This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem.

Cite this: DOI: 10.1039/c2nj40151g

a-Fe2O3 nanoparticles anchored on graphene with 3D quasi-laminated

architecture: in situ wet chemistry synthesis and enhanced

electrochemical performance for lithium ion batteriesw

Dezhi Chen,ab Wei Wei,a Ruining Wang,a Jingchao Zhua and Lin Guo*a

Received (in Victoria, Australia) 1st March 2012, Accepted 3rd May 2012

DOI: 10.1039/c2nj40151g

A novel a-Fe2O3/graphene composite is prepared by a simple in situ wet chemistry approach.

The a-Fe2O3 particles with diameter around 130 nm are homogeneously anchored on graphene

nanosheets to form a 3D quasi-laminated architecture. Such a well-organized flexible structure

can offer sufficient void space to facilitate the electrolyte penetration, alleviate the effect of the

volume change of a-Fe2O3 particles and avoid particle–particle aggregation during lithium

insertion/desertion. In addition, graphene not only improves the electric conductivity of the

composite electrode but also maintains the structural integrity of the composite electrode during

long-term cycling. As anode material for Li-ion batteries, the a-Fe2O3/graphene composite

electrode exhibits a stable capacity of 742 mAh g�1 up to 50 cycles. The synthesis technique is

suitable for practical large-scale production of graphene-based metal oxide composites as

advanced electrode materials for rechargeable Li-ion batteries.

1. Introduction

Lithium-ion batteries (LIBs) are some of the most popular

types of rechargeable batteries for portable electronics. In

addition, LIBs are also growing in popularity for military,

electric vehicle and aerospace applications. In order to meet

the ever-growing need for high capacity and high power, it

urgently demands numerous efforts to develop new high-

performance electrode materials for next-generation LIBs.1–4

As a new class of promising anodematerial for high energy density

LIBs, transition-metal oxides have been widely investigated

since 2000.5 In comparison with the conventional graphite

anodes (372 mAh g�1), these oxide electrodes have shown

higher theoretical Li-ion storage capacity (4500 mAh g�1).6

Among these potential anode materials, hematite (a-Fe2O3),

the most stable form of iron oxides, has attracted considerable

attention due to its low-cost, environmental-friendliness, natural

abundance and high theoretical capacity of 1007 mAh g�1 (6 mol

of Li per mole of a-Fe2O3).7–11 However, similar to most of the

transition-metal oxides, a-Fe2O3 also suffers from low reversible

capacity and poor cycling performance in practical cells

because a-Fe2O3 typically breaks into small metal clusters,

leading to a large volume change, the crumbling of active

materials and conduction network breakage during repeated

lithium uptake and removal reactions.10,12,13 The strategy of

preparing composites with nanostructure carbon-based materials

had been proposed to circumvent these obstacles and achieve

good cyclic performance as well as to maintain high specific

capacity.7,9,14–16 Herein, these carbon materials can effectively

buffer the strain from the volume change, improve the structural

stability of the electrode, increase the electrical conductivity, and

enhance electrochemical Li-ion storage performance.16 As the

new star in nanostructure carbon materials, graphene, a single

atomic layer of carbon connected by sp2 hybridized bonds, has

received intense attention for its unique properties such as

superior electrical conductivity, excellent mechanical flexibility,

large surface area and high thermal stability.17–22 Graphene-

based composites with transition-metal oxides, such as

Co3O4,23 Mn3O4,

24 NiO,25 CuO,26 Fe3O4,27 and TiO2,

28 have

been fabricated to prepare anode materials for rechargeable

LIBs. These composites showed high reversible capacity, long

cycle life, and good rate performance owing to their spectacular

electrochemical and physical properties along with the combined

virtues between the heterogeneous components.

