5336
J. Phys. Chem. C 2009, 113, 5336–5339
Carbon/ZnO Nanorod Array Electrode with Significantly Improved Lithium Storage Capability Jinping Liu,* Yuanyuan Li, Ruimin Ding, Jian Jiang, Yingying Hu, Xiaoxu Ji, Qingbo Chi, Zhihong Zhu, and Xintang Huang* Center for Nanoscience and Nanotechnology, Department of Physics, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: January 15, 2009; ReVised Manuscript ReceiVed: February 16, 2009
Carbon/ZnO nanorod arrays on nickel substrate have been fabricated over a large area by the simple carbonization of preadsorbed glucose on ZnO arrays at 500 °C in argon gas. The uniform coating of average 6 nm carbon shell on ZnO nanorod surface is confirmed. The novel array architecture possesses both the electroactivity of carbon and the electrochemical advantages of array structure on conductive substrate. When used as anode for Li ion batteries, it displays significantly improved performance in terms of cycling stability and rate capability. The observed lithium storage ability ranges among the best reported to date for ZnObased anode. We believe that the novel carbon-coating route is general and can be extendable to other metal oxide nanoarray electrodes. Introduction There is currently a great interest in developing green technologies to power high efficiency plug-in hybrid electric vehicles (PHEVs).1 Li ion batteries (LIBs) have emerged as one of the most promising candidates due to their high energy density and long lifespan.1-4 Metal oxides have long been known as attractive Li insertion alternatives to graphite anode.5 Downsizing these materials to a nanoscale level is believed to be capable of increasing the active reaction sites and accelerating the electron/Li ion transport.6 Therefore, much effort has been devoted to explore the Li storage capability of various metal oxide nanostructures and study the structure-electrochemistry relationships.7-11 ZnO, as a versatile functional material,12 is a well-known Li insertion compound but was investigated only during the early years of Li battery research.13-16 Bulk ZnO generally suffers from poor kinetics and severe capacity fade upon cycling, even at low rates.13 For nanosized ZnO, the loss of electrical contact arising from the volume expansion-contraction during the charge-discharge process is still a serious problem, even though reducing the size could limit the volume change to some extent.14,15 Accordingly, high reversible capacity and rate capability of ZnO material have never been demonstrated.17 Also, little consideration on the improvement of ZnO anode performance has been made to date. We reported recently that the in situ incorporation of homogeneous ZnAl2O4 buffer into ordered ZnO nanosheets can significantly improve the reversible capacity.17 However, the rate capability remains limited by the relativity poor electronic conductivity of ZnO. Coating highly crystalline ZnO nanostructures with conductive carbon would be an effective means of eliminating this problem because of carbon’s unique electrochemical advantages.18-20 Here, we present a nanostructured electrode made up of vertically aligned carbon-coated ZnO nanorods on Ni foil and demonstrate its use in LIBs. The benefits of using one* To whom correspondence should be addressed. Fax: +86-02767861185. E-mail:
[email protected] (J.L.);
[email protected] (X.H.).
