Three-dimensional interconnected cobalt oxide-carbon hollow spheres arrays as cathode materials for hybrid batteries

Three-dimensional interconnected cobalt oxide-carbon hollow spheres arrays as cathode materials for hybrid batteries

Progress in Natural Science: Materials International 26 (2016) 253–257 Contents lists available at ScienceDirect HOSTED BY Progress in Natural Scie...

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Progress in Natural Science: Materials International 26 (2016) 253–257

Contents lists available at ScienceDirect

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Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi

Original Research

Three-dimensional interconnected cobalt oxide-carbon hollow spheres arrays as cathode materials for hybrid batteries Jiye Zhan, Xinhui Xia n, Yu Zhong, Xiuli Wang, Jiangping Tu n State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

art ic l e i nf o

a b s t r a c t

Article history: Received 30 March 2016 Accepted 20 April 2016 Available online 7 June 2016

Hierarchical porous metal oxides arrays is critical for development of advanced energy storage devices. Herein, we report a facile template-assisted electro-deposition plus glucose decomposition method for synthesis of multilayer CoO/C hollow spheres arrays. The CoO/C arrays consist of multilayer interconnected hollow composite spheres with diameters of ∼350 nm as well as thin walls of ∼20 nm. Hierarchical hollow spheres architecture with 3D porous networks are achieved. As cathode of high-rate hybrid batteries, the multilayer CoO/C hollow sphere arrays exhibit impressive enhanced performances with a high capacity (73.5 mAh g  1 at 2 A g  1), and stable high-rate cycling life (70 mAh g  1 after 12,500 cycles at 2 A g  1). The improved electrochemical performance is owing to the composite hollowsphere architecture with high contact area between the active materials and electrolyte as well as fast ion/electron transportation path. & 2016 Chinese Materials Research Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Cobalt oxide Hollow spheres Cathode Arrays Hybrid battery

1. Introduction Hybrid batteries HBs are considered as one of the most important power sources due to their fast recharge capability, high power density and long cycling life. Great efforts have been dedicated to searching for advanced cathode materials for highperformance hybrid batteries (HBs) with high energy/power density [1–5]. In recent years, the cathode materials of HBs was wrongly called as pseudocapacitive cathode in the literature [6–8]. Brousse et al. and Gogotsi group [6,7] pointed out that they belong to different categories because the cathode of HBs usually exhibit obvious redox couples with distinct charge/discharge plateaus, not liner behavior of capacitors. Accordingly, HBs usually consist of redox-reaction active cathodes (such as metal oxides/hydroxides [9–13], and metal sulfides [14]) and carbon materials anodes through electrochemical double layer [14,15]. Metal oxides/hydroxides are typical redox cathodes of HBs with high electrochemical reactivity, and good cycling life. But their low reaction kinetics greatly hinders their commercial application. CoO is considered as one of the most promising cathode candidates because of its high specific capacity [1,8]. However, the practical application of CoO is still hampered by some issues (e.g., low electrical conductivity and slow ion diffusion), resulting in n

Corresponding authors. E-mail addresses: [email protected] (X. Xia), [email protected] (J. Tu). Peer review under responsibility of Chinese Materials Research Society.

poor rate capability and reversibility, and unsatisfactory cycling life. To tackle the above problems, free-standing arrays electrodes has been constructed to enhance the power/energy densities and electrochemical stability [16–18]. Compared with the bulk powder counterparts, the integrated arrays electrodes present several advantages as follows: 1. The array structures possess combined properties of higher porosity and better electrical contact with the conductive substrates; 2. The arrays architecture can buffer large volume changes, leading to improved mechanical stability and cycling life; 3. No addition of polymer binders can reduce the inner resistance of electrodes; 4. Larger surface area and shorter ions/ electrons diffusion path are obtained in the arrays electrodes. The integrated electrodes design has been verified successful in lots of metal oxides (e.g., CoO [18,19], Co3O4 [20,21], CoO [22,23]) nanostructure systems and noticeable electrochemical enhancements are achieved for Li ion batteries and supercapacitors. Currently, integrated hollow nanosphere arrays have been investigated due to their unique geometry and porous configuration [24]. Currently, there has been no research about fabrication of 3D multilayer interconnected CoO/C hollow spheres arrays and their application for HBs. In this work, we report 3D porous multilayer CoO/C hollow sphere arrays (HSAs) by a facile combination of template-assisted electro-deposition (ED) and glucose carbonization method. Porous structure with hollow cores and interconnected nanowalls is achieved in one electrode. As cathodes of HBs, the designed CoO/C HSAs exhibit high discharge capacities and good high-rate properties owing to the unique hollow sphere array structure with faster ions/electrons transfer and large

http://dx.doi.org/10.1016/j.pnsc.2016.05.017 1002-0071/& 2016 Chinese Materials Research Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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contact area. The proposed method is applicable for preparation of other high-performance hierarchical porous arrays for applications in energy storage and conversion.

