Efficient Separation of Aluminum Foil and Cathode Materials from

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Efficient Separation of Aluminum Foil and Cathode Materials from Spent Lithium-Ion Batteries using a Low-Temperature Molten Salt Mengmeng Wang, Quanyin Tan, Lili Liu, and Jinhui Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06694 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

Efficient Separation of Aluminum Foil and Cathode

Materials

from

Spent

Lithium-Ion

Batteries using a Low-Temperature Molten Salt Mengmeng Wang,† Quanyin Tan,† Lili Liu,† Jinhui Li*† † State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China Mailing address: Mengmeng Wang: Room 825, Sino-Italian Environmental and Energy-efficient Building, School of Environment, Tsinghua University, Haidian District, Beijing 100084, China Quanyin Tan: Room 805, Sino-Italian Environmental and Energy-efficient Building, School of Environment, Tsinghua University, Haidian District, Beijing 100084, China Lili Liu: Room 805, Sino-Italian Environmental and Energy-efficient Building, School of Environment, Tsinghua University, Haidian District, Beijing 100084, China * Corresponding Author Jinhui Li: Room 804, Sino-Italian Environmental and Energy-efficient Building, School of Environment, Tsinghua University, Haidian District, Beijing 100084, China E-mail address: [email protected] Tel.: +86-10-62794143 Fax: +86-10-62772048

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ACS Paragon Plus Environment

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ABSTRACT The effective separation of aluminum (Al) foil and cathode materials is a critical issue for the recycling of spent lithium-ion batteries (LIBs). Previous studies have shown that the strong binding force provided by the organic binder polyvinylidene fluoride (PVDF) between the cathode materials and the Al foil of spent LIBs makes it difficult to peel off the cathode materials from the Al foil, reducing the effectiveness of the metal recovery process. This study reports on a low-temperature molten salt technology for melting the organic binder PVDF. The results showed that an aluminum chloridesodium chloride (AlCl3-NaCl) system—a non-toxic reaction medium—could melt PVDF efficiently, and is environmentally friendly as well, because of the phase transformation of molten salt caused by heat storage. The optimal conditions for peeling off cathode materials were: temperature of 160 ℃, molten salt: cathode electrode mass ratio of 10:1, and holding time of 20 min. The highest peeling off percentage of cathode materials was 99.8 wt%. The purpose of this study was to provide an environmentally friendly, low-cost and effective solution for the separation of cathode materials and Al foil from spent LIBs. Efficient Separation of Aluminum Foil and Cathode Materials from Spent Lithium-Ion Batteries using a Low-Temperature Molten Salt KEYWORDS: Polyvinylidene fluoride; Metal recovery; AlCl3-NaCl; Heat storage; Non-toxic; Low-cost; Environmentally friendly

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■ INTRODUCTION The accelerated development of the new “green-energy” electric vehicle industry has led to a sharp increase in the production of lithium-ion batteries (LIBs), and the number of spent LIBs has been increasing accordingly.1-5 Untreated or poorly treated, spent LIBs are a potential environmental hazard, as well as a huge waste of resources, because the valuable metals in spent LIBs have significant economic value. 6-11 Therefore, it is urgent to develop effective technologies for spent LIBs disposal, for both environmental protection and resource recovery.12-14 Currently, most research on recycling spent LIBs focuses on the recovery of valuable metals from the cathode materials.15-27 However, the high stability and strong bonding capability of the organic binder polyvinylidene fluoride (PVDF) make it difficult to separate the aluminum (Al) foil from the cathode materials, in the recycling process for spent LIBs.28-30 If an effective approach can be found, to dissolve or decompose this organic binder, the Al foil and cathode materials could be separated with a relatively simple treatment, ensuring not only that the Al foil will be recovered in a single form, but also that the cathode materials will be completely separated from the Al foil, greatly facilitating the subsequent metal recovery process. The most common methods of separating the cathode materials from the Al foil, used in previous studies, are as follows: (1) dissolving the Al foil with a strong acid or alkali, so that Li, Co, and Al are all dissolved into the solution in the form of metal ions, and the metals are separated and recovered by an extraction or precipitation process.31, 32

