we achieved selective separation of lithium The precipitate product was subjected to calcination for 2 h at 500 ◦ C
ions using DMT/OA/H2O DES and isolated transition metal-rich pre­ to eliminate crystal water and form (Ni0.5Co0.2Mn0.3)3O4. This process
cipitates after filtration. Further characterizations were conducted to was performed to validate the feasibility of using the regenerated pre­
analyze the leaching residue. The precipitate was identified as oxalate cursor for producing a new NCM battery. XRD analysis confirmed the
dihydrate because its XRD pattern (Fig. 4a, red line) matched extremely successful formation of (Ni0.5Co0.2Mn0.3)3O4, as the resulting pattern
well with those of NiC2O4⋅2H2O, CoC2O4⋅2H2O, and MnC2O4⋅2H2O. (Fig. 5a) exhibited a spinel structure consistent with Co3O4. The quality
This view was also supported by the infrared spectrum (Fig. 4b). The of the precipitate also indicated the successful formation of (Ni0.5C­
characteristic absorption peaks at 3389.5 cm− 1 (–OH stretching), o0.2Mn0.3)3O4, as shown in Table 2. Subsequently, the calcined product
1620.2 cm− 1 (C–O antisymmetric stretching), 1361.7 cm− 1, and was mixed with lithium carbonate (Li2CO3) and heated to 850 ◦ C at a
1317.4 cm− 1 (C– – O symmetric stretching) appeared in the spectrum, rate of 5 ◦ C/min, maintaining this temperature for 10 h. XRD diffraction
which is in agreement with MeC2O4⋅2H2O. The thermogravimetric- confirmed the formation of NCM523, as the crystal structure of the
differential scanning calorimetry (TG-DSC) (Fig. 4c) results indicate calcined product matched well with the standard LiNiO2 pattern
that as the temperature increases, the precipitate undergoes a dehy­ (Fig. 5b).
dration and decomposition process, consistent with the thermal weight The resulting black powder was used as the regenerated cathode
loss behavior of dihydrate oxalate salts. XPS analysis was performed to material and assembled into a new button battery. The performance of
determine the composition of the precipitate powder. Fig. 4d–f displays the regenerated batteries was evaluated using a constant current char­
the XPS curve fitting of the Ni, Co, Mn 2p3/2 bands in the raw material, ge–discharge testing system. The cycle performance data (Fig. 5c)
indicating that manganese, nickel, and cobalt primarily exist in the form showed that the newly generated LIBs exhibited an initial charge ca­
of Mn4+, Ni2+ (partially Ni3+), and Co3+ with peak values of 643.5, pacity of approximately 166.8 mAh/g and an initial discharge capacity

Fig. 4. (a) XRD pattern, (b) FTIR pattern, and (c) TG-DSC curve of the precipitated product. XPS spectra of (d) Ni 2p, (e) Co 2p, and (f) Mn 3p valence bands for
pristine and leached NCM.

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Fig. 5. XRD patterns of the (a) intermediate and (b) new cathode materials. (c) Initial charge–discharge curve of the spent, commercial, and regenerated NCM523
battery at 0.2C. (d) Coulombic efficiency and discharge capacity over 40 cycles.

