Author's Accepted Manuscript

Porous α-Fe2O3 nanorods supported on carbonnanotubes-graphene foam as superior anodefor lithium ion batteries

Minghua Chen, Jilei Liu, Dongliang Chao, JinWang, Jinghua Yin, Jianyi Lin, Hong Jin Fan, ZeXiang Shen

PII: S2211-2855(14)20085-1DOI: http://dx.doi.org/10.1016/j.nanoen.2014.08.011Reference: NANOEN464

To appear in: Nano Energy

Received date: 25 July 2014Revised date: 18 August 2014Accepted date: 19 August 2014

Cite this article as: Minghua Chen, Jilei Liu, Dongliang Chao, Jin Wang, JinghuaYin, Jianyi Lin, Hong Jin Fan, Ze Xiang Shen, Porous α-Fe2O3 nanorods supportedon carbon nanotubes-graphene foam as superior anode for lithium ion batteries,Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2014.08.011

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Porous α-Fe2O3 nanorods supported on carbon nanotubes-graphene

foam as superior anode for lithium ion batteries

Minghua Chen a,b,#, Jilei Liu b,c, #, Dongliang Chao b, Jin Wang b, Jinghua Yin a, Jianyi Lin c, Hong

Jin Fan b,c,*, and Ze Xiang Shen b,c,*

a School of Applied Science, Harbin University of Science and Technology, Harbin 150080, P.R. China

b Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore

c Energy Research Institute @ NTU, Nanyang Technological University, Singapore 639798

# These authors contribute equally to this work.

* Corresponding authors. fanhj@ntu.edu.sg (H.J.F); zexiang@ntu.edu.sg (Z.X.S)

Abstract

A novel flexible and lightweight Fe2O3-based lithium-ion battery anode has been developed

by growing porous α-Fe2O3 nanorods onto carbon nanotubes-graphene foam (CNT-GF). The

CNT-GF 3D network provides a highly conductive, high surface areas and lightweight

scaffold for the active Fe2O3 nanorods. Such unique electrodes for lithium-ion battery exhibit

an 80% initial columbic efficiency, high-rate capabilities, and >1000 mA h/g capacities at 200

mA/g up to 300 cycles without obvious fading. These properties can be attributed to the fast

electrochemical reaction kinetics and electron transport rendered by the conductive 3D

network. Our structural design protocol can be extended to many other nanostructured metal

oxides or sulfides, and thus provides a new strategy for construction of high-performance

electrodes for energy storage.

Keywords: Iron oxide; graphene foam; carbon nanotubes; electrochemical energy storage;

lithium ion battery

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1. Introduction

Because of their high working voltages, high capacities, and low toxicity, lithium ion batteries

(LIBs) have great promise and wide applications in grid energy storage, and power sources

for electric vehicles and portable electronics.1, 2 Currently, the most commercially popular

anode is graphite, but new materials are needed to meet the demand of LIBs with higher

energy density and higher power density. Meanwhile, other requirements such as flexibility,

lightweight, low cost, and environmental friendliness are also being pursued for the

fast-developing modern electronics.3, 4 Many studies are focused on the nanostructured metal

oxides (Fe2O3, MnO2, SnO2, TiO2, and Co3O4) as alternative anode materials for LIBs.5-12

Among them, Fe2O3 has attracted considerable attention due to its high theoretical capacity

(1005 mA h/g), natural abundance, and environmental friendliness. However, the low

electronic conductivity, strong agglomeration and low stability during the electrochemical

cycles have hampered its practical application in LIBs.13-16 Enhancing the electrical

conductivity and constructing open channels for Li ion diffusion are effective strategies to

improve the reversible capacity, rate capability and cycle life. This can be achieved by

synthesis of nanosized Fe2O3 and hybridization with more conductive carbon nanomaterials.

A number of recent reports are available on the Fe2O3-based nanostructured anode materials

hybridized with carbon component as conductive matrices, such as thin-layer carbon shell,

carbon nanotube (CNT) or reduced graphene oxide (rGO).17-22 However, the results are still

not good enough particularly in the rate capability and long-cycle stability. It is well known

the electrochemical performance is highly dependent not only on the intrinsic crystalline

texture, but also on the morphology, surface properties and assembled nanostructure of the

active material. There is still large room for improvement by smart nanostructure design and

functional synergy.