Recently, the design and fabrication of three-dimensional

multifunctional architectures from the appropriate nanoscale

building blocks have been attracting increasing attention and

enriching ‘‘bottom-up’’ approaches toward future electrode

materials.29 Braun and co-authors30 reported that cathodes

made from a self-assembled three-dimensional bicontinuous

nanoarchitecture consisting of an electrolytically active material

sandwiched between rapid ion and electron transport pathways

have very large battery charge and discharge rates with minimal

a School of Chemistry and Environment, Beihang University, Beijing100191, People’s Republic of China. E-mail: guolin@buaa.edu.cn;Fax: +86-10-82338162; Tel: +86-10-82338162

b School of Environmental and Chemical Engineering, NanchangHangkong University, Nanchang 330063, People’s Republic of China

w Electronic supplementary information (ESI) available. See DOI:10.1039/c2nj40151g

NJC Dynamic Article Links

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capacity loss. Mullen et al.31 reported that graphene-based nano-

sheets with a sandwich structure can significantly improve the

electrochemical performance due to their favorable structures.

Graphene-encapsulated Fe3O4 nanoparticles with 3D laminated

structure exhibited a stable capacity of 650 mAh g�1 with no

noticeable fading for up to 100 cycles.32 However, the composite of

graphene-based nanosheets and a-Fe2O3 with 3D hierarchical

structure has not been reported via a simple in situ wet

chemistry method up to now. Although the excellent electro-

chemical performance of graphene-based a-Fe2O3 composites

has been achieved by the hydrothermal/solvothermal methods

and the microwave irradiation synthesis,33–39 other simple

synthetic methods of graphene-based composite are still worth

developing. Furthermore, although the graphene-based 2D

templates have provided a well-defined functionalized surface

for good distribution of nanoparticles, the graphene-based

composites are randomly stacked to make electrodes, inevitably

leading to the particle–particle aggregation40 and reducing the

reversible capacity after multiple cycles.35,38,41 Hence, in the present

work, we demonstrate a simple strategy to achieve optimum

electrochemical properties by constructing a 3D quasi-laminated

architecture from self-assembly of a-Fe2O3 and graphene nano-

sheets by the aid of surfactant under atmospheric pressure. In

comparison with the bare a-Fe2O3, the novel a-Fe2O3/graphene

composite electrode delivers a stable reversible capacity of

742mAh g�1 up to 50 cycles as anodematerial for LIBs. The simple

in situ wet chemistry approach presents operational simplicity and

capability for a large-scale production of graphene-based metal

oxide composites as electrode materials for LIBs.

2. Experimental section

2.1 Preparation of a-Fe2O3/graphene composite

All of the reagents were of analytical grade and were used

without further purification. Deionized (DI) water was used in

all experiments. Graphite oxide was synthesized from natural

graphite powder (499.8%, Alfa Aesar) by a modified Hummers

method.42–44 In a typical synthesis of the composite, 50mg graphite

oxide was first dispersed in 50 mL of deionized water at room

temperature by ultrasonication (500 W) for 1 hour. Then other

50 mL of deionized water containing 0.555 g polyvinylpyrrolidone

(PVP, K30, Fluka) and 0.005 mol iron chloride hexahydrate

(FeCl3�6H2O) was added to the graphene oxide suspension with

constant stirring. Following this, the hybrid solution was gradually

heated to 100 1C from room temperature (about 10 min) in an oil

bath under magnetic stirring by open reflux condensation mode.

Afterward, 0.5 mL of hydrazine monohydrate liquid (N2H4�H2O,

80%) was added into the mixture dropwise. The reaction was

continued for 24 h at 100 1C. Finally, the black mixture was

collected by centrifugation. After washing with adequate deionized

water and ethanol to remove the PVP and residual N2H4�H2O, the

a-Fe2O3/graphene composite was obtained by drying at 80 1C for

24 h. The bare a-Fe2O3 (ESIw, Fig. S1) nanoparticles were preparedusing the same procedure in the absence of graphene oxide.