2. Experimental The multilayer CoO/C HSAs were prepared by a polystyrene sphere (PS) template-assisted ED method plus glucose decomposition. The simplified growth process was shown in Fig. 1a. The PS (particle size of ∼300 nm) spheres template was assembled on the nickel substrate described in detail in the previous work [25]. Then, the CoO was prepared through a simple cathodic ED method, which was conducted in a three-electrode cell, the above PS spheres template electrode as the working electrode, saturated calomel electrode (SCE) as the reference electrode and a Pt foil as the counter electrode. The electrolyte consisted of 0.5 M Co(NO3)2

þ0.1 M NaNO3. The ED was performed at a cathodic current density of 1.5 mA cm  2 for 30 min. Then, PS spheres template was etched by immering the sample in toluene for 12 h. Then, the sample was annealed at 400 °C in argon for 2 h to form the CoO HSAs. After that, the sample was immersed into 0.05 M glucose for 12 h plus an annealing process at 500 °C for 2 h in argon to form porous CoO/C HSAs. The mass of CoO and C was about 2.1 and 0.1 mg cm  2, respectively. The morphology and microstructure of the samples were characterized by X-ray power diffraction (XRD, Rigaku D/max 2550 PC, Cu Kα), a scanning electron microscopy (SEM, Hitachi S-4700 and FESEM, FEI Sirion-100), transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 200 kV) and Raman spectroscopy (WITec-CRM200 Raman system with a laser wavelength of 532 nm). The electrochemical performances were tested in full HBs. The devices were assembled based on the CoO/C HSAs as the positive

Fig. 1. (a) Schematics of growth of CoO/C hollow sphere arrays. SEM-TEM images of (b,c) polystyrene sphere template (high magnification images in inset) and (d,e) CoO/C hollow sphere arrays.

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electrode (cathode) and AC-based electrode as the negative electrode (anode). The AC-based anode was fabricated according to the Ref. [14]. The cathode and anode were separated by a porous nonwoven cloth separator and assembled into a full coin-type cell, in which the capacities were determined by the cathode because the capacity of anode was excessive to let the cathode perform its best. 6 M KOH was used as the electrolyte. A series of electrochemical tests including cyclic voltammetry (CV), and galvanostatic charge/ discharge measurements were performed on CHI660c electrochemical workstation and Land battery program-control test system with a current range up to 10 A.

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3. Results and discussion SEM image (Fig. 1b) demonstrates that the PS spheres template is self-assembled into to close-packed template. The adopted PS spheres have diameters of ∼300 nm and are perpendicular to the substrate. TEM image (Fig. 1c) verifies that the PS spheres are homogeneous and show dense structure. SEM-TEM images of the CoO/C HSAs on nickel foil substrates are shown in Fig. 1d-e. Notice that a 3D porous interconnected hollow sphere structure is well formed and the individual hollow sphere exhibits a size of ∼350 nm (Fig. 1d). The hollow sphere structure is clearly distinguished in Fig. 1e. The wall thicknesses of hollow spheres are about 20 nm and the interconnected hollow spheres of ∼350 nm are verified (Fig. 1e). Furthermore, the CoO/C HSAs have 3D porous

Fig. 2. (a,b) Cross-sectional SEM images of CoO/C hollow sphere arrays. (c-e) TEM-HRTEM images of CoO/C hollow spheres (SAED pattern in inset). (f) EDS mappings of Co, O and C.

