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Cleaner recycling of cathode material by in-situ thermite reduction
Journal of Cleaner Production xxx (xxxx) xxx
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Cleaner recycling of cathode material by in-situ thermite reduction Wenqiang Wang a, Yingchao Zhang a, Lei Zhang a, Shengming Xu a, b, c, * a
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education, Tsinghua University, Beijing, 100084, China c Beijing Key Lab of Fine Ceramics, Tsinghua University, Beijing, 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2019 Received in revised form 11 November 2019 Accepted 15 November 2019 Available online xxx
The extensive application of lithium-ion batteries (LIBs) generates a large amount of hazardous spent LIBs, for which sustainable recycling is seriously required by the demand of both environmental protection and resources conservation. An environmental friendly, economic effective and industrial applicable process has been developed for spent LiNixCoyMnzO2 (x þ y þ z ¼ 1) cathode recycling. Here, the current collector of aluminium preforms as the in-situ reductant of thermite reduction transforming valuable metals in LiNixCoyMnzO2 cathode into LiAlO2, Li2O, NiO, CoO, and MnO. Then, Al in the in the thermite reduction product can be removed by alkaline leaching without large amount of explosive H2 emission. Afterwards, valuable metals of Li, Ni, Co, and Mn were effectively leached into H2SO4 solution with leaching efficiencies of 99.78%, 98.62%, 99.29% and 99.91%, respectively. Comparing with the traditional carbothermic reduction methods, this method enjoys advantages of no need for peeling off the cathode materials, none adding of carbonaceous materials, avoiding large amount of H2 emission and lower reduction temperature. The in-situ thermite reduction process demonstrates a greener, simpler, safer, more energy conserving and industrial applicable approach for sustainable recycling of spent LIBs. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Zuotai Zhang Keywords: Sustainable recycling Spent Li-Ion battery Current collector Al foil Metal recovery
1. Introduction Lithium-ion batteries (LIBs) are widely applied in portable electric devices, and are increasingly in demand of large scale applications, such as electric vehicles(EVs) and renewable energy storage. The global demand of LIBs for vehicles is expected to be $221 billion from 2015 to 2024(Hu et al., 2017). Considering the cycle life of LIBs in EVs (usually less than 1000 cycles), a large number of spent LIBs is going to be produced. However, in spent LIBs, hazardous contents of electrolyte and metal contents are contained, bringing serious threats to both the environment and human health. On the other hand, spent LIBs are rich in valuable metal content such as Li, Ni, Co, etc., for which, spent LIBs are also treated as a significant secondary resource. Therefore, the recycling of spent LIBs is significant and inevitable to both environmental protection and primary resources saving. Recycling of spent LIBs cathode has attracted numerous attentions due to the high content of valuable metals, for which hydrometallurgical or combination of pyro and hydro technologies
* Corresponding author. Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China. E-mail address: [email protected] (S. Xu).
are intensively studied(Lv et al., 2018). The cathode is usually consisted of active materials, binder, electron-conducting carbon and Al current collector. Generally, two procedures are needed to recover valuable metals from spent cathode: separating of active material from current collector and valuable contents extraction. For the separating procedure, the difficulty locates in peeling off the active material from current collector, because they are made with the requirement of long-lasting adhesion by use of binder. Three kinds of methods are used for their separation: (1) Mechanical separation: active materials can be roughly separated from current collector by crushing and screening (Wang et al., 2016). (2) Dissolution of current collector: Al foil can be selectively dissolved in NaOH solution leaving active materials and the binder unchanged in residue(Chen, J. et al., 2016; Hu et al., 2017). However, the dissolution procedure is dangerous due to the violent reaction between Al foil and NaOH solution and the release of large amount of explosive H2. (3) Removal of binder: binder in the cathode can be removed by solvent extraction or thermal decomposition. The binder of polyvinylidene fluoride (PVDF) can be dissolved in organic solvents like N-methyl-2-pyrrolidone (NMP)(Peng et al., 2019). The high cost and the toxicity of NMP limit the industrial application of the solvent dissolving method. Thermal degradation method of binder is more favorable in industry. PVDF can be
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Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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completely decomposed when temperature reaches to 600 C(Yang et al., 2016). For the extraction of valuable contents, metal elementsare firstly dissolved into solution by leaching, followed by separation, purification and product preparation procedures in a hydrometallurgical process. Among these steps, the leaching procedure plays the most significant role. In leaching solutions, the stable forms of transition metals are in valence of þ2, while their valence in active materials are higher (>þ2). Generally, two categories of methods have been proposed to reduce the valence. One is reducing the valence by adding reductants into leaching solutions like H2O2(Yao et al., 2018), Na2SO3(Meng et al., 2019), active metals(Guan et al., 2017), and organic reductants(Chen, X. et al., 2016), among which, H2O2 is environmental favorable because of no impurity introducing. However, the consumption of large amount of H2O2 originated from decomposition(Gu et al., 2017) has made the cost too high. Another choice is to reduce the valence to a lower state before leaching. Carbonaceous materials can be employed for reducing the valence of transition metals under heating at temperature of 650e1000 C(Hu et al., 2017; Li et al., 2016; Wang et al., 2017). After thermal reduction, valuable metals can be dissolved into leaching solutions without adding of reductants. However, disadvantages exist that significant mass loss of Li2CO3 is observed in carbothermic reduction procedure when temperature is higher than 750 C. About 30% of lithium was lost at 750 C caused by volatilization in the carbothermic reduction s et al., 2006). Besides, the high temprocedure of LiMnO4(Konda perature procedure causes a high energy consumption and results in the emission of hazardous gases like CO2 and CO. Therefore, to make the procedure cleaner, the temperature of thermal reduction should be as lower as possible. In our previous study(Wang et al., 2019b), a simplified process for spent LiCoO2 cathode recovery based on thermite reduction was proposed. However, considering the scarcity and high cost of cobalt resources, lithium nickel manganese cobalt oxides (LiNixCoyMnzO2 (x þ y þ z ¼ 1)) recently have attracted lots of attentions and been widely applied in electric vehicles for their high theoretical energy density and reasonable cost, which means huge amount of spent LiNixCoyMnzO2 cathodes is going to be generated. Therefore, the scalability of thermite reduction to the recycling of spent LiNixCoyMnzO2 cathodes enjoys a wider industrial application prospect. Besides, the complicated contents of LiNixCoyMnzO2 cathodes indicate the reduction behavior and the extraction procedures will be different from that of LiCoO2. Therefore, a novel process for sustainable recovery of valuable metals from spent LiNixCoyMnzO2 cathode is developed. In the new process, the current collector preforms as the reductant converting transition metals into lower valence states. Afterwards, valuable metals of Li, Ni, Co and Mn can be fully leached into sulfuric acid solution without external reductant adding. Comparing with traditional carbothermic reduction processes, in the present one, the in-situ aluminum current collector preforms as the reductant of thermal reduction avoiding the complex peeling off active materials. Additionally, because the chemical character of Al is more active than carbonaceous materials, thermal reduction can be accomplished under lower temperature, resulting in a lower energy consumption. Besides, the hazardous emission of CO2 and CO released by carbothermic reduction is also avoided due to the feature of none-carbonaceous-adding. Therefore, the in-situ reduction process using Al foil as the reductant will be simpler and cleaner than traditional carbothermic reduction processes for recycling of spent LIBs.
2. Experimental 2.1. Materials and reagents The spent LiNixCoyMnzO2 cathodes are supplied by Shenzhen Ruicycle Environmental Technology Corp. Ltd., who dismantled the spent LIBs. Sodium hydroxide is used as the leaching reagent to dissolve Al, sodium phosphate is treated as the precipitation reagent of lithium, and sulfuric acid is used as the leaching solution for extraction of valuable metals from the alkaline leaching residue. Chemical reagents like H2SO4, NaOH, Na3PO4, etc. used in this work are all in analytical grade. All solutions were prepared using deionized water.
2.2. Experimental procedure The flowsheet designed for the recycling of spent cathode materials is shown in Fig. 1, which mainly includes dismantling, crushing, in-situ thermite reduction, alkaline leaching, and acid leaching. The cathode obtained from dismantling procedure was used as the raw material of this work. The cathode was firstly crushed into powders as the feed of thermite reduction. In thermite reduction procedure, the chemical valences of transition metal elements in cathode were reduced, and new phases were generated. Afterwards, alkaline leaching was carried out to remove the aluminum content, following by extraction of valuable metals through acid leaching. The details of each procedure are described as following:
Fig. 1. Flowsheet of in-situ thermite reduction for recycling of spent LIBs cathode.
