Electrochemical and Solid-State Letters, 8 共9兲 A481-A483 共2005兲
A481
1099-0062/2005/8共9兲/A481/3/$7.00 © The Electrochemical Society, Inc.
Preparation and Electrochemical Properties of Silicon/Carbon
Composite Electrodes
Xuelin Yang, Zhaoyin Wen,*,z Xiujian Zhu, and Shahua Huang
Shanghai Institute of Ceramics, Graduate School, Chinese Academy of Sciences, Shanghai 200050, China
Silicon/carbon composites were prepared at room temperature by dehydration of a mixture of sucrose and silicon powders using
concentrated H2SO4, and the electrochemical behavior of this kind of material as an anode for lithium secondary batteries was
investigated. The composites demonstrated a high initial coulombic efficiency of 82% and a large charge capacity above
560 mAh g−1 over 75 cycles. The improved electrochemical characteristics are attributed to the uniform distribution of silicon
particles in an amorphous carbon matrix, which, to a certain extent, could buffer the volume change during cycling.
© 2005 The Electrochemical Society. 关DOI: 10.1149/1.1999915兴 All rights reserved.
Manuscript submitted April 17, 2005; revised manuscript received May 21, 2005. Available electronically July 29, 2005.
Since the successful commercialization in the 1990s of lithiumion batteries using carbonaceous materials as the anode, the search
for new anode materials to improve energy density has been
ongoing.1-4 Silicon has been identified as a possible candidate for
the next generation anode materials because it has demonstrated
improved lithium storage capacity 共4200 mAh g−1兲 over graphite
共372 mAh g−1兲.5-9 However, its commercial use is still hindered by
fast capacity fading resulting from the strong volume changes
共⬎300%兲 upon cycling.10-12 A promising approach to overcome this
deterrent is to create a composite microstructure in which silicon
particles are homogeneously dispersed in a ductile, inactive
matrix.13,14 Small volume expansion, a relative light mass, and good
electronic conductivity make carbon the best matrix.
Generally, Si-C composites include carbon-coated silicon by
thermal vapor deposition 共TVD兲15,16 and carbon-netted silicon by
the pyrolysis of organic compounds.17 During the VD process, benzene or toluene vapors decomposed on the surface of silicon particles to form the carbon-coating layer at the temperature of 1000°C.
Since carbon coating can keep the integrity of active particles, the
reversibility of structural changes during cycling provides an improved reversible capacity. In the later case, silicon particles were
uniformly dispersed in poly共vinyl chloride兲 共PVC兲18,19 or pitch20
which were predissolved in acetone. After evaporation of the solvent, to obtain the solid blend a pyrolysis reaction was performed at
900°C. The pyrolyzed carbon acted not only as the matrix but also
the blinder, thus the composite exhibited a large capacity with a
good cycling performance. Obviously, all the carbon was obtained at
very high temperatures in the above two processes. Furthermore,
some inert gases such as N2, Ar, or reductive gas 共H2兲 were needed
to protect silicon from being oxidized at such a high temperature.
This is because there was no successful preparation of siliconcarbon composite in the air atmosphere at room temperature.
Here we report the preparation of a carbon-netted silicon composite at room temperature by dehydration of the mixture of silicon
and sucrose using concentrated H2SO4. The method is simple, controllable, and is the first example of Si-C composite synthesized at
low temperature without protective gas. Therefore, the cost of preparation could be reduced to a large extent.
Experimental
The Si-C composite was prepared using the mechanically milled
crystal silicon powders 共⬎99%, ⬍100 nm兲, sucrose 共AR兲, and concentrated H2SO4 共⬎98%兲 as the starting materials. Silicon powders
were homogeneously dispersed in a sucrose solution with the help of
an ultrasonic bath, and the obtained suspension was further agitated
under infrared light until it was converted into a syrup. The concentrated H2SO4 was slowly added into the as-prepared syrup, mixed
homogeneously, and dehydrated for 2 h. The resulting product was
* Electrochemical Society Active Member.
z
E-mail: [email protected]
filtered after dilution by distilled water. Then it was rinsed with
distilled water until pH 7 was reached and dried under vacuum for 8
h, ground, and sieved. The samples were characterized by field
emission scanning electron microscopy 共FESEM, JSM-6700F
JEOL兲, transmission electron microscope 共TEM, JEM-2010兲, and
X-ray diffraction 共XRD, Rigaku RINT-2000兲 with Cu K␣ radiation.
