1. No category

Journal of Power Sources 281 (2015) 425e431
Contents lists available at ScienceDirect
Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
Preparation of nanographite sheets supported Si nanoparticles by in
situ reduction of fumed SiO2 with magnesium for lithium ion battery
Yi Zhang, Yizhe Jiang, Yudong Li, Beibei Li, Zhihui Li, Chunming Niu*
Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi'an
Jiaotong University, 99 Yanxiang Rd., Xi'an 710054, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Explore low cost methods for the
preparation
of
Si/nanographite
sheets (Si/NanoGs) composite materials for Li ion battery.
Prepared nanographite supported Si
nanoparticles by direct Mg thermal
reduction of mixtures of fumed SiO2
and nanographite.
Si nanoparticles with average diameter of 20 nm and sheet like
morphology are formed on the surface of nanographite sheets.
Si/NanoGs materials show excellent
electrochemical performance.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 October 2014
Received in revised form
3 January 2015
Accepted 5 February 2015
Available online 7 February 2015
This work explores low cost methods for the preparation of Si/nano-graphite sheets (NanoGs) composite
materials for Li ion battery. The Si/NanoGs composites are prepared by magnesium thermal reduction of
mechanical mixture of fumed SiO2 and NanoGs under Ar atmosphere. The structure of the samples is
characterized by XRD, Raman spectroscope, SEM, and HRTEM. The results show that highly crystallized Si
nanoparticles with a sheet-like morphology are uniformly distributed on the surface of NanoGs (both
edge and a-b plane). The average diameter of Si nanoparticles is ~20 nm. Electrochemical characterization shows that an electrode has an initial lithium storage capacity of 1702.9 mAh g1 at a current of
100 mA g1. The storage capacity decreases to 975.7 mAh g1 after 100 cycles. Since cheap commercial
fumed SiO2 and graphite are used and the process is simple and easy to scale up, with further
improvement, the method has potential for use in large scale production of high energy density Si/
NanoGs anode materials at low cost.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Silicon
Nano-graphite sheets
Magnesium thermal reduction
Anode
Li ion battery
1. Introduction
The energy storage needs for various technological applications
including portable electronics, transportation, and electric grid
* Corresponding author.
E-mail address: [email protected] (C. Niu).
http://dx.doi.org/10.1016/j.jpowsour.2015.02.020
0378-7753/© 2015 Elsevier B.V. All rights reserved.
have been increasing exponentially in recent years. Lithium-ion
batteries (LIBs) dominate portable electronics market, are replacing nickel metal hydride in hybrid electric vehicles (HEVs), the
primary choice for pure electric vehicles (EVs), and in early stage of
the development for electric grid storage [1e4]. Although major
properties of LIBs are advantages compared with those of other
battery technologies, they are still unsatisfactory, in particularly as
426
Y. Zhang et al. / Journal of Power Sources 281 (2015) 425e431
the power source for next generation of EVs, which demand significant improvement in energy density, power density (charging
rate), cycle life and cost [5].
Graphite is the most commonly used anode material for commercial LIBs due to its high stability, high conductivity and low
price. However, the energy density of LIBs based on the graphite
anode is too low to overcome the problem of limited driving ranges
for EVs. There is no room for improvement because the energy
density of graphite anode has almost reached its theoretical specific
capacity of 372 mAh g1 [6]. Therefore, there is a great need for
alternative anode materials. A number of materials, such as nanosized carbon [7], silicon [8], Sn [9], metal oxides [10e13] have
been investigated.
