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Yolk-shell ZnO-C microspheres with enhanced electrochemical performance as anode material for lithium ion batteries
Electrochimica Acta 125 (2014) 659–665
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Yolk-shell ZnO-C microspheres with enhanced electrochemical performance as anode material for lithium ion batteries Qingshui Xie a , Xiaoqiang Zhang a , Xiaobiao Wu b , Huayi Wu b , Xiang Liu a , Guanghui Yue a , Yong Yang b , Dong-Liang Peng a,∗ a Department of Materials Science and Engineering, College of Materials, Fujian Key Laboratory of Advanced Materials, Xiamen University, Xiamen 361005, China b State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 29 November 2013 Received in revised form 27 January 2014 Accepted 1 February 2014 Available online 15 February 2014 Keywords: Yolk-shell Zinc oxide Carbon Anode Lithium ion batteries
a b s t r a c t Three ZnO-C samples with distinct structures including yolk-shell microspheres, hollow microspheres and solid microspheres are fabricated through a facile chemical solution reaction followed by calcination in argon. When employed as the anode materials for lithium ion batteries, yolk-shell ZnO-C microspheres exhibit the best electrochemical properties than the hollow and solid microspheres. After 150 cycles, yolk-shell ZnO-C microspheres demonstrate a relative high capacity of 520 mA h g−1 at a current density of 100 mA g−1 with a Coulombic efficiency of about 99.3%. The excellent cycling stability and good rate capability of yolk-shell ZnO-C microspheres stem from the synergistic effect of the unique yolk-shell structures and extra carbon support. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, hollow micro/nanostructures have been widely investigated as anode materials for lithium ion batteries due to their large surface area and internal void space which can provide the large electrode/electrolyte contact area and effectively accommodate the huge volume variation during the Li+ intercalation/deintercalation process, respectively [1–5]. More recently, yolk-shell structures as one of the emerging hollow structures have received increasingly attention in lithium ion batteries [6–8]. For example, Lou’s group reported the successful preparation of yolk-shell V2 O5 microspheres through templating method, which revealed a high reversible capacity of about 187 mA h g−1 at 300 mA g−1 after 100 cycles [8]. Son et al. found that yolk-shell Co3 O4 microspheres displayed significantly enhanced electrochemical performance in comparison with the hollow Co3 O4 microspheres synthesized by the same route, which was ascribed to the better structure stability of the appealing yolk-shell construction [9]. Yolk-shell Sn/C powders prepared by Zhang et al. showed superior lithium storage capacity and excellent cycling stability [10]. Inspired by above studies, to fabricate various electrode
∗ Corresponding author. Tel.: +86 592 2180155; fax: +86 592 2183515. E-mail address: [email protected] (D.-L. Peng). http://dx.doi.org/10.1016/j.electacta.2014.02.003 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
materials with yolk-shell construction would be an effective strategy to improve their electrochemical properties. However, it is still a great challenge to develop a facile and cost-effective route to large-scale synthesize various yolk-shell structures till now. Transition metal oxides have gained more and more interest as electrode materials in lithium ion batteries in the last decades because of their higher theoretical capacity and safety compared with the conventional carbon materials [11–14]. Among them, ZnO has some special advantages such as low cost, easy preparation, morphologic diversity, environmental benignity and so on. Unfortunately, the poor electronic conductivity, large volume change during lithium/delithium process and the resulting severe capacity fading hinder its practical application. Hitherto, some efforts have been made to improve its cycling performance including the proper nanostructuring and decoration by metal, metal oxide and carbon [15–20]. For example, Huang’s group directly fabricated ZnO nanosheets on copper current-collecting substrates, which delivered the higher capacity and better cycling stability than commercial ZnO powders [19]. Although the reversible capacity of ZnO electrodes has been improved to some extent after appropriate construction and modification, the cyclability of ZnO anodes is still not satisfactory for developing advanced electrode materials for next-generation lithium ion batteries. Therefore, to further improve the cycling stability and reversible capacity of ZnO
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electrode materials through elaborate construction and decoration is still highly desirable. In this article, three different ZnO-C structures including yolkshell microspheres, hollow microspheres and solid microspheres were synthesized through a two step method. In the initial synthesis process, zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres were fabricated through a simple chemical solution reaction. Then the carboxylic acid groups in zinc citrate function as the in situ carbon source to prepare three different ZnO-C samples in the subsequent carbonization treatment. To the best of our knowledge, this is the first report on the successful synthesis of yolk-shell ZnO-C microspheres. The investigation of electrochemical properties suggests that all three samples exhibit similarly excellent cycling stability and yolk-shell ZnO-C microspheres possess the highest reversible capacity and best rate capability than the hollow and solid counterparts. 2. Experimental 2.1. Synthesis In a typical synthesis, amorphous zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres were firstly prepared through a facile chemical solution route as described in our previous literature [21]. Then the as-obtained zinc citrate microspheres with different structures were carbonized at 500 ◦ C for 2 h with a heating rate of 2 ◦ C/min in argon than air. After heat treatment, the furnace was cooled to room temperature naturally and then the black powders can be harvested. 