2.2 Characterization

XRD analyses were carried out on an X-ray diffractometer

(D/MAX-1200, Rigaku Denki Co. Ltd., Japan). The XRD

patterns with Cu Ka radiation (l= 1.5406 A) at 40 kV and

40 mA were recorded in the range of 2y =151–651. The XRD

specimens were prepared by means of flattening the powder on

the small slides. Scanning electron microscope images were

achieved by a FEI Quanta 250 field-emission gun environ-

mental scanning electron microscope at 10 kV with the

samples obtained from the thick suspension dropping on the

silicon slice. Transmission Electron Microscopy images were

obtained using a JEM-2100F transmission electron micro-

scope (JEOL Ltd., Japan) operated at 200 kV. Raman spectra

were recorded from 200 to 1900 cm�1 on a LabRAM HR800

Laser Raman spectroscope (HORIBA Jobin Yvon CO. Ltd.,

France) using a 632.5 nm argon ion laser. All samples were

deposited on silicon wafers in powder form without using any

solvent. Thermogravimetric analysis was performed in air

using a Pyris Diamond TG analyzer (PerkinElemer Inc.,

U.S.A.). The samples were heated from 50 1C to 700 1C at

10 1C min�1. Fourier transform infrared (FT-IR) spectra of the

samples were recorded on an Avatar 360 spectrophotometer

(Thermo Nicolet, U.S.A.). The test specimens were prepared by

the KBr disk method.

2.3 Electrochemical measurements

The electrochemical properties of the bare a-Fe2O3 particle

and the a-Fe2O3/graphene composite as anode materials in

lithium ion cells were evaluated by a galvanostatic charge/

discharge technique. The test electrodes were prepared by

mixing 80 wt% active material with 10 wt% carbon black

and 10 wt% polyvinylidene fluoride (PVDF) dissolved in

N-methyl-2-pyrrolidone (NMP) to form a slurry, which was

then coated onto copper foil (current collector), dried at 80 1C

for 10 h. Then, the copper foil was punched into small disks

with diameter of 14 mm. The mass loading is around 2.0 mg.

CR2016 type coin cells were finally assembled in a highly-pure

argon-filled glovebox using the test electrodes and the metallic

lithium counter/reference electrode, a polypropylene separator

(Celgard 2400) and an electrolyte of 1 mol L�1 LiPF6 in ethylene

carbonate and diethyl carbonate (EC/DMC, 1/1 vol) (Tianjin

Jinniu Power Sources Material Co., Ltd. China). Charge–

discharge measurements were carried out galvanostatically at a

current density of 100 mA g�1 in the voltage range from 0.005 V

to 3.0 V using a battery test system (LAND CT2001A model,

Wuhan Jinnuo Electronics. Ltd., China). Note that the mass of

graphene was included when calculating the specific reversible

capacity of the a-Fe2O3/graphene composite. The electrochemical

impedance measurements were performed on a CHI660D

electrochemical workstation (Shanghai Chenhua Co. Ltd.,

China) at an AC voltage of 5 mV amplitude in the range of

100 kHz to 0.01 Hz.

3. Results and discussion

The fabrication process of the a-Fe2O3/graphene composite is

schematically illustrated in Fig. 1. Firstly, an aqueous solution

of a certain amount of FeCl3 and PVP was mixed with

graphene oxide dispersion to prepare reaction solution. When

the solution was gradually heated to 100 1C under magnetic

stirring, hydrazine monohydrate was added into the mixture

dropwise. The hydrazine monohydrate can not only reduce the

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graphene oxide nanosheets but also promote the hydrolysis of

FeCl3. After reaction at 100 1C for 24 h, the a-Fe2O3/graphene

composite was collected.

Fig. 2a presents the powder XRD pattern of the a-Fe2O3/

graphene composite and bare a-Fe2O3. All the peaks match

well with that of hematite (JCPDS File Card No. 33–0664).

A diffraction hump appears between 201 and 301 in the powder

XRD pattern of the composite, which may be originated from

graphene nanosheets. Raman spectroscopy is an important

tool to identify the crystal phase and determine the quality of

crystalline or amorphous materials. The Raman spectra of the

a-Fe2O3/graphene composite and free a-Fe2O3 particle are

presented in Fig. 2b. The fundamental Raman scattering

peaks for pure a-Fe2O3 particle are observed at 223, 242,

290, 408, 497 and 608 cm�1, which can be assigned to the 2A1g

and 4Eg Raman modes for the typical hematite phase.45 The

peak observed at 1303 cm�1 is attributed to two-magnon

scattering of hematite.45 No other iron oxides, such as magnetite

or maghemite, were detected, indicating the high purity of the

product. The Raman spectrum of the a-Fe2O3/graphene composite

is actually that of a combination of bare a-Fe2O3 and graphene,

and shows typical peaks of hematite and characteristic peaks of the

D and G bands from graphene at around 1348 and 1588 cm�1.

