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Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Materials Letters journal homepage: www.elsevier.com/locate/matlet
Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles Q1
Jae-Chan Kim, In-Sung Hwang, Seung-Deok Seo, Dong-Wan Kim n Department of Materials Science and Engineering, Ajou University, Suwon 443-749, Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 7 February 2013 Accepted 5 April 2013
Nanocomposite electrodes of cobalt (II) oxide nanoparticles (CoO NPs)/multiwalled carbon nanotubes (MWCNTs) were fabricated by a two-step process consisting of electrophoretic deposition followed by chemical vapor deposition. Results of the electrochemical analysis demonstrate that the CoO NP-bearing MWCNT electrodes could deliver reversible charge capacities of 600 and 550 mA h g−1 after 50 and 100 cycles, respectively, at the rate of 715 mA g−1. The superior cyclability is attributed to the unique selfsupported one-dimensional structure of the MWCNTs and the uniformly dispersed CoO NPs that constitute these nanocomposite electrodes. & 2013 Published by Elsevier B.V.
Keywords: Cobalt oxide Carbon nanotube Nanocomposites Anode Lithium ion batteries
1. Introduction Transition metal oxides have attracted much attention in the past decade as anode materials for Li-ion batteries because of their high theoretical capacities, which are greater than that of commercial graphite (at 372 mA h g−1) [1,2]. These transition metal oxides including Co(II)O react with lithium by a conversion reaction to form cobalt nanocrystals that are well dispersed in the Li2O matrix [2]. This process inevitably causes drastic volume variations in the matrix, resulting in severe capacity fading during the cycling process [3]. It is well recognized that electrode materials with predesigned nanostructures could accommodate the volume change, and wellincorporated active materials in nanocomposite electrodes could offer more stable cyclability [4,5]. Among the various reports, Du et al. have demonstrated that anodes composed of carbon nanotube (CNT)–transition metal oxide composites could react with lithium effectively because of the synergistic action of the metal oxides with the unique properties of CNTs (including strong mechanical properties, excellent electronic conductivities, and large surface areas) [6]. Moreover, nanoparticles could retain their sizes, and agglomeration could be prevented because of their tight anchoring by the CNTs. Extensive efforts have been taken toward exploiting the advantages offered by metal oxide–CNT composites, and these composites have been synthesized by various methods like hydrothermal processing, chemical vapor deposition (CVD), ball-milling, and wet chemical processing [7–10].
Q4
n
Corresponding author. Tel.: +82 31 219 2468; fax: +82 31 219 3248. E-mail addresses: [email protected], [email protected] (D.-W. Kim).
Herein, we report the fabrication of heterostructured CoO nanoparticle (NP)/multiwalled CNT (MWCNT) anodes free of binders. These nanocomposite electrodes were found to exhibit high capacity delivery with good cycle retention.
2. Experimental The CoO NP/MWCNT composites were synthesized by a simple two-step process. First, the MWCNTs (from Hanwha Nanotech Co. Ltd.) were deposited electrophoretically on stainless steel (SS) substrates by a previously reported procedure [11]. Then, CoO NPs were decorated on the surface of the MWCNTs by a CVD process (Supplementary information). Field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL), transmission electron microscopy (TEM, JEM-2100F, JEOL), X-ray diffraction (XRD, Miniflex II, Rigaku), and X-ray photoelectron spectroscopy (XPS, ESCA2000, VG Microtech) were carried out to determine the phase, structure, and morphology of the CoO NP/MWCNT nanocomposites. The masses of the samples SS substrate, MWCNTs on SS, and CoO NPs/MWCNT on SS were accurately measured at each step using a microbalance (mass resolution¼ 0.1 μg, UTM5, Mettler Toledo). For the electrochemical evaluation of the CoO NP/MWCNT nanocomposite electrodes, we assembled a Swagelok-type cell containing a separator film (Celgard 2400), a liquid electrolyte (1 M LiPF6 EC/DMC, 1:1 v/v), and a lithium metal foil (counter electrode). Galvanostatic cycling was conducted at the rate of 715 mA g−1 in the voltage range 0.01–3.00 V using an automatic cycler (WBCS 3000, Wonatech). Cyclic voltammetry (CV) was performed in the voltage range 0.01–3.00 V with a scan rate of 0.3 mV s−1. The average CoO NPs:MWCNTs ratio was maintained at 1:0.9 by mass in each step.
