Ketjen Black composite as anode material for lithium ion batteries

Accepted Manuscript Title: High performance of mesoporous ␥-Fe2 O3 nanoparticle/Ketjen Black composite as anode material for lithium ion batteries Author: Hui Dong Yunlong Xu Mandi Ji Huang Zhang Zhen Zhao Chongjun Zhao PII: DOI: Reference:

S0013-4686(14)02025-8 http://dx.doi.org/doi:10.1016/j.electacta.2014.10.022 EA 23533

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

10-9-2014 4-10-2014 6-10-2014

Please cite this article as: Hui Dong, Yunlong Xu, Mandi Ji, Huang Zhang, Zhen Zhao, Chongjun Zhao, High performance of mesoporous rmgamma-Fe2O3 nanoparticle/Ketjen Black composite as anode material for lithium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.10.022 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.

High performance of mesoporous γ-Fe2O3 nanoparticle/Ketjen Black composite as anode material for lithium ion batteries Hui Dong, Yunlong Xu1, Mandi Ji, Huang Zhang, Zhen Zhao, Chongjun Zhao Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China

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*Corresponding author. Tel.: +86 21 64252019; Fax: +86 21 64252019.

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E-mail address: [email protected] (Y. L. Xu).

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Highlights

A mesoporous γ-Fe2O3/KB composite was synthesized via solvothermal method.

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KB was used as a carbon template to improve electrochemical performance of

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γ-Fe2O3.

3D network structure can relieve volume change and improve the ionic transport.

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The composite exhibited an ultrahigh capacity and high rate performance.

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Abstract

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A type of γ-Fe2O3 nanoparticle/Ketjen Black (KB) composite material is synthesized by a solvothermal method combined with precursor thermal

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transformation. The structure and morphology are characterized by XRD, raman spectra, TG, nitrogen sorption, SEM, TEM and EDS. The results show that the composite has a uniform nanoporous network and well-dispersed γ-Fe2O3 particles with a size of ca. 5 nm are embedded in the mesopores of KB. The γ-Fe2O3/KB

exhibits superior eletrochemical performances to the bare γ-Fe2O3, especially at high current rate. The discharge capacity of the composite is 1100 mAh·g-1 at the first cycle and remains 988.8 mAh·g-1 after 100 cycles at 0.2C. Moreover, it also maintains a high discharge capacity of 697.8 mAh·g-1 at 2C and 410.1 mAh·g-1 at 5C after 100 cycles, respectively. Such improved electrochemical performances could be attributed to the superior conductivity and favorable structure of KB, which contributes to the improvement in electronic conductivity and structure stability of γ-Fe2O3 during the lithium ion insertion/ desertion process.

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Keywords: γ-Fe2O3, Ketjen Black, mesoporous, ultrahigh capacity, rate capability

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1. Introduction

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Iron oxide is a very promising material due to its wide practical application to

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magnetic recording medium [1,2], gas-sensitive materials [3], energy storage materials

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[4,5] and biomedical materials [6,7]. Recently, the iron oxide has been intensively investigated as an anode material for lithium ion batteries because of its non-toxicity,

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low cost, and high capacity [8-13]. Many types of iron oxides have been reported for battery application, such as

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alpha Fe2O3 (α-Fe2O3) [14,15], gamma Fe2O3 (γ-Fe2O3) [16,17] and Fe3O4 [18].

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Among them, the γ-Fe2O3 can be categorized into the defect spinel, in which a part of

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the octahedral sites in Fe3O4 are vacant [19]. However, as an anode material, γ-Fe2O3 usually exhibits an unfavorable performance due to large volume expansion, irreversible phase transformation and poor electronic conductivity [20]. Recently, various approaches have been raised to improve the electrochemical performance of Fe2O3 [21]. Sho et al. synthesized the nano-sized γ-Fe2O3 and found that the

irreversible phase transformation was suppressed in the nanostructured material [22]. A mesoporous Fe2O3 nanoparticle/carbon aerogel composite was successfully prepared by Liu et al., which exhibits excellent cycling stability, specific capacity and rate capability [23]. Gao et al. report the synthesis of carbon-coated α-Fe2O3 nanorods with an interesting hierarchical tubular structure, which improves the electrical conductivity and structural stability of Fe2O3 [24]. Additionally, Xu et al. have

