Download New Type of Carbon/Carbon Composite as Anode Material for High

Journal of New Materials for Electrochemical Systems 9, 353-358 (2006)
© J. New Mat. Electrochem. Systems
New Type of Carbon/Carbon Composite as Anode Material
for High Power Li-ion Cells
∗
1
J.M. Skowroński1,2 , S. Błażewicz3 , M. Walkowiak2
Poznań University of Technology, Institiute of Chemistry and Technical Electrochemistry, ul Piotrowo 3, 60-965 Poznań, Poland
2
Central Laboratory of Batteries and Cells, ul. Forteczna 12, 61-362 Poznań, Poland
3
Academy of Mining and Metallurgy, Department of Special Ceramics, al A. Mickiewicz 30, 30-059 Kraków, Poland
Received: February 14, 2006, Accepted: May 27, 2006
Abstract: Phenol formaldehyde resin filled with two-directionally arrayed graphite fibers was thermally treated at 2200◦ C in
argon to give carbon-carbon composite plates. Anodes for lithium-ion cells were prepared by both simple cutting out disks electrodes
and milling C/C composite plates followed by mixing the resulting carbon particles with polymer binder. All the powder type anodes
consisted of carbon particles smaller than 32 μm in diameter. A very high cyclic reversibility was reached (94%) for the first cycle
of disk anodes, but discharge capacity appeared to be unsatisfactory. The number increased 2.5 times for anodes composed of milled
composites, whereas their cyclic reversibility persisted unchanged. These anodes exhibited particularly advantageous behavior when
discharged at high rates. Their discharge capacities appeared to be 40% and 19% higher as compared to anodes composed of milled
graphite fibers as well as graphite flakes, respectively. It was shown that the milling operation carried out excessively long exerted a
negative influence on the electrochemical parameters of anodes. The electrochemical results are discussed with respect to XRD and
SEM data.
Keywords: inorganic compounds, nanostructures, electron microscopy, X-ray diffraction, electrochemical properties.
graphite materials have also a number of disadvantages,
among which the most important are slow lithium ion diffusion rate and incompatibility with some electrolytes. One
of the ways to improve the electrochemical performance of
graphitic materials is their coating with a layer of unorganized carbon, and thus creating a carbon/carbon-type
composite. There are roughly two kinds of methods of
achieving this goal. The first one (carbonization methods) is based on mixing graphite material with carbon
precursor followed by carbonization at a high temperature. In the second group of methods (vapor deposition
methods), carbon is deposited onto the graphite particles
from vapor phase. The first report on C/C composites
as potential anodes for Li-ion cells comes from the year
1995. In their work Kuribayashi et al. [1] studied materials obtained by heat treatment of the mixture of various
graphitic materials and pitch blended phenol resins. The
authors classified their composites as core-shell structured
materials. More recently, Liu et al. [2] examined another
material of this type, in which the graphite core was encapsulated in non-graphitizable coke shell. They found this
electrode material to have excellent characteristics, such as
1. INTRODUCTION
Along with the development of the so-called lithium-ion
(rocking chair, shuttle-cock, swing) energy storage technology at the end of the twentieth century, carbon materials have found new rich area of application. This is due
to the fact that in spite of intensive investigations, carbons (especially graphites) are still considered as the best
option for anode materials for Li-ion batteries. Among
carbon materials studied as potential anodes are natural
and synthetic graphites (including their derivatives such
as expanded graphites), graphite fibers, non-graphitized
carbons (both soft and hard), doped carbons, carbon composites and nanotubes. In spite of the existence of so many
forms of carbon, various kinds of graphite are most commonly chosen as practical anodes in commercial batteries.
This is caused by certain favorable features of graphites
such as high reversible capacity, relatively low irreversible
capacity, flat voltage profile with the average lithium intercalation voltage close to zero volts (vs. Li/Li+ ). However,
∗ To
whom corresponding to: E-mail: [email protected],
Fax: +48 61 665 2571
353
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high capacity, enhanced kinetics of lithium transport and
compatibility with the electrolyte. Very promising results,
especially in terms of reduced irreversible capacity, were
obtained also by Yoshio et al. [3], who synthesized their
graphite-carbon composite using so-called thermal vapor
deposition technique. There exist reports on carbon fibers
(not graphitized) as the core component in C/C composites regarded from the point of view of lithium intercalation
[4-7]. Takamura et al. [6] proved that a poor cycleability
of a mesophase carbon fibers (prepared at 950◦ C) of a low
crystalline perfection can be considerably improved after
coating with epoxy resin carbon. Recently, Skowroński et
al. [7] showed that the cyclic discharge/charge efficiency
as high as 93% can be attained in the first cycle for PANbased fibers after coating with pyrolytic carbon at 960◦ C
followed by heat treatment at 2200◦ C. Up to now there
have been no reports on lithium intercalation into C/C
composites having graphitized fibers as the core.
In the present paper, a new type of C/C composite is
reported as potential anode material for Li-ion cells. The
presented material is produced in an innovatory way by
carbonization method, using graphite fibers as the core
component embedded in the non-graphitizable carbon matrix.
It is shown in this work that graphite fibers undergo peculiar transformation when heated together with the carbon precursor. It is suggested that this phenomenon can
be regarded as a general rule which applies to all C/C
composites obtained by carbonization methods (but not
for those obtained by vapor deposition methods) and can
convincingly account for their enhanced rate performance.
2. EXPERIMENTAL
Carbon/carbon composites were obtained from high
modulus graphite fibers (Thornel K-1100) and phenolformaldehyde resin used as carbon precursor. The first
step in the production of polymer-based composites was
the formation of a single layer prepreg with unidirectional arrangement of fibers infiltrated with the solution of
phenol-formaldehyde resin in ethanol. The prepregs were
air dried at 50 o C under vacuum, to remove the solvent.
Next, appropriately cut layers of the unidirectional fiber
prepreg were put into the form, pressed and cured at about
160 o C. Compression molding was accomplished by placing the preform into a matched die preheated up to 90
o
C. By stacking prepregs in two directions, 2D composites
were obtained. These samples were carbonized at 1000
o
C followed by heat treatment up to 2200 o C in an argon
atmosphere. In the resulting C-C composite the content
of phenol resin-based hard carbon was 30 %. This composite (denoted K-1100/ZF) was milled gently (sample K1100/ZF-R32) in an agate mortar and intensively (sample K-1100/ZF-M32) during 6 h in a mortar grinder ‘pulverisette 2’ (Fritch). For comparison, the graphite fibers
were examined both in the original state (sample K-1100)
and intensively milled (sample K