C microspheres as anode materials for lithium ion batteries

C microspheres as anode materials for lithium ion batteries

Electrochimica Acta 56 (2011) 6752–6756 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elec...

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Electrochimica Acta 56 (2011) 6752–6756

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

CuO/C microspheres as anode materials for lithium ion batteries X.H. Huang a,b,∗ , C.B. Wang a , S.Y. Zhang a , F. Zhou a,∗∗ a b

Academy of Frontier Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China School of Physics & Electronic Engineering, Taizhou University, Taizhou 318000, China

a r t i c l e

i n f o

Article history: Received 18 February 2011 Received in revised form 13 May 2011 Accepted 13 May 2011 Available online 26 May 2011 Keywords: CuO/C Microsphere Anode Lithium ion battery

a b s t r a c t CuO/C microspheres are prepared by calcining CuCl2 /resorcinol-formaldehyde (RF) gel in argon atmosphere followed by a subsequent oxidation process using H2 O2 solution. The microstructure and morphology of materials are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transition electron microscopy (TEM). Carbon microspheres have an average diameter of about 2 ␮m, and CuO particles with the sizes of 50–200 nm disperse in these microspheres. The electrochemical properties of CuO/C microspheres as anode materials for lithium ion batteries are investigated by galvanostatic discharge–charge and cyclic voltammetry (CV) tests. The results show that CuO/C microspheres deliver discharge and charge capacities of 470 and 440 mAh g−1 after 50 cycles, and they also exhibit better rate capability than that of pure CuO. It is believed that the carbon microspheres play an important role in their electrochemical properties. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction The research of 3d transition-metal oxides (MO, M = Fe, Co, Ni, Cu, etc.) as anode materials for lithium ion batteries has received much attention. These oxides are attractive alternatives to graphite, and they can deliver high reversible capacities [1–14]. However, they often exhibit poor kinetics and severe capacity fading upon cycling, mainly because of the low conductivity and the severe pulverization arising from the large volume change during the cycling. To overcome these problems, forming composite with carbon is a very useful way. Carbon can buffer the volume changes of active particles and thus alleviates their pulverization, and meanwhile, carbon has high conductivity and it is able to keep the active particles well electrically contacted [15–27]. Both of the two aspects are very beneficial to the electrochemical properties of the 3d transition-metal oxides. In the present work, to improve the electrochemical performance of CuO anode materials, CuO/C composite microspheres are prepared by calcining CuCl2 /RF gel in flowing argon followed by an oxidation treatment using H2 O2 solution. In this composite material, most of the CuO particles are embedded in the carbon aerogel microspheres. As the carbon aerogel microspheres can offer a good electrical contact and a good buffer effect to the CuO particles

∗ Corresponding author at: School of Physics & Electronic Engineering, Taizhou University, Taizhou 318000, China. Tel.: +86 576 88661937. ∗∗ Co-corresponding author: Tel.: +86 25 84893083; fax: +86 25 84893083. E-mail addresses: [email protected] (X.H. Huang), [email protected] (F. Zhou). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.072

inside, this CuO/C composite microspheres exhibit enhanced electrochemical performance when they are used as anode materials of lithium ion batteries. 2. Experimental CuO/C composite microspheres were prepared by calcining CuCl2 /RF gel in argon atmosphere followed by a subsequent oxidation treatment using H2 O2 solution, as described in Fig. 1. Preparation of carbon aerogel microspheres from RF gel had been reported elsewhere [28,29]. In the present work, 17 g copper chloride dihydrate, 6.5 g resorcinol, 9 mL 37 wt% formaldehyde, and 0.5 mL 37 wt% concentrated hydrochloric acid were dissolved in 100 mL deionized water. The mixed solution was sealed with several layers of polyethylene films and placed in a water bath at 80 ◦ C for 3 h to generate a CuCl2 /RF gel. The gel was dried in air in a drying oven at 80 ◦ C, and then transferred into a quartz tube furnace and calcined at 750 ◦ C for 2 h in flowing argon. In this carbonization process, the RF gel turned to carbon aerogel microspheres, and the Cu2+ was reduced to Cu as well, forming Cu/C composite microspheres. Subsequently, the Cu/C microspheres were dispersed in 15 wt% H2 O2 solution and placed in a drying oven at 95 ◦ C for more than 24 h until the solution was completely evaporated, and in this process Cu was oxidized to CuO. Finally, the CuO/C microspheres were further dried at 300 ◦ C in vacuum to remove the residual water inside the microspheres. The microstructure of the materials was characterized by means of powder X-ray diffraction (XRD, Rigaku D/max-rA). The morphology of the microspheres was observed by scanning electron microscopy (SEM, FEI Sirion-100) and transmission electron

