Electrochemistry Communications 28 (2013) 79–82
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Enhanced performance of SiO/Fe2O3 composite as an anode for rechargeable
Li-ion batteries
Mingjiong Zhou, Mikhail L. Gordin, Shuru Chen, Terrence Xu, Jiangxuan Song, Dongping Lv, Donghai Wang ⁎
Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA 16802, USA
a r t i c l e
i n f o
Article history:
Received 22 November 2012
Received in revised form 9 December 2012
Accepted 10 December 2012
Available online 20 December 2012
Keywords:
SiO
Iron oxide
Anode
Lithium-ion batteries
a b s t r a c t
Iron oxide (Fe2O3) was utilized to enhance the electrochemical properties of SiO as a promising anode for Li-ion
batteries. An SiO/Fe2O3 composite, composed of SiO coated with Fe2O3 nanoparticles, was synthesized by
mechanical milling and characterized by X-ray diffraction, scanning electron microscopy, transmission electron
microscopy, and energy-dispersive X-ray spectroscopy. The electrochemical properties of the SiO/Fe2O3 composite, SiO, and mechanically milled SiO as anodes for Li-ion batteries were then investigated. The SiO/Fe2O3 composite showed superior performance compared with the two Fe2O3-free SiO samples, including an increased
initial coulombic efficiency, enhanced rate capability, and better capacity retention.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Lithium-ion batteries have been applied in a variety of portable electronic devices and are being pursued as power sources for hybrid electric vehicles and electric vehicles [1,2]. Commercially available Li-ion
batteries generally use graphitic carbon with a theoretical capacity of
372 mAh/g as their anode material [3,4]. These anodes cannot enable
the creation of batteries that achieve the high energy density required
for vehicles and other emerging applications, which has motivated
research into new high-capacity anode materials. Materials that can
electrochemically form intermetallic alloys with Li are promising candidates due to their much higher theoretical capacities [5–8]. Among
these, silicon monoxide (SiO) has been extensively investigated because of its low cost, high specific capacity, and low charge–discharge
potential. However, it also suffers from a low initial coulombic efficiency
and capacity fading upon cycling. Many efforts have been made to overcome these problems and improve the performance of SiO, such as the
synthesis of SiO-carbon composites [9–11] and metal-doped SiO [12].
However, such approaches still have several critical issues such as unsatisfactory cyclability, complicated synthetic processes, and relatively
high cost for commercialization. In the present work, we prepared an
Fe2O3-coated SiO composite by mechanical milling and demonstrated
its improved electrochemical performance compared with SiO and mechanically milled SiO.
Silicon monoxide (Aldrich Chemical Company, −325 mesh, 99%)
was mixed with iron oxide (5 wt.%) (Fe2O3, Aldrich Chemical Company,
b50 nm) by planetary ball milling (Across International PQ-N04,
500 rpm for 12 h) to obtain the SiO/Fe2O3 composite. For comparison,
a milled SiO sample was also obtained following the same process as
used for the SiO/Fe2O3 composite. All samples were characterized
using X-ray diffraction (XRD, Rigaku Dmax-2000) utilizing Cu·Kα radiation, scanning electron microscopy (SEM, FEI NanoSEM 630), transmission electron microscopy (TEM, JEOL EM-2010F), and energy
dispersive X-ray spectroscopy (EDS). Slurries were prepared containing
the active materials (SiO/Fe2O3 composite, SiO, or milled SiO), Super-P
carbon, and sodium-carboxymethyl cellulose (Na-CMC) in a 2:2:1
ratio, with the Na–CMC as a 1 wt.% solution in distilled water. The slurries were coated onto copper foil, dried at 100 °C for 10 h in a vacuum
oven, and cut into 1/2 in. diameter electrodes for testing. The amount
of material on the Cu foil was in the range of 2–3 mg, thus the active
material loading is calculated to be ca. 0.8–1.2 mg. The electrochemical
properties of the electrodes were studied using coin cells (2016,
R-type). Li foils and polypropylene films were used as counter electrodes and separators, respectively. The electrolyte used was 1 M
LiPF6 in a mixture of ethylene carbonate (EC), diethylene carbonate
(DEC), and dimethyl carbonate (DMC) (2:1:2, v/v). The galvanostatic
charge and discharge tests were carried out at a constant current
between 0.01 and 1.5 V using a battery tester (Arbin BT2000). Cyclic
voltammetry (CV) was performed at a scan rate of 0.1 mV/s within
the range of 0.01–1.5 V using the same battery tester. Electrochemical
impedance spectra (EIS) of the electrodes were collected by a CHI-660
workstation, using an alternating current with an amplitude of 5 mV
⁎ Corresponding author. Tel.: +1 814 863 1287.