2.2. Characterization PANalytical X’ pert PRO x-ray diffractometer (Cu K␣ radiation 40 kV, 30 mA) was employed to characterize the crystal phase of the products. Scanning electron microscopy (LEO-1530) and transmission electron microscopy (JEM-2100, 200 kV) were used to investigate the morphology and microstructure of the products. The thermogravimetric analysis (TGA) was carried out on a SDTQ600 thermal analyzer. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution were evaluated on a TriStar 3020 system. 2.3. Electrochemical measurements Two-electrode cells were used to characterize the electrochemical performance of the as-produced products. The working electrode was made up of 75 wt% active materials (ZnO-C microsheres with different structures), 15 wt% carbon black, and 10 wt% poly (vinyl difluoride) (PVDF). Li metal and 1 M LiPF6 dissolved in a mixed solution composed of equal volume of ethylene carbonate (EC) and diethyl carbonate (DEC) were employed as the counterelectrode and electrolyte, respectively. All the cells were assembled in an argon-filled glove box. The investigation of discharge-charge cycling performance was conducted on a multichannel battery testing system (Newware, China). Cyclic voltammetry (CV) was carried out using Autolab electrochemical workstation (NOVA 1.8) at a scanning rate of 0.1 mV s−1 at room temperature. 3. Results and discussion 3.1. Morphology and structure characterization Fig. 1 displays the morphology and microstructure of the as-obtained yolk-shell ZnO-C microspheres. The panoramic SEM image (Fig. 1a) implies that the sample is made up of dispersed
microspheres with an average diameter on the order of 1.6 m. From a broken microsphere as shown in the inset in Fig. 1a, the inner core, outer shell and interstitial void space can be obviously discerned, suggesting the yolk-shell structure of microsphere. From the magnified SEM micrograph (Fig. 1b), it can be observed that the shell is composed of numerous nanoparticles with an average size of 20 nm. Energy dispersive spectroscopy (EDS) microanalysis confirms the presence of Zn, O, C elements (Fig. S1). The TEM investigation was carried out to provide detailed structure of the samples. As depicted in Fig. 1c, the yolk-shell structure is further validated. The thickness of the shell and the diameter of the core are about 280 and 400 nm, respectively. The lattice fringes with a spacing of 0.281 nm in HRTEM image (Fig. 1d) recorded from the shell relate to the spacing of the (100) plane of ZnO. Nanocarbon in yolkshell ZnO-C microspheres is difficult to be directly distinguished even under high-magnification TEM observation in view of the carbon stem from the in situ carbonization of carboxylic acid groups in zinc citrate. Differently, for conventional carbon-coated materials, nanocarbon-layer can be clearly seen under HRTEM observation [18]. The corresponding SAED pattern (the inset in Fig. 1d) demonstrates the polycrystalline nature of the obtained yolk-shell ZnO-C microspheres. The morphology and microstructure of the products acquired from the carbonization of zinc citrate hollow microspheres and solid microspheres are displayed in Fig. 2. It is visible that ZnOC microspheres obtained from the carbonization of zinc citrate hollow microspheres possess the hollow structure (the inset in Fig. 2a and Fig. 2c). The average diameter of the hollow microspheres and thickness of the shell are about 1.6 m and 250 nm, respectively. The surface of the microsphere (Fig. 2b) is consisted of many nanoparticles with the particle size ranging from 20 to 30 nm, which is similar with that of yolk-shell ZnO-C microspheres. The SEM and TEM images revealed in Fig. 2d and 2f show the solid structures of ZnO-C microspheres acquired from the carbonization of zinc citrate solid microspheres. The average diameter of the solid microspheres is about 1.45 m, which is slight smaller than the yolk-shell and hollow microspheres. From the magnified SEM image shown in Fig. 2e, the size of the nanoparticle building blocks for ZnO-C solid microspheres is approximate 45 nm. Based on the above observation and analysis, it can be pronounced that ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres can be easily fabricated via the good morphology inheritance of zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres by directly carbonizing various zinc citrate microspheres in argon. Shown in Fig. 3 are the XRD patterns of the as-obtained ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. All the diffraction peaks can be indexed to hexagonal ZnO (JCPDS card no. 89-0510). The absence of the diffraction peaks originated from carbon indicates the amorphous nature of such derived carbon. Apart from ZnO diffraction peaks, no additional peaks can be detected, suggesting the high purity of the products. In addition, the intensities of diffraction peaks for ZnO-C yolk-shell microspheres and hollow microspheres are stronger than ZnO-C solid counterparts, which may be ascribed to the higher content of carbon in ZnO-C solid microspheres (TGA results discussed below). The carbon contents in the products are determined based on the TGA measurement. As shown in Fig. 4, the different amounts of weight loss below 350 ◦ C in all three samples are caused by the removal of physically adsorbed water. The amorphous carbon decomposes between 350 and 500 ◦ C, which is similar with the results of SnO2 -C hollow microspheres and MoS2 -C nanotubes [22,23]. The carbon contents of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres are calculated to be 6.34%, 6.30% and 9.82%, respectively. The contents of carbon for ZnO-C yolk-shell and hollow microspheres are almost
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Fig. 1. The low (a) and high (b) SEM micrographs of yolk-shell ZnO-C microspheres. The inset in Fig. 1a reveals a broken microsphere. The scale bar represents 200 nm. The TEM (c) and HRTEM (d) images of yolk-shell ZnO-C hollow microspheres. The inset in Fig. 1d is the corresponding SAED pattern of yolk-shell ZnO-C microspheres.
Fig. 2. (a-c) The SEM and TEM micrographs of ZnO-C hollow microspheres. The scale bar in the inset represents 200 nm. (d-f) The SEM and TEM images of ZnO-C solid microspheres.