This approach, however, not only necessitates a tedious metal separation step, but 3



also produces refractory acid/base wastewater. (2) Directly calcining the cathode materials and Al foil by high-temperature roasting (400~600 ℃) to decompose the PVDF. This is the most common approach for spent LIB recycling.33 However, it is easy to accidentally trigger the volatilization of elemental fluorine in the form of hydrogen fluoride gas, causing equipment corrosion and atmospheric pollution. (3) Dissolving the PVDF with an organic solvent to recover the cathode materials and Al foil directly, based on the “like dissolves like” principle.34-37 Although this method is relatively efficient and widely accepted in the field of spent LIB recycling, the use of volatile organic solvents not only increases the cost of recycling, but also leads to adverse health effects for the practitioners. In our previous studies, green solvent-ionic liquid was employed to achieve efficient separation of Al foil and cathode materials, but the high cost and complex synthesis steps of ionic liquids limited the further use of this method, negating its potential industrial application.38 Therefore, the development of a low-cost, high-efficiency, and environmentally friendly technology for separating cathode materials from Al foil has practical significance for the recycling of spent LIBs. Molten salt technology, a robust thermal but non-flame process, has been emerging as a green and novel technique for e-waste disposal and recovery.39 Molten salt has the inherent capability of destroying organic constituents in various wastes, while retaining inorganic and radioactive materials in situ.40 For example, the copperrich metallic fraction of waste printed circuit boards was efficiently recovered by a molten salt KOH–NaOH eutectic. The glass, oxides, and plastics in waste printed circuit boards were dissolved and destroyed without oxidizing the valuable metals, during 4


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molten salt processing.41, 42 However, few reports have focused on spent LIBs recovery using molten salt technology. This study proposed using a low-temperature molten salt technology to achieve the separation of cathode materials and Al foil from spent LIBs. We explored the peeling efficiency of several different molten salt systems applied to cathode materials, and can report that the molten salt of a chloride system has more separation advantages than hydroxide or nitrate systems. The experimental results show that such a lowtemperature molten salt technology can achieve highly efficient separation of cathode materials and Al foil, and that the Al foil can be recovered intact, simplifying the recycling process for an entire spent LIBs and increasing the usefulness of the recycled products.

■ EXPERIMENTAL SECTION Materials and Reagents Spent LIBs were provided by Huaxin Environmental Co. Ltd, Beijing, China. The cathode electrodes of lithium cobalt oxide (LiCoO2) used in this study were obtained by manual disassembly. Reagents used in the experiment, including aluminum chloride (AlCl3 · 6H2O), sodium chloride (NaCl), sodium nitrate (NaNO3), potassium nitrate (KNO3), sodium hydroxide (NaOH), and potassium hydroxide (KOH), were all purchased from the Chemical Reagent Company of Beijing, China. The AlCl3 ·6H2O was dried at 60 ℃ to remove partially bound water before use.

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Experimental Procedure The proposed flowchart for separating cathode materials and Al foil from spent LIBs via a low-temperature molten salt is shown in Figure 1. Prior to the manual dismantling stage, the spent LIBs were first discharged completely with 5 wt% NaCl solution for 24 h, to avoid short circuiting or self-ignition. After drying, the spent batteries were manually dismantled, and the metallic shell, organic separators, cathode electrode, and anode electrode separated out. The cathode electrode was cut into strips, which were used as experimental raw material in the subsequent steps. Anode Electrode Metal Shell Organic Separators

Spent LIBs

Moisture evaporation

Manual dismantling Cathode Electrode

Molten salt AlCl3-NaCl

Cutting Cathode Electrode Sheet

Heating treatment

Recycle


Dissolution

Cathode Material PVDF Aluminum Foil

Whole

PVDF Cathode Material

PVDF

Whole Cathode Material PVDF

Manual Separation

Aluminum Foil PVDF

Cathode Material PVDF

Washing

Molten salt AlCl3-NaCl PVDF

Vacuum filtration PVDF

Aluminum Foil PVDF Cathode Material

Figure 1. Flowchart for proposed method of separating cathode materials and Al foil intact, using low-temperature molten salt. The mole ratios and melting points of molten salt in several different systems are shown in Table S1. The experimental device diagram used in this study is shown in Figure S1. First, the molten salt was weighed according to the applicable mole ratio (as 6


