[email protected] Hybrid Materials with High Capacity and Robust Cycling Performance for Li-ion Batteries

[email protected] Hybrid Materials with High Capacity and Robust Cycling Performance for Li-ion Batteries

Accepted Manuscript Title: Hierarchical Porous Acetylene Black/ZnFe2 O4 @Carbon Hybrid Materials with High Capacity and Robust Cycling Performance for...

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Accepted Manuscript Title: Hierarchical Porous Acetylene Black/ZnFe2 O4 @Carbon Hybrid Materials with High Capacity and Robust Cycling Performance for Li-ion Batteries Author: Junjie Cai Chun Wu Ying Zhu Pei Kang Shen Kaili Zhang PII: DOI: Reference:

S0013-4686(15)30866-5 http://dx.doi.org/doi:10.1016/j.electacta.2015.11.095 EA 26095

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

3-10-2015 17-11-2015 18-11-2015

Please cite this article as: Junjie Cai, Chun Wu, Ying Zhu, Pei Kang Shen, Kaili Zhang, Hierarchical Porous Acetylene Black/[email protected] Hybrid Materials with High Capacity and Robust Cycling Performance for Li-ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.11.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hierarchical Porous Acetylene Black/[email protected] Hybrid Materials with High Capacity and Robust Cycling Performance for Li-ion Batteries Junjie Caia, Chun Wua, Ying Zhua, Pei Kang Shenb,* and Kaili Zhanga,* a

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong

b

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou, 510275, PR China. * Corresponding Author: [email protected] and [email protected]

Graphical Abstract

A hierarchical porous acetylene black/[email protected] hybrid material is prepared by direct thermal decomposition and self- assembly, and potential application of this hybrid nanostructure was demonstrated for lithium-ion battery anode materials.

ABSTRACT: A hierarchical porous acetylene black/[email protected] hybrid material is prepared by direct thermal decomposition of a mixture of Zn-Fe-oleate complex mixed with acetylene black and subsequent calcination that promotes the conversion reaction to generate ZnFe2O4 nanoparticles. In the hybrid structure, well-dispersed ZnFe2O4 nanoparticles are anchored on the acetylene black substrate and these nanocomposites are further covered and interlinked by amorphous carbon layer, resulting in self-assembly into large hierarchical porous granules. Utilization of the conductive carbon in the nanocomposite can enable better electrons transfer. In addition, this unique structure effectively prevents the aggregation of the ZnFe2O4 nanoparticles and buffers the large volume change of the active material as well as avoiding undesired side effect of the electrode. The hierarchical porous acetylene black/[email protected] nanocomposite exhibits favorable electrochemical performance, including high reversible capacity retention, good cycling stability, and high rate performance, which suggests that this rational hybrid material has alluring prospect for superior Li-ion batteries. KEYWORDS:ZnFe2O4; Hierarchical porous; Carbon coating; Hybrid materials; Li-ion batteries

