Self-Assembly of NiO-Coated ZnO Nanorod Electrodes with Core

Jan 18, 2013 - Xiangjun Lu , An Xie , Yong Zhang , Haichang Zhong , Xuecheng ... Zengan Qi , Zengsheng Ma , Wenjuan Jiang , Renwen Hu , Jinliang Duan...
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Self-Assembly of NiO-Coated ZnO Nanorod Electrodes with Core− Shell Nanostructures as Anode Materials for Rechargeable LithiumIon Batteries Mao-Sung Wu* and Hsin-Wei Chang Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan ABSTRACT: A porous nickel hydroxide shell was selfassembled by hydrolysis of aqueous nickel chloride in the presence of hexagonal ZnO nanorod template at room temperature. The nickel hydroxide shell converted to cubic NiO after heat treatment at 500 °C. Galvanostatic charge and discharge results indicated that the NiO-coated ZnO nanorod electrode is capable of delivering a higher capacity than the bare ZnO nanorod and carbon-coated ZnO nanorod electrodes especially in high-rate charge and discharge processes. The presence of porous NiO shell might prevent the disintegration of ZnO nanorods because of the large volume change during charge and discharge. In addition, the porous NiO shell ensured good electrical contact of ZnO with the current collector and facilitated the charge transfer and transport of lithium ions. The amount of NiO coated on ZnO nanorods significantly affected the electrochemical performance of ZnO electrode toward lithium. Insufficient NiO shell would not provide enough electrical conductivity and protection against disintegration. When the NiO content was higher than 32.6 wt %, the electrode performance could be significantly improved.

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INTRODUCTION Rechargeable lithium-ion batteries with high-energy density have received much research effort because of the increasing power demand of portable electronic devices and electrical vehicles. Nanosized metal oxides have been considered to be one of the most promising anode materials for rechargeable lithium-ion batteries because they can provide higher lithium storage capacity than traditional graphite-based anodes which are usually used as the anode materials for rechargeable lithiumion batteries.1 The lithium storage mechanism in metal oxides differs from lithium intercalation/deintercalation in graphitebased anodes or lithium-alloying processes in alloy anodes. The reaction mechanism of metal oxides with lithium includes the formation and decomposition of Li2O accompanying the reduction and oxidation of metal nanoparticles, respectively.1 However, it is very difficult to achieve the delithiation reaction of Li2O in the absence of catalysts such as metal nanoparticles. Previous reports indicated that nanosized metal oxide is thought to be capable of facilitating the reduction and oxidation between Li2O and metal nanoparticles during the charge and discharge processes.1 ZnO has a large theoretical capacity of about 1000 mAh g−1 compared to the traditional graphite-based anode materials, but it has rarely been used as anode materials for lithium-ion batteries. The major reason is that pure ZnO suffers from poor kinetics and severe capacity fade during cycling test resulting from its low electronic conductivity and large volume change during the charge and discharge processes.2−5 Therefore, the © 2013 American Chemical Society

pristine ZnO material is difficult to meet the energy demand of future electronic devices. Many efforts have been devoted to overcome the shortcomings of ZnO anodes. Porous ZnO/ ZnAl2O4 composite film can be obtained by calcining Zn−Al layered double hydroxide film exhibiting better cycling stability than pristine ZnO.6 The improved electrochemical performance can be attributed to the buffering effect of the inactive matrix ZnAl2O4 by relieving the stress induced by the large volume change during charge and discharge cycling.6 Dai et al. demonstrated that doping with a small amount of Al2O3 (less than 3 wt %) can significantly improve the electrochemical performance of ZnO resulting from the improved conductivity of ZnO after doping.7 The nanocomposites such as ZnO−Se and ZnO1−xSx have turned out to be the effective methods for improving the ZnO anode toward lithium.8,9 It was reported that coating with Ni, NiO, Au, or C can increase the electronic conductivity and alleviate inner stress, which conspicuously enhances the electrochemical performance of ZnO.10−14 SnO2coated ZnO showed improved performance resulting from the high capacity of SnO2.15 The TiO2 coating is believed to reduce the degree of reactions during the charge−discharge process since the inactive coating layer prevents the electrode from having direct contact with the electrolyte.16 Synthesizing nanostructured ZnO in various forms including thin film, Received: August 9, 2012 Revised: January 14, 2013 Published: January 18, 2013 2590

