Materials Letters 193 (2017) 220–223

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Materials Letters

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Mesoporous ZnCo2O4-ZnO hybrid nanotube arrays as advanced anodesfor lithium-ion batteries

http://dx.doi.org/10.1016/j.matlet.2017.01.1220167-577X/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: xiahui@njust.edu.cn (H. Xia).

1 Both authors contributed equally to this work.

Jizi Liu a,1, Qiuying Xia a,1, Yadong Wang b, Hui Xia a,⇑a School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, Chinab School of Engineering, Nanyang Polytechnic, 180 Ang Mo Kio Ave 8, Singapore 569830, Singapore

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 November 2016Received in revised form 21 January 2017Accepted 25 January 2017Available online 28 January 2017

Keywords:NanotubeAnodeHybridEnergy storage and conversion

Mesoporous ZnCo2O4-ZnO hybrid nanotube arrays are prepared by a diffusion-controlled solid–solidapproach and applied as anodes for lithium-ion batteries. Due to the unique hierarchical hollow struc-ture, the ZnCo2O4-ZnO hybrid arrays exhibit a large reversible capacity up to 1145 mAh g

�1 as well asgood rate capability (433 mAh g�1 at a current density of 5 A g�1) and good cycle performance, makingthem promising as anodes for advanced lithium-ion batteries.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Lithium-ion batteries (LIBs) have become one of the mostimportant green and clean energy-storage techniques for hybridelectric vehicles (HEVs), electric vehicles (EVs), and portable elec-tronics [1,2]. However, the energy density and power density ofcurrent LIBs cannot meet the requirements for the fast-developing electrical devices. In recent years, great efforts havebeen devoted to exploring alternative anode materials to replacegraphite in LIBs for further increasing their energy density andpower density [3–5]. ZnCo2O4 has been considered as an attractiveanode material candidate due to its high theoretical capacity(�1000 mAh g�1) and environmental friendliness [6–8]. Untilnow, various nanostructures of ZnCo2O4, such as nanoparticles[6], nanowires [7], nanosheets [8], and microspheres [9] have beendeveloped to improve the electrochemical performance. However,the poor rate capability and fast capacity fading still hinder theirpractical application in LIBs. Recent studies suggest that the directgrowth of three-dimensional (3D) nanoarchitectured electrodes onconductive substrates is a good strategy to improve the electro-chemical performance due to large surface areas, fast charge trans-port, and improved structural stability for 3D architectures [10–11].

Herein, we report the fabrication of 3D mesoporous ZnCo2O4-ZnO hybrid nanotube arrays on carbon cloth by a diffusion-controlled solid–solid approach. As expected, the ZnCo2O4-ZnOhybrid nanotube arrays exhibited superior electrochemical perfor-mance compared to the bare Co3O4 nanowire arrays, demonstrat-ing the advantage of this unique nanoarchitecture design. Thiswork brings new opportunities for developing high-performanceanodes for LIBs.

2. Experimental section

Co(CO3)0.5(OH)�0.11H2O nanowire arrays (NWAs) were pre-pared by a facile hydrothermal method according to literature[12]. After that, a ZnO layer was conformably deposited onto theCo(CO3)0.5(OH)�0.11H2O NWAs by the atomic layer deposition(ALD) technique (Beneq TFS-200, Finland). The as-prepared Co(CO3)0.5(OH)�0.11H2O@ZnO NWAs were then annealed at 400 �Cfor 2 h in a quartz tube furnace to obtain mesoporous ZnCo2O4-ZnO hybrid nanotube arrays (NTAs). For comparison, Co3O4 NWAswere synthesized by directly annealing the Co(CO3)0.5(OH)�0.11H2O NWAs in air.

The obtained samples were characterized by X-ray diffraction(XRD, Bruker AXS D8 Advance) with Cu Ka radiation. The sampleswere further investigated by field emission scanning electronmicroscopy (FESEM, Quanta 250F) and transmission electronmicroscopy (TEM) equipped with energy dispersive X-ray spec-troscopy (EDS). Nitrogen adsorption/desorption measurementswere conducted at 77 K (ASAP Tristar II 3020 model).