1. INTRODUCTION Lithium ion batteries currently represent the state-of-the-art technology in rechargeable batteries and dominate the portable electronics market, but their further application in electrical vehicles and large-scale electrical grids requires much higher energy and power densities.1-4 The increasing demand for superior Li-ion batteries has motivated ongoing research of new electrode materials with high capacity and rapid charge/discharge as well as stable cycling ability.5-7 Transition metal oxides, especially cobalt oxides and iron oxides, have been extensively studied due to their relative high theoretical specific capacity of ~900 mAh g-1 compared with 372 mAh g-1 for commercial graphite.8-15 Recently, ZnFe2O4, a binary transition metal oxide, has attracted considerable attention because of its higher theoretical capacity of 1000.5 mAh g-1 originated from the formation of the Li–Zn alloy during the discharge process that can contribute to extra capacity. 16-18 Whereas, their poor electrical conductivity and slow Li ions diffusion in bulk solid materials greatly limit the rate capabilities for fast charge/discharge. More seriously, the dramatic volume change during charge/discharge processes results in the pulverization and exfoliation from the current collector, leading to poor cycling performance and rapidly declining capacity. Thus, there are still many challenges associated with the practical application when considering their poor rate and cycling capability.19-21 To address these issues, tremendous efforts have been made to develop rational hybrid nanostructures that incorporate nanosized metal oxides with conductive carbon materials. 22 On one hand, it is well accepted that downsizing the electro-active materials into nanoscale can better mitigate the strain of Li ions insertion/extraction and exceptionally shorten the Li ions and electrons transport pathways. 23-24 On the other hand, the utilization of carbon materials can further improve the electronic conductivity of composites and buffer the volume change of the active materials as well as avoiding some side effects caused by decreasing the metal oxides into nanoscale. It is well known that nanoparticles tend to form agglomerates in practical operation due to their high surface energy. As a result, it is difficult to achieve good cycling and rate performance by the simply utilization of nanoparticles as electrodes. Moreover, the high surface area of nanoparticles would inevitably increase undesirable electrode/electrolyte interface reactions and cause additional consumption of Li ions to form solid electrolyte interphase (SEI) film, leading to low initial columbic efficiency. It has been proved to be very promising to use carbon as an inactive material to stabilize the active component25-31. In previous studies, conventional composites commonly consist of metal oxides nanoparticles well dispersed on carbon matrix, such as carbon nanotubes25-26, graphene27-28 and hierarchical porous carbon29-31. This is an effective way to prevent the aggregation of nanoparticles since carbon can immobilize the nanoparticles on its surface through either the non-covalent interactions or the chemical bonding. However, this

kind of hybrid nanostructure only can restrict the repetitive volume expansion and contraction of the active materials during charge/discharge processes to a limited degree, thus offering limited stability and capacity enhancements. Compared with the aforementioned strategy, carbon coated nanostructured metal oxides do much better in confining the volume change of the active material due to the advantages contributed by the exterior carbon shell.32-34 Firstly, carbon shell has considerable elasticity to accommodate the strain of volume change of the core during Li ions insertion/extraction, ensuring the nanoparticles to expand without rupturing the carbon shell, which maintains their structural integrity and enhances cycling stability. In addition, from the viewpoint of surface electrochemistry, core-shell structures can effectively decrease the surface energy of the nanomaterials, which protects against side reactions between the electrode and electrolyte. This would avoid a high level of electrochemical irreversibility and improve the first cycle columbic efficiency. Although all of the above strategies have considerable advantages, there is no single solution that can satisfy the needs to improve the cyclability, rate capability and relative low initial coulombic efficiency of metal oxides anodes simultaneously. Rational combination of multiple strategies by integrating metal oxides nanoparticles onto hierarchical porous carbon substrate and coating amorphous carbon layer on nanoparticles might help to realize high performance anode materials. Herein, we demonstrate a facile thermal decomposition and subsequent calcination process for the synthesis of hierarchical porous acetylene black/[email protected] hybrid materials. In this rational architecture, well dispersed ZnO and FeO nanoparticles (NPs) are first in-situ grown on the surface of acetylene black (AB) by the direct thermal decomposition of a Zn-Fe-oleate complex. Chained acetylene black particles are selected as substrates for supporting the metal oxides nanoparticles because of their open structure and high specific surface area, providing multiple accessible sites for the thermal decomposition. Besides, the ultralow cost of acetylene black help maintain the low cost of the synthesized composite when compared with utilizing other carbon materials, such as carbon nanotube and graphene. As the thermal decomposition progresses, these acetylene black/ZnO/FeO nanocomposites will be covered by amorphous carbon layer derived from organic molecules for buffering the volume change and avoiding the nanoparticles to directly contact with the electrolyte. Meanwhile, nanocomposites are self-assembled into large hierarchical porous granules with the help of interlinking by the carbon layer. A subsequent calcination in air is carried out for promoting the ZnO and FeO to convert into ZnFe2O4. The rational combination of multiple strategies (including dispersing nanoparticles to carbon matrix and coating nanoparticles with carbon layers) for realizing an AB/[email protected] hierarchical porous granule could offer great enhancements in specific capacity retention, cycling performance, and rate capability in comparison with pristine ZnFe2O4 nanoparticles when used as an anode material for Li-ion batteries.