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electrode was rinsed in deionized water. The nickel hydroxide electrode with hollow nanotube arrays was formed by dipping the zinc oxide template in a solution of 1 M nickel chloride for 20 min, and then the electrode was rinsed in deionized water. Finally, the electrode was immersed in 6 M sodium hydroxide solution for 60 min to remove the remnants of zinc oxide, and then the electrode was rinsed several times in deionized water. The resultant electrodes were annealed at 500 °C for 3 h in nitrogen atmosphere. The amount of active materials including NiO, ZnO, and carbon was measured by a microbalance (Mettler, XS105DU) with an accuracy of 0.01 mg. The surface morphology of nanorod electrode was examined with a scanning electron microscope (SEM, JEOL JSM-6700F) with an accelerating voltage of 3 keV. The core−shell structure of the nanorods was identified by a transmission electron microscope (TEM) with an accelerating voltage of 200 keV. The crystal structure of NiO and ZnO was identified by an Xray diffractometer (XRD, Rigaku D/MAX2500) with a Cu Kα target (wavelength = 1.54056 Å). Samples were stripped from electrodes for XRD and TEM measurements. The electrochemical performance of the prepared anodes toward lithium was investigated in a homemade three-compartment cell. The electrolyte used was a PC-based (propylene carbonate) electrolyte containing LiClO4 (1 M). Galvanostatic charge and discharge tests were performed on a charge/discharge unit (Land CT2001A). The electrodes were charged and discharged at various current densities to the cutoff potentials of 3.0 and 0.05 V versus Li/Li+, respectively. Cyclic voltammetric measurements were taken using a potentiostat/galvanostat (CH Instruments CHI 608). The potential was swept linearly with time at a scan rate of 0.1 mV s−1 in a potential range of 0.05−3.0 V versus Li/Li+. The electrochemical impedance spectroscopy (EIS) was carried out at open-circuit condition using a potentiostat/galvanostat (CH Instruments, CHI 608) with ac amplitude of 10 mV at a frequency range of 0.1−1 × 105 Hz.

nanorod/nanowire arrays, nanosheets, and nanoflower-like structures attracts considerable attention because of their large electrode/electrolyte interfaces, short diffusion path, and better strain accommodation.12−14,17−21 Although most of the reported ZnO materials for lithium-ion batteries have improved lithium storage capacity compared to graphite materials, there is still a need for improved performance in areas such as high-cycle-life stability and highcapacity retention during high-rate charge and discharge processes. In this work, a nanostructured anode made up of NiO-coated ZnO nanorods on stainless steel (SS) substrate is proposed for application in lithium-ion batteries. The benefits of using a one-dimensional nanostructure in a battery are greatly magnified by the introduction of a conductive and porous NiO shell. As previously mentioned, an anode made of nanosized metal oxide is very helpful in facilitating the reduction and oxidation between Li2O and the metal nanoparticle during charging and discharging processes. Therefore, the synthesis of nanosized porous NiO-coated ZnO nanorod electrodes has become one of the important issues for lithium-ion batteries. NiO is capable of delivering high capacity during lithiation and delithiation processes.1,22−24 Our previous study has shown that the nickel hydroxide shell, which exhibits a porous structure, can be self-assembled around the hexagonal ZnO nanorods during hydrolysis of nickel chloride solution.25 After being heat-treated at a higher temperature, the nickel hydroxide may convert into nickel oxide. In this work, the ZnO nanorod electrodes were cathodically electrodeposited on the SS substrate as a template for the hydrolysis of nickel chloride. In addition, the electrochemical performance of the NiO-coated anodes toward lithium is analyzed and compared to the pristine ZnO and carbon-coated ZnO anodes.

