Electrochimica Acta 54 (2009) 2851–2855
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Evaluation of ZnO nanorod arrays with dandelion-like morphology as negative electrodes for lithium-ion batteries Hongbo Wang a , Qinmin Pan a,∗ , Yuexiang Cheng a , Jianwei Zhao b , Geping Yin a a b
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210008, PR China
a r t i c l e
i n f o
Article history: Received 7 October 2008 Accepted 12 November 2008 Available online 21 November 2008 Keywords: ZnO nanorod arrays Dandelion-like morphology Hydrothermal synthesis Negative electrodes Lithium-ion batteries
a b s t r a c t In this study, ZnO nanorod arrays have been evaluated for the negative electrodes of lithium-ion batteries. The ZnO nanorod arrays with dandelion-like morphology were directly grown on copper substrates by a hydrothermal synthesis process at 80 ◦ C. X-ray diffraction, scanning electron microscopy, galvanostatic discharge–charge, and cyclic voltammetry were employed to characterize the structure and electrochemical property of the arrays. The array electrodes showed a stable capacity over 310 mAh g−1 after 40 cycles, and good capacity retention as the anodes of lithium-ion batteries. It was believed that the unique dandelion-like binary-structure played an important role in the electrochemical performance of the array electrodes. The present ﬁnding opens the possibility to fabricate micro/nanometer hierarchical ZnO ﬁlms that might be applied in lithium-ion batteries. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Transition metal oxides (MO) (M = Co, Ni, Cu, Fe, etc.) have been attracted considerable interest among lithium-ion battery experts because they exhibited higher reversible capacities and better safety compared to the conventional carbonaceous materials [1–4]. It is now established that crystallinity, particle size, and morphology play signiﬁcant roles in the electrochemical performance of MO anode materials. Therefore, nanostructured MO with various dimensions and morphologies had been investigated as anode materials as they could provide large electrolyte/electrode contact area and short diffusion path for Li ions and electrons [5,6]. ZnO has a theoretical capacity of 978 mAh g−1 , but it has rarely been used as anode materials in lithium-ion batteries because ZnO showed poor cycleability compared with other transition metal oxides. It is believed that low electronic conductivity and large volume-change during lithium insertion–extraction process are responsible for the poor electrochemical performance of ZnO electrode [8,9]. Despite some effort had been devoted to overcome the problems, slight improvement had been achieved.
∗ Corresponding author. Tel.: +86 451 8641 3721 fax: +86 451 8641 4661. E-mail address: [email protected]
(Q. Pan). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.11.019
It had been proven that controlling the morphologies of transition metal oxides could improve their electrochemical performance such as cycling characteristics and rate capability . For example, Chen et al. revealed that dandelion-like CuO microspheres exhibited high reversible capacity and good cycleability . Our previous work also demonstrated that ﬂower-like morphology was favorable to lithium insertion and extraction within CuO electrode [12,13]. Therefore, it is necessary to explore the possibility to overcome the above-mentioned problems associated with ZnO electrode by introducing various nanostructures. Recently, self-supported nanowire (NW) arrays grown directly on a current-collector represent an attractive architecture for Li ion battery electrode [2,14–17]. The array-electrode not only share the advantages of other nanostructured electrodes such as large electrolyte/electrode contact area, fast Li ion diffusion, and good strain accommodation [18–20], but also allow easy diffusion of electrolyte into the inner region of the electrode, direct contact between nanowires and current-collector. As a result, the nanowire array-electrode showed reduced internal resistance and improved high-power-density. To the best of our knowledge, however, no study reports ZnO nanowire or ZnO nanorod arrays for lithium-ion batteries. Moreover, the procedures for nanowire arrays fabrication involved the utilization of template, which limit their large-scale production. Here, we propose a simple template-free method for the large-scale fabrication of ZnO nanorod arrays with dandelion-like
H. Wang et al. / Electrochimica Acta 54 (2009) 2851–2855
Fig. 1. SEM images of a typical ZnO nanorod array, (a) ×500, (b) ×2000 and (c) a magniﬁed view on microspheres.