2. EXPERIMENT SECTION 2.1 Sample Preparation. In a typical synthesis, 2 mmol of zinc(II) acetylacetonate (Zn(acac)2) and 4 mmol of iron (III) acetylacetonate (Fe(acac)3) was solventlessly mixed with 16 mmol oleic acid under vigorously stirring for 2 h. The zinc-iron-oleate complexes were then mixed thoroughly with 0.1 g AB by using a pestle and mortar. The precursor was heated to 600 oC at a heating rate of 10 oC min-1 under a N2 atmosphere and kept for 3 h to obtain AB/ZnO/FeO/carbon composite. For converting the ZnO and FeO into ZnFe2O4, the composite was subsequently calcined at 300 oC in air for 5 h and the hierarchical porous AB/[email protected] nanocomposite was finally obtained. The controlled experiments on various content of ZnFe2O4 sample were attained by adjusting the additive amount of AB, with other conditions being fixed. 2.2 Characterization. X-ray diffraction (XRD) measurements were carried out on a D/Max-III (Rigaku Co. Ltd., Japan) using Cu Ka radiation and operated at 34 kV and 26 mA. The morphologies were observed by a field emission scanning electron microscopy (FESEM Hitachi S4800). Transmission electron microscopy (TEM) investigations were carried out on TEM JEM-2010HR at 200 kV (JEOL Ltd., Japan) and constituent content was analyzed by energy-dispersive X-ray spectroscopy (EDS) (Oxford Co. Ltd., England). The X-ray photoelectron spectroscopic (XPS) measurements were performed in an ESCALAB 250 spectrometer to analyze the composition of the specimens. A thermo gravimetric analyzer (TA-DTG Q600) was used to determine the content of ZnFe2O4 in hybrid materials. The pore structures of the samples were characterized by analysis of the N2 adsorption–desorption isotherms, performed at 77 K with Micromeritics ASAP 2420 in a relative pressure (P/P0) range from 10−6 to 1 after degassing the samples at 150 °C for 5 h. 2.3 Electrochemical Measurement. For the electrochemical measurements, the as-prepared hierarchical porous AB/[email protected] samples were mixed with acetylene black and polyvinylidene fluoride (PVDF) at a weight ratio of 8:1:1 in nmethyl-2-pyrrolidene (NMP) solution. The mixed slurry was coated on a copper foil and dried in a vacuum oven at 120 oC for 10 h. The electrodes were cut and pressed into 1.54 cm2 disc. The mass loading of each electrode is in the range of 1.2 to 1.7 mg cm-2. The 2032 coin cells were assembled in an argon-filled glove box, using pure Li metal as the counter electrode and micro-porous membrane (Celguard 2400, USA) as the separator. The electrolyte was 1 M LiPF 6 in a mixture of ethylene carbonate/dimethyl carbonate (EC/DMC; 1:1 v/v). The cells were galvanostatically charged and discharged between 0.01 V and 3 V versus lithium at room temperature on a program-controlled test system (Shenzhen Neware Battery Co., China). The cyclic voltammetric and electrochemical impedance spectroscopy measurements were performed on an electrochemical workstation (CHI660e, China). The specific capacity of AB/[email protected] composite was calculated based on the whole weight of the hybrid materials.