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Fig. 3. The XRD patterns of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres.
the same and ZnO-C solid microspheres possess the highest content of carbon. N2 adsorption-desorption isotherms were performed to gain insight into the textural features of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. As shown in Fig. 5, ZnO-C yolk-shell microspheres reveal the highest BrunauerEmmett-Teller (BET) surface area of 99.7 m2 g −1 than the hollow microspheres (86.6 m2 g −1 ) and solid microspheres (53.2 m2 g −1 ). The pore size distributions (the inset in Fig. 5) of above three samples were determined based on the Barrett–Joyner–Halenda method, which indicates the presence of numerous nanopores in all three samples. Accordingly, the large surface area possess more electrochemical active sites and the porosity can provide an effective channel for the diffusion of the electrolyte, which is beneficial to improve the electrochemical properties of electrode materials [24]. 3.2. Electrochemical properties The electrochemical properties of all three samples used as anode materials for lithium ion batteries were investigated. Fig. 6a reveals the cyclic voltammograms (CVs) of yolk-shell ZnO-C microspheres at a scan rate of 0.1 mV s−1 in the voltage ranging from 0.01 to 3 V. In the first lithium process, two cathodic peaks located at 0.5 and 0.25 V can be found clearly. The relative weak peak at 0.5 V originates from the reduction of ZnO into Zn and
Fig. 4. TG curves for ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres.
Fig. 5. N2 adsorption-desorption isotherms of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. The insets show the corresponding pore size distributions.
the formation of amorphous Li2 O, while the strong peak nearby 0.25 V is caused by the generation of Li-Zn alloy together with the decomposition of electrolyte and the resulting growth of organic-like solid electrolyte interphase (SEI) layer [16,19,25,26]. In the subsequent delithium process, three weak oxidation peaks centred at 0.27, 0.52 and 0.63 V can be carefully discerned which can be attributed to a multistep dealloying of Li-Zn alloy [26,27]. The broad oxidation peak located at 1.35 V corresponds to the formation of ZnO [28]. Recently, some researches demonstrated that there is slight difference in the position of the broad oxidation peak for various ZnO or ZnO-based electrodes [16,19,29,30]. For example, Baibarac et al. reported that the broad oxidation peak of ZnO nanoneedle electrodes located at 1.54 V during the first cycle [29]. The broad oxidation peaks of ZnO porous nanosheets and powders centered at 1.38 and 1.46 V, respectively [19]. ZnO/MnO2 sea urchans-like arrays and ZnO nanorod arrays fabricated by Yuan exhibited the broad oxidation peaks nearby 1.33 and 1.36 V, respectively [30]. From the above analysis, it is reasonable to conclude that the position of the broad oxidation peak originated from the regeneration of ZnO during charge process may relate to the morphology and composition of ZnO or ZnO-based electrode
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Fig. 6. (a) Cyclic voltammograms of yolk-shell ZnO-C microspheres tested at 0.1 mV s−1 in 0.01-3 V. (b) Discharge-charge curves of yolk-shell ZnO-C microspheres at 100 mA g−1 . (c) Discharge capacities and Coulombic efficiencies vs cycle number of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres at 100 mA g−1 . (d) Rate performance of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres.
materials. In the second scan, there is only a peak nearby 0.6 V in the cathodic curve. And the oxidation peaks in the anodic curve slightly move to higher potential. From the second cycle onwards, the CV curves almost superpose in shape, implying the good reversibility of the electrochemical reactions. The CV results of ZnO-C hollow microspheres and solid microspheres are similar with that of ZnO-C yolk-shell microspheres (Fig. S2). According to above results and previous studies, the electrochemical reactions of yolk-shell ZnO-C microspheres are proposed as follows [19,31]: ZnO + 2Li+ + 2e− ↔ Zn + Li2 O
(1)
Zn + xLi+ + xe− ↔ Lix Zn (x ≤ 1)
(2)
The galvanostatic discharge-charge curves for yolk-shell ZnOC electrode at 100 mA g−1 in the voltage range of 0.01-3.0 V are depicted in Fig. 6b. Two plateaus in the first discharge curve can be observed. The long plateau located at 0.5 V is attributed to the reduction of ZnO into Zn as well as the formation of Li2 O, while the relative weak plateau appeared at 0.25 V may be assigned to the generation of Li-Zn alloy and the decomposition of electrolyte, which is in good agreement with the above CV results [15,20,32]. In the second discharge curve, the long plateau is replaced by a slope between 1.1 and 0.3 V, which is similar with other ZnO-based electrodes reported previously [16,18,19]. However, no obvious plateaus can be observed in the charge curves. After the first cycle, the curves are similar in shape, implying the excellent reversibility of the electrochemical reactions. The initial discharge and charge capacities of yolk-shell ZnO-C microspheres are 1432 and 798 mA h g−1 , respectively. The low Coulombic efficiency of about 55.7% at first cycle is common for the most metal oxide electrode materials, which is mainly caused by the formation of SEI film [27,33]. The voltage-capacity profiles of ZnO-C hollow microspheres (Fig. S3) and solid microspheres (Fig. S4) are analogous to that of the above ZnO-C yolk-shell microspheres. The initial discharge/charge capacities of ZnO-C hollow microspheres and solid microspheres are 1397/682 and 1322/676 mA h g−1 with the corresponding Coulombic efficiencies of 48.8% and 51.1%, respectively. Yolk-shell