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reported a simple surfactant-free solvothermal method combined with thermal

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transformation for the synthesis of monodisperse γ-Fe2O3 magnetic mesoporous

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microspheres [25], which exhibits a high initial discharge capacity and good

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reversible performance, unfortunately there is no obvious improvement on the

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conductivity.

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Ketjen Black (KB) has large specific surface area (1270 m2·g−1), superior

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conductivity and relatively narrow pore diameter distribution [26]. Therefore, KB can

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be used as a carbon template to relieve the volume expansion and improve the

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conductivity of γ-Fe2O3. In our previous work, the KB has been proposed as

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conductive template to improve the rate and low temperature performances of

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LiFePO4 [27]. However, to our knowledge, there are no reports on the KB considered

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as conductive and structural template to enhance the performances of γ-Fe2O3. In this paper, γ-Fe2O3 nanoparticle/KB composite was obtained via a

solvothermal process combined with precursor thermal transformation. Our results suggest an interesting approach for controlling the structure/morphology and improving the high-rate cyclability of as-prepared γ-Fe2O3 anode material.

2. Experimental 2.1 Material Preparation 2.1.1 Preparation of pure γ-Fe2O3 The pure γ-Fe2O3 was prepared through a solvothermal method according to the Ref. [25]. The stoichiometric amount of Fe(NO3)3·9H2O and tartaric acid were dissolved in dimethylformamide (DMF) by magnetic stirring. The resulting solution

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was transferred into a 100 mL Teflon lined stainless steel autoclave, sealed, and

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heated to 160°C then kept at this temperature for 8 h. After cooling to room

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temperature naturally, the powders ware collected by centrifugation, washed with

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ethanol, and dried as the precursor at 60°C. The pure γ-Fe2O3 was obtained by

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calcination treatment of the prepared ferrous tartrate precursor powder at 320°C in air

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for 1 h.

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2.2.2 Preparation of γ-Fe2O3/KB material

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The scheme of γ-Fe2O3/KB composite forming mechanism is shown in Fig. 1.

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The stoichiometric amount of Fe(NO3)3·9H2O and tartaric acid were dissolved in

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10mL deionized water. Then KB was immersed with the above solution. After

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repeated filtration, the precursor mixture was dried in a vacuum oven and then impregnated in DMF. The resulting solution was transferred into a 100 mL Teflon

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lined stainless steel autoclave, sealed, heated to 160°C and retained at this temperature for 8 h. After cooling to room temperature naturally, the product was collected by centrifugation, washed with ethanol, and dried to powder as the precursor

at 60°C. The γ-Fe2O3/KB composite was obtained by calcination treatment of the ferrous tartrate precursor powder at 320°C in air for 1 h. 2.2 Sample characterization The crystal structures of the samples were characterized by X-ray diffraction measurement (XRD, D/MAX 2550 V, Japan) using Cu Kα radiation (γ=0.154 nm) and raman spectra ( LabRAM HR). The morphology of the γ-Fe2O3/KB composite and the

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size of γ-Fe2O3 particles were observed by scanning electronic microscopy (SEM,

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S-3400N and S-4800) and field emission transmission electron microscopy (TEM,

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JEM-2100, Japan). The distribution of the elements of the samples was analyzed by

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energy dispersive spectrometer, which combined with scanning electron microscopy

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(SEM, Hitachi S4800, Japan). The surface areas and pore distributions of as-prepared

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samples were determined via nitrogen adsorption with the Micromeritics Tristar

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surface area and porosity analyzer (BET, BJH, TriStar 3000, USA). The samples were

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degassed under vacuum (100 mTorr) at 150°C for 16 h prior to analysis. The surface

