Investigation on the First-Cycle Charge Loss of Graphite Anodes by Coating of the Pyrolytic Carbon Using Tumbling CVD

Article abstract of DOI:10.1149/1.1639156

The first-cycle charge loss of graphite coated with pyrolytic carbon using a tumbling chemical vapor deposition (CVD) process has been studied for the active material of anodes in lithium-ion secondary batteries. Through the coating of pyrolytic carbons on the surface of graphite particles, carbon materials with a core (graphite)-shell (pyrolytic carbon) structure can be obtained. The irreversible capacity of these carbons at first charge/discharge cycle has been examined using a charge-discharge cycler. The coating of the pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity, which leads to improvement of first cycle coulombic efficiency from 87.21 to 93.32%. The irreversible capacity formed above 0.8 V vs. Li/Li⁺ at the first charging cycle is proportional to the surface area of the carbon, that is, the coating time of pyrolytic carbon. The plateau of the first-cycle charging curve disappears distinctly with the coating of pyrolytic carbon. The reduction of the initial irreversible capacity in graphite coated with pyrolytic carbon results from this disappearance of the plateau. This means that the coating layer of pyrolytic carbon is more compatible with the electrolyte than the surface of bare graphite. From electrochemical impedance spectroscopy spectra, the increase of contact resistance in bare graphite electrodes occurs at 0.8-0.2 V vs. Li/Li ⁺ in the first charging and the coating of pyrolytic carbon on the surface of graphite can suppress the increment of contact resistance.

Full text of DOI:10.1149/1.1639156

Journal of The Electrochemical Society, 151 ͑2͒ A291-A295 ͑2004͒
A291
0
013-4651/2004/151͑2͒/A291/5/$7.00 © The Electrochemical Society, Inc.
Investigation on the First-Cycle Charge Loss
of Graphite Anodes by Coating of the Pyrolytic Carbon
Using Tumbling CVD
Young-Soo Han,a,z Jae-Han Jung, and Jai-Young Lee *
b
c,
a
LG Electronics Institute of Technology, Devices and Materials Laboratory, Seoul 137-724, South Korea
LG Chemical, Limited, Battery Research Center, Taejon 305-380, South Korea
b
c
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,
Taejon 305-701, South Korea
The first-cycle charge loss of graphite coated with pyrolytic carbon using a tumbling chemical vapor deposition ͑CVD͒ process has
been studied for the active material of anodes in lithium-ion secondary batteries. Through the coating of pyrolytic carbons on the
surface of graphite particles, carbon materials with a core ͑graphite͒-shell ͑pyrolytic carbon͒ structure can be obtained. The
irreversible capacity of these carbons at first charge/discharge cycle has been examined using a charge-discharge cycler. The
coating of the pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity, which leads to
ϩ
improvement of first cycle coulombic efficiency from 87.21 to 93.32%. The irreversible capacity formed above 0.8 V vs. Li/Li
at the first charging cycle is proportional to the surface area of the carbon, that is, the coating time of pyrolytic carbon. The plateau
of the first-cycle charging curve disappears distinctly with the coating of pyrolytic carbon. The reduction of the initial irreversible
capacity in graphite coated with pyrolytic carbon results from this disappearance of the plateau. This means that the coating layer
of pyrolytic carbon is more compatible with the electrolyte than the surface of bare graphite. From electrochemical impedance
ϩ
spectroscopy spectra, the increase of contact resistance in bare graphite electrodes occurs at 0.8-0.2 V vs. Li/Li in the first
charging and the coating of pyrolytic carbon on the surface of graphite can suppress the increment of contact resistance.
©
2004 The Electrochemical Society. ͓DOI: 10.1149/1.1639156͔ All rights reserved.
Manuscript submitted March 17, 2003; revised manuscript received August 27, 2003. Available electronically January 9, 2004.