ZnO-C microspheres reveal the highest discharge/charge capacities and Coulombic efficiency in the initial stage compared with other two ZnO-C counterparts. Fig. 6c displays the cycling performance and the corresponding Coulombic efficiencies of all three samples measured at 100 mA g−1 . In the first 25th cycles, a large capacity fading can be found in all three samples, while in the following cycles all three samples show similarly excellent cycling stability. After 150 cycles, ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres exhibit a reversibility capacity of 520, 427 and 341 mA h g−1 with the Coulombic efficiencies of 99.3%, 99.7% and 99.7%, respectively. Yolk-shell ZnO-C microspheres exhibit the highest discharge capacity. For comparison, the electrochemical properties of other ZnO nanostructures or ZnO-based electrode materials are also provided in Table 1. For instance, a reversible capacity of 392 mA h g−1 can be acquired at 120 mA g−1 for Au-ZnO flower-like nanostructures after 50 cycles [16]. Ultrathin ZnO nanotubes fabricated by Park et al. demonstrate a reversible capacity of 386 mA h g−1 after 50 cycles at 0.5 C [15]. Carbon/ZnO nanorod arrays deliver a discharge capacity of 330 mA h g−1 after 50 cycles [18]. It is obvious that yolk-shell ZnO-C microspheres show a relative higher reversible capacity and better cycling stability in comparison with other ZnO or ZnO-based electrode materials. The rate capabilities of all three samples were also measured and the results are depicted in Fig. 6d. Compared to ZnO-C hollow microspheres and solid microspheres, ZnO-C yolkshell microspheres display enhanced rate capability. The average reversible capacities of ZnO-C yolk-shell microspheres range from 764, 465, 339 to 212 mA h g−1 as the current densities increase from 100, 200, 500 to 1000 mA g−1 . When the current density is back to 100 mA g−1 , a reversible capacity of 416 mA h g−1 can be acquired again. The enhanced electrochemical performance of yolk-shell ZnO-C microspheres can be attributed to its special yolk-shell structures and extra carbon support. First, yolk-shell ZnO-C microspheres have the largest surface area than the hollow and solid counterparts, which can provide largest electrode/electrolyte contact area and most active sites. In addition, the porosity of the samples can
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Table 1 The electrochemical properties of various ZnO or ZnO-based electrode materials. Materials
Morphology
Reversible capacity (mA h g−1 )
Cycles
Ref
ZnO ZnO ZnO ZnO/C ZnO/Au ZnO/Se ZnO/SnO2 ZnO/Fe2 O3 ZnO/NiO/C ZnO/C
Ultrathin nanotubes Dandelion-like nanorod arrays Porous nanosheets Nanorod arrays Flower-like nanostructures Nanocomposites Nanocomposites Flower-like films Flower-like films Yolk-shell/hollow/solid microspheres
386 310 400 330 392 <400 497 776 488 520/427/341
50 40 100 50 50 100 40 50 50 150
[15] [26] [19] [18] [16] [34] [35] [36] [20] Our work
provide an effective channel for the diffusion of the electrolyte, benefiting to the improvement of the reversible capacity [24]. Second, yolk-shell structures possess better structure stability than hollow and solid structures during the repeated Li+ insertion/extraction process. Compared to solid structures, the large void space in yolk-shell structures can effectively accommodate the huge volume variation during discharge-charge process and then prevent the electrode from pulverization to some extent, which is in favor of the cycling stability [2,37]. Additionally, in recent years, studies have found that yolk-shell structures have more robust shells and improved mechanical strength in comparison with the hollow structures due to the synergistic effect of the cores and shells, which can provide better tolerance for structural degradation during lithium/delithium process [38–40]. For example, yolk-shell CoMn2 O4 microspheres revealed better electrochemical properties than hollow microspheres fabricated under the same conditions [41]. Third, the building blocks of yolk-shell ZnO-C microspheres are in nano-scale dimension, which can reduce the Li+ diffusion length and then improve the kinetics of electrochemical reactions [37]. Finally, the nanocarbon originated from the in situ carbonization of carboxylic acid groups in zinc citrate can not only improve the conductivity of ZnO electrodes, but also provide extra support to the structures, which plays a positive role in the high reversible capacity and excellent cyclability of the electrode materials [42,43]. 4. Conclusions In summary, ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres were fabricated through a facile route using zinc citrate microspheres with different structures as precursors. The carboxylic acid groups in zinc citrate function as the in situ carbon source in the carbonization process. When applied as the anode materials for lithium ion batteries, all three samples exhibit similarly excellent cycling stability. Yolk-shell ZnO-C microspheres possess the highest discharge/charge capacities and Coulombic efficiency during the first lithium-delithium process compared with other two ZnO-C counterparts. And after 150 cycles, a high reversible capacity of 520 mA h g−1 can be acquired for yolkshell ZnO-C microspheres. The relative higher reversible capacity and superior cyclability of yolk-shell ZnO-C microspheres are due to its largest surface area, better structural stability and extra carbon support, which can provide more electrochemical active sites, effectively alleviate the huge volume change during Li+ uptake and removel process and enhance the conductivity of active materials. This novel and facile method may be extended to prepare other yolk-shell micro/nanostructures which may find applications in lithium ion batteries. Acknowledgements The authors gratefully acknowledge financial support from the National Basic Research Program of China (No. 2012CB933103),
