Effect of carbon coating on electrochemical performance of treated natural graphite as lithium-ion battery anode material

Article abstract of DOI:10.1149/1.1393344

Carbon-coated natural graphite has been prepared by thermal vapor decomposition treatment of natural graphite at 1000°C. Natural graphite coated with carbon showed much better electrochemical performance as an anode material in both propylene carbonate-ba

Full text of DOI:10.1149/1.1393344

Effect of Carbon Coating on Electrochemical Performance of
Treated Natural Graphite as Lithium −Ion Battery Anode
Material
Masaki Yoshio, Hongyu Wang, Kenji Fukuda, Yoichiro Hara and Yoshio Adachi
J. Electrochem. Soc. 2000, Volume 147, Issue 4, Pages 1245-1250.
doi: 10.1149/1.1393344
Receive free email alerts when new articles cite this article - sign up in the
box at the top right corner of the article or click here
Email alerting
service
To subscribe to Journal of The Electrochemical Society go to:
http://jes.ecsdl.org/subscriptions
© 2000 ECS - The Electrochemical Society
Downloaded on 2012-12-08 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

Journal of The Electrochemical Society, 147 (4) 1245-1250 (2000)
1245
S0013-4651(99)08-083-0 CCC: $7.00 © The Electrochemical Society, Inc.
Effect of Carbon Coating on Electrochemical Performance of Treated
Natural Graphite as Lithium-Ion Battery Anode Material
Masaki Yoshio,a,*,z Hongyu Wang,a Kenji Fukuda,b Yoichiro Hara,b and Yoshio Adachic
aDepartment of Applied Chemistry, Saga University, Saga 840-8502, Japan
bMitsui Mining Company, Limited, Wakamatsu-ku, Kitakyushu 808, Japan
cKyushu National Industrial Research Institute, Tosu, Saga 841-0052, Japan
Carbon-coated natural graphite has been prepared by thermal vapor decomposition treatment of natural graphite at 1000ЊC. Nat-
ural graphite coated with carbon showed much better electrochemical performance as an anode material in both propylene car-
bonate-based and ethylene carbonate-based electrolytes than “bare” natural graphite. The effect of carbon coating on the electro-
chemical performance was investigated by solid-state 7Li-NMR in conjunction with standard electrochemical techniques.
© 2000 The Electrochemical Society. S0013-4651(99)08-083-0. All rights reserved.
Manuscript submitted August 23, 1999; revised manuscript received November 1, 1999.
Accompanying the development of lithium-ion batteries, world-
wide efforts have been devoted to the application of carbonaceous
materials as anodes (negative electrodes) in these batteries. Among
the wide spectrum of carbonaceous materials, natural graphite ap-
pears to be the most suitable candidate because of its many advan-
tages, such as its high capacity of 372 mAh/g (with a stoichiometry
of LiC6), low and flat potential profile, and reasonable cost. As far
as we are aware, graphite and graphitized carbon have been used in
many lithium-ion battery commercial products on the market to date.
However, graphite has an important disadvantage which may limit
its further utilization as the anode material in lithium-ion batteries:
its sensitivity to certain electrolytes. Ethylene carbonate (EC) and
propylene carbonate (PC) are the most commonly used high permit-
tivity solvents for lithium-ion batteries. It is well known that EC-
based electrolytes are always used with graphite anodes in lithium-
ion batteries, whereas PC-based electrolytes are not compatible with
graphite anodes because PC decomposes on graphite’s surface ac-
companied by graphite exfoliation.1-5 Nevertheless, EC-based elec-
trolytes exhibit inferior low temperature performance compared to
PC-based electrolytes, mainly due to their different melting points
(mpEC 39ЊC, mpPC Ϫ49ЊC). Therefore, one problem to be solved is:
How can we apply graphite anodes in PC-based electrolytes?
In our opinion, a one-word answer to this question is “modifica-
tion.” It is the modification of graphite or PC-based electrolytes, or
both, that may lead to acceptable stability.
