Hollow carbon spheres with encapsulation of Co3O4 nanoparticles as anode material for lithium ion batteries

Hollow carbon spheres with encapsulation of Co3O4 nanoparticles as anode material for lithium ion batteries

Electrochimica Acta 78 (2012) 440–445 Contents lists available at SciVerse ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/loca...

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Electrochimica Acta 78 (2012) 440–445

Contents lists available at SciVerse ScienceDirect

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

Hollow carbon spheres with encapsulation of Co3 O4 nanoparticles as anode material for lithium ion batteries Liang Zhan a,b,∗ , Yanli Wang a , Wenming Qiao a , Licheng Ling a , Shubin Yang b,∗∗ a b

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China Mechanical Engineering and Material Science, Rice University, Houston 77005, USA

a r t i c l e

i n f o

Article history: Received 24 March 2012 Received in revised form 13 May 2012 Accepted 8 June 2012 Available online 15 June 2012 Keywords: Hollow carbon spheres Cobalt oxide Anode electrode Lithium ion batteries

a b s t r a c t Based on the high theoretical capacity of Co3 O4 for lithium storage, a noval type of monodisperse hollow carbon spheres with encapsulation of Co3 O4 nanoparticles (HCSE-Co3 O4 ) were designed and synthesized. The monodisperse hollow carbon spheres not only can provide enough void volume to accommodate the volume change of encapsulated Co3 O4 nanoparicles, but also can prevent the formation of solid electrolyte interface (SEI) films on the surface of Co3 O4 nanoparticles and following direct contact of Co and SEI films upon lithium extraction. The HCSE-Co3 O4 electrode exhibit highly reversible capacity, excellent cycle performance and rate capability attributed to the unique structure. The reversible capacity of HCSECo3 O4 electrode is as high as 500 mAh g−1 at a current density of 744 mA g−1 , while that of bare Co3 O4 electrode is only around 80 mAh g−1 . © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Lithium ion batteries, as power sources for portable electronic devices and electric/hybrid vehicles, have attracted tremendous attentions in the scientific and industrial fields due to their high electromotive force and high energy density. To meet the increasing demand of batteries with higher energy density and longer cycle life, many efforts have been made recently to develop new electrode materials or design novel structures of electrode materials [1–8]. For anode materials, transition-metal oxides such as Co3 O4 [9–11], Fe3 O4 [12–14], CuO [15] and RuO2 [16] provide a good promising to substitute conventional carbonaceous materials due to their high theoretical capacities. For example, Co3 O4 can store more than eight lithium atoms per formula unit, corresponding to a reversible capacity of 890 mAh g−1 [9–11,17]. Unfortunately, most of the transition-metal oxides suffer from the problem of poor cycle performance, on one hand resulting from the large specific volume change during cycling process which leads to the aggregation of metal oxide and even pulverization of the electrode [10,18], on the other hand forming the unstable solid electrolyte interface

∗ Corresponding author at: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China. Tel.: +86 21 64252924; fax: +86 21 64252914. ∗∗ Corresponding author. E-mail addresses: [email protected] (L. Zhan), [email protected] (S. Yang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.06.017

(SEI) film on the surface of transition-metal oxides [18,19]. It has been demonstrated that the thick SEI films formed on the surface of metal oxides during discharge process could be completely decomposed by the catalysis of transition metals upon lithium extraction, which not only leads to rapid capacity decay of transition-metal oxides but also to severe safety problem for lithium ion batteries [18,19]. Recently, several strategies have been proposed to improve the cycle performance of transition-metal oxides by decreasing the particle size [20,21], using the metal oxide films or alloys [19,20], or dispersing metal oxides into an inactive/active matrix [18,22,23]. One of the most promising strategies is to disperse nanosized transition-metal oxides into a carbon matrix, where carbon acts as both structural buffer and electrochemically active material during the lithium insertion/extraction [9–12,18]. For example, CuO/graphite [24] and Co3 O4 /porous carbon composites [9,25,26] with high capacity and improved cyclability were achieved as anode materials for lithium ion batteries. However, it is still a challenge to provide enough void volume to compensate the volume change of transition-metal oxides and at the same time to avoid the direct contact of transition metal with SEI films during cycling processes, which is very important to improve the cycle performance and safety of transition-metal oxides. In our present works, we design and elaborate a new type of monodisperse hollow carbon spheres with encapsulation of Co3 O4 nanoparticles (denoted as HCSE-Co3 O4 ) via layer-by-layer coating, in which hollow carbon spheres not only could provide enough