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3. Results and discussion 3.1. Characterization of CuO/C microspheres The CuCl2 /RF gel was undergoing a carbonization process when it was heat-treated in argon atmosphere at 750 ◦ C, and at the same time, Cu2+ was reduced to Cu metal. Fig. 2(a) shows the XRD pattern of this carbonization product. The peaks at 43.4◦ , 50.5◦ , and 74.3◦ can be assigned to the (1 1 1), (2 0 0), and (2 2 0) reflections of Cu, respectively. The diffraction peaks of carbon are not obvious, indicating that the carbon should be amorphous. After the subsequent oxidation process by H2 O2 solution, the product’s XRD pattern (Fig. 2(b)) exhibits only diffraction peaks of CuO, according to JCPDS 80-1917. No peaks of Cu or Cu2 O can be observed, indicating that Cu was fully oxidized to CuO by H2 O2 solution.

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microscopy (TEM, JEOL JEM 200CX). The carbon content in the composite was precisely analyzed by thermogravimetric analysis (TGA) carried out on a Thermo SDT-Q600 thermal analyzer. The specific surface area of CuO/C composite was determined by the Brunauer–Emmett–Teller (BET) method using Micromeritics ASAP2010 analyzer. The working electrodes were prepared by coating the slurry consisting of 80 wt% active materials, 12 wt% acetylene black, and 8 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone (NMP) on copper foam. The electrodes were dried at 95 ◦ C for 24 h in vacuum and then pressed under 15 MPa. Coin-type (CR2025) test cells were assembled in an argon-filled glove box, using the lithium metal foil as the counter electrode, a microporous membrane (Cellgard 2300) as the separator, and 1 mol L−1 solution of LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by weight) as the electrolyte. The cells were aged for 24 h before the electrochemical measurements. Test cells were galvanostatically discharged and charged on a battery testing instrument (LAND CT2001A) using different specific current densities over a voltage range of 0.02–3 V. Cyclic voltammetry (CV) tests of the cells were carried out on a CHI660D electrochemical workstation at a scan rate of 0.1 mV s−1 between 0 and 3 V. All electrochemical tests were performed at room temperature.

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Fig. 1. Schematic illustration of the synthetic procedure of CuO/C microspheres.

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Carbon aerogel microspheres were obtained by heating the RF gel in argon, as shown in the SEM image of CuO/C composite (Fig. 3(a)). It is obvious that each carbon microsphere has a diameter of about 2 ␮m, and some CuO particles locate on the surfaces of the spheres. According to the magnified image of a microsphere (Fig. 3(b)), there are lots of nanopores on the surface, which originated from the emission of gases during the carbonization process. Electrolyte can permeate into the inner of spheres through these nanopores, so the particles inside can take part in the electrochemical reaction. The morphology of the microspheres was further analyzed by TEM, and the images are present in Fig. 4. The diameter of the microsphere is about 2 ␮m, as shown in Fig. 4(a), which is in agreement with SEM results. However, it was difficult to observe the inner morphology because the electron beam cannot penetrate through such thick spheres. To overcome this problem, the microspheres were ground into pieces using an agate mortar pestle, and the TEM image was recorded from these fragments, which is given in Fig. 4(b). It is clear that CuO particles have sizes of 50–200 nm, and they disperse homogeneously in the carbon matrix. Therefore, it can be concluded that in the CuO/C composite microspheres, most of the CuO particles are embedded in the spheres. To investigate the precise content of carbon in the CuO/C composite, thermogravimetric analysis (TGA) was carried out in air. The sample was heated from 30 to 750 ◦ C at a rate of 10 ◦ C min−1 . Fig. 5 shows the TGA curves of the CuO/C composite. As can be seen, the