In this paper, we report a novel flexible, lightweight nanostructured LIB anode composed

of porous α-Fe2O3 nanorods that are well dispersed onto a three-dimensional (3D) carbon

nanotubes-graphene foam (CNT-GF) substrate. The purpose of this structure design is as

follows. First, while GFs have emerged as important material for energy storage device

electrodes,23-26 the pore size is larger than 100 μm and thus a high percentage of volume space

is still not utilized. By growing CNTs, which are also highly conductive and lightweight, onto

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the surface of GF, the specific surface area can be drastically increased (from ~23.5 to 42.0 m2

g-1 according to BET result). Second, the bottom-up growth of both CNTs and FeOOH

nanorods endow rigid contacts at the interfaces of Fe2O3/CNT and CNT/GF. This is beneficial

to both electron conduction and structural stability of whole composite material as a

binder-free electrode. Also because of the direct growth, the Fe2O3 nanorods are uniform

dispersed within the 3D network, which eliminates the agglomeration during repeated cycles.

The above unique structural advantages make our 3D porous α-Fe2O3 nanorods/CNT-GF

composite a high-performance LIB anode with superior high-rate capability, cycling stability,

and good ductility. To the best of our knowledge, this is the first time to report the growth of

porous α-Fe2O3 nanorods on the CNT-GF composite substrates for LIB applications.

2. Results and Discussion

2.1 Fabrication and structures

Fig. 1a-c schematically shows the synthesis strategy for the Fe2O3/CNT-GF hybrid electrode.

First, GF was prepared by chemical vapor deposition (CVD) using a 3D porous nickel foam

as a template.10 The as-prepared GF exhibits good mechanical flexibility and very low density

(∼ 0.6 mg cm-2), and consists of a continuous and interconnected 3D network with smooth

branches of 60−100 μm and macropores of 150−500 μm (Fig. 1d). Secondly, the bi-metallic

(cobalt and nickel) catalyst nanoparticles were uniformly deposited on the GF surface by

hydrothermal synthesis method. Then, CNT was grown by CVD on the GF surface. As can be

clearly seen in a SEM image in Fig. 2a, a dense forest of CNTs fully and uniformly wraps

around the 3D GF scaffold (Fig. 1e and Fig. S1). The CNTs as grown are multi-walled,

sinuous and highly entangled, with a length of tens μm and an average outer diameter of ∼50

nm. Thirdly, the FeOOH nanorods were grown directly on 3D CNT-GF by solvothermal

synthesis method. Finally, the Fe2O3/CNT-GF was obtained by annealing at 400 ºC in air. The

Fe2O3/CNT-GF hierarchical structure consists of 3D porous CNT-GF uniformly decorated

with Fe2O3 nanorods on the entire surface (Fig. 1f and Fig. 2b). The Fe2O3 nanorods possess

uniform size distribution with a length of more than 100 nm and a diameter of around 50 nm

(Fig. 1f). Importantly, in the hierarchical structure porous Fe2O3 nanorods are in contact with

the CNT. The open network contains interconnected micro- and mesopores, which can

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accommodate electrolyte for the electrochemical reactions. Fig. S2a-c show the color change

of the sample from grey GF (Fig. S2a) to black CNT-GF (Fig. S2b) and finally to reddish

Fe2O3/CNT-GF (Fig. S2c). The Fe2O3/CNT-GF sample has a good mechanical stability and

good flexibility (Fig. S3). The samples can be bent to large angles with no breaking or any

peeling-off during repeated bending. Hence, they can be indeed applied as self-supported and

binder-free electrodes without necessity of conventional polymer binders, additives and metal

substrates. For comparison, the Fe2O3 nanorods were also directly deposited on 3D GF (i.e.,

without CNT) (see SEM images in Fig. S4).

Fig. 1 (a-c) Schematics of the sample fabrication stage: (a) GF, (b) CNT-GF, and (c) Fe2O3/CNT-GF. (d-f) The corresponding SEM images: (d) GF, (e) CNT grown on GF, and (f) Fe2O3 nanorods grown on one CNT.

The XRD patterns of Fe2O3/CNT-GF, CNT-GF and Fe2O3 are shown in Fig. 2c. The

patterns of pure α-Fe2O3 shows clearly sharp peaks indexed to hematite Fe2O3 (JCPDS no.