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Fig. 1. SEM and TEM images of (a) and (b) cobalt [email protected], (c) and (d) cobalt [email protected]@PDVB, (e) and (f) [email protected]@carbon and (g) and (h) HCSE-Co3 O4 . The inset in (f) is the SAED pattern of HSE-Co.

void volume to accommodate the volume change of cobalt oxide but also could effectively prevent the SEI forming on the surface of Co3 O4 nanoparticles and the aggregation of Co3 O4 nanoparticles during charging and discharging processes. One thus expects highly reversible capacity, excellent cycle performance and high safety of the HCSE-Co3 O4 composite as anode materials for lithium ion batteries.

2. Experimental 2.1. Synthesis The overall fabrication procedure of HCSE-Co3 O4 is shown in Scheme 1. The overall synthetic procedure of HCSE-Co3 O4 composite mainly includes four steps. In briefly, cobalt [email protected]

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Scheme 1. Schematic illustration of the fabrication of HCSE-Co3 O4 .

0 spheres were firstly fabricated via the hydrolysis of tetraethylsiloxane (TEOS) on the surface of cobalt alkoxide, which was synthesized by cobalt acetate reaction with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP), the details can be seen elsewhere [27]. The weight ratio between TEOS and cobalt alkoxide was fixed to 40:1. And then 3-(methacryloxy) propyltrimethoxysilanemodified cobalt [email protected] spheres were used as the seeds to prepare cobalt [email protected]@poly (divinylbenzene) (PDVB) spheres by dispersion polymerization of divinylbenzene (DVB) in an ethanol solution. The resultant sample was subsequently pyrolyzed at 700 ◦ C for 2 h under Ar atmosphere, further thermally treated at 400 ◦ C for 4 h in air. 2 M NaOH solution was finally used to remove the silica, thus yielding the monodisperse HCSE-Co3 O4 . 2.2. Characterization The morphology, microstructure and composition of the samples were investigated by scanning electron microscopy (SEM, LEO 1530) and transmission electron microscopy (TEM, Philips EM 420) and X-ray diffraction (XRD) measurements. Electrochemical experiments were carried out by using standard R2032 type coin cells. The working electrodes were prepared by mixing the HCSE-Co3 O4 or bare Co3 O4 , carbon black and poly(vinyl difluoride) (PVDF) at a weight ratio of 80:10:10 and pasted on pure Cu foil. Pure lithium foil (Aldrich) was used as the counter electrode. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by volume) obtained from Ube Industries Ltd. The cells were assembled in an argon-filled glove box. Galvanostatical discharge–charge experiments were tested at different current densities in the voltage range of 0.01–3.00 V. Cyclic voltammetry measurements were carried out on an electrochemical workstation (Model 2273, Princeton Applied Research, USA) at a scanning rate of 0.1 mV s−1 .

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Two-theta (degree) Fig. 2. XRD patterns of HCSE-Co3 O4 and HSE-Co.