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EXPERIMENTAL SECTION The ZnO template composed of hexagonal nanorods was electrodeposited directly onto the SS foil by applying a cathodic potential of −0.8 V versus a saturated Ag/AgCl electrode at 60 °C for 10 min. The cathodic deposition of ZnO template was carried out in a homemade three-compartment cell. A saturated Ag/AgCl electrode was used as the reference electrode and a platinum foil (2 × 2 cm) was the counter electrode. The plating solution was composed of 0.1 M zinc nitrate and 0.1 M potassium nitrate and was stirred using a Teflon stir bar on a magnetic plate during the deposition process. Prior to cathodic deposition, SS foil was cut into pieces of 2 × 2 cm, which were then soaked in acetone and were ultrasonically vibrated for 5 min to remove any trace of contaminants. Deionized water was then used to rinse the SS foils in ultrasonic vibration for another 5 min. After deposition, the template was rinsed several times in deionized water, and then the template was dried at 150 °C for 30 min in air. Carbon-coated ZnO nanorod electrode was prepared as described elsewhere.11 For the synthesis of carbon-coated ZnO nanorod electrode, SS foil-supported ZnO was immersed into a glucose aqueous solution (0.3 M) at room temperature for 15 h to allow the adsorption of glucose molecules onto the nanorod surface. After this, the foil was withdrawn from the bath, was dried at 60 °C, and was further annealed at 500 °C for 5 h in nitrogen atmosphere to allow the carbonization of glucose.11 The Ni(OH)2-coated ZnO nanorod electrode was formed by dipping the SS foil-supported ZnO template in a solution of 1 M nickel chloride for various periods of time, and then the

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RESULTS AND DISCUSSION Figure 1 shows the SEM micrographs of pristine ZnO and carbon-coated ZnO nanorod arrays. ZnO tends to grow as a hexagonal nanorod during cathodic deposition. The ZnO electrode consists of spaced nanorods of 60−120 nm in diameter after annealing at 500 °C for 5 h in nitrogen atmosphere (Figure 1a). The formation of carbon layer around the ZnO nanorod is evidenced by the SEM. The uniform and continuous coating of carbon layer is shown in Figure 1b. After coating, the hexagonal-shaped ZnO nanorods remain almost the same, while the carbon-coated ZnO shows a rough surface and has a larger diameter than the pristine ZnO. Liu et al. have demonstrated the carbonization of preadsorbed glucose on ZnO arrays at 500 °C in argon gas.11 They used Raman spectrum to identify the presence and partial graphitization of carbon.11 In this work, energy-dispersive X-ray spectroscopy (EDS) analysis showed that the carbon content in the carboncoated ZnO nanorod arrays is approximately 21.5 wt % after annealing at 500 °C for 5 h in nitrogen atmosphere. The carbon content can be readily modified by changing the concentration of glucose solution. Figure 2 shows the NiO-coated ZnO nanorod electrodes obtained for various durations of hydrolysis. The NiO-coated ZnO nanorod electrode was self-assembled by using the partial hydrolysis of nickel ions in the presence of ZnO nanorod template followed by heat treatment at 500 °C for 5 h in 2591