characteristic peaks of DES and Re-DES were nearly identical (Fig. 6a).
Table 2
Additionally, no substantial changes were observed in the 1H NMR
Mass change of precipitation products before and after roasting.
spectra between the recovered Re-DES and the initial DES, indicating
Theoretical data (Ni, Co, Mn)C2O4⋅2H2O (Ni, Co, Mn)3O4 that the structure of Re-DES remained unchanged (Fig. 6b). The solution
1.0 g 0.435 g exhibited similar leaching behavior during secondary leaching using Re-
Actual data Precipitation Roasted products DES, confirming that Re-DES maintained its excellent leaching ability
1.0 g 0.429 g (Fig. 6c).
The overall recovery route proposed in this study is depicted in
Fig. 7. The first step involved in the process was preparing the DES.
of approximately 138.4 mAh/g, which was a significant improvement Subsequently, the spent cathode powder was added to the DES and
over the spent battery. Further analysis revealed that the regenerated leached by stirring at 60 ◦ C for 15 min. The precipitated products ob­
battery demonstrated stable performance over 40 charge and discharge tained by centrifugation from the DES solution can be used directly as
cycles (Fig. 5d). Optimizing the recycling method and adjusting the heat precursors for the production of new LIBs. The lithium-rich DES leaching
treatment process can potentially enhance the battery performance, solution can be reused for the next leaching process after simple treat­
initially demonstrating the feasibility of this recycling process. Overall, ment, enabling a closed-loop cycle. During this process, the concentra­
the precipitate obtained through in situ regeneration using DESs serves tion of lithium ions becomes enriched with each cycle. Na3PO4/NaCO3
as a promising precursor, highlighting DES as an efficient and cost- can be added to precipitate lithium ions as Li3PO4/Li2CO3 for recovery
effective green leaching agent for recovering cathode metals from after a certain number of cycles, ensuring the complete recovery of
waste LIBs, offering a new pathway for their recycling. valuable metals. Additionally, the leaching efficiency of different cath­
odes and the recovery efficiency of transition metal elements were
3.7. DES recycling strategy and application calculated using our proposed recovery process. This method was
proven to be effective in separating cathode materials with varying
Equally important is the regeneration of DES. In this study, DES was compositions (Fig. 6d). This work demonstrates the combination of
regenerated easily by replenishing the consumed OA and water after green and sustainable DES with efficient recovery of LIB cathode ma­
centrifugation of the precipitated products. Further characterization of terials for a closed-loop process. Future investigations should focus on
Re-DES was conducted. FTIR analysis demonstrated that the exploring the thermal treatment conditions for regenerating cathode

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Fig. 6. (a) FTIR pattern and (b) 1H NMR spectra of the Re-DES and initial DES. (c) Secondary leaching by Re-DES. (d) Leaching efficiency of different cathodes.

Fig. 7. The overall recovery route proposed in this study.