Graphite has been used widely for lithium ͑Li͒ ion secondary
surface structure of graphite particles. In the this study, we examine
the cause of the first-cycle charge loss by investigating the electro-
chemical phenomena of graphite coated with pyrolytic carbon.
batteries because of its advantages of practical energy density,
1
charge-discharge reversibility, and safety aspects. Graphite consists
of hexagonal arrays of carbon atoms arranged in benzene-type struc-
tures. The hexagonal arrays form ordered layer structures and the
layer planes are separated by 3.354 Å. The surface of graphite par-
ticles consists of the basal planes and the prismatic ͑edge͒ planes of
the hexagonal arrays. It has been recognized that the chemical reac-
tivity of the prismatic plane was much higher than that of the basal
Experimental
Coating of pyrolytic carbons.—The new type of carbon materials
with core ͑graphite͒—shell ͑pyrolytic carbon͒ structure was ob-
7
tained by the tumbling CVD process. For improving the uniformity
of the pyrolytic carbon shell, graphite particles tumbled in the rotat-
ing reactor tube during the pyrolytic carbon tumbling were depos-
ited on the surface of graphite particles. The rotation speed of the
reactor tube varied from 10 to 50 rpm. Four grooves parallel to the
tube axis were placed on the inner wall of the tube to promote the
tumbling action of the graphite particles. The coating of pyrolytic
carbon on graphite was carried out by introducing a gas mixture of
liquid propane gas ͑LPG͒ and argon under 1 atm pressure in the
temperature range from 1000 to 1200°C. The coating time varied
from 0.5 to 3 h. The concentration of LPG varied from 10 to 100%,
and the total flow rate varied from 1 to 6 L/min. The specific surface
area of each sample was determined with Brunauer-Emmett-Teller
2
,3
plane.
The irreversible capacity loss of graphite anodes in Li-ion sec-
ondary batteries originates from the decomposition of electrolyte to
form both a solid electrolyte interface ͑SEI͒ layer and gaseous prod-
ucts on the electrode during the initial charge/discharge cycles. Win-
4
ter et al. reported that both the total surface area and the average
ratio of the basal plane and edge thickness dimension influenced the
5
irreversible capacity loss. Bar-Tow et al. showed that the mecha-
nism of electrolyte decomposition was different on the basal and
prismatic planes in an electrolyte consisting of LiAsF in ethylene
6
carbonate ͑EC͒—diethyl carbonate ͑DEC͒. The SEI layer formed on
the prismatic planes is rich in inorganic compounds, whereas that
formed on the basal planes is rich in organic compounds. Zaghib
͑
BET͒ measurement with nitrogen gas. Prior to measurements, all
samples were heated at 300°C for 10 h under vacuum to remove
surface-adsorbed moisture and hydrocarbons.
6
et al. demonstrated that the distribution of basal and prismatic
Cell assembly and electrochemical measurement.—Carbon elec-
trodes were prepared by coating slurries of the above new type
carbon powder and polyvinylidene fluoride ͑PVDF͒ dissolved in
N-methyl pyrrolidinone on copper foils. After coating, the electrodes
were dried at 150°C for 3 h in vacuum (10 Torr͒ and then pressed
at about 150 kg/cm . The diameter and thickness of the circular-
shaped carbon electrodes were 16 mm and about 50 ␮m, respec-
tively. Coin-type test cells were constructed from these electrodes. A
microporous film ͑Celgard 2400͒ wetted with electrolyte ͓1 M
LiPF6 dissolved in a 50/50 volume percent ͑vol%͒ mixture of EC
DEC; Merck & Co., Inc.͔ was sandwiched between the carbon-
aceous cathode and Li metal foil anode. All cells were assembled in
an argon-filled glove box. These cells were charged and discharged
in the potential range of 0 to 2 V vs. Li/Li using a galvanostatic
cycler ͑Toscat-3100U; Toyo Corp.͒. The constant current density
was 18.6 mA/g ͑0.05 C͒.
planes was associated with the extent of electrolyte decomposition
and the prismatic planes ͑edge sites͒ were active sites for chemical/
electrochemical reactions.
