Materials Transactions, Vol. 54, No. 6 (2013) pp. 1018 to 1024
© 2013 The Japan Institute of Metals and Materials
EXPRESS REGULAR ARTICLE
Microstructure and Charge­Discharge Characteristics
of Ag­AgCl Coated Natural Bamboo Carbon
Chih-Hsien Wang, Fei-Yi Hung+, Truan-Sheng Lui and Li-Hui Chen
Department of Materials Science and Engineering, Center for Micro/Nano Science and Technology,
National Cheng Kung University, Tainan, Taiwan 701
Bamboo carbon decomposed by low temperature has a high surface area and contains micro-holes; it belongs to one of the amorphous
carbon materials. Due to the large lithium storage space and high discharge performance rate, lithium batteries enjoy high power consumption.
However, there are also some defects of the first higher irreversible capacity and voltage delay. In this study, a natural bamboo carbon powder
was used as an experimental material. After adding AgNO3 and treating the surface with heat at 450 and 650°C, fine Ag/AgCl phases are coated
on the surface of carbon powders. C­Ag and C­AgCl are formed to increase the capacity and reduce the first irreversibility. Also, the
concentration of Ag on the carbon surface increased with the increment of temperature from 450 to 650°C. This not only increased the
conductivity but also enhanced the surface bonding of C­Ag powders to promote the performance of charge­discharge cycles.
[doi:10.2320/matertrans.M2012254]
(Received July 17, 2012; Accepted March 5, 2013; Published May 25, 2013)
Keywords: bamboo carbon, silver, silver­chloride, charge­discharge
1.
Introduction
Carbon material has many advantages such as low cost,
large current discharge capability, stable structure, safety etc.
However, the capacity of commercial carbon materials which
are produced by petroleum is less than the theoretical
capacity of graphite (372 mA hg¹1).1) Therefore, the development of high capacity and excellent cycle life anode materials
is an important issue in the application of lithium­ion
batteries.
Bamboo carbon is a green material. It is conducive to the
passage of lithium ions and creates more on lithium storage
locations due to the pore structure, nano-channels and large
specific surface area. Hence, the lithium storage space and
large-current discharge capability of the bamboo carbon are
greater than that of graphite. However, the bamboo carbon
has a huge irreversible reaction that affects the electrochemical performance. Surface modification of carbon materials is
an effective method to improve the electrochemical characteristics of carbonaceous electrodes. In recent years, some
methods of surface modification have been attempted viz.,
surface oxidation,2) surface fluorination,3) metal or metaloxide coating4,5) and carbon coating.6)
According to our previous study,5) carbon coating is a
highly efficient interface reaction. Therefore, natural bamboo
carbons were used as a negative electrode in this study and
the carbon surface was modified by the ball milling7,8) and
surface treatment methods. The charge­discharge characteristics of natural bamboo carbon before and after the surface
modification has been explored. Ag has the best conductivity.
In this study, the Ag/AgCl phases were coated on the carbon
surface with good bonding interface. The Ag/AgCl-carbon
phases, which were formed with a heat treatment (450°C;
650°C), had excellent stability and conductivity thus
providing a higher volumetric capacity and longer lifecycle
than conventional graphite anodes. In addition to discussing
the effects of Ag/AgCl coating on heat treatment, the coating
+
Corresponding author, E-mail: [email protected]
mechanism and lithium ions’ intercalation­deintercalation
reaction in the electrode was investigated. Thus the charge­
discharge life cycle of C­Ag and C­AgCl was clarified for
possible applications to batteries.
2.
Experimental Procedures
A natural bamboo carbon was prepared by sintering at
750°C and then soaked in 10 mass% of HCl solution,
followed by ultrasonic treatment with deionized water. It was
defined as C (non-coating). The ratio of the mixture of C and
AgNO3 was 10 to 1. After applying ball milling for 12 h, the
carbon powders were removed to be dried and calcined in
the furnace at 450 and 650°C respectively in Ar/H2 for 1 h
until it cooled down to room temperature. The size of
D50 particles was controlled below 5 µm. Bamboo carbon
powders were modified by two heat treatments, namely CA450 and CA-650.