The intensity ratio of the D band and the G band is consistent with

a previous report on GO reduction.46

Fig. 3a presents the FT-IR spectra of free a-Fe2O3 particles,

the a-Fe2O3/graphene composite and pristine PVP. Three

intense absorption bands are observed at 3440, 2923 and

1633 cm–1 in Fig. 2a, which are assigned to the asymmetrical

stretching vibration, symmetrical stretching vibration and

deformation vibration of physically adsorbed H2O molecules

in the sample, respectively. For bare a-Fe2O3 (Fig. 3a(1)) and

a-Fe2O3/graphene composite (Fig. 3a(2)) samples, the sharp

Fig. 1 Scheme of preparation of a-Fe2O3/graphene composite.

Fig. 2 XRD pattern (a) and Raman spectra (b) of a-Fe2O3 particle

(1) and a-Fe2O3/graphene composite (2).

Fig. 3 (a) FT-IR spectrum of (1) a-Fe2O3 particle, (2) a-Fe2O3/

graphene composite and (3) pristine PVP K30; (b) TGA curves of

a-Fe2O3 particle, a-Fe2O3/graphene composite.

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bands at 576 and 478 cm–1 can be assigned to the stretching

vibrations of the Fe3+�O2� bond in the FeO6 octahedron and

FeO4 tetrahedron, respectively.47 However, different from the

spectrum of pristine PVP in Fig. 2b(3), no significant absorption

band was observed at 2946, 1675, 1429 and 1284 cm�1 for bare

a-Fe2O3 particle and a-Fe2O3/graphene composite samples. It

shows that the PVP in bare a-Fe2O3 and a-Fe2O3/graphene

composite samples were removed almost completely. TGA curves

of bare a-Fe2O3 and a-Fe2O3/graphene composite samples, as

shown in Fig. 3b, were used to quantify the amount of graphene

in the composite. From the TGA curves, we can see that the bare

a-Fe2O3 sample remains stable over the entire temperature range,

however, the a-Fe2O3/graphene composite shows rapid mass loss

between 300 and 550 1C due to the oxidation of carbon.

According to the change of weight, it is estimated that the mass

percentage of a-Fe2O3 particles in the composite is 75%.

Typical SEM images of the a-Fe2O3/graphene composite

are shown in Fig. 4. Because of the natural 2D sheet morphology,

graphene decorated with nanoparticles is prone to stack each

other to construct 3D layer-by-layer architecture under control

of a surfactant.48 As shown in Fig. 4a, it can be clearly observed

that graphene nanosheets are hybridized with a-Fe2O3 particles

to preserve a 3D quasi-laminated structure, and the nanosized

a-Fe2O3 particles anchored on graphene uniformly (Fig. 4b).

From the digital photo of the product dried at 80 1C for 24 h

(ESIw, Fig. S2), we can clearly observe that the dried composite

shows the morphology of curly film, which is also a better proof

to support the 3D structure claimed previously. The typical

a-Fe2O3 particles are sphere in shape and around 130 nm in

diameter, which can be further confirmed by the TEM image in

Fig. 5. Fig. 5a clearly shows the a-Fe2O3 particles embedded in

graphene layers. TEM image in Fig. 5b exhibits the interface

between graphene and a-Fe2O3 particles. As presented in

Fig. 5c, the lattice fringes having an interlayer distance of

0.252 nm agree well with the spacing between (110) planes of

a-Fe2O3 crystals.

In our experiments, PVP is very important to control the

interfacial interactions between graphene and the metal oxide

materials during the formation of a-Fe2O3/graphene composite

architecture. On the one hand, the PVPmolecules can attach onto

graphene oxide/graphene through the hydrophobic interactions

to improve the dispersion of graphene oxide/grapheme.49

Actually, Shan et al. had reported that graphene protected

by PVP could be stably dispersed in water, and exhibited good

electronic properties.50 On the other hand, they also interact

with the metal oxide precursor through the hydrophilic head

groups to control nucleation and growth of the metal oxide.51

Fig. 4 FESEM images of a-Fe2O3/graphene composite with different

magnifications.

Fig. 5 TEM images of a-Fe2O3/graphene composite with different

magnifications. (c) Corresponding high-resolution TEM image from

the marked region.