Electrodes containing 60% active materials 共Si wt 20%兲, 20%
poly共vinylidene fluoride兲 共PVDF兲 dissolved in N-methyl pyrrolidinone 共NMP兲, and 20% acetylene black 共AB兲 on copper substrates
were used for all electrochemical tests. These electrodes were made
by spreading a slurry of the electrode ingredients onto 18 ␮m thick
copper foil and drying at 120°C under vacuum for 10 h. The
CR2025 coin cells were assembled in an argon-filled glove box
共VAC, USA兲 using lithium metal foil as the counter electrode. The
electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate 共EC兲
and dimethyl carbonate 共DMC兲 共1:1 by volume兲. The cells were
galvanostatically discharged and charged in the range of 0.02-1.5 V
at a current density of 0.1 mA cm−2. Cyclic voltammetry 共CV兲 was
performed on a Solartron 1287 potentiostat.
Results and Discussion
As seen in Fig. 1a, the silicon powders we used consisted of fine
particles with their size less than 100 nm, and most of them aggregated. It is very difficult to disperse the silicon particles homogeneously in the sucrose solution by simple mechanical mixing. Therefore, ultrasonic waves were performed throughout the mixing
process, and this was effective in avoiding agglomeration. On the
other hand, mechanical stirring was also performed throughout the
drying process, which led to a uniform distribution of silicon particles in the syrup before dehydration. Figure 1b shows the TEM
image of the Si-C composite. The silicon particle size was less than
30 nm, with a distinct contrast from the matrix as seen from the
image. Further energy-dispersive X-ray spectroscopy 共EDXS兲 of the
black dot in Fig. 1c confirmed the presence of silicon and carbon.
Therefore, we can conclude that the nanosized silicon particles
共black dots兲 were uniformly distributed in the matrix of amorphous
carbon which would effectively buffer the volume changes of silicon
particles during insertion/extraction cycling.
Trace A in Fig. 2 shows the characteristic XRD pattern of the
dehydrated product of sucrose without adding silicon powders. It
displays a typical amorphous feature without any reflection peaks
corresponding to the sucrose peaks, confirming that sucrose was
dehydrated completely, and the resulting product was amorphous
carbon. Si-C composite showed a typical pattern of crystal silicon
with higher intensity 共Fig. 2, trace B兲, demonstrating that elemental
silicon was preserved even after the dehydration by concentrated
H2SO4 and there is no other detectable phase besides the amorphous
carbon in the composite.
Shown in Fig. 3 are the cyclic voltammograms 共CVs兲 of Si-C
composite cycled between 0 and 1.5 V, the sequence of cycles was
run without breaking. Clearly, the CV curve for the first cycle is
very different from those that followed mainly in the appearance of
Downloaded on 2016-05-16 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).
A482
Electrochemical and Solid-State Letters, 8 共9兲 A481-A483 共2005兲
Figure 2. XRD patterns of the dehydration product of sucrose 共trace A兲 and
silicon-carbon composite 共trace B兲.
decreased continuously. The new peak resulted from the gradual
activation of a composite electrode at each cycle, and the decrease
of current implied that more and more Li+ was irreversibly trapped
in composite materials through the reactions between lithium and
trace impurities containing Si-O22 or -OH.21 These two reduction
peaks and two oxidation peaks located at 0.34 and 0.49 V, respectively, should correspond to the lithiation and delithiation in silicon
Figure 1. 共a兲 SEM morphology of silicon powders, 共b兲 TEM image of
silicon/carbon composite, and 共c兲 EDX spectra of the black dot in the
Fig. 1b.
a broad reduction peak near to 0.7 V and a very strong reduction
peak toward 0 V. The broad peak could be attributed to the formation of a solid-electrolyte interphase 共SEI兲 on the active particles.21
It occurred only in the first cycle. Since the second cycle, a new
reduction peak appeared at around 0.2 V and the current of this peak
Figure 3. CVs of Si-C composite electrode: 共a兲 the first cycle scanning at
0.05 mV s−1; 共b兲 the 11th to 15th cycles at 0.08 mV s−1 and the 16th to 20th
cycles at 0.1 mV s−1.
Downloaded on 2016-05-16 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).
Electrochemical and Solid-State Letters, 8 共9兲 A481-A483 共2005兲
A483
Figure 5. Cycling performance of pure silicon and silicon/carbon composite
as anode active materials.
with electronic conductivity, which not only buffered the great volume change during the cycling process but also avoided possible
agglomeration of uniformly distributed silicon particles.
Conclusions
We have made a Si-C composite anode at room temperature by a
process based on the dehydration reaction of sucrose. The electrochemical performance of the composite anode material was superior
to a pure silicon anode with greatly improved reversibility, and the
dehydration of sucrose by concentrated H2SO4 would be also a
simple and effective method for the preparation of other carbonbased materials.
Acknowledgments
This work was financially supported by a key project of the
Natural Science Foundation of China 共NSFC兲, no. 20333040.
