Preparation and Electrochemical Performance of Nano-Co 3 O 4 Anode Materials from Spent Li-Ion Batteries for Lithium-Ion Batteries

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1 Available online at SciVerse ScienceDirect J. Mater. Sci. Technol., 2013, 29(3), 215e220 Preparation and Electrochemical Performance of Nano-Co 3 O 4 Anode Materials from Spent Li-Ion Batteries for Lithium-Ion Batteries Chuanyue Hu *, Jun Guo, Jin Wen, Yangxi Peng Department of Chemistry and Materials Science, Hunan Institute of Humanities, Science and Technology, Loudi , China [Manuscript received December 19, 2011, in revised form September 1, 2012, Available online 28 January 2013] A hydrometallurgical process for the recovery of cobalt oxalate from spent lithium-ion batteries was used to recycle cobalt compound by using alkali leaching, reductive acid leaching and chemical deposition of cobalt oxalate. The recycled cobalt oxalate was used to synthesize nano-co 3 O 4 anode material by sol-gel method. The samples were characterized by thermal gravity analysis and differential thermal analysis (TGA/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and charge/discharge measurements. The influence of molar ratio of Co 2þ to citric acid and calcination temperature on the structure and electrochemical performance of nano-co 3 O 4 was evaluated. As the molar ratio of Co 2þ to citric acid is 1:1, the face-centered cubic (fcc) Co 3 O 4 powder shows the discharge capacity of ma h g 1, the high coulombic efficiency of 99.7% in the first cycle at the current density of 125 ma g 1, and the excellent cycling performance with the reversible capacity of ma h g 1 after 20 cycles at the current density of 250 ma g 1. KEY WORDS: Spent lithium-ion batteries; Sol-gel method; Reductive acid leaching; Nanostructure cobalt oxide; Electrochemical behavior 1. Introduction Lithium-ion batteries (LIB) are dominantly used as electrochemical power sources in the mobile telephones, personal computers, video cameras and other modern-life appliances due to their favorable characteristics of lightweight and high energy density. However, the spent LIB not only contains metals such as nickel, cobalt, copper, and lithium, but also produces large amounts of metal-containing hazardous waste. Therefore, the research of recycling technologies for spent LIB is attracting great attention, both for environmental protection and resource conservation. Currently, LiCoO 2 is the most widely used active cathode material for lithium-ion secondary batteries. However, LiNi x M- n y Co 1 x y O 2 and LiMn 2 O 4 cathode materials are gradually used to substitute for LiCoO 2 in order to improve the safety and electrochemical performance of lithium-ion batteries. This means that the active cathode materials in spent LIB contain cobalt, nickel, and manganese with variable concentrations, thus making * Corresponding author. Assoc. Prof., Ph.D.; Tel.: þ ; Fax: þ ; address: huchuanyue@vip.sina.com.cn (C. Hu) /$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. chemical processes for recovery of the spent LIB more complex. In order to efficiently recover the valuable metals (Co, Ni, Mn and Li), various physical and chemical processes have been reported, involving crushing, dismantling, sieving, alkali leaching, acid leaching, solvent extraction, chemical precipitation and electrochemistry [1e4]. Cobalt and lithium are easily leached from spent lithium-ion batteries by using HNO [5] 3, HCl [6] and H 2 SO [7,8] 4 as leaching agents. But generally, reducing agents like hydrogen peroxide (H 2 O 2 ) is required to promote the dissolution of transition metal oxide cathode material. For the purification of Co(II) from the leach liquor, the separation methods such as chemical precipitation and electrolysis were proposed. However, the important drawback of the above methods is the low purity of the cobalt compounds. Recently, nano-sized transition metal oxides such as NiO, CuO, Co 3 O 4,Fe 2 O 3 and Fe 3 O 4 have been widely studied for alternative anode materials to replace graphite [9e15]. The coreshell nanostructure was used to improve the electrochemical performance of metal oxides [16]. This means that most the impurities from the spent LIB can be used to prepare the metal oxide anode material for lithium-ion batteries, which indicates that the recycled cobalt compounds can be directly used to synthesize cobalt oxide anode material. Among the candidates of transition metal oxides, cobalt oxide (Co 3 O 4 ) has shown the highest capacity and best cycle performance. Many routes have been applied to synthesize Co 3 O 4 powders with various morphologies, such as the flower-like sphere fabricated by means of a simple calcination process from a-phase cobalt