the National Outstanding Youth Science Foundation of China (Grant No. 50825101), the National Natural Science Foundation of China (Grant Nos. 51171158 and 51371154) and the Fundamental Research Funds for the Central Universities of China (Grant no. 201312G003).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2014.02.003.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Yolk-shell ZnO-C microspheres with enhanced electrochemical performance as anode material for lithium ion batteries Qingshui Xie a , Xiaoqiang Zhang a , Xiaobiao Wu b , Huayi Wu b , Xiang Liu a , Guanghui Yue a , Yong Yang b , Dong-Liang Peng a,∗ a Department of Materials Science and Engineering, College of Materials, Fujian Key Laboratory of Advanced Materials, Xiamen University, Xiamen 361005, China b State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 29 November 2013 Received in revised form 27 January 2014 Accepted 1 February 2014 Available online 15 February 2014 Keywords: Yolk-shell Zinc oxide Carbon Anode Lithium ion batteries
a b s t r a c t Three ZnO-C samples with distinct structures including yolk-shell microspheres, hollow microspheres and solid microspheres are fabricated through a facile chemical solution reaction followed by calcination in argon. When employed as the anode materials for lithium ion batteries, yolk-shell ZnO-C microspheres exhibit the best electrochemical properties than the hollow and solid microspheres. After 150 cycles, yolk-shell ZnO-C microspheres demonstrate a relative high capacity of 520 mA h g−1 at a current density of 100 mA g−1 with a Coulombic efficiency of about 99.3%. The excellent cycling stability and good rate capability of yolk-shell ZnO-C microspheres stem from the synergistic effect of the unique yolk-shell structures and extra carbon support. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, hollow micro/nanostructures have been widely investigated as anode materials for lithium ion batteries due to their large surface area and internal void space which can provide the large electrode/electrolyte contact area and effectively accommodate the huge volume variation during the Li+ intercalation/deintercalation process, respectively [1–5]. More recently, yolk-shell structures as one of the emerging hollow structures have received increasingly attention in lithium ion batteries [6–8]. For example, Lou’s group reported the successful preparation of yolk-shell V2 O5 microspheres through templating method, which revealed a high reversible capacity of about 187 mA h g−1 at 300 mA g−1 after 100 cycles [8]. Son et al. found that yolk-shell Co3 O4 microspheres displayed significantly enhanced electrochemical performance in comparison with the hollow Co3 O4 microspheres synthesized by the same route, which was ascribed to the better structure stability of the appealing yolk-shell construction [9]. Yolk-shell Sn/C powders prepared by Zhang et al. showed superior lithium storage capacity and excellent cycling stability [10]. Inspired by above studies, to fabricate various electrode
∗ Corresponding author. Tel.: +86 592 2180155; fax: +86 592 2183515. E-mail address: [email protected] (D.-L. Peng). http://dx.doi.org/10.1016/j.electacta.2014.02.003 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
materials with yolk-shell construction would be an effective strategy to improve their electrochemical properties. However, it is still a great challenge to develop a facile and cost-effective route to large-scale synthesize various yolk-shell structures till now. Transition metal oxides have gained more and more interest as electrode materials in lithium ion batteries in the last decades because of their higher theoretical capacity and safety compared with the conventional carbon materials [11–14]. Among them, ZnO has some special advantages such as low cost, easy preparation, morphologic diversity, environmental benignity and so on. Unfortunately, the poor electronic conductivity, large volume change during lithium/delithium process and the resulting severe capacity fading hinder its practical application. Hitherto, some efforts have been made to improve its cycling performance including the proper nanostructuring and decoration by metal, metal oxide and carbon [15–20]. For example, Huang’s group directly fabricated ZnO nanosheets on copper current-collecting substrates, which delivered the higher capacity and better cycling stability than commercial ZnO powders [19]. Although the reversible capacity of ZnO electrodes has been improved to some extent after appropriate construction and modification, the cyclability of ZnO anodes is still not satisfactory for developing advanced electrode materials for next-generation lithium ion batteries. Therefore, to further improve the cycling stability and reversible capacity of ZnO
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electrode materials through elaborate construction and decoration is still highly desirable. In this article, three different ZnO-C structures including yolkshell microspheres, hollow microspheres and solid microspheres were synthesized through a two step method. In the initial synthesis process, zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres were fabricated through a simple chemical solution reaction. Then the carboxylic acid groups in zinc citrate function as the in situ carbon source to prepare three different ZnO-C samples in the subsequent carbonization treatment. To the best of our knowledge, this is the first report on the successful synthesis of yolk-shell ZnO-C microspheres. The investigation of electrochemical properties suggests that all three samples exhibit similarly excellent cycling stability and yolk-shell ZnO-C microspheres possess the highest reversible capacity and best rate capability than the hollow and solid counterparts. 2. Experimental 2.1. Synthesis In a typical synthesis, amorphous zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres were firstly prepared through a facile chemical solution route as described in our previous literature [21]. Then the as-obtained zinc citrate microspheres with different structures were carbonized at 500 ◦ C for 2 h with a heating rate of 2 ◦ C/min in argon than air. After heat treatment, the furnace was cooled to room temperature naturally and then the black powders can be harvested. 