Several papers have been published on modifying PC-based elec-
trolytes using additives such as catechol carbonate, crown ether, eth-
ylene sulfite, and sulfur dioxide.6-9 However, from the viewpoint of
graphite modification, one approach is to coat carbon onto the
graphite surface to protect it from PC. In fact, the prototype of this
concept, the core-shell-structured carbon composite was first applied
as an anode material in lithium-ion batteries by Kuribayashi et al.10
More recently, other groups have also reported the electrochemical
performance of core-shell-structured carbon composites.11,12 Re-
ported processes of preparing composite carbons include common
procedures such as mixing the carbon precursors with graphite or
graphitized carbon and heating the slurry mixtures at temperatures
of about 1000ЊC or above. In contrast, we have prepared carbon-
coated natural graphite by thermal vapor decomposition (TVD). In
this paper, we discuss the excellent electrochemical performance of
TVD-treated natural graphite in both EC-based and PC-based elec-
trolytes. The effect of carbon coating has also been studied.
and 1 L/min, respectively. Natural graphite was stirred in the reac-
tion tube to make a fluid-bed layer and to expose the surfaces of the
graphite particles to the vapor. The temperature of the reaction tube
was maintained at 1000ЊC. At such a high temperature, the toluene
vapor flowed into the reaction tube, decomposed, and deposited on
the graphite surface as a carbon coating. The carbon coating thick-
ness was controlled by the feed time of the toluene vapor. In the
studies, three carbon-coated natural graphite samples with 17.6,
13.4, and 8.6 wt % of carbon coatings, respectively, were tested as
well as the original natural graphite. Selected physical properties for
all of these samples are listed in Table I.
Graphite electrodes were prepared by spreading the graphite
powder slurry (90 wt %) and poly(vinylidene fluoride) (10 wt %)
dissolved in 1-methyl-2-pyrrolidinone onto a copper foil substrate.
The electrodes were then dried overnight at 105ЊC under vacuum
and pressed between two flat steel plates at about 0.2 ton/cm2.
Both EC-based and PC-based electrolytes were used in our stud-
ies. The electrolyte EC:dimethyl carbonate (DMC) (1:2 by volume)/
1 M LiPF6 (Ube) was chosen as representative for EC-based elec-
trolytes. The PC-based electrolytes were 1 M LiPF6 (Tomiyama, bat-
tery grade) in PC/DMC (Wako, high purity) mixed solvents. The
water content in all these electrolytes was < 20 ppm.
The electrochemical measurements included galvanopotentiosta-
tic charge-discharge tests and cyclic voltammetric experiments on
the graphite electrodes. In the galvanopotentiostatic charge-discharge
tests, two-electrode cells were used. The two-electrode cells includ-
ed a lithium metal electrode and a graphite electrode, held apart by a
separator (Celgard 2400) and glass fiber.
The test procedures were as follows. (i) The cells were dis-
charged (intercalation of lithium into the graphite electrode) from
the open-circuit voltage (OCV) to 0 V at a constant current density
of 0.4 mA/cm2, (ii) the cell voltages were held at 0 V until the cur-
rent density decreased to < 0.08 mA/cm2, (iii) the cells were rested
for 10 min, (iv) the cells were charged (deintercalation of lithium
from the graphite electrodes) at a constant current density of
0.4 mA/cm2 to1.5 V, and (v) the cells were rested for 10 min; then
the cycles were repeated.
In the cyclic voltammetric experiments, three-electrode cells were
used. In these cells, lithium metal electrodes were used as both the
counter and reference electrodes, and graphite electrodes were used
as the working electrodes. The cells were cycled between the OCV
(ca. 3 V vs. Liϩ/Li) and 0 V vs. Liϩ/Li at a scan rate of 0.1 mV/s.
All cells were fabricated in a glove box filled with a dry argon
atmosphere.
To gain more insight into our studies on carbon-coated natural
graphite, all graphite samples, together with carbon sample MCMB
6-10 (Mesocarbon Microbeads, Osaka Gas, nominal diam 6 ␮m,
heat-treated at 1000ЊC) as a reference, were fully lithiated electro-
chemically in the electrolyte of EC:DMC (1:2 by volume)/1 M
Experimental
Carbon-coated natural graphite (Mitsui Mining Co., Ltd., Japan)
was prepared by the TVD technique. Toluene vapor and nitrogen
carrier gas were fed into a reaction tube at flow rates of 2 mL/min
* Electrochemical Society Active Member.
z
E-mail: yoshio@ccs.ce.saga-u.ac.jp
Downloaded on 2012-12-08 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

1246
Journal of The Electrochemical Society, 147 (4) 1245-1250 (2000)
S0013-4651(99)08-083-0 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 1. SEM images of graphite sam-
ples: (a) original natural graphite; (b) nat-
ural graphite coated with 17.6 wt %
carbon.