E-mail address: [email protected] (D. Wang).
1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.elecom.2012.12.013
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M. Zhou et al. / Electrochemistry Communications 28 (2013) 79–82
Fig. 1. (a) SEM image of as-purchased SiO. (b) SEM image of SiO/Fe2O3 composite. (c) HRTEM image and corresponding EDS elemental mappings of (d) Si, (e) O, and (f) Fe for the SiO/Fe2O3
composite. (g) Magnified HRTEM image of the SiO/Fe2O3 composite (h) XRD patterns for Fe2O3, SiO, and SiO/Fe2O3 composite. The dashed line marks the position of the (311) peak on the patterns of the Fe2O3 and SiO/Fe2O3 composite.
M. Zhou et al. / Electrochemistry Communications 28 (2013) 79–82
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in the frequency range of 0.1 Hz–100 kHz. The EIS of electrodes before
the initial lithiation and lithiated electrodes after the initial lithiation
were collected at open circuit potentials of approximately 2.9 V and
0.05 V (vs. Li+/Li), respectively.
3. Results and discussion
Fig. 1 shows SEM images of as-purchased SiO and SiO/Fe2O3 composite, together with TEM images and the corresponding scanning
TEM-EDS elemental mappings of the SiO/Fe2O3 composite. The XRD
patterns for SiO, Fe2O3, and SiO/Fe2O3 composite are also plotted in
Fig. 1. SiO particles are irregularly shaped with large flat surfaces, as
shown in Fig. 1a. SiO/Fe2O3 composite particles are smaller and aggregated together (Fig. 1b). TEM studies show SiO particles in the composite are coated by nanoparticles with diameters less than 10 nm, as
shown in Fig. 1c. The corresponding selected area STEM-EDS elemental
mappings of the SiO/Fe2O3 composite show that Si, O, and Fe are distributed homogeneously (Fig. 1d-f). A high-resolution TEM (HRTEM)
image (Fig. 1g) clearly shows lattice fringes with spacing of 0.21 nm
on the nanoparticles, which is consistent with the interplanar distance
of the (400) lattice plane of Fe2O3. STEM-EDS and HRTEM studies thus
show that the SiO particles in the composite are coated uniformly with
crystalline Fe2O3 nanoparticles. The XRD pattern of the SiO shows it is
amorphous. The XRD pattern of the SiO/Fe2O3 composite was similar
to that of SiO, except for a weak reflection peak at a 2θ of around 36°,
corresponding to the (311) reflection peak of crystalline Fe2O3.
The electrochemical properties of the SiO-based anodes are shown
in Fig. 2. Fig. 2a shows the first charge/discharge curves of the SiO/
Fe2O3 composite, SiO, and milled SiO at current density of 160 mA/g.
The initial discharge and charge capacities of the SiO are 2365 and
1386 mAh/g, respectively, which correspond to an initial coulombic efficiency of 59%. The milled SiO exhibits better initial capacities (discharge and charge capacities of 2607 and 1635 mAh/g, respectively),
which may be attributed to the reduced particle size after milling, and
has a similar initial coulombic efficiency (62%). The SiO/Fe2O3 composite shows further-increased initial discharge and charge capacities of
2773 and 1893 mAh/g, respectively, with an initial coulombic efficiency
of 68%.
The cycling performance of SiO, milled SiO, and SiO/Fe2O3 composite
was also compared at the same current density of 160 mA/g. As shown
in Fig. 2b, both SiO and milled SiO show very poor capacity retention.