0167-577X/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2013.04.013
Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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The amount of carbon nanotube relative to the total weight of composite was ∼42 % based on the thermogravimetric analysis in Fig. S1, which was in good agreement with the mass ratio measured using a microbalance. The loading amount of the CoO NPs/MWCNT on SS was found to be 265 μg cm−2. The specific capacities were calculated based on the total mass of CoO NPs and MWCNTs.
3. Results and discussion Fig. 1a shows the typical XRD pattern acquired from the CoO NP/ MWCNT composites. The characteristic (002) reflection corresponding to the stacking of graphene layers in the MWCNTs was detected at 2θ of about 25.81. The peaks at 36.41, 42.41, and 61.61 (2θ) could be indexed to reflections from (111), (200), and (220) planes of pure CoO with a cubic structure (Fm3m, JCPDS #48-1719), respectively. The considerable peak broadening and reduced intensity observed confirm the nanocrystalline nature and the ultrafine crystallite size of CoO. The deposition of CoO NPs was carried out in an atmosphere of Ar. It is believed that this relatively reducing atmosphere led to preferable formation of CoO, and not Co3O4. In order to provide a clear evidence of the presence of CoO phase, the analysis was conducted in detail by XPS. A magnified view of the Co 2p peak obtained from the CoO NP/MWCNT composite is shown in Fig. 1b. The peaks corresponding to
Co 2p3/2 and Co 2p1/2 were located at 781.3 and 796.7 eV, respectively. The distance between the peaks (ΔE2) was 15.4 eV, which indicated the +2 oxidation state of Co [2]. Moreover, the shake-up satellite peak at 787.2 eV was positioned at a binding energy of 6 eV (ΔE1) above the Co 2p3/2 peak. We could confirm that pure CoO was present in the absence of pure Co, Co2O3, and Co3O4 impurities in the nanocomposite samples, which was in agreement with the results obtained from the XRD analysis. Fig. 2a shows the FESEM image of the surface of the electrophoretically deposited MWCNTs on the SS substrate. It can be observed that most MWCNTs assembled and networked homogeneously with negligible aggregation. After the deposition of CoO on the MWCNTs, the uniform coverage by tiny NPs was clearly observed and irregularly particles were absent (Fig. 2b). The simplified illustrations of an individual MWCNT and a MWCNT decorated CoO NPs are provided in the insets of Fig. 1a and b, respectively. Fig. 3a and b shows the typical bright-field TEM images of the CoO NPs/MWCNTs at low and high magnifications, respectively. Most individual MWCNTs were distinctly found to be covered by the NPs (Fig. 3a). The NPs exhibited a mean diameter of ∼13 nm (Fig. 3b). The features in the fast Fourier transform (FFT) patterns acquired from the nanocomposites shown in Fig. 3c could be assigned to the (111), (200), and (222) reflections of the face-centered cubic structure of CoO. The distinct crystalline lattice with an interplanar distance of 0.248 nm (Fig. 3d) matched well with the lattice parameter of the
Fig. 1. The typical (a) XRD pattern and (b) Co 3p peak in the XPS profile acquired from CoO NP/MWCNT nanocomposite samples.
Fig. 2. SEM images of (a) electrophoretically deposited MWCNTs and (b) MWCNTs decorated with CoO NPs. Insets of (a) and (b) show schematic illustrations of MWCNT and CoO NP/MWCNT nanocomposites on SS substrates, respectively.
Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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J.-C. Kim et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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(111) plane of cubic CoO, which was consistent with the results obtained from the XRD and XPS analyses. It is believed that cobalt precursor preferentially sticks to the structural defects on acidfunctionalized MWCNTs during the early stages of the CVD process to create CoO-based seeds, on which CoO particles form by isotropic growth. As an anode material for lithium ion batteries, the CoO NP/ MWCNT composite demonstrated a superior lithium storage performance. Cyclic voltammograms showed the lithium reactivity of the CoO NP/MWCNT nanocomposite. Two strong peaks at 0.45 and 0.1 V were observed in the first cathodic scan, corresponding to the conversion of CoO to Co (following the reaction CoO +2Li++2e−- Co+Li2O) and the formation of an irreversible solid electrolyte interface (SEI) layer, respectively [1,2]. After this first discharge, a good cycle performance of the CoO NP/MWCNT nanocomposites was confirmed by subsequent CV studies. In the galvanostatic cycling curve between 0.01 and 3 V at a charge/discharge current density of 715 mA g−1 (at the rate of 1 C, based on the theoretical capacity of CoO, 715 mA h g−1), the
3
voltage dropped from the open-circuit potential to a plateau value of ∼0.5 V and then decreased slowly to 0.01 V, corresponding to the reduction of Co2+ to Co0, as indicated in the cyclic voltammogram furnished in Fig. 4a. The initial discharge and charge capacities of the nanocomposites were measured as 1250 mA h g−1 and 790 mA h g−1, respectively, accounting for an initial Coulombic efficiency of 63%, which is common for most metal oxide anode materials. The initial capacity loss may be mainly attributed to irreversible processes such as the inevitable formation of the SEI layer and electrolyte decomposition [12]. After further cycling, the specific capacities of CoO NP/MWCNT nanocomposite samples were measured at 10th and 50th cycles. The electrodes with different compositions of CoO/MWCNTs could be obtained by controlling the deposition time. We also evaluated the cycle performance of each electrode at the same current density (Fig. S2). A high reversible capacity of 550 mA h g−1 (based on the total mass of CoO NPs and MWCNTs) was obtained even after 100 cycles, which is much higher than values reported for commercial carbon-based anodes. The capacity of ∼350 mA h g−1
Q3 Fig. 3. (a) TEM, (b) HRTEM, and (c) FFT patterns acquired from MWCNTs decorated with CoO NPs. (d) An enlarged image of red square shown in (b) clearly exhibiting the lattice planes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. The typical (a) cyclic voltammograms and (b) charge–discharge curves of the CoO NP/MWCNT nanocomposite anodes.
Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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was observed in Co3O4 NPs/MWCNT composite electrode (60 wt% of MWCNT) at a current rate of 45 mA g−1 during 14 cycles by Shan and Gao [13]. It was also reported that 50 wt% CoO-loaded carbon hollow spheres delivered the capacities of 535 and 420 mA h g−1 at 160 and 800 mA h g−1, respectively [14]. Therefore, the electrochemical performance of composite electrodes in this work is also superior to other reported cobalt oxide/CNT nanocomposite anodes. The characteristics of the CoO NPs/MWCNTs electrodes can offer a reasonable explanation for the large reversible capacity and good cycling retention. First, the good adhesion between the conductive MWCNT networks, which are directly connected to the SS current collector and the CoO NPs could facilitate the transportation of electrons and lithium ions. Next, the tiny CoO NPs uniformly dispersed on the MWCNTs could provide functional spaces for buffering the volume variation and thus accommodate the strain during repeated lithiation and delithiation [11]. Our results suggest that the CoO NP/MWCNT composites are effective for overcoming the drawbacks of transition metal oxide-based anodes and show promise for application in practical lithium ion batteries. 4. Conclusions We prepared CoO NP/MWCNT nanocomposites by the direct anchoring of CoO on the surface of electrophoretically predeposited MWCNT networks via a CVD process. The CoO NP/MWCNT nanocomposite anodes that were free of binders displayed a large reversible capacity of over 550 mA h g−1 at a high rate of 715 mA g−1 even after 100 cycles. The enhancement in the electrochemical performance could be attributed to the uniform distribution of tiny CoO NPs (13 nm in diameter), the existence of highly conductive MWCNT networks, and the creation of favorable open spaces that buffer the huge volume change during lithiation and delithiation. Acknowledgment This research was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (20090094046, 2012M1A2A2671802 and 2012R1A2A2A01045382).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.04.013.