The factors responsible for the irreversible capacity loss are still
a subject of intense research and debate. Both the catalytic proper-
ties and physical structure of graphite are believed to play important
roles in the irreversible capacity loss. In an effort to investigate the
irreversible capacity loss of graphite electrode for Li-ion secondary
batteries, we made new-type carbon materials with a core
Ϫ3
2
͑
graphite͒-shell ͑pyrolytic carbon͒ structure by a tumbling chemical
7
vapor deposition ͑CVD͒ process. In previous work, we reported
that the coating of pyrolytic carbon could reduce significantly the
irreversible capacity without the loss of reversible capacity. The
coating of pyrolytic carbon is a useful process for modifying the
ϩ
The irreversible capacity was defined as the difference between
the charge and discharge capacity at the first cycle. For a detailed
examination of the charge loss we subdivided the first charge curve
*
Electrochemical Society Active Member.
E-mail: hyszzz@lge.com
z

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Journal of The Electrochemical Society, 151 ͑2͒ A291-A295 ͑2004͒
Figure 2. Irreversible capacity, Cirr , in A-B, B-C, C-D sectors of the graph-
ite coated with pyrolytic carbons at various coating times.
Figure 1. Typical charge-discharge profile of a graphite anode for the first
cycle. The charge curve has been subdivided into three potential sectors;
A-B, B-C, C-D.
surface area. The irreversible capacity of graphite that is highly or-
dered carbon mainly results from the SEI layer formation on the
surface. So the surface area and structure are very important factors
for estimating the irreversible capacity of graphite. The irreversible
in three sectors ͑A-B, B-C, and C-D͒ as shown in Fig. 1. Cirr,A-B and
Cirr,B-C are the irreversible capacities in the sector A-B ͑above 0.8 V
vs. Li/Li ) and sector B-C ͑between 0.8 and 0.2 V vs. Li/Li ),
ϩ
ϩ
ϩ
capacity formed in above 0.8 vs. Li/Li ͑in sector A-B͒ is results
respectively. Cirr,C-D means the irreversible capacity formed below
from a reaction of SEI layer formation that increases proportionally
with the surface area of carbon material. The irreversible reactions
in sector A-B are mainly associated with the surface area rather than
the surface structure of graphite ͑highly ordered carbon͒ and pyro-
lytic carbon ͑disordered carbon͒. These reactions are side reactions
from the reduction of solvent on the surface of carbon materials, that
is, the formation of the SEI layer which consists of Li CO ,
ϩ
the potential of 0.2 V vs. Li/Li . In order to estimate the internal
resistance of the carbon electrodes at each potential, the impedance
spectrum was investigated using electrochemical impedance spec-
troscopy ͑EIS͒. A Solatron 1255 frequency response analyzer was
used in conjunction with the Solatron 1286 electrochemical inter-
face. The electrochemical impedance measurement was carried out
by applying an ac voltage of 5 mV over the frequency range from 1
2
3
8,9
CH OCO Li, Li O, etc.
2
2
2
mHz to 100 kHz for the carbon electrodes whose potentials were 0,
Figure 4 shows the variation of irreversible capacity in sector
B-C for the coating time of pyrolytic carbon. The Cirr,B-C decreases
drastically up to 1 h and then saturates for the coating time. As the
coating time increases, the surface of the graphite is covered gradu-
ally with pyrolytic carbon and the irreversible capacity which is
ϩ
0
.2, and 0.8 V vs. Li/Li at the first charging.
Results and Discussion
New type of carbon materials with a core-shell structure were
ϩ
synthesized from the coating of pyrolytic carbon on the surface of
graphite in the tumbling reactor tube. The electrochemical properties
of these new carbons are characterized with charge-discharge cy-
cling. Figure 2 shows the irreversible capacities obtained from the
each sector A-B, B-C, and C-D in the first charge/discharge curve of
the graphite coated with pyrolytic carbon at various coating times.
As the coating time increases, the total irreversible capacity shows
minimum behavior. The irreversible capacities of the sector A-B and
C-D, Cirr,A-B and Cirr,C-D increase gradually with the coating time.
However, the irreversible capacity of the sector B-C, Cirr,B-C shows
the saturation after drastic decrease as the coating time increases.
The irreversible capacities of each sector are listed with the first
cycle coulombic efficiencies and BET surface areas in Table I. The
first cycle coulombic efficiency of the graphite coated with pyrolytic
carbon for 1 h is 93.32% which is much higher than that of the bare
graphite ͑87.21%͒. This result shows that the coating of pyrolytic
carbon on the surface of graphite by tumbling CVD is very effective
for improving the initial coulombic efficiency and especially de-
creasing the irreversible capacity which is formed in the potential
formed in the potential range from 0.8 to 0.2 V vs. Li/Li decreases.