LiPF6 (Ube) and examined by 7Li-NMR spectroscopy. Carbon lithi-
ation was performed by discharging the two-electrode cells at a con-
stant current density of 0.4 mA/cm2 to 5 mV and holding the cells at
this potential for ca. 5 h. Then these cells were disassembled inside
the glove box and the graphite electrodes were washed with dried
DMC solvent. After evaporation of the DMC solvent from these
electrodes under vacuum at room temperatures for ca. 3 h (in the
side box of the glove box), the graphite was scraped off the copper
foil substrates and sealed in NMR sample tubes inside the glove box.
mograms (CVs) of carbon-coated and original natural graphite elec-
trodes in PC-based electrolytes. The effects of carbon coating can be
clearly observed. In the cyclic voltammogram of original bare natur-
al graphite, which is drawn as a thin solid curve, there is a large irre-
versible peak near 0.5 V vs. Liϩ/Li which can be definitely ascribed
to PC decomposition and graphite exfoliation. In addition, there are
three small peaks at potentials lower than 0.2 V vs. Liϩ/Li during the
cathodic sweep. These small peaks are due to the intercalation of
lithium ions into the graphite electrode and the transformations be-
tween different stages of the intercalation compounds (from dilute
stage 1 to stage 4 (LiC36), from liquid-like stage 2 (LiC18) to stage 2
(LiC12), and from stage 2 (LiC12) to stage 1 (LiC6), respective-
7
The sample tubes were set into a Li-NMR spectrometer (external
magnetic field B0 ϭ 7.05 T, resonance frequency for 7Li 116.7 MHz;
DSX-300, Bruker). Line shifts were measured with LiCl aqueous
solution as the external standard at room temperature; magic angle
spinning spectroscopy was not used.
Results and Discussion
Scanning electron microscopy (SEM) images of the original nat-
ural graphite and graphite coated with 17.6 wt % carbon are shown
in Fig. 1a and b, respectively. These two images show that the outer
surface morphology of graphite particles is retained even after car-
bon coating. Therefore, we conclude that TVD is a very effective
technique for uniformly depositing carbon coating on the surfaces of
graphite particles. However, graphite particle size appears to in-
crease after carbon coating, which also can be seen from Table I.
A transmission electron microscopy (TEM) image of the carbon
coating on the graphite surface is presented in Fig. 2. The highly
ordered structure with well-defined layer planes is clearly evident. It
is believed that the lattice image of the layers is observed along the
[110] direction of the coated carbon with TEM. In other words, the
coated carbon covers the original graphite particles with highly ori-
ented (110) planes toward their surface. This kind of treatment of
natural graphite, by covering the active surface, is thought to be an
alternative way to suppress PC decomposition and exfoliation of the
graphite.
Figure 2. TEM image of the carbon coating on the surface of natural
graphite.
Because the principal goal of coating carbon on the natural
graphite surface is to prevent PC decomposition and graphite exfoli-
ation, all carbon-coated natural graphite samples, along with the
original bare natural graphite, were first tested in the PC-based elec-
trolytes to study the effect of carbon coating on the suppression of
PC decomposition. Figure 3 shows typical first-cycle cyclic voltam-
Table I. Some physical properties of the original and carbon-
coated graphite samples.
Amount of
Brunauer, Emmett, Average
Carbon coating and Teller method
particle
size
(␮m)
on graphite
(wt %)
surface area
(m2/g)
d002
Lc(002)
(nm)
(nm)
101.
18.6
13.4
17.6
5.6
4.5
3.1
2.5
12.6
18.5
20.4
22.3
0.33544 102.4
0.33555 183.8
0.33553 135.2
0.33553 193.3
Figure 3. CVs of the graphite electrodes in PC-based electrolyte PC:DMC
(1:1.86 by volume)/1 M LiPF6 for the first cycle: (a) natural graphite coated
with 17.6 wt % carbon; (b) original natural graphite. Scan rate: 0.1 mV/s.