Three types of powder (C, CA-450, CA-650) morphology
were examined by SEM (Philip XL-40FEG), and then the
electrodes were scanned by backscattered electron imaging
(BEI) to distinguish Ag/AgCl. The content of carbon and
modified carbon was also calculated semi-quantitatively by
using an energy dispersive spectrometer (EDS). Meanwhile
an X-ray diffractometer (XRD, RigakuD-max/IIB, CuK
0.15418 nm) identified the powder structure (scan angle of
2­90°, scan rate of 2° min¹1), and the particle size of the
powders was measured by a particle size analyzer (HORIBA300 type).
The electrochemical test was performed by a two-electrode
semi-cell. The working electrode was carbon and the counter
electrode was lithium metal. In the initial stage of the charge­
discharge cycle, lithium ions first intercalated into carbon
during the discharge process and then deintercalated from
carbon into lithium metal in the later charge process. An
electrode was prepared by mixing 90 mass% carbon,
6 mass% PVDF (polyvinylidene fluoride) and 4 mass%
conductive carbon black, and subsequently dissolved in
NMP (N-methyl-2-pyrrolidone). The slurry was coated onto
Microstructure and Charge­Discharge Characteristics of Ag­AgCl Coated Natural Bamboo Carbon
copper foil (15 µm). The coated films were dried for 2 h at
100°C and were cut to a diameter of 13 mm. The electrolyte
was 1 M LiPF6 (lithium hexafluorophosphate) in EC (ethylene carbonate) + DEC (diethyl carbonate) 1 : 1 in terms of
volume. The assembled cells exhibited 0.2C discharge
(intercalation) and 0.2C charge (deintercalation) at first, and
then a cycle life test was implemented at 1C discharge
(intercalation) and 1C charge (deintercalation) (Arbin
BT2000). The voltage range was 0.01­2 V, and the electrochemical impedance spectroscopy (EIS) (Potentiostats
2273) was performed at a frequency of 1 MHz­0.01 Hz.
3.
1019
(a)
Results and Discussion
3.1 Microstructure of powder
Figure 1(a) shows the morphology of the bamboo carbons
after ball milling. It shows that the form of the particles is
irregular and some are in void and fold structures. The size of
the particles (D50) was measured as 2.5 µm. Table 1 shows
the element analysis of powders C: the weight percentage
of carbon is 84.87 mass%, H is 2.17 mass% and O is about
10 mass%. Because the bamboo carbon can be decomposed
at low temperature (750°C), the proportion of H and O was
high and they existed on the carbon surface in the form of
hydroxyl and carboxyl groups. The atomic size and charges
of other elements are different from carbon’s, and this could
have affected the charge­discharge behavior to yield a larger
irreversible capacity.9) On the other hand, Fig. 1(b)(c) show
the morphologies of CA-450 and CA-650 after the heat
treatment. Tiny phases were generated on the carbon surface;
the morphology and size of the C powders did not change
obviously after modification. To corroborate the distribution
of surface elements, the electrode’s mapping was carried out.
We found that the elements of Ag and Cl were uniformly
distributed on the surface of the electrodes (Fig. 2(a)(b)).
Notably, the amount of Cl in the CA-650 electrode was
significantly less than the CA-450 electrode. EDS analysis
for CA-450 and CA-650 are listed in Table 2. The amount of
Cl is significantly more than Ag in the CA-450 electrode.
