Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Electrochimica Acta Electrochemical performance of a-Fe203 nanorods as anode material for lithium-ion cel
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochimica
Acta
Electrochemical performance of a-Fe203 nanorods as anode material for
lithium-ion cells
Hao Liu, Guoxiu Wang, Jinsoo Park, Jiazhao Wang, Huakun Liu, Chao Zhang
School of Mechanical, Materials and Mechatronic Engineering, and Institute for Superconducting and Electronic Materials, University of Wollongong,
New South Wales 2522, Australia
Article history:
Received 14 July 2008
Received in revised form
25 September 2008
Accepted 30 September 2008
Available online 17 October 2008
a-Fe203 nanorods were synthesized by a facile hydrothermal method The as-prepared a-Fe203 nanorods have a high quality crystalline nanostructure with diameters in the range of 6(0-80nm and lengths extending from 300 to 500nm The crystal structure of the a-Fe203 nanorods was characterized by X- ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) The a-Fe203 nanorod anodes exhibit a stable specific capacity of 8300 mAh/g This indicates significantly improved electrochemical performance in lithium-ion cells, compared to that of commercial microcrys-
a-Fe203 nanorods
One-dimensional structure
Anode material
Lithium ion batteries
© 2008 Elsevier Ltd All rights reserved
1 Introduction
Due to its low cost and the abundance of its raw materials in
nature, Fe,O3 has been widely investigated in many technological
fields for applications such as energy materials for lithium ion stor-
age, gas sensors, catalysts, and magnetic applications [1-6] It is
well known that the particle sizes and shapes of nanoscale materi-
als affect their properties and potential applications Iron oxides
have been synthesized in a variety of morphologies, including
nanoparticles [7,8], nanowires [9,10], nanorods [4,11,12], nanotubes
[13,14], nanoflakes [15], and novel core-shell structures [16]
Many transition metal oxides have been investigated as anode
materials for lithium ion batteries [17,18] The Fe203 crystal lattice
can store six Li ions per formula unit, and the theoretical capac-
ity of Fe.03 is as high as 1005 mAh/g, which is much higher than
that of commercial graphite anode materials (372 mAh/g) Thus, the
investigation of Fe203 as a lithium ion storage material should be
potentially important to in the search for new anode materials with
high capacity for lithium-ion batteries [4,19-23] The mechanism
of lithium ion intercalation/de-intercalation in Fe203 materials can
be described by the following equation:
FeaOa + 6LI <> 3LiạO + 2Fe
* Corresponding author Fax: +61 2 42215731
E-mail address: swang@uow.edu.au (G Wang)
0013-4686/$ - see front matter © 2008 Elsevier Ltd All rights reserved
doi:10.1016/j.electacta.2008.09.071
The extraction of lithium ion from LiOz should be thermo- dynamically impossible However, it does become feasible for nanosize materials, as has been demonstrated previously [17] Capacity fading is the main issue for all transition metal oxides used as anode materials for lithium-ion batteries Using nanoscale Fe203 material is a feasible approach to improve its properties as
an anode material because nanostructured materials can provide high reactivity for lithium ion insertion/extraction
In this paper, we report a facile method with low cost starting materials (FeCl3 and urea) to synthesize a-Fe,03 nanorods as anode material for lithium ion batteries The electrochemical performance
of the a-Fe203 nanorods has been significantly improved compared
to that of commercial microcrystalline Fe,03 powders
2 Experimental The a-Fe203 nanorods were synthesized via a hydrothermal method 0.324g iron chloride (FeCl3, 2mmol) and 0.3¢ urea (CO(NH2)2, 5mmol) were dissolved in 15 ml distilled water by magnetic stirring The solution was sealed in a 30 ml Teflon-lined stainless steel autoclave and kept at 120°C for 10h After cooling down to room temperature, the precipitate was washed three times
with distilled water and another three times with ethanol, then
dried in vacuum oven at 50°C overnight The final Fe203 nanorods were obtained by sintering the precursor at 500°C for 2h The a-Fe203 nanorod anode electrodes were fabricated by mixing the active materials with acetylene black (AB) and a