2.2. Characterization PANalytical X’ pert PRO x-ray diffractometer (Cu K␣ radiation 40 kV, 30 mA) was employed to characterize the crystal phase of the products. Scanning electron microscopy (LEO-1530) and transmission electron microscopy (JEM-2100, 200 kV) were used to investigate the morphology and microstructure of the products. The thermogravimetric analysis (TGA) was carried out on a SDTQ600 thermal analyzer. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution were evaluated on a TriStar 3020 system. 2.3. Electrochemical measurements Two-electrode cells were used to characterize the electrochemical performance of the as-produced products. The working electrode was made up of 75 wt% active materials (ZnO-C microsheres with different structures), 15 wt% carbon black, and 10 wt% poly (vinyl difluoride) (PVDF). Li metal and 1 M LiPF6 dissolved in a mixed solution composed of equal volume of ethylene carbonate (EC) and diethyl carbonate (DEC) were employed as the counterelectrode and electrolyte, respectively. All the cells were assembled in an argon-filled glove box. The investigation of discharge-charge cycling performance was conducted on a multichannel battery testing system (Newware, China). Cyclic voltammetry (CV) was carried out using Autolab electrochemical workstation (NOVA 1.8) at a scanning rate of 0.1 mV s−1 at room temperature. 3. Results and discussion 3.1. Morphology and structure characterization Fig. 1 displays the morphology and microstructure of the as-obtained yolk-shell ZnO-C microspheres. The panoramic SEM image (Fig. 1a) implies that the sample is made up of dispersed
microspheres with an average diameter on the order of 1.6 m. From a broken microsphere as shown in the inset in Fig. 1a, the inner core, outer shell and interstitial void space can be obviously discerned, suggesting the yolk-shell structure of microsphere. From the magnified SEM micrograph (Fig. 1b), it can be observed that the shell is composed of numerous nanoparticles with an average size of 20 nm. Energy dispersive spectroscopy (EDS) microanalysis confirms the presence of Zn, O, C elements (Fig. S1). The TEM investigation was carried out to provide detailed structure of the samples. As depicted in Fig. 1c, the yolk-shell structure is further validated. The thickness of the shell and the diameter of the core are about 280 and 400 nm, respectively. The lattice fringes with a spacing of 0.281 nm in HRTEM image (Fig. 1d) recorded from the shell relate to the spacing of the (100) plane of ZnO. Nanocarbon in yolkshell ZnO-C microspheres is difficult to be directly distinguished even under high-magnification TEM observation in view of the carbon stem from the in situ carbonization of carboxylic acid groups in zinc citrate. Differently, for conventional carbon-coated materials, nanocarbon-layer can be clearly seen under HRTEM observation [18]. The corresponding SAED pattern (the inset in Fig. 1d) demonstrates the polycrystalline nature of the obtained yolk-shell ZnO-C microspheres. The morphology and microstructure of the products acquired from the carbonization of zinc citrate hollow microspheres and solid microspheres are displayed in Fig. 2. It is visible that ZnOC microspheres obtained from the carbonization of zinc citrate hollow microspheres possess the hollow structure (the inset in Fig. 2a and Fig. 2c). The average diameter of the hollow microspheres and thickness of the shell are about 1.6 m and 250 nm, respectively. The surface of the microsphere (Fig. 2b) is consisted of many nanoparticles with the particle size ranging from 20 to 30 nm, which is similar with that of yolk-shell ZnO-C microspheres. The SEM and TEM images revealed in Fig. 2d and 2f show the solid structures of ZnO-C microspheres acquired from the carbonization of zinc citrate solid microspheres. The average diameter of the solid microspheres is about 1.45 m, which is slight smaller than the yolk-shell and hollow microspheres. From the magnified SEM image shown in Fig. 2e, the size of the nanoparticle building blocks for ZnO-C solid microspheres is approximate 45 nm. Based on the above observation and analysis, it can be pronounced that ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres can be easily fabricated via the good morphology inheritance of zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres by directly carbonizing various zinc citrate microspheres in argon. Shown in Fig. 3 are the XRD patterns of the as-obtained ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. All the diffraction peaks can be indexed to hexagonal ZnO (JCPDS card no. 89-0510). The absence of the diffraction peaks originated from carbon indicates the amorphous nature of such derived carbon. Apart from ZnO diffraction peaks, no additional peaks can be detected, suggesting the high purity of the products. In addition, the intensities of diffraction peaks for ZnO-C yolk-shell microspheres and hollow microspheres are stronger than ZnO-C solid counterparts, which may be ascribed to the higher content of carbon in ZnO-C solid microspheres (TGA results discussed below). The carbon contents in the products are determined based on the TGA measurement. As shown in Fig. 4, the different amounts of weight loss below 350 ◦ C in all three samples are caused by the removal of physically adsorbed water. The amorphous carbon decomposes between 350 and 500 ◦ C, which is similar with the results of SnO2 -C hollow microspheres and MoS2 -C nanotubes [22,23]. The carbon contents of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres are calculated to be 6.34%, 6.30% and 9.82%, respectively. The contents of carbon for ZnO-C yolk-shell and hollow microspheres are almost
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Fig. 1. The low (a) and high (b) SEM micrographs of yolk-shell ZnO-C microspheres. The inset in Fig. 1a reveals a broken microsphere. The scale bar represents 200 nm. The TEM (c) and HRTEM (d) images of yolk-shell ZnO-C hollow microspheres. The inset in Fig. 1d is the corresponding SAED pattern of yolk-shell ZnO-C microspheres.
Fig. 2. (a-c) The SEM and TEM micrographs of ZnO-C hollow microspheres. The scale bar in the inset represents 200 nm. (d-f) The SEM and TEM images of ZnO-C solid microspheres.