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shown in Table S1), and mixed manually in a crucible. A strip of the cathode electrode was placed in the middle of the molten salt in the crucible. The crucible was then placed in a tube furnace and heated to a set temperature (±3 ℃) at a heating rate of 5 ℃/min in a nitrogen atmosphere. After being held at a stable temperature for a period of time, the tube furnace was allowed to cool naturally to room temperature. When the intact cathode electrode material was removed as a whole from the crucible, the Al foil and the cathode materials could be separated by gentle shaking. Cathode materials could thereby can be recovered completely and used as the characterization products. The obtained cathode materials were washed and weighed to calculate the corresponding peeling percentage. PVDF melted in molten salt can be separated out by a series of continuous steps, including molten salt dissolution-washing-vacuum filtration, and the molten salt can be steam-dried and reused. The insoluble polymer PVDF was thus recovered. Analysis Methods Peeling percentages of cathode materials (η) using AlCl3-NaCl molten salt were determined by Eq. (1): η (wt%)=

W1 × 100% W2

(1)

where W1 is the obtained actual weight of the cathode materials, and W2 is the theoretical weight of the cathode materials on the cathode electrode. The surface morphologies of the obtained experimental samples were characterized with field emission scanning electron microscopy (FESEM; Carl Zeiss MERLIN Compact, Germany). The crystal structures of the cathode material obtained 7



in this study were characterized by X-ray diffraction (XRD; Philips PW 1700, USA) using Cu Kα radiation (γ=1.5418 Å) with 30 kV voltage and 30 mA current. Thermo gravimetric analyzer (TGA/DSC; Mettler Toledo, Switzerland) was used for thermogravimetric analysis of the samples, combined with differential scanning calorimetry (TGA-DSC) under an argon atmosphere.

■ RESULTS AND DISCUSSTION Effects of Different Molten Salt Systems Figure 2 shows the peeling off effects of different molten salt systems on cathode materials from the Al foil of the spent LIBs. As can be seen from Figure 2a, heating the cathode electrode at 160 ℃ did not alone melt the organic binder, and the PVDF remained firmly bonded between the cathode materials and the Al foil after the heat treatment. This result shows that at 160 ℃, a medium-free system could not effect a separation of the cathode materials from the Al foil. Neither could a heat treatment at the same temperature where the cathode electrode had been corroded by NaOH-KOH molten salt, as can be seen in Figure 2b; the Al foil and cathode materials were still difficult to separate. This result can be attributed to the corrosion of the Al foil by strong alkali reagents under high-temperature conditions. From Figure 2c, it can be seen that the NaNO3-KNO3 molten salt also had no significant effect on the peeling off of the cathode materials at 160 ℃. This result was predictable, as the melting point of a NaNO3-KNO3 molten salt system is about 220 ℃, confirming that a nitrate system was not suitable for stripping cathode materials from a cathode electrode at low temperature. Figure 2d shows that after AlCl3-NaCl molten salt treatment at the temperature of 8


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160 ℃, however, the Al foil and cathode materials were well separated, and the cathode materials could be peeled off as a whole, maintaining a relatively cohesive structure and indicating that the AlCl3-NaCl molten salt system had an extremely significant effect on dissolving PVDF at a relatively low temperature. As shown in Table S1, the melting point of the AlCl3-NaCl molten salt system was 154 ℃. Since the melting point of PVDF is about 172 ℃, the molten salt system of AlCl3-NaCl can competently separate Al foil and cathode materials under the temperature of 160 ℃ after the phase transformation. Compared with nitrate, the chloride system showed the obvious advantages of low reaction temperature and low energy consumption. Under actual production conditions, lower reaction temperatures would mean lower energy costs and safer operating areas.