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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(1) Crushing of cathode: The spent LIBs cathode was firstly sheared into small scraps in size of 2 2 cm without peeling off active material from current collector. Then, the obtained scraps were milled into fine powders using a vibromill with crushing time of 4 min. The obtained powders (marked as “prepared cathode powders”) were treated as the raw material of the later procedure without any further treatment. (2) In-situ thermite reduction: the prepared cathode powders are used as the feeds of thermite reduction procedure conducting in a tube furnace (YQL-1400, China). The prepared cathode powders were placed in the furnace and heated to a certain temperature with heating rate of 8 C∙min1 under argon atmosphere. After a period of holding time, the cathode materials were transformed into thermite reduction products. (3) Alkaline leaching: In each run, certain amount of NaOH solution coupled with accurately weighted thermite reduction products were placed into the reactor of a flask. A magnetic mixer was used to agitate the solution. After agitation at 350 r/min for a certain time, the leaching solution was drawn out by a syringe from the flask and followed by filtering through a filters with pore diameter of 0.22 mm for analysis of the ion concentration. Finally, the solution after leaching was filtrated for a complete separation from the residue using a vacuum pump. Afterwards, the obtained residue was washed for 3 times. The obtained alkaline leaching residue was dried in an oven at 60 C for 24 h. (4) Acid leaching: In each run, 150 mL H2SO4 solution coupled with accurately weighted alkaline leaching residue were added into the flask. The agitation, sampling and filtration operations were as same as that in the alkaline leaching procedure. The theoretical value of sulfuric acid was the amount for dissolution of all valuable metals into solution, which can be calculated as the chemical reaction equations listed as Eq. (1)~(4).
3
using Pt powder before observing the morphology using a scanning electronic microscope (SEM, SU-8000, Hitachi), while the distribution of the elements was figured out by an Energy dispersive spectrometer (EDS). The chemical states were characterized using an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher). The metal element concentration in the leaching solutions were analyzed using an ICP-OES. Leaching efficiency hi was calculated as Equation (5).
hi ¼
ci V 100% m0 ui
(5)
where ci (g∙L-1) represents the element concentration of “i” sampled by the springe, while V represents the related volume of the solution after leaching, m0 is the weight of the cathode powders used, and ui is the content (in weight) of element “i”. 3. Results and discussion 3.1. Measurement of the cathode powders The size distribution of the prepared cathode powders was presented in Fig. S1, from which it is obvious that the particle size distribution interval can be divided into two different ranges: 0.040e0.832 mm and 1.096e181.97 mm. Particles in the small size range account for 65.1% in volume ratio, which should be attributed to active materials. While particles in the bigger size range should be Al foil scraps adhered with some of active materials. The cathode powder contains a range of elements, including Al, Li, Ni, Co, Mn and C, which is shown in Table 1. The content of valuable metals accounts for 6.47%, 16.94%, 16.53%, and 13.14% for Li, Ni, Co and Mn, while the content of Al account for 13.28%. The reaction of thermite reduction can be described as Equations (6) and (7), which means, theoretically, the mole ratio of Al to LiNixCoyMnzO2 should be higher than 1/3. As calculated from the metal content, the mole ratio of Al to LiNixCoyMn1-x-yO2 is higher than 0.5 (>1/3), which means the in-situ Al content originated from the waste Al foil is enough for the thermite reduction procedure.
NiO þ H2SO4/NiSO4þH2O
(1)
CoO þ H2SO4/CoSO4þH2O
(2)
MnO þ H2SO4/MnSO4þH2O
(3)
6LiNixCoyMn1-x-yO2þ2Al/3Li2Oþ6xNiOþ6yCoOþ6(1-x-y) MnO þ Al2O3
(6)
Li3PO4þH2SO4/Li2SO4þH3PO4
(4)
Li2O þ Al2O3/2LiAlO2
(7)
Each experiment was repeated three times in the whole alkaline leaching and acid leaching procedures to avoid random errors, and mean values of the analytical results would be treated as the final experimental results. 2.3. Characterization The size distribution of the prepared powders was analyzed using a particle size analyzer (mastor2000, Malvern Panalytical). The element contents of the prepared cathode powders were completely dissolved by the mixing solution of HCl and HNO3 (volume ratio 3:1) and the contents were analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000, Perkin Elmer), while the content of carbon in cathode material was characterized using an element analyzer (PE2400-II, Perkin Elmer). The thermogravimetric and differential scanning calorimetry of the thermite reduction procedure were carried out using a thermogravimetric analyzer (TGA, SDT-Q600, TA) with heating rate of 8 C∙min in argon (99.999%) flow. An X-ray powder diffraction (XRD, D8 ADVANCE, Bruker) was applied for measuring the phase information of samples. The samples were firstly ejected
3.2. In-situ thermite reduction of cathode powders Simultaneous thermogravimetric and differential scanning calorimetry (TG/DSC) method. The TG/DSC analysis was used to figure out the temperature range of the thermite reduction procedure as shown in Fig. 2. As a comparison, thermogravimetric/ differential thermal analysis (TG/DTA) curves of commercial PVDF are shown in Fig. S2. A weight loss of 2.13% from 30 to 530 C can be found in Fig. 2, which may be ascribe to the volatilization of bound water and the decomposition of PVDF. In the DSC curve, the endothermic peak at 76.6 C conforms the volatilization of bound water. The significant
Table 1 Main content of the prepared powders. Element
Li
Ni
Co
Mn
Al
C
Contents/wt%
6.47
16.94
16.53
13.14
13.28
3.14
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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Fig. 2. TG/DSC curves of spent cathode materials in thermite reduction procedure.
TG change of commercial PVDF around 460 C in Fig. S2 conforms the weight loss is associated with the decomposition of PVDF in the cathode powders. Additionally, the TG curve of cathode materials maintains in the same level in the range of 530 to about 700 C in Fig. 2 indicating no mass loss in this temperature range. However, in the DSC curve, an exothermic peak appears at 557.7 C, which should be ascribed to the reaction of thermite reduction. Besides, the endothermic peak appearing at 656.4 C is resulted from the melting of the unreacted Al foil scraps (melting point 660.2 C(George E. Totten, 2003)). These results indicate the thermite reduction of cathode powders may be implemented in the range of 500 and 700 C. Fig. 3 shows the SEM images before and after thermite reduction. As shown in Fig. 3(a), the particles related to the cathode material before reduction presents in agglomerates morphology because of the existence of PVDF binder. After thermite reduction conducted under 520 C, PVDF binder degrades resulting in the
Fig. 3. SEM image of cathode material before (a) and after (b) reduction; (c) EDS mapping corresponding to the enlarged view of thermite reduction product.
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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collapse of the agglomerates. Therefore, the morphology of the particles becomes more loose after thermite reduction as shown in Fig. 3(b). The EDS mappings corresponding to the enlarged view of the thermite reduction products are shown in Fig. 3(c). As can be seen, Ni, Co, Mn and O are distributed in similar patterns, which indicates the transition metals may be in the form of metal oxides in the reduction products. The active materials can not be reduced by graphite under 520 C, therefore, carbon originated from the electron-conducting carbon in the cathode is remained after thermite reduction as shown in Fig. 3 (c). To figure out the accurate temperature of the thermite reduction, the phase transformation of reduction products under different temperatures was tested, and the XRD patterns of reduction products are shown in Fig. 4. As can be seen, the XRD pattern of reduction product generated under 480 C in Fig. 4 (b) matches to LiNixCoyMnzO2 and Al, which was almost unchanged from the raw material of the cathode powders shown in Fig. 4(a). The diffraction peaks corresponding to LiNixCoyMnzO2 disappeared when the reduction was performed under 520 C, at the same time, some new diffraction peaks matching to NiO and CoO emerged as shown in Fig. 4(c). A weak diffraction peak at 18.7 matching to LiAlO2 can be figured out in Fig. 4(c), whose intensity becomes stronger and more complete when reduction temperature was higher than 550 C as shown in Fig 4(d)~(f), conforming the generation of LiAlO2. Besides, diffraction peaks of metallic Ni and Co also appear in Fig 4(d)~(f), indicating the generated NiO and CoO was further reduced. The diffraction peaks of MnO appear in the XRD pattern of reduction product generated under 750 C. In the meantime, the diffraction peaks of Al eliminated. In order to achieve a high leaching efficiency of valuable metals in the later procedures, the goal of the reduction procedure is to destroy the phase structure of LiNixCoyMnzO2 by reducing the chemical valences of transition metals. Therefore, reduction conducting under 520 C can meet the requirement. The influence of the thermite reduction time on the phase transformation of reduction products was tested, and the results are shown in Fig. 5. As can be seen, the XRD pattern with reduction
Fig. 4. XRD patterns (a) cathode materials; thermite reduction products produced at different temperature for 60 min: (b) 480 C, (c) 520 C, (d) 550 C, (e) 600 C, (f) 750 C.