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the instability of lithium with water during charge and discharge processes. Heat treatment at a higher temperature obviates the disadvantage of the reaction between lithium and water. In this work, the annealing temperature is therefore set at 500 °C for the removal of water in the structure of nickel hydroxide. After annealing at 500 °C for 5 h, the Ni(OH)2 may convert to NiO. The NiO content, which is determined by EDS, increases with increasing the hydrolysis time. The NiO contents obtained from hydrolysis for 1, 5, 10, and 20 min are 4.7, 8.2, 32.6, and 44.6 wt %, respectively. As can be seen from Figure 2a, only a small amount of NiO can be coated on the ZnO nanorods after hydrolysis for 1 min. When the hydrolysis time is increased to 5 min, the ZnO nanorods are coated with a NiO shell. Obviously, the NiO-coated ZnO nanorods tend to form a tubular structure at a hydrolysis time longer than 10 min (Figure 2c and d). The core−shell structure is further confirmed by the TEM image shown in Figure 4. The hydrolysis time is 20 min. The results suggest that ZnO/NiO core−shell morphology shown in Figure 4a was obtained by hydrolysis of nickel ions in the presence of ZnO nanorod. A top-view TEM image shown in Figure 4b reveals the hollow hexagonal nanotubes after removal of ZnO core. A new structure of ZnO anode, NiO-coated ZnO with spaced nanorod arrays, is proposed for improving the electrochemical performance ZnO anode toward lithium. This structure may facilitate the electron conduction and electrolyte transport during charge and discharge processes. Figure 5 illustrates the schematic diagram of the NiO-coated ZnO nanorod arrays on SS substrate for lithium-ion batteries. The porous NiO shell can accommodate the large volume changes during charge/discharge cycling and can provide a high specific surface area for redox reaction and a shortened ion diffusion path in the solid phase. In addition, it is reasonable that a porous NiO shell is more easily accessible to the electrolyte allowing lithium ions to react with the ZnO core. Therefore, the spaced ZnO nanorod arrays with porous NiO shell might be helpful in enhancing its electrochemical behavior toward lithium. Figure 6 displays the XRD patterns of ZnO, carbon-coated ZnO, and NiO-coated ZnO nanorod arrays after annealing at 500 °C. The XRD pattern of the ZnO nanorod arrays can be assigned as hexagonal ZnO (JCPDS No. 36-1451). There is no characteristic peak of carbon in the carbon-coated ZnO indicating that the carbon shell formed on the ZnO surface has amorphous nature. The crystal structure of carbon-coated ZnO is similar to that of ZnO after annealing at 500 °C. Except for the diffraction peaks of the ZnO, the XRD results reveal that the diffraction pattern of NiO shell closely resembles cubic NiO (JCPDS 47-1179). Figure 7 shows the cyclic voltammograms (CVs) of ZnO and NiO-coated ZnO anodes for three cycles. According to the previous studies, the electrochemical process of ZnO toward lithium should proceed with the following reactions:6,17

Figure 1. SEM micrographs of pristine ZnO and carbon-coated ZnO nanorod arrays.

nitrogen atmosphere. The formation mechanism of ZnO/ Ni(OH)2 core−shell structure is illustrated in Figure 3. When the ZnO nanorod arrays are immersed in an aqueous solution of nickel chloride, the nickel ions dissolved from nickel chloride prefer to adsorb on the walls of the hexagonal ZnO nanorods.25 When the nickel ions are adsorbed on the ZnO surface, the hydrolysis of Ni2+ commences simultaneously to form Ni(OH)2 and H+ as follows:26 Ni 2 + + 2H 2O → Ni(OH)2 + 2H+

(1) +

The ZnO nanorods are then etched by H . This etching reaction may accelerate the hydrolysis of Ni2+.26 Therefore, the unreacted ZnO core of the nanorods shrinks in size during the hydrolysis process. The shrinking rate at the top of the ZnO rods is higher than that at the bottom because the top can receive a greater number of nickel ions than the bottom in a diffusion-controlled reaction. The porous Ni(OH)2 shell grows in size until the ZnO core is entirely consumed. In this work, the hydrolysis time was limited to less than 20 min. Therefore, only a small amount of ZnO wall can be etched during the hydrolysis of Ni2+. The Ni(OH)2-coated ZnO may contain structural water in the solid oxide phase. The water content in the structure induces challenges in lithium-ion battery application because of

ZnO + 2Li+ + 2e− ↔ Li 2O + Zn

(2)

Zn + Li+ + e− ↔ LiZn

(3)

As can be seen from Figure 7a, in the first cathodic scan, there is a strong peak at 0.47 V, which is related to the first electrochemical process of the ZnO material. This process contains the reduction of ZnO into Zn and the formation of LiZn alloy. The cathodic current density peak decreases in the subsequent cycle. This can be attributed to the structural or 2592