33-0664).27-29 On the other hand, the as-grown CNT-GF shows two typical diffraction peaks at

26.5 and 54.7°, corresponding to the (002) and (004) reflections of graphitic carbon,

respectively (JCPDS no. 75-1621).30 After Fe2O3 decoration, the obvious diffraction peaks of

graphitic carbon and α-Fe2O3 can be observed, and all diffraction peaks match well with those

of the hematite. No other impurities were found.

Raman spectra in Fig. 2d show five characteristic peaks at 227, 290, 408, 607 and 1313

cm-1 for pure Fe2O3. The first four bands are assigned to the A1g and Eg modes of α-Fe2O3,

while the last band at 1313 cm-1 is ascribed to a two-magnon scattering due to the interaction

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of two magnons established on antiparallel close spin sites.27, 31, 32 The Raman peaks of

CNT-GF are located at 1350 and 1580 cm-1, corresponding to the D and G band due to the

Fig. 2 Structural characterization. SEM images of (a) CNT-GF and (b) Fe2O3/CNT-GF (inset: low-magnification view); (c) XRD patterns and (d) Raman spectra of the pure Fe2O3, CNT-GF and Fe2O3/CNT-GF. (e) TEM image of the Fe2O3 nanorods on one curved CNT (inset: TEM image of the porous Fe2O3 nanorod). (f) HRTEM image of the porous Fe2O3 nanorod and the corresponding SEAD pattern in inset.

disorder induced features of lattice defects and the E2g vibrational mode within aromatic

carbon rings, respectively.24, 25 In the spectrum of the Fe2O3/CNT-GF sample, the Raman

modes of Fe2O3 can be clearly identified by comparing with the peaks of pure Fe2O3,

indicating the successful decoration of Fe2O3 on the substrate. However, the typical peak at

1313 cm-1 vanishes, probably due to the overlapping of D band of CNT-GF with the α-Fe2O3

d20 nm

5 nm

0.25 nm

110e f

1 μm1 μm

10 μm

a

d

b

c

200 nm

6

magnon scattering peak. The D and G bands are barely shifted after Fe2O3 decoration while

the G band intensity is relatively increased.

The detailed microstructure of Fe2O3/CNT-GF sample is characterized using TEM and

HRTEM. The TEM image in Fig. 2e shows an interconnected and core-branch structure of the

Fe2O3 nanorods/CNTs. As shown in the inset, the nanorods supported on a CNT-GF branch

have a typical length of about 100 nm and a width of around 50 nm. Each nanorod contains

numerous nanopores in the size range of 5−10 nm. The distance of the lattice fringes in the

HRTEM image (Fig. 2f) of one Fe2O3 nanorod is 0.25 nm, corresponding to the (110)

d-spacing of hematite (JCPDS no. 33-0664).

The specific surface areas of pure GF, CNT-GF and Fe2O3/CNT-GF were determined from

nitrogen isotherm adsorption-desorption curves (see Fig. S5). As expected, after the CNTs

growth, the specific surface area shows an evident increase from 23.5 m2/g (GF) to 42.0 m2/g

(CNT-GF). As for the final hybrid material Fe2O3/CNT-GF, the surface area increases to 55

m2/g. This large increase should arise from the mesoporous Fe2O3 nanorods. Overall, the

highly porous architecture (from microporous Fe2O3 to mesoporous CNT network and finally

to macroporous GF foam) is beneficial in facilitating electrochemical reactions and increasing

the utilization efficiency of active materials. Hence, good Li-ion storage performance is

expected, and to be presented below.

2.2 Electrochemical properties for lithium-ion storage

The electrochemical property of Fe2O3/CNT-GF electrode for LIBs was systematically

investigated using a lithium foil electrode as reference electrode in the coin-cell batteries.

Cyclic voltammograms (CV) curves of the Fe2O3/CNT-GF electrode at a scan rate of 0.5 mV

s-1 for five cycles in the 0 to 3.0 V (vs. Li/Li+) voltage window are shown in Fig. 3a. A

substantial difference between the first and the subsequent cycles is noticed. In the first

discharge process, a strong reduction peak appears at 0.50 V, corresponding to the Li+ ions

insertion into Fe2O3 and the further reduction reaction leading to the formation of metallic Fe

and Li2O, as also elaborated in previous work.33 The high intensity peak extends to very low

potential and disappears in the subsequent cycles. This can be attributed to the formation of

solid-electrolyte interface (SEI) films during the first cycle. The first anodic scan shows an