2 216 C. Hu et al.: J. Mater. Sci. Technol., 2013, 29(3), 215e220 hydroxide precursors [17], hollow Co 3 O 4 spheres prepared with a electrostatic spray deposition (ESD) technique with a precursor suspension of cobalt alkoxide with a hollow structure [18], macroporous Co 3 O 4 platelets with a microwave-assisted synthesis method [19], etc. In this study, nano-sized Co 3 O 4 powders have been synthesized by sol-gel method with recycled cobalt oxalate from spent LIB. The recycled cobalt oxalate was not further purified. The effects of synthesizing conditions on the powder characteristics were investigated and the chargeedischarge behavior as an anode material was discussed. 2. Experimental 2.1. Materials and reagents Spent LIBs used in different mobile phones were collected for this study, and they were dismantled through a manual procedure to remove both plastic and steel shells. Both the anode and cathode films were crushed in the range of 1e5 mm in size and then bathed in N-methyl pyrrolidinone (NMP) solvent at 100 C for 6 h with high speed stirring. The mixed powder of active anode and cathode materials was effectively separated from copper and aluminum support substrates (which were recovered here in the metallic form), and dried at 120 C for 24 h. The dried mixture powder was finally ground for 30 min to obtain a fine powder that was screened in sequential sieves with openings ranging from 10 to 500 mm Precipitation of cobalt oxalate First, the mixture of active powder materials was leached with 2 mol L 1 NaOH solution to selectively dissolve aluminum. Then the filter residue was leached with 6 wt% H 2 SO 4 and 1.5 wt% H 2 O 2 at 60 C for 3 h. The ph of leach liquor was adjusted to 5 with 2 mol L 1 NaOH solution in order to make the impurity ions form Al(OH) 3, MnOOH, Cu(OH) 2 and Ni(OH) 2 precipitation. Lastly, (NH 4 ) 2 C 2 O 4 saturation solution was added into the leach liquor containing Co 2þ and the ph of mixed solution was adjusted to a ph value of 2 to form CoC 2 O 4 precipitation. The CoC 2 O 4 products were washed with deionized water for several times and dried at 100 C for 24 h. The recycled CoC 2 O 4 precipitation was used to synthesize nano-co 3 O 4 without being further purified Sample synthesis 200) operated at an acceleration voltage of 5 kv. Powder X-ray diffraction (XRD, Philips X Pert MPD Diffractometer, CuKa radiation) was used for phase identification. Scan rate of 4 min 1 and step size of 0.04 were applied to record the pattern. The operation voltage and current were 40 kv and 45 ma, respectively. Thermal gravity analysis (TGA) /differential thermal analysis (DT-2) data for decomposition and oxidation of cobalt citric acid gel precursors were obtained at a heating rate of 5 C min 1 in air with a TGA/DTA50 thermal gravimetric analyzer (Shimadzu Corporation, Japan) Electrochemical characterization Electrochemical performances of the nano-co 3 O 4 particles were investigated with two-electrode coin-type cells (CR 2025). The working electrodes were prepared by a slurry coating procedure. The slurry consisted of 800 wt% Co 3 O 4 powder, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone (NMP), and was incorporated on copper foil with 12 mm in diameter. After being dried at 110 C for 24 h in vacuum, the foil was pressed under a pressure of Pa. The loading stress of Co 3 O 4 is kg m 2 Test cells were assembled in an argon-filled glove box with the metallic lithium foil as both the reference and counter electrodes, 1 mol L 1 LiPF 6 in ethylene carbonate (EC)- dimethylene carbonate (DMC)- ethyl methyl carbonate (EMC) (1:1:1 in volume) as the electrolyte, and a polypropylene (PP) micro-porous film (Celgard 2300) as the separator. The galvanostatic chargeedischarge tests were conducted on a blue-key battery program-control test system (BK6061 Testing System, Guangzhou Blue-key Electronic Industry Co., Ltd) at a current density of 125 ma g 1 and 250 ma g 1 in the voltage range of 0.02e3.0 V (versus Li/ Li þ ) at room temperature (25 1 C). The electrochemical impedance spectroscopy (EIS) measurements of the cells were performed on an electrochemical workstation (CHI 660C) in the frequency range from 0.01 to 100 khz. 3. Resultants and Discussion 3.1. Thermal analysis of cobalt citric acid gel precursor The TGA/DTA measurement of cobalt citric acid gel precursor was carried out to find out suitable calcination temperature for the precursor and the results are shown in Fig. 1. The double peaks located at 160 C and C on the DTA curve are The recycled CoC 2 O 4 powder was first dissolved in 4 mol L 1 HNO 3 acid solution. This solution was mixed until homogeneous and citric acid was added while stirring, until the molar ratio of Co 2þ to citric acid was up to the designed molar ratio (molar ratio, 1:0.4, 1:0.7, 1:1). Then the solution was heated to 70e80 C until a gel formed. Lastly, the gel was transferred to an alumina boat and heated at 2 C/min to 500e650 C under air atmosphere and was decomposed at that temperature for 1 h to form Co 3 O 4 products Sample characterization Morphology of the fabricated sample was characterized by field emission scanning electron microscopy (SEM, FEI Quanta- Fig. 1 TGA/DTA profiles of cobalt citric acid gel precursor at a heating rate of 5 C min 1 in air.