However, the coating effect on the Cirr,B-C is reduced because the
surface of graphite particles are fully covered with pyrolytic carbon
after 1 h of coating time. The reduction of the total irreversible
capacity in graphite anodes by the coating of pyrolytic carbon re-
sults mainly from that of the irreversible capacity which is formed in
ϩ
the potential range from 0.8 to 0.2 V vs. Li/Li .
The charge/discharge curves in Fig. 5a are the first cycle curves
of half cells made with bare graphite and graphite coated with py-
rolytic carbon. Figure 5b shows the magnified initial part of the first
Table I. First cycle coulombic efficiency, BET surface area, and
irreversible capacity of the graphite coated with pyrolytic car-
bons.
Uncoated Coated Coated Coated Coated
Coating time
st cycle coulombic
͑0 h͒
͑0.5 h͒ ͑1 h͒
͑2 h͒
͑3 h͒
ϩ
1
87.21
90.91 93.32 92.12 91.21
range from 0.8 to 0.2 V vs. Li/Li ͑in the sector B-C͒.
efficiency ͑%͒
BET surface area ͑m /g͒
Figure 3a is the variation in the BET surface area of the graphite
coated with pyrolytic carbon for various coating times. As the coat-
ing time increases, the BET surface area shows a trend of parabolic
increase. The irreversible capacities of sector A-B are plotted in Fig.
2
14.9
15.2
15.7
18.3
22.8
Irreversible
capacity
͑mAh/g͒
Total
A-B
B-C
C-D
77.98
7.87
65.19
4.92
59.59 48.23 48.5
7.89 8.05
46.48 34.78 33.33 33.67
5.22 5.4 6.14 6.59
51.56
9.03 11.3
3b as a function of coating time. They also show a parabolic incre-
ment, which is coincident with the increasing trend of the BET

Journal of The Electrochemical Society, 151 ͑2͒ A291-A295 ͑2004͒
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Figure 5. ͑a͒ First charge-discharge profiles and ͑b͒ magnified initial charg-
ing parts in the first charge curves of the bare graphite and coated graphite
anodes.
Figure 3. Variation of ͑a͒ the BET surface area and ͑b͒ the irreversible
capacity in sector A-B of the graphite coated with pyrolytic carbons at vari-
ous coating times.
cycle charge curves in Fig. 5a. As the coating time increases, the
ϩ
charge curves shift from left to right above 0.8 V vs. Li/Li and
ϩ
from right to left below 0.8 V vs. Li/Li . In particular, the plateau
ϩ
around 0.8 V vs. Li/Li that is observed in the charge curve of the
bare graphite anode disappears by the coating of pyrolytic carbon.
These phenomena are closely associated with the change of irrevers-
ible capacity loss. That is, the coating of pyrolytic carbon on the
surface of graphite slightly increases the irreversible capacity
ϩ
formed above 0.8 V vs. Li/Li but decreases drastically the irrevers-
ϩ
ible capacity formed below 0.8 V vs. Li/Li .
1
0
Besenhard et al. reported the plateau appeared about 0.8 V vs.
ϩ
Li/Li in the first cycle charge curve of bare graphite anodes due to
the co-intercalation of solvated ions with Li ions into graphene lay-
ers as shown in Fig. 6. These reactions between the electrolyte and
the prismatic plane of graphite form SEI layers which consist of
Li (solv) C and cause large irreversible capacity loss. The carbon
x
y
6
atoms in a graphene layer have strong covalent bonding. But the
bonding between the graphene layers has a van der Waals force that
is very weak. So when the basal plane of graphite contacts an elec-
trolyte the co-intercalation of solvated ions does not occur, but it can
occur when the prismatic plane of graphite contacts the electrolyte.
On the other hand, the bonding force between graphene layers of
pyrolytic carbon is much larger than that of graphite because the
graphene layers of pyrolytic carbon are cross-linked to each other.