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J. Liu et al. /Materials Letters 193 (2017) 220–223 221

The electrochemical measurements were carried out on Swage-lok cells. All the samples were directly used as the electrodes with-out any binders or conducting additives. Celgard 2400 was used asthe separator, 1 M LiPF6 in ethylene carbonate and diethyl carbon-ate (EC/DEC with a volume ratio of 1:1) solution was used as theelectrolyte, and pure lithium foil was used as both counter and ref-erence electrodes. Galvanostatic charge–discharge measurementswere performed using a NEWARE battery tester in a voltage win-dow of 0.05 V –3 V (vs. Li/Li+). Electrochemical impedance spec-troscopy (EIS) measurements were conducted at an open circuitpotential on a CHI660D electrochemical workstation.

Fig. 1. Schematic illustration of the fabrication of the mesoporous ZnCo2O4-ZnONTAs and XRD patterns of (a) Co(CO3)0.5OH�0.11H2O, (b) Co(CO3)0.5OH�0.11H2-O@ZnO, (c) Co3O4, and (d) ZnCo2O4-ZnO samples.

Fig. 2. FESEM images of (a,b) Co(CO3)0.5OH�0.11H2O NWAs (Inset is the EDS spectrum), (spectrum), and (f) mesoporous ZnCo2O4-ZnO NTAs.

3. Results and discussion

The fabrication of the mesoporous ZnCo2O4-ZnO NTAs and theXRD patterns for different products are shown in Fig. 1. The Co(CO3)0.5(OH)�0.11H2O NWAs were grown on carbon cloth by ahydrothermal method as confirmed by the XRD pattern inFig. 1a. A uniform ZnO layer was then coated on the Co(CO3)0.5(-OH)�0.11H2O NWAs by ALD, and Fig. 1b indicates the ZnO layeris in amorphous state. Following by a heat treatment of Co(CO3)0.5(OH)�0.11H2O@ZnO NWAs at 400 �C for 2 h, the meso-porous ZnCo2O4-ZnO hybrid NTAs were obtained. As shown inFig. 1d, the diffraction peaks can be well indexed to the cubicZnCo2O4 (JCPDS No. 23-1390) and hexagonal ZnO (JCPDS No. 36-1451), revealing the existence of both phases in the sample. Whilepure Co3O4 NWAs can be obtained by directly annealing the Co(CO3)0.5(OH)�0.11H2O in air (Fig. 1c).

The FESEM images of the Co(CO3)0.5(OH)�0.11H2O NWAs inFig. 2a and b show that the nanowires are about 70–90 nm indiameter and their surface are very smooth. The FESEM image ofCo3O4 is shown in Fig. 2c, indicating that the nanowire morphologyof Co(CO3)0.5(OH)�0.11H2O is well kept after calcination while itssurface becomes rough due to the release of CO2 and H2O.Fig. 2d and e show the FESEM images of the Co(CO3)0.5(OH)�0.11H2O @ZnO core-shell NWAs. The increased diameters of thenanowires (130–150 nm) reveal the successful coating of ZnO layerof about 30–40 nm in thickness. The corresponding EDS spectra(Insets in Fig. 2a and d) confirm the existence of Zn element inthe NWAs after ALD, and the ZnCo2O4/ZnO mass ratio is about2:1. After calcination, the final ZnCo2O4-ZnO hybrid NTAs showsimilar morphology while the diameter is slightly reduced (80–100 nm).

Fig. 3a shows the TEM image of a single Co3O4 nanowire,demonstrating a mesoporous structure and a diameter of about80 nm. Interestingly, a hollow interior is observed for the ZnCo2-O4-ZnO hybrid nanowire as shown in Fig. 3b, presenting a nan-otube structure. The walls of the nanotubes are composed ofinterconnected nanoparticles, showing a mesoporous feature forthe nanotubes. Nitrogen adsorption/desorption isotherms ofCo3O4 and ZnCo2O4-ZnO samples shown in Fig. 3c further confirmtheir mesoporous structures. As displayed in Fig. 1, the Co3O4 and

c) Co3O4 NWAs, (d,e) Co(CO3)0.5OH�0.11H2O@ZnO core-shell NWAs (Inset is the EDS

222 J. Liu et al. /Materials Letters 193 (2017) 220–223

ZnO can diffuse to the interface and react with each other to formZnCo2O4 during the heat treatment. The hollow structure isobtained due to the Kirkendall effect that the diffusion speed ofCo atoms is faster than that of Zn atoms [13]. The redundantZnO phase forms a shell on the ZnCo2O4 core, resulting in acore-shell nanotube arrays. The HRTEM image of the edge areaof the nanotube in Fig. 3d shows clear lattice fringes with an inter-planar spacing of 0.26 nm, corresponding to the (002) planes ofhexagonal ZnO. The lattice fringes with an interplanar spacing of0.24 nm on the center area of the nanotube in Fig. 3e can beassigned to the (311) planes of cubic ZnCo2O4. Fig. 3f shows thescanning transmission electron microscopy (STEM) image and cor-responding element distribution of the ZnCo2O4-ZnO hybrid nan-otube, where the core-shell structure is apparently distinguishedfrom the Zn and Co distribution.