5
Fig. 5. XRD patterns of thermite reduction products produced at 520 C with different reduction time: (a) 10 min, (b) 30 min, (c) 60 min, (d) 90 min.
time of 10 min shown in Fig. 5(a) matches to diffraction peaks of LiNixCoyMnzO2 and Al, while the diffraction peaks of NiO and CoO were not detected. The intensity of peaks assigning to LiNixCoyMnzO2 weakens by increasing reduction time. As shown in Fig. 5(b), the peaks of LiNixCoyMnzO2 weakens to a very low level, indicating the structure of LiNixCoyMnzO2 was destroyed with reduction time of 30 min. In the same time, new peaks of NiO and CoO appear. Further increasing the reduction time to 60 min and 90 min shown in Fig. 5(c) and (d), the diffraction peaks of LiNixCoyMnzO2 eliminated and peaks matching to NiO and CoO were strengthened. Therefore, 60 min is suitable to reduce transition metals in LiNixCoyMnzO2. The influence of Al content on the phase transformation of reduction products was also tested as shown in Fig. S3, from which it can be known that the in-situ content of Al is enough for reduction of transition metals in LiNixCoyMnzO2. The XPS spectra of thermite reduction product are presented in Fig. 6. The spectra of Ni 2p shown in Fig. 6(a) indicates there are two resolved doublets of Ni 2p3/2 and Ni 2p1/2(Gaskell et al., 2007; Peck and Langell, 2012). The Ni 2p3/2 part can be fitted with a main feature at 855.5 eV corresponding to NiO, and a less intense peak at about 861.9 eV is corresponded to the satellite of Ni 2p3/2. This same phenomenon can be observed in the spectra of Ni 2p1/2 with peaks at 872.8 eV assigning to NiO and 880.0 eV assigning to the satellite(Wang et al., 2019a). The spectra of Co 2p shown in Fig. 6(b) is very similar to that of Ni 2p, which can also be fitted with two resolved doublets of Co 2p3/2 and Co 2p1/2. The Co 2p3/2 part can be fitted with a main peak at 781.0 eV and a satellite at 787.5 eV, while the signals at 796.5 eV and 802.5 eV are assigning to Co 2p1/2 and the corresponding satellite, indicating the existence of CoO(Xu et al., 2018). The spectrum of Mn 2p as shown in Fig. 6(c) reveals the peaks of Mn 2p3/2 and Mn 2p1/2 at 642.3 eV and 653.5eV, assigning to manganese oxides(Gu et al., 2015; Hsieh et al., 2011). The satellite at 646.6 eV is assigned to MnO. Therefore, from the XPS spectra of Ni 2p, Co 2p and Mn 2p shown in Fig. 6(a)~(c), it is obvious that NiO, CoO, and MnO are the existence forms of transition metals in the reduction product. Additionally, the XPS spectrum of C 1s was shown in Fig. 6(d), three peaks observed at 284.8 eV, 286.0 eV and 290.3 eV, respectively, corresponding to CeC, CeOeC, and CeF bonds(Yang et al., 2011). The peak assigning to graphite-like carbon can be found at 284.8 eV, which corresponds to conductive carbon in the cathode materials(Ismail et al., 2001). The CeOeC bond may originate from the oxidation of carbon
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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Fig. 6. XPS spectra of thermite reduction product produced under 520 C with reduction time of 60 min.
during the reduction procedure(Sun et al., 2013), while the CeF bond is comes from the incomplete degradation of binder. 3.3. Alkaline leaching of Al After in-situ thermite reduction of cathode powders, valuable metals are transformed into NiO, CoO, MnO, Li2O, LiAlO2 and Al2O3. In order to recover pure products of valuable metals, Al, as the impurity element, should be removed by alkaline leaching. The influence of alkaline leaching parameters is shown in Fig. 7. As shown in Fig. 7(a), the effect of alkaline concentration on leaching efficiency of lithium and aluminum are given. It is obvious that the leaching efficiencies of both lithium and aluminum increased with the increasing of NaOH concentration. The Al leaching efficiency reaches to 100% when NaOH concentration increases to 2.5 mol∙L1. The effect of temperature on the leaching efficiencies of lithium and aluminum are shown in Fig. 7(b). It can be seen that temperature plays a significant role in the leaching of Al. The leaching efficiency of Al is only 41.99% when leaching is conducted at 25 C. Increasing the leaching temperature can remarkably enhance the leaching efficiency of Al. When leaching is conducted at 85 C, the leaching efficiency of aluminum increased to 100%. In the meantime, the leaching efficiency of Li also increases along with the temperature. The similar leaching behavior of lithium and aluminum is ascribed to the reduction product of LiAlO2. The incomplete leaching efficiency of lithium should be ascribed to the existence of LiF (insoluble in alkaline solution) or by the adsorption by manganese oxide. However, incomplete leaching
of Li is inconvenient for the recovery of Li. The distributed lithium in both alkaline solution and leaching residue will make the recovery of Li more complex. To avoid the dispersion of Li, reagents are introduced to precipitate Li in the residue. The precipitation efficiency of Li in different leaching systems are shown in Fig. S4. It is obvious that 0.2 mol∙L1 Na3PO4 solution is efficient for Li precipitation. The effect of leaching time on the leaching efficiencies of lithium and aluminum in the mixture solution of NaOH and Na3PO4 are shown in Fig. 7(c). The leaching efficiency of Al reaches to 100% in 90 min. While the leaching efficiency of Li decreases with time. This may be ascribed to the nucleation and growth of Li3PO4 crystals. After precipitation, the Li leaching efficiency reduced to 4.37% after 120 min. The effect of Na3PO4 concentration are shown in Fig. 7(d). The Al leaching efficiency is not influenced by the concentration of Na3PO4. While the lithium leaching efficiency reduced with the increasing of Na3PO4 concentration, and 0.2 mol∙L1 seems to be pregnant enough for precipitation of Li. Therefore, under the optimum leaching condition of temperature at 85 C, 120 min, using the leaching solution of 2.5 mol∙L1 NaOH and 0.2 mol∙L1 Na3PO4, the leaching efficiencies of Al and Li are 100% and 4.37%, respectively. The XRD patterns of samples before and after alkaline leaching are shown in Fig. 8. It is obvious that the diffraction peaks of Al disappeared in the pattern after NaOH leaching, indicating Al is successfully removed. Besides, new peaks of Li3PO4 can be observed in the XRD pattern shown in Fig. 8(c) after leaching with mixed solution of NaOH and Na3PO4, demonstrating Li is effectively precipitated by Na3PO4.
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Fig. 7. Leaching efficiency of Li and Al: (a) effect of NaOH concentration (leaching condition: 85 C, liquid to solid ratio ¼ 20 mL∙g1, 120 min); (b) effect of leaching temperature (leaching condition: liquid to solid ratio ¼ 20 mL∙g1, 2.5 mol∙L1 NaOH, 120 min); (c) effect of leaching time (leaching condition: 85 C, liquid to solid ratio ¼ 20, mix solution of 2.5 mol∙L-1 NaOHþ0.2 mol∙L1 Na3PO4); (d) effect of Na3PO4 concentration (leaching condition: 85 C, liquid to solid ratio ¼ 20 mL∙g1, 90 min, mix solution of 2.5 mol∙L1 NaOHþ0.2 mol∙L1 Na3PO4).
Fig. 8. XRD patterns: (a) reduction products before alkaline leaching; (b) residue of alkaline leaching (2.5 mol∙L1 NaOH, 85 C, 120 min); (c) residue of alkaline leaching (2.5 mol∙L1 NaOH and 0.2 mol∙L1 Na3PO4, 85 C, 120 min).
