Electrochemical behavior of nanoporous/nanofibrous Si anode materials prepared by mechanochemical reduction

Article abstract of DOI:10.1016/j.jallcom.2009.10.023

A mixture of nanoporous and nanofibrous silicon (NPNF-Si) is prepared by the mechanochemical reduction of SiCl₄ using Li₁₃Si₄ as reducing agent in the presence of Super P carbon (SP) and the following thermal treatment and washing process. Morphologies and structures of the as-obtained composites are investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). After carbon coating via pyrolysis of polyvinyl chloride (PVC), the resulting NPNF-Si/SP/C composite is evaluated for the use as anode material in lithium-ion battery. It exhibits the first discharge capacity of 746.6 mAh g^-1 at current density of 0.2 mA mg^-1 and capacity retention of 91.1% after 30 cycles. The good electrochemical performance arises from the particular Si nano-structure, which can alleviate the volume effect during lithium insertion and extraction.

Full text of DOI:10.1016/j.jallcom.2009.10.023

Journal of Alloys and Compounds 490 (2010) 84–87
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Electrochemical behavior of nanoporous/nanofibrous Si anode materials
prepared by mechanochemical reduction
Rongguan Lv, Jun Yang∗, Pengfei Gao, Yanna NuLi, Jiulin Wang
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A mixture of nanoporous and nanofibrous silicon (NPNF-Si) is prepared by the mechanochemical reduc-
tion of SiCl4 using Li13Si4 as reducing agent in the presence of Super P carbon (SP) and the following
thermal treatment and washing process. Morphologies and structures of the as-obtained composites
are investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM)
and X-ray diffraction (XRD). After carbon coating via pyrolysis of polyvinyl chloride (PVC), the resulting
NPNF-Si/SP/C composite is evaluated for the use as anode material in lithium-ion battery. It exhibits
the first discharge capacity of 746.6 mAh g−1 at current density of 0.2 mA mg−1 and capacity retention of
91.1% after 30 cycles. The good electrochemical performance arises from the particular Si nano-structure,
which can alleviate the volume effect during lithium insertion and extraction.
Received 21 July 2009
Received in revised form
24 September 2009
Accepted 3 October 2009
Available online 13 October 2009
Keywords:
Nanoporous/nanofibrous silicon
Mechanochemical reduction
Anode material
© 2009 Elsevier B.V. All rights reserved.
Lithium-ion battery
1. Introduction
at C/20 rate within 10 cycles [8]. Barbara reported on a new three-
dimensional Si nanowires (200–300 nm in diameter) synthesized
Rechargeable lithium-ion batteries are widely used for portable
electronic devices duo to their high energy density and low main-
tenance requirement. However, the theoretical Li-storage capacity
of commercial carbon anodes is limited to 372 mAh g−1 [1]. In
recent year, there is a great interest in developing alternative anode
materials to carbonaceous for lithium-ion batteries. Among them,
Si is one of the most promising anode materials due to its high
theoretical lithium storage capacity of 4200 mAh g−1 and low elec-
trochemical potential vs. Li/Li+ [2,3]. One of the major challenges
associated with its practical application is the poor electrochemi-
cal reversibility upon cycling, deriving from the very large volume
change during the lithium insertion/extraction process.
To improve the cycling stability of Si-based negative electrodes,
the current strategies are focusing on two sides. One is to fabricate
composite microstructures in which the Si particles are finely dis-
persed in various active/inactive matrixes. To date, carbon is one of
the most effective matrixes to disperse Si. For instance, Si particles
embedded in a disordered carbon matrix deriving from pyroly-
sis of various organic polymers were reported by many groups
[4–7]. However, the capacity fade mostly remains remarkable. The
other is to reduce the particle and crystallite size to the nanometer
regime (such as nano-particles, nanowires, nanosized films, etc.).
Recently, Si nanowires were prepared by using the VLS process and
the electrode exhibited a reversible capacity of about 3100 mAh g−1
by CVD method. This architecture can accommodate the large vol-
ume change during lithium insertion/extraction [9]. Si nanowires
with homogeneous diameters of 10–20 nm have been synthesized
by DC arc discharge method with metal catalyst [10]. It is also
shown that some porous Si-based anode materials can effectively
alleviate the volume expansion [11–14]. For example, Zheng et al.
reported a nanoporous Si/graphite composite [11]. More nanosized
Si composites have been prepared via ball-milling technique using
various Si sources and reducing agents (such as C, Mg, Al, Li and
so on) [15–19]. In view of the importance of the reducing agent
for the mechanochemical reaction under ball-milling, we choose
Li13Si4 as the reducing agent here, because it acts not only as the
reducing agent but also as nucleation center for Si reduction from
other Si source.
In this work, nanoporous and nanofibrous silicon/carbon black
(Super P) composite (abbreviated as NPNF-Si/SP hereafter) was pre-
pared via a mechanochemical reaction between SiCl4 and Li13Si4
under ball-milling and subsequent thermal treatment and wash-
ing process. For the first time to our knowledge, Si nanofibers
can be prepared by such a route. The microstructures and the
electrochemical behaviors of NPNF-Si/SP anode materials were
investigated.
2. Experimental
2.1. Material preparation
∗
Corresponding author. Tel.: +86 21 54747667; fax: +86 21 54747667.
E-mail address: yangj723@sjtu.edu.cn (J. Yang).
For the material preparation, 1.85 ml SiCl4 (Aladdin-reagent Corp., China, 99.9%
pure), 0.8394 g Li13Si4 powders (SIMIT, CAS, China) (i.e., with 20% excess SiCl4 for
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.023