(a)

Medium-free (160℃)

(c)

(b)

NaNO3-KNO3 (160℃)

NaOH-KOH (160℃)

(d)

AlCl3-NaCl (160℃)

Figure 2. Effects of different molten salt systems on the peeling of cathode materials: (a) medium-free, (b) NaOH-KOH, (c) NaNO3-KNO3, and (d) AlCl3-NaCl. (Conditions: temperature of 160 ℃, holding time of 20 min, and salt: cathode electrode mass ratio of 10:1). Effects of Different Reaction Parameters When the heating temperature was 140 ℃, the cathode material was difficult to peel off from the surface of the Al foil (Figure 3a). When the heating temperature was raised to 150 ℃, the peeling off percentage of the cathode material from the surface of the Al 9



foil was about 63.1 wt%. When the heating temperature was further increased to 160 ℃, the peeling off percentage of the cathode material was close to the highest possible value: 99.8 wt%. This result fully indicates that in the AlCl3-NaCl molten salt system, temperature significantly influenced the peeling off percentage of the cathode material, by causing the PVDF to melt. Increasing the mass ratio of molten salt can also promote the peeling of cathode materials, because the larger proportions of molten salt can accumulate more heat in a shorter time (Figure 3b). Moreover, prolonging the time can also promote PVDF melting and fully strip off the cathode materials (Figure 3c). Therefore, the optimal conditions for peeling off cathode material from Al foil were found to be: temperature of 160 ℃, holding time of 20.0 min, and AlCl3-NaCl molten salt: cathode electrode mass ratio of 10:1.

Peeling off percentage (wt%)

80

100

60 40 20 0 130 140 150 160 Temperature (oC)

(b)

100 (c)


100 (a)


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90

80

70

5

10 15 Mass ratio

20

95

90

5

10 15 20 Time (min)

Figure 3. Effects of different parameters on peeling off percentages of cathode material: (a) temperature (conditions: AlCl3-NaCl molten salt: cathode electrode mass ratio of 10:1 and holding time of 20 min), (b) AlCl3-NaCl molten salt to cathode electrode mass ratio (conditions: temperature of 160 ℃ and holding time of 20 min), and (c) time 10


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(conditions: temperature of 160 ℃ and AlCl3-NaCl molten salt: cathode electrode mass ratio of 10:1). Thermodynamic Behavior Analysis The thermodynamic behaviors of AlCl3-NaCl molten salt and the cathode electrode were both analyzed by TG-DSC (Figure 4). The TG curve of AlCl3-NaCl molten salt in Figure 4a shows that only slight weight loss was observed at 160 ℃, a result that can be attributed to the evaporation of bound water in AlCl3 · 6H2O, as the AlCl3-NaCl molten salts are extremely stable at 160 ℃. The DSC curves in Figure 4a show that the AlCl3-NaCl molten salt began to absorb heat at 125 ℃, indicating that AlCl3-NaCl molten salts have heat storage behavior and will exhibit a phase transition under thermodynamic conditions. The TG curve of the cathode electrode in Figure 4b presents a mass loss that can be attributed to the evaporation and decomposition of electrolytes on the surface of the cathode electrode. Below the temperature of 100 ℃, the chemical properties of the LiCoO2 crystals, PVDF, and the conductive reagent acetylene black are extremely stable. Therefore, the weight loss of the cathode electrode in Figure 4b may be caused by the decomposition of the electrolyte lithium hexafluorophosphate on the surface of the cathode electrode.43 Lithium hexafluorophosphate is extremely reactive and decomposes rapidly when heated at room temperature, resulting in weight loss of the cathode electrode. The DSC results in Figure 4b also show that the cathode electrode needs to absorb heat under thermodynamic conditions, indicating that there is a phase transition at lower temperatures, which can be attributed to the melting point of PVDF. 11



148 oC

80

-5

-10

60

40

0

AlCl3-NaCl 50

Endothermic peak 1 100 150 200 o Temperature ( C)