interestingly found that morphology of the obtained sample (HSECo) has significantly changed from uniform bulk sphere into hollow spherical shell embedded with many nanoparticles. These shells have an average diameter of 350 nm and a thickness of 75 nm. The nanoparticles in the shells have diameters from 40 to 70 nm (Fig. 1(e) and (f)), whose selected area diffraction (SAED) pattern presents obvious spot rings, corresponding to the lattice spacings of metallic cobalt, which should be resulted from the decomposition of cobalt alkoxide [28]. Fig. 1(g) and (h) shows the representative SEM and TEM images of the sample obtained after oxidation in air and removal of the silica shell between cobalt oxide and carbon by a NaOH solution. It can be clearly seen that these monodisperse hollow carbon spheres exhibit uniform size and thin shell. Combined the SEM image of an occasionally broken sphere (inset in Fig. 1(g)) with the TEM image of integral sphere shown in Fig. 1(h), it is well known that these Co3 O4 nanoparticles (as confirmed by XRD analysis described below) are encapsulated into the hollow carbon spheres. Moreover, it is very facile to adjust the void volume in hollow carbon spheres and the ratio between Co3 O4 and carbon by tuning the thicknesses of the silica and PDVB shells during fabrication. The XRD patterns of HCSE-Co3 O4 and HSE-Co are compared in Fig. 2. It is disclosed that the obtained HSE-Co product is a mixture of Co and carbon. The broad hump at 22.3◦ corresponds to the diffraction of amorphous carbon, and the other three peaks at 44.2, 51.5 and 75.9◦ are indexed to face-centered cubic (fcc) Co [29], well

3. Results and discussion To elucidate the morphology and structure of the samples during fabrication, SEM and TEM measurements were combinedly carried out. As shown in Fig. 1, the cobalt alkoxide synthesized by cobalt acetate reaction with ethylene glycol displays the uniform size and spherical shape. After hydrolysis of TEOS on the surface of cobalt alkoxide, the core-shell cobalt [email protected] spheres were obtained. These present a diameter of about 330 nm and smooth spheres surface similar to that of cobalt alkoxide (Fig. 1(a) and (b)). Fig. 1(c) and (d) shows the typical SEM and TEM images of cobalt [email protected]@PDVB, which reveal the increasing diameter of the spheres and clear double shells structure, demonstrating the PDVB shells were successfully coated on cobalt [email protected] spheres. After pyrolysis at 700 ◦ C under Ar atmosphere, it is

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Fig. 3. SEM image of bare Co3 O4 nanoparticles.

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Fig. 4. Cyclic voltammograms of (a) HCSE-Co3 O4 and (b) bare Co3 O4 electrodes.

consistent with the results of their SAED pattern. In comparison, the diffraction peaks of the HCSE-Co3 O4 are perfectly indexed to the ˚ consistent cubic spinel Co3 O4 with a lattice parameter of a = 8.01 A, with the values in standard card of Co3 O4 (JCPDS card no. 42-1467), demonstrating the metallic Co in hollow spheres was completely transferred into Co3 O4 after the mild oxidation. Such ideal structure and component of HCSE-Co3 O4 should result in excellent electrochemical performance when it is applied as anode material for lithium ion batteries (see below). Cyclic voltammetry experiments were initially conducted to evaluate the electrochemical performance of the HCSE-Co3 O4 at a scanning rate of 0.1 mV s−1 over the voltage range from 0.01 to 3.00 V. For comparison, bare Co3 O4 nanoparticles prepared by oxidizing cobalt alkoxide (Fig. 3) were also tested under the same

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electrochemical conditions. Their cyclic voltammograms (CV) are shown in Fig. 4. In the case of HCSE-Co3 O4 , three reduction peaks and two oxidation peaks are observed in the first scanning. Two of the reduction peaks are around 0.9 and 0.6 V, which should be ascribed to the reduction of Co3 O4 to Co and Li2 O and the formation of SEI films on the surface of hollow carbon spheres [26], respectively. In comparison, only one dominant reduction peak appears in the CV of bare Co3 O4 in the first scanning, which is similar to that reported in the literatures [30,31]. This is attributed to the overlap of the two peaks corresponding to the reduction of Co3 O4 to Co and Li2 O as well as the formation of SEI films on the surface of Co3 O4 nanoparticles. The third reduction peak at 0.0 V and the first broad oxidation peak between 0.6 and 1.5 V typically correspond to the lithium insertion and extraction from carbon shells. And the second

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Fig. 5. Galvanostatic discharge–charge profiles of (a) HCSE-Co3 O4 and (b) bare Co3 O4 electrodes; (c) cycle performance and (d) rate capabilities of HCSE-Co3 O4 and bare Co3 O4 electrodes.