R. Lv et al. / Journal of Alloys and Compounds 490 (2010) 84–87
85
Table 1
Compositions of the composite samples in weight percent.
Sample
Si (wt.%)
SP (wt.%)
Carbon (wt.%)
Si
100
80.8
30.0
–
19.2
7.1
–
–
62.9
NPNF-Si/SP
NPNF-Si/SP/C
the redox reaction) and 0.2000 g carbon black named Super P (SP) (40 nm, Timical)
were loaded in an 80 ml argon filled stainless-steel vial with eight 10 mm and seven
8 mm diameter hardened steel balls. Then the milling was performed on a Plane-
tary Mono Mill P-6 (Fritsch, Germany) at a rotation speed of 450 rpm for 20 h. To
remove the excessive SiCl4, the as-milled composite was placed in a quartz tube in
an argon filled glove box, and heated at the rate of 5 ◦C min−1 under a constant flow
of Ar and maintained at 700 ◦C for 6 h, then naturally cooled down to room temper-
ature. The obtained NPNF-Si/SP mixed with LiCl was washed with deionized water
and isolated by centrifugation for 4 times. By this process, LiCl was removed and
NPNF-Si/SP composite was obtained. The sample was finally dried under vacuum at
100 ◦C for 4 h. For comparison, Si was prepared using the above-mentioned proce-
dure without SP. The NPNF-Si/SP composite was added into the polyvinyl chloride
(PVC, (CH2–CHCl)n, average Mw ∼ 233,000 Aldrich) solution (dissolved in tetrahy-
drofunan) and homogeneously mixed under ultrasonic action. After stirring for 1 h,
the resulting slurry was spread onto a flat surface, dried at 80 ◦C, and then the pre-
cursor was heated at 5 ◦C min−1 to 900 ◦C under Ar atmosphere. After pyrolysis at
900 ◦C for 2 h, the furnace was cooled automatically. The resulting composite was
named as NPNF-Si/SP/C hereafter.
Fig. 1. XRD patterns of the composites of NPNF-Si/SP (a), and NPNF-Si/SP/C (b).
ized water, LiCl was completely removed from composite, thus
nanosized pores are formed. Fig. 2 presents SEM images of
as-prepared Si and NPNF-Si/SP. As shown in Fig. 2a, short
Si fibers, irregular porous and compact Si particles coexist in
the as-prepared Si powders. When SP was added for ball-
milling, the longer Si nanofibers and more distinct nanoporous
Si particles are obtained (see Fig. 2b). The Si nanofibers have
diameters of 50–200 nm and lengths ranging from several ten
nanometers to several micrometers. The mechanism of the Si
nanofibers’ growth remains unclear and requires further investi-
gation. The nanoporous Si morphology is exhibited in the inset
2.2. Characterizations
The resulting samples were analyzed by X-ray diffraction (XRD) on a Rigaku
diffractometer D/MAX-2200/PC equipped with Cu K␣ radiation. The carbon contents
of the composites were measured using a Carbon-Sulfur Analyzer (Dekai Instrument,
China). The weight ratios of the composite samples are listed in Table 1, assum-
ing no loss of silicon and SP during pyrolysis. The morphologies of the samples
were observed by scanning electron microscopy (SEM) on a JEOL field-emission
microscope (JSM-7401F) and transmission electron microscopy (TEM) on a JEOL
high-resolution electron microscope (JEM-2010). The specific surface area and pore
volume were determined from the results of N2 adsorption/desorption experiments
using ASAP2010M+C (Micromeritics Inc., USA).
2.3. Cells assembling and electrochemical test
The electrodes in thickness of ca. 30 ␮m were composed of 80 wt.% active materi-
als (the as-prepared composite), 10 wt.% polyvinylidene fluoride (PVDF) binder, and
10 wt.% SP as a conducting agent. The electrode performance was evaluated using
CR2016 coin cells with ENTEK ET20-26 membrane separator, lithium-foil counter
electrode, and electrolyte of 1 M LiPF6 in the mixture of ethylene carbonate (EC) and
dimethyl carbonate (DMC) (1:1 in volume ratio). The cells were assembled in an
argon filled glove box (Mbraun, Unilab, Germany) with oxygen and water contents
less than 10 ppm. The charge/discharge test of the cells was performed using LAND
CT2001A system. The cells were charged (delithiation) and discharged (lithiation) in
the galvanostatic mode at a constant current of 0.2 mA mg−1 between 0.01 and 1.4 V
vs. Li/Li+. The specific capacity was calculated on the basis of the total composite
weight.
3. Results and discussion
Direct ball-milling of SiCl4 and Li13Si4 led to the formation of
coarse and clumpy powders. To solve this problem, SP was added
as dispersing agent. At the same time, SP can improve the elec-
tronic conductivity and accommodate part of the volume change
of silicon during lithium insertion/extraction. The XRD patterns of
the NPNF-Si/SP and the NPNF-Si/SP/C are shown in Fig. 1. For NPNF-
Si/SP composite, the main diffraction peaks at scattering angels (2Â)
of about 28.4◦, 47.4◦, 56.2◦ and 69.2◦ correspond to (1 1 1), (2 2 0),
(3 1 1) and (4 0 0) planes of silicon crystallites and the amorphous
SP presents no diffraction response. The high temperature prepa-
ration of NPNF-Si/SP/C composite at 900 ◦C has almost no effect on
the crystallinity of the sample. SiC phase is not observed. The broad
peak near 24◦ is attributed to the carbon pyrolyzed from PVC.
On the basis of Sandu’s work [16], we list the fol-
lowing mechanochemical reaction between SiCl4 and Li13Si4:
SiCl4(l) + Li13Si4(s) → Si(s) + LiCl(s). This redox reaction will produce
nano-dispersed LiCl in composite. After washing by deion-
Fig. 2. SEM images of Si (a), and NPNF-Si/SP (b).

