C Hybrids YolkâShell Microspheres with

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MOF-derived ZnO/Ni3ZnC0.7/C Hybrids Yolk-Shell Microspheres with Excellent Electrochemical Performances for Lithium Ion Batteries Yacong Zhao, Xin Li, Jiandi Liu, Chunge Wang, Yanyan Zhao, and Guang-Hui Yue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12562 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

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ACS Applied Materials & Interfaces

MOF-derived ZnO/Ni3ZnC0.7/C Hybrids Yolk-Shell Microspheres with Excellent Electrochemical Performances for Lithium Ion Batteries Yacong Zhao, Xin Li, Jiandi Liu, Chunge Wang, Yanyan Zhao, Guanghui Yue* Department of Materials Science and Engineering, College of materials, Xiamen University, Xiamen 361005, China KEYWORDS: ZnO/Ni3ZnC0.7/C, metal-organic frameworks, yolk-shell structure, anodes, lithium ion batteries

ABSTRACT: In this study, ZnO/Ni3ZnC0.7/C spheres were synthesized successfully via a simple method based on metal-organic frameworks (MOFs). The experimental results show that the reaction time has a great influence on the structure of the material. ZnO/Ni3ZnC0.7/C spheres with controlled solid and yolk-shell structures have been obtained by altered the reaction time. When applied as the anode materials, both of the solid and yolk-shell ZnO/Ni3ZnC0.7/C composites present excellent electrochemical performance. In addition, what is worth mentioning that the yolk-shell composites structure property is superior to the solid ones in terms of lithium storage. The stable reversible capacity of yolk-shell ZnO/Ni3ZnC0.7/C can be retained at 1002 mA h g-1 at 500 mA g-1 when completion 750 cycles, and it also exhibits superior high rate

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performance. In contrast, the solid ZnO/Ni3ZnC0.7/C under the same conditions of testing, shows a reversible capacity of 824 mA h g-1.

1.

INTRODUCTION

Lithium ion batteries are widely applied in the field of modern digital products (as mobile phones, laptops) because of their many advantages, such as high energy density and long lifetime. With the gradual improvement of the performance requirements for energy storage devices (such as hybrid electric vehicles, electric bicycle and flexible/wearable electronics), the existing commercial lithium-ion batteries become increasingly difficult to keep pace with the demands of the common use.1, 2 Commercialized graphite used as a lithium-ion battery anode materials has a low theoretical capacity (372 mA h g-1) and limited rate capability which becomes one of the stumbling blocks for the further development of lithium ion batteries.3, 4 In order to enhance the electrochemical properties of lithium ion batteries, researchers have been trying various novel methods. In recent years, transition metal oxides (TMOs), have been discovered great potentials as electrode materials for lithium-ion batteries (LIBs) owing to their high theoretical capacity, low cost, and benign environmental friendliness.5-7 As a good alternative, ZnO delivers a higher theoretical capacity of lithium storage (987 mA h g-1) as well as lithium-ion diffusion coefficient in comparison with others.8 Although these kinds of materials have high theoretical capacity, their poor cycling performance arisen from the large volume variation in cycling processes and poor conductivity become the major stymie in practical application.9-11 To overcome aforementioned problems, researchers have sought a number of methods. One of the promising strategies is to mix two or more components together. Compared to materials with


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a single component, hybrid and multicomponent structured compounds may gain more perfect performance through a reinforcement or modification of each other, that is, the synergistic effect.12 Besides, during the repeated lithiation and de-lithiation processes, not all of the active materials react with Li+ at the same time, so that the remaining ones which have not a reaction with Li+ can effectively mitigate the stress and accommodate large volume change to hinder electrode material crush. Many of hybrid materials, such as, flower-like NiO/Ni structure,13 ZnO/Ni/C hollow microspheres,14 multilayer CuO@NiO hollow spheres15 have been reported with excellent electrochemical performance as anode materials for LIBs. When mixed with a certain metal, the metal of active substance can not only improve the electrical conductivity of the electrode material, but also act as a catalytic agent which benefits for improving the electrochemical properties of electrode material. Sun’ group reported a kind of Ni/NiO hybrid nanomembranes which showed the best rate capability in reported NiO-based anodes for Li-ion batteries up until that time.16 The other strategy is to synthesize special structure providing free space to with relieve relieving the structural strain to preventing the structure collapse during cyclic process, such as yolk-shell and hollow structure. Furthermore, the special structure not only can provide large electrode-electrolyte contact area and more active sites, but also effectively reduce distance for Li+ ions diffusion. Based on the advantages of these structures, the electrochemical properties of such electrode materials have been greatly improved. For instance, hollow polyhedral ZnxCo3-xO4,17 multi-shelled hollow spheres α-Fe2O318, 19 and yolkshell structure iron oxide@carbon20 with well-designed structures all showed highly reversible performance on lithium storage when regarded as anodes for lithium ion batteries. Zou and coworkers prepared porous ZnO/ZnFe2O4/C octahedra with hollow interiors using FeIII-modified MOF-5 that can reach a capacity of about 762 mA h g-1 at a current density of 10 A g-1.21 Hollow