References [1] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM. Nano-sized transitionmetal oxides as negative-electrode materials for lithium-ion batteries. Nature 2000;407:496–9. [2] Peng C, Chen B, Qin Y, Yang S, Li C, Zuo Y, et al. Facile ultrasonic synthesis of CoO quantum dot/graphene nanosheet composites with high lithium storage capacity. ACS Nano 2012;6:1074–81. [3] Kang JG, Ko YD, Park JG, Kim DW. Origin of capacity fading in nano-sized Co3O4 electrodes: Electrochemical impedance spectroscopy study. Nanoscale Res Lett 2008;3:390–4. [4] Park KS, Kang JG, Choi YJ, Lee S, Kim DW, Park JG. Long-term, high-rate lithium storage capabilities of TiO2 nanostructured electrodes using 3D self-supported indium tin oxide conducting nanowire arrays. Energy Environ Sci 2011;4:1796–801. [5] Kang JG, Lee GH, Park KS, Kim SO, Lee S, Kim DW, et al. Three-dimensional hierarchical self-supported multi-walled carbon nanotubes/tin (IV) disulfide nanosheets heterostructure electrodes for high power Li ion batteries. J Mater Chem 2012;22:9330–7. [6] Du N, Zhang G, Chen B, Wu J, Ma X, Liu Z, et al. Porous Co3O4 nanotubes derived from Co4(CO)12 clusters on carbon nanotube templates: a highly efficient material for Li-battery applications. Adv Mater 2007;19:4505–9. [7] Liang Y, Wang H, Diao P, Chang W, Hong G, Li Y, et al. Oxygen reduction electrocatalyst based on strongly coupled cobalt oxide nanocrystals and carbon nanotubes. J Am Chem Soc 2012;134:15849–57. [8] Hwang IS, Kim JC, Seo SD, Lee S, Lee JH, Kim DW. A binder-free Ge-nanoparticle anode assembled on multiwalled carbon nanotube networks for Li-ion batteries. Chem Commun 2012;48:7061–3. [9] Du H, Jiao L, Wang Q, Peng W, Song D, Wang Y, et al. Structure and electrochemical properties of ball-milled Co-carbon nanotube composites as negative electrode material of alkaline rechargeable batteries. J Power Sources 2011;196:5751–5. [10] Lang J, Yan X, Xue Q. Facile preparation and electrochemical characterization of cobalt oxide/multi-walled carbon nanotube composites for supercapacitors. J Power Sources 2011;196:7841–6. [11] Seo SD, Lee GH, Lim AH, Min KM, Kim JC, Shim HW, et al. Direct assembly of tin–MWCNT 3D-networked anode for rechargeable lithium ion batteries. RSC Adv 2012;2:3315–20. [12] Wu FD, Wang Y. Self-assembled echinus-like nanostructures of mesoporous CoO nanorod@CNT for lithium-ion batteries. J Mater Chem 2011;21:6636–41. [13] Shan Y, Gao L. Multiwalled carbon anotubes/Co3O4 nanocomposites and its electrochemical performance in lithium storage. Chem Lett 2004;33:1560–1. [14] Li F, Zou QQ, Xia YY. CoO-loaded graphitable carbon hollow spheres as anode materials for lithium-ion battery. J Power Sources 2008;177:546–52.
Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles Q1
Jae-Chan Kim, In-Sung Hwang, Seung-Deok Seo, Dong-Wan Kim n Department of Materials Science and Engineering, Ajou University, Suwon 443-749, Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 7 February 2013 Accepted 5 April 2013
Nanocomposite electrodes of cobalt (II) oxide nanoparticles (CoO NPs)/multiwalled carbon nanotubes (MWCNTs) were fabricated by a two-step process consisting of electrophoretic deposition followed by chemical vapor deposition. Results of the electrochemical analysis demonstrate that the CoO NP-bearing MWCNT electrodes could deliver reversible charge capacities of 600 and 550 mA h g−1 after 50 and 100 cycles, respectively, at the rate of 715 mA g−1. The superior cyclability is attributed to the unique selfsupported one-dimensional structure of the MWCNTs and the uniformly dispersed CoO NPs that constitute these nanocomposite electrodes. & 2013 Published by Elsevier B.V.
Keywords: Cobalt oxide Carbon nanotube Nanocomposites Anode Lithium ion batteries
1. Introduction Transition metal oxides have attracted much attention in the past decade as anode materials for Li-ion batteries because of their high theoretical capacities, which are greater than that of commercial graphite (at 372 mA h g−1) [1,2]. These transition metal oxides including Co(II)O react with lithium by a conversion reaction to form cobalt nanocrystals that are well dispersed in the Li2O matrix [2]. This process inevitably causes drastic volume variations in the matrix, resulting in severe capacity fading during the cycling process [3]. It is well recognized that electrode materials with predesigned nanostructures could accommodate the volume change, and wellincorporated active materials in nanocomposite electrodes could offer more stable cyclability [4,5]. Among the various reports, Du et al. have demonstrated that anodes composed of carbon nanotube (CNT)–transition metal oxide composites could react with lithium effectively because of the synergistic action of the metal oxides with the unique properties of CNTs (including strong mechanical properties, excellent electronic conductivities, and large surface areas) [6]. Moreover, nanoparticles could retain their sizes, and agglomeration could be prevented because of their tight anchoring by the CNTs. Extensive efforts have been taken toward exploiting the advantages offered by metal oxide–CNT composites, and these composites have been synthesized by various methods like hydrothermal processing, chemical vapor deposition (CVD), ball-milling, and wet chemical processing [7–10].
Q4
n
Corresponding author. Tel.: +82 31 219 2468; fax: +82 31 219 3248. E-mail addresses: [email protected], [email protected] (D.-W. Kim).
Herein, we report the fabrication of heterostructured CoO nanoparticle (NP)/multiwalled CNT (MWCNT) anodes free of binders. These nanocomposite electrodes were found to exhibit high capacity delivery with good cycle retention.
2. Experimental The CoO NP/MWCNT composites were synthesized by a simple two-step process. First, the MWCNTs (from Hanwha Nanotech Co. Ltd.) were deposited electrophoretically on stainless steel (SS) substrates by a previously reported procedure [11]. Then, CoO NPs were decorated on the surface of the MWCNTs by a CVD process (Supplementary information). Field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL), transmission electron microscopy (TEM, JEM-2100F, JEOL), X-ray diffraction (XRD, Miniflex II, Rigaku), and X-ray photoelectron spectroscopy (XPS, ESCA2000, VG Microtech) were carried out to determine the phase, structure, and morphology of the CoO NP/MWCNT nanocomposites. The masses of the samples SS substrate, MWCNTs on SS, and CoO NPs/MWCNT on SS were accurately measured at each step using a microbalance (mass resolution¼ 0.1 μg, UTM5, Mettler Toledo). For the electrochemical evaluation of the CoO NP/MWCNT nanocomposite electrodes, we assembled a Swagelok-type cell containing a separator film (Celgard 2400), a liquid electrolyte (1 M LiPF6 EC/DMC, 1:1 v/v), and a lithium metal foil (counter electrode). Galvanostatic cycling was conducted at the rate of 715 mA g−1 in the voltage range 0.01–3.00 V using an automatic cycler (WBCS 3000, Wonatech). Cyclic voltammetry (CV) was performed in the voltage range 0.01–3.00 V with a scan rate of 0.3 mV s−1. The average CoO NPs:MWCNTs ratio was maintained at 1:0.9 by mass in each step.