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Fig 1 X-ray diffraction patterns of the commercial and nanorod Fe203
binder, poly(vinylidene fluoride) (PVdF), in weight ratios of
60:20:20, 50:30:20, and 40:40:20, respectively, in N-methyl-2-
pyrrolidone (NMP) solvent In contrast, the commercial Fe203
(Aldrich) electrodes were made with the ratio of active mate-
rials:AB:PVdF = 60:20:20 The resultant slurries were uniformly
pasted on Cu foil with a blade These prepared electrode sheets
were dried at 120°C in a vacuum oven for 12h and pressed under
approximately 200 kg/cm2 CR2032-type coin cells were assembled
in a glove box for electrochemical characterization The electrolyte
was 1M LiPF, in a 1:1 mixture of ethylene carbonate and dimethyl
carbonate Li metal foil was used as the counter and reference elec-
trode
The microstructure and morphology of the a-Fe203 nanorods
were characterized by X-ray diffraction (XRD, Philips 1730) in the 20
degree range from 15° to 60°, scanning electron microscopy (SEM,
JEOL JEM-3000), and transmission electron microscopy (TEM, JEOL
2011) The cells were galvanostatically charged and discharged at a
current density of 0.1 C within the range of 0.01-3 V Cyclic voltam-
metry (CV) curves were collected at 0.1 mV/s within the range of
0.01-3.0V In the electrochemical impedance spectroscopy (EIS)
measurement, the excitation voltage applied to the cells was 5 mV
and the frequency range was between 100 kHz and 10 mHz Both the
CV and EIS measurements were carried out on an electrochemistry
workstation (CHIG660C)
3 Results and discussion
Fig 1 shows XRD patterns of the commercial and nanorod Fe203
that were collected using Cu Ka radiation (A =0.15406 nm) The
diffraction patterns confirm that both the crystal structures are
coincident with the standard hematite (a-Fe203) structure, JCPDS
card No 33-0664 No impurity was detected from the XRD pattern
of the nanorods, indicating that the nanorods have a single-phase
rhombohedral crystal structure after the 500°C annealing
The SEM images revealed the morphology of the commercial
and nanorod a-Fe203, as shown in Fig 2(a) and (b) From Fig 2(a),
it can be seen that the particle shapes of the commercial microcrys-
talline Fe20O3 are not regular The sizes of the commercial Fe203
particles are in range of 150-300nm It is clear that the size dis-
tribution of the nanorods is uniform (as shown in Fig 2(b)) The
diameters of the nanorods are in the range of 60-80nm, and the
length of the nanorods is around 300-500 nm The TEM image in
Fig 3(a) is in agreement with the SEM image (Fig 2(b)) and confirms
the size distribution at higher magnification It also shows that the
nanorods are partially agglomerated The agglomeration is mainly due to the thermal treatment at 500°C Fig 3(b) is a high resolu- tion TEM (HRTEM) image of a typical single crystalline nanorod The HRTEM image clearly shows the interplanar spacing of 0.27 nm for the (104) crystal planes, which is well matched with the standard d;,o4 value of rhombohedral hematite
Fig 4(a) shows the cyclic voltammetry profile of the commer- cial microcrystalline Fe,03 powder anode for the first 3 cycles at the scanning rate of 0.1 mV/s It is clear that there is a substan- tial difference between the first and the subsequent cycles In the first cycle, there is a spiky peak that appears at about 0.5V in the cathodic process, which could be associated with electrolyte decomposition and the reversible conversion reaction of lithium ion intercalation to form Li2O An anodic peak is also present at about 1.75 V, corresponding to the reversible oxidation of Fe® to FeỶ* During the anodic process, both the peak current and the integrated area of the anodic peak are decreased, indicating capac- ity loss during the charging process In the subsequent cycles, the cathodic/anodic peak potentials shift to 0.95 and 1.80V, respec- tively Fig 4(b) shows the cyclic voltammetry profile of the Fe203 nanorods for the first 3 cycles Compared to the microcrystalline Fe2Q3 electrode, the peak current and integrated peak area of the nanorods are much higher, indicating that the Fe203 nanorod elec- trode has higher capacity and reactivity The CV curves of the Fe203 nanorod electrode are stable and well matched after the second cycle This enhancement of the reactivity of the Fe,03 nanorods
Fig 2 SEM images of the (a) commercial and (b) nanorod Fe203.