Figure 4. Variation of the irreversible capacity in sector B-C of the graphite
coated with pyrolytic carbons at various coating times.
11
That is, the pyrolytic carbon has a turbostratic structure. Therefore,
the co-intercalation of solvated ions cannot occur at the prismatic

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Journal of The Electrochemical Society, 151 ͑2͒ A291-A295 ͑2004͒
Figure 7. Nyquist plots of the bare graphite and coated graphite anodes at
ϩ
the potential of ͑a͒ 0.8, ͑b͒ 0.2, and ͑c͒ 0 V vs. Li/Li in the first charging.
Figure 6. Model of co-intercalation of solvated ions between the graphene
layers. ͑Adapted from Ref. 10.͒
sector is mainly due to the loss of Li ions inserted within graphite.
Internal defects ͑micropore, buckled layer, grain boundary, etc.͒ of
carbon materials can play roles of irreversible sites for lithium ions.
The irreversible capacity loss is proportional to the amount of
lithium ions trapped in the internal defects. In general, disordered
carbons ͑pyrolytic carbon͒ have many more internal defects than
plane of pyrolytic carbon having a turbostratic structure. In this
work, the decrease of irreversible capacity in graphite anodes by
coating of pyrolytic carbon can be explained as follows. Covering
prismatic planes of graphite with pyrolytic carbon having a turbos-
tratic structure effectively inhibits the co-intercalation of solvated
ions and it suppresses the formation of Li (solv) C SEI layer
x
y
6
which causes the irreversible capacity in the sector B-C.
In order to estimate the internal impedance of graphite elec-
trodes, the Nyquist plots obtained from the results of EIS analysis
are shown in Fig. 7. In the Nyquist plot, the first and second semi-
circles appear to be due to the contact resistance and charge-transfer
resistance, respectively. The last linear part represents the Warburg
impedance.12 Figure 7a shows the Nyquist plots of bare graphite and
ϩ
coated graphite anodes at 0.8 V vs. Li/Li of the first charge cycle
ϩ
͑
͑
B point in Fig. 1͒. Figure 7b and c shows those at 0.2 V vs. Li/Li
ϩ
C point in Fig. 1͒ and at 0 V vs. Li/Li ͑full charge state͒ of first
charge cycle, respectively. The effects of coating do not appear
ϩ
when the charge proceeds until 0.8 V vs. Li/Li as shown in Fig. 7a.
However, the effects of coating appear apparent after charging in the
B-C sector as shown in Fig. 7b. The contact resistance of the coated
graphite electrode is much smaller than that of the bare graphite
electrode. As mentioned above, this demonstrates that the SEI layer
of Li (solv) C does not form on the prismatic surface of graphite
x
y
6
by coating of pyrolytic carbon. EIS spectra of fully charged elec-
trodes in Fig. 7c do not differ greatly from those in Fig. 7b. This
means that the coating effects on the electrochemical resistances of
graphite electrode can be neglected in sector C-D.
Figure 8 is a plot for the variation of irreversible capacity in
sector C-D. Sector C-D is the potential range below 0.2 V vs.
ϩ
Li/Li , in which lithium ions begin to intercalate between graphene
Figure 8. Variation of the irreversible capacity in sector C-D of the graphite
layers of a graphite anode. Thus the irreversible capacity of this
coated with pyrolytic carbons at various coating times.

Journal of The Electrochemical Society, 151 ͑2͒ A291-A295 ͑2004͒
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ϩ
highly ordered carbons ͑graphite͒. From the viewpoint of the irre-
versible capacity loss owing to internal irreversible sites pyrolytic
carbon can have more irreversible capacity than graphite. Therefore,
as shown in Fig. 8, the linear increment of irreversible capacity in
sector C-D with coating time is due to the linear increment in the
amount of pyrolytic carbon covered on the surface of graphite, that
causes the increase of internal irreversible sites.
graphite particles. Below 0.2 V vs. Li/Li , the irreversible capacity
loss resulted from the trapping of lithium ions at internal irreversible
sites of pyrolytic carbon, which were associated with the internal
structure of carbon electrode materials.
LG Electronics Institute of Technology assisted in meeting the publica-
tion costs of this article.
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