3 C. Hu et al.: J. Mater. Sci. Technol., 2013, 29(3), 215e endothermic and correspond to a sample weight loss of 19.5% from 142 to 260 C on the TGA curve. It is due to the dehydration of chemically bonded water in the cobalt citric acid gel precursor. A sharp exothermic peak observed at C on the DTA curve indicates the decomposition and oxidation reaction of cobalt citric acid gel. The weight remains 68.5% after the second weight loss process. Another two small endothermic peaks locate at C on the DTA curve, corresponding to a lightweight loss of 0.5%, indicating the carbothermal reduction reaction from the pyrolysis carbon of citric acid gel. Based on the above results, a calcination temperature range of 550e650 C was used in this work in order to decompose the cobalt citric acid gel and avoid reduction of Co 3 O Effects of calcination temperature of cobalt citric acid gel precursor Co 3 O 4 compounds were synthesized by calcining the cobalt citric acid gel precursors. For the prepared conditions of cobalt citric acid gel precursor, the molar ratio of Co 2þ to citric acid was 1:0.4. Fig. 2 shows the XRD patterns of Co 3 O 4 powder prepared with different calcinations temperature. The Co 3 O 4 powder prepared at 550e650 C for 1 h exhibits a single phase diffraction patterns and is indexed to a face-centered cubic (fcc) Co 3 O 4 phase of fd3m space group (JCPDS ). It can be seen that the peaks intensity, especially for the (111) and (222) peak intensity, of Co 3 O 4 powder prepared at 650 C is much stronger than that of the sample prepared at 550 and 650 C, indicating the Co 3 O 4 prepared at 650 C exhibits better crystal structure. The electrochemical performance of the Co 3 O 4 materials at different calcination temperature was evaluated by galvanostatic charge/discharge between 0.02 and 3 V vs. Li/Li þ at a constant current density of 125 ma g 1, and the results are shown in Fig. 3. The discharge capacity of Co 3 O 4 prepared at 650 Cis ma h g 1 and the coulombic efficiency is 59.1% in the first cycle at the current density of 125 ma g 1. The coulombic efficiency is the ratio of charge capacity and discharge capacity. On the one hand, the extra capacity is attributed to the formation of a surface-electrolyte interphase (SEI) film due to electrolyte decomposition [20]. The larger specific surface area of nano- Co 3 O 4 powder provides more contact area between Co 3 O 4 and electrolyte and offers more sites to accommodate Li þ, indicating the higher initial discharge capacity. The low reversible capacity of carbon black, on the other hand, should not be neglected, because in our experiments, the electrode consists of 80% active Fig. 3 Dischargeecharge curves of Co 3 O 4 particles at different calcination temperatures for 1 h at the current density of 125 ma g 1. material, 10% carbon black and 10% PVDF binder. Fortunately, the contribution of carbon black toward the capacity of Li-ion battery becomes smaller and smaller along with the increase in the number of cycles since the capacity of carbon black decreases very sharply during charge/discharge process [21]. The formation of surface-electrolyte interphase (SEI) film is the main reason for the low coulombic efficiency and large capacity loss during the initial cycle, which happens in all 3d transition metal oxides including CuO, NiO, Co 3 O 4 [9e16]. During the first discharge, each profile of the compounds presents a long plateau at around 1.05 V vs. Li/Li þ, and then the voltage decreases gradually down and another voltage plateau occurs near 0.5 V vs. Li/Li þ, which is a characteristic of conversion reaction mechanism as expressed by the following equation [22e25]. Co 3 O 4 þ 8Li þ þ 8e % discharge charge 3Co þ 4Li 2 O The charge process exhibits a higher and sloping voltage profile, with two charge voltage plateaus around 1.9 V vs. Li/Li þ and 2.5 V vs. Li/Li þ. That is, the redox reaction of Co 3 O 4 is a multi step electron capture and loss procedure during the charge process [26]. More details can be observed in the differential capacity plots (dx/dv), which are shown in Fig. 4. The reduction peak at 0.5 V vs. Li/Li þ in the discharge curve can be ascribed to the formation of SEI film. For the conversion reactions of transition metal oxides electrodes, the SEI film has been Fig. 2 XRD patterns of Co 3 O 4 compounds synthesized at different calcination temperatures for 1 h. Fig. 4 Differential capacity (dx/dv) plots of Co 3 O 4 particles prepared at 650 C for 1 h at the current density of 125 ma g 1 in the first dischargeecharge cycle.