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extended peak between 1.6 and 1.9 V, which corresponds to an oxidation of Fe to Fe3+ to

reform Fe2O3. In the subsequent cycles,

Fig. 3 Electrochemical properties. (a) CV curves for Fe2O3/CNT-GF electrode at the scanning rate of 0.5 mV/s for five cycles; (b) Charge/discharge curves measured between 0.01 and 3.0 V at a current of 200 mA g-1; (c) Rate capability of Fe2O3/GF and Fe2O3/CNT-GF electrode at different current densities; (d) Nyquist plots of Fe2O3/GF and Fe2O3/CNT-GF at fully charge stage (Randles equivalent circuit in inset). RS: ohmic resistance of solution and electrodes; Rct: charge transfer resistance; Q: double layer capacitance; ZW: Warburg impedance.

the cathodic lithium insertion mainly occurs at 1.4 and 0.70 V, and the anodic lithium

extraction occurs at 1.6 and 2.2 V. Both of them are due to the highly reversible

electrochemical reduction/oxidation (Fe2O3 ↔ Fe) reactions. The simplified electrochemical

reactions can be expressed as follows.33

2 3 x 2 3Fe O Li Li Fe Ox xe+ −+ + → (1)

( ) ( )x 2 3 2 2 3Li Fe O 2 Li 2 Li Fe Ox x e+ −+ − + − → (2)

02 2 3 2Li Fe O 4Li 4 2Fe +3Li Oe+ −+ + ↔ (3)

As compared to the CV curves of pure Fe2O3 electrode (see Fig. S6a), the CV curves in Fig.

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3a for Fe2O3/CNT-GF electrode also show another pair of redox peaks located at 0.13 and

0.35 V, which correspond to the lithiation and dilithiation of graphite foam, respectively.30

Fig. 3b shows the discharge and charge voltage profiles of different cycles at a current

density of 200 mA g-1 within a cut-off voltage window of 0.01−3.0 V. An obvious long

plateau at 0.80 V can be observed in the first discharge voltage profile up to a capacity of

about 520 mAh/g (lower than theoretical 680 mAh/g, ca. 3 moles of Li per Fe2O3) (based on

the total mass of active material). The followed sloping drop to low potentials corresponds to

irreversible reactions (e.g., the formation of SEI). The initial discharge and charge capacities

of the 1st cycle are found to be 1310 and 1028 mA h/g, respectively, corresponding to a first

initial coulombic efficiency of more than 80%. In general, high initial irreversible loss is

commonly observed due to the formation of SEI as well as the severe structure destruction.

These are caused by the Li insertion and Fe ion reduction (to Fe + Li2O), leading to

consequent disconnection of the active material with the current collector. Nevertheless, to

our best knowledge this high initial coulombic efficiency of 80% is quite outstanding

compared with previous reports (no more than 50%).31, 34, 35 In the subsequent discharge

profiles, the plateaus rises to 1.4 and 0.9 V, which correspond to the two right-shifted peaks in

CV curves due to a change in crystalline structure after the first cycle.

A high rate capability is essential for high power density applications. Fig. 3c shows the

capabilities for both Fe2O3/CNT-GF and Fe2O3/GF electrodes in the range of 200−3000 mA/g.

Evidently, the Fe2O3/CNT-GF electrode shows a more stable high-rate profile with specific

capacities ranging from 900 mA h/g at 200 mA/g to 450 mA h/g at 3000 mA/g after 10 cycles.

These values are consistently higher than those of the Fe2O3/GF counterpart, as well as other

Fe2O3 nanoparticles powders and composites with rGO, CNT and GF.28, 29, 36, 37 In particular,

the Fe2O3/CNT-GF composite electrode can retain a capacity of 500 mA·h/g under the high

rate of 3000 mA/g, while there was only 280 mA·h/g capacity observed for the Fe2O3/GF

electrode.