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porous CoFe2O4nanocubes can manifest 815 mA h g-1 at a current density of 20 C and standout cycling performance.22 Zhang’ group successfully used carbonaceous spheres as template to synthesize yolk bi-shell structure MnxCo1-xFe2O4 hollow microspheres owning high performance of lithium storage.23 Metal-organic frameworks (MOFs), namely metal centers as network nodes and organic ligands as linkers, have high surface area and high porosity. Facts have proved that MOFs have tunable morphology and structure through a combination of different metal ions and organic ligands.22As a kind of excellent material, MOFs have been demonstrated considerable applications in many fields, such as catalysis,24-26 gas storage,27, 28 drug delivery,29 health monitoring,30 separations31 and so on. It has been proved that MOFs are effective templates or precursors to synthesize yolkshell structure transition metal oxides. Zhang et al. succeed in the preparation of MOF-derived ZnO@ZnO quantum dots/C core-shell nanorod arrays on a carbon cloth.8 Starfish-shaped Co3O4/ZnFe2O4 hollow nanocomposite can be fabricated by using MOF as precursors.32 Notably, the kind of MOF-derived transition metal oxides has some advantages compared with each other in details. As we know, the presence of carbon in the electrode material is beneficial to improve the electrochemical performances of the active material. The ligands of MOFs can be regarded as the carbon source introducing carbon via a thermolysis procedure. There is no need to introduce a carbon source to increase carbon content after the target product is synthesized. So, as far as the method of operation is concerned, it is convenient and economical. What’s important, compared to other templates/precursors, MOFs can offer the variable pore size and unique morphology through combination of distinct organic bridging ligands and metal ions, which may provide a way to produce a variety of functional materials for their possible applications. It is amazing that these MOFs derived transition metal oxides present enhanced


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lithium storage properties when used for anode materials of lithium ion battery. Liu et al. reported that yolk-shell structured NiO@C can retain stable reversible capacity at a current of 962 mA h g-1 when completed 200 cycles, and it also displays perfect rate performance.33 Ge et al. succeed in preparing core/shell structured ZnO/ZnCo2O4/C composites derived from MOF template, of which capacity retained 669 mA h g-1 after 250 cycles at a current rate of 500 mA h g-1.34 Hu et al. prepared novel CuO/Cu2O hollow polyhedrons and its capacity as high as 740 mA h g-1 at 100 mA g-1 after 250 cycles.35 For the purpose of improving lithium storage performance of ZnO anodes, we demonstrate a feasible and facile strategy to synthesize ZnO/Ni3ZnC0.7/C hybrid yolk-shell microspheres from metal-organic frameworks. The metallic Ni in the Ni3ZnC0.7 phase can serve as a catalyst for the Li2O decomposition. The catalytic effect of Ni plays a key role in improving the electrochemical properties of ZnO.36 2. EXPERIMENTAL SECTION 2.1. Synthesis All the chemicals used are analytical grade without further purification. 0.1329 g nickel nitrate hexahydrate, 0.1359 g zinc nitrate hexahydrate, 0.064 g 1, 3, 5 - benzenetricarboxylic acid and 0.125 g glucose were dissolved in 72 ml mixture of glycol and N, N-dimethylformamide (DMF) (1:1, V/V) under magnetic stirring to form a homogeneous solution. After stirring for a period of time, the mixed solution was transferred to 100 ml Teflon-lined autoclave and heated at 150。C in an oven for a period of time (32h and 80h, respectively). Then the product could be harvested through the centrifugation with ethanol and DMF for several times. Subsequently, the precursor


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was dried at 60 。C in air for overnight and annealed at 450 。C in argon flow for 2 h with a 。

heating rate of 1 C min-1.

