Superior Lithium Electroactive
Mesoporous Si@Carbon Core-Sh ll
Nanowires for Lithium Battery Anode
Material
Hyesun Kim and Jaephil Cho*
Department of Applied Chemistry, Hanyang UniVersity, Ansan, 426-791 Korea
Received June 27, 2008; Revised Manuscript Received September 10, 2008
ABSTRACT
Mesoporous Si@carbon core-sh ll nanowires with a diameter of∼6.5 nm were prepared for a lithium battery anode material using a SBA-15
template. As-synthesized nanowires demonstrated excellent first charge capacity of 3163 mA h/g with a Coulombic efficiency of 86% at a rate
of 0.2 C (600 mA/g) between 1.5 and0Vincoin-typehalf-cells. Moreover, the capacity retention after 80 cycles was 87% and the rate capability
at 2 C (6000 mA/g) was 78% the capacity at 0.2 C.
Si metal has been known to reach a highest lithium capacity
of ∼4200 mA h/g corresponding to Li
4.4
Si;
1
however a large
volume change (>300%) during lithium alloying and deal-
loying can result in pulverization of the particles and
electrical disconnection from the current collector.
2
The
electrical disconnection leads to a rapid capacity fade of the
cell. Many reports have focused on reducing such volume
change via composites with carbon and Si nanoparticles.
2,9
These methods showed some improvement of the capacity
retention because the carbon acts as an electron conductor
between the pulverized particles. However, in order to
achieve relatively good capacity retention, the carbon content
in the composite should be greater than 50 wt %, and the
capacity retention after 50 cycles less than 1500 mA h/g.
Alternatively, reports on one-dimensional (1D) silicon
nanowires for anode materials are very rare. Si nanowires
prepared by the laser ablation system described by Lieber
et al.
10
had a wire diameter of ∼10-30 nm,
11
and the first
discharge and charge capacities of the nanowires were ∼1300
and ∼900 mA h/g, respectively, even though the reason for
the low specific capacities was not reported. Si nanowires
with a diameter <50 nm prepared by vapor-liquid-solid
(VLS) template-free growth was reported by Cui et al.
12
They
reported a reversible capacity of ∼2000 mA h/g at a rate of
0.2 C between 2 and 0 V, and no capacity fade up to 10
cycles. Semiconducting nanowire growth by the VSL,
solution-liquid-solid (SLS), and template-free synthesis
have been reported.
13,21
However, these methods did not
produce well-aligned nanowires with ordered separation
distance. It is very important to array the Si nanowires with
uniform interwire distance (same pore size) so that the
ordered pores can act as a buffer layer for the uniform
volume changes. One such example is mesoporous SnO
2
and
mesoporous tin phosphates prepared by soft templates,
21
which showed excellent capacity retention compared to the
nanosized counterpart, thus demonstrating the role of me-
sopores.
Recently, Ryoo’s group reported preparation of ordered
mesoporous carbon or metal oxides using highly ordered
mesoporous silica templates.
22
After an annealing process,
the parent silica materials were selectively removed. These
framework compositions should be stable under conditions
used to dissolve the mold, that is, stability in relatively
concentrated NaOH or HF for silica.
23
This method is highly
reproducible and can use silica templates. To prepare the
nanowires, a hexagonal SBA-15 silica template with p6 mm
symmetry was used, which contains two-dimensional, paral-
lel cylindrical pores arranged with a hexagonal symmetry.
In this study, mesoporous silicon-carbon nanowires
prepared by a SBA-15 hard template are reported. Even
though metallic semiconductors cannot be used in this
template due to reaction at higher temperature, butyl
terminators in the Si-C
4
H
9
precursor are converted to carbon
shell layers later in the annealing process. Hence, the carbon
layer blocks a direct reaction between the Si core and the
SiO
2
template. These mesoprous carbon@shell nanowires
show not only excellent lithium reactivity with a reversible
capacity of 3163 mA h/g at a rate of 0.2C but also excellent
rate capability at 2 C rate.
Figure 1a shows the schematic diagram of the preparation
procedure for the mesoporous Si@carbon core-shell nanow-* Corresponding author, jpcho@hanyang.ac.kr.