3.4. Acid leaching of Li, Ni, Co and Mn H2SO4 solution is used to extract valuable metals of Li, Ni, Co and Mn from the residue obtained in the alkaline leaching. Parameters like time, H2SO4 dosage, temperature and solid to liquid ratio are
tested as shown in Fig. 9. From Fig. 9(a), it is obvious that the leaching efficiency of Li exceeds 97.8% in 3 min, while that of Ni, Co and Mn are 87.6%, 89.5% and 91.5%, respectively, which reveals the leaching reaction proceeded very fast. The leaching efficiencies of Li, Ni, Co and Mn are all higher than 94% within 5 min, which increased to higher than 99.9% in 60 min. In the following experiments, 60 min was chosen as the leaching time. The effect of the H2SO4 dosage on the leaching efficiencies is shown in Fig. 9(b). When the H2SO4 dosage is lower than the theoretical amount, the leaching efficiencies are significantly influenced. When the amount is higher than 1.2 times of theoretical amount, the leaching efficiencies of Li, Ni, Co and Mn significantly increased to higher than 99.9%. As can be seen in Fig. 9(c), the leaching temperature significantly influences the leaching efficiencies. When the temperature is 20 C, the leaching efficiency of Ni, Co and Mn are 79.6%, 80.6% and 90.3%, respectively. The leaching efficiencies of valuable metals increase with the rising of leaching temperature. When the temperature reaches 60 C, the leaching efficiencies of Li, Ni, Co and Mn increased to 99.78%, 98.62%, 99.29% and 99.91%, respectively. Therefore, the leaching temperature is set at 60 C in the following experiments. The effect of solid to liquid ratio on leaching efficiencies of valuable metals and metal ion concentrations are presented in Fig. 9(d) and (e). It can be seen in Fig. 9(d) that the influence of solid to liquid ratio has a slight effect on the leaching efficiencies, indicating the thermite reduction products are easy to be handled, which is beneficial for industrial production. The SEM images and EDS analysis of acid leaching residue are shown in Fig. S5. It can be seen from Fig. S5(a), the microstructure of acid leaching residue is
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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Fig. 9. Leaching efficiencies of Li, Ni, Co and Mn in acid leaching procedure: (a) effect of leaching time (80 C, solid/liquid 50 g∙L1, 2 times of H2SO4 dosage); (b) effect of H2SO4 dosage (80 C, 60 min, solid/liquid 50 g∙L1); (c) effect of leaching temperature (60 min, solid/liquid 50 g∙L1, 1.2 times of H2SO4 dosage); (d) effect of solid to liquid ratio (60 C, 1.2 times of H2SO4 dosage, 60 min); (e) effect of solid to liquid ratio on metal concentration (60 C, 1.2 times of H2SO4 dosage, 60 min); (f) photographs of leaching solution generated with different solid to liquid ratio.
Table 2 Comparison of reduction processes for recycling of spent cathode materials. Raw material
Pretreatment of Al foil
reductant
temperature
products
ref
Pure LiCoO2 LiMn2O4 LiMn2O4 LiMn2O4 LiNixCoyMnzO2 LiNixCoyMnzO2
e mechanical separation manual e alkaline leaching no need
graphite graphite graphite graphite lignite Al foil
1000 C 800 C 650 C 700 C 650 C 520 C
Co MnO MnO MnO Ni, Co, MnO NiO, CoO, MnO
Li et al., (2016) Xiao et al. (2017b) s et al., (2006) Konda Xiao et al. (2017a) (Hu et al., 2017; Zhang et al., 2018) this work
in fibrous structure with length of about 0.5 mm. The EDS results shown in Fig. S5(b) and (c) indicate no signal of Ni, Co or Mn is detected in both mappings and the spectra, demonstrating the complete leaching of Ni, Co and Mn. The metal ion concentrations shown in Fig. 9(e) clearly demonstrated the advantages of the high leaching efficiencies under big solid to liquid ratio. Under the ratio of 200 g∙L1, the concentrations of Li, Ni, Co and Mn are 12.28 g∙L1, 41.22 g∙L1, 40.07 g∙L1 and 33.36 g∙L1, respectively. The photographs of leaching solutions generated under different solid to liquid ratio explicitly reveals the concentrated metal concentrations as shown in Fig. 9(f). The leaching solution with high concentration of metal ions is preferred by the industrial production, which can make the production more effective and make the energy consumption lower. The leaching solution with content of Li, Ni, Co and Mn can be treated using Co-extraction method as reported in our previous work(Yang et al., 2017). Besides, technologies like solvent extraction (Hu et al., 2017), adsorption(Gomaa et al., 2018)for separation and purification of metal ions have been well studied, which can be applied for the following separation procedures of the process. 4.2.1. Comparison with other methods As thermal decomposition for separation of Al foil from active
materials and thermal reduction of active materials are hotspots in the field of spent LIBs recycling. Here, a comparison is presented in Table 2, which lists the recent researches on the thermal process for spent LIBs recycling. It is obvious that the difficulty in separation of Al foil from active material is successfully avoided in present in-situ thermite reduction process, which will make the recycling process more compact. Besides, comparing with approaches of carbothermic reduction, the thermite reduction process enjoys advantages of none carbonaceous materials adding, without the release of CO2 and CO, and the lowest reduction temperature. These features indicate a lower energy consumption and more compact flowsheet can be achieved for recycling of spent LIBs by using in-situ thermite reduction approach. GEM High-tech Co., Ltd and Brunp Co., Ltd are the biggest two LIB disposers in China, whose recycling processes are both consisted of pretreatment (dismantling, shredding, and separation), and hydrometallurgical extraction procedures(Gu et al., 2017). In the leaching procedure, H2O2 is needed to be added into the solution to reduce the transition metals. By using of the present in-situ thermite reduction process, the separation of Al foil can be avoided and the consumption of H2O2 can be eliminated, making the process more simplified. In other words, the process developed here is industrial applicable.
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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4. Conclusions In summary, a sustainable approach for recovery of valuable metals from spent LIBs by in-situ thermite reduction followed by alkaline leaching and acid leaching has been developed. By using this approach, transition metals in LIBs cathodes can be reduced to new forms in low chemical valences, which can be leached in H2SO4 solution without reductant adding. The cathode was first crushed into powders without separation of Al foil, following by heating under 520 C for 60 min to conduct in-situ thermite reduction. The thermite reduction product was characterized by SEM-EDS, XRD and XPS, which reveals the active material of LiNixCoyMnzO2 was transformed into LiAlO2, NiO, CoO and MnO. Al content in thermite reduction product was in oxidation state which can be removed by alkaline leaching without emission of large amount of explosive H2. In the meantime, Li was precipitated in the residue along with transition metals. Valuable metals in alkaline leaching residue were completely leached into H2SO4 solution with high ion concentrations for Li, Ni, Co and Mn of 12.28 g∙L1, 41.22 g∙L1, 40.07 g∙L1 and 33.36 g∙L1, respectively, which is beneficial for metal recovery. Comparing with other reduction processes, the thermite reduction process shows advantages of lowest reduction temperature, none-carbonaceous-adding and no need for peeling off active materials from aluminum current collector. This work suggests a novel possibility for recycling of cathode material from spent LIBs with advantages of flowsheet compact, energy saving, environmental friendly and industrial applicable. Author contribution statement Wenqiang Wang: Conceptualization, Validation, Formal analysis, Writing - Original Draft. Yingchao Zhang & Lei Zhang: Review & Editing, Visualization. Shengming Xu: Supervision, Project administration, and Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support of the Key Program of the National Natural Science Foundation of China (51834008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.119340. References Chen, J., Li, Q., Song, J., Song, D., Zhang, L., Shi, X., 2016. Environmentally friendly recycling and effective repairing of cathode powders from spent LiFePO4 batteries. Green Chem. 18 (8), 2500e2506. Chen, X., Fan, B., Xu, L., Zhou, T., Kong, J., 2016. An atom-economic process for the recovery of high value-added metals from spent lithium-ion batteries. J. Clean. Prod. 112, 3562e3570. Gaskell, K.J., Starace, A., Langell, M.A., 2007. ZnxNi1-xO rocksalt oxide Surfaces: novel environment for Zn2þ and its effect on the NiO band structure. J. Phys. Chem. C 111 (37), 13912e13921.