This indicates that Cl not only reacts with Ag to form AgCl,
but also bonds with C easily to form COCl or CCl2. This is
due to the existence of the functional groups and the
microvoids.10,11) However, the amount of Ag was more than
Cl in the CA-650 electrode (treated at 650°C). It is known
that AgCl was decomposed into silver and chlorine in the
reducing atmosphere. Through BEI scanning of the electrode
surface, it was found that the light spots were aggregated in
the CA-450 and CA-650 electrodes. Table 3 shows the EDS
analysis of the light spot zone. The main components are
carbon and silver in the CA-450 electrode, and a small
quantity of chlorine. Notably, the light spot zones of the CA650 electrode were carbon and silver, and no chlorine was
detected. According to the results above, we can know that
when the temperature rises, C­AgCl or C­Ag compounds
will form on the surface of bamboo carbons. To understand
the structure of C­AgCl or C­Ag phases, XRD identification
was carried out.
The results of XRD are shown in Fig. 3. The structure of
carbon (C) is amorphous and its degree of crystallization is
low. Diffraction peaks at (002) and (100) were broad, and
2μ m
(b)
2μ m
(c)
2μ m
Fig. 1
SEM photograph: (a) C (b) CA-450 (c) CA-650 powders.
Table 1
EA analysis of C powders.
Element
N
C
H
O
mass (%)
0.48
84.87
2.17
Bal.
indicate that some graphite layers were stacked. The d(002)
became larger and the solid-state diffusion became faster.
This contributed to the acceleration of the charge­discharge
rate. The intensity of the peak was higher in the low-angle
C.-H. Wang, F.-Y. Hung, T.-S. Lui and L.-H. Chen
Intensity (a.u.)
1020
(a)
Fig. 2
(b)
Diffraction angle, 2θ / degree
SEM-BEI and mapping: (a) CA-450 and (b) CA-650 electrodes.
Fig. 3
Table 2 EDS analysis of CA-450 and CA-650 electrodes.
Element
C
CA-450
Mass%
82.27
CA-650
At%
89.58
Mass%
82.29
91.05
3.24
0.39
7.03
0.87
Cl
3.34
1.23
1.01
0.38
O
8.70
7.11
7.20
5.98
F
2.45
1.69
2.47
1.73
EDS analysis of bright spot on CA-450 and CA-650 electrodes.
Element
CA-450
CA-650
Mass%
At%
Mass%
At%
C
7.96
38.96
14.57
54.91
Ag
87.64
47.75
81.59
34.23
Cl
1.41
2.34
O
2.98
10.95
0
3.84
Table 4 Charge/discharge measurements at 0.2 C and 0.01­2 V and special
surface area (SSA) analysis.
C
At%
Ag
Table 3
XRD patterns for the original and modified carbon powders.
0
10.86
region, which reveals that the carbon materials have microporous characteristics.12) Furthermore, the XRD pattern of
CA-450 not only shows the existence of carbon phase but
also silver cubic phase and silver chloride cubic phase.
According to Scherrer’s equation, the grain size of AgCl is
about 43.2 nm. XRD patterns of CA-650 show an increasing
trend in the heat-treatment temperature, which increased the
volume ratio and the degree of crystallization of silver phase,
but silver chloride phase was not significantly affected. When
the temperature increased from 450 to 650°C in the reducing
CA-450
CA-650
DC (mA hg¹1)
488
1062
848
CC (mA hg¹1)
195
576
439
1st IR (%)
SSA (m2 g¹1)
60
46
48
137.0
141.8
145.7
atmosphere, AgCl could decompose more easily into silver
and chlorine, and resulting in an increase of silver phase.
Based on this, C­Ag phase was obtained in the coating
mechanism of the modified bamboo carbon powders with
Ag/AgCl and improved the stability of crystallization.
3.2 Charge­discharge behavior
The coating of the carbon electrode with Ag/AgCl
improved the initial charge­discharge performance. Table 4
shows the first charge­discharge capacity and irreversibility.