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Figure 2. NiO-coated ZnO nanorod electrodes obtained from the hydrolysis of nickel chloride solution for (a) 1, (b) 5, (c) 10, and (d) 20 min.

mitigated by the NiO shell. The anodic current peaks at 1.02 and 1.49 V have also been magnified by the NiO coating. This is the reason that the porous NiO shell protects ZnO nanorods from pulverization caused by large-volume expansion during lithiation and acts as a current collector to conduct electrons for facilitating the reaction between Li2O and Zn. Figure 8 shows the SEM images of NiO-coated ZnO nanorod electrodes after charge and discharge cycles. NiOcoated ZnO nanorod electrodes were prepared by hydrolysis of Ni2+ for 1 and 20 min corresponding to 4.7 wt % and 44.6 wt % NiO, respectively. Clearly, the morphology of NiO-coated ZnO nanorod electrode with insufficient amount of NiO shell (Figure 8a) has changed drastically compared with that of the fresh electrode (Figure 2a). This morphological change suggests that the lithium storage process in ZnO electrode is unlike the process in lithiated compounds such as graphite materials because the morphology of the graphite materials after lithium intercalation does not change significantly with only a small volumetric expansion. Large-volume change during charge/discharge cycling test fully destroys the structure of ZnO nanorod arrays leading to severe electrode pulverization and, finally, to a thorough loss of electrical contact. It was previously reported that the presence of carbon shell on the surface of ZnO nanorods can effectively alleviate the strains caused by the volume variation of ZnO cores and can prevent

textural modifications of the ZnO nanorods because of the formation of Li2O and Zn. The potentials of these reactions are very close, so the figure shows only one strong peak. The cathodic peak at 0.8 V can be ascribed to the growth of the gellike solid electrolyte interphase (SEI ) layer. In the first anodic scan, the three peaks, located at 0.31, 0.56, and 0.71 V, are ascribed to the multistep dealloying process of LiZn alloy.5,6 Another broad peak ranging from 1.0 to 1.75 V should be related to the formation of ZnO by the redox reaction between Li2O and ZnO according to the literature.3,27 After the first cycle, the curves almost coincide in shape indicating more reversible electrode reactions. The CV curves of the NiOcoated ZnO anode shown in Figure 7b are similar to those of ZnO anode. The additional redox peaks at 1.56 and 2.10 V can be assigned to the conversion reaction between NiO and lithium.10 These redox peaks are also closely related to the intercalation and deintercalation of lithium ion in NiO.28 Another strong cathodic peak appears at 0.8 V, which may be ascribed to the SEI formation and to the reaction between NiO and lithium by the following reaction:1 NiO + 2Li+ + 2e− ↔ Ni + Li 2O

(4)

The cathodic current peak of NiO-coated ZnO electrode at 0.47 V is lower than that of ZnO electrode representing that the structural or textural modifications of the ZnO nanorods are 2593

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effect of the NiO shell by alleviating the pulverization of ZnO rod resulting from the large-volume change during charge and discharge cycling. Figure 9 shows the galvanostatic charge and discharge curves of ZnO and carbon-coated ZnO electrodes at various cycles. Both electrodes were charged and discharged at a 1 C current (about 900 mA g−1 and 700 mA g−1 for the ZnO and carboncoated ZnO electrodes, respectively). In the first discharge process (lithiation), the capacity is much higher than its corresponding charge capacity (delithiation). On the basis of the first discharge reaction, the calculated theoretical capacity of ZnO would be 988 mAh g−1. However, the experimentally observed value is about 2100 mAh g−1, which is much higher than the theoretical capacity. The extra capacity can be caused by the formation of SEI film and polymeric gel-type layer on the Zn nanoparticles because of decomposition of the solvent in the electrolyte.27 The reversible capacity of the bare ZnO electrode is approximately 698 mAh g−1, and the irreversible capacity (the difference between the lithiation and delithiation capacities) is about 1044 mAh g−1. After the first charge/ discharge cycle, an irreversible capacity exists still for each sequential cycle, but the irreversible capacity is relatively small compared with the first cycle. The large irreversible capacity in the first cycle can be attributed to the decomposition reactions of the electrolyte and formation of the SEI film onto the ZnO surface. As revealed in Figure 9, the potential curve of the second lithiation process differs significantly from the first. According to the charge and discharge behaviors of transitionmetal oxides reported by Poizot et al., this difference can result from some drastic, lithium-driven, structural or textural modifications during the first lithiation process.1 The reversible and irreversible capacities of the carbon-coated ZnO electrode are approximately 935 and 1211 mA h g−1, respectively, which are higher than those of bare ZnO electrode. The higher irreversible capacity can be attributed to an increase in the surface area induced by coating carbon shell around ZnO nanorods. In the carbon-coated ZnO array electrode, the preservation of the array structure during the Li+ lithiation/ delithiation processes helps to keep the electrical continuity leading to better lithium-storage capacity.11 Figure 10 shows the galvanostatic charge and discharge curves of NiO-coated electrodes at various cycles. All electrodes were charged and discharged at a 1 C current (about 900 mA g−1). Clearly, the NiO content influences the charge and