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George, E., Totten, D.S.M., 2003. Handbook of Aluminum, Physical Metallurgy and Processes. CRC press, New York. Gomaa, H., Shenashen, M.A., Yamaguchi, H., Alamoudi, A.S., El-Safty, S.A., 2018. Extraction and recovery of Co2þ ions from spent lithium-ion batteries using hierarchical mesosponge g-Al2O3 monolith extractors. Green Chem. 20 (8), 1841e1857. Gu, F., Guo, J., Yao, X., Summers, P.A., Widijatmoko, S.D., Hall, P., 2017. An investigation of the current status of recycling spent lithium-ion batteries from consumer electronics in China. J. Clean. Prod. 161, 765e780. Gu, X., Yue, J., Chen, L., Liu, S., Xu, H., Yang, J., Qian, Y., Zhao, X., 2015. Coaxial MnO/Ndoped carbon nanorods for advanced lithium-ion battery anodes. J. Mater. Chem. 3 (3), 1037e1041. Guan, J., Li, Y., Guo, Y., Su, R., Gao, G., Song, H., Yuan, H., Liang, B., Guo, Z., 2017. Mechanochemical process enhanced cobalt and lithium recycling from wasted lithium-ion batteries. ACS Sustain. Chem. Eng. 5 (1), 1026e1032. 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Lv, W., Wang, Z., Cao, H., Sun, Y., Zhang, Y., Sun, Z., 2018. A critical Review and analysis on the recycling of spent lithium-ion batteries. ACS Sustain. Chem. Eng. 6 (2), 1504e1521. Meng, K., Cao, Y., Zhang, B., Ou, X., Li, D.-m., Zhang, J.-f., Ji, X., 2019. Comparison of the ammoniacal leaching behavior of layered LiNixCoyMn1exeyO2 (x ¼ 1/3, 0.5, 0.8) cathode materials. ACS Sustain. Chem. Eng. 7 (8), 7750e7759. Peck, M.A., Langell, M.A., 2012. Comparison of nanoscaled and bulk NiO structural and environmental characteristics by XRD, XAFS, and XPS. Chem. Mater. 24 (23), 4483e4490. € m, M., 2019. Selective extraction of Peng, C., Liu, F., Wang, Z., Wilson, B.P., Lundstro lithium (Li) and preparation of battery grade lithium carbonate (Li2CO3) from spent Li-ion batteries in nitrate system. J. Power Sources 415, 179e188. Sun, Y., Hu, X., Luo, W., Xia, F., Huang, Y., 2013. 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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Cleaner recycling of cathode material by in-situ thermite reduction Wenqiang Wang a, Yingchao Zhang a, Lei Zhang a, Shengming Xu a, b, c, * a
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education, Tsinghua University, Beijing, 100084, China c Beijing Key Lab of Fine Ceramics, Tsinghua University, Beijing, 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2019 Received in revised form 11 November 2019 Accepted 15 November 2019 Available online xxx
The extensive application of lithium-ion batteries (LIBs) generates a large amount of hazardous spent LIBs, for which sustainable recycling is seriously required by the demand of both environmental protection and resources conservation. An environmental friendly, economic effective and industrial applicable process has been developed for spent LiNixCoyMnzO2 (x þ y þ z ¼ 1) cathode recycling. Here, the current collector of aluminium preforms as the in-situ reductant of thermite reduction transforming valuable metals in LiNixCoyMnzO2 cathode into LiAlO2, Li2O, NiO, CoO, and MnO. Then, Al in the in the thermite reduction product can be removed by alkaline leaching without large amount of explosive H2 emission. Afterwards, valuable metals of Li, Ni, Co, and Mn were effectively leached into H2SO4 solution with leaching efficiencies of 99.78%, 98.62%, 99.29% and 99.91%, respectively. Comparing with the traditional carbothermic reduction methods, this method enjoys advantages of no need for peeling off the cathode materials, none adding of carbonaceous materials, avoiding large amount of H2 emission and lower reduction temperature. The in-situ thermite reduction process demonstrates a greener, simpler, safer, more energy conserving and industrial applicable approach for sustainable recycling of spent LIBs. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Zuotai Zhang Keywords: Sustainable recycling Spent Li-Ion battery Current collector Al foil Metal recovery
1. Introduction Lithium-ion batteries (LIBs) are widely applied in portable electric devices, and are increasingly in demand of large scale applications, such as electric vehicles(EVs) and renewable energy storage. The global demand of LIBs for vehicles is expected to be $221 billion from 2015 to 2024(Hu et al., 2017). Considering the cycle life of LIBs in EVs (usually less than 1000 cycles), a large number of spent LIBs is going to be produced. However, in spent LIBs, hazardous contents of electrolyte and metal contents are contained, bringing serious threats to both the environment and human health. On the other hand, spent LIBs are rich in valuable metal content such as Li, Ni, Co, etc., for which, spent LIBs are also treated as a significant secondary resource. Therefore, the recycling of spent LIBs is significant and inevitable to both environmental protection and primary resources saving. Recycling of spent LIBs cathode has attracted numerous attentions due to the high content of valuable metals, for which hydrometallurgical or combination of pyro and hydro technologies
* Corresponding author. Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China. E-mail address: [email protected] (S. Xu).
are intensively studied(Lv et al., 2018). The cathode is usually consisted of active materials, binder, electron-conducting carbon and Al current collector. Generally, two procedures are needed to recover valuable metals from spent cathode: separating of active material from current collector and valuable contents extraction. For the separating procedure, the difficulty locates in peeling off the active material from current collector, because they are made with the requirement of long-lasting adhesion by use of binder. Three kinds of methods are used for their separation: (1) Mechanical separation: active materials can be roughly separated from current collector by crushing and screening (Wang et al., 2016). (2) Dissolution of current collector: Al foil can be selectively dissolved in NaOH solution leaving active materials and the binder unchanged in residue(Chen, J. et al., 2016; Hu et al., 2017). However, the dissolution procedure is dangerous due to the violent reaction between Al foil and NaOH solution and the release of large amount of explosive H2. (3) Removal of binder: binder in the cathode can be removed by solvent extraction or thermal decomposition. The binder of polyvinylidene fluoride (PVDF) can be dissolved in organic solvents like N-methyl-2-pyrrolidone (NMP)(Peng et al., 2019). The high cost and the toxicity of NMP limit the industrial application of the solvent dissolving method. Thermal degradation method of binder is more favorable in industry. PVDF can be
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completely decomposed when temperature reaches to 600 C(Yang et al., 2016). For the extraction of valuable contents, metal elementsare firstly dissolved into solution by leaching, followed by separation, purification and product preparation procedures in a hydrometallurgical process. Among these steps, the leaching procedure plays the most significant role. In leaching solutions, the stable forms of transition metals are in valence of þ2, while their valence in active materials are higher (>þ2). Generally, two categories of methods have been proposed to reduce the valence. One is reducing the valence by adding reductants into leaching solutions like H2O2(Yao et al., 2018), Na2SO3(Meng et al., 2019), active metals(Guan et al., 2017), and organic reductants(Chen, X. et al., 2016), among which, H2O2 is environmental favorable because of no impurity introducing. However, the consumption of large amount of H2O2 originated from decomposition(Gu et al., 2017) has made the cost too high. Another choice is to reduce the valence to a lower state before leaching. Carbonaceous materials can be employed for reducing the valence of transition metals under heating at temperature of 650e1000 C(Hu et al., 2017; Li et al., 2016; Wang et al., 2017). After thermal reduction, valuable metals can be dissolved into leaching solutions without adding of reductants. However, disadvantages exist that significant mass loss of Li2CO3 is observed in carbothermic reduction procedure when temperature is higher than 750 C. About 30% of lithium was lost at 750 C caused by volatilization in the carbothermic reduction s et al., 2006). Besides, the high temprocedure of LiMnO4(Konda perature procedure causes a high energy consumption and results in the emission of hazardous gases like CO2 and CO. Therefore, to make the procedure cleaner, the temperature of thermal reduction should be as lower as possible. In our previous study(Wang et al., 2019b), a simplified process for spent LiCoO2 cathode recovery based on thermite reduction was proposed. However, considering the scarcity and high cost of cobalt resources, lithium nickel manganese cobalt oxides (LiNixCoyMnzO2 (x þ y þ z ¼ 1)) recently have attracted lots of attentions and been widely applied in electric vehicles for their high theoretical energy density and reasonable cost, which means huge amount of spent LiNixCoyMnzO2 cathodes is going to be generated. Therefore, the scalability of thermite reduction to the recycling of spent LiNixCoyMnzO2 cathodes enjoys a wider industrial application prospect. Besides, the complicated contents of LiNixCoyMnzO2 cathodes indicate the reduction behavior and the extraction procedures will be different from that of LiCoO2. Therefore, a novel process for sustainable recovery of valuable metals from spent LiNixCoyMnzO2 cathode is developed. In the new process, the current collector preforms as the reductant converting transition metals into lower valence states. Afterwards, valuable metals of Li, Ni, Co and Mn can be fully leached into sulfuric acid solution without external reductant adding. Comparing with traditional carbothermic reduction processes, in the present one, the in-situ aluminum current collector preforms as the reductant of thermal reduction avoiding the complex peeling off active materials. Additionally, because the chemical character of Al is more active than carbonaceous materials, thermal reduction can be accomplished under lower temperature, resulting in a lower energy consumption. Besides, the hazardous emission of CO2 and CO released by carbothermic reduction is also avoided due to the feature of none-carbonaceous-adding. Therefore, the in-situ reduction process using Al foil as the reductant will be simpler and cleaner than traditional carbothermic reduction processes for recycling of spent LIBs.