The SiO/Fe2O3 composite shows considerably better cyclability than either, with 71% charge capacity retention (1335 mAh/g) with respect
to the first charge capacity after 50 cycles. The rate capability of SiO/
Fe2O3 composite is also superior to that of SiO and milled SiO (Fig. 2c).
The composite still has a capacity of ~600 mAh/g at a high current density of 4.8 A/g, while the SiO can only achieve a low capacity of
~200 mAh/g at this current density.
The capacity, cyclability and rate performance of Si-based anodes are
strongly dependent on the electrical conductivity of the electrodes,
which has been well demonstrated in SiO–C composites and other
Si-based anodes [9–17]. Fe2O3 is known to semi-reversibly react with
Li to form metallic Fe and Li2O [18–22]. We thus propose that metallic
Fe forms during lithiation of SiO/Fe2O3, increasing the conductivity of
the composite and thereby significantly improving its performance.
Cyclic voltammetry was performed to verify the conversion reaction
of Fe2O3 in the composite, as shown in Fig. 3a. Consistent with existing
literature, the cathodic peaks at 0.01 V (all cycles) and 0.24 V (second
and third cycles) are attributed to lithiation of SiO and Si, respectively,
while the anodic peaks at 0.5 V (first cycle) and 0.4 V (subsequent cycles) are attributed to delithiation of Si [13,23–25]. Critically, two small
and broad peaks are also observed in the range of 0.9 to 0.5 V during
the first cathodic scan, which are due to the lithiation of Fe2O3 and indicate formation of metallic Fe [26]. These peaks disappear in subsequent
cycles, implying that the lithiation of Fe2O3 is only semi-reversible and
that some metallic Fe remains in the composite even after delithiation.
Fig. 2. (a) Charge and discharge curves of SiO, milled SiO, and SiO/Fe2O3 composite during
the first cycle. (b) Cycling performance of SiO, milled SiO, and SiO/Fe2O3composite. Squares
denote discharge capacities, while circles denote charge capacities. (c) Galvanostatic rate
capability of SiO, milled SiO, and SiO/Fe2O3 composite. Values shown are discharge
capacities.
EIS was carried out to verify the conductivity-improving effect of the
metallic Fe. Fig. 3b shows Nyquist plots of the SiO and SiO/Fe2O3 composite cells before and after their first lithiation. The EIS spectra before
lithiation are very similar, with both showing large semicircles in the
high-to-medium frequency region, indicating that both have a similar
conductivity. After lithiation, EIS spectra of both cells show additional
small semicircles in the medium frequency region, which may be due
to Li-ion migration through the interface between the surface film and
the bulk [27], and an overall decrease in the semicircle size, indicating
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M. Zhou et al. / Electrochemistry Communications 28 (2013) 79–82
Li-ion batteries. XRD, SEM, TEM, and EDS show that the composite
consists of SiO particles uniformly coated with crystalline Fe2O3
nanoparticles. Compared with SiO and milled SiO, the SiO/Fe2O3 composite shows enhanced electrochemical performance, including an
increased initial coulombic efficiency, higher reversible capacity, improved capacity retention, and superior rate capability. The improved
performance of the composite can be attributed to its increased conductivity due to formation of metallic Fe upon lithiation of Fe2O3 during the
charge/discharge processes.
Acknowledgment
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S.
Department of Energy under contract no. DE-AC02-05CH11231, subcontract no. 6951378 under the Batteries for Advanced Transportation
Technologies (BATT) Program.
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Fig. 3. (a) Cyclic voltammogram of three charge/discharge cycles of an SiO/Fe2O3 composite electrode, collected at a scan rate of 0.1 mV/s. Peaks associated with lithiation of Fe2O3
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electrodes at open circuit potential of approximately 2.9 V (vs. Li+/Li) and using lithiated
SiO/Fe2O3 composite and lithiated SiO electrodes after the initial discharge to the potential
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increased conductivity. However, the conductivity of the SiO/Fe2O3
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cell, which is attributed to the above-identified formation of metallic
Fe during lithiation.
4. Conclusions
We have prepared an Fe2O3-coated SiO composite using mechanical
milling and investigated its electrochemical properties as an anode for
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