Figure 3. (a) TEM image of NiO-coated ZnO nanorod and (b) topview TEM image of hollow NiO nanotubes obtained after removal of ZnO core.

the disintegration.11 In this work, the surface morphology of nanorods with sufficient amount of NiO shell remains almost unchanged (Figure 8b) compared to the fresh electrode (Figure 2d) suggesting that the porous NiO shell has a similar function as the carbon shell. This can be attributed to the protective

Figure 4. Schematic formation of ZnO/Ni(OH)2 core−shell structure during hydrolysis of nickel chloride using ZnO nanorods as template. 2594

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Figure 5. Schematic diagram of NiO-coated ZnO nanorod arrays on SS substrate for high-performance lithium-ion batteries.

Figure 6. XRD patterns of ZnO, carbon-coated ZnO (21.5 wt % C), and NiO-coated ZnO (44.6 wt % NiO) nanorod electrodes after annealing at 500 °C for 5 h.

discharge performances in the NiO-coated ZnO electrodes. The reversible capacity increases with increasing the NiO content and levels off. When the NiO content is higher than 32.6 wt % (hydrolysis time is 10 min), the porous NiO shell can provide sufficient protection against volume expansion/ contraction of ZnO core during lithiation and delithiation processes. Figure 11 shows the reversible capacity variations of the ZnO, the carbon-coated ZnO, and the NiO-coated ZnO electrodes for 15 galvanostatic charge/discharge cycles. The electrodes were charged and discharged at a 1 C current. The capacity of pristine ZnO electrode decreases rapidly in the first 10 cycles of charge and discharge and then decays slowly to a lower capacity value. A decrease in capacity indicates that the electrode resistance increases with increasing cycle number. The main reasons for the decrease in reversible capacity during the charge and discharge processes are the following: (1) the metallic Zn nanoparticles might be isolated by nonconducting materials such as Li2O or a passive film and (2) the lithiation and delithiation processes could result in a disintegration of the ZnO nanorods as suggested by the observation of ZnO particles falling from the film surface to the bottom of the cell. Therefore, an improved electrode design may be needed to

Figure 7. CV curves of ZnO and NiO-coated ZnO (8.2 wt % NiO) anodes for three cycles.

obtain better stability. Interestingly, the cycle-life stability of ZnO electrode can be significantly improved by coating carbon shell or porous NiO shell on the ZnO nanorods. Insufficient amount of NiO shell and carbon coating has little contribution to the cycle-life performance. Cycle-life stability of ZnO coated with 8.2 wt % NiO is comparable to that of ZnO coated with 21.5 wt % carbon. When the NiO content is higher than 32.6 wt %, the NiO-coated ZnO electrodes exhibit a very stable capacity during cycle. Pure NiO electrode with hollow nanotubes obtained by removal of ZnO cores also shows a very stable cycle-life performance. The effect of surface modification of carbon or NiO on the electrochemical performance of ZnO nanorod electrodes can be further characterized with the help of EIS under open-circuit 2595