2. Experimental 2.1. Materials and reagents The spent LiNixCoyMnzO2 cathodes are supplied by Shenzhen Ruicycle Environmental Technology Corp. Ltd., who dismantled the spent LIBs. Sodium hydroxide is used as the leaching reagent to dissolve Al, sodium phosphate is treated as the precipitation reagent of lithium, and sulfuric acid is used as the leaching solution for extraction of valuable metals from the alkaline leaching residue. Chemical reagents like H2SO4, NaOH, Na3PO4, etc. used in this work are all in analytical grade. All solutions were prepared using deionized water.
2.2. Experimental procedure The flowsheet designed for the recycling of spent cathode materials is shown in Fig. 1, which mainly includes dismantling, crushing, in-situ thermite reduction, alkaline leaching, and acid leaching. The cathode obtained from dismantling procedure was used as the raw material of this work. The cathode was firstly crushed into powders as the feed of thermite reduction. In thermite reduction procedure, the chemical valences of transition metal elements in cathode were reduced, and new phases were generated. Afterwards, alkaline leaching was carried out to remove the aluminum content, following by extraction of valuable metals through acid leaching. The details of each procedure are described as following:
Fig. 1. Flowsheet of in-situ thermite reduction for recycling of spent LIBs cathode.
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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(1) Crushing of cathode: The spent LIBs cathode was firstly sheared into small scraps in size of 2 2 cm without peeling off active material from current collector. Then, the obtained scraps were milled into fine powders using a vibromill with crushing time of 4 min. The obtained powders (marked as “prepared cathode powders”) were treated as the raw material of the later procedure without any further treatment. (2) In-situ thermite reduction: the prepared cathode powders are used as the feeds of thermite reduction procedure conducting in a tube furnace (YQL-1400, China). The prepared cathode powders were placed in the furnace and heated to a certain temperature with heating rate of 8 C∙min1 under argon atmosphere. After a period of holding time, the cathode materials were transformed into thermite reduction products. (3) Alkaline leaching: In each run, certain amount of NaOH solution coupled with accurately weighted thermite reduction products were placed into the reactor of a flask. A magnetic mixer was used to agitate the solution. After agitation at 350 r/min for a certain time, the leaching solution was drawn out by a syringe from the flask and followed by filtering through a filters with pore diameter of 0.22 mm for analysis of the ion concentration. Finally, the solution after leaching was filtrated for a complete separation from the residue using a vacuum pump. Afterwards, the obtained residue was washed for 3 times. The obtained alkaline leaching residue was dried in an oven at 60 C for 24 h. (4) Acid leaching: In each run, 150 mL H2SO4 solution coupled with accurately weighted alkaline leaching residue were added into the flask. The agitation, sampling and filtration operations were as same as that in the alkaline leaching procedure. The theoretical value of sulfuric acid was the amount for dissolution of all valuable metals into solution, which can be calculated as the chemical reaction equations listed as Eq. (1)~(4).
3
using Pt powder before observing the morphology using a scanning electronic microscope (SEM, SU-8000, Hitachi), while the distribution of the elements was figured out by an Energy dispersive spectrometer (EDS). The chemical states were characterized using an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher). The metal element concentration in the leaching solutions were analyzed using an ICP-OES. Leaching efficiency hi was calculated as Equation (5).
hi ¼
ci V 100% m0 ui
(5)
where ci (g∙L-1) represents the element concentration of “i” sampled by the springe, while V represents the related volume of the solution after leaching, m0 is the weight of the cathode powders used, and ui is the content (in weight) of element “i”. 3. Results and discussion 3.1. Measurement of the cathode powders The size distribution of the prepared cathode powders was presented in Fig. S1, from which it is obvious that the particle size distribution interval can be divided into two different ranges: 0.040e0.832 mm and 1.096e181.97 mm. Particles in the small size range account for 65.1% in volume ratio, which should be attributed to active materials. While particles in the bigger size range should be Al foil scraps adhered with some of active materials. The cathode powder contains a range of elements, including Al, Li, Ni, Co, Mn and C, which is shown in Table 1. The content of valuable metals accounts for 6.47%, 16.94%, 16.53%, and 13.14% for Li, Ni, Co and Mn, while the content of Al account for 13.28%. The reaction of thermite reduction can be described as Equations (6) and (7), which means, theoretically, the mole ratio of Al to LiNixCoyMnzO2 should be higher than 1/3. As calculated from the metal content, the mole ratio of Al to LiNixCoyMn1-x-yO2 is higher than 0.5 (>1/3), which means the in-situ Al content originated from the waste Al foil is enough for the thermite reduction procedure.
NiO þ H2SO4/NiSO4þH2O
(1)
CoO þ H2SO4/CoSO4þH2O
(2)
MnO þ H2SO4/MnSO4þH2O
(3)
6LiNixCoyMn1-x-yO2þ2Al/3Li2Oþ6xNiOþ6yCoOþ6(1-x-y) MnO þ Al2O3
(6)
Li3PO4þH2SO4/Li2SO4þH3PO4
(4)
Li2O þ Al2O3/2LiAlO2
(7)
Each experiment was repeated three times in the whole alkaline leaching and acid leaching procedures to avoid random errors, and mean values of the analytical results would be treated as the final experimental results. 2.3. Characterization The size distribution of the prepared powders was analyzed using a particle size analyzer (mastor2000, Malvern Panalytical). The element contents of the prepared cathode powders were completely dissolved by the mixing solution of HCl and HNO3 (volume ratio 3:1) and the contents were analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000, Perkin Elmer), while the content of carbon in cathode material was characterized using an element analyzer (PE2400-II, Perkin Elmer). The thermogravimetric and differential scanning calorimetry of the thermite reduction procedure were carried out using a thermogravimetric analyzer (TGA, SDT-Q600, TA) with heating rate of 8 C∙min in argon (99.999%) flow. An X-ray powder diffraction (XRD, D8 ADVANCE, Bruker) was applied for measuring the phase information of samples. The samples were firstly ejected
3.2. In-situ thermite reduction of cathode powders Simultaneous thermogravimetric and differential scanning calorimetry (TG/DSC) method. The TG/DSC analysis was used to figure out the temperature range of the thermite reduction procedure as shown in Fig. 2. As a comparison, thermogravimetric/ differential thermal analysis (TG/DTA) curves of commercial PVDF are shown in Fig. S2. A weight loss of 2.13% from 30 to 530 C can be found in Fig. 2, which may be ascribe to the volatilization of bound water and the decomposition of PVDF. In the DSC curve, the endothermic peak at 76.6 C conforms the volatilization of bound water. The significant
Table 1 Main content of the prepared powders. Element
Li
Ni
Co
Mn
Al
C
Contents/wt%
6.47
16.94
16.53
13.14
13.28
3.14
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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Fig. 2. TG/DSC curves of spent cathode materials in thermite reduction procedure.
TG change of commercial PVDF around 460 C in Fig. S2 conforms the weight loss is associated with the decomposition of PVDF in the cathode powders. Additionally, the TG curve of cathode materials maintains in the same level in the range of 530 to about 700 C in Fig. 2 indicating no mass loss in this temperature range. However, in the DSC curve, an exothermic peak appears at 557.7 C, which should be ascribed to the reaction of thermite reduction. Besides, the endothermic peak appearing at 656.4 C is resulted from the melting of the unreacted Al foil scraps (melting point 660.2 C(George E. Totten, 2003)). These results indicate the thermite reduction of cathode powders may be implemented in the range of 500 and 700 C. Fig. 3 shows the SEM images before and after thermite reduction. As shown in Fig. 3(a), the particles related to the cathode material before reduction presents in agglomerates morphology because of the existence of PVDF binder. After thermite reduction conducted under 520 C, PVDF binder degrades resulting in the
Fig. 3. SEM image of cathode material before (a) and after (b) reduction; (c) EDS mapping corresponding to the enlarged view of thermite reduction product.