The capacity of CA-450 and CA-650 were greater than the
theoretical capacity of graphite (372 mA hg¹1). Moreover,
CA-450 showed the highest charge capacity (deintercalation,
up to 576 mA hg¹1) and the charge capacity of CA-650 was
439 mA hg¹1. They were larger than the charge capacity of
the original carbon material (195 mA hg¹1).5) The first cycle
irreversibility of CA-450 and CA-650 was 46 and 48%
respectively, both of which were less than the 60% of the
original carbon material. When the lithium ions intercalated
into carbon, the electrolyte reacted on the carbon electrode
and decomposed to form the passive layers on the electrode
surface. This would have caused some lithium ions to be lost
Microstructure and Charge­Discharge Characteristics of Ag­AgCl Coated Natural Bamboo Carbon
2.5
C
CA-450
CA-650
Voltage, V / V
2
1.5
1
0.5
0
0
200
400
600
800
1000
1200
Capacity, C / mAh g-1
Fig. 4 Discharge­charge curves of the first cycle for the carbon powders at
0.2C and 0.01­2 V.
in the reaction. Therefore the charge capacity (deintercalation) was less than the discharge capacity (intercalation),
hence the irreversibility of electrode.
Since carbon has a large surface area, the solid electrolyte
interface (SEI) layers which are produced through the
response of the electrode and electrolyte increased. According to the literature13) and Table 4 (special surface area),
a carbon surface which is coated with halide, exhibits the
enhanced kinetics of lithium intercalation and leads to an
increase in the capacity of modified carbon when the SEI
forms. In addition, the modified carbon powders formed a
more conductive interface due to the high conductivity of
silver for lithium ions. During the process of electrochemical
discharge, the Ag reacted with Li to form AgLi at a lower
voltage (<0.2 V), which led to the irruption of more Li to
form Ag5Li8.14) It’s also clear that Li migration with the
increase of Ag on the carbon surface. Silver increased the
electron transport rate and enhanced the diffusion velocity of
lithium ions between the carbon particles. The carbon surface
was coated with Ag/AgCl to change the intercalation­
deintercalation voltage during the charge­discharge life
cycle.
Figure 4 shows the first charge­discharge curve of the
carbon. The discharge (intercalation) curve of C powders is
smooth and the SEI layers are produced through the response
of carbon materials and electrolyte when below 1 V. The
charge (deintercalation) curve has two stages of 0.01­0.9 V
and 0.9­2 V, and the lithium deintercalation mainly occurred
in the 0.01­0.9 V stage. Since lithium ions do not depart
from carbon powders easily, the potential platform was not
obvious, and the intercalation potential was higher than the
deintercalation, resulting in serious voltage hysteresis.15­17)
For the modified powders, the discharging curves of CA-450
and CA-650 were similar. The difference compared with C
powder is that the discharge (intercalation) curve has a longer
platform at 0.8 V. This reveals that Ag/AgCl joined the
formation of SEI layers and AgCl was decomposed to LiCl
and Ag.
1021
The silver particles alloyed with doped lithium at 0 V,
but the amount of lithium intercalation was limited. Also,
it was distinguishable from several platforms below 0.8 V,
showing that the discharge (intercalation) curve declined
gradually from 0.4 to 0 V and a structure of Li-intercalated
carbon compound formed.18) With regards charging (deintercalation), the difference between the original carbon and
modified carbon is that the obvious platform produced
more than 0.5 V and this proves that the surface of modified
carbon has better ionic conductivity. The voltage curve of
CA-450 and CA-650 lies almost entirely between 0 and 1 V,
but the CA-450 had a more obvious delithiated platform
than CA-650 between 1 and 2 V, showing that the surface
of CA-450 is modified by chlorine that enhanced the kinetics
of lithium deintercalation from the carbon electrode. From
Fig. 4 and Table 4, we can see that the coating with silver
chloride and silver obviously increased the initial reversible
capacity of the modified carbon electrode due to the
reduction in polarization during the intercalation and
deintercalation of lithium. Furthermore, the initial coulomb
efficiency increased from 40 to 55­60%. It was confirmed
that the chloride modified SEI layers and the better kinetics
of lithium intercalation decreased internal resistance and
produced the obvious delithiated platform thus decreasing
voltage hysteresis.