2.2. Characterizations The structures and compositions of as-prepared products were analyzed by using an X-ray diffractometer (PANalytical X’ pert PRO, Cu Kα radiation 40 kV, 30 mA). And JEM-2100 (200 kV) and TECNAI F30 (300 kV) were applied to acquire transmission electron microscopy images. Scanning electron microscopy (SEM) images were obtained from Hitachi SU-70 and LEO 1530. Fourier transform infrared (FTIR) spectra were recorded by Nicolet Nexus-670 FTIR spectrometer. Thermogravimetric (TG) evaluation was conducted by SDT-Q600 thermal analyzer in air. The element analysis is carried out by Vario EL Ⅲ Elemental Analyzer. The Brunauer Emmett-Teller (BET) surface area and pore size distributions were carried out by a TriStar 3020 system. 2.3. Electrochemical investigations For electrochemical measurements, the active materials, carbon black and polyvinyl difluoride (PVDF) were mixed in a weight ratio of 70:20:10. Then Nmethylpyrrolidone (NMP) was added dropwise to the mixture and stirred until homogeneous. The mixed slurry was pasted on copper 。 foils and subsequently dried at 80 C for 12 h under vacuum. After that, coin-type cells were

assembled in an argon-filled glove box. The coin-type cells are basically composed of metallic lithium foil as counter electrode, Celgard 2400 polypropylene as separator, and 1 M LiPF6 dissolved in a mixture made of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as electrolyte. The galvanostatic discharge-charge cycling investigations were carried


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out on a Newware multichannel battery testing system. The cyclic voltammetry (CV) studies were performed on an electrochemical workstation (Potentiostat/Galvanostat Model 263A). And the electrochemical impedance spectra (EIS) were conducted on an Autolab electrochemical workstation (NOVA 1.9) in the range of 0.01 Hz to 100 kHz. The specific capacities of ZnO/Ni3ZnC0.7 active materials are calculated by deduction the mass of carbon. 3. RESULTS AND DISCUSSION The XRD pattern of precursor is displayed in Figure 1a. In the XRD pattern of precursor powder, there is a set of peaks from metallic nickel. Accordingly, we can deduce that Ni2+ was reduced to metallic nickel by some reductive group in solvent during thermal solvent process. From Fourier transform infrared spectra (Figure S1), it have been proved that the existence of MOFs in precursor. According to the test results, we consider the rest of peaks (except for metallic nickel peaks) in the XRD pattern of precursor should belong to MOFs. As displayed in Figure 1b, the XRD pattern of as-prepared powder demonstrates that the product is composed of ZnO and Ni3ZnC0.7. Two sets of diffraction peaks were distinguished by different signs. After analysis, we can know one set of the diffraction peaks corresponds to hexagonal ZnO (JCPDF #89-0510) and the other belongs to cubic Ni3ZnC0.7 (JCPDF #28-0713). No impurity peak can be found from the figure which reveals high purity of sample. The XRD patterns of products with 32h and 80h reaction time are depicted in Figure S2. It shows that the phase of as-obtained ZnO/Ni3ZnC0.7/C compound do not vary with the altering of reaction time. Thermogravimetric (TG) analyses of yolk-shell and solid structure ZnO/Ni3ZnC0.7/C are shown in Figure S3. A weight loss below about 200 。 C is associated with desorption of physically adsorbed water in the sample. Subsequent, an upward peak emerges in the TG curves of solid


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and yolk-shell ZnO/Ni3ZnC0.7/C complexes, which may be due to Zn and Ni of the Ni3ZnC0.7 generate oxidation reaction.37The tentative inference on this result was further verified by the XRD pattern (FigureS4) of yolk-shell structure products after TG analysis. The XRD pattern shows that there are only ZnO and NiO peaks, that is, Ni3ZnC0.7 phase is completely 。