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Figure 10. Galvanostatic charge and discharge curves of NiO-coated ZnO electrodes obtained from the hydrolysis of nickel chloride solution for (a) 1 min (4.7 wt % NiO), (b) 5 min (8.2 wt % NiO), (c) 10 min (32.6 wt % NiO), and (d) 20 min (44.6 wt NiO) at various cycles.

condition. Figure 12 shows the Nyquist plots of pristine ZnO, carbon-coated ZnO, and NiO-coated ZnO electrodes. The cells were fully lithiated before EIS measurement. A simple equivalent circuit used to simulate the impedance spectra is shown in the inset of Figure 12. The simulated curve is nearly overlapped with the measured curve for each electrode indicating that the equivalent circuit is applicable and reliable to these ZnO electrodes. The resistance values obtained from

Figure 8. SEM images of NiO-coated ZnO nanorod electrodes after charge and discharge cycles. NiO-coated ZnO nanorod electrodes were prepared by hydrolysis of Ni2+ for (a) 1 min (4.7 wt % NiO) and (b) 20 min (44.6 wt % NiO). The inset shows the high-magnification SEM images.

Figure 9. Galvanostatic charge and discharge curves of (a) ZnO and (b) carbon-coated ZnO (21.5 wt % carbon) electrodes at various cycles. 2596

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resistance, and Cdl is the double-layer capacitance. Zd is the Warburg diffusion impedance. In the low-frequency range, the linear section resembles the solid-state lithium-ion diffusion. The pulverization of ZnO nanorods during charge and discharge cycling produces fresh surface area which reacts with electrolyte to form SEI film. The SEI film resistance is therefore increased. Carbon or NiO coating on the ZnO nanorods can reduce the SEI film resistance of electrode because of the protection against disintegration. The highly porous NiO shell can provide sufficient protection against volume expansion/contraction of ZnO core and can allow for the fast transport of lithium ions during lithiation and delithiation processes. Therefore, the charge-transfer resistance of NiO-coated ZnO nanorod electrode is much lower than that of pristine and carbon-coated ZnO nanorod electrode. Figure 13a shows the capacity retention of the ZnO, the carbon-coated ZnO, and the NiO-coated ZnO electrodes at various C rates. The weight of active material for each electrode was measured to be about 0.2 mg. All electrodes were charged and discharged for 15 cycles at 1 C rate before measuring the capacity retention at various C rates. As revealed in the inset of

Figure 11. Reversible capacity variations of the ZnO, the carboncoated ZnO, and the NiO-coated ZnO electrodes for 15 galvanostatic charge/discharge cycles.

Figure 12. Nyquist plots of the pristine ZnO, the carbon-coated ZnO, and the NiO-coated ZnO electrodes at fully lithiated condition. The equivalent circuit used to fit the spectra is presented in the inset.

the impedance spectra are shown in Table 1. At high frequencies, the intercept at Z′ is a combinational resistance Table 1. Fitting Results Obtained from EIS Data in Figure 12 electrodes

Rs (Ω)

Rsei (Ω)

Rct (Ω)

pristine ZnO ZnO/C (21.5 wt %C) ZnO/NiO (32.6 wt % NiO)

5.3 1.0 1.2

15.9 4.8 3.7

37.4 27.0 13.2

of ionic resistance in electrolyte and intrinsic resistance of active materials (Rs). In this work, variations in Rs may result only from the intrinsic resistance of active materials probably because of the same electrolyte used. Clearly, the electrical conductivity of ZnO electrode is significantly improved by coating ZnO nanorods with carbon or NiO. A depressed semicircle in the high-frequency range is caused by the resistance and capacitance from the SEI film and chargetransfer process. Rsei is the SEI film resistance, and Csei is the corresponding capacitance to Rsei. Rct is the charge-transfer