In particular, silicon has been showed to be promising due to its
high theoretical capacity of 4200 mAh g1, which is more than ten
times higher than that of the graphite. Different from intercalation
chemistry of the graphite anode, lithiation and delithiation of Si
anode is an alloying process, resulting in a much great structural
and volume change (~400%) of Si, which leads to fracture and
pulverization of Si, henceforth quick capacity fading and reduction
of cycle life [14,15]. A number of strategies have been employed to
improve the performance of Si anode, including nanostructurization of Si, such as Si nanoparticles [16,17], Si nanowires
[18], Si nanotubes [19], Si thin film [20,21], nanoporous Si, [22,23]
and stabilization of Si nanostructures by dispersion on a support
[24,25] or embedding in a matrix. Carbon in various forms has been
used as the support or matrix, including amorphous carbon [26],
carbon nanotubes [27], graphene [28]. However, synthesis of nanosized silicon remains to be a great challenge, often involving high
temperature or high energy pyrolysis of expensive precursors such
as silane/polysilane/halosilane precursors [29,30] or laser ablation
of bulk Si [31]. Moreover, it is not trivial to prepare uniformly
dispersed or embedded Si/Carbon composites. Recently, magnesium thermal reduction of SiO2 has attracted attentions [32,33]. Du
et al. [34] fabricated SiO2 on the graphene by hydrolysis of tetraethyl orthosilicate (TEOS) and then prepared Si/graphene composite anode materials by magnesium thermal reduction of SiO2.
Wu et al. [35] synthesized SiO2 spheres using modified Stober
method [36] and then wrapped them with a 3D interconnected
network of graphene oxide (GO) sheets by a layer-by-layer assembly approach to obtain SiO2@GO network, which was then
reduced by magnesium to yield Si@G network anode materials.
In this work, we explore methods for preparation high energy
density Si/NanoGs composite materials for LIBs using cheap commercial materials of fumed SiO2 and graphite. The Si/NanoGs
composites were prepared by magnesium thermal reduction of
mixture of fumed SiO2 and NanoGs. The structural properties of
samples were characterized by XRD, Raman spectroscope, SEM and
HRTEM. Lithium storage capacity and cycle ability of the samples
were evaluated in a testing button cell using Li metal as the counter
electrode.
nanometer thick graphite sheets (NanoGs). Finally, the sample was
filtered, washed with water repeatedly and dried in vacuum oven at
100 C to yield NanoGs.
NanoGs, SiO2 (20 nm, SigmaeAldrich) and Mg powder (3 mm,
SigmaeAldrich) in molar ratio of 10:6:15 were mixed and grinded
under Ar atmosphere in a glove box to obtain a uniform mixture,
then the mixture was spread evenly into a corundum boat. The
corundum boat was placed into a tube furnace and heated under an
argon gas flow to 650 C at a ramp rate of 5 C min1. After heated
at 650 C for 3.0 h, the sample was stirred in 1 M HCl solution for
12 h to dissolve MgO and Mg2Si. The resulted product was filtered,
washed and dried at 80 C to obtain Si/NanoGs composites. To
remove residual SiO2, the sample was further treated with 5% hydrofluoric acid.
2.2. Characterization
The morphology of Si/NanoGs composites was characterized by
scanning electron microscopy (SEM, FEI, Quanta 250) equipped
with a EDS attachment. HRTEM images were recorded with a JEM2100 HT high-resolution transmission electron microscope. X-ray
diffraction patterns were recorded with a Rigaku diffractometer
(XRD, Rint-2000, Rigaku) operated at a voltage of 40 kV and a
current of 30 mA using Cu Ka radiation (l ¼ 1.5418 Å). The surface
area and micropore analysis were carried out using a Quantachrome Autosob IQ analyzer. The Raman spectra were measured using
a Renishaw Raman RE01 scope with a 514 nm excitation argon laser.
2.3. Electrochemical measurements
The working electrode was prepared by steps of 1) Mix Si/
NanoGs composites (70 wt%), carbon black 20 wt%) and polyacrylic
acid (PAA, Mw ¼ 100000, SigmaeAldrich, 10 wt%) to form a uniform
slurry, 2) Coat the slurry on the surface of a Cu foils, 3) Dry the
coating in a vacuum oven at 120 C for 12 h, and 4) Punch the
coated Cu foil into 14 mm diameter disks to yield working electrode. The testing button cells were constructed in a glove box
(Lab2000, Etelux, China) using Li foil as the counter electrode, a
Cellgard 2400 microporous membrane as the separator and 1.0 M
LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) (1:1, v/v) as the electrolyte. The cycling experiments
were performed using a battery test system (BT2000, Arbin, USA) at
various currents within a voltage range between 0.01 and 3.0 V.