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bare Co3O4 HCSE-Co3O4

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provide enough void volume to accommodate the volume change of Co3 O4 during cycling processes. Thirdly, hollow carbon sphere could prevent the formation of SEI films on the surface of Co3 O4 nanoparticles and following direct contact of Co and SEI films, and thus leads to the improvement of the safety of Co3 O4 nanoparticles. In addition, the HCSE-Co3 O4 particles are spherical in shape, which provides a high packing density to allow a high volumetric energy density. 4. Conclusions

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Zre (ohm) Fig. 6. Nyquist plots of HCSE-Co3 O4 and bare Co3 O4 electrodes obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range 100 kHz to 0.01 Hz.

oxidation peak at 2.1 V is arisen from the reformation of Co3 O4 from Co and Li2 O [30]. These CV behaviors of HCSE-Co3 O4 demonstrate that both hollow carbon shells and Co3 O4 are electrochemically active components for Li ion storage, which will result in the high capacity of HCSE-Co3 O4 . After the first scanning, the reduction and oxidation peaks of HCSE-Co3 O4 are very steady, while those of bare Co3 O4 decrease obviously. The CV cycle-ability of HCSE-Co3 O4 should be attributed to the unique hollow carbon spheres, which can effectively prevent the SEI forming on the surface of Co3 O4 nanoparticles and the aggregation of Co3 O4 nanoparticles during charging and discharging processes. The galvanostatic discharge-charge profiles of HCSE-Co3 O4 and bare Co3 O4 electrodes at the current density of 74 mA g−1 are presented in Fig. 5(a) and (b), respectively. It is remarkable to note that the HCSE-Co3 O4 electrode not only delivers a very high reversible capacity (732 mAh g−1 ) in the voltage range from 0.01 to 3.00 V, two times higher than that of graphite (372 mAh g−1 ) and close to that of bare Co3 O4 (764 mAh g−1 ), but also exhibits an excellent cycle performance after the second cycle. It can be seen from Fig. 5(c) that the reversible capacity of HCSE-Co3 O4 electrode is stable at 750 mAh g−1 after 20 cycles, whereas those of bare Co3 O4 and commercial Co3 O4 electrodes rapidly decay to 447 and 120 mAh g−1 [10], respectively. At the higher current density, the capacity retention of HCSE-Co3 O4 electrode is more evident (Fig. 5(d)). For instance, at the current density of 744 mA g−1 , the reversible capacity of HCSE-Co3 O4 electrode is still as high as 500 mAh g−1 , while that of bare Co3 O4 electrode decays to around 80 mAh g−1 . In order to understand why HCSE-Co3 O4 electrode exhibits such a superior electrochemical performance compared to bare Co3 O4 electrode, AC impedance measurements were performed after 20 cycles. Nyquist plots (Fig. 6) show that the diameter of the semicircle for HCSE-Co3 O4 electrode in the high-medium frequency region is much smaller than that of bare Co3 O4 electrode, suggesting that HCSE-Co3 O4 electrode possesses lower contact and charge-transfer impedances. Above results confirm that the hollow carbon spherical shells not only can preserve the high conductivity of the overall electrode, but also largely improve the electrochemical activity of Co3 O4 during the cycle processes because of the following reasons. Firstly, the monodisperse hollow carbon spheres act as a barrier to efficiently prevent the aggregation of the Co3 O4 nanoparticles during charging and discharging processes. Secondly, hollow carbon spheres could