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Fig. 3. SEM images of (a) the CuO/C microspheres, and (b) the surface of a CuO/C microsphere.

composite shows rapid weight loss between 400 and 600 ◦ C, which corresponds to the oxidation of carbon. Therefore, the change in weight before and after the oxidation of carbon directly translates into the content of carbon in the composite. By using this method, it was estimated that the content of carbon in the composite was about 18 wt%. The specific surface area was determined with BET method, and the result is that the CuO/C microspheres have a BET surface area of 486 m2 g−1 .

Fig. 4. TEM images of (a) the CuO/C microspheres, and (b) the fragments of microspheres caused by a grinding process.

Fig. 7 displays the galvanostatic discharge–charge curves for the first three cycles and the 50th cycle of the CuO/C electrode measured between 0.02 and 3 V at a current density of 100 mA g−1 . Notice that the plateaus on the voltage profiles coincide well with the cathodic and anodic peaks in the CV curves. During the first discharge, there are three different potential plateaus around 1.9, 1.1, and 0.8 V, respectively. However, the plateaus during the first charge are not so obvious, and the main sloping potential ranges are around 2.5 and 2.7 V. After the 1st cycle, the curves of each cycle are similar in shape, indicating that the electrode reactions

3.2. Electrochemical performance of CuO/C microspheres 0.1

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CuO/C microspheres were used as anode materials of lithium ion batteries and their electrochemical properties were investigated. CV curves of the CuO/C electrode were measured between 0 and 3 V at a scan rate of 0.1 mV s−1 , which are shown in Fig. 6. There are three peaks at 1.9, 1.1, and 0.8 V during the first reduction scan. According to Tarascon and coworkers’ research, the peak at 1.9 V corresponds to the formation of an interstitial compound of Li in CuO, while the peak at 1.1 V corresponds to the convention from CuO to Cu2 O, and the peak at 0.8 V corresponds to the further decomposition of Cu2 O into Cu and Li2 O [4]. There are two oxidation peaks at about 2.5 and 2.7 V appearing in the first oxidation scan, which related to the oxidation process 2Cu + Li2 O → Cu2 O + 2Li and the partial oxidation of Cu2 O to CuO, respectively [3,4]. It can be seen that in the second scan, the reduction peaks locate at 1.9, 1.2, and 0.8 V, respectively, and the oxidation curve coincides with the first one, indicating that the electrode reactions become more reversible.

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become more reversible. The slopes in each discharge curve are around 1.9, 1.2, and 0.8 V, respectively, and in each charge curve are around 2.5 and 2.7 V. The CuO/C microspheres deliver a first discharge capacity of 1150 mAh g−1 (determined on basis of the total mass of CuO and C), much higher than the theoretical value, which is 675 mAh g−1 for pure CuO (calculated from the electrode reaction CuO + 2Li → Cu + Li2 O). The extra capacity should come from the irreversible side reactions including the formation of the solid electrolyte interface (SEI) during the first discharge. The SEI is a gel-like organic layer, and it consists of ethylene-oxide-based oligomers, LiF, Li2 CO3 , and lithium alkyl carbonate [30]. The first charge capacity is 560 mAh g−1 , and the second discharge capacity is 580 mAh g−1 . It can be calculated that the initial Coulombic efficiency of CuO/C electrode is about 50%, lower than the reported value [3,4], which may be attributed to the nanopores in carbon microspheres. The nanopores are capable of storing some lithium, but the Coulombic efficiency is low [31]. After 50 cycles, the discharge and charge capacities remain 470 and 440 mAh g−1 , respectively. The capacities of the 1–50th cycles are plotted in Fig. 8. It can be seen that about 80% of the reversible capacity (charge capacity) remains after 50 cycles. To investigate the effect of carbon on the electrochemical performance, a comparison between CuO/C composite and pure CuO was made. Pure CuO was prepared by just calcining the CuO/C composite microspheres in oxygen at 750 ◦ C for 30 min, and its 3

Fig. 8. Cycling performance of CuO/C microspheres at the current density of 100 mA g−1 .