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Fig 3 TEM images of Fe203 nanorods: (a) low magnification view and (b) high
resolution TEM image
in the lithium ion intercalation/de-intercalation processes could
remarkably improve the electrochemical performance of Fe,03 as
an anode material
The Nyquist plots of the ac impedance for the microcrystalline
Fe203 powders and the Fe203 nanorods, which were measured
in the open circuit voltage state using fresh cells, are shown in
Fig 5 Both profiles exhibit a semicircle in the high frequency
region and a straight line in the low frequency region In the low
frequency region, the straight beeline represents typical Warburg
behaviour, which is related to the diffusion of lithium ions in the
active anode material The depressed semicircle in the moderate
frequency region is attributed to the charge transfer process The
numerical value of the diameter of the semicircle on the Z; axis
gives an approximate indication of the charge transfer resistance
(Ret ) Comparing the semicircles of the samples in the moderate fre-
quency region, the charge transfer resistance of the microcrystalline
powder electrode is as high as 750 2, while that of the nanorod
electrode is only about 250 Q This effect can be attributed to the
facile charge transfer at the nanorod/electrolyte interface and also
within the Fe.03 nanorods, due to their one-dimensional structure
and nanosize scale
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Fig 4 (a) Cyclic voltammetry (CV) curves of microcrystalline Fez03 powder elec- trode (b) CV curves of Fe203 nanorod electrode Scanning rate: 0.1 mV/s in the range
of 0.01-3.0V
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Fig 5 Nyquist plots of ac impedance spectra in the frequency range between
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Fig 6 (a) First cycle charge/discharge profiles of the commercial Fe;Os powder
and FezOs nanorod electrodes with 20%, 30%, and 40% conductive carbon content
(b) The electrochemical cycling performance of microcrystalline Fez03 powder and
nanorod electrodes containing different weight percentages of carbon additive as
anodes in lithium-ion cells (charge/discharge rate: 0.1 C)
The a-Fe,03 nanorods were tested as anode materials in
lithium-ion cells with different weight ratios of conductive car-
bon (20%, 30% and 40%) The electrochemical performances of the
electrodes are shown in Fig 6 The capacity of the microcrys-
talline Fe203 electrode at the first cycle was 1285 mAh/g The extra
capacity beyond the theoretical value is probably due to the decom-
position of non-aqueous electrolyte during the discharge process In
contrast, the initial capacities of the a-Fe.0O3 nanorods containing
20, 30, and 40 wt% carbon were 1281, 1333, and 1332 mAh/g, respec-
tively, which were almost the same initial discharge capacity as the
commercial material It is clear that the first charge capacities of the
nanorods are remarkably improved The first charge capacity of the
commercial and the 20%, 30%, and 40% carbon containing nanorod
electrodes were 603, 881, 920 and 955 mAh/g, respectively This
improvement might be attributable to the high surface area and
high activity of the nanostructured materials After 30 cycles, the
discharge capacities of the four samples decreased to 112, 425, 570
and 763 mAh/g, namely, 8.7%, 33.2%, 42.8% and 57.3% capacity reten-
tion, respectively, compared to the first cycle The cyclability of the
a-Fe203 nanorod electrodes was dramatically improved compared
to that of the microcrystalline powders as shown in Fig 6(b) This
improvement is in agreement with the CV investigation Besides
the conductive carbon content, it has been reported that the par-
ticular carbonaceous sources and binder content also affect the nano-Fe,03 performance as anode material for lithium ion batter- ies [24,25] Nanosize materials have large surface areas and high surface energy For lithium ion battery applications, the large sur- face areas of nanostructured materials can provide more sites for lithium ion intercalation/de-intercalation The improvement seen
in the nanorods electrodes may be also attributed to the shorter pathways in the nanorods for lithium ion diffusion Thus, the elec- trochemical performance of the one-dimensional nanostructured electrodes is remarkably enhanced
4 Conclusion
In summary, single crystalline a-Fe203 nanorods, which have diameters in the range of 60-80 nm, were prepared by a hydrother- mal method Both the nanorods and commercial Fe203 were tested
as anode materials for lithium ion batteries Electrochemical mea-
surements, such as CV and EIS, demonstrated that the nanorods
had higher reactivity than the commercial microcrystalline Fe,03 powders The Fe203 nanorods exhibited a 763 mAh/g capacity after
30 cycles, which is remarkably higher than that of the microcrys- talline powder electrode This investigation indicates that there are good prospects for using Fe203 nanorods as anode materials in lithium-ion cells
Acknowledgment This work was financially supported by the Australian Research Council (ARC) through an ARC Discovery project (DP0772999)
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