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area was calculated using the Brunauer-Emmet-Teller (BET) method and the

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Barrett-Joyner-Halenda (BJH) pore size distribution was determined using the

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desorption branch of the isotherm. The carbon content of the γ-Fe2O3/KB composite was evaluated by thermogravimetric analysis (SDT Q600, Switzerland) at a heating

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rate of 10°C/min from ambient temperature to 800°C in air. 2.3 Electrochemical characterization Study of the electrochemical performance was performed by assembling 2032 coin cell. The synthesized powders were thoroughly mixed with 10%wt

polyvinylidene

fluoride

(PVDF)

and

10%wt

acetylene

black

(AB)

in

N-methyl-2-pyrrolidone (NMP). The obtained slurries were then brushed onto copper foils substrate and dried in a vacuum oven at 120°C for 12h, respectively. The cells were assembled in an argon-filled glove box using lithium foil as counter electrode, Celgard 2400 microporous polyethylene membrane as separator, 1M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) as electrolyte, and

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then left to age for at least 12h before charge/discharge test. The charge/discharge

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cycling was performed on a battery test instrument (CT2001A, LAND Battery

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Program-control Test System, China) over a voltage range of 0.1–3.0V. The

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electrochemical behaviors of the individual composite electrodes were evaluated by

(CHI,

660B,

CHENHUA,

China).

Electrochemical

impedance

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Workstation

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cyclic voltammetry (CV), at a scan rate of 0.1mV·s-1, on an Electrochemical

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spectroscopies (EIS) of the cells were also conducted on the Electrochemical

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Workstation. The EIS spectra were potentiostatically collected by using a DC

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potential equal to the open circuit voltage of the cell and an AC oscillation of 5mV

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over a frequency range of 105Hz–0.01Hz.

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3. Results and discussion

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3.1 Structure and morphology Fig. 2(a) shows the XRD patterns of γ-Fe2O3, γ-Fe2O3/KB composite, KB and

the standard diffraction pattern of γ-Fe2O3, respectively. The diffraction peaks of pure γ-Fe2O3 and γ-Fe2O3/KB composite match well with the characteristic diffractions of γ-Fe2O3 phase (JCPDS No. 39-1346), which indicates that the introduction of KB

does not change the crystal structure of γ-Fe2O3. The slight protuberance between 15° and 30° in the XRD pattern of γ-Fe2O3/KB can be attributed to the amorphous KB, which is in the accordance with the broad diffraction peak of KB in the range of 15° – 30°. Besides, Raman spectra are employed to confirm the phase of γ-Fe2O3. In Fig. 2(b), it shows the presence of bands at 220, 285, 380, 590, 1310 and 1580 cm-1. Among these only 380 and 1580 cm-1 can be ascribed to γ-Fe2O3 and the others are

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due to α-Fe2O3. This can be explained by the degradation of γ-Fe2O3 to α-Fe2O3

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induced by the laser irradiation [28].

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In order to calculate carbon content of γ-Fe2O3/KB, the TG curve under an

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ambient atmosphere is shown in Fig. 3. The slight initial weight loss up to 300°C of

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γ-Fe2O3/KB composite may be due to the evaporation of surface adsorbed water,

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while the weight loss in the temperature range from 300 to 500°C is about 28.02%,

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which can be ascribed to the oxidization and subsequent decomposition of KB. Hence,

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the carbon contents of γ-Fe2O3/KB composite are estimated to be 28.02%.