3. RESULTS AND DISCUSSIONS The formation of the AB/[email protected] hybrid structure is illustrated in Scheme 1. The synthesis process mainly consists of two steps: (i) direct thermal decomposition of precursor, (ii) converting ZnO and FeO component into ZnFe2O4 by calcination. During the thermal decomposition, nanocomposites derived from precursor would self-assemble into large hierarchical porous granules. The crystallographic structure and phase purity of as-prepared samples at each step are examined by XRD as shown in Figure 1. After direct decomposition of the precursor, ZnO and FeO components were found in the resultant as evidenced by the clear diffraction

peaks from the hexagonal ZnO (JCPDS no. 36-1451) and wustite FeO ((JCPDS no. 46-1312), shown in Fig. 1(a). Considering the molar ratio of starting materials and based on completely converting the precursor into ZnO and FeO, the calculated molar ratio of Zn, Fe and O elements should be 1:2:3. The EDS result of this composite in Fig. S1 shows that the atomic ratio is 7.81% for Zn, 15.43% for Fe and 21.93% for O in the composite, which is consistent with the above calculated result. In addition, a broad peak is observed at 20 to 30 degree similar to the pristine acetylene black but with weak relative intensity (see Fig. S2). This peak could be ascribed to acetylene black and the formation of amorphous carbon layer derived from the carbonization of olelate complex. On one hand, oleic acid molecules with long carbon chain can serve as surfactant to cap the nanoparticles preventing the aggregation during the thermal decomposition process. On the other hand, these organic molecules around the nanoparticles are in situ transformed to a thin carbon layer coating on the nanoparticles via carbonization at 600 oC for 3 h under N2 atmosphere. Apparently, the peaks from the FeO and ZnO phase entirely disappear in Fig. 1(b). Instead of these peaks, pure ZnFe2O4 phase is observed in the nanocomposite and the amorphous carbon peaks still remain. The average size of ZnFe2O4 nanoparticles in the composite is calculated from the largest diffraction peak (311) by using Scherrer's formula and the estimated average crystal size is about 32 nm. Since the structure of FeO is not chemically stable and is prone to oxidation to form Fe2O335, it is believed that the wustite phase FeO would be oxidized and converted into Fe2O3 during the calcination under 300 oC in air. The TG curve of AB/ZnO/[email protected] in Fig. 1(c) shows clearly that the weight of the sample is slightly increased from 250 oC to 350 oC, suggesting that the occurrence of the oxidation reaction. As the calcination progresses, the Fe2O3 component further reacts with ZnO via the thermochemical reaction to form ZnFe2O4. As shown in the TG curve, a sharp mass loss occurred between 350 oC to 500 oC is corresponding to the combustion of the carbon component. The total weight loss is 31%, which is equivalent weight of acetylene black and amorphous carbon in the composite. The result shows that carbon is stable even being calcined at 300 oC under air atmosphere. Obviously, this calcination temperature is very important for the synthesis of ZnFe2O4/carbon hybrid material because it must simultaneously satisfy both the conversion reaction and the carbon component retaintion in the composite. The morphologies of the hierarchical porous AB/[email protected] hybrid structure are first observed by SEM and the chemical composition of the sample is determined by EDS attached to SEM. Figure 2(a) shows a representative large granule with a diameter of about 10 µm whose morphology is quite different from the pristine acetylene black shown in Fig. S3. The surface of the granule is rough and small particles are easily visible on its surface, indicating that the granule is composed of AB/ZnFe2O4-NPs nanocomposite. Figure 2(b), a high magnification SEM image, clearly confirms that the nanocomposite is covered and interlinked by a continuous carbon layer. In addition, the elements mapping images present a uniform distribution of the Zn, Fe, O and C elements in the nanocomposite, revealing that the ZnFe2O4 nanoparticles are well dispersed in the carbon matrix. The existence of ZnFe2O4 and carbon in the nanocomposite is further confirmed by observing the peaks of Zn 2p3 Fe 2p, O 1s, and C 1s from the XPS spectra in