Fig. 3. TEM images of (a) a single Co3O4 nanowire and (b) a single ZnCo2O4-ZnO nanoHRTEM images of the square areas of d (d) and e (e) in (b). (e) STEM image and corresp

Fig. 4. (a) and (b) Charge-discharge curves of the bare carbon cloth, ZnCo2O4-ZnO, and Cothe ZnCo2O4-ZnO and Co3O4 electrodes.

Fig. 4a and b show the charge and discharge profiles of the barecarbon cloth, ZnCo2O4-ZnO, and Co3O4 electrodes. The carbon clothdelivers nearly no capacity and can serve as current collector. Theinitial discharge and charge capacities of the ZnCo2O4-ZnO elec-trode are 1383 and 1145 mAh g�1, respectively, which are largerthan 1195 and 1010 mAh g�1 of the Co3O4 electrode, demonstrat-ing the high electrochemical activity of the ZnCo2O4-ZnO electrode.The coulombic efficiency for the first charge/discharge cycle of theZnCo2O4-ZnO electrode was calculated to be 82.8%. During the dis-charge process, ZnCo2O4 and ZnO are reduced to metallic Zn andCo, then Zn can further react with Li to form LiZn alloy. While inthe charge process, Zn and Co can be oxidized to form the respec-tive metal oxides (ZnO, CoO and Co3O4) with Li removal.

Fig. 4c compares the rate capabilities of the ZnCo2O4-ZnO NTAsand Co3O4 NWAs electrodes. The ZnCo2O4-ZnO electrode delivers

tube. (c) N2 adsorption/desorption isotherms of Co3O4 and ZnCo2O4-ZnO samples.onding elemental mapping for the ZnCo2O4-ZnO nanotube.

3O4 electrodes. (c) Rate performance, (d) Cycle performance, and (e) Nyquist plots of

J. Liu et al. /Materials Letters 193 (2017) 220–223 223

much higher reversible capacity compared to the Co3O4 electrodeat various current densities, demonstrating its superior rate capa-bility. The cycle performances of the ZnCo2O4-ZnO NTAs andCo3O4 NWAs electrodes tested at 0.2 A g�1 for 100 cycles are com-pared in Fig. 4d. The ZnCo2O4-ZnO NTAs electrode retains a highreversible capacity of 1064 mAh g�1 after 100 cycles, correspond-ing to 92.9 % capacity retention. The Co3O4 NWAs electrode, how-ever, exhibits much lower capacity retention of 85.7%. The EISspectra in Fig. 4d show that the charge transfer resistance ofZnCo2O4-ZnO NTAs electrode is 155X, which is smaller than212X of the Co3O4 NWAs electrode. The high capacity, excellentcycling stability, and good rate capability of the ZnCo2O4-ZnO elec-trode can be attributed the following aspects: (1) The good electri-cal contact between the ZnCo2O4-ZnO NTAs and the carbon clothensures good expressway for charge transfer. (2) The unique 3Dhollow architecture can provide large surface area, more electro-chemical active sites, and short pathways for facile ions diffusion.(3) The hollow structure can well accommodate the strain inducedby the volume change during electrochemical reactions.

4. Conclusions

In summary, mesoporous ZnCo2O4-ZnO hybrid NTAs have beensuccessfully prepared by a facile diffusion-controlled solid–solidmethod. The ZnCo2O4-ZnO NTAs electrode exhibits high reversiblecapacity (up to 1145 mAh g�1), good rate capability (433 mAh g�1

at 5 A g�1), and good cycle performance, which are superior to

those of the bare NWAs Co3O4 electrode. This work provides adapt-able strategy for the construction of advanced 3D electrodes forhigh performance energy storage devices.

Acknowledgement

This work was supported by National Natural Science Founda-tion of China (No. 51301064, 51572129, U1407106).

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Mesoporous ZnCo2O4-ZnO hybrid nanotube arrays as advanced anodes for lithium-ion batteries1 Introduction2 Experimental section3 Results and discussion4 ConclusionsAcknowledgementReferences