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Fig. 3. The XRD patterns of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres.
the same and ZnO-C solid microspheres possess the highest content of carbon. N2 adsorption-desorption isotherms were performed to gain insight into the textural features of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. As shown in Fig. 5, ZnO-C yolk-shell microspheres reveal the highest BrunauerEmmett-Teller (BET) surface area of 99.7 m2 g −1 than the hollow microspheres (86.6 m2 g −1 ) and solid microspheres (53.2 m2 g −1 ). The pore size distributions (the inset in Fig. 5) of above three samples were determined based on the Barrett–Joyner–Halenda method, which indicates the presence of numerous nanopores in all three samples. Accordingly, the large surface area possess more electrochemical active sites and the porosity can provide an effective channel for the diffusion of the electrolyte, which is beneficial to improve the electrochemical properties of electrode materials [24]. 3.2. Electrochemical properties The electrochemical properties of all three samples used as anode materials for lithium ion batteries were investigated. Fig. 6a reveals the cyclic voltammograms (CVs) of yolk-shell ZnO-C microspheres at a scan rate of 0.1 mV s−1 in the voltage ranging from 0.01 to 3 V. In the first lithium process, two cathodic peaks located at 0.5 and 0.25 V can be found clearly. The relative weak peak at 0.5 V originates from the reduction of ZnO into Zn and
Fig. 4. TG curves for ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres.
Fig. 5. N2 adsorption-desorption isotherms of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. The insets show the corresponding pore size distributions.
the formation of amorphous Li2 O, while the strong peak nearby 0.25 V is caused by the generation of Li-Zn alloy together with the decomposition of electrolyte and the resulting growth of organic-like solid electrolyte interphase (SEI) layer [16,19,25,26]. In the subsequent delithium process, three weak oxidation peaks centred at 0.27, 0.52 and 0.63 V can be carefully discerned which can be attributed to a multistep dealloying of Li-Zn alloy [26,27]. The broad oxidation peak located at 1.35 V corresponds to the formation of ZnO [28]. Recently, some researches demonstrated that there is slight difference in the position of the broad oxidation peak for various ZnO or ZnO-based electrodes [16,19,29,30]. For example, Baibarac et al. reported that the broad oxidation peak of ZnO nanoneedle electrodes located at 1.54 V during the first cycle [29]. The broad oxidation peaks of ZnO porous nanosheets and powders centered at 1.38 and 1.46 V, respectively [19]. ZnO/MnO2 sea urchans-like arrays and ZnO nanorod arrays fabricated by Yuan exhibited the broad oxidation peaks nearby 1.33 and 1.36 V, respectively [30]. From the above analysis, it is reasonable to conclude that the position of the broad oxidation peak originated from the regeneration of ZnO during charge process may relate to the morphology and composition of ZnO or ZnO-based electrode
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Fig. 6. (a) Cyclic voltammograms of yolk-shell ZnO-C microspheres tested at 0.1 mV s−1 in 0.01-3 V. (b) Discharge-charge curves of yolk-shell ZnO-C microspheres at 100 mA g−1 . (c) Discharge capacities and Coulombic efficiencies vs cycle number of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres at 100 mA g−1 . (d) Rate performance of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres.
materials. In the second scan, there is only a peak nearby 0.6 V in the cathodic curve. And the oxidation peaks in the anodic curve slightly move to higher potential. From the second cycle onwards, the CV curves almost superpose in shape, implying the good reversibility of the electrochemical reactions. The CV results of ZnO-C hollow microspheres and solid microspheres are similar with that of ZnO-C yolk-shell microspheres (Fig. S2). According to above results and previous studies, the electrochemical reactions of yolk-shell ZnO-C microspheres are proposed as follows [19,31]: ZnO + 2Li+ + 2e− ↔ Zn + Li2 O
(1)
Zn + xLi+ + xe− ↔ Lix Zn (x ≤ 1)
(2)
The galvanostatic discharge-charge curves for yolk-shell ZnOC electrode at 100 mA g−1 in the voltage range of 0.01-3.0 V are depicted in Fig. 6b. Two plateaus in the first discharge curve can be observed. The long plateau located at 0.5 V is attributed to the reduction of ZnO into Zn as well as the formation of Li2 O, while the relative weak plateau appeared at 0.25 V may be assigned to the generation of Li-Zn alloy and the decomposition of electrolyte, which is in good agreement with the above CV results [15,20,32]. In the second discharge curve, the long plateau is replaced by a slope between 1.1 and 0.3 V, which is similar with other ZnO-based electrodes reported previously [16,18,19]. However, no obvious plateaus can be observed in the charge curves. After the first cycle, the curves are similar in shape, implying the excellent reversibility of the electrochemical reactions. The initial discharge and charge capacities of yolk-shell ZnO-C microspheres are 1432 and 798 mA h g−1 , respectively. The low Coulombic efficiency of about 55.7% at first cycle is common for the most metal oxide electrode materials, which is mainly caused by the formation of SEI film [27,33]. The voltage-capacity profiles of ZnO-C hollow microspheres (Fig. S3) and solid microspheres (Fig. S4) are analogous to that of the above ZnO-C yolk-shell microspheres. The initial discharge/charge capacities of ZnO-C hollow microspheres and solid microspheres are 1397/682 and 1322/676 mA h g−1 with the corresponding Coulombic efficiencies of 48.8% and 51.1%, respectively. Yolk-shell