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Fig. 4 shows the N2 adsorption isotherms and pore distributions of mesoporous

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carbon (KB) and γ-Fe2O3/KB composite. According to the IUPAC classification [29],

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the two samples exhibit a typical V isotherm with H1 hysteresis loop, indicating that both KB and γ-Fe2O3/KB composite have a mesoporous structure. The BET surface

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area, pore volume and pore diameter of the samples are listed in Table 1. The obtained values of specific surface area are 1256.32 m2 g−1 for KB template and 422.75 m2 g−1 for γ-Fe2O3/KB composite, while the pore volumes are 2.56 cm3 g−1 and 0.52 cm3 g−1, respectively. Moreover, the KB template has a primary pore size of 7.17 nm and

γ-Fe2O3/KB has a pore size distribution with a maximum at 3.92 nm. These decreases in parameters may be caused by the γ-Fe2O3 nanoparticles embedded in the mesopores of KB. To further confirm the structure and morphology of γ-Fe2O3/KB composite, SEM and TEM measurements are carried out. Fig. 5(a) and Fig. 5(b) show the morphology of pure γ-Fe2O3, which is in accordance with the result of Ref. [25]. The loose and

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porous structure of KB is shown in Fig. 5(c) and the similar morphology is observed

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in γ-Fe2O3/KB composite, which indicates the preparation process will not change the

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monolithic construction of KB. From the TEM and HR-TEM images of γ-Fe2O3/KB

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composite, the dark particles in Fig. 5(e) are γ-Fe2O3 and the highly ordered

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crystalline lines of γ-Fe2O3 nanoparticles are clearly seen in Fig. 5(f). It also can be

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seen that the uniform γ-Fe2O3 nanoparticles with a diameter of about 5 nm are

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decorated in the mesoporous networks of KB. Such special a novel structure ensures

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that γ-Fe2O3 nanoparticles can contact with the KB continuous conductive framework

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more profitably, which can boost the transportation of ions and electrons.

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In order to better see the uniform dispersion of γ-Fe2O3 nanoparticles in KB, we

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performed the elemental mapping of γ-Fe2O3/KB by Energy Dispersion Spectroscopy (EDS) measurement. Fig. 6(a) shows the SEM image of the tested area, the integral

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distribution of the Fe, O and C elements in the tested area is presented in Fig. 6(b), Fig. 6(c,e,f) exhibit the separate distributions of the different elements Fe, O and C in the area, respectively. As shown in Fig. 6(c,e), elements Fe and O have homogeneous

distributions, which suggest that well-dispersed γ-Fe2O3 nanoparticles are decorated in mesopores of KB. 3.2 Electrochemical properties Fig. 7 shows the CV curves of γ-Fe2O3/KB tested between 0.01–3.0V for the initial 7 cycles at a scan rate of 0.1mV s-1 at 25°C. Each circle of the CV curve consist of a broad oxidation peak around 1.8V and a reduction peak around 0.8V, which are

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corresponded the oxidation of Fe0 to Fe3+ and reduction of iron from Fe3+ to Fe0 [30].

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This electrochemical reaction mechanism between Li+ and Fe2O3 in LIBs can be

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described as the following reaction.

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In Fig. 7, we can clearly see that the first circle is different from other circles.

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The reduction peak of the initial circle marked as “1” is at 0.5V, which is associated

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with the electrolyte decomposition to form SEI films and the reduction of Fe3+ to Fe0

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besides the irreversible phase transformation during lithium insertion/extraction

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process in the initial cycle [31]. After the initial discharge process, all the reduction

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peaks marked as “3” are at around 0.8V and the shape of reduction peaks changed

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slightly as the cycle index being increased, which indicates the γ-Fe2O3/KB composite

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electrode shows good reversibility of the oxidation reaction from Fe0 to Fe3+. Fig. 8 shows the initial two discharge-charge curves of the γ-Fe2O3/KB and

γ-Fe2O3 electrodes at 0.1C rate and a voltage range of 0.01-3.0 V (vs. Li+/Li). From Fig. 8, we can see that the first charge/discharge curve shows a flat voltage plateau at around 0.8V (vs. Li+/Li), which is caused by the reduction of iron oxide in the

electrode materials, and this electrochemical reaction is the main contribution to their discharge capacities. It also can be observed that the first discharge capacity of γ-Fe2O3/KB and γ-Fe2O3 at a constant current rate of 0.1C is 1998.9 mAh·g-1 and 1041.3 mAh·g-1, respectively, which is higher than the theoretical value（1007 mAh·g-1）. This kind of phenomenon may be ascribed to the formation of a gel-like organic layer on the surface of the iron oxide and KB [32] and the absorption of Li+ in

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nano/micro pores or the further Li storage via an interfacial reaction due to the charge

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separation at the metal/Li2O phase boundary [33,34] . The second discharged capacity

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decreases to 1110 mAh·g-1 for γ-Fe2O3/KB composite and 834 mAh·g-1 for γ-Fe2O3

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since irreversible SEI formation is nearly completed after the first lithium insertion

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and extraction process.