Figure 13. (a) Capacity retention of the ZnO, the carbon-coated ZnO, and the NiO-coated ZnO electrodes at various C rates. The inset shows the cycling performance of ZnO and NiO-coated ZnO (32.6 wt % NiO) electrodes at various C rates. (b) The variation of capacity retention with weight of active material for ZnO and NiO-coated ZnO electrodes at 10 C rate. 2597

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reversible capacity of NiO-coated ZnO anode with NiO content higher than 32.6 wt % could reach as high as 1100 mAh g−1 at 1 C current discharge, higher than that of carboncoated ZnO (935 mAh g−1) and pristine ZnO (698 mAh g−1) electrodes. The ZnO coated with sufficient amount of NiO shell exhibited high-rate capability and cycle-life stability compared with the pristine and carbon-coated ZnO electrodes.

Figure 13a, when the current density returns back to 1 C, the capacity retention for each electrode rebounds to about 97% of the initial value. The capacity retention of carbon-coated ZnO (21.5 wt % C) electrode is similar to that of pristine ZnO electrode at various C rates, which decreases with increasing the discharging current. The capacity retention of carbon-coated ZnO and pristine ZnO electrode discharged at 10 C decreases to about 58%. When the carbon content is decreased to 3.9 wt %, the high-rate performance is slightly increased to 63% at 10 C current. However, insufficient amount of carbon coating on ZnO nanorods cannot prevent the disintegration of ZnO nanorods. A thick carbon coating on ZnO rods impedes the transport of lithium ions and therefore decreases the high-rate performance. When the NiO content is higher than 32.6 wt %, the capacity retention of NiO-coated ZnO electrode discharged at 10 C can reach about 73%. Pure NiO electrode with hollow nanotubes exhibits the highest capacity retention probably because of the porous nanotube structure that expedites the transport of lithium ions through the hollow nanotube and porous shell. This improved high-rate performance suggests that the porous NiO shell plays an important role in enhancing the electrolyte transport and electron conduction. Therefore, the proposed porous NiO-coated ZnO anode has a better rate capability than the pristine ZnO and carbon-coated ZnO electrodes. The difference in rate capability between ZnO and NiO-coated ZnO electrodes is pronounced when the weight of active material on the anode is increased shown in Figure 13b. The capacity retention of NiO-coated ZnO electrode is slightly decreased, while that of pristine ZnO electrode is significantly decreased with increasing the weight of active material. The higher the weight of active material on the anode, the larger is the film thickness. Because of the poor electronic conductivity of pristine ZnO material, electron transport in ZnO film electrode can be significantly impeded by increasing the film thickness leading to a decrease in rate capability. As mentioned above, a drastic morphological change in the ZnO electrode occurs during the lithiation process suggesting that the pristine nanorods can no longer be maintained. The existence of protective NiO shell around ZnO nanorods exhibits a superior high-rate capability and cycle-life stability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 886-7-3830674. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Science Council, Taiwan, Republic of China (Project No. NSC 101-2221-E-151-055-MY2).

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REFERENCES

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CONCLUSIONS An electrode structure composed of porous NiO-coated ZnO anode is capable of delivering high lithium-storage capacity and sustaining long-term stability. NiO-coated ZnO nanorod electrodes were self-assembled by hydrolysis of nickel chloride in the presence of ZnO nanorods followed by heat treatment at 500 °C in nitrogen atmosphere. The morphology of pristine ZnO nanorods underwent drastic changes during the lithiation process. SEM results indicated that the surface morphology of ZnO nanorods coated with sufficient amount of NiO remained almost unchanged during the charge/discharge process. The presence of porous NiO shell around the ZnO nanorods might have three major advantages: (1) porous shell allows an electrolyte to easily reach the ZnO core; (2) NiO shell provides a better electron conduction enhancing the charge-transfer reaction; and (3) porous NiO shell acts as structural buffer layer to effectively mitigate the strain caused by the large volume expansion of ZnO nanorods and to prevent the disintegration. The galvanostatic charge/discharge result revealed that the ZnO anode with uniform coating of NiO is capable of delivering higher capacity and cycle-life stability than the pristine ZnO and carbon-coated ZnO anodes. The 2598

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