(b) 100

Heat flow (W/g 103)

160 oC

-15 250

0.0

-0.5 95 -1.0

90

Cathode electrode 50

Endothermic peak 2 100 150 200 Temperature (oC)

Heat flow (W/g 103)

Weight (wt%)

(a) 100

Weight (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1.5 250

Figure 4. TG-DSC curves of (a) AlCl3-NaCl molten salt; (b) cathode electrode. Surface Morphology and Element Distribution Figure 5a shows the cathode electrode of spent LIBs before molten salt treatment. SEM results (Figure 5a-I) show that the surface of the cathode electrode was composed of LiCoO2 particles of varying sizes. Further enlargement (Figure 5a-II) shows that the surface of the cathode electrode was covered with a flocculent layer, which can be regarded as the organic binder PVDF film. The EDAX results (Figure 5a-III) show that the fluorine content on the surface of the cathode electrode was 3.8 wt%, confirming the existence of PVDF. No Al content was detected on the surface of the cathode electrode, indicating that photoelectrons could not penetrate the particles of the cathode material covering the Al foil surface. Figure 5b shows the cathode electrode of the spent 12


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LIBs after molten salt treatment. SEM results (Figure 5b-I) show that the LiCoO2 particles became clear, possibly because the PVDF melted. Further enlargement (Figure 5b-II) shows that there was still some PVDF residue on the surface of the cathode electrode. EDAX results (Figure 5b-III) also confirmed that fluorine, and therefore PVDF, remained on the surface of the cathode electrode after the molten salt treatment, but its content was greatly reduced. The significant increase in Al content can be attributed to the reactions and to the residues of chloride molten salts. Figure 5c is an Al foil product obtained after molten salt treatment. SEM results (Figure 5c-I) show that there were some particles on the surface of the Al foil. EDAX (Figure 5c-III) results confirm that the surface elements of the Al foil were C, O, F, and Co. The above elements were composed of acetylene black, LiCoO2, and PVDF. Thus, although the surface of the obtained Al foil was extremely smooth at the macro level, some residues of cathode material and melted PVDF were still visible in the micro view. It is well known that the separation of Al foil from cathode material is the result of the deactivation of PVDF, an organic binder in the cathode electrode. The above results of SEM-EDAX indicate that fluorine remained on the surface of the Al foil and the particles of cathode material, after the molten salt treatment. Since fluorine is a characteristic element of PVDF, this result also shows that PVDF was still distributed on the surface of the cathode materials and Al foil, after the molten salt treatment. Therefore, the deactivation during the treatment was caused, not by the decomposition of PVDF, but more likely by the change in physical properties (melting).

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(a-I)

(a-II)

(a-III)

10 μm (b-I)

1 μm (b-II)

(b-III)

10 μm (c-I)

1 μm (c-II)

(c-III)

10 μm

1 μm

Figure 5. SEM-EDAX results of different samples: (a) cathode electrode before molten salt treatment, (b) cathode electrode after molten salt treatment, and (c) Al foil after molten salt treatment. (I) SEM image (10 μm), (II) SEM image (1 μm), and (III) EDAX result. Crystal Structure Analysis Further XRD results (Figure 6) show that the crystalline structure of the separated cathode material LiCoO2 was intact, and that the original structure of the LiCoO2 remained unchanged, a result consistent with the JCPDF card (No: 00-016-0427), indicating that the AlCl3-NaCl molten salt did not destroy the crystal structure of LiCoO2, and had no negative effect on subsequent metal recovery.