Electrochemical impedance spectra (EIS) were conducted to demonstrate the improved

charge transfer kinetics in our hybrid electrode materials. As shown in Fig. 3d, the Nyquist

plots for both Fe2O3/GF and Fe2O3/CNT-GF possess much smaller diameters of the

semicircles in the high-medium frequency region than that of the pure Fe2O3. The smaller

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diameter indicates lower contact and charge−transfer resistances for both carbon-supported

samples. Furthermore, a simple equivalent circuit model is applied to fit the AC impedance

spectra (inset in Fig. 3d). The simulated values of SEI film resistance Rb and charge−transfer

resistance Rct for the Fe2O3/CNT-GF electrode are 60 and 9.0 Ω, respectively, both lower than

the Fe2O3/GF (111 and 15.2 Ω), and than the pure Fe2O3 (360 and 40.5 Ω). This is in good

accordance to the expectation that the CNT-decorated GF architecture can improve the

conductivity of the electrode and effectively enhance the electrochemical activity. The CNT

network is favorable for the access of electrolyte and hence the Li ion diffusion. Furthermore,

it is noted that, in Fig. 3a the two cathodic peaks of Fe2O3/CNT-GF are well separated at 0.75

and 1.3 V, whereas in Fig.S4a and many other CV files in literature, these two peaks are not

clearly resolved, but merge into a broaden peak at around 0.8 V. This difference is also

indicative of the fast kinetics of both processes in Eqs. (1) and (2) for Fe2O3/CNT-GF due to

improved mass transport.

Fig. 4 (a) Cycling performance at a current of 200 mA/g for the Fe2O3/GF and Fe2O3/CNT-GF LIB anode. (b) Low-magnification and (c) high-magnification SEM images of Fe2O3/CNT-GF after 300 cycles illustrating the preservation of the core-branch structure during long cycles.

We now check the cycling stability of the Fe2O3/CNT-GF 3D composite electrode. For

comparison, two control coin cells based on pure Fe2O3 and the Fe2O3/GF composite

materials as the electrodes were also assembled and tested under the same condition as

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Fe2O3/CNT-GF. Fig. 4a shows the plots of capacity versus cycle number at a current density

of 200 mA/g. The Fe2O3/CNT-GF electrode exhibits a high capacity around 1000 mAh/g up

to 300 cycles without evident fading. In contrast, the Fe2O3/GF electrode maintains around

700 mAh/g and the pure Fe2O3 drops to 260 mAh/g after 130 cycles (Fig. S6b). Typical SEM

images of the Fe2O3/CNT-GF electrode after 300 cycles show that the 3D network structure

and the well-dispersed nanorods are still clearly observable after long cycles (see Figs. 4b and

c). This further verifies the excellent structural stability of the 3D CNT-GF network branched

with Fe2O3 porous nanorods, for which a synergistic effect probably plays a role.

By re-examining the discharge profiles in Fig. 4a, one can see that the curves of both

Fe2O3/CNT-GF and Fe2O3/GF electrodes show an obvious decrease at initial cycles, followed

by a steady and gradual increase during subsequent cycling. This is in sharp contrast to the

fast degradation of the pure Fe2O3 nanorod electrode (Fig. S6b). Indeed this phenomenon

(increasing of capacity) has also been reported previously for metal oxides supported on

carbon electrodes in long term cycles.27, 33, 38 We have proposed possible reasons in our early

work.30 A recent experiment conducted by Hu et al. provided more direct evidence on the

origin of extra capacity. 39 It might be attributed to the ‘formation of the electrode’, which

takes several cycles to form stable SEI films on the discharge intermediates, i.e. nanosized

metallic Fe and amorthous Li2O. These SEI films establish an intimate electrical contact with

the current connector, and improve the lithium-ion accessibility in the electrodes during the

cycling processes.

3. Conclusion

We have presented a controllable fabrication of porous α-Fe2O3 nanorods supported on 3D

CNT-GF composite current collector as lightweight and binder-free anode for lithium-ion

battery. In this hierarchal structure, the Fe2O3 nanorods are uniform dispersed onto the 3D

network of CNT-GF, and thus prevents agglomeration during the Li-ion charge-discharge

process. The CNT-GF current collector possesses superior structural properties over

individual rGO, CNT and GF, such as high specific surface area and surface/bulk ratio, large

pore volume, and good permeability; these properties are favorable for lithium ions diffusion

and electron transport. Hence, the Fe2O3/CNT-GF electrode exhibits high columbic efficiency,

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superior rate capacity, high electrode stability and fast lithium storage kinetics. Such

structural design and fabrication of electrode materials will have important applications for

high performance lithium-ion batteries.