We have designed and fabricated a new type of hollow carbon spheres with encapsulation of Co3 O4 nanoparticles. It was demonstrated that the monodisperse hollow carbon spheres not only could provide enough void to accommodate the volume change of encapsulated Co3 O4 nanoparticles but also could prevent the formation of SEI films on the surface of Co3 O4 nanoparticles and following direct contact of Co and SEI films upon lithium extraction. As a result, the HCSE-Co3 O4 exhibit remarkable lithium storage performance including highly reversible capacity, excellent cycle performance and rate capability. They thus shed light on the utility of hollow carbon spheres to improve the electrochemical performance and safety of a variety of transition-metal oxide nanoparticles. Acknowledgements We thank the National Natural Science Foundation of China (20806024, 51002051, 50730003, 50672025), Fundamental Research Funds for the Central Universities (WA1014016) and Research Fund of China 863 program (2008AA062302) for financial support. References [1] B. Kang, G. Ceder, Nature 458 (2009) 190. [2] N. Liu, L.B. Hu, T.M. Matthew, J. Ariel, Y. Cui, ACS Nano 5 (2011) 6487. [3] A.R. Armstrong, L. Christopher, P.M. Panchmatia, M.S. Islam, P.G. Bruce, Nature Materials 10 (2011) 223. [4] A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, Nature Materials 9 (2010) 461. [5] F.Y. Cheng, J. Liang, Z.L. Tao, J. Chen, Advanced Materials 23 (2011) 1695. [6] T.T. Truong, Y. Qin, Y. Ren, Z.H. Chen, M.K. Chan, J.P. Greeley, K. Amine, Y.G. Sun, Advanced Materials 23 (2011) 4947. [7] N.A. Kaskhedikar, J. Maier, Advanced Materials 21 (2009) 2664. [8] J.T. Han, Y.H. Huang, J.B. Goodenough, Chemistry of Materials 23 (2027) (2011). [9] Y.Y. Liang, Y.U. Li, H.L. Wang, J.G. Zhou, j. Wang, Nature Materials 10 (2011) 780. [10] L.J. Zhi, Y.S. Hu, B.E. Hamaoui, X. Wang, I. Lieberwirth, U. Kolb, J. Maier, K. Mullen, Advanced Materials 20 (2008) 1727. [11] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. [12] X.J. Zhu, Y.W. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, ACS Nano 5 (2011) 3333. [13] J. Li, H.M. Dahn, L.J. Krause, D.B. Le, J.R. Dahn, Journal of the Electrochemical Society 155 (2008) A812–A816. [14] L. Taberna, S. Mitra, P. Poizot, P. Simon, J.M. Tarascon, Nature Materials 5 (2006) 567. [15] S.F. Zheng, J.S. Hu, L.S. Zhong, W.G. Song, L.J. Wan, Y.G. Guo, Chemistry of Materials 20 (2008) 3617. [16] O. Delmer, P. Balaya, L. Kienle, J. Maier, Advanced Materials 20 (2008) 501. [17] B.K. Guo, N. Liu, J.Y. Liu, H.J. Shi, Z.X. Wang, L.Q. Chen, Electrochemical and Solid-State Letters 10 (2007) A118. [18] S.A. Needham, G.X. Wang, K. Konstantinov, Y. Tournayre, Z. Lao, H.K. Liu, Electrochemical and Solid-State Letters 9 (2006) A315. [19] W.M. Zhang, X.L. Wu, J.S. Hu, Y.G. Guo, L.J. Wan, Advanced Functional Materials 18 (2008) 3941. [20] Y.H. Huang, J.B. Goodenough, Chemistry of Materials 20 (2008) 7237. [21] L.Q. Sun, M.J. Li, R.H. Cui, H.M. Xie, R.S. Wang, Journal of Physical Chemistry C 114 (2010) 3297. [22] H. Qiao, L.F. Xiao, Z. Zheng, H.W. Liu, F.L. Jia, L.Z. Zhang, Journal of Power Sources 185 (2008) 486. [23] Y. Sharma, G.V.S. Rao, B.V.R. Chowdari, Advanced Functional Materials 17 (2855) (2007). [24] H. Huang, E.M. Kelder, J. Schoonman, Journal of Power Sources 97–98 (2001) 114.

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