Cyclic voltammetry (CV) measurements were performed on an
electrochemical station (PGSTAT 302N, Metrohm, Switzerland) in
the 0e2.0 V window at a scan rate of 0.1 mV s1. The electrochemical impedance spectroscopy (EIS) was measured using the
same station with a voltage amplitude of 10 mV in the frequency
range of 10 MHz to 0.01 Hz.
3. Results and discussion
2. Experimental
3.1. Phase composition
2.1. Preparation of Si/NanoGs composites
The XRD patterns of the Si/NanoGs composite are displayed in
Fig. 1c. For comparison, the XRD patterns of NanoGs and asprepared pure silicon are also presented (Fig. 1a, b). As can be
seen from Fig. 1c, two peaks at 2q ¼ 26.4 and 54.5 in Si/NanoGs
composite's pattern can be assigned to NanoGs corresponding to
(002) and (004) reflections of graphite (JCPDS No. 65-6212) [37],
and three peaks at 2q ¼ 28.4 , 47.3 and 56.1 can be assigned to Si
corresponding to (111), (220) and (311) reflections, respectively,
which are three strongest reflections of elemental silicon (JCPDS
No. 27-1402) [38]. The average crystallite size calculated by using
Debye Scherrer equation D ¼ K*l/b*cosq is around 20 nm. The
The natural graphite flake (Nanjing XFNANO Materials Tech Co.,
Ltd) were stirred in a mixture of concentrated sulfuric acid and
fuming nitric acid (4:1, v/v) at room temperature for 24.0 h, then
filtrated and washed until the pH level of the effluent reaches 6.
After dried at 100 C overnight, the sample, graphite intercalation
compound (GIC) was heat-treated at 1000 C for 10 s in a muffle
furnace to obtain expanded graphite (EG). The EG was added to a
mixture of water and alcohol (7:3), and subjected to ultrasonication
treatment in a bath sonicator for 48 h to further break it into
Y. Zhang et al. / Journal of Power Sources 281 (2015) 425e431
427
nanoscopic sizes of the Si [41], high strain induced by Si-NanoGs
interaction and the formation of interfacial SieC bonds. Note that
Raman shift corresponding to TO mode of 3C-SiC is at 796 cm1
[42].
3.2. Morphology
Fig. 1. XRD diffraction patterns of a) NanoGs, b) Silicon and c) Si/NanoGs composite.
results clearly showed that SiO2 has been successfully converted
into Si. Sharpness of the Si reflections suggests high crystallinity of
formed Si phase. No peaks corresponding to MgO and Mg2Si (by
product of reaction) were detected from X-ray diffraction.
The further evidence for the formation of the Si/NanoGs composites was given from the Raman patterns. The Raman spectrum of
the Si/NanoGs composite (Fig. 2c) exhibits five Raman shifts at
295 cm1, 506 cm1, 938 cm1, 1349 cm1 and 1574 cm1. Two of
them, 1349 cm1 and 1574 cm1 are from NanoG, and related to D
band and G band of graphite, respectively. A blue shift of 4 cm1 for
D band and 6 cm1 for G band can be attributed to structural
change of NanoGs induced during magnesium reduction. Note that
the magnesium reduction of SiO2 is highly exothermic, could cause
local heating, resulting in reaction between newly formed Si
nanoparticles with NanoGs. This is a complicate issue we intent to
study in detail for a future publication. Three peaks at 295 cm1,
506 cm1 and 938 cm1 are from Si formed by magnesium reduction of SiO2, and related to two transverse acoustic phonon (2TA)
mode, the first-order optical phonon (TO) mode and two transverse
optical phonons (2TO) mode of Si, respectively [39,40]. A relative
large blue shift of 5 cm1 for the band corresponding to 2TA mode,
13 cm1 for the band corresponding to 2TO mode and 25 cm1 for
the band corresponding to 2TA mode could be resulted from the
Fig. 3 shows SEM images of EG, NanoGs, and the Si/NanoGs
composite. EG was prepared by flush heating graphite intercalation
compound at 1000 C. Rapid gas release from decomposition H2SO4
and HNO3 causes graphite exfoliate, resulting in the formation of
warm like structure (Fig. 3aeb) with large volume expansion,
which is highly porous. NanoGs was prepared by further breakdown of EG through ultrasonication. The thickness of NanoGs sheet
is around 20 nm (Fig. 3c), and width is from 5 to 10 mm (Fig. 3d). Si
nanoparticles were supported on NanoGs surface by in situ magnesium reduction of 20 nm SiO2 nanoparticles. As shown in Fig. 3e,
f, a large number of Si nanoparticles with diameter from 15 to
20 nm are uniformly distributed on the surface (a-b plane) and at
the edges of NanoGs.