The charge (deintercalation) capacity of CA-450 and CA650 as a function of cycle numbers is shown in Fig. 5(a).5)
After the 30th charge­discharge cycle, the current was at 1C
rate, the charge (deintercalation) capacity’s retention of CA650 and CA-450 were 79.9 and 59.2%. Because the SEI
layers of the first charge­discharge cycle did not fully form,
fast cyclic degradation occurred. The beginning of the
charge­discharge cycles experienced larger capacity degradation due to the current (1C). This current raised the reactive
rate of lithium ions and was able to reduce the degradation
rate of charge­discharge cycles. CA-450 has a better lithium
intercalation capability, but its higher chloride content had an
insulating effect which hindered charge transfer reaction and
decreased the amount of lithium intercalation. By increasing
the charge­discharge current and cycles, the internal
resistance increased, and this resulted in a decline in capacity
and the quality of cycle life. On the contrary, a large amount
of Ag also increased the lithium diffusion rate on the carbon
surface, and stimulated the diffusion of lithium ions within
the carbon particles, effectively preventing the generation
of dendrites.14) Comparing the columbic efficiencies with
charge capacity during 30 cycles, the charge­discharge
current was 1C, as shown in Fig. 5(b). It shows that CA650 has better columbic efficiency and stability during the
initial cycle stage, which indicates that the surface structure
of CA-650 is stable and has a better cyclability than the
other carbons. This reveals a close relationship between the
modified structure and the cyclability.
To understand the charge­discharge cyclic characteristics
of the modified carbons, electrochemical impedance spectroscopy (EIS) was carried out after the 30th cycle
(Fig. 6(a)). There were two obvious semicircles from high
to middle frequency in CA-650. The high frequency
semicircle was attributed to the passivation film formed on
the carbon. The semicircle in the middle frequency region
1022
C.-H. Wang, F.-Y. Hung, T.-S. Lui and L.-H. Chen
300
(a)
CA-450
CA-650
(a)
-Zimg (ohms)
Capacity, C / mAh g-1
250
200
150
100
50
0
0
20 40 60 80 100 120 140 160 180 200 220
Zre, Z / ohms
Cycle number, N
CA-450
CA-650
(b)
120
CA-450
CA-650
(b)
220
110
200
Zre, Z / ohms
Coulomb efficiency, Ce / %
240
100
90
180
160
140
80
120
70
100
4
5
6
7
8
9
10
ω −1/2
60
0
10
20
30
Cycle number, N
Fig. 5 (a) Cycle performance (b) coulomb efficiency of CA-450 and CA650 were subjected to discharge and charge at 1C rate and 0.01­2 V.
was derived from the charge-transfer process at the electrolyte/electrode interface. The sloping line at low frequencies
was attributed to the diffusion of lithium ions within the
carbon electrode. Moreover, the charge-transfer impedance of
CA-450 after the 30th cycle was higher than that of CA-650.
The main reason is that CA-650 forms a higher conductive
interface of Ag which improves the intercalation and
deintercalation of lithium ions. In order to understand the
ability of lithium to diffuse within the carbon materials, the
plot of Zre and ½¹1/2 was drawn, and · (Warburg factor)
values can be obtained by the fitting method (Fig. 6(b)). The
· value is equivalent to the slope of Zre with the reciprocal
of the square root of frequency (½¹1/2) in the low frequency
domain, while the diffusion coefficient was inversely
proportional to the · value.14,19) The · value can be
calculated, which in the case CA-450 was 10.87 and CA650 was 9.85, respectively. The diffusion of lithium ions in
the CA-650 electrode was higher than that of CA-450. It
was confirmed that the C­Ag system had excellent charge­
discharge characteristics.