Downloaded on 2012-12-08 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

Journal of The Electrochemical Society, 147 (4) 1245-1250 (2000)
1247
S0013-4651(99)08-083-0 CCC: $7.00 © The Electrochemical Society, Inc.
ly).13-16 During the anodic sweep, there are also three small peaks
which can be attributed to the deintercalation of lithium ions from
the graphite electrode. These peaks appear symmetrical to the three
small peaks in the cathodic sweep. However, the reversible capacity,
which is mainly composed of the integrated area under the three
peaks is rather small. We observed that a large part of the graphite
was stripped from the copper foil substrate and fell to the bottom of
the three-electrode cell after the first-cycle sweep. Thus, it is expect-
ed that the small reversible capacity of natural graphite in the PC-
based electrolyte, e.g., PC:DMC (1:1.86 by volume)/1M LiPF6,
originates from the residual graphite bound to the copper foil sub-
strate. This agrees with the conclusion of Aurbach et al. that besides
the exfoliation of graphite, fractures leading to electronic disconnec-
tion of graphite particles from the bulk contribute to electrode degra-
dation.17 In contrast to the cyclic voltammogram of the original nat-
ural graphite in PC-based electrolyte, that of carbon-coated natural
graphite demonstrates different performance in PC-based elec-
trolyte. In the CV of natural graphite coated with 17.6 wt % carbon,
as drawn in the thick solid curve, the irreversible peak near 0.5 V vs.
Liϩ/Li is barely visible, whereas the reversible peaks due to stage
transformations of lithium-graphite intercalation compounds are
prominent and clear. The disappearance of the irreversible peak
results from the effective suppression of PC decomposition and
graphite exfoliation by carbon coating. However, other than this phe-
nomenon, the CV shows no signs of lithium insertion into or dein-
sertion from the carbon-coating phase. By comparing the CVs of the
carbon-coated and original natural graphite electrodes in the PC-
based electrolyte, the superiority of carbon-coated natural graphite
to the original bare natural graphite becomes clear.
tained. Moreover, the plateaus due to lithium intercalation-deinter-
calation can be clearly observed below 0.2 V.
For comparison with the charge-discharge characteristics of the
graphite electrodes in the PC-based electrolytes as discussed above,
the charge-discharge tests were also carried out for graphite elec-
trodes in the EC-based electrolyte. Figure 5 shows typical first-cycle
charge-discharge curves of the carbon-coated and original natural
graphite electrodes in EC:DMC (1:2 by volume)/1 M LiPF6. In the
first-cycle discharge curve of the original natural graphite electrode,
two shoulder-like plateaus at about 1.4 and 0.6 V can be seen. These
plateaus can be attributed to the decomposition of the electrolyte to
build up a solid electrolyte interphase (SEI) on the graphite surface.
For the carbon-coated natural graphite electrodes, these types of
plateaus are difficult to detect in the first-cycle discharge curves.
Instead, the discharge curves drop down smoothly from the OCV to
about 0.2 V. For all the graphite electrodes, coated with carbon or
not, in the potential range from 0.2 to 0 V the charge-discharge
curves demonstrate plateaus relating to lithium intercalation-deinter-
calation. Moreover, for natural graphite coated with 17.6 wt % car-
bon, the irreversible capacity is remarkably decreased compared to
that of the original natural graphite.
To gain more insight into the electrochemical performance of
carbon-coated natural graphite, we tested the charge-discharge char-
acteristics of graphite electrodes in electrolytes with different com-
ponents. Table II lists some of these results. The blanks in this table
indicate cases for which the plateaus at ca. 0.7 V due to PC decom-
position and exfoliation of graphite electrodes are very long, as
Corresponding to the CVs, the effect of carbon coating can also
be observed in the charge-discharge tests. As shown in Fig. 4a, in the
first-cycle discharge curve of the original natural graphite in PC-
based electrolyte, a very long plateau can be observed near 0.7 V.