dimensional (1D) nanostructure array in battery devices are greatly magnified by the introduction of conductive carbon shell.The novel array architecture possesses both the electroactivity of carbon and the electrochemical advantages of array structure on conductive substrate.To our knowledge, this is the first report on the application of carbon decorated nanorod array electrode in LIBs. Our work presents a new concept for optimizing the structure of LIB electrodes. Experimental Section Pristine ZnO arrays on Ni foil were prepared as described elsewhere.21 One side of the foil was protected from solution by uniformly coating with rubberized fabric, designed for electric contacting. For the synthesis of carbon/ZnO nanorod array, Ni foil-supported pristine ZnO arrays were immersed into a 50-mL glucose aqueous solution (0.3 M) for 15 h. The free space between neighboring nanorods would allow easy adsorption of glucose molecules onto the nanorod surface. After this, the foil was taken out, dried at 60 °C, and further annealed in Ar gas at 500 °C for 5 h to allow the carbonization of glucose (Figure 1a). We chose 500 °C as the carbonization temperature because at this temperature the generated carbon could be partially graphitizd to benefit the LIB performance and the deleterious interfacial reaction between ZnO and Ni foil could also be avoided. Since the carbon-coating process is simple and limitation-free, it is general and can be extended to nanoarray electrodes of some other metal oxides such as SnO2. Product was characterized using powder X-ray diffraction (XRD) (Bruker D-8 Avance), transmission electron microscopy (TEM) (JEM-2010FEF, 200 kV), scanning electron microscopy (SEM) (JSM-6700F, 5.0 kV), and Raman spectroscopy (Witech CRM200, 532 nm). Thermogravimetric analysis (TGA) was carried out on an SDT600 apparatus with a heating rate of 10 °C min-1 in air. The Swagelok-type battery was assembled in an Ar-filled glovebox (Mbraun, Unilab, Germany) by directly using the carbon/ZnO array (∼1.1 mg) on Ni as the anode (ZnO arrays on one side of the Ni foil were removed for electrical contacting), a Li metal circular foil (0.59 mm thick, 14 mm
10.1021/jp900427c CCC: $40.75 2009 American Chemical Society Published on Web 03/11/2009
Carbon/ZnO Nanorod Array Electrode
J. Phys. Chem. C, Vol. 113, No. 13, 2009 5337
Figure 1. (a) Schematic diagram of the fabrication process of carbon/ZnO arrays. (b) SEM image of the arrays. Inset is an optical image of the electrode. (c) XRD and (d) Raman spectrum of the carbon/ZnO nanorod arrays.
Figure 2. (a) HRTEM image and SAED pattern of an individual carbon/ZnO nanorod. The circle indicates the presence of carbon shell. (b) TEM image of a single nanorod with uniform and continuous carbon coating.
diameter) as the counter and reference electrodes, a microporous polypropylene membrane as the separator, and 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as the electrolyte. For the comparative study of the electrochemical performance, ZnO random nanorods were also prepared. The random nanorods were obtained by scraping ZnO arrays from the Ni substrate. For electrochemical test of the sample, electrode was made in the conventional way: a slurry was first obtained by thoroughly mixing 75 wt % ZnO disordered nanorods, 15 wt % carbon black, and 10 wt % polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidene (NMP) solvent. The slurry was then casted onto the Ni surface at room temperature and further dried in vacuum overnight at 100 °C. Before being used, the film electrode was cut into circular disks of 14 mm in diameter and pressed between two stainless steel plates at 20 MPa. The discharge-charge cycling
was performed at room temperature by using a multichannel battery tester (model SCN, USA). The capacity values were calculated based on the mass of active carbon/ZnO. Results and Discussion Figure 1b shows the representative SEM image of the product. The nanorods have sharp tips of less than 50 nm and are separated apart sufficiently. Inset in Figure 1b is an optical image of 14 mm sized carbon/ZnO array electrode. Peaks in XRD pattern (Figure 1c) can be well indexed to c-axis-oriented ZnO expect for those from Ni substrate.21 Figure 1d depicts the Raman spectrum of the array, which shows two obvious peaks located at 1340 (D-band) and 1588 cm-1 (G-band), confirming the presence and partial graphitization of carbon.22 TGA analysis confirms that the carbon content is ∼3.0 wt %. The characteristic peak of ZnO at 438 cm-1 is also observed. The decoration of
5338 J. Phys. Chem. C, Vol. 113, No. 13, 2009
Figure 3. Cycling performances of carbon/ZnO array (circle), pristine ZnO array (star), and disordered ZnO nanorods (triangle) at 0.25 C (hollow symbols, discharge; solid symbols, charge). Inset is the voltage/ capacity curve of carbon/ZnO array for the first and second cycles. (b) Rate performance of the carbon/ZnO array. (c) Cycling stability of carbon/ZnO array at 0.75 C. Inset shows the SEM image of carbon/ ZnO array after cycled at 0.75 C for 50 times.