。

decomposed and oxidized into ZnO and NiO. The reduction of mass from 310 C to 500 C is due to the decomposition of amorphous carbon.38 So the carbon content of solid and yolk-shell ZnO/Ni3ZnC0.7/C are at last 12.16% and 11.16%, respectively. On the basis of elemental analysis (Table S1), the total weight percentage of carbon in the yolk-shell structure ZnO/Ni3ZnC0.7/C is about 10.89%, which is approximately consistent with the TG result. N2 adsorption-desorption isotherm was executed to obtain a more comprehensive information on the structure of ZnO/Ni3ZnC0.7/C. As depicted in FigureS5, the Brunauer Emmett-Teller (BET) surface area of the hybrid ZnO/Ni3ZnC0.7/C yolk-shell structure microspheres reach to 112 m2 g1

. Notably, the specific surface area of ZnO/Ni3ZnC0.7/C is much larger than many formerly

reported TMOs products.35, 39, 40The pore size distribution (the inset in FigureS5) reveals that average pore diameters of the sample are in the range of 2-10 nm. It is favorable for electrode materials to own a large specific surface area and porosity in terms of electrochemical properties, because it can provide more electrochemical active sites and channels for the diffusion of the electrolyte, respectively. The morphology and microstructure of the prepared two kinds of Zn-MOF/Ni precursors are investigated by SEM and TEM. Figure 2 a-b and c-d are corresponding to the precursors with reaction time of 32h and 80h, respectively. It is apparent that both of the Zn-MOF/Ni microspheres are composed of solid nanoparticles. The TEM image (Figure 2c) of 32h Zn-


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MOF/Ni precursors reveals that it is solid structure. Compared to 32h Zn-MOF/Ni precursors in SEM, we can find that 80h Zn-MOF/Ni precursors showed apparent collapse on the surface. It indicated that the structure of Zn-MOF/Ni spheres varied from their reaction time. The TEM (f) further shows that 80h Zn-MOF/Ni precursor is yolk-shell structure. From the information obtained from both of TEM images, we can further verify this inference which the reaction time have a certain effect on the structure of Zn-MOF/Ni spheres. 。

The morphology and structure of the ZnO/Ni3ZnC0.7/C spheres yielded by calcinations at 450 C is thoroughly characterized by SEM and TEM. As revealed in Figure 3a-b, d-e, Zn-MOF/Ni precursors were transformed to ZnO/Ni3ZnC0.7/C with well-retained spherical shape after heat treatment, meanwhile the reason why the shell surface of the spheres becomes more rough is happen to compact assembly of nano-sized small particles during heat treatment. Energy dispersive spectroscopy (EDS) measurement of yolk-shell structure ZnO/Ni3ZnC0.7/C (Figure S6) proves the presence of Zn, Ni, O and C, while the molar ratio of Zn/Ni is 1.97.Combined with the carbon content measured by elemental analyzer, we calculated that the atomic ratio of Zn/Ni/O/C is about 82.13:41.72:68.22:90.67.And it also can be known that the weight percents of ZnO, Ni3ZnC0.7 and free carbon in the composites are about 55.51%, 34.76% and 9.72%, respectively. As-obtained ZnO/Ni3ZnC0.7/C products are uniform with spherical shape around 0.8-1.1 nm. It can also be discovered that some fragments adhere to the surface on both of ZnO/Ni3ZnC0.7/C products, which is formed by collecting the particles adhering on the surface of precursors during the heat treatment process. The panoramic TEM image reveals that 32h ZnO/Ni3ZnC0.7/C is solid without discernible porosity (Figure 3c). The panoramic SEM image (Figure 3d) shows 80h ZnO/Ni3ZnC0.7/C obviously present yolk-shell structure known from the partially broken shell vividly. The enlarged SEM image deciphers (Figure 2e) that the core of