This long plateau is due to PC decomposition and exfoliation of the
graphite electrode. This plateau is so long that it exceeded the test-
ing time scale in our experiment, and no charge capacity could be
obtained. In contrast, in the first-cycle charge-discharge curve of nat-
ural graphite coated with 17.6 wt % of carbon in the same PC-based
electrolyte (Fig. 4b), considerable charge capacity could be ob-
Figure 4. Charge-discharge curves of graphite electrodes in PC-based elec-
trolyte PC:DMC (1:1.86 by volume)/1 M LiPF6 for the first cycle: (a) origi-
nal natural graphite; (b) natural graphite coated with 17.6 wt % carbon.
Figure 5. Charge-discharge curves of graphite electrodes in EC-based elec-
trolyte EC:DMC (1:2 by volume)/1 M LiPF6 for the first cycle.
Downloaded on 2012-12-08 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

1248
Journal of The Electrochemical Society, 147 (4) 1245-1250 (2000)
S0013-4651(99)08-083-0 CCC: $7.00 © The Electrochemical Society, Inc.
Table II. Charge-discharge characteristics of graphite electrodes in different electrolytes.
Amount of
carbon coating
(wt %)
EC:DMC
(1:2)
1 M LiPF6
PC:DMC
(1:4)
1 M LiPF6
PC:DM
(1:3)
1 M LiPF6
PC:DMC
(1:2.33)
1 M LiPF6
PC:DMC
(1:2.08)
1 M LiPF6
PC:DMC
(1:2)
1 M LiPF6
PC:DMC
(1:1.86)
1 M LiPF6
101.
18.6
13.4
17.6
392.0a
341.7b
50.3c
408.6
313.1
95.5
450.9
314.7
136.2
69.8%
467.1
328.0
139.1
70.2%
383.4
291.8
91.6
582.9
307.9
275.0
52.8%
434.5
318.5
116.0
73.3%
403.5
308.7
94.8
—
—
—
—
—
—
87.2%d
411.6
347.5
64.1
76.6%
436.0
330.4
105.6
75.8%
337.0
295.1
42.5
87.4%
360.7
333.4
27.3
84.4%
381.2
335.1
46.1
87.9%
357.4
337.8
19.6
677.8
270.7
407.1
39.9%
427.4
311.8
115.6
73.0%
76.1%
377.1
332.0
45.1
76.5%
397.3
316.8
80.5
430.3
305.4
124.9
71.0%
422.5
303.3
119.2
71.8%
94.5%
92.4%
88.0%
79.7%
a The discharge capacity at the first cycle (mAh/g).
b The charge capacity at the first cycle (mAh/g).
c The irreversible capacity at the first cycle (mAh/g).
d Coulombic efficiency at the first cycle.
shown in Fig. 4a. From Table II, some general understanding can be
acquired; the more PC content in the electrolyte, the more dramati-
cally PC decomposes and the more easily graphite electrodes exfo-
liate. In addition, the more carbon that is coated on the graphite’s
surface, the more compatible are the graphite electrodes toward PC-
based electrolytes.
provides a buffer phase between the graphite and the electrolyte. The
decomposition of the electrolytes on this phase to build up the SEI
film passivating the electrodes is much milder than for pure graphite.
As the carbon coating thickness increases, the coverage of this phase
over the graphite particles becomes denser and thus protects the
graphite from electrolyte attack more effectively. From Fig. 7, it can
be observed that as the carbon coating amount increases, the
coulombic efficiency increases.