4 218 C. Hu et al.: J. Mater. Sci. Technol., 2013, 29(3), 215e220 suggested to be formed upon discharge process at low potentials [27]. The higher reduction peak at 1.2 V vs. Li/Li þ during the first discharge process in dx/dv plots reveals a faster kinetics for the phase transformation of cobalt. It also can be considered that the grain size of the product is very fine and the specific surface area is large, so the reaction is relatively severe. To further understand the mechanism of the enhanced electrode reaction kinetics of the porous Co 3 O 4 electrode, EIS measurements were conducted from 0.01 Hz to 100 khz. Since the fcc phase Co 3 O 4 powder has been mixed with the conductive reagent for the preparation of the electrode, it is not reasonable to compare the electrochemical impedance of the Co 3 O 4 powder with the Co 3 O 4 films. Thus, only the Nyquist plots of the nanostructure Co 3 O 4 electrodes after charging to 1.3 V (vs. Li/ Li þ ) after the first cycle at the current density of 125 ma g 1 are shown in Fig. 5. The plots exhibit the similar curves consisting of a semicircle at high frequency, a semicircle at medium frequency and a following 45 line at low frequency. It is supposed that the high frequency semicircle corresponds to the SEI film resistance, the medium frequency semicircle corresponds to the charge-transfer resistance. As seen from Fig. 5, all samples exhibit the similar high frequency semicircle, indicating that similar SEI film are formed on the surface of Co 3 O 4 electrodes. The impedance of SEI film is about 115 U, which is the major reason of the low Coulombic efficiency during the first cycle. However, the medium frequency semicircle radius is different each other, which indicates that the lithium-ions and electrons can transfer more easily in the sample synthesized at 600 C and 650 C for the smaller semicircle, thus leading to the enhanced electrode reaction kinetics and improved electrochemical performance. Fig. 6 shows the cycling performance of Co 3 O 4 synthesized with different calcination temperatures. The results show that the Co 3 O 4 electrodes synthesized at 600 and 650 C exhibit the good cycling performance. The reversible capacity is ma h g 1 and ma h g 1, respectively, after 20 cycles at the current density of 250 ma g Effects of molar ratio of cobalt to citric acid Three samples of Co 3 O 4 were prepared with different molar ratios of cobalt to citric acid at 650 C for 1 h. The influence of different molar ratios on the structure and electrochemical performance was evaluated. Fig. 7 shows the XRD patterns of Co 3 O 4 powders prepared with different molar ratios of cobalt to Fig. 6 Cycling performance of Co 3 O 4 electrodes prepared at different calcination temperatures for 1 h at the current density of 250 ma g 1. citric acid. The results show that all Co 3 O 4 materials are indexed to an fcc Co 3 O 4 phase of fd3m space group (JCPDS ). Fig. 8(a) shows the first discharge-charge curves of Co 3 O 4 electrodes. When the molar ratio of n(co 2þ ):n(citric acid) is 1:0.4, 1:0.7 and 1:1, the obtained Co 3 O 4 electrode shows the discharge capacity of ma h g 1, ma h g 1 and ma h g 1, the Coulombic efficiency of 59.1%, 70.3% and 99.7%, respectively. Nyquist plots of the Co 3 O 4 electrodes are shown in Fig. 8(b). The plots exhibit the similar curves consisting of a semicircle in high frequency, a semicircle in medium frequency and a following 45 line at low frequency. Cycling performance of Co 3 O 4 electrodes is shown in Fig. 8(c). When the molar ratio of Co 2þ to citric acid is 1:1, Co 3 O 4 electrode shows the smallest high frequency semicircle and the smallest medium frequency, which indicates the higher coulombic efficiency, and also exhibits excellent cycling performance. The reversible capacity is ma h g 1 after 20 cycles at the current density of 250 ma g 1. Fig. 9 shows SEM micrographs of Co 3 O 4 powder prepared with different molar ratios of cobalt to citric acid. Three samples are aggregated of primary particles and present different morphology. The stick clusters particles and the average size of Co 3 O 4 particles decrease with decreasing of molar ratio of cobalt to citric acid. TEM image of Co 3 O 4 powder with the molar ratio of cobalt to citric acid of 1:1, is shown in Fig. 10. As seen from Fig. 10, the Co 3 O 4 is assembled by spherical particles and the average diameter is about 80 nm. Fig. 5 Nyquist plots of the Co 3 O 4 electrodes prepared at different calcination temperatures for 1 h at the current density of 125 ma g 1 after the first cycle. Fig. 7 XRD patterns of Co 3 O 4 compounds synthesized with different molar ratios of cobalt to citric acid at 650 C for 1 h. Molar ratio of cobalt to citric acid: (a) 1:0.4; (b) 1:0.7; (c) 1:1.