ZnO-C microspheres reveal the highest discharge/charge capacities and Coulombic efficiency in the initial stage compared with other two ZnO-C counterparts. Fig. 6c displays the cycling performance and the corresponding Coulombic efficiencies of all three samples measured at 100 mA g−1 . In the first 25th cycles, a large capacity fading can be found in all three samples, while in the following cycles all three samples show similarly excellent cycling stability. After 150 cycles, ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres exhibit a reversibility capacity of 520, 427 and 341 mA h g−1 with the Coulombic efficiencies of 99.3%, 99.7% and 99.7%, respectively. Yolk-shell ZnO-C microspheres exhibit the highest discharge capacity. For comparison, the electrochemical properties of other ZnO nanostructures or ZnO-based electrode materials are also provided in Table 1. For instance, a reversible capacity of 392 mA h g−1 can be acquired at 120 mA g−1 for Au-ZnO flower-like nanostructures after 50 cycles [16]. Ultrathin ZnO nanotubes fabricated by Park et al. demonstrate a reversible capacity of 386 mA h g−1 after 50 cycles at 0.5 C [15]. Carbon/ZnO nanorod arrays deliver a discharge capacity of 330 mA h g−1 after 50 cycles [18]. It is obvious that yolk-shell ZnO-C microspheres show a relative higher reversible capacity and better cycling stability in comparison with other ZnO or ZnO-based electrode materials. The rate capabilities of all three samples were also measured and the results are depicted in Fig. 6d. Compared to ZnO-C hollow microspheres and solid microspheres, ZnO-C yolkshell microspheres display enhanced rate capability. The average reversible capacities of ZnO-C yolk-shell microspheres range from 764, 465, 339 to 212 mA h g−1 as the current densities increase from 100, 200, 500 to 1000 mA g−1 . When the current density is back to 100 mA g−1 , a reversible capacity of 416 mA h g−1 can be acquired again. The enhanced electrochemical performance of yolk-shell ZnO-C microspheres can be attributed to its special yolk-shell structures and extra carbon support. First, yolk-shell ZnO-C microspheres have the largest surface area than the hollow and solid counterparts, which can provide largest electrode/electrolyte contact area and most active sites. In addition, the porosity of the samples can
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Table 1 The electrochemical properties of various ZnO or ZnO-based electrode materials. Materials
Morphology
Reversible capacity (mA h g−1 )
Cycles
Ref
ZnO ZnO ZnO ZnO/C ZnO/Au ZnO/Se ZnO/SnO2 ZnO/Fe2 O3 ZnO/NiO/C ZnO/C
Ultrathin nanotubes Dandelion-like nanorod arrays Porous nanosheets Nanorod arrays Flower-like nanostructures Nanocomposites Nanocomposites Flower-like films Flower-like films Yolk-shell/hollow/solid microspheres
386 310 400 330 392 <400 497 776 488 520/427/341
50 40 100 50 50 100 40 50 50 150
[15] [26] [19] [18] [16] [34] [35] [36] [20] Our work
provide an effective channel for the diffusion of the electrolyte, benefiting to the improvement of the reversible capacity [24]. Second, yolk-shell structures possess better structure stability than hollow and solid structures during the repeated Li+ insertion/extraction process. Compared to solid structures, the large void space in yolk-shell structures can effectively accommodate the huge volume variation during discharge-charge process and then prevent the electrode from pulverization to some extent, which is in favor of the cycling stability [2,37]. Additionally, in recent years, studies have found that yolk-shell structures have more robust shells and improved mechanical strength in comparison with the hollow structures due to the synergistic effect of the cores and shells, which can provide better tolerance for structural degradation during lithium/delithium process [38–40]. For example, yolk-shell CoMn2 O4 microspheres revealed better electrochemical properties than hollow microspheres fabricated under the same conditions [41]. Third, the building blocks of yolk-shell ZnO-C microspheres are in nano-scale dimension, which can reduce the Li+ diffusion length and then improve the kinetics of electrochemical reactions [37]. Finally, the nanocarbon originated from the in situ carbonization of carboxylic acid groups in zinc citrate can not only improve the conductivity of ZnO electrodes, but also provide extra support to the structures, which plays a positive role in the high reversible capacity and excellent cyclability of the electrode materials [42,43]. 4. Conclusions In summary, ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres were fabricated through a facile route using zinc citrate microspheres with different structures as precursors. The carboxylic acid groups in zinc citrate function as the in situ carbon source in the carbonization process. When applied as the anode materials for lithium ion batteries, all three samples exhibit similarly excellent cycling stability. Yolk-shell ZnO-C microspheres possess the highest discharge/charge capacities and Coulombic efficiency during the first lithium-delithium process compared with other two ZnO-C counterparts. And after 150 cycles, a high reversible capacity of 520 mA h g−1 can be acquired for yolkshell ZnO-C microspheres. The relative higher reversible capacity and superior cyclability of yolk-shell ZnO-C microspheres are due to its largest surface area, better structural stability and extra carbon support, which can provide more electrochemical active sites, effectively alleviate the huge volume change during Li+ uptake and removel process and enhance the conductivity of active materials. This novel and facile method may be extended to prepare other yolk-shell micro/nanostructures which may find applications in lithium ion batteries. Acknowledgements The authors gratefully acknowledge financial support from the National Basic Research Program of China (No. 2012CB933103),
the National Outstanding Youth Science Foundation of China (Grant No. 50825101), the National Natural Science Foundation of China (Grant Nos. 51171158 and 51371154) and the Fundamental Research Funds for the Central Universities of China (Grant no. 201312G003).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2014.02.003.
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