morphology on copper substrates. The goal is achieved by immersing zinc-coated copper plates in an ammonia solution at 80 ◦ C. We demonstrate that the ZnO nanorod arrays exhibit desirable electrochemical performance as the anodes of lithium-ion batteries. 2. Experimental The fabrication of ZnO nanorod arrays on copper plates was carried out as follows. At ﬁrst, zinc ﬁlms were coated on copper plates by cathodic deposition from an aqueous solution containing 65 g L−1 ZnCl2 , 190 g L−1 KCl, 30 g L−1 H3 BO3 , brighteners 921A 32 mL L−1 and 921B 2 mL L−1 (Xiamen Honggong Chem. Ltd., China). A sheet of pure zinc plate (99.9%) was used as the active anode. The electroplating was performed at a current density of 1.0 A dm−2 at room temperature for 140 s. After electrodeposition, the copper plates were washed with deionized water and dried in air . The thickness of zinc ﬁlms was about 0.65 m. Then the zinc-coated copper plates were immersed in a 80 mL aqueous solution containing 1.0 g ammonia (NH4 OH, 25%) in Teﬂon-lined stainless steel autoclaves (100 mL) followed by heating at 80 ◦ C for 4 h . After the hydrothermal reaction, the resulting copper plates were rinsed with deionized water and dried under vacuum. The preparation of bulk ZnO was carried out according to the procedure described in the literature .
The ZnO nanorod arrays were cut into 1.1 cm circular plates in diameter and used as the negative electrodes of lithium-ion batteries. Lithium foils were used as the counter and the reference electrodes. The electrolyte solution was 1.0 M LiPF6 in EC/DMC (1:1 by volume). Coin cells were assembled in a glovebox ﬁlled with argon. The electrochemical performance of the array electrodes was evaluated by galvanostatic discharge–charge measurement using a computer-controlled battery tester between 0 and 3.0 V. Cyclic voltammograms (CVs) were recorded on a CHI604 potentiostat at a scan rate of 0.5 mV s−1 . All the potentials indicated here were referred to the Li/Li+ electrode potential. Scanning electron microscopy (SEM) images were obtained with a QUANTA200 (FEI) scanning electron microscope. X-ray diffraction (XRD) analysis was carried out by using Philips PW-1830. 2.1. Determination of ZnO nanorods weight Before cell-assembly, the obtained ZnO nanorod arrays were dried in vacuum and weighed. After discharge–charge measurement, the cells were disassembled and the ZnO electrodes were peeled. Then the electrodes were washed successively in 1.0 M HCl solution, deionized water and acetone. After being dried in vacuum, the resulted plates were weighed. The weight difference between
H. Wang et al. / Electrochimica Acta 54 (2009) 2851–2855
Fig. 2. XRD pattern of the ZnO nanorod arrays synthesized in this study.