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yolk-shell structure is consisted of solid nanoparticles and the shell structure with an average thickness about 40-50 nm. TEM image (Figure 3f) clearly discloses the yolk-shell structure with an obvious gap between the shell and the inner core, which is consistent with the features of SEM images presenting. It is worth mentioning that the gap we can clearly identify between the shell and the inner core of yolk-shell ZnO/Ni3ZnC0.7/C is larger than the precursors’. In addition, it can also be observed that the yolk-shell structure is highly porous. During the thermolysis of MOFs, the release of various gases (such as CO2, CO, NO) makes the yolk-shell sphere highly porous, which is conducive to increase the material active sites and improve the ability to maintain the structure stability of the material. The marked lattice spacing of 0.1830 and 0.2155 nm in a high resolution TEM lattice image of an individual yolk-shell structure ZnO/Ni3ZnC0.7/C (Figure 3g) match well to the plane spacing of (200) and (111) of Ni3ZnC0.7 (JCPDF #28-0713), respectively, which are in good consistent with the peak observed in the XRD. Line scan (Figure 3i) results show the most element of the outer shell and inner core are Ni and Zn, respectively, which indicate that the surface of ZnO/Ni3ZnC0.7/C is basically made up of Ni3ZnC0.7 and the main composition of core is ZnO. The high-angle annular dark-field (HAADF) scanning TEM (STEM) images of yolk-shell structure ZnO/Ni3ZnC0.7/C is shown in Figure 3j. The relative elemental mappings of Ni and Zn are exhibited in Figure 3k and l. As shown in above Figures, Ni signal emerge more intensely than Zn in the external shell, and the Zn signal reflected in the core area is more intense compared with that in the external shell area. In order to further confirm the structure of these solid and yolk-shell ZnO/Ni3ZnC0.7/C spheres, these two powders were dispersed into 4.0 M NaOH solution stirring for 6 h to selectively remove ZnO. XRD patterns (Figure S7a) of the solid ZnO/Ni3ZnC0.7/C treated by NaOH proved that ZnO had been successfully dissolved. The TEM images (Figure S7b, d) reveal that Ni3ZnC0.7 phase is


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uniformly distributed in the spheres both of solid and yolk-shell structure, which is consistent with the analysis results of line scan and element mapping. Furthermore, it can be discerned the presence of free carbon from magnified TEM image (Figure S7c).Based on the above analysis, we can draw conclusions that the main composition of the outer shell is Ni3ZnC0.7, ZnO and Ni3ZnC0.7are co-existence and uniform distribution in the core. In accordance with the above analysis of the test results, we summarize the synthesis process of ZnO/Ni3ZnC0.7/C, as shown in Scheme 1. During the solvothermal process, the Ni2+ is reduced to metallic nickel; the Zn2+ combines with H3BTC to form the Zn-MOF. At the outset, the sphere of Zn-MOF/Ni precursor firstly forms. A period of time later, a gap gradually emerges in the sphere, at the same time the sphere is divided into two parts, namely, the outer shell layer and inner solid core area. The transformation from the solid sphere to yolk-shell structure is on the basis of the Ostwald ripening mechanism.32 The SEM images (Figure S8) of time-dependent precursors disclose that that the gap between inner core and outer shell will gradually increase when prolonged the reaction time. In the process of thermal decomposition, material composition has a transformation accompanied with the release of some gas molecules (such as CO2, CO, NO). Therefore, after the heat treatment, the gap in the yolk-shell structure becomes larger and its characteristic becomes more obvious. A small amount of element Zn is produced during the pyrolysis process of the organic precursors, and then Zn and C dissolved in the Ni crystallites at a high temperature, which brings about the formation of Ni3ZnC0.7.36 In order to know about relational electrochemical performance of the obtained solid and yolkshell structure ZnO/Ni3ZnC0.7/C products used as anode material of lithium ion battery, a series of electrochemical tests are implemented. As shown in Figure 4a, cyclic voltammograms (CV) curves disclose the lithium storage mechanism of the yolk-shell ZnO/Ni3ZnC0.7/C composites


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during charge-discharge process. During the first cathodic procedure, a sharp reduction peak at 0.25 V and a weak reduction peak at 0.57 V can be seen, which arise from the conversion of ZnO into metal zinc and subsequent generation of Li-Zn alloys along with the formation of solid electrolyte interphase (SEI) films. In the subsequent anodic process, there are four oxidation peaks appearing in 0.29V, 0.33V, 0.54V, 0.67V, which correspond to a multi-step de-alloying reaction of Li-Zn alloys (LiZn, Li2Zn3, LiZn2 and Li2Zn5).41Whereafter, a broad oxidation peak appear in 1.32 V, which may relate to the regeneration of ZnO.14 During the second process, an apparent reduction peak exists at around 0.77 V, and the oxidation peaks still maintain the original position and shape. The forms of CV curves remain essentially consistent after the second loop, suggesting that the active material owns high reversibility. The first, second, and third cycle discharge-charge voltage profiles of yolk-shell structure ZnO/Ni3ZnC0.7/C electrode at a current density of 500 mA g-1 in the voltage range from 0.01 to 3 V is described in Figure 4b. According to Figure 4b, there is a potential plateau at about 0.5 V in the first discharge curve. Then the 0.5 V plateau vanished during the first discharge process, which demonstrated a mechanism of heterogeneous reaction of lithium insertion and extraction.36After that, the potential plateau trended to become steeper from the second discharge curve onwards. The initial discharge and charge capacities are 1743 and 1015 mA h g-1, respectively. In the galvanostatic discharge-charge profile of solid structured ZnO/Ni3ZnC0.7/C (FigureS9), it also appeared a potential plateau. From the second cycle onwards, it appeared the same situation as in the previous case, that is, the potential plateau was substituted by a steep slope. As shown in Figure 4c, it can be seen a distinct improvement in terms of capacity when the solvothermal time changed from 32h to 80h, which is basically due to the emergence of yolk-