0167-577X/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2013.04.013
Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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The amount of carbon nanotube relative to the total weight of composite was ∼42 % based on the thermogravimetric analysis in Fig. S1, which was in good agreement with the mass ratio measured using a microbalance. The loading amount of the CoO NPs/MWCNT on SS was found to be 265 μg cm−2. The specific capacities were calculated based on the total mass of CoO NPs and MWCNTs.
3. Results and discussion Fig. 1a shows the typical XRD pattern acquired from the CoO NP/ MWCNT composites. The characteristic (002) reflection corresponding to the stacking of graphene layers in the MWCNTs was detected at 2θ of about 25.81. The peaks at 36.41, 42.41, and 61.61 (2θ) could be indexed to reflections from (111), (200), and (220) planes of pure CoO with a cubic structure (Fm3m, JCPDS #48-1719), respectively. The considerable peak broadening and reduced intensity observed confirm the nanocrystalline nature and the ultrafine crystallite size of CoO. The deposition of CoO NPs was carried out in an atmosphere of Ar. It is believed that this relatively reducing atmosphere led to preferable formation of CoO, and not Co3O4. In order to provide a clear evidence of the presence of CoO phase, the analysis was conducted in detail by XPS. A magnified view of the Co 2p peak obtained from the CoO NP/MWCNT composite is shown in Fig. 1b. The peaks corresponding to
Co 2p3/2 and Co 2p1/2 were located at 781.3 and 796.7 eV, respectively. The distance between the peaks (ΔE2) was 15.4 eV, which indicated the +2 oxidation state of Co [2]. Moreover, the shake-up satellite peak at 787.2 eV was positioned at a binding energy of 6 eV (ΔE1) above the Co 2p3/2 peak. We could confirm that pure CoO was present in the absence of pure Co, Co2O3, and Co3O4 impurities in the nanocomposite samples, which was in agreement with the results obtained from the XRD analysis. Fig. 2a shows the FESEM image of the surface of the electrophoretically deposited MWCNTs on the SS substrate. It can be observed that most MWCNTs assembled and networked homogeneously with negligible aggregation. After the deposition of CoO on the MWCNTs, the uniform coverage by tiny NPs was clearly observed and irregularly particles were absent (Fig. 2b). The simplified illustrations of an individual MWCNT and a MWCNT decorated CoO NPs are provided in the insets of Fig. 1a and b, respectively. Fig. 3a and b shows the typical bright-field TEM images of the CoO NPs/MWCNTs at low and high magnifications, respectively. Most individual MWCNTs were distinctly found to be covered by the NPs (Fig. 3a). The NPs exhibited a mean diameter of ∼13 nm (Fig. 3b). The features in the fast Fourier transform (FFT) patterns acquired from the nanocomposites shown in Fig. 3c could be assigned to the (111), (200), and (222) reflections of the face-centered cubic structure of CoO. The distinct crystalline lattice with an interplanar distance of 0.248 nm (Fig. 3d) matched well with the lattice parameter of the
Fig. 1. The typical (a) XRD pattern and (b) Co 3p peak in the XPS profile acquired from CoO NP/MWCNT nanocomposite samples.
Fig. 2. SEM images of (a) electrophoretically deposited MWCNTs and (b) MWCNTs decorated with CoO NPs. Insets of (a) and (b) show schematic illustrations of MWCNT and CoO NP/MWCNT nanocomposites on SS substrates, respectively.
Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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(111) plane of cubic CoO, which was consistent with the results obtained from the XRD and XPS analyses. It is believed that cobalt precursor preferentially sticks to the structural defects on acidfunctionalized MWCNTs during the early stages of the CVD process to create CoO-based seeds, on which CoO particles form by isotropic growth. As an anode material for lithium ion batteries, the CoO NP/ MWCNT composite demonstrated a superior lithium storage performance. Cyclic voltammograms showed the lithium reactivity of the CoO NP/MWCNT nanocomposite. Two strong peaks at 0.45 and 0.1 V were observed in the first cathodic scan, corresponding to the conversion of CoO to Co (following the reaction CoO +2Li++2e−- Co+Li2O) and the formation of an irreversible solid electrolyte interface (SEI) layer, respectively [1,2]. After this first discharge, a good cycle performance of the CoO NP/MWCNT nanocomposites was confirmed by subsequent CV studies. In the galvanostatic cycling curve between 0.01 and 3 V at a charge/discharge current density of 715 mA g−1 (at the rate of 1 C, based on the theoretical capacity of CoO, 715 mA h g−1), the
3
voltage dropped from the open-circuit potential to a plateau value of ∼0.5 V and then decreased slowly to 0.01 V, corresponding to the reduction of Co2+ to Co0, as indicated in the cyclic voltammogram furnished in Fig. 4a. The initial discharge and charge capacities of the nanocomposites were measured as 1250 mA h g−1 and 790 mA h g−1, respectively, accounting for an initial Coulombic efficiency of 63%, which is common for most metal oxide anode materials. The initial capacity loss may be mainly attributed to irreversible processes such as the inevitable formation of the SEI layer and electrolyte decomposition [12]. After further cycling, the specific capacities of CoO NP/MWCNT nanocomposite samples were measured at 10th and 50th cycles. The electrodes with different compositions of CoO/MWCNTs could be obtained by controlling the deposition time. We also evaluated the cycle performance of each electrode at the same current density (Fig. S2). A high reversible capacity of 550 mA h g−1 (based on the total mass of CoO NPs and MWCNTs) was obtained even after 100 cycles, which is much higher than values reported for commercial carbon-based anodes. The capacity of ∼350 mA h g−1
Q3 Fig. 3. (a) TEM, (b) HRTEM, and (c) FFT patterns acquired from MWCNTs decorated with CoO NPs. (d) An enlarged image of red square shown in (b) clearly exhibiting the lattice planes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. The typical (a) cyclic voltammograms and (b) charge–discharge curves of the CoO NP/MWCNT nanocomposite anodes.
Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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was observed in Co3O4 NPs/MWCNT composite electrode (60 wt% of MWCNT) at a current rate of 45 mA g−1 during 14 cycles by Shan and Gao [13]. It was also reported that 50 wt% CoO-loaded carbon hollow spheres delivered the capacities of 535 and 420 mA h g−1 at 160 and 800 mA h g−1, respectively [14]. Therefore, the electrochemical performance of composite electrodes in this work is also superior to other reported cobalt oxide/CNT nanocomposite anodes. The characteristics of the CoO NPs/MWCNTs electrodes can offer a reasonable explanation for the large reversible capacity and good cycling retention. First, the good adhesion between the conductive MWCNT networks, which are directly connected to the SS current collector and the CoO NPs could facilitate the transportation of electrons and lithium ions. Next, the tiny CoO NPs uniformly dispersed on the MWCNTs could provide functional spaces for buffering the volume variation and thus accommodate the strain during repeated lithiation and delithiation [11]. Our results suggest that the CoO NP/MWCNT composites are effective for overcoming the drawbacks of transition metal oxide-based anodes and show promise for application in practical lithium ion batteries. 4. Conclusions We prepared CoO NP/MWCNT nanocomposites by the direct anchoring of CoO on the surface of electrophoretically predeposited MWCNT networks via a CVD process. The CoO NP/MWCNT nanocomposite anodes that were free of binders displayed a large reversible capacity of over 550 mA h g−1 at a high rate of 715 mA g−1 even after 100 cycles. The enhancement in the electrochemical performance could be attributed to the uniform distribution of tiny CoO NPs (13 nm in diameter), the existence of highly conductive MWCNT networks, and the creation of favorable open spaces that buffer the huge volume change during lithiation and delithiation. Acknowledgment This research was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (20090094046, 2012M1A2A2671802 and 2012R1A2A2A01045382).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.04.013.
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Please cite this article as: Kim J-C, et al. Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.013i
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