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Figure 6. XRD pattern of the cathode material peeled off from the cathode electrodes of spent LIBs. Exploration of the Melting Mechanism of Polyvinylidene Fluoride A possible melting mechanism of PVDF in AlCl3-NaCl molten salt is shown in Figure 7. Molten salt has a non-aqueous high-temperature flux. Its main feature is dissociation into ions during melting. Since the positive and negative ions are combined by the Coulomb force (Figure 7), they can be used as a reaction medium at high temperatures.44 Generally, heat is stored in the reaction medium in the form of sensible heat, latent heat, or both. Sensible heat is stored by increasing the temperature of the heat-storage medium. The large amount of fusion heat required to melt a material from a solid to a liquid results in latent heat—absorbed stored heat.45-47 When AlCl3-NaCl is used as the molten salt system, the phase transformation point of the AlCl3-NaCl molten salt system is about 153 ℃. Therefore, when the heating temperature exceeds 153 ℃, the AlCl3-NaCl mixture is converted from a solid compound to a liquid compound by

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absorbing a large amount of heat. When the temperature continues to rise to 160 ℃, the PVDF melts, resulting in efficient separation of the cathode material and Al foil. XPS characterization was conducted to further confirm the melting mechanism of PVDF in the chloride molten salt. The XPS high-resolution spectra of the surface elements of the cathode electrode before and after the chloride molten salt treatment are shown in Figure 8. The XPS spectra of F1s in Figure 8a show that the fluorine species on the surface of the cathode electrode mainly corresponded to LiF (684.87 eV) and PVDF (687.20 eV). After the molten salt treatment (Figure 8b), the surface concentration of the fluorine species of the cathode electrode was remarkably lowered, but LiF (684.63 eV) and PVDF (687.65 eV) were still detected, indicating that the fluorine species on the surface of the cathode electrode did not change before or after treatment in the molten salt, even though its concentration decreased obviously. Therefore, the fluorine in the PVDF was more likely to undergo physical property changes after being treated in the molten salt medium. The XPS spectra of C 1s in Figures 8c and 8d show that some fluoride species were combined with C in the form of PVDF before and after the molten salt treatment; the difference was the variance in concentration, consistent with the F 1s results in Figures 8a and 8b. In summary, PVDF has a melting point of only 172 ℃. After the heat storage effect, the molten salt, having an external temperature of 160 ℃, was sufficient to melt the PVDF. The thermal decomposition temperature of PVDF can be as high as 350 ℃ or more. The results of Zhang

48

et al., for example, confirming that in the absence of a

medium, in order for the PVDF in the cathode electrode to be directly decomposed by 16


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heating, a temperature of 500 ℃ was required. Therefore, the deactivation of PVDF in the chloride molten salt can be considered a result of the melting of PVDF.

AlCl3 Al3+

Na+ NaCl

Na+ Al3+

Cl-

Cl-

Phase transition

NaCl Cathode electrode

AlCl3 Al foil


Cathode material

Heating 160 oC Molten salt AlCl3-NaCl

Al3+ ClNa+

Na+ Cl-


Al3+

Recycle

Figure 7. Possible melting mechanism of PVDF in AlCl3-NaCl molten salt.

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Figure 8. XPS high-solution spectra of different spent LIB samples: (a) F 1s spectra of cathode electrode before molten salt treatment, (b) F 1s spectra of cathode electrode after molten salt treatment, (c) C 1s spectra of cathode electrode before molten salt treatment, and (d) C 1s spectra of cathode electrode after molten salt treatment. Environmental Implication and Impact To highlight the environmental implications and impact of low-temperature molten salt technology for separating cathode materials and Al foils from spent LIBs, four evaluations were performed: process comparison, potential environmental impacts, cycling performance of molten salt, and operating cost. First, the advantages of the use of molten salt were analyzed using ionic liquid and N-methyl pyrrolidone as references. The results in Table S2 show that AlCl3-NaCl molten salt required a relatively low 18