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Fig. 9 presents the rate performance of all samples at different rate from 0.2C to

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1C. It is clearly seen that the capacities of all the samples are decreased as the density

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increases, especially the pure γ-Fe2O3, while γ-Fe2O3/KB composite shows superior

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capacity retention at different rate. The capacities of pure γ-Fe2O3 tested at 0.2C, 0.5C,

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0.8C and 1C are 604.5 mAh·g-1, 460 mAh·g-1 , 323.6 mAh·g-1, 203.6 mAh·g-1,

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respectively. Correspondingly, the γ-Fe2O3/KB composite delivers discharge capacity of 974 mAh·g-1 at 0.2C, 868.1 mAh·g-1 at 0.5C, 830.3 mAh·g-1 at 0.8C and 780.4

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mAh·g-1 at 1C. After eliminating the capacities provided by KB, the capacities of sole γ-Fe2O3 are 892.9 mAh·g-1, 794.7 mAh·g-1, 763 mAh·g-1, 721 mAh·g-1 at the same condition. In Fig. 9, it can be seen that the capacity of γ-Fe2O3/KB composite can recover to the initial value as long as the current density reverses back to a low rate.

This phenomenon reveals that the structure and electrochemical properties of γ-Fe2O3 can be maintained by this novel mesoporous structure constructed by KB. Fig. 10(a) shows the cycle performance of the γ-Fe2O3, γ-Fe2O3/KB and KB at 0.2C during 100 cycles and the cut-off voltage is set between 0.01 and 3V. From Fig. 10(a), it can be identified that the cyclability of the γ-Fe2O3/KB is much better than the pristine γ-Fe2O3. The initial discharge capacities of γ-Fe2O3, γ-Fe2O3/KB and KB

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are 1100 mAh·g-1, 871 mAh·g-1 and 303 mAh·g-1, and the discharge capacities still

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remain 988.8 mAh·g-1, 506.3 mAh·g-1 and 240.1 mAh·g-1 after 100 cycles,

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respectively. Eliminating the capacity of KB, the capacity of sole γ-Fe2O3 can be

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calculated as 921.5 mAh·g-1. Thus the capacity of γ-Fe2O3 in the composite is much

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higher than that of bulk γ-Fe2O3. Consequently, the γ-Fe2O3/KB electrode shows more

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prominent cycle performance with good capacity retention. Meanwhile, on the whole,

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the coulombic efficiency of γ-Fe2O3/KB is higher than pure γ-Fe2O3. Normally, the

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repeated volume change during the lithium ions insertion/extraction process results in

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the particle pulverization and electrode deterioration. Hence, the good cycle

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performance of the composite can be overwhelmingly attributed to its higher porosity

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created by KB skeletons and high dispersion state of nano-structured γ-Fe2O3 particles embedded in the KB network, which can effectively buffer the volume expansion

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during the charge-discharge process. Furthermore, we also investigated the cycle performance of γ-Fe2O3/KB and γ-Fe2O3 at high current rate as shown in the Fig. 10(b). The initial capacities of γ-Fe2O3/KB composite are 855.4 mAh·g-1 at 2C and 580.1 mAh·g-1 at 5C rate and it retains 81.6% and 71% of capacity after 100 cycles at

2C and 5C rate, respectively. For comparison purpose, we choose the carbon black (CB) as the control experiment which used the same weight percentage of carbon black as conductive additive to demonstrate the specific function of 3D network of KB, and the results are shown in Fig. 10(b). Compared with γ-Fe2O3/KB, it is clearly seen that the cycle performance of γ-Fe2O3/KB composite at 2C and 5C is better than that of γ-Fe2O3/CB, which indicates the role of mesoporous KB in composite is

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superior to CB.