86
R. Lv et al. / Journal of Alloys and Compounds 490 (2010) 84–87
Table 2
EDX analysis results of NPNF-Si/SP composite after washing process.
Element
Weight%
Atomic%
C
O
Si
20.01
3.05
76.94
36.2
4.2
59.6
Fig. 3. BJH pore size distribution for the NPNF-Si/SP.
of Fig. 2b. As determined by Brunauer–Emmett–Teller (BET) mea-
surement, the specific surface area of the NPNF-Si/SP composite
is 118.4 m2 g−1. The BET result demonstrates the formation of
porous structure in the Si particles. BJH pore size distribution
for NPNF-Si/SP composite is given in Fig. 3. The most pores dis-
tribute from 10 to 50 nm, which may accommodate the volume
change.
Fig. 5. Cycling behaviors of the Si-based electrodes at the constant current density
of 0.2 mA mg−1. Solid and hollow marks represent discharge and charge capacity,
respectively.
A TEM image shown in Fig. 4(a) reveals that the nanoporous
and nanofibrous parts are blended and enwound in the NPNF-Si/SP
composite. The selected area electron diffraction (SAED) pattern
(inset of Fig. 4(a)) and the energy dispersive X-ray (EDX) spectrum
(Fig. 4(b)) were recorded simultaneously with TEM analysis. The
SAED pattern indicates the polycrystalline nature of Si. The EDX
result is shown in Table 2, which confirms the complete elimination
of LiCl product from the composite. In addition, besides polycrys-
talline Si (59.6 at.%) and amorphous SP carbon (36.2 at.%), a small
amount of oxygen (4.2 at.%) is detected. It should be related to oxi-
dation of the fresh silicon during the sample preparation.
Fig. 5 shows the cycling stability for the different Si-based anode
materials. The first discharge capacity of as-prepared Si sample
is 2692.9 mAh g−1 and its cycling stability is poor. In the case of
the NPNF-Si/SP composite electrode, the first discharge capacity
is 2012.5 mAh g−1 and the first efficiency is 66.7%. The capacity
loss in the first cycle mainly arises from the SEI formation relating
with the specific surface area of 118.4 m2 g−1 [20,21]. Further-
more, after 9 cycles, the charge capacity of NPNF-Si/SP electrode
becomes 449.4 mAh g−1, which is higher than that of as-prepared
Si electrode (200.9 mAh g−1). The relatively good electrochemical
performance of NPNF-Si/SP composite should be ascribed to the
nanoporous and nanofibrous structure, which can suppress part
of the volume change [9,11]. Additionally, nanosized dispersion of
SP in NPNF-Si/SP composite is also beneficial to accommodate the
volume change during lithium insertion/extraction.
In view that incorporation of pyrolyzed carbon with silicon
remarkably improves the cycling stability of the Si-based mate-
rial, the NPNF-Si/SP/C composite was prepared by carbon coating
pyrolyzed from PVC. Fig. 5 shows that the first discharge capacity of
NPNF-Si/SP/C electrode is 746.6 mAh g−1 and the first efficiency is
68.8%. Apparently, the cycling stability of NPNF-Si/SP/C electrode
has been improved distinctly compared with that of as-prepared
NPNF-Si/SP electrode, which mainly derives from the enhanced
electronic conductivity by incorporation of pyrolyzed carbon. Tak-
ing account of a Li-insertion capacity of ca. 290 mAh g−1 for
pyrolyzed carbon from PVC, about 70% of the theoretical capacity
for Si can be effectively utilized. The architecture of NPNF-Si/SP/C
composite can accommodate the volume change and preserve the
integrity of the electrode structure. Then, 91.1% of the first charge
Fig. 4. TEM image of NPNF-Si/SP and the corresponding SAED pattern (inset) (a), and EDX spectrum of NPNF-Si/SP (b).