Fig. S4. N2 gas adsorption–desorption measurement (Fig. 2(c)) shows that the BET surface area is 137 m2/g for the AB, and 49 m2/g for the AB/[email protected] granule. Expressly, low surface area would be beneficial to the initial coulombic efficiency because of the reduced contact area between electrode and electrolyte. In Fig. 2(d), the pristine AB sample presents a narrow pore distribution under 4 nm which is originated from large quantities of pore defects on its surface. Besides the similarity of pore distribution under 4 nm, the AB/[email protected] also shows a wide pore distribution in the range of 10 to 100 nm, consisting of different degrees of porosity. The dramatic decrease of the specific surface area and the difference in their pore distribution imply that the carbon coating and the granulation make the AB/ZnFe2O4 nanocomposite self-assemble into the micrometer aggregations. The oleate complex acts as binder to wet the AB and it melts when the temperature is increased. The melt binder coats AB and sticks them together to form granules by a combination of viscous and capillary forces. Further transform of the organic molecules into dense carbon layer strengthens the interlinking of the AB/ZnFe2O4 NPs composite that better preserves the structure of the hybrid materials. Although their BET surface and pore distribution are quite different, the pore volume of AB/[email protected] almost does not change compared to pristine acetylene, suggesting that the granulation process can maintain the void space in the composite. These void spaces are available to accommodate the expansion of ZnFe2O4 during the lithiation process and facilitate the electrolyte to infiltrate. Typical TEM images of the AB/[email protected] hybrid structure are depicted in Fig. 3 and Fig. S5. Figure 3(a) shows that the nanocomposites are nearly spherical in shape and interconnected. The carbon layer derived from the organic molecules works as the binder to promote the granulation process. It is easy to identify that nanoparticles with a uniform size distribution of ~40 nm are well dispersed on the surface of AB, indicating that they are ZnFe2O4 NPs according to the calculated average size from XRD result. Multitudinous uniform nanoparticles are highly interconnected via a point-to-point mode among them. Since the surface of AB often has defects and carries many chemical groups, ZnFe2O4 nanoparticles tend to tightly anchor on its surface through either the noncovalent interactions or the chemical bonding. This immobilization effect of carbon material leads to the configuration of uniformdispersed nanoparticles on the carbon substrate. The higher magnification images in Fig. 3(b, c) demonstrate that these adjacent ZnFe2O4 nanoparticles are not only well anchored on AB, but also perfectly encapsulated in amorphous carbon layer. Especially, amorphous carbon layer around the ZnFe2O4 nanoparticles and graphitized carbon in the edge can be identified in Fig. 3(c). The graphitized carbon with clear lattice fringes which is separated by 0.34 nm corresponds to the (200) lattice plane of graphite. This graphitized carbon can be ascribed to AB since it is partially graphitized. This result further confirms the sandwich structure of AB substrate, ZnFe2O4 NPs and amorphous carbon layer. The carbon layers perfectly cover the AB/ZnFe2O4 NPs nanocomposites and only limited edge area of AB substrate is exposed as shown in Fig. 3(d). High-resolution TEM image in Fig. 3(d) evidences that there are lattice fringes of the ZnFe2O4 NPs coated with a thin amorphous carbon layer with a thickness of 2.6 nm. The carbon layer can improve the electronic conductivity and effectively limit the volume change and it is thin enough for the easy penetration of Li ions. The selected-area electron diffraction (SAED) pattern (see inset in Fig. 3(d)) shows indexed diffraction rings patterns of the ZnFe2O4 and AB. However, perfect coating of carbon layer on the nanoparticles surface leads to blurred diffraction rings, only (311), (400), (440) and (444) facets are barely identified from inside to outside, respectively. These results are well consistent with the XRD data, suggesting a typical polycrystalline structure of the product. In addition, the (002) facets of graphitized carbon also can be observed in SAED pattern which is in agreement with the Fig. 3(c).