Fig. 4 shows EDS analysis results of the Si/NanoGs composite.
Two strong peaks in EDS spectrum belong to C and Si, and one weak
peak belongs to O. The composition of the Si/NanoGs composite
calculated from EDS analysis is 30.61 wt% Si, 5.42 wt% O and
63.97 wt% C. The O signal could be from SiO2 residue and O bonded
to carbon. The content of Si in the Si/NanoGs composite was also
calculated from beginning NanoGs weight of 0.1000 g and final
weight of Si/NanoGs sample of 0.1420 g, which gave a Si content of
29.6 wt%, very close to the result of EDS analysis.
To investigate the microstructure of the Si/NanoGs composite,
we carried out high resolution TEM study. Fig. 5 shows TEM images
of the Si/NanoGs composite at different magnification. The darker
spots in Fig. 5a, b represent silicon nanoparticles. In consistent with
SEM observation, Si nanoparticles are uniformly distributed on the
surface of NanoGs. The average diameter of Si nanoparticles is
20 nm. Fig. 5ced shows HR-TEM images which simultaneously
resolved lattice fringe of a Si nanoparticle and small portions of
NanoGs support. The distances of 0.314 nm and 0.342 nm are corresponding to the lattice fringe distances of Si (1 1 1) planes and
graphite (0 0 1) planes. It is very interesting to note that most Si
nanoparticles are plate like as if a thin coating deposited on the
surface of NanoGs support. This can be explained by strong interaction between Si and carbon because Si is a carbide forming material. The larger of interface contact, the lower interface energy.
To investigate the porous structure of the materials, we carried
out BJH micropore analysis by nitrogen adsorption. The result
revealed a surface area of 95.12 m2/g, an average pore diameter of
1.88 nm and a total pore volume 0.67 cm3/g, suggesting a high Si
nanoparticles dispersion and sufficient space to allow Si volume
expansion and electrolyte diffusion.
3.3. Electrochemical performance
Fig. 2. Raman spectra of a) NanoGs, b) Silicon, and c) Si/NanoGs composite.
Electrochemical performance of Si/NanoGs composite electrode
was evaluated in a standard testing button cell using Si/NanoGs as
the working electrode and Li metal as the counter electrode. The
mass of active materials on electrode is 0.417 mg/cm2. The results
are shown in Fig. 6aed. Fig. 6a depicts the dischargeecharge
properties of the Si/NanoGs composite electrode at a current density of 100 mA g1 in a voltage range of 0.01e3.0 V vs Li/Liþ,
including curves from the 1st, 2nd, 10th, 25th, 50th and 100th cycles. The Si/NanoGs composite displays a long discharge flat curve
with a plateau below 0.2 V in the first cycle, which corresponds to
delithiation process form amorphous LixSi phase [43]. The
discharge (delithiation) and charge (lithiation) capacities of the Si/
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Y. Zhang et al. / Journal of Power Sources 281 (2015) 425e431
Fig. 3. SEM images of a & b) EG, c & d) NanoGs and e & f) the Si/NanoGs composite.
Fig. 4. EDS spectrum of Si/NanoGs composite.
Y. Zhang et al. / Journal of Power Sources 281 (2015) 425e431
429
Fig. 5. HRTEM images of Si/NanoGs composite at different magnifications.