Fig. 9. SEM image of the pure CuO particles prepared by burning the CuO/C composite microspheres in oxygen.

SEM image is given in Fig. 9. Obviously, the pure CuO particles are 50–200 nm in size, and aggregate together severely. The rate capabilities of CuO/C microspheres and pure CuO were compared, which are present in Fig. 10. Both of the cells were cycled between 0.02 and

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Fig. 10. Comparison of rate capabilities of CuO/C microspheres and pure CuO.

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3 V at various current densities from 100 to 1000 mA g−1 . The cells were first cycled at the current density of 100 mA g−1 . In the first few cycles, pure CuO deliver higher capacities than those of CuO/C microspheres. This is because the capacity of CuO/C composite is determined on basis of the total mass of CuO and C. Therefore, the theoretical capacity of CuO/C must be lower than that of pure CuO. However, the capacities of pure CuO decrease very quickly, and at the end of this stage, the discharge and charge capacities of CuO are 530 and 500 mAh g−1 , lower than those of CuO/C microspheres (540 and 515 mAh g−1 ). In the subsequent cycles, the CuO/C microspheres always exhibit higher capacities than those of CuO. At the higher current densities of 200, 500, and 1000 mA g−1 , the final discharge capacities of each period for CuO/C microspheres are 485, 300, 165 mAh g−1 , while the charge capacities are 460, 280, and 155 mAh g−1 . For CuO, the final discharge capacities of each period are 390, 210, 110 mAh g−1 , while the charge capacities are 365, 200, and 100 mAh g−1 . At last, when the current density returns back to the initial 100 mA g−1 , the final discharge and charge capacities of CuO/C microspheres are 470 and 440 mAh g−1 , respectively, which are higher than those of CuO (335 and 310 mAh g−1 ). The enhanced electrochemical performance of CuO/C composite is attributed to the carbon microspheres. In this composite, most of the CuO particles disperse homogenously in the carbon microspheres, and this has three advantages to the electrochemical performance. First, as carbon has good buffer ability, the pulverization of CuO particles arising from the large volume change during the cycling can be restrained. Second, the carbon matrix can prevent the CuO particles from aggregating and forming larger particles, which ensures that more CuO particles are able to take part in the electrochemical reaction. Third, carbon is a good conductor, so in the CuO/C composite, it keeps the CuO particles better electrically contacted, and also offers much more conductive pathways, which are very helpful to the capacity, cyclability, and rate capability. These aspects lead to the improvements of the electrochemical properties. 4. Conclusions CuO/C composite microspheres have been prepared successfully. In this composite, most of the CuO particles with the sizes of 50–200 nm disperse homogenously inside the carbon aerogel microspheres of about 2 ␮m in diameter. The content of carbon in the composite is about 18 wt%. Electrochemical tests of this composite show that about 80% of the reversible capacity (charge capacity) remains after 50 cycles at the specific current density of 100 mA g−1 , and the composite also exhibit better rate capability than that of pure CuO. The carbon microspheres have the abilities of alleviating the pulverization, suppressing the aggregation, and enhancing the conductivity of the CuO particles. All of these aspects are beneficial to the electrochemical performance.