To investigate the electrochemical properties of the as-prepared AB/[email protected] electrode during the cycling process, cyclic voltammograms (CV) is first implemented. Figure 4(a) shows the CV curves for the first 6 cycles at a scan rate of 0.1mV s-1 in the potential range from 0.0 to 3.0V. Typically, a small peak appears in the first anodic scan at 0.9 V (vs Li/Li+), which is usually attributed to the occurrence of side reactions on the electrode surfaces and interfaces caused by the irreversible decomposition of the electrolyte to form a SEI. Another well-defined reduction peak observed at 0.6 V can be ascribed to the complete reduction of ZnFe2O4 to Zn(0), Fe(0) and the formation of Li2O. Two small peaks below 0.5 V are probably corresponding to the formation of Li–Zn alloys by further lithiation of Zn(0). In the first cathodic scan, two broad overlapping oxidation peaks at 1.6 and 1.9 V correspond to the reversible oxidation of Fe(0) to Fe(III) and the Li-Zn alloys dealloying, respectively. These results are in good agreement with the Li ion storage mechanism of ZnFe2O4 component as discussed by previous literature16-21, which is proposed as follows: ZnFe2O4+8Li+ +8e- ↔ Zn+Fe+4Li2O (1) Zn+Li++e-↔LiZn (2) Fe2O3+6Li++6e-↔ Fe+3Li2O (3) In the subsequent cycles, a new reversible peak is observed below 0.3 V, corresponding to the Li ion intercalation/de-intercalation process of the acetylene black and outer amorphous carbon layer. For the ZnFe2O4 component, the reduction peak is observed at 0.8 V and its two oxidation peaks remain but become broader. Both reduction and oxidation peaks are positively shifted, which would be caused by a structure rearrangement of the electrode materials at the first cycle36 and the polarization increased during the subsequent cycles37. In addition, their peak intensity drops significantly compared to the first cycle, indicating that the formation of SEI film and the conversion reactions of the active materials is irreversible in a certain degree. Importantly, it is noteworthy that both the peak current and the integrated area intensity almost do not change after the first cycle, which means there is almost no capacity loss during the subsequent charge/discharge processes. It suggests that a stable SEI film forms on the interfaces of carbon shells in the first cycle, preventing the direct contact of encapsulated ZnFe2O4 nanoparticles with electrolyte to guarantee the structural integrity of interior ZnFe2O4 during subsequent charge/discharge cycles, thus leading to a relative high coulombic efficiency and superior reversibility of the electrodes. Figure 4(b) shows the galvanostatic charge/discharge curves of the AB/[email protected] electrode at a current density of 100 mA g-1 in the potential range from 0.01 to 3.0 V. It is noted that the first discharge profile has two long plateau at approximately 0.7 to 0.5 V, resulting from the complete reduction of Fe(III) and Zn(II) to metallic Fe and Zn, which is in good agreement with the results confirmed by CV measurement. The initial discharge and charge capacities of the AB/[email protected] hierarchical porous composite are 1099 and 762 mAh g-1 respectively, based on the total mass of the nanocomposite. This measured discharge capacity is very close to the theoretical capacity of 1000.5 mAh g-1 for ZnFe2O4 even though there are 31 % carbon content in the composite determined by the TG analysis and the pure AB only contributes a reversible capacity of ~250 mAh g-1 (see Fig. S6). According to previous literature38, the additional capacity in the initial discharge is associated with the formation of SEI film on the surface of electrode due to electrolyte decomposition. However, this additional capacity is irreversible and the metallic Fe and Zn cannot be completely converted into the original oxide during the first charge process, resulting in the initial irreversible capacity loss of the electrode. Starting from the second cycle, the reversible capacity of AB/[email protected] gradually increases in the initial 80 cycles. This is likely due to the reversible growth of the polymeric gel-like film by the kinetically activated electrolyte degradation, which has been also reported in other transition metal oxide composites28-31. In the subsequent cycles, the capacity reaches stable and a high reversible capacity of 803 mAh g-1 is still retained after 150 cycles, indicating relatively constant lithiation and delithiation processes for the AB/[email protected] electrode. For comparison, the cycling performance of AB/[email protected], mechanically mixed AB/ZnFe2O4 NPs and bare ZnFe2O4 NPs (see Fig. S7) is also tested at a current density of 100 mA g-1. Figure 4(c) shows clearly that AB/[email protected] electrode has much higher capacity retention compared with the other two electrodes even after 150 cycles. According to the disadvantages of using nanoparticles alone as electrode, it is not surprising that the capacity of bare ZnFe2O4 NPs electrode significantly decreases and reaches 213 mA h g-1 after 50 cycles, indicating low capacity retention and poor cycling performance. In contrast, a relative stable cycling performance can be obtained by using mechanically mixed AB/ZnFe2O4 NPs electrode which finally shows a capacity of 475 mAh g-1 after 100 cycles. This result suggests that the strategy of nanoparticles well dispersed and embedded in carbon matrix can buffer the repetitive volume change of the active materials during charge/discharge processes to a limited degree, resulting in a stable cyclability. However, when considering the initial coulombic efficiency which is 69.3% for AB/[email protected] electrode and 60.2% for mechanically mixed AB/ZnFe2O4 NPs electrode, it strongly demonstrates that carbon coating can effectively improve the initial coulombic efficiency by preventing the active material from direct exposure to the electrolyte. As a result, the side effect of the electrode and electrolyte is reduced. Through cycling performance research of these three electrodes, it evidently proves that the significant improvements of capacity retention and coulombic efficiency can be both achieved by integrating nanoparticles onto hierarchical porous carbon substrate and carbon coating. The rate performance of AB/[email protected] composite is shown in Fig. 4(d), in which the current density is increased stepwise from 0.1 to 2 A g-1 for every 20 successive cycles in the voltage range of 0.01 to 3.0 V. A reversible discharge capacity of the AB/[email protected] decreases from 731 to 229 mA h g-1 as the current density increases from 0.1 to 2.0 A g-1. It is worth noting that it delivers a reversible discharge capacity of 375 mAh g-1 after being cycled at 1 A g-1 which is still higher than the theoretical capacity of graphite. Importantly, when the current density is finally returned to its initial value of 0.1 A g-1, a discharge capacity of 470 mAh g-1 is recovered and it is further increased to 830 mAh g-1 after another 100 cycles which is higher than its initial reversible value, implying very good reversibility. As expected, the composite electrode exhibits a considerable rate capability mainly because

