Preparation of carbon gel microspheres containing silicon powder
for lithium ion battery anodes
Takahiro Hasegawa, Shin R. Mukai
*
, Yoji Shirato, Hajime Tamon
Department of Chemical Engineering, Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan
Received 30 March 2004; accepted 25 May 2004
Available online 20 July 2004
Abstract
Carbon gel microspheres containing silicon powder were successfully prepared by simply adding silicon powder to the water
phase during the inverse emulsion polymerization of resorcinol with formaldehyde, followed by freeze-drying and carbonization in
an inert atmosphere. SEM images and XRD profiles of the obtained carbon gel microspheres suggest that the silicon powder was
successfully encased within the carbon gel microspheres. From the results of galvanostatic lithium ion charging and discharging
experiments, it was found that the reversible capacity of silicon and its cyclic characteristics could be improved by encasing its
powder in carbon cryogel microspheres. It is assumed that the mesoporous structure of the carbon cryogel acted as a buffering
matrix which relieves the morphological changes of silicon powder which occurs during charging/discharging.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Carbon microbeads; B. Carbonization, Coating; D. Electrochemical properties
1. Introduction
Currently, carbon materials are commonly used as
anodes in lithium ion battery systems, but an upper limit
in the lithium ion capacities of such materials exists [1–
5]. For the further improvement of lithium ion batteries,
the development of alternative anode materials which
possess larger capacities is required. In recent studies [6–
18], binary lithium alloy systems such as Li–Si or Li–Sn
systems have been thoroughly investigated for the
development of new anode materials with high capaci-
ties. The Li–Si system has a possibility of achieving ex-
tremely high capacities up to 4200 mA h/g, since alloys
with the maximum lithium content can be formed in this
system (Li
4:4
Si) [6–12]. However, this material has a
serious drawback when used as the anode, which is the
pulverization of silicon caused by its volume expansion/
shrinkage during lithium ion insertion/extraction. Such
morphological changes cause loose contacts between the
anode material and the current collector, which results
in poor charge/discharge cyclic characteristics. In order
to improve the cyclic characteristics of this material, it is
necessary to relieve such morphological changes.
Yoshio and coworkers [19–23] reported that carbon-
coated silicon synthesized by the thermal vapor depo-
sition (TVD) method shows better cyclic characteristics
when compared with conventional silicon anodes. This
result suggests that the morphological changes of silicon
can be relieved by encasing it within a carbonaceous
material, which has an appreciable flexibility to act as a
buffer. Carbon gels, which are synthesized by the sol–gel
polycondensation of resorcinol (R) with formaldehyde
(F) followed by drying and carbonization [24,25], are
also promising materials for encasing silicon powder
because of the following reasons. Silicon powder can be
easily dispersed in an RF solution and the resulting gels
can be formed into designated shapes, therefore the
silicon powder is likely to be successfully encased within
the resulting carbon gels. Moreover, it is thought that
the mesoporous structure of the carbon gels formed by
nano-sized carbon particles is favorable for buffering the
morphological changes of silicon.
In this work, silicon powder was encased within
carbon gel microspheres by modifying the method used
by Yamamoto et al. [26]. Carbon gel microspheres
*
Corresponding author. Tel.: +81-75-383-2684; fax: +81-75-383-
2654.
E-mail address: mukai@cheme.kyoto-u.ac.jp (S.R. Mukai).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2004.05.050
Carbon 42 (2004) 2573–2579
www.elsevier.com/locate/carbon