carbon on the surface of single-crystalline ZnO nanorod is evidenced by the high-resolution TEM (HRTEM), as shown in Figure 2a and its inset. The average thickness of carbon shell is ∼6 nm. The uniform and continuous coating of carbon is further confirmed in Figure 2b (see arrows). The carbon/ZnO nanorod array electrode was examined with Li ion insertion and extraction to demonstrate its electrochemical performance in energy storage (Figure 3a). The assembled cell was cycled between 0.05 and 2.5 V at room temperature with a current rate of 0.25 C (C was defined as 3 Li+ per hour; 987.8 mA g-1).17 The shape of the voltage/capacity curve (inset in Figure 3a) shows that carbon/ZnO nanorod array electrode reacts, as expected,13-16 toward Li via a conversion reaction
Liu et al. (plateau at 0.5 V, formation of Zn and Li2O13) and subsequent alloy/dealloy process. The first discharge and charge capacities are 1150 and 640 mAh g-1, respectively, with an initial Coulombic efficiency of 55.7%. Although there is a large irreversible capacity ratio (44.3%) due to the irreversible conversion reaction, our first Coulombic efficiency is still higher than those reported before.13-17 We also note that carbon/ZnO electrode delivers reversible capacities of ∼500 and 330 mAh g-1 after 20 and 50 cycles, respectively (Figure 3a). In contrast, even with the first discharge capacities above 1000 mAh g-1, capacities larger than 300 mAh g-1 could not be retained after only 10 cycles for previously reported ball-milled nanoparticles15 and disordered nanorods.16 To further demonstrate the advantages of carbon-decorated array electrode, anodes composed of pristine ZnO nanorod array and random nanorod film were prepared and tested separately. The pristine ZnO nanorod array exhibits better cycling performance than random nanorods, but both show rapid capacity fading as compared to carbon/ZnO array. The charge capacities of pristine ZnO nanorod array and random nanorods after 10 cycles are only 440 and 205 mAh g-1, respectively. Another remarkable feature of the carbon/ZnO array electrode is its drastically improved rate capability. High rate capability is important for many applications of batteries such as electric vehicles and portable powers.7 Figure 3b displays rate performance of the carbon/ZnO array electrode at 0.35, 0.75, and 2 C rates. The charge capacities at these rates are 550, 471, and 385 mAh g-1, respectively. The reversible capacity at 2 C is still larger than that of graphite (372 mAh g-1). Under 0.75 C cycling, the carbon/ZnO electrode exhibits discharge and charge capacities of 360 and 324 mAh g-1, respectively, after 30 cycles, as shown in Figure 3c. The charge capacity retention after 30 cycles at 0.75 C is 68.8%, comparable to that cycled at 0.25C for 30 times. On the contrary, pristine ZnO array and disordered nanorods deliver very limited capacities (below 300 mAh g-1) at higher rates even after five cycles. During the battery test, Zn nanograins migrate through the Li2O matrices faster than other metal nanoparticles such as Sn upon the volume variation and therefore aggregate much faster.23 This leads to severe electrode pulverization and finally a thorough loss of electrical contact, as observed for conventional ZnO electrodes.13-15 Although additive buffers could limit the volume change,17 the whole electrical conductivity of the active materials remains poor, not beneficial to rapid ionic and electronic diffusion. In our work, the carbon shell on the ZnO nanorods is believed to have two major roles: First, carbon itself is an electronic conductor,18 which ensures good electrical contact of ZnO with the current collector and enhances the charge transfer/Li+ transport. With the full and uniform coating of carbon, electrons can easily reach all the positions where Li+ ion intercalation takes place.19a This feature is particularly helpful when the battery is cycled at high currents. Second, carbon has enough mechanical strength to act as structural buffer.20 Thus, the presence of carbon shell on the nanorod surface would effectively alleviate the strains caused by the volume variation of ZnO nanorod cores and prevent the disintegration. As confirmed, we observed that even after 50 cycles at 0.75 C the carbon/ZnO electrode preserved the array configuration (inset in Figure 3c). However, pristine ZnO array could only maintain the array structure for less than 25 cycles; as shown in Figure 4, large volume change during the discharge-charge process fully destroyed the array structure. For carbon/ZnO, the preservation of the array structure during the Li+ insertion/extraction processes helps to keep the electrical
Carbon/ZnO Nanorod Array Electrode
Figure 4. SEM image of the pristine ZnO nanorod array electrode after 25 cycles.