of the use of acetylene black as a conductive substrate to support the nanoparticles, enabling better electrons transfer. In addition, AB/[email protected] hierarchical porous granule with interconnected channels and pores can facilitate the electrolyte to infiltrate into the electrode and inevitably lead to more efficient contact between Li ions and active materials, thus also enhancing the Li ions transportation. The long-term cycling performance of AB/[email protected] sample working at a large current density of 1 A g-1 has been investigated upon 200 cycles and the evolution of the specific capacity is displayed in Fig. 4(e). The first 5 cycles are performed at 0.1 A g-1 and then 200 cycles at 1 A g-1. It can be seen that the reversible capacities at 0.1 A g-1 is 685 mAh g-1. When the current density of 1 A g-1 is implemented, its capacity declines rapidly to 386 mAh g- 1 and then a very slow capacity increase to 430 mA g-1 after 200 cycles, further demonstrating good cycling stability of this hybrid nanocomposite even at high charge/discharge rates. Such favorable rate performance and cycling stability at high large current density originate from the rational design. In order to explain the cycling and rate performance of the AB/[email protected], electrochemical impedance spectroscopy (EIS) measurement is conducted in a frequency range of 105 to 10 -2 Hz at the A.C amplitude of 5 mV and a controlled potential of 0.7 V is selected. Nyquist plots of the ZnFe2O4 NPs electrodes with/without carbon are obtained from EIS measurement as shown in Fig. 5(a). Generally, the diameter of semi-circle at high to medium frequency in the Nyquist plots of the electrodes is assigned to the chargetransfer resistance (Rct) and the inclined line in the low-frequency region represents the Warburg impedance (Zw) related to Li ions diffusion in the solid. Expressly, the diameter of the semicircle for AB/[email protected] electrode in the high-medium frequency region is significantly smaller than that of bare ZnFe2O4 nanoparticles. It is not surprising that AB/[email protected] electrode possesses a lower contact and charge-transfer resistance since AB provides not only a support for anchoring well-dispersed ZnFe2O4 nanoparticles but also works as a highly conductive substrate, enabling better electrons transfer. Moreover, the close-packed ZnFe2O4 NPs and the outer carbon layer lead to a good interparticle electrical contact, thus improved charge transfer and decreased electrical resistivity could be reasonably achieved. At the following low frequency region,the slope of the AB/[email protected] electrode is smaller than that of bare ZnFe2O4, indicating that the carbon layer blocks the easy penetration of Li ions into ZnFe2O4 NPs and therefore causes a higher diffusion resistance of AB/[email protected] electrode.39 The comparison of Nyquist plots of the fresh electrode and the electrode after 150 cycles are presented in Fig. 5(b). Apparently, two semicircles appear in the high to medium frequency in the Nyquist plots for the cycled electrode and the total electrical resistivity is slightly increased compared to the fresh one. According to the literature, the first semicircle could be ascribed to the formation of the SEI film on the outer surface of carbon layer which has poor electrical conductivity and the second semicircle is the usual charge-transfer resistance. However, the slope of the AB/[email protected] electrode increased after cycling, which is believe that the formation of a stable amorphous SEI film after cycling enhanced the Li ions diffusion of the AB/[email protected] It is true that the formation of SEI film slightly increases the internal electrical resistivity, which might be against the superior rate capability at high current density. However, a stable SEI film during cycling is crucial for achieving a robust cycling performance since it can avoid the ZnFe2O4 nanoparticles from direct exposure to electrolytes and maintain its morphology and structure during subsequent charge/discharge cycles. All evidences demonstrate that the AB/[email protected] hybrid nanostructure is an effective way to enhance the electrochemical performance of Li-ion batteries.