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shell structure. The initial discharge capacities of solid and yolk-shell structure are 1355 and 1743 mA h g-1, respectively. Correspondingly, their initial Coulombic efficiencies are 59.0% and 58.2%, respectively. The primary reasons for irreversible capacity losses are the generation of the solid electrolyte interphase (SEI) films and the reduction of metal oxide to metal along with the formation of Li2O.23 The capacity of solid and yolk-shell ZnO/Ni3ZnC0.7/C are 824 and 1002 mA h g-1 at the current density of 500 mA g-1 after 750 loops, respectively. For solid ZnO/Ni3ZnC0.7/C, its Coulombic efficiency is kept above 95.3% from the second cycle onwards. At the same time, the Coulombic efficiency of yolk-shell structure ZnO/Ni3ZnC0.7/C can gradually reach above 93% and maintained until the last lap. Besides the excellent specific capacity and superior cyclic performance, the rate performance is also an important parameter to measure the performance of Li ion battery. The electrochemical performance at the different rate was investigated after the active materials were tested at a current density of 200 mA g-1 after around 441 cycles (FigureS10). Figure 4d depicts the capacities of the ZnO/Ni3ZnC0.7/C at different current rates range from 200 mA g-1 to 3 A g-1. The average discharge capacities of yolk-shell ZnO/Ni3ZnC0.7/C are 885, 701, 525, 411, 334 and 246 mA h g-1 at current densities of 200, 500, 1000, 1500, 2000 and 3000mA g-1, respectively. Subsequently, according to this order back to 200 mA g-1, a specific capacity of ca. 923 mA h g-1 is recovered at a current density of 200 mA g-1 after 100 cycles. On the basis of experimental result above-mentioned, yolk-shell ZnO/Ni3ZnC0.7/C composite shows more excellent rate capability than solid ZnO/Ni3ZnC0.7/C. And yolk-shell ZnO/Ni3ZnC0.7/C demonstrates the stable capacities no matter at both low and high current densities. It is worth emphasizing that no matter solid and yolk-shell ZnO/Ni3ZnC0.7/C, they both exhibit prominent lithium storage properties.


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The prominent lithium ion storage performance of ZnO/Ni3ZnC0.7/C microspheres, especially yolk-shell ZnO/Ni3ZnC0.7/C, two main possibilities are as shown below. First, the metallic Ni in the Ni3ZnC0.7 phase played a key role on improving the electrochemical properties of ZnO, which can serve as a catalyst for the Li2O decomposition (Li2O + Ni → Li + Ni) and speed the reversible reaction of ZnO + Li ↔ Zn + Li2O.36 Second, the content of carbon existing in ZnO/Ni3ZnC0.7/C microspheres can improve electronic conductivity of electrode material. Good conductivity favors more active substances involved in the electrochemical reaction, which is conducive to improving the specific capacity.42 In addition, there are also some reasons which lead to electrochemical performance of yolk-shell ZnO/Ni3ZnC0.7/C better than solid one. On the one hand, the unique structure of yolk-shell ZnO/Ni3ZnC0.7/C can effectively relieve the structural strain preventing the structure collapse during cyclic process. Furthermore, the yolkshell structure not only can supply broad electrode-electrolyte contact area and more active sites, but also effectively shorten path of Li+ ions diffusion. On the other hand, it can be seen that the diameter of semicircle of yolk-shell ZnO/Ni3ZnC0.7/C in the high-medium frequency region is smaller than that of solid structure one (Figure 4e). It indicates that yolk-shell hybrid ZnO/Ni3ZnC0.7/C possesses a smaller charge transfer resistance, which means it owns a lower charge transfer resistance. 4. CONCLUSION Yolk-shell ZnO/Ni3ZnC0.7/C spheres were successfully synthesized by a simple synthesis strategy, in which Zn-MOF/Ni were produced by the solvothermal reaction as a template. With the extension of reaction time, the construction of the harvested Zn-MOF/Ni precursor gradually transformed from the solid into yolk-shell structure. In the process of high temperature pyrolysis,