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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collapse of the agglomerates. Therefore, the morphology of the particles becomes more loose after thermite reduction as shown in Fig. 3(b). The EDS mappings corresponding to the enlarged view of the thermite reduction products are shown in Fig. 3(c). As can be seen, Ni, Co, Mn and O are distributed in similar patterns, which indicates the transition metals may be in the form of metal oxides in the reduction products. The active materials can not be reduced by graphite under 520 C, therefore, carbon originated from the electron-conducting carbon in the cathode is remained after thermite reduction as shown in Fig. 3 (c). To figure out the accurate temperature of the thermite reduction, the phase transformation of reduction products under different temperatures was tested, and the XRD patterns of reduction products are shown in Fig. 4. As can be seen, the XRD pattern of reduction product generated under 480 C in Fig. 4 (b) matches to LiNixCoyMnzO2 and Al, which was almost unchanged from the raw material of the cathode powders shown in Fig. 4(a). The diffraction peaks corresponding to LiNixCoyMnzO2 disappeared when the reduction was performed under 520 C, at the same time, some new diffraction peaks matching to NiO and CoO emerged as shown in Fig. 4(c). A weak diffraction peak at 18.7 matching to LiAlO2 can be figured out in Fig. 4(c), whose intensity becomes stronger and more complete when reduction temperature was higher than 550 C as shown in Fig 4(d)~(f), conforming the generation of LiAlO2. Besides, diffraction peaks of metallic Ni and Co also appear in Fig 4(d)~(f), indicating the generated NiO and CoO was further reduced. The diffraction peaks of MnO appear in the XRD pattern of reduction product generated under 750 C. In the meantime, the diffraction peaks of Al eliminated. In order to achieve a high leaching efficiency of valuable metals in the later procedures, the goal of the reduction procedure is to destroy the phase structure of LiNixCoyMnzO2 by reducing the chemical valences of transition metals. Therefore, reduction conducting under 520 C can meet the requirement. The influence of the thermite reduction time on the phase transformation of reduction products was tested, and the results are shown in Fig. 5. As can be seen, the XRD pattern with reduction
Fig. 4. XRD patterns (a) cathode materials; thermite reduction products produced at different temperature for 60 min: (b) 480 C, (c) 520 C, (d) 550 C, (e) 600 C, (f) 750 C.
5
Fig. 5. XRD patterns of thermite reduction products produced at 520 C with different reduction time: (a) 10 min, (b) 30 min, (c) 60 min, (d) 90 min.
time of 10 min shown in Fig. 5(a) matches to diffraction peaks of LiNixCoyMnzO2 and Al, while the diffraction peaks of NiO and CoO were not detected. The intensity of peaks assigning to LiNixCoyMnzO2 weakens by increasing reduction time. As shown in Fig. 5(b), the peaks of LiNixCoyMnzO2 weakens to a very low level, indicating the structure of LiNixCoyMnzO2 was destroyed with reduction time of 30 min. In the same time, new peaks of NiO and CoO appear. Further increasing the reduction time to 60 min and 90 min shown in Fig. 5(c) and (d), the diffraction peaks of LiNixCoyMnzO2 eliminated and peaks matching to NiO and CoO were strengthened. Therefore, 60 min is suitable to reduce transition metals in LiNixCoyMnzO2. The influence of Al content on the phase transformation of reduction products was also tested as shown in Fig. S3, from which it can be known that the in-situ content of Al is enough for reduction of transition metals in LiNixCoyMnzO2. The XPS spectra of thermite reduction product are presented in Fig. 6. The spectra of Ni 2p shown in Fig. 6(a) indicates there are two resolved doublets of Ni 2p3/2 and Ni 2p1/2(Gaskell et al., 2007; Peck and Langell, 2012). The Ni 2p3/2 part can be fitted with a main feature at 855.5 eV corresponding to NiO, and a less intense peak at about 861.9 eV is corresponded to the satellite of Ni 2p3/2. This same phenomenon can be observed in the spectra of Ni 2p1/2 with peaks at 872.8 eV assigning to NiO and 880.0 eV assigning to the satellite(Wang et al., 2019a). The spectra of Co 2p shown in Fig. 6(b) is very similar to that of Ni 2p, which can also be fitted with two resolved doublets of Co 2p3/2 and Co 2p1/2. The Co 2p3/2 part can be fitted with a main peak at 781.0 eV and a satellite at 787.5 eV, while the signals at 796.5 eV and 802.5 eV are assigning to Co 2p1/2 and the corresponding satellite, indicating the existence of CoO(Xu et al., 2018). The spectrum of Mn 2p as shown in Fig. 6(c) reveals the peaks of Mn 2p3/2 and Mn 2p1/2 at 642.3 eV and 653.5eV, assigning to manganese oxides(Gu et al., 2015; Hsieh et al., 2011). The satellite at 646.6 eV is assigned to MnO. Therefore, from the XPS spectra of Ni 2p, Co 2p and Mn 2p shown in Fig. 6(a)~(c), it is obvious that NiO, CoO, and MnO are the existence forms of transition metals in the reduction product. Additionally, the XPS spectrum of C 1s was shown in Fig. 6(d), three peaks observed at 284.8 eV, 286.0 eV and 290.3 eV, respectively, corresponding to CeC, CeOeC, and CeF bonds(Yang et al., 2011). The peak assigning to graphite-like carbon can be found at 284.8 eV, which corresponds to conductive carbon in the cathode materials(Ismail et al., 2001). The CeOeC bond may originate from the oxidation of carbon
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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Fig. 6. XPS spectra of thermite reduction product produced under 520 C with reduction time of 60 min.
during the reduction procedure(Sun et al., 2013), while the CeF bond is comes from the incomplete degradation of binder. 3.3. Alkaline leaching of Al After in-situ thermite reduction of cathode powders, valuable metals are transformed into NiO, CoO, MnO, Li2O, LiAlO2 and Al2O3. In order to recover pure products of valuable metals, Al, as the impurity element, should be removed by alkaline leaching. The influence of alkaline leaching parameters is shown in Fig. 7. As shown in Fig. 7(a), the effect of alkaline concentration on leaching efficiency of lithium and aluminum are given. It is obvious that the leaching efficiencies of both lithium and aluminum increased with the increasing of NaOH concentration. The Al leaching efficiency reaches to 100% when NaOH concentration increases to 2.5 mol∙L1. The effect of temperature on the leaching efficiencies of lithium and aluminum are shown in Fig. 7(b). It can be seen that temperature plays a significant role in the leaching of Al. The leaching efficiency of Al is only 41.99% when leaching is conducted at 25 C. Increasing the leaching temperature can remarkably enhance the leaching efficiency of Al. When leaching is conducted at 85 C, the leaching efficiency of aluminum increased to 100%. In the meantime, the leaching efficiency of Li also increases along with the temperature. The similar leaching behavior of lithium and aluminum is ascribed to the reduction product of LiAlO2. The incomplete leaching efficiency of lithium should be ascribed to the existence of LiF (insoluble in alkaline solution) or by the adsorption by manganese oxide. However, incomplete leaching
of Li is inconvenient for the recovery of Li. The distributed lithium in both alkaline solution and leaching residue will make the recovery of Li more complex. To avoid the dispersion of Li, reagents are introduced to precipitate Li in the residue. The precipitation efficiency of Li in different leaching systems are shown in Fig. S4. It is obvious that 0.2 mol∙L1 Na3PO4 solution is efficient for Li precipitation. The effect of leaching time on the leaching efficiencies of lithium and aluminum in the mixture solution of NaOH and Na3PO4 are shown in Fig. 7(c). The leaching efficiency of Al reaches to 100% in 90 min. While the leaching efficiency of Li decreases with time. This may be ascribed to the nucleation and growth of Li3PO4 crystals. After precipitation, the Li leaching efficiency reduced to 4.37% after 120 min. The effect of Na3PO4 concentration are shown in Fig. 7(d). The Al leaching efficiency is not influenced by the concentration of Na3PO4. While the lithium leaching efficiency reduced with the increasing of Na3PO4 concentration, and 0.2 mol∙L1 seems to be pregnant enough for precipitation of Li. Therefore, under the optimum leaching condition of temperature at 85 C, 120 min, using the leaching solution of 2.5 mol∙L1 NaOH and 0.2 mol∙L1 Na3PO4, the leaching efficiencies of Al and Li are 100% and 4.37%, respectively. The XRD patterns of samples before and after alkaline leaching are shown in Fig. 8. It is obvious that the diffraction peaks of Al disappeared in the pattern after NaOH leaching, indicating Al is successfully removed. Besides, new peaks of Li3PO4 can be observed in the XRD pattern shown in Fig. 8(c) after leaching with mixed solution of NaOH and Na3PO4, demonstrating Li is effectively precipitated by Na3PO4.
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Fig. 7. Leaching efficiency of Li and Al: (a) effect of NaOH concentration (leaching condition: 85 C, liquid to solid ratio ¼ 20 mL∙g1, 120 min); (b) effect of leaching temperature (leaching condition: liquid to solid ratio ¼ 20 mL∙g1, 2.5 mol∙L1 NaOH, 120 min); (c) effect of leaching time (leaching condition: 85 C, liquid to solid ratio ¼ 20, mix solution of 2.5 mol∙L-1 NaOHþ0.2 mol∙L1 Na3PO4); (d) effect of Na3PO4 concentration (leaching condition: 85 C, liquid to solid ratio ¼ 20 mL∙g1, 90 min, mix solution of 2.5 mol∙L1 NaOHþ0.2 mol∙L1 Na3PO4).