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To further investigate the role of KB combination on the electrochemical

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performance of γ-Fe2O3 anode material, electrochemical impedance spectroscopy

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(EIS) measurements were carried out in a frequency range from 0.01 to 105 HZ and

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the Nyquist plots are given in Fig. 11(a). All the EISs were carried out at the fully

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charged state after 100 cycles. It can be seen the Nyquist plots of γ-Fe2O3 and

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γ-Fe2O3/KB both consist of a sloping line in the low frequency range and a depressed

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semicircle in high frequency. The EIS spectra are fitted using an equivalent circuit as

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shown in the insert of Fig. 11(a). Specifically, Re is the electrolyte resistance, Rct

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represents the charger-transfer resistance at the electrolyte/electrode interface between

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electrolyte and electrode, CPE means the double layer capacitance and the sloped lines at low frequency can be regarded as the Warburg impedance (W), which is

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related to the Li-ions diffusion in the bulk electrode. The values of Re, Rct, W are derived using ZsimpWin software and listed in the Table 2 .The Rct value of γ-Fe2O3/KB is much smaller than that of γ-Fe2O3, which means the lower charger-transfer resistance of the γ-Fe2O3/KB.

The diffusion coefficient values of the lithium ions (DLi+) in the bulk electrode can be calculated from the following Eq. 1 [35].

DLi  

R 2T 2

(1)

2 A2 n 4 F 4C 2  w 2

where R is the gas constant (8.314Jmol-1 K-1), T is the absolute temperature in Kelvin(298.15K), A is the area of the electrode surface, n is the number of electrons

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per molecule during oxidization, F is the Faraday’s constant (96,500C mol-1), C is the concentration of lithium ion in solid and σw is the Warburg coefficient, ρ is density of

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the synthesized materials, M is molecular weight of Fe2O3. The σw can be calculated

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by the following Eq. 2 [35].

（2）

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Z re  R e  R ct   w  1 / 2

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where Zre is the real part of the impedance, Re is the resistance of the electrolyte, Rct is

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the charge transfer resistance and ω is the angular frequency in the low frequency

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region. Both Re and Rct are kinetics parameters independent of frequency. And σw is

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the slope for the plot of Zre vs. the reciprocal root square of the lower angular frequency (  1/ 2 ). The plot of Zre vs. the reciprocal root square of the lower angular

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frequencies (  1/ 2 ) for the two samples is shown in Fig. 11(b). The σw values of

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γ-Fe2O3/KB and γ-Fe2O3 are 5.75Ω cm2· s1/2 and 30.26Ω cm2· s1/2, as shown in Table

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2. According to the Eq. 2, the DLi+ value is in inverse proportion to the σw value. As a consequence, the value of DLi+ for γ-Fe2O3/KB is higher than that of γ-Fe2O3. The decreased charger-transfer resistance and enhanced diffusion coefficient can be mainly attributed to the combination of conducive KB, which can maintain the nanostructure of Fe2O3, construct a 3D network to shorten the lithium ion diffusion

length hinders particle agglomeration and enlarge the electrode/electrolyte contact surface Above all, the superior rate performance of γ-Fe2O3/KB is due to the combination with mesoporous KB, and the effects of KB to the composite may be concluded as followings: (i) the 3D network structure of KB acts as a fancy conductive bridge, which can improve the electronic and ionic transport in the

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γ-Fe2O3 material. (ii) the mesoporous structure of KB offers the cushion-like effect to

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relieve volume expansion of γ-Fe2O3 during the lithium ion insertion and desertion.

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(iii) the carbon template restrains the agglomeration of iron oxide NPs, maintaining

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small particle size and good dispersion of the γ-Fe2O3 in the stage of the γ-Fe2O3/KB

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composite formation.