4. CONCLUSIONS In summary, the hierarchical porous AB/[email protected] hybrid nanostructure has been successfully prepared by a facile thermal decomposition and subsequent calcination. The synthesis process is relatively simple, low-cost and suitable for large scale production of superior electrode material for Li-ion batteries with high capacity and rapid charge/discharge as well as stable cycling ability. Considering these advantages, it is believed that this rational design can be extended to a series of combinations of metal oxides for superior Li-ion batteries, such as Fe3O4, Co3O4, ZnCo2O4, etc. by replacing the corresponding metal salts in the system. In this study, the obtained hierarchical porous AB/[email protected] hybrid nanostructure exhibits improved reversible capacity, cycling and rate performances which could be attributed to the synergetic effect of carbon matrix and carbon coating. Due to the simple synthesis strategy and good electrochemical performance, the hierarchical porous AB/[email protected] hybrid materials as anode show alluring prospect for advanced Li-ion batteries.

ACKNOWLEDGMENTS This work was supported by Hong Kong Research Grants Council (grant no. CityU 125412) and NSAF (grant No. U1330132).

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Scheme 1. Schematic of the AB/ZnFe2O [email protected] hybrid structure via direct thermal decomposition and subsequent calcination: (a) the conversion from the precursor to AB/[email protected] and (b) self-assembly process of hierarchical porous granule.

Figure 1. XRD patterns of (a) AB/ZnO/[email protected] and (b) AB/ZnFe2O [email protected]; (c) TG curve of the AB/ZnO/[email protected]

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Figure 2. (a-b) SEM-EDS mapping images of AB/[email protected]; (c) N 2 gas adsorption–desorption curves and (d) pore distribution of acetylene black and AB/[email protected], respectively.

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Figure 3. TEM (a-b) and HR-TEM (c-d) images of the AB/[email protected]; inset in image (d) is the SAED pattern.

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Figure 4. Electrochemical performance of the as-prepared AB/ZnFe2O [email protected] electrodes: (a) cyclic voltammogram curves; (b) galvanostatic charge/discharge curves; (c) cycling performance of the different samples; (d) rate performance; (e) cycling performance at a current density of 1 A g-1.

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Figure 5. Nyquist plots from EIS (a) the comparison of AB/[email protected] and bare ZnFe2O 4 NPs and (b) the comparison of AB/ZnFe2O [email protected] before and after 150 cycles.

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