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The cooperative interactions between the surfactant, the GO,

and the precursors lead to the homogeneous mixing of the

components. After the addition of hydrazine monohydrate,

the as-grown a-Fe2O3 particles are then coated on graphene

surfaces and form a 3D quasi-laminated architecture with the

aid of PVP. Contrarily, because the dispersion of graphene

oxide in water depends on the charge stabilization in solution,

the addition of FeCl3 in solution can affect the diffusion of

double layer formation and leads to agglomeration of graphene

oxide.33 It is difficult to prepare a uniform a-Fe2O3/graphene

composite without PVP (ESIw, Fig. S3).To investigate the electrochemical performance of the

a-Fe2O3/graphene composite as an anode for LIBs, discharge/

charge cycling was carried out in the voltage range of

0.005–3.0 V (vs. Li+/Li) at a current density of 100 mA g�1

and room temperature. The initial, second, tenth and fiftieth

charge–discharge curves of pristine a-Fe2O3 particles and

a-Fe2O3/graphene composite electrodes are shown in Fig. 6a

and b. In the initial discharge curve, an extended potential

plateau at about 0.8 V vs. Li+/Li is observed for the a-Fe2O3

particle and a-Fe2O3/graphene composite electrodes, similar to

the literature results.52,53 As shown in Fig. 6a, the bare a-Fe2O3

electrode shows an initial discharge capacity of 1719 mAh g�1

and a charge capacity of 809 mAh g�1 in the first cycle. The

irreversible capacity loss is 53%. From the length of the lithium

intercalation plateau observed in the first discharge curve, we

can see that the pure a-Fe2O3 particle electrode is close to a

theoretical capacity of 1007 mAh g�1 from the reduction

reaction of Fe3+ to Fe0, corresponding to a maximum lithium

uptake of 6 Li per a-Fe2O3. The equation of electrochemical

reversible reaction is as follows: a-Fe2O3 + 6e� + 6Li+ 3

2Fe0 + 3Li2O.52,53 Furthermore, the excessive plateau capacity

of the nanosized a-Fe2O3 electrode over the theoretical capacity

may be ascribed to the severe side reaction of the enlarged

surface area with the electrolyte54 and the formation of the solid

electrolyte interphase (SEI).55 The following sloping region

might be attributed to the formation of a surface polymeric

layer, which in general contributes to additional capacity and

results in a large irreversible capacity.56 Furthermore, the

interfacial reaction due to the charge separation at the

Fe/Li2O phase boundary and the inherent poor electrical/ionic

conductivity of the a-Fe2O3/Fe/Li2O matrix formed during the

charge/discharge process may also cause irreversible capacity.57,58

During the second cycle, the discharge capacity and the corres-

ponding charge capacity decreased to 783 and 462 mAh g�1.

Then, it quickly fades to 98 and 97 mAh g�1 in the fiftieth cycle,

respectively.

Compared with the pristine a-Fe2O3, in the initial cycle,

the a-Fe2O3/graphene composite electrode has a discharge

capacity of 990 mAh g�1 with the corresponding charge

capacity of 719 mAh g�1 (Fig. 6b), and the Coulombic

efficiency was rose to 72.6%. The lower initial capacity than

the bare a-Fe2O3 electrode may be attributed to that the

electrochemically active material for Li-ion storage is mainly

the a-Fe2O3, not the graphene. This result is different from

other reports where graphene is also the active phase for

energy storage.38 Furthermore, the discharge voltage plateau

is not as flat as for the a-Fe2O3/graphene composite compared

to the pure a-Fe2O3, probably because of reduction in crystallinity

and/or change in surface site energetics during the formation

process of the composite.59–61 An increased Coulombic efficiency

of 85.5% was achieved in the second cycle with a discharge

capacity of 827 mAh g�1 and a charge capacity of 708 mAh g�1.

During the tenth and fiftieth cycle, the Coulombic efficiency

further increased to 95.8% and 97.8%, respectively.

Fig. 7a shows the cycling performance of the pure a-Fe2O3

particle and a-Fe2O3/graphene composite electrodes at a

current density of 100 mA g�1 for 50 cycles. The discharge capacity

of the bare a-Fe2O3 electrode decreases from 809 mAh g�1 to

97 mAh g�1, which is only about 6% of the initial capacity,

showing poor capacity retention. On the contrary, the

a-Fe2O3/graphene composite electrode has a high discharge

capacity of 827 mAh g�1 in the second cycle. Subsequently, the

capacity dropped slowly to 742 mAh g�1 during the initial

30 cycles, and then maintained stably at 742 mAh g�1 without

any capacity loss up to the 50th cycle.