Fig. 8. XRD patterns: (a) reduction products before alkaline leaching; (b) residue of alkaline leaching (2.5 mol∙L1 NaOH, 85 C, 120 min); (c) residue of alkaline leaching (2.5 mol∙L1 NaOH and 0.2 mol∙L1 Na3PO4, 85 C, 120 min).
3.4. Acid leaching of Li, Ni, Co and Mn H2SO4 solution is used to extract valuable metals of Li, Ni, Co and Mn from the residue obtained in the alkaline leaching. Parameters like time, H2SO4 dosage, temperature and solid to liquid ratio are
tested as shown in Fig. 9. From Fig. 9(a), it is obvious that the leaching efficiency of Li exceeds 97.8% in 3 min, while that of Ni, Co and Mn are 87.6%, 89.5% and 91.5%, respectively, which reveals the leaching reaction proceeded very fast. The leaching efficiencies of Li, Ni, Co and Mn are all higher than 94% within 5 min, which increased to higher than 99.9% in 60 min. In the following experiments, 60 min was chosen as the leaching time. The effect of the H2SO4 dosage on the leaching efficiencies is shown in Fig. 9(b). When the H2SO4 dosage is lower than the theoretical amount, the leaching efficiencies are significantly influenced. When the amount is higher than 1.2 times of theoretical amount, the leaching efficiencies of Li, Ni, Co and Mn significantly increased to higher than 99.9%. As can be seen in Fig. 9(c), the leaching temperature significantly influences the leaching efficiencies. When the temperature is 20 C, the leaching efficiency of Ni, Co and Mn are 79.6%, 80.6% and 90.3%, respectively. The leaching efficiencies of valuable metals increase with the rising of leaching temperature. When the temperature reaches 60 C, the leaching efficiencies of Li, Ni, Co and Mn increased to 99.78%, 98.62%, 99.29% and 99.91%, respectively. Therefore, the leaching temperature is set at 60 C in the following experiments. The effect of solid to liquid ratio on leaching efficiencies of valuable metals and metal ion concentrations are presented in Fig. 9(d) and (e). It can be seen in Fig. 9(d) that the influence of solid to liquid ratio has a slight effect on the leaching efficiencies, indicating the thermite reduction products are easy to be handled, which is beneficial for industrial production. The SEM images and EDS analysis of acid leaching residue are shown in Fig. S5. It can be seen from Fig. S5(a), the microstructure of acid leaching residue is
Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340
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Fig. 9. Leaching efficiencies of Li, Ni, Co and Mn in acid leaching procedure: (a) effect of leaching time (80 C, solid/liquid 50 g∙L1, 2 times of H2SO4 dosage); (b) effect of H2SO4 dosage (80 C, 60 min, solid/liquid 50 g∙L1); (c) effect of leaching temperature (60 min, solid/liquid 50 g∙L1, 1.2 times of H2SO4 dosage); (d) effect of solid to liquid ratio (60 C, 1.2 times of H2SO4 dosage, 60 min); (e) effect of solid to liquid ratio on metal concentration (60 C, 1.2 times of H2SO4 dosage, 60 min); (f) photographs of leaching solution generated with different solid to liquid ratio.
Table 2 Comparison of reduction processes for recycling of spent cathode materials. Raw material
Pretreatment of Al foil
reductant
temperature
products
ref
Pure LiCoO2 LiMn2O4 LiMn2O4 LiMn2O4 LiNixCoyMnzO2 LiNixCoyMnzO2
e mechanical separation manual e alkaline leaching no need
graphite graphite graphite graphite lignite Al foil
1000 C 800 C 650 C 700 C 650 C 520 C
Co MnO MnO MnO Ni, Co, MnO NiO, CoO, MnO
Li et al., (2016) Xiao et al. (2017b) s et al., (2006) Konda Xiao et al. (2017a) (Hu et al., 2017; Zhang et al., 2018) this work
in fibrous structure with length of about 0.5 mm. The EDS results shown in Fig. S5(b) and (c) indicate no signal of Ni, Co or Mn is detected in both mappings and the spectra, demonstrating the complete leaching of Ni, Co and Mn. The metal ion concentrations shown in Fig. 9(e) clearly demonstrated the advantages of the high leaching efficiencies under big solid to liquid ratio. Under the ratio of 200 g∙L1, the concentrations of Li, Ni, Co and Mn are 12.28 g∙L1, 41.22 g∙L1, 40.07 g∙L1 and 33.36 g∙L1, respectively. The photographs of leaching solutions generated under different solid to liquid ratio explicitly reveals the concentrated metal concentrations as shown in Fig. 9(f). The leaching solution with high concentration of metal ions is preferred by the industrial production, which can make the production more effective and make the energy consumption lower. The leaching solution with content of Li, Ni, Co and Mn can be treated using Co-extraction method as reported in our previous work(Yang et al., 2017). Besides, technologies like solvent extraction (Hu et al., 2017), adsorption(Gomaa et al., 2018)for separation and purification of metal ions have been well studied, which can be applied for the following separation procedures of the process. 4.2.1. Comparison with other methods As thermal decomposition for separation of Al foil from active
materials and thermal reduction of active materials are hotspots in the field of spent LIBs recycling. Here, a comparison is presented in Table 2, which lists the recent researches on the thermal process for spent LIBs recycling. It is obvious that the difficulty in separation of Al foil from active material is successfully avoided in present in-situ thermite reduction process, which will make the recycling process more compact. Besides, comparing with approaches of carbothermic reduction, the thermite reduction process enjoys advantages of none carbonaceous materials adding, without the release of CO2 and CO, and the lowest reduction temperature. These features indicate a lower energy consumption and more compact flowsheet can be achieved for recycling of spent LIBs by using in-situ thermite reduction approach. GEM High-tech Co., Ltd and Brunp Co., Ltd are the biggest two LIB disposers in China, whose recycling processes are both consisted of pretreatment (dismantling, shredding, and separation), and hydrometallurgical extraction procedures(Gu et al., 2017). In the leaching procedure, H2O2 is needed to be added into the solution to reduce the transition metals. By using of the present in-situ thermite reduction process, the separation of Al foil can be avoided and the consumption of H2O2 can be eliminated, making the process more simplified. In other words, the process developed here is industrial applicable.
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4. Conclusions In summary, a sustainable approach for recovery of valuable metals from spent LIBs by in-situ thermite reduction followed by alkaline leaching and acid leaching has been developed. By using this approach, transition metals in LIBs cathodes can be reduced to new forms in low chemical valences, which can be leached in H2SO4 solution without reductant adding. The cathode was first crushed into powders without separation of Al foil, following by heating under 520 C for 60 min to conduct in-situ thermite reduction. The thermite reduction product was characterized by SEM-EDS, XRD and XPS, which reveals the active material of LiNixCoyMnzO2 was transformed into LiAlO2, NiO, CoO and MnO. Al content in thermite reduction product was in oxidation state which can be removed by alkaline leaching without emission of large amount of explosive H2. In the meantime, Li was precipitated in the residue along with transition metals. Valuable metals in alkaline leaching residue were completely leached into H2SO4 solution with high ion concentrations for Li, Ni, Co and Mn of 12.28 g∙L1, 41.22 g∙L1, 40.07 g∙L1 and 33.36 g∙L1, respectively, which is beneficial for metal recovery. Comparing with other reduction processes, the thermite reduction process shows advantages of lowest reduction temperature, none-carbonaceous-adding and no need for peeling off active materials from aluminum current collector. This work suggests a novel possibility for recycling of cathode material from spent LIBs with advantages of flowsheet compact, energy saving, environmental friendly and industrial applicable. Author contribution statement Wenqiang Wang: Conceptualization, Validation, Formal analysis, Writing - Original Draft. Yingchao Zhang & Lei Zhang: Review & Editing, Visualization. Shengming Xu: Supervision, Project administration, and Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support of the Key Program of the National Natural Science Foundation of China (51834008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.119340. References Chen, J., Li, Q., Song, J., Song, D., Zhang, L., Shi, X., 2016. Environmentally friendly recycling and effective repairing of cathode powders from spent LiFePO4 batteries. Green Chem. 18 (8), 2500e2506. Chen, X., Fan, B., Xu, L., Zhou, T., Kong, J., 2016. An atom-economic process for the recovery of high value-added metals from spent lithium-ion batteries. J. Clean. Prod. 112, 3562e3570. Gaskell, K.J., Starace, A., Langell, M.A., 2007. ZnxNi1-xO rocksalt oxide Surfaces: novel environment for Zn2þ and its effect on the NiO band structure. J. Phys. Chem. C 111 (37), 13912e13921.
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Please cite this article as: Wang, W et al., Cleaner recycling of cathode material by in-situ thermite reduction, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119340