Experimental Synthesis of Graphene Foam (GF)

The growth of the 3D graphene foams was achieved by chemical vapour deposition (CVD) using First

Nano’s EasyTube 3000 System with a modified recipe from previous methods. Briefly, the 8 × 8 cm

size nickel foams were directly used as the scaffold templates and were loaded into a 5 inch quartz tube

reactor inside a horizontal tube furnace. The furnace was heated to 1000 ºC under an Ar (500 sccm) and

H2 (200 sccm) atmosphere and stayed at the peak temperature for 5 min in order to clean the nickel

foam surfaces and eliminate the thin surface oxide layer. After the annealing procedure, a small amount

of CH4 was introduced into the reaction tube at ambient pressure. The flow rates of CH4, H2 and Ar

were 50, 100 and 800 sccm, respectively. After 2 min growth, the samples were rapidly cooled to room

temperature at a rate of 100 C/min under a constant flow of Ar (500 sccm) and H2 (200 sccm).

Free-standing GF foam was obtained via acidic etching of nickel backbone in Fe(NO3)3/HCl mixture

solution.

Synthesis of Carbon nanotube - Graphene Foam (CNT-GF) hybrid films

The above mentioned GF was used as the substrate for the subsequent growth of CNTs. The NiCo

catalyst was deposited on GF via a hydrothermal process. Briefly, 1 mmol of Ni(NO3)2 • 6H2O and 2

mmol of Co(NO3)2 • 6H2O were dissolved into 40 ml DI water to form a clear pink solution, followed

by the addition of 12 mmol of urea. GF were immersed into the above solution to form GF/NiCo

precursor composite by keeping at 120 ºC for 2h in an autoclave. The GF/NiCo precusor was then

annealed in air at 350 ºC for 1 min and used directly as the catalyst-loaded substrate for the growth of

CNTs at 750 ºC in a gas flow of C2H4, H2 and Ar with 20, 40 and 100 sccm, respectively. Hydrogen

was introduced prior to growth process in order to activate catalyst effectively. The areal mass density

of CNT-GF hybrid films was around 0.65 mg/cm2.

Synthesis of porous nanorods Fe2O3/CNT-GF

The above 3D CNT-GF was used as the backbone for the growth of porous Fe2O3 nanorods by

hydrothermal synthesis methods. Before growth, the 3D CNT-GFs were pre-treated with HNO3 in an

autoclave at 120 ºC for 12 h. The porous Fe2O3 nanorods were prepared by a modified solvothermal

synthesis method. The solvothermal solution was first prepared by dissolving 1 mmol Fe(NO3)3·9H2O

(Merck AR, >99.0%) and 15 mmol urea (AR, >99.0%) in DI water (50 ml) and the following

ultrasonication for 0.5 h. Then 5 mL of 30 % HCl was added and kept continuously stirring for about

20 min to obtain a yellow solution. The obtained solution was then transferred into 100 mL

Teflon-lined stainless steel autoclave. The 3D CNT-GFs were immersed into the reaction solution and

the autoclave was kept at 120 ºC for 15h. Then the sample was collected and rinsed with distilled water

12

and ethanol in turn for three times. Finally, the samples were annealed at 400 ºC in air for 2 h to obtain

Fe2O3/CNT-GF. The Fe2O3 mass loading per electrode is around 1.32 mg. The areal mass loading

density of Fe2O3/CNT-GFs composite and pure Fe2O3 is 1.85 and 1.2 mg/cm, respectively. Therefore,

the weight ratio of CNT-GF and Fe2O3 is around 1 : 2. For comparison, the Fe2O3 nanoparticles were

grown on a 3D GF without CNT, which was denoted as Fe2O3/GF.

Sample Characterization

The sample morphologies were studied by field-emission scanning electron microscope (FE-SEM

JEOL JSM-6700F; JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy

(HRTEM, JEOL JEM-2010F) operating at 200 kV. The crystal structure of the samples was examined

by using x-ray powder diffraction (Bruker D8 ADVANCE XRD). The Raman spectra were recorded on

a WITEC-CRM200 Raman system (WITEC, Germany), using 532 nm laser (2.33 eV) as the excitation

source. The Si peak at 520 cm−1 was used as a reference to calibrate the wavenumber.

Battery Fabrication and Electrochemical Measurements

Standard CR2032-type coin cells were assembled in an argon-filled glove box (Mbraun, Unilab,

Germany) by using the as fabricated Fe2O3/CNT-GF as the working electrode (diameter of 12 mm)

without any binder or additives. Lithium metal circular foil (0.59 mm thick) was used as the

counter-electrode. A polypropylene (PP) film (Cellgard 2400) was as the separator while 1 M LiPF6 in

ethylene carbonate (EC) and dimethyl carbonate (DME) (1:1 by volume) was the electrolyte. The

cyclic voltammetry (CV) measurements (scanning rate: 0.5 mV s-1, voltage range: 0−3.0 V) and

electrochemical impedance spectroscopy (EIS) (amplitude of the sine perturbation signal: 0.5 mV,

frequency range: 0.01-100 kHz) were performed using an electrochemical workstation (CHI1760D).