the original plates and those washed after drying was estimated to be the mass of ZnO nanorod arrays. 3. Results and discussion Fig. 1 shows SEM images of a synthesized ZnO nanorod array. It is observed that the copper surface is completely coated with dandelion-like microspheres of 4–7 m in diameter, exhibiting a highly porous and ﬂocky appearance. The high magniﬁcation image in Fig. 1 shows that the microspheres are built from quasi-oriented nanorods of few tens nanometers in diameter, indicating the presence of hierarchical structures on both micrometer (microspheres) and nanometer (nanorods) scales. The unique dandelion-like architecture will ensure that every nanorod is in contact with copper substrate and also interfaced with electrolyte. In addition, the open space between neighboring nanorods will allow easy diffusion of electrolyte into the inner region of the arrays. These features are very important for a high-power-density electrode. The crystal structure of the ZnO nanorod arrays was investigated by XRD, as shown in Fig. 2. All of the diffraction peaks can be indexed as pure hexagonal phase of wurtzite-type ZnO with a crystallite size of 20–30 nm (space group P63mc, JCPDS No. 36-1451). The above results conﬁrm that dandelion-like ZnO nanorod arrays can be easily constructed via hydrothermal treatment of zinc-coated copper plates. Then the nanorod arrays were subjected to a systematic electrochemical analysis. For the purpose of comparison, the electrochemical performance of bulk ZnO electrodes was also included. Fig. 3a shows the cyclic voltammograms of a ZnO array at a scan rate of 0.5 mV s−1 . In the ﬁrst scan, there is only one strong cathodic peak at 0.15 V, which can be assigned to the reduction of ZnO into Zn and the formation of lithium–zinc alloy, and the decomposition of electrolyte and hence the growth of an organic-like-layer . The potentials of these reactions are so close that they only show a strong peak. After the initial scan, the cathodic peak splits into a weak peak at 0.35 V and a strong peak at 0.70 V, respectively. The weak peak corresponds to the decomposition of electrolyte and the formation of lithium–zinc alloy; while the peak at 0.70 V is associated with the reduction of ZnO into Zn and the generation of amorphous Li2 O. During the subsequent anodic scan, there are four different oxidation peaks in the potential range of 0–0.8 V, which can be attributed to a multi-step dealloying process of lithium–zinc alloy (LiZn, Li2 Zn3 , LiZn2 and Li2 Zn5 ) and the decomposition of the
Fig. 3. Cyclic voltammograms of (a) ZnO nanorod arrays and (b) bulk ZnO electrodes at a scan rate of 0.5 mV s−1
organic-like-layer [24,25]. In addition, a broad oxidation peak is found at 1.54 V, which may relate to the decomposition of Li2 O . In the subsequent cycles, the CV curves show very good reproducibility and almost no change in peak shape is observed, suggesting high reversibility of the ZnO nanorod arrays. In contrast, the bulk ZnO electrode shows two broad anodic peaks and a strong cathodic peak. The shape and potential of these peaks do not change signiﬁcantly during scanning process (Fig. 3b). The discharge–charge proﬁles of a ZnO nanorod array are illustrated in Fig. 4a. Generally, the plateaus on the voltage proﬁles are corresponding to the CV peaks shown in Fig. 3a. The ZnO arrayelectrode delivers an initial discharge capacity of 1461 mAh g−1 , while its charge capacity is close to 980 mAh g−1 . We also have compared the capacities of bulk ZnO electrodes (Fig. 4b). It is interesting to ﬁnd that the ZnO nanorod arrays exhibit much higher lithium storage capability than the bulk ZnO. Such enhancement of capacity is due to the large surface area and short diffusion distance offered by the nanorod arrays. It is reasonable that the large surface area of the ZnO nanorod arrays provides sufﬁcient space for lithium storage, including lithium–zinc alloy formation, the growth of the organic-like-layer  as well as interfacial interaction toward lithium . Considering that the ZnO mass obtained in our case might include the weight of trace unreacted Zn, the speciﬁc capacity for the ZnO nanorod arrays should be higher than the measured values.
H. Wang et al. / Electrochimica Acta 54 (2009) 2851–2855 Table 1 Comparison on the cycling performance of ZnO nanorod arrays and bulk ZnO electrodes. Cycling number (n)
ZnO nanorod arrays (mAh g−1 ) Bulk ZnO (mAh g−1 )
Fig. 4. Voltage proﬁles of (a) ZnO nanorod arrays and (b) bulk ZnO electrodes at a current density of 0.1 mA cm−2 .