Figure 4. Discharge and charge profiles obtained with 共a兲 pure silicon and
共b兲 silicon/carbon composite as anode active materials.
Chinese Academy of Sciences assisted in meeting the publication costs of
this article.
distributed in the carbon matrix. After ten cycles, the shapes of the
CVs are almost concurrent even though the scanning rate was further increased to 0.08 and 0.1 mV s−1. Furthermore, there was no
obvious shift of peak voltage, implying that there was no obvious
polarization, and the composite electrode exhibited good reversibility after the initial several cycles.
Figure 4 compared the charge/discharge profiles of electrodes
using pure silicon powder and silicon/carbon composite as active
materials, respectively. As seen in Fig. 4a, the first discharge capacity amounted to 3042 mAh g−1, but the charge capacity was only
2108 mAh g−1. Within only five cycles, the capacity loss was close
to 90% of the first charge capacity. The fast capacity fade was predominately attributed to volume change between lithiation and
delithiation states of silicon particles, especially the volume contraction after the delithiation process. On the other hand, as shown in
Fig. 4b, the composite electrode just charged 1115 mAh g−1 in the
first cycle, but the coulombic efficiency was close to 82%. While the
cycling extended, the discharge capacity dropped slightly, indicating
an increased polarization caused by the hindered electrochemical
kinetics. Although the charge/discharge capacity gradually suffered
from a loss after the first several cycles, a charge capacity above
560 mAh g−1 after 75 cycles were still be achieved as shown in Fig.
5. In contrast, the cycling performance of electrode made from pure
silicon powers was also shown in Fig. 5. Within only five cycles, the
capacity loss is close to 90% of the first charge capacity. The improved performance could be attributed to the amorphous carbon
1. K. Ozawa, Solid State Ionics, 69, 212 共1994兲.
2. Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, and T. Miyasaka, Science, 276,
1395 共1997兲.
3. J. R. Dahn, T. Zheng, Y. H. Liu, and J. S. Xue, Science, 270, 590 共1995兲.
4. K. T. Lee, Y. S. Jung, and S. M. Oh, J. Am. Chem. Soc., 12, 5652 共2003兲.
5. M. Winter, J. O. Besenhard, and M. E. Spahr, Adv. Mater. (Weinheim, Ger.), 10,
725 共1998兲.
6. C. J. Wen and R. A. Huggins, J. Solid State Chem., 37, 271 共1981兲.
7. R. A. Huggins, J. Power Sources, 81-82, 13 共1999兲.
8. R. A. Huggins, Solid State Ionics, 113-115, 57 共1998兲.
9. M. Winter and J. O. Besenhard, Electrochim. Acta, 45, 31 共1999兲.
10. N. Tamura, R. Ohahita, and M. Fujimoto, J. Electrochem. Soc., 150, A679 共2003兲.
11. W. J. Weydanz, M. Wohlfahrt, and R. A. Huggins, J. Power Sources, 81-82, 237
共1999兲.
12. B. C. Kim, H. Uono, T. Satou, T. Fuse, T. Ishihara, M. Ue, and M. Senna, J.
Electrochem. Soc., 152, A532 共2005兲.
13. H. Y. Lee and S. M. Lee, Electrochem. Commun. 6, 465 共2004兲.
14. N. Kurita and M. Endo, Carbon, 40, 253 共2002兲.
15. N. Dimov, K. Fukuda, T. Umeno, S. Kugino, and M. Yoshio, J. Power Sources,
114, 88 共2003兲.
16. X. Q. Yang, J. McBreen, W. S. Yoon, M. Yoshio, H. Y. Wang, K. J. Fukuda, and T.
Umeno, Electrochem. Commun., 4, 893 共2002兲.
17. N. Dimov, S. Kugino, and M. Yoshio, Electrochim. Acta, 48, 1579 共2003兲.
18. Y. Liu, K. Hanai, J. Yang, N. Imanishi, A. Hirano, and Y. Takeda, Electrochem.
Solid-State Lett., 7, A369 共2004兲.
19. J. Yang, B. F. Wang, K. Wang, Y. Liu, J. Y. Xie, and Z. S. Wen, Electrochem.
Solid-State Lett., 6, A154 共2003兲.
20. Z. S. Wen, J. Yang, B. F. Wang, K. Wang, and Y. Liu, Electrochem. Commun., 5,
165 共2003兲.
21. W. R. Liu, Z. Z. Gu, W. S. Young, D. T. Shieh, H. C. Wu, M. H. Yang, and N. L.
Wu, J. Power Sources, 140, 139 共2004兲.
22. S. Ohara, J. Suzuki, K. Sekine, and T. Takamura, J. Power Sources, 136, 303
共2004兲.
References
Downloaded on 2016-05-16 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).