Fig. 6 (a) Impedance spectra (b) the relationship between Zre and ½¹1/2 at
the low-frequency region of the carbon after and 30th discharge­charge
cycles.
3.3 C­Ag coating mechanism
To understand the interface phenomenon of lithium­ion
transfer, Fig. 7 shows the electrode morphology of CA-450
and CA-650 after 30 charge­discharge cycles. The surfaces
of the CA-450 and CA-650 electrodes were covered with a
compound layer due to the initial irreversible reaction and
repeated charge­discharge reaction. Large white deposits
formed on the electrodes and it consisted of decomposition
products such as ROCO2Li mixtures.20) Ag and Cl still
distributed evenly on the electrode according to mapping
observation. EDS (Table 5) analysis on the electrodes
revealed that there was significant more Ag than Cl. The
charge­discharge cycles show that AgCl gradually decomposed to distribute in the electrolyte or compounds.
Furthermore, XRD identified the structure of the CA-450
and CA-650 electrodes after 30 charge­discharge cycles
(Fig. 8). With the charge­discharge reaction, the (002) peak
of C phase was not obvious, due to various compounds
covering the electrode. In addition to the original Ag phase,
the distinctive peak of Li2CO3 formed at the same time in
CA-650, but the peak was not obvious in CA-450 (Table 5,
Microstructure and Charge­Discharge Characteristics of Ag­AgCl Coated Natural Bamboo Carbon
1023
Intensity (a.u.)
Ag
Li2CO3
Cu
Diffraction angle, 2θ / degree
(a)
Fig. 8 XRD patterns of electrodes after 30th discharge­charge cycles.
(b)
Fig. 7 BEI and mapping of electrodes after 30th discharge­charge cycles:
(a) CA-450 (b) CA-650.
Element
CA-450
Electrochemical performance
Table 5 EDS analysis of CA-450 and CA-650 electrodes after 30th
discharge­charge cycles.
CA-650
Mass%
At%
Mass%
At%
C
32.20
52.47
33.36
52.76
Cl
1.05
0.58
0.86
0.46
Ag
10.98
1.99
9.72
1.71
O
13.9
17.02
20.21
17.0
F
3.77
3.88
2.39
2.39
P
38.10
24.08
36.66
22.48
the oxygen content of CA-650 is greater). The main reason
for this effect is that the formation of Li2CO3 and the
reductive decomposition of EC had generated CO32¹.21) The
Li2CO3 which formed on the electrode surface contributed
to suppress the decomposition of electrolyte and allowed
the intercalation and deintercalation of lithium ions. This
can protect carbon materials from being destroyed by the
intercalation of solvent molecules22) and thus promotes the
cycle life. Figure 9 is a schematic illustration of the natural
bamboo carbon after surface modification. In the early stage,
fine AgCl particles were coated on the carbon surface.
Because of the ball milling process, carbon materials
combined well with the modified substance. With the
increase of the heat treatment temperature (AgCl decomposed), a large number of Ag particles were precipitated
among AgCl particles resulting in better crystallization
of Ag attached to the carbon surface. Meanwhile, the
modified effect contributed to the charging and discharging
performance.
High capacity
Low irreversibility
Good cyclability
High conductibility
CA-Ag
CA-AgCl
High capacity
Low irreversibility
C
Low capacity
High irreversibility
low
Temperature
high
Fig. 9 A schematic illustration of the surface coating mechanism.
4.
Conclusion
After implementing natural bamboo carbon surface treatment, Ag/AgCl formed on the carbon surface, enhancing
the first reversible capacity of carbon and improving the
irreversible nature. With increasing heat treatment temperature, the Ag phase increased as well as the crystallization
being enhanced. After 30 charge­discharge cycles at 1C,
the impedance of CA-650 with rich Ag phase had lower
impedance than CA-450. Under multiple cyclic tests, the
derivative of non-active material reduced the charge­
discharge cycle life.