As we know, rapid decay of the capacity of the bare

transition metal oxide electrode is mainly attributed to the

large volume charge, which causes structural damage of the

metal oxide particles and the breakage of the electronic

conducting network during Li ion insertion and extraction

processes.62,63 However, in the a-Fe2O3/graphene composite

Fig. 6 Electrochemical performance of bare a-Fe2O3 particle and

a-Fe2O3/graphene composite electrodes under room temperature.

Discharge-charge profiles of (a) a-Fe2O3 particle and (b) a-Fe2O3/

graphene composite electrodes at the current density of 100 mA g�1.

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electrode, the nanosized a-Fe2O3 particles are homogeneously

anchored on graphene to form a 3D quasi-laminated architecture.

Such a well-organized flexible interleaved structure of the a-Fe2O3

nanoparticles anchored on graphene uniformly can facilitate the

electrolyte penetration and increase the interfacial area between

the electrode and electrolyte firstly, secondly, it can reduce the

effect of volume expansion and alleviate the stress of a-Fe2O3

particles upon lithium insertion/desertion, finally, it can prevent

the detachment and agglomeration of a-Fe2O3 nanoparticles

and the large active surfaces of the nanoparticles are able to

participate in Li/electron diffusion more efficiently during the

frequent discharge/charge cycles.27,32,40 In addition, owing to the

superior electrical conductivity and excellent mechanical flexibility,

graphene also serves as an efficient conducting matrix to provide a

highly conductive network for electron transfer and maintain the

structural integrity of composite electrodes.32,38

In order to verify the good electrochemical performance of

composites in comparison with pure metal oxide particles, AC

impedance spectra measurements were carried out. Fig. 7b

shows AC impedance spectra of the sample electrodes

measured at an open potential of 0.8 V. The impedance spectra

of the pure a-Fe2O3 and a-Fe2O3/graphene composite electrodes

were obtained in the 4th discharge cycle. The high-frequency

semicircle is attributed to SEI film resistance, and the spectra in the

medium frequency include features that are usually semicircular in

shape and related to charge transfer resistance.64 From Fig. 7b, it

can be observed that the diameter of the semicircle for the

composite electrode in the high-medium frequency region is much

smaller than that of the pure a-Fe2O3 electrode, which indicates

that the composite possesses lower contact and charge-transfer

resistances when the surface area of both samples is the same in the

measurement of the impedance. This result shows that graphene in

composite not only improves the high conductivity of the overall

electrode, but also largely enhances the electrochemical activity of

a-Fe2O3 during the cycle processes.

4. Conclusions

In summary, a-Fe2O3 nanoparticles anchored on graphene

with a 3D quasi-laminated architecture have been synthesized

by a simple and effective in situ wet chemistry method under

atmospheric pressure. Without any thermal annealing, the

a-Fe2O3/graphene composite electrode exhibits a significantly

improved cycle life than that of the bare a-Fe2O3 electrode and

a stable and high reversible capacity of 742 mA h g�1 over

50 cycles. The excellent electrochemical performance of a-Fe2O3/

graphene composites can be attributed to the combined effects of

the 3D quasi-laminated flexible architecture and high conductivity

and structure stability of electrode by graphene. The facile in situ

wet chemistry approach is suitable for practical large-scale

production of graphene-based metal oxide composites as advanced

electrode materials for rechargeable LIBs.

Acknowledgements

The authors are thankful for financial support from National

Basic Research Program of China (No. 2010CB934700),

NSFC (11079002 & 20973019) and the Fundamental Research

Funds for the Central Universities.

Notes and references

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Fig. 7 (a) Cycling performance of a-Fe2O3 particle and a-Fe2O3/

graphene composite electrodes at the current density of 100 mA g�1

(hollow-charge, solid-discharge). (b) Nyquist plots of a-Fe2O3 particle

and a-Fe2O3/graphene composite electrodes obtained by applying

a sine wave with amplitude of 5.0 mV over the frequency range from

100 kHz to 0.01 Hz.

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