Fig. 5 displays the dependence of discharge and charge capacities on cycling number for both the nanorod array and the bulk ZnO at a rate of 0.1 mA cm−2 . The charge capacity of the nanorod arrays decreases to 596 mAh g−1 in the second cycle and 481 mAh g−1 in the third. The degradation is slower in subsequent cycles and a stable capacity of 419 mAh g−1 will reach after the 5th cycle. The
ZnO arrays keep a capacity larger than 310 mAh g−1 even after 40 cycles, which is about 4 times higher than the stabilized capacity of the bulk ZnO (Table 1). By comparison, we can tell that the cycling performance of the ZnO nanorod arrays is much better than the bulk ZnO. The only comparable report is the nickel-coated ZnO prepared by electroless plating . Nevertheless, the present ZnO arrays are easier and simpler in fabrication procedure. These features promised them favorable electrodes for lithium-ion batteries. It is known that cycling performance of MO anodes is signiﬁcantly affected by the volume-change of active particles during lithium insertion–extraction process. If active particles could not tolerate the volume-change, they will pulverize into smaller particles and electrode is strongly polarized as a result. According to the procedure proposed for a Cr2 O3 electrode , we can estimate that the volume of a ZnO electrode will increase to 163% after lithium insertion, indicating the presence of large volumechange. Since there is no additive (e.g., carbon black or polymer binder) in the ZnO nanorod arrays, the good cycleability of the arrays should be ascribed to their unique dandelion-like morphology. At ﬁrst, the nanorod arrays can ensure that every ZnO nanorod participates in the electrochemical reaction, because each nanorod is in electric contact with Cu substrate and also interfaced with the electrolyte solution. Second, the dandelion-like microspheres enlarge the electrolyte/ZnO contact area, shorten the Li ion diffusion length in ZnO nanorod, and accommodate the strain induced by the volume-change during the lithium insertion–extraction process [11,20]. Third, it was reported that binary hierarchical structures were beneﬁcial to the mechanical stability of the nanoscale building of a self-assembly system . Owing to the ZnO microspheres are built from quasi-oriented nanorods, the binary-structure will act as barrier to the aggregation of nanorods and thus increasing the mechanical stability of the nanorod arrays. Therefore, the dandelion-like ZnO nanorod arrays combine the advantages of nanomaterials and the volume-change tolerance of microspheres, which is extremely favorable to lithium insertion and extraction. We also reveal that reaction time in the ammonia solution plays an important role in the capacity of ZnO nanorod arrays, as shown in Table 2. As the reaction time in the ammonia solution is only 1 h, the arrays’ capacity is very low. Then the capacity increases when the reaction time is prolonged. The capacity reaches its maximum value as the reaction undergoes for 3–4 h, and then the capacity decreases gradually upon increased time, indicating that excessive reaction time in the ammonia solution exerts little inﬂuence on the capacity.
Table 2 Effect of reaction time in ammonia on the capacities of the ZnO nanorod arrays.
Fig. 5. Cycling performance of ZnO nanorod arrays at a rate of 0.1 mA cm−2 .
Reaction time in ammonia solution (h)
ZnO weight (mg cm−2 )
Reversible capacity at 5th cycle (Ah cm−2 )
Reversible capacity at 5th cycle (mAh g−1 )
3 4 5.5 8 12
0.36 0.50 0.57 0.47 0.42
162.6 219.1 199.4 187.0 153.1
451.7 438.2 349.8 397.9 364.5
H. Wang et al. / Electrochimica Acta 54 (2009) 2851–2855
4. Conclusions In summary, ZnO nanorod arrays with dandelion-like morphologies were fabricated on copper plates via hydrothermal synthesis and they exhibited desirable electrochemical performance as the anodes of lithium-ion batteries. The unique hierarchical structures of the nanorod arrays combine the advantages of large electrolyte/electrode contact area, fast charge carrier diffusion, enhanced electrical contact, and good strain accommodation. Our results reveal that binary structures on both micro- and nanometer scales are favorable for improving the electrochemical performance of ZnO anode materials.
            
This work was supported by National Science Foundation of China (Grant No. 50803013).
   
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