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Zn-MOF/Ni then converted to porous ZnO/Ni3ZnC0.7/C microsphere in a flow of inert gas. As a lithium-ion battery anode material, electrochemical properties of ZnO/Ni3ZnC0.7/C yolk-shell structure composites is superior to the solid ones’. The stable reversible capacity of yolk-shell ZnO/Ni3ZnC0.7/C can be retained at 1002 mA h g-1 after 750 cycles at the current density of 500 mA g-1, and it also exhibits commendable rate performance. Under the same test conditions, solid ZnO/Ni3ZnC0.7/C perform 824 mA h g-1. Experimental results shed light on the significance of reasonable design of the structure and composition of the electrode material for improvement of electrochemical performance. As a simple and effective synthetic avenue, mental organic frameworks provide a new tactics to produce nano/micro-functional materials with rational design of components and structures in the field of energy storage, catalysis, sensors, and so on.


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FIGURESAND CAPTIONS

Figure 1. XRD patterns of Zn-MOF/Ni precursors (a) and yolk-shell ZnO/Ni3ZnC0.7/C microsphere (b) with the reaction time of 80h.

Figure 2. The SEM and TEM images of solid (a-c) and yolk-shell (d-f) structured Zn-MOF/Ni microspheres.


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Figure 3.The SEM (a-b and d-e), TEM (c and f) micrographs of the as-prepared ZnO/Ni3ZnC0.7/C solid and yolk-shell hybrid microspheres, respectively. (g-l) images of ZnO/Ni3ZnC0.7/C yolk-shell microspheres. HRTEM (g) micrographs, HAADF STEM image (h, j), STEM-EDS line-scan analysis (i) and the elemental mappings of Ni (k), and Zn (l).


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Figure 4. (a) Cyclic voltammograms (CV), (b) discharge-charge curves and (d) rate capabilities at different current densities of yolk-shell structure ZnO/Ni3ZnC0.7/C electrodes. (c) Cycling properties and corresponding Coulombic efficiencies at 500mA g-1, (e) electrochemical impedance spectra (EIS) of solid and yolk-shell structure ZnO/Ni3ZnC0.7/C microspheres.


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SCHEMES

Scheme 1. Schematic illustration about the formation process of solid and yolk-shell structured Zn-MOF/Ni and ZnO/Ni3ZnC0.7/C ASSOCIATED CONTENT Supporting Information FTIR spectra of yolk-shell structured ZnO/Ni3ZnC0.7/C, its precursors and H3BTC ligand; the XRD pattern, TG analysis of solid and yolk-shell structured ZnO/Ni3ZnC0.7/C; XRD pattern of yolk-shell structure products after TG analysis; BET, EDS spectrum, element analysis, cyclic performance and comparison of cyclic performance of yolk-shell ZnO/Ni3ZnC0.7/C composite; XRD pattern and TEM images of ZnO/Ni3ZnC0.7/C treated by 4.0M NaOH solution; SEM


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images of Zn-MOF/Ni precursors at different times; galvanostatic discharge-charge profile of solid structured ZnO/Ni3ZnC0.7/C. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G. H. Yue). Tel: 86-592-2180155; Fax: 86-592-2183515. ACKNOWLEDGMENT This work was jointly supported by the National Science Foundation of China (No. 11572271, No. 51302236), the National Basic Research Program of China (No. 2012CB933103). REFERENCES 1.

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Insert Table of Contents Graphic and Synopsis Here

The figure is about cycling properties corresponding Coulombic efficiencies at 500mA g-1of solid and yolk-shell structure ZnO/Ni3ZnC0.7/C microspheres. And the insert picture is schematic illustration about the formation process of solid and yolk-shell structured Zn-MOF/Ni and ZnO/Ni3ZnC0.7/C.


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C Hybrids YolkâShell Microspheres with

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