continuity, leading to better cycling stability. In addition, carbon is a very stable electroactive anode material for LIBs. The solid electrolyte interphase (SEI) film on carbon surface was reported to be relatively stable as compared to that on bare transition metal oxides.18b,24 The stable SEI film will also be beneficial for the maintenance of inner ZnO array structure. We further point out that direct growth of nanorod arrays on current collectors has additional electrochemical advantages.9,11b,25 Those include, for example, robust mechanical adhesion and electrical contact between nanorods and substrate, sufficient electrochemical reaction of every nanorod, and the direct electron transport (without pass of detrimental grain boundaries25) along the length direction to the current collector. All these contribute to the improved battery performance. Conclusions In summary, the simple synthesis and battery application of carbon/ZnO nanorod array on Ni substrate have been investigated. The carbon layer on ZnO nanorod surfaces not only improves the conductivity but also acts as a structural buffer. As a result, the electrode architecture exhibits substantially superior performance over previously reported ZnO anodes. Our work offers exciting opportunities for the development of new anode materials for LIBs by optimizing the electronic and physical properties of materials. Acknowledgment. We gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 50872039 and 50802032). References and Notes (1) Kang, K.; Meng, Y. S.; Berger, J.; Grey, C. P.; Ceder, G. Science 2006, 311, 977. (2) (a) Hassoun, J.; Reale, P.; Scrosati, B. J. Mater. Chem. 2007, 17, 3668. (b) Lv, R. T.; Zou, L.; Gui, X. C.; Kang, F. Y.; Zhu, Y. Q.; Zhu,
J. Phys. Chem. C, Vol. 113, No. 13, 2009 5339 H. W.; Wei, J. Q.; Gu, J. L.; Wang, K. L.; Wu, D. H. Chem. Commun. 2008, 2046. (c) Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, Z. X.; Lim, C. T.; Subba Rao, G. V.; Chowdari, B. V. R. AdV. Funct. Mater. 2007, 17, 2792. (d) Li, Y. M.; Li, J. H. J. Phys. Chem. C 2008, 112, 14216. (3) (a) Shen, J. M.; Feng, Y. T. J. Phys. Chem. C 2008, 112, 13114. (b) Kim, H.; Cho, J. J. Mater. Chem. 2008, 18, 771. (c) Cho, J. J. Mater. Chem. 2008, 18, 2257. (d) Park, M.-S.; Kang, Y.-M.; Dou, S.-X.; Liu, H.K. J. Phys. Chem. C 2008, 112, 11286. (e) Cakan, R. D.; Titirici, M.-M.; Antonietti, M.; Cui, G. L.; Maier, J.; Hu, Y.-S. Chem. Commun. 2008, 3759. (4) (a) Wu, C. Z.; Hu, Z. P.; Wang, W.; Zhang, M.; Yang, J. L.; Xie, Y. Chem. Commun. 2008, 3891. (b) Murugan, A. V.; Muraliganth, T.; Manthiram, A. J. Phys. Chem. C 2008, 112, 14665. (c) Ren, M. M.; Zhou, Z.; Gao, X. P.; Peng, W. X.; Wei, J. P. J. Phys. Chem. C 2008, 112, 5689. (5) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nature 2000, 407, 496. (6) Arico`, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (7) Nam, K. T.; Kim, D.