From the data listed in Table II, we provide two figures (Fig. 6
and 7) to show the relationships between the amount of carbon coat-
ing and the irreversible capacity and coulombic efficiency, respec-
tively. From Fig. 6, it can be seen that for each kind of electrolyte, as
the coating amount increases, the irreversible capacity becomes
smaller. Some previous studies have indicated that the irreversible
capacity of carbonaceous anode materials is closely related to their
specific surface areas.2,18 In this study, the trend appears to be simi-
lar because as the carbon coating amount on the surface of natural
graphite particles increases, the specific surface area of carbon-coat-
ed graphite decreases as illustrated in Table I, and then the irre-
versible capacity decreases accordingly. However, it is assumed that
the remarkable decrease of irreversible capacity with the increase of
carbon coating amount cannot be simply explained by the fact that
the specific surface area of graphite decreases with increasing car-
bon coating amount. Rather, it is believed that the carbon coating
As illustrated above, the electrochemical performance of carbon-
coated natural graphite is quite different from that of the original nat-
ural graphite, so it can be expected that for a single particle of car-
bon-coated natural graphite, the electrochemical performance of the
shell part must differ from that of the core part. That is, the carbon
coating may store some lithium in a distinct way compared with that
of natural graphite in the core. However, in our previous attempts
using techniques such as in situ X-ray diffraction, cycle voltamme-
try, and charge-discharge tests, it was difficult to differentiate the
role each part plays in determining the overall electrochemical per-
formance (especially in the case of EC-based electrolytes). Here, we
applied a more powerful technique, 7Li-NMR, to clarify the storage
7
sites for lithium in carbon-coated natural graphite. In the Li-NMR
Figure 6. Relationship between carbon coating amount and irreversible
Figure 7. Relationship between carbon coating amount and coulombic effi-
capacity of graphite electrodes for the first cycle in different electrolytes.
ciency of graphite electrodes for the first cycle in different electrolytes.
Downloaded on 2012-12-08 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

Journal of The Electrochemical Society, 147 (4) 1245-1250 (2000)
1249
S0013-4651(99)08-083-0 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 8. 7Li-NMR spectra of fully
lithiated carbons: (a) original natural
graphite; (b) natural graphite coated
with 17.6 wt % carbon; (c) MCMB 6-
10.
7
graphite (Fig. 8a), the peak at 43.74 ppm in Li-NMR spectra of
fully lithiated carbon-coated natural graphite can be ascribed to lithi-
um intercalated into graphite (LiC6).19,20 For the peak at 15.83 ppm,
spectra of fully lithiated carbon-coated natural graphite, as shown in
Fig. 8b, there are two peaks at 43.74 and 15.83 ppm, respectively.
Compared with the 7Li-NMR spectra of fully lithiated natural
Figure 9. 7Li-NMR spectra of fully lithiat-
ed natural graphite coated with different
amounts of carbon.
Downloaded on 2012-12-08 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

1250
Journal of The Electrochemical Society, 147 (4) 1245-1250 (2000)
S0013-4651(99)08-083-0 CCC: $7.00 © The Electrochemical Society, Inc.
graphite show that there are two types of storage sites for lithium
insertion in this kind of carbon material: the graphite core part for
lithium intercalation and the soft-carbon-type shell part for lithium
storage. Moreover, the ratio of the peaks’ intensities (the intensity of
the peak at ca. 15 ppm to that of the peak at ca. 44 ppm) in the 7Li-
NMR spectra of fully lithiated carbon-coated natural graphite bears
a linear relationship to the amount of carbon coating.
Saga University assisted in meeting the publication costs of this article.
References
1. A. N. Dey and B. P. Sullivan, J. Electrochem. Soc., 117, 222 (1970).
2. R. Fong, U. von Sacken, and J. R. Dahn, J. Electrochem. Soc., 137, 2009 (1990).
3. G. Eichinger, J. Electroanal. Chem., 74, 183 (1976).
4. M. Arakawa and J. Yamaki, J. Electroanal. Chem., 219, 273 (1987).
5. H. Nakamura, H. Komatsu, and M. Yoshio, J. Power Sources, 62, 219 (1996).
6. C. Wang, H. Nakamura, K. Komatsu, M. Yoshio, and H. Yoshitake, J. Power
Sources, 74, 142 (1998).
7. Z. X. Chu, R. S. McMillan, and J. J. Murray, J. Electrochem. Soc., 140, L101
(1993).
8. Y. Ein-Eli, S. R. Thomas, and V. R. Koch, J. Electrochem. Soc., 144, 1159 (1997).
9. G. H. Wrodnigg, T. M. Wrodnigg, J. O. Besenhard, and M. Winter, Electrochem.
Commun., 1, 148 (1999).
10. I. Kuribayashi, M. Yokoyama, and M. Yamashita, J. Power Sources, 54, 1 (1995).
11. G. Okuno, K. Kobayakawa, Y. Sato, T. Kawai, and A. Yokoyama, Denki Kagaku
oyobi Kogyo Butsuri Kagaku, 65, 226 (1997).