NANO
LETTERS
2008
Vol. 8, No. 11
3688-3691
10.1021/nl801853x CCC: $40.75 2008 American Chemical Society
Published on Web 10/25/2008

ires using the SBA-15 template. Butyl-capped Si precursor
was impregnated into the template and annealed at 900 °C
under vacuum. The template was removed using HF. Figure
1b displays the transmission electron microscopy (TEM)
image of carbon@Si nanorods with a diameter of 4 nm and
a length of 20 nm obtained after the first impregnation,
annealing at 900 °C, and HF etching. The (111) lattice fringe
and selected area diffraction pattern (SADP) (inset of c)
confirm the formation of the diamond cubic Si phase.
However, after the fourth impregnation, fully grown nanow-
ires are observed (Figure 1d). The light lines in Figure 1d
are considered projections of the mesopore channels, whereas
the dark lines are the Si walls. In addition, a very thin
amorphous carbon layer is observed (Figure 1e) and CHS
analysis confirmed that the carbon content was 6 wt %. The
ordering of the carbon layer was examined by surface-
enhanced Raman spectral analysis (Figure 1f). The mode at
1582 cm
-1
, referred to as the G mode, was assigned to the
“in-plane” displacement of carbons strongly coupled in
the hexagonal sheets.
24
When disorder was introduced into
the graphite structure, the bands broadened. Further, the band
near 1357 cm
-1
is typically called the “disorder-induced”
or D mode, and the integrated intensity ratio I
D
/I
G
is
indicative of the degree of carbonization.
24
A smaller
intensity ratio indicates a higher degree of carbonization. The
value was 0.09 for ordered synthetic graphite, but that value
of the Si nanowires was 1.45, indicating the formation of
disordered carbon layer.
Figure 2a exhibits a low-angle X-ray diffraction (XRD)
pattern of the synthesized nanowires after removing the
templates. Two resolved peaks indexed as (100) and (110)
confirmed a well-ordered hexagonal mesoporous structure
with a space group of p6 mm. The intense (100) peak
corresponds to a d-spacing of 8.8 ( 0.1 nm, and the high-
angle XRD pattern of the annealed sample clearly showed
the presence of Si nanocrystals (inset of Figure 1a). The
crystal size of the sample, calculated using the Scherrer
formula, was estimated to be approximately 6.5 nm.
The nitrogen-adsorption isotherm of the annealed sample,
with a Brunauer-Emmett-Teller (BET) surface area of 74
m
2
/g, is shown in Figure 2b. The nitrogen adsorption-desorp-
tion isotherms of both templates were type IV with a sharp
capillary condensation step with high relative pressures and
H1 hysteresis loops, indicative of large channel-like pores
in a narrow size distribution. The average pore size in the
annealed sample was approximately 2.3 nm (Barrett-Joyner-
Halenda (BJH) analysis), which is consistent with the XRD
and TEM data (Figures 1 and 2, respectively). The pore wall
thickness was estimated from the difference in the d-spacings,
and the pore size was determined from the desorption branch
of the isotherm using the BJH method. Accordingly, the
calculated pore-wall size of the nanowires was 6.5 nm,
consistent with the TEM result.
Figure 3a shows voltage profiles of the mesoporous
carbon@Si nanowires after 1, 30, 60, and 80 cycles, and
the first discharge and charge capacities were 3664 and 3163
mA h/g, respectively, with an initial Coulombic efficiency
of 86%. An irreversible capacity ratio of 14% may be due
to the side reaction with the electrolytes. In the side reaction,
solvent molecules and salt anions are reduced on the active
surface, forming insoluble Li salts that precipitate to form a
passivating film surface.
25
In addition, the relatively large
surface area of the nanowires leads to the intensified surface
reactions and, hence, to their high irreversible capacity.
Another source that may contribute to the irreversible
capacity is the formation of SiO
x
, and the decomposition
plateau of SiO
x
to Si and Li
2
O should appear near 0.8 V.
26
However, the cycling curve during the first cycle does not
show decomposition, indicating the formation of the SiO
x
Figure 1. (a) Schematic view of the preparation of Si@carbon
core-shell nanowires. (b) TEM image of the Si@carbon core-shell
nanorods obtained from first impregnation. (c) Expanded TEM
image of (b) (inset is SADP of (c)). (d) TEM image of the
Si@carbon core-shell nanowires obtained from fourth impregna-
tion. (e) Expanded TEM image of (d) and (f) Raman spectrum of
Si@carbon core-shell nanowires.
Figure 2. (a) Low- and high-angle (inset) XRD pattern of the
Si@carbon core-shell nanowires. (b) Nitrogen adsorption-desorption
isotherm of the Si@carbon core-shell nanowires. (c) Pore size
distribution of the Si@carbon core-shell nanowires.
Nano Lett.,Vol. 8, No. 11,2008 3689