Acknowledgements This work is supported by China Postdoctoral Science Foundation (No. 20090461109), Jiangsu Postdoctoral Science Research Foundation (No. 0902015C), and Science Foundation of Nanjing University of Aeronautics and Astronautics (No. 909388). We would like to acknowledge them for financial support. References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. [2] J.M. Tarascon, M. Armand, Nature 414 (2001) 359. [3] S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Poizot, J.M. Tarascon, J. Electrochem. Soc. 148 (2001) A285. [4] A. Débart, L. Dupont, P. Poizot, J.B. Leriche, J.M. Tarascon, J. Electrochem. Soc. 148 (2001) A1266. [5] P. Poizot, S. Laruelle, S. Grugeon, J.M. Tarascon, J. Electrochem. Soc. 149 (2002) A1212. [6] H. Li, Z. Wang, L. Chen, X. Huang, Adv. Mater. 21 (2009) 4593. [7] M.S. Wu, Y.P. Lin, Electrochim. Acta 56 (2011) 2068. [8] Y. Sun, X.Y. Feng, C.H. Chen, J. Power Sources 196 (2011) 784. [9] J.Y. Xiang, J.P. Tu, Y.Q. Qiao, X.L. Wang, J. Zhong, D. Zhang, J. Phys. Chem. C 115 (2011) 2505. [10] Y. Lu, Y. Wang, Y. Zou, Z. Jiao, B. Zhao, Y. He, M. Wu, Electrochem. Commun. 12 (2010) 101. [11] J. Jiang, J. Liu, R. Ding, X. Ji, Y. Hu, X. Li, A. Hu, F. Wu, Z. Zhu, X. Huang, J. Phys. Chem. C 114 (2010) 929. [12] X.H. Huang, J.P. Tu, B. Zhang, C.Q. Zhang, Y. Li, Y.F. Yuan, H.M. Wu, J. Power Sources 161 (2006) 541. [13] P. Zhang, Z.P. Guo, H.K. Liu, Electrochim. Acta 55 (2010) 8521. [14] M.M. Rahman, J.Z. Wang, X.L. Deng, Y. Li, H.K. Liu, Electrochim. Acta 55 (2009) 504. [15] P. Lian, X. Zhu, H. Xiang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 56 (2010) 834. [16] Y.J. Mai, X.L. Wang, J.Y. Xiang, Y.Q. Qiao, D. Zhang, C.D. Gu, J.P. Tu, Electrochim. Acta 56 (2011) 2306. [17] X.H. Huang, J.P. Tu, C.Q. Zhang, X.T. Chen, Y.F. Yuan, H.M. Wu, Electrochim. Acta 52 (2007) 4177. [18] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, J. Power Sources 195 (2010) 313. [19] J.Y. Xiang, J.P. Tu, J. Zhang, J. Zhong, D. Zhang, J.P. Cheng, Electrochem. Commun. 12 (2010) 1103. [20] Y. He, L. Huang, J.S. Cai, X.M. Zheng, S.G. Sun, Electrochim. Acta 55 (2010) 1140. [21] W.J. Yu, P.X. Hou, L.L. Zhang, F. Li, C. Liu, H.M. Cheng, Chem. Commun. 46 (2010) 8576. [22] B. Wang, X.L. Wu, C.Y. Shu, Y.G. Guo, C.R. Wang, J. Mater. Chem. 20 (2010) 10661. [23] H. Wang, L.F. Cui, Y. Yang, H.S. Casalongue, J.T. Robinson, Y. Liang, Y. Cui, H. Dai, J. Am. Chem. Soc. 132 (2010) 13978. [24] H. Kim, D.H. Seo, S.W. Kim, J. Kim, K. Kang, Carbon 49 (2011) 326. [25] Q. Pan, L. Qin, J. Liu, H. Wang, Electrochim. Acta 55 (2010) 5780. [26] H. Wang, Q. Pan, J. Zhao, W. Chen, J. Alloys Compd. 476 (2009) 408. [27] Q. Bao, C.M. Li, L. Liao, H. Yang, W. Wang, C. Ke, Q. Song, H. Bao, T. Yu, K.P. Loh, J. Guo, Nanotechnology 20 (2009) 065203. [28] T. Hasegawa, S.R. Mukai, Y. Shirato, H. Tamon, Carbon 42 (2004) 2573. [29] X.H. Huang, J.P. Tu, C.Q. Zhang, F. Zhou, Electrochim. Acta 55 (2010) 8981. [30] G. Gachot, S. Grugeon, M. Armand, S. Pilard, P. Guenot, J.M. Tarascon, S. Laruelle, J. Power Sources 178 (2008) 409. [31] A. Mabuchi, K. Tokumitsu, H. Fujimoto, T. Kasuh, J. Electrochem. Soc. 142 (1995) 1041.