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materials and further details of DES recycling during the recycling Chen, W., Li, X., Chen, L., Zhou, G., Lu, Q., Huang, Y., et al., 2021b. Tailoring
hydrophobic deep eutectic solvent for selective lithium recovery from the mother
process.
liquor of Li2CO3. Chem. Eng. J. 420.
Chen, Y., Wang, Y., Bai, Y., Duan, Y., Zhang, B., Liu, C., et al., 2021c. Significant
4. Conclusions improvement in dissolving lithium-ion battery cathodes using novel deep eutectic
solvents at low temperature. ACS Sustain. Chem. Eng. 9, 12940–12948.
Du, K., Ang, E.H., Wu, X., Liu, Y., 2022. Progresses in sustainable recycling technology of
In this study, we designed and prepared a novel ternary DES spent lithium-ion batteries. Energy Environ. Mater. 5, 1012–1036.
composed of DMT, OA, and water for the selective recovery of valuable Fan, Y., Kong, Y., Jiang, P., Zhang, G., Cong, J., Shi, X., et al., 2023. Development and
metals from cathode materials (NCM523) under mild conditions. The challenges of deep eutectic solvents for cathode recycling of end-of-life lithium-ion
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for 15 min, owing to its intrinsic reducing ability and abundant coor­ from spent lithium ion batteries (LIBs) using organic acids as leaching reagents: a
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duction of a new NCM523 battery. Charge–discharge tests conducted Hanada, T., Goto, M., 2022. Cathode recycling of lithium-ion batteries based on reusable
hydrophobic eutectic solvents. Green Chem. 24, 5107–5115.
under a constant current reveal that the initial charge and discharge Huang, C., Xia, X., Chi, Z., Yang, Z., Huang, H., Chen, Z., et al., 2022. Preparation of
capacities of the new battery are 166.8 and 138.4 mAh/g, respectively, single-crystal ternary cathode materials via recycling spent cathodes for high
which are comparable to those of commercial NCM523 batteries. This performance lithium-ion batteries. Nanoscale 14, 9724–9735.
Lai, Y., Zhu, X., Li, J., Gou, Q., Li, M., Xia, A., et al., 2023. Recovery and regeneration of
highlights the potential of DES as a green solvent for the sustainable
anode graphite from spent lithium-ion batteries through deep eutectic solvent
recycling of LIB cathode materials. The strategy proposed in this study treatment: structural characteristics, electrochemical performance and regeneration
effectively circumvents the need for harsh temperatures and corrosive mechanism. Chem. Eng. J. 457, 141196.
acids in the recycling process. This approach promotes resource effi­ Lei, S., Cang, Y., Cao, X., Sun, W., Weng, Y., Yang, Y., 2020. Separation of lithium and
transition metals from leachate of spent lithium-ion batteries by solvent extraction
ciency and reduces the environmental impact of heavy metals by rein­ method with Versatic 10. Sep. Purif. Technol. 250, 117258.
troducing nonrenewable resources into the supply chain, aligning with Lei, S., Zhang, Y., Song, S., Xu, R., Sun, W., Xu, S., et al., 2021. Strengthening valuable
the principles of a circular economy. metal recovery from spent lithium-ion batteries by environmentally friendly
reductive thermal treatment and electrochemical leaching. ACS Sustain. Chem. Eng.
9, 7053–7062.
CRediT authorship contribution statement Li, T., Xiong, Y., Yan, X., Hu, T., Jing, S., Wang, Z., et al., 2022. Closed-loop cobalt
recycling from spent lithium-ion batteries based on a deep eutectic solvent (DES)
with easy solvent recovery. J. Energy Chem. 72, 532–538.
Yi Luo: Conceptualization, Formal analysis, Data curation, Writing – Liu, T., Chen, J., Shen, X., Li, H., 2021. Regulating and regenerating the valuable metals
original draft. Chengzhe Yin: Investigation, Methodology, Project from the cathode materials in lithium-ion batteries by nickel-cobalt-manganese co-
administration. Leming Ou: Funding acquisition, Methodology, Writing extraction. Sep. Purif. Technol. 259.
Luo, Y., Yin, C., Ou, L., Zhang, C., 2022. Highly efficient dissolution of the cathode
– review & editing. materials of spent Ni-Co-Mn lithium batteries using deep eutectic solvents. Green
Chem. 24, 6562–6570.
Declaration of competing interest Luo, Y., Ou, L., Yin, C., 2023a. Extraction of precious metals from used lithium-ion
batteries by a natural deep eutectic solvent with synergistic effects. Waste Manag.
164, 1–8.
The authors declare the following financial interests/personal re­ Luo, Y., Ou, L., Yin, C., 2023b. A green and efficient combination process for recycling
lationships which may be considered as potential competing interests: spent lithium-ion batteries. J. Clean. Prod. 136552.
Ma, C., Svärd, M., Forsberg, K., 2022. Recycling cathode material LiCo1/3Ni1/3Mn1/3O2
Leming Ou reports financial support was provided by National Natural
by leaching with a deep eutectic solvent and metal recovery with antisolvent
Science Foundation of China. crystallization. Resour. Conserv. Recycl. 186, 106579.
Morina, R., Callegari, D., Merli, D., Alberti, G., Mustarelli, P., Quartarone, E., 2022.
Data availability Cathode active material recycling from spent lithium batteries: a green (circular)
approach based on deep eutectic solvents. ChemSusChem 15, e202102080.
Neumann, J., Petranikova, M., Meeus, M., Gamarra, J.D., Younesi, R., Winter, M., et al.,
Data will be made available on request. 2022. Recycling of lithium-ion batteries-current state of the art, circular economy,
and next generation recycling. Adv. Energy Mater.
Ou, Y., Yang, Y., Wang, L., Gao, W., Li, K., Zhang, Y., et al., 2023. Process mechanism for
Acknowledgement production of green lixiviant thiosulfate at atmospheric pressure. ACS Sustain.
Chem. Eng.
This work was financially supported by the National Natural Science Padwal, C., Pham, H.D., Jadhav, S., Do, T.T., Nerkar, J., Hoang, L.T.M., et al., 2022. Deep
eutectic solvents: green approach for cathode recycling of Li-ion batteries. Adv.
Foundation of China (No. 51674291). Energy Sustain. Res. 3, 2100133.
Peeters, N., Binnemans, K., Riaño, S., 2020. Solvometallurgical recovery of cobalt from
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