1024
C.-H. Wang, F.-Y. Hung, T.-S. Lui and L.-H. Chen
Acknowledgements
The authors are grateful to the Center for Micro/Nano
Science and Technology (D101-2700) of National Cheng
Kung University, Taiwan and NSC 101-2221-E-006-114 for
the financial support.
REFERENCES
1) T. Ohzuku, K. Iwakoshi and K. Sawai: J. Electrochem. Soc. 140 (1993)
2490­2498.
2) Y. P. Wu, C. Jiang, C. Wan and R. Holze: J. Appl. Electrochem. 32
(2002) 1011­1017.
3) T. Nakajima, J. Li, K. Naga, K. Yoneshima, T. Nakai and Y. Ohzawa:
J. Power Sources 133 (2004) 243­251.
4) S. Kim, Y. Kadomo, H. Ikuta, Y. Uchimoto and M. Wakihara:
Electrochem. Solid-State Lett. 4 (2001) A109­A112.
5) C. H. Wang, F. Y. Hung, T. S. Lui and L. H. Chen: Mater. Trans. 51
(2010) 186­191.
6) H. Y. Lee, J. K. Baek, S. W. Jang, S. M. Lee, S. T. Hong and K. Y. Lee:
J. Power Sources 101 (2001) 206­212.
7) H. Y. Lee and S. M. Lee: Electrochem. Commun. 6 (2004) 465­469.
8) T. Zheng, Q. Zhong and J. R. Dahn: J. Electrochem. Soc. 142 (1995)
L211­L214.
9) S. Nakanishi, F. Mizuno, K. Nobuhara, T. Abe and H. Iba: Carbon 50
(2012) 4794­4803.
10) K. Naga, T. Nakajima, Y. Ohzawa, Z. Mazej, B. Žemva and H. Groult:
J. Electrochem. Soc. 154 (2007) A347­A352.
11) K. Matsumoto, J. Li, Y. Ohzawa, T. Nakajima, Z. Mazej and B. Žemva:
J. Fluorine Chem. 127 (2006) 1383­1389.
12) A. Gibaud and J. S. Dahn: Carbon 34 (1996) 499­503.
13) T. Nakajima, M. Koh, R. N. Singh and M. Shimada: Electrochim. Acta
44 (1999) 2879­2888.
14) E. Ronnebro, J. Yin, A. Kitano, M. Wada and T. Sakai: Solid State
Ionics 176 (2005) 2749­2757.
15) J. Machnikowski, E. Frackowiak, K. Kierzek, D. Waszak, R. Benoit
and F. Béguin: J. Phys. Chem. Solids 65 (2004) 153­158.
16) Z. Ma, X. Yuan, D. Li, X. Liao, H. Hu, J. Ma and J. Wang:
Electrochem. Commun. 4 (2002) 188.
17) H. Q. Xiang, S. B. Fang and Y. Y. Jiang: Carbon 37 (1999) 709­711.
18) S. R. Mukai, T. Hasegawa, M. Takagi and H. Tamon: Carbon 42 (2004)
837­842.
19) T. Zhang, H. P. Zhang, L. C. Yang, B. Wang, Y. P. Wu and T.
Takamura: Electrochim. Acta 53 (2008) 5660­5664.
20) J. S. Shin, C. H. Han, U. H. Jung, S. I. Lee, H. J. Kim and K. Kim:
J. Power Sources 109 (2002) 47­52.
21) Y. K. Choi, K. Chung, W. S. Kim, Y. E. Sung and S. M. Park: J. Power
Sources 104 (2002) 132­139.
22) W. S. Kim, K. Chung, J. H. Cho, D. W. Park, C. U. Kim and Y. K.
Choi: J. Ind. Eng. Chem. 9 (2003) 699­703.