5 C. Hu et al.: J. Mater. Sci. Technol., 2013, 29(3), 215e Fig. 8 (a) Chargeedischarge curves of Co 3 O 4 anodes in the first cycle at the current density of 125 ma g 1, respectively; (b) Nyquist plots of the Co 3 O 4 anodes after the first cycle at 0.2 V, respectively; (c) cycling performance of Co 3 O 4 anodes at the current density of 250 ma g 1. Molar ratio of Co 2þ to citric acid is 1:0.4; 1:0.7; 1:1. Fig. 9 SEM images of Co 3 O 4 powder prepared with different molar ratio of Co 2þ to citric acid. Molar ratio of Co 2þ to citric acid: (a) 1:0.4; (b) 1:0.7; (c) 1:1. Co 3 O 4 powder calcined at 650 C for 1 h shows the discharge capacity of ma h g 1, the Coulombic efficiency of 99.7% in the first cycle at the current density of 125 ma h g 1, and the excellent cycling performance with the reversible capacity of ma h g 1 after 20 cycles at the current density of 250 ma g 1. The particle average diameter is about 80 nm. Acknowledgments This work was financially supported by Project Supported by the Planned Science and Technology Project of Hunan Province, China (Nos. 2011FJ3160, 2011GK2002) and Project Supported by Scientific Research Fund of Hunan Provincial Education Department (10B054). REFERENCES Fig Conclusion TEM images of Co 3 O 4 powder at 650 C for 1 h. The molar ratio of Co 2þ to citric acid is 1:1. A relatively simple hydrometallurgical process has been used for the separation and recovery of cobalt from the spent lithiumion batteries with efficient removal of impurities of Al, Ni, Mn and Cu. A sol-gel method has been used to prepare the nanostructure Co 3 O 4. Citric acid was used as sol-gel reagent. The influences of molar ratio of Co 2þ to citric acid and calcination temperature on the structure and electrochemical performance of Co 3 O 4 were evaluated. An fcc Co 3 O 4 powder was successfully synthesized. When the cobalt citric acid gel precursor was prepared with the molar ratio of n(co 2þ ):n(citric acid) ¼ 1:1, the [1] L. Li, R.J. Chen, F. Sun, F. Wu, J.R. Liu, Hydrometallurgy 108 (2011) 220e225. [2] L. Sun, K.Q. Qiu, J. Hazard. Mater. 194 (2011) 378e384. [3] L. Chen, X.C. Tang, Y. Zhang, L.X. Li, Z.W. Zeng, Y. Zhang, Hydrometallurgy 108 (2011) 80e86. [4] E.M. Garcia, J.S. Santos, E.C. Pereira, M.B.J.G. Freitas, J. Power Sources 185 (2008) 549e553. [5] S. Castillo, F. Ansart, C. Laberty-Robert, J. Portal, J. Power Sources 112 (2002) 247e254. [6] M. Contestabile, S. Panero, B. Scrosati, J. Power Sources 92 (2001) 65e69. [7] S.M. Shin, N.H. Kim, J.S. Sohn, D.H. Yang, Y.H. Kim, Hydrometallurgy 79 (2005) 172e181. [8] B. Swain, J. Jeong, J.C. Lee, G.H. Lee, J.S. Sohn, J. Power Sources 167 (2007) 536e544. [9] X.H. Huang, J.P. Tu, C.Q. Zhang, F. Zhou, Electrochim. Acta 55 (2010) 8981e8985. [10] M. Wan, D.L. Jin, R. Feng, L.M. Si, M.X. Gao, L.H. Yue, Inorg. Chem. Commun. 14 (2011) 38e41.

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