NanoGs composite at the first cycle are 2182.8 mAh g1 and
1553.9 mAh g1 with a Coulombic efficiency of 71.2%. The large
irreversible capacity of the first cycle can be attributed to the
formation of a solid electrolyte interface (SEI) layer on the surface of
the electrode [43]. As the cycle number increased, the capacity
decreased continuously, but with a much slower rate. Fig. 6b shows
Fig. 6. a) Dischargeecharge profiles of Si/NanoGs composite anode, b) cyclic voltammetry curves of Si/NanoGs composite electrode for the first three cycles, c) Rate capability of Si/
NanoGs composite, d) Cycling performance of Si/NanoGs composite compared with pure Si.
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Y. Zhang et al. / Journal of Power Sources 281 (2015) 425e431
the cyclic voltammogram curves of the Si/NanoGs composite
anode, measured in the voltage range of 0e2.0 V (vs. Li/Liþ) at a
scanning rate of 0.1 mV s1. In the first cycle, a broad cathodic peak
from 0.6 to 0.9 V indicates the formation of SEI on the surface of the
electrode due to the decomposition of electrolyte. The peaks appear
at 0.13 V during discharge and between 0.27 and 0.53 V during
charge, representing the phase transformation between amorphous Si and LixSi. In the following cycles, the cathodic peak in the
range from 0.6 to 0.9 V disappears, indicating a stable SEI layer has
been maintained. The peaks at 0.13 V and 0.53 V gradually evolved,
corresponding to generation of LieSi alloy phases. The result is in
agreement with those previously reported in the literatures [43].
Fig. 6c shows the rate performance of the Si/NanoGs composite
anode at various current densities. The cell was first cycled at
100 mA g1 for 10 cycles, in which an average reversible specific
capacity of about 1690.9 mAh g1 was measured. At the current
densities of 200, 500, 1000, and 2000 mAh g1, the Si/NanoGs
composite can still deliver high reversible specific capacities of
1385.5, 1083.4, 915.3, and 672.2 mAh g1, respectively. After the
current returns to 100 mA g1, the reversible specific capacity of the
electrode is recovered to 1125.9 mAh g1. Fig. 6d shows the specific
capacity vs. the cycle numbers of the Si/NanoGs composite anode
and pure silicon at a current density of 100 mA g1. It can be seen
that though silicon shows high charge capacity (3283.5 mAh g1) in
the first cycle, the subsequent decay is so rapid that the capacity
drops down to 77.1 mAh g1 at the 100th cycle. The initial charge
capacity of the Si/NanoGs composite is 1702.9 mAh g1 while it is
975.7 mAh g1 after 100 cycles. The overall capacity of the Si/
NanoGs composite anode decreased at a much slower rate than that
pure Si anode. The Coulombic efficiency is a high 96.0% after first
cycle, kept as if a constant through the cycle testing. Although it is
difficult to make direct comparison of these performance with
those reported in the literature because of difference in Si content
and testing conditions, overall performances of our electrode from
capacity improvement, capacity retention and the Coulombic efficiency are comparable to some of most recent results [44e46]. The
improvement of capacity retention of our electrode can be attributed to nanometer dimension of Si particles and their interaction
with NanoGs. The average diameter of the silicon nanoparticles
prepared by magnesium thermal reduction of SiO2 is around 20 nm,
such small Si particle size resulted in a small absolute volume
changes during the chargeedischarge process, which is relatively
easy to be accommodated by surrounding porous environment
provided by flexible NanoGs. The NanoGs also provided a conductive network, which promotes electron transfer during the chargeedischarge. To characterize this effect, we carried out EIS
analysis. A depressed semicircle at high frequency which extended
~60 U in Nyquist plot is about 1/4 the size of the depressed semicircle in Nyquist plot of pure Si nanoparticles electrode, suggesting
significant charge transfer impedance reduction.
4. Conclusions
battery at low cost since the cheap commercial fumed SiO2 and
graphite are used and the process is simple and easy to scale up.
Acknowledgments
The authors acknowledge the financial support from the China
Postdoctoral Science Foundation Grant (2014M552437) and CNSF
Grant (51221005). The authors thank Ms. Juan Feng and Mr.
Chuansheng Ma for their help with SEM and HRTEM characterization carried out at the International Center for Dielectric
Research (ICDR), Xi'an Jiaotong University.
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