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4. Conclusions

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In this paper, we successfully synthesized the γ-Fe2O3 nanoparticle/KB

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composite as anode material for lithium ion battery via a solvothermal approach

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combined with precursor thermal transformation. Compared with bare γ-Fe2O3, the

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composite inherited the mesoporous structure from KB exhibits ultrahigh capacity and

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excellent high rate cyclability. The improving electrochemical performance of γ-Fe2O3 can be ascribed to the addition of the 3D network structure of KB, which can

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improve the electronic and ionic transport in the γ-Fe2O3 anode and offer the cushion-like effect to relieve volume expansion during the lithium ion insertion and desertion process. These results certainly provide the way to develop high performance anodes and can be extended to the rest of the active materials.

Acknowledgements This work is supported by Shanghai Leading Academic Discipline Project (B502), Shanghai Nanotechnology Special Foundation (No.11nm0500900) and

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Shanghai Key Laboratory Project (08DZ2230500).

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anode material for lithium ion batteries, Electrochimica Acta 97 (2013) 271. [24] G. X. Gao, L. Yu , H. B. Wu, X. W. Lou, Hierarchical tubular structures constructed by carbon coated α-Fe2O3 nanorods for highly reversible lithium storage, Small 9 (2013) 1741. [25] J. S. Xu, Y. J. Zhu, Monodisperse Fe3O4 and γ-Fe2O3 magnetic mesoporous microspheres as anode materials for lithium-ion batteries, Appl. Mater. Inter. 4 (2012) 4752.

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Figure Captions

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composite.

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Fig.1. Schematic representation of the formation of mesoporous γ-Fe2O3/KB

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Fig.2. (a) XRD patterns of pure KB, γ-Fe2O3 and γ-Fe2O3/KB composite, (b) Raman spectra of γ-Fe2O3/KB composite..

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Fig.3. TG curve of mesoporous γ-Fe2O3/KB composite. Fig.4. (a) Nitrogen adsorption-desorption isotherms and (b) pore diameter distributions of KB and γ-Fe2O3/KB composite.

Fig.5. (a,b) SEM of pure γ-Fe2O3, (c,d) SEM images of KB and γ-Fe2O3/KB composite, (e,f) TEM and HR-TEM of γ-Fe2O3/KB. Fig.6. Elemental mapping for the particles of γ-Fe2O3/KB. Fig.7. Cyclic voltammogram curves of the γ-Fe2O3/KB anode material for the initial 7 cycles. Fig.8. Discharge/charge curves of the γ-Fe2O3/KB and γ-Fe2O3 electrodes at 0.1C for

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the initial two cycles.

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Fig.9. Rate capability of KB, γ-Fe2O3 and γ-Fe2O3/KB.

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Fig.10. (a) Discharge capacity of the samples at 0.2 C and (b) cycling performance of

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γ-Fe2O3, γ-Fe2O3/KB and γ-Fe2O3/CB at high current 2C and 5C rate for 100 cycles.

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Fig.11.(a) EIS spectra of γ-Fe2O3/KB composite in the frequency range between 0.01

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γ-Fe2O3 and γ-Fe2O3/KB.

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Hz and 100 kHz; (b) The relationships between Z’ and  1/ 2 at low frequency for

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composite.

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Table 1 Pore volume, BET surface area and pore size of KB and γ-Fe2O3/KB

SBET (m2 g−1)

KB

1256.32

2.56

7.17

γ-Fe2O3/KB

422.75

0.52

3.92

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Samples

Vtotal (cm3 g−1)

DPore (nm)

Table 2 Impedance parameters of the as-obtained samples. a

σw (Ωcm2 s-1)

Samples

Re (Ω)

Rct (Ω)

γ-Fe2O3

59.44

243.2

30.26

γ-Fe2O3/KB

8.22

65.73

5.75

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Re: electrolyte resistance; Rct: charge transfer resistance; σw: Warburg impedance

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