12. W. Qiu, G. Zhang, S. Lu, and Q. Liu, Chin. J. Power Sources, 23, 7 (1999).
13. J. R. Dahn, Phys. Rev. B: Condens. Matter, 44, 9170 (1991).
14. T. Ohzuku, Y. Iwakoshi, and K. Sawai, J. Electrochem. Soc., 140, 2490 (1993).
15. N. Takami, A. Satoh, M. Hara, and T. Ohsaki, J. Electrochem. Soc., 142, 2564
(1995).
16. D. Aurbach, M. Levi, and E. Levi, J. Electroanal. Chem., 421, 79, 89 (1997).
17. D. Aurbach, Y. Ein-Eli, O. Chusid, Y. Carmeli, M. Babai, and H. Yamin, J. Elec-
trochem. Soc., 141, 603 (1994).
18. F. Disma, L. Aymard, L. Dupont, and J.-M. Tarascon, J. Electrochem. Soc., 143,
3959 (1996).
Figure 10. Relationship between the ratio of the peaks’ (ca. 15 and 44 ppm)
intensities in 7Li-NMR spectra and the coating amount of carbon on the
graphite surface.
we take the 7Li-NMR spectra of fully lithiated MCMB 6-10 (Fig. 8c)
as a reference; i.e., there is one peak at ca. 11.43 ppm. Because these
peaks’ chemical shifts are near each other and MCMB 6-10 is a kind
of soft carbon, we can attribute the peak at 15.83 ppm to the pres-
ence of lithium in the crystallized soft-carbon-type shell part.20-25
7
Moreover, from the Li-NMR spectra of fully lithiated natural
graphite coated with different amounts of carbon, as shown in Fig. 9,
we find that the ratios of these two peaks’ intensities change with the
amount of carbon coating. Figure 10 shows the relationship between
the ratio of the peaks’ intensities (the intensity of the peak at ca. 15
ppm to that of the peak at ca. 44 ppm) and the amount of carbon
coating. A linear relationship can be clearly observed. In turn, from
7
the ratio of these two peaks’ intensities in the Li-NMR spectra of
the fully lithiated carbon-coated natural graphite, the coating amount
can be calculated from the straight line in Fig. 10.
19. J. Conard and H. Estrade, Mater. Sci. Eng., 31, 173 (1977).
20. K. Zaghib, K. Tatsumi, Y. Sawada, S. Higuchi, H. Abe, and T. Ohsaki, J. Elec-
trochem. Soc., 146, 2784 (1999).
21. Y. Jung, M. Suh, H. Lee, M. Kim, S. Lee, S. Shim, and J. Kwak, J. Electrochem.
Soc., 144, 4279 (1997).
22. K. Tatsumi, T. Akai, T. Imamura, K. Zaghib, N. Iwashita, S. Higuchi, and Y. Sawa-
da, J. Electrochem. Soc., 143, 1923 (1996).
23. N. Takami, A. Satoh, T. Ohsaki, and M. Kanda, Electrochim. Acta, 42, 2537 (1997).
24. S. Yamazaki, T. Hashimoto, T. Iriyama, Y. Mori, H. Shiroki, and N. Tamura, J. Mol.
Struc., 441, 165 (1998).
Conclusions
Carbon-coated natural graphite was prepared by a TVD tech-
nique. This material shows superior electrochemical performance as
an anode for lithium-ion batteries in both EC- and PC-based elec-
trolytes compared to original natural graphite. The amount of carbon
coating on the natural graphite can affect the electrochemical per-
formance significantly; as the coating amount increases, the irre-
versible capacity decreases while the coulombic efficiency increas-
es. 7Li-NMR spectra of the fully lithiated carbon-coated natural
25. Y. Mori, T. Iriyama, T. Hashimoto, S. Yamazaki, F. Kawakami, H. Shiroki, and T.
Yamabe, J. Power Sources, 56, 205 (1995).
Downloaded on 2012-12-08 to IP 138.87.11.21 address. Redistribution subject to ECS license or copyright; see www.esltbd.org