Galvanostatic charge discharge cycles were carried out by Neware battery tester at different current

densities between 0.01 V and 3.0 V (vs Li/Li+) at room temperature. For comparison with the

free-standing F2O3/CNT-GF, pure Fe2O3 nanorods sample was physically mixed with carbon black

(super P) as the conductive agent and polyvinylidence fluoride (PVDF) dissolved in

N-methyl-2-pyrrolidone (NMP) as the binder in a weight ratio of 80:10:10 to form a slurry, which was

then coated onto a copper foil.

Supplementary Information Supporting Information is available online.

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Author biography

Minghua Chen is an associate professor at Harbin University of Science and Technology. He worked as a visiting scientist at the school of Physical and Mathematical Sciences, Nanyang Technological University (NTU) from 2013 to 2014. He received his B. Sc in Electronic Science (2006), M.Sc. in Solid-state Electronics (2009) and Ph.D. in Materials Science (2013), from Harbin University of Science and Technology (Harbin, China). His current research interests are synthesis, characterization of various carbon materials (graphite, graphene and carbon nanotubes) and polymer composites materials and their applications for energy storage and conversion devices.

Jilei Liu is currently pursuing his Ph.D. in Division of Physics and Applied Physics at Nanyang Technological University under the supervision of Prof. Zexiang Shen and Prof. Jianyi Lin. He received his B.Sc in Materials Physics from Hunan University (Changsha, China, 2008) and M.Sc from Shanghai Institute of Ceramics, Chinese Academy of Sciences (Shanghai, China, 2011). His current research interests are synthesis, characterization of carbon materials (graphene, CNTs and graphene/CNTs hybrids) and their applications in electrocatalysts, electrochemical capacitors and rechargeable batteries.

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Application, Ministry of Education, Harbin University of Science and Technology. She received the B.Sc in physics from the Jilin University in 1982, the M.Sc from Harbin University of Science and Technology in 1988, and the Ph.D. from Harbin Institute of Technology in 2001. Her main interests are structure and property of nano-dielectric, structure and reliability property of microelectronic device.

Prof. Jianyi Lin is currently a project consultant at Institute of Chemical and Engineering Sciences (ICES), A*STAR, Singapore, and an Adjunct Professor in the Department of Physics, National University of Singapore (NUS). He graduated from Xiamen University, China and received PhD in Chemistry from Stanford University in 1991. His research and expertise areas lie in surface science, heterogeneous catalysis and nano-materials, which include hydrogen production and storage, PEM fuel cell, supercapacitor and Li-ion battery studies.

Dr. Hong Jin Fan is currently an associate professor at Nanyang Technological University (NTU). He received PhD from National University of Singapore in 2003, followed by postdoc at Max-Planck-Institute of Microstructure Physics, Germany and University of Cambridge. He joined in NTU since 2008. His research interests include semiconductor nanowires and heterostructured nanomaterials, atomic layer deposition (ALD), and battery and solar energy conversion materials and technologies. He is an editorial board member of Nanotechnology and Editor of Materials Research Bulletin.

18

Dr. Ze Xiang Shen is a Professor in the School of Physical and Mathematical Sciences, and the School of Materials Science and Engineering, Nanyang Technological University. He is the Program Chair of the Interdisciplinary Graduate School. He concurrently holds the position of Co-Director, Centre for Disruptive Photonics Technologies. His research areas include carbon related materials, especially graphene. His work involves spectroscopic and theoretical study of few-layer graphene and folded graphene, graphene based composites for energy harvesting and nanoelectronics, as well as fundamentals on electronic structures, doping, and intercalation. He also works on developing near-field Raman spectroscopy/imaging techniques and the study of plasmonics structures.

19

Graphical Abstract

Highlights:

1. Carbon nanotubes (CNTs) forests are grown onto graphene foam (GF), forming CNT-GF composite foam.

2. CNTs are decorated with branches of porous α-Fe2O3 nanorods, forming 3D hierarchical nanostructured electrode

3. Lightweight, high-rate capacity, stable anode material for Li-ion storage.