R. Lv et al. / Journal of Alloys and Compounds 490 (2010) 84–87
87
cathodic peaks appear at around 0.90, 0.68 and 0.01 V, respectively.
The first two low current peaks originate from the formation of
SEI films, which disappear from the subsequent cycles. One addi-
tional cathodic peak at 0.20 V appears from the second cycle. Upon
charging, two anodic peaks located at 0.34 and 0.50 V gradually
evolve after the first cycle and become more distinct for the follow-
ing cycles. These two anodic peaks correspond to the extraction of
lithium from the silicon. Furthermore, the magnitude of the current
peaks increased with cycling, correlating to the capacity’s increase
during the initial cycles in Fig. 6.
4. Conclusions
In summary, nanoporous and nanofibrous Si-based composite
has been firstly prepared by mechanochemical reduction using
SiCl4 and Li13Si4 as starting materials with small amount of SP
and subsequent thermal and washing treatments. As anode mate-
rial for lithium-ion battery, the composite NPNF-Si/SP electrode
showed fast capacity loss upon cycling. After carbon coating via the
pyrolysis of PVC, the capacity retention was significantly improved.
The initial discharge capacity of the composite NPNF-Si/SP/C elec-
trode is 746.6 mAh g−1, and the first coulombic efficiency is 68.8%.
After 30 cycles, 91.1% of the first charge capacity remained. More
investigation will focus on the Si structure optimization and the
improvement in electrochemical reversibility.
Fig. 6. Galvanostatic discharge–charge curves of the cell with NPNF-Si/SP/C in the
potential region of 0.01–1.4 V (vs. Li/Li+) at the current density of 0.2 mA mg−1
.
Acknowledgements
This work was financially supported by National 863 Project
(2006AA03Z232) and National 973 Programs (2007CB209700).
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Fig. 7 shows the cyclic voltammograms (CVs) of NPNF-Si/SP/C
electrode measured at the scan rate of 0.2 mV s−1. The initial poten-
tial of the electrode is about 2.0 V vs. Li/Li+ in the open-circuit state.
In the case of the first reduction half-cycle (lithium insertion), three