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Fig. 3. (a) XRD patterns of CoO/C hollow sphere arrays. (b) Raman spectrum of CoO/C hollow sphere arrays in the region of 800–2000 cm  1.

arrays networks with pores reaching out in all directions (Fig. 2a and b). The whole architecture is open and porously connected. This characteristic is believed to be beneficial for fast transfer of ions/electrons and provides large active sites for electrochemical reactions. In addition, the hollow spheres are composed of nanoparticles of 5–10 nm (Fig. 2c and d). The lattice fringes with a lattice spacing of ∼0.24 nm correspond well to the (111) planes of CoO phase (JCPDS 78-0431) (Fig. 2d), supported by the selected area electron diffraction patterns (inset in Fig. 2c). As shown in

Fig. 2e, an amorphous carbon layer of ∼5 nm is noticed and well coated on the surface of CoO. But no SAED patterns of carbon is detected, implying the amorphous nature of carbon in the composite arrays. The existence of Co, O and C is confirmed by the EDS mappings of Co, O and C (Fig. 2f). The phase/composition of all samples are also monitored by the XRD and Raman tests. From the XRD pattern (Fig. 3a), except for the peaks of nickel foil substrate, the other diffraction peaks are indexed well to the cubic CoO phase (JCPDS 78-0431). The

Fig. 4. Electrochemical performances of CoO/C hollow sphere arrays: (a) CV curves at different scanning rates; (b) discharge profiles and (c) corresponding specific capacities at different current densities; (d) cycling performance at 2 A g  1 (inset: SEM images of CoO/C hollow sphere arrays after 12,500 cycles at 2 A g  1).

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presence of carbon is supported by the Raman spectrum, which shows two obvious peaks located at 1355 cm  1 (d-band) and 1598 cm  1 (G-band) (Fig. 3b), characteristic of amorphous carbon [26]. Thus, based on these results, it is reasonable that the multilayer CoO/C HAS have been successfully prepared via the PS template-assisted ED plus glucose decomposition method. The electrochemical performances of the CoO/C HSAs are tested as cathodes for HBs. Fig. 4a shows the cyclic voltammetry (CV) curves of the assembled HBs at different scan rates. Only one redox couple is observed in the CV curves, implying that the capacities are mainly governed by faradaic redox reactions. The involved reaction at the cathode can be simply expressed as follows. CoO þOH   e  2CoOOH

(1)

The reaction at the anode can be illustrated as follows. C þK þ þ e  2K þ //C  þ



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network are combined in the integrated arrays of CoO/C. Due to the unique hollow sphere array structure, the obtained CoO/C HSAs show high-capacity and good cycling performance when applied as cathode of HBs. The proposed integrated design of hollow sphere arrays is proven as an effective way for construction of high-performance cathodes for high-rate HBs.

Acknowledgements This work is supported by National Natural Science Foundation of China (Grants no. 51502263), and by Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

(2) þ

K //C represents the K is absorbed on the surface of activated carbon. The whole reaction of the HBs is as follows. CoO þOH  þ Cþ K þ 2CoOOH þK þ //C 

(3)

Discharge curves and corresponding specific capacities of the CoO/C HSAs at various current densities are shown in Fig. 4b-c. The CoO/C HSAs exhibit good high-rate capability with a capacity of 73.5 mAh g  1 at 2 A g  1, 67.5 mAh g  1 at 4 A g  1, 60.6 mAh g  1 at 8 A g  1, 58.9 mAh g  1 at 10 A g  1, and 51 mAh g  1 at 20 A g  1, respectively. 70% of capacity is retained when the charge/discharge current density changes from 2 A g  1 to 20 A g  1. The obtained capacity values are higher than those of CoO/TiO2 arrays [27], CoO films [19,28], and Co3O4 spheres powders [15], but a little lower than the CoS nanowire arrays [14]. Furthermore, the CoO/C HSAs present very good high-rate cycling life (Fig. 4d). After 12,500 cycles, a specific capacity of 70 mAh g  1 at 2 A g  1 is obtained with the high capacity retention of 95%. The good high capacity achieved at high rates implies that this type of electrode is a promising candidate for high-power HBs applications. The good electrochemical performance is mainly attributed to the hierarchical hollow sphere arrays structure. Firstly, it can provide large specific surface area to facilitate the transportation of ion/ electron. Secondly, the carbon shell layer can enhance the electrical conductivity and improve reaction kinetics leading to good rate performance. Thirdly, the hollow spheres are intimately connected with each other to ensure every sphere to participate in the electrochemical reaction, resulting in higher utilization of CoO. And fourthly, the hollow spheres arrays architecture could keep stable by buffering the volume expansion, demonstrated by the SEM morphology after 12,500 cycles at 2 A g  1 (inset in Fig. 4d). In view of these positive characteristics, therefore, the CoO/C HSAs exhibit impressive high-rate performance.

4. Conclusion In summary, we have proven the rational synthesis of 3D porous CoO/C HSAs via an electrodeposited template plus glucose decomposition route. Hollow sphere and interconnected porous

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