-W.; Yoo, P. J.; Chiang, C-Y.; Meethong, N.; Hammond, P. T.; Chiang, Y.-M.; Belcher, A. M. Science 2006, 312, 885. (8) Guo, Y.-G.; Hu, Y.-S.; Maier, J. Chem. Commun. 2006, 2783. (9) Taberna, P. L.; Mitra, S.; Poizot, P.; Simon, P.; Tarascon, J.-M. Nat. Mater. 2006, 5, 567. (10) (a) Lou, X. W.; Deng, D.; Lee, J. Y.; Archer, L. A. Chem. Mater. 2008, 20, 6562. (b) Lou, X. W.; Deng, D.; Lee, J. Y.; Archer, L. A. J. Mater. Chem. 2008, 18, 4397. (c) Lou, X. W.; Deng, D.; Lee, J. Y.; Feng, J.; Archer, L. A. AdV. Mater. 2008, 20, 258. (d) Wang, Y.; Cao, G. Z. J. Mater. Chem. 2007, 17, 894. (11) (a) Du, N.; Zhang, H.; Chen, B. D.; Wu, J. B.; Ma, X. Y.; Liu, Z. H.; Zhang, Y. Q.; Yang, D. R.; Huang, X. H.; Tu, J. P. AdV. Mater. 2007, 19, 4505. (b) Li, Y. G.; Tan, B.; Wu, Y. Y. Nano. Lett. 2008, 8, 265. (c) Du, N.; Zhang, H.; Chen, J. E.; Sun, J. Y.; Chen, B. D.; Yang, D. R. J. Phys. Chem. C 2008, 112, 14836. (12) (a) Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. (b) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829. (c) Shen, G. Z.; Bando, Y.; Liu, B. D.; Golberg, D.; Lee, C. J. AdV. Funct. Mater. 2006, 16, 410. (13) Li, H.; Huang, X.; Chen, L. Solid State Ionics 1999, 123, 189. (14) Wang, J.; King, P.; Huggins, R. A. Solid State Ionics 1986, 20, 185. (15) Belliard, F.; Irvine, J. T. S. J. Power Sources 2001, 97-98, 219. (16) Zheng, Z. F.; Gao, X. P.; Pan, G. L.; Bao, J. L.; Qu, J. Q.; Wu, F.; Song, D. Y. Chinese J. Inorg. Chem 2004, 20, 488. (17) Liu, J. P.; Li, Y. Y.; Huang, X. T.; Li, G. Y.; Li, Z. K. AdV. Funct. Mater. 2008, 18, 1448. (18) (a) Dimov, N.; Kugino, S.; Yoshio, M. Electrochim. Acta 2003, 48, 1579. (b) Zhang, W.-M.; Wu, X.-L.; Hu, J.-S.; Guo, Y.-G.; Wan, L.-J. AdV. Funct. Mater. 2008, DOI: 10.1002/adfm.200801386. (c) Kim, H.; Cho, J. Nano Lett. 2008, 8, 3688. (d) Cui, G. L.; Gu, L.; Zhi, L. J.; Kaskhedikar, N.; van Aken, P. A.; Mullen, K.; Maier, J. AdV. Mater. 2008, 20, 3079. (19) (a) Ng, S. H.; Wang, J.; Wexler, D.; Konstantinov, K.; Guo, Z. P.; Liu, H. K. Angew Chem., Int. Ed. 2006, 45, 6896. (b) Kwon, Y.; Cho, J. Chem. Commun. 2008, 1109. (c) Zhang, W.-M.; Hu, J.-S.; Guo, Y.-G.; Zheng, S.-F.; Zhong, L.-S.; Song, W-.G.; Wan, L.-J. AdV. Mater. 2008, 20, 1160. (20) Cui, G. L.; Hu, Y.-S.; Zhi, L. J.; Wu, D. Q.; Lieberwirth, I.; Maier, J.; Mullen, K. Small 2007, 12, 2066. (21) Liu, J. P.; Huang, X. T.; Li, Y. Y.; Ji, X. X.; Li, Z. K.; He, X.; Sun, F. L. J. Phys. Chem. C 2007, 111, 4990. (22) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61, 14095. (23) Connor, P. A.; Belliard, F.; Behm, M.; Tovar, L. G.; Irvine, J. T. S. Ionics 2002, 8, 172. (24) Aurbach, D.; Levi, M. D.; Levi, E.; Schechter, A. J. Phys. Chem. B 1997, 101, 2195. (25) Chan, C. K.; Peng, H.; Liu, G.; Mcilwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. Nanotechnol. 2008, 3, 31.
JP900427C