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heating temperature compared with ionic liquids, leading to lower energy consumption. Another advantage of molten salt may be short reaction times compared to Nmethylpyrrolidone. Potential environmental impact analysis (Table S2) shows that the molten salt always maintained a solid phase before and after reaction, and therefore no waste liquid was discharged during the separation process. In addition, because molten salt is in a solid-phase state, the cathode electrode can be separated without cleaning or washing after treatment, and no waste water was generated in the entire process, greatly facilitating the separation of cathode material and Al foil as well as the direct recovery of metal in the subsequent step. 38 The cycling performance of the AlCl3-NaCl molten salt in Figure S2 indicates that the peeling off percentages of the cathode materials did not decrease noticeably even after the AlCl3-NaCl molten salt was recycled 4 times, indicating that AlCl3-NaCl molten salt can be recycled continuously, a phenomenon that has significant practical significance for reducing the operating cost and making industrial application practical. SEM images of AlCl3-NaCl molten salt before and after use are shown in Figures S3a and S3b, respectively. Also, there is no generation of waste residue in the process of stripping the cathode material with molten salt. Another major advantage of chloride molten salts was the overall stripping of the cathode material. In the actual experiment, the entire intact cathode electrode can be placed into the molten salt medium, and after a series of steps, such as heating and cooling, the cathode material can be peeled off from the Al foil still intact, under the action of external force. This phenomenon can be attributed to the hardening effect after the 19



melting of PVDF, avoiding any loss of cathode material powder during collection or separation treatment. Operating cost analysis shows that the reagent cost of AlCl3-NaCl molten salt for treating 1 kg of spent LIBs was only US$0.67 (Table S3). Considering the reagent’s cost and safety, the overall cost of using AlCl3-NaCl molten salt as reaction reagent is much lower than the cost of using ionic liquids. Although there is a risk of volatilization in the use of AlCl3·6H2O, the gas absorption devices can well control the potential hazards and risks in the reaction process of molten salt. The above comprehensive analysis shows that the presented low-temperature molten salt technology for separating cathode materials and Al foil in spent LIBs has excellent environmental benefits, economic benefits, and recyclability, making it a potential and promising technology for industrial application in spent LIBs recovery.

■ CONCLUSIONS In this study, the separation of cathode materials and Al foil in spent LIBs was successfully achieved using a low-temperature AlCl3-NaCl molten salt. The experimental results show that an AlCl3-NaCl molten salt system has a better peeling off effect than the commonly used hydroxide or nitrate systems. The separation mechanism of cathode materials from Al foil appears to be that the heat storage of the phase transformation of AlCl3-NaCl molten salt causes the melting of the organic binder PVDF. The optimal conditions for peeling off cathode material from Al foil were: temperature of 160 ℃, holding time of 20 min, and AlCl3-NaCl molten salt: cathode electrode mass ratio of 10:1. In addition, low-temperature molten salts also present 20


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excellent economic benefits, environmental benefits, and recycling performance, so that they could be applied in actual industrial situations. The low-temperature molten salt technology proposed in this study, for separating cathode materials and Al foil, has extremely important application potential for solving the problem of resource recycling of spent LIBs.

■ SUPPORTING INFORMATION Supporting Information is available free of charge via Internet at http://pubs.acs.org. Figure S1. Experimental device diagram used in this study; Figure S2. Cycling performance of AlCl3-NaCl molten salt (conditions: temperature of 160 ℃, holding time of 20 min, and AlCl3-NaCl molten salt: cathode electrode mass ratio of 10:1); Figure S3. SEM images of AlCl3-NaCl molten salt before (Figure S3a) and after (Figure S3b) use; Table S1. Melting points and compositions of the molten salt systems used in this study; Table S2. Environmental benefits assessment of different peeling off processes; Table S3. Economic analysis of the proposed low temperature molten salt process. (PDF)

■ AUTHOR INFORMATION Corresponding author Jinhui Li* Tel.: +86-10-62794143.

Fax: +86-10-62772048.

E-mail address: [email protected] Notes The authors declare no competing financial interest. 21



■ ACKNOWLEDGEMENTS This research is supported by major project of “The Beijing Social Science Fund” (17ZDA27).

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■ For Table of Contents Use Only Spent LIBs AlCl3/NaCl Al Foil

Cathode Electrode

160 ℃

Cathode Materials

Synopsis: Efficient separation of aluminum foil and cathode materials from spent lithium-ion batteries was achieved using a low-temperature molten salt, presenting the advantages of low cost and sustainable use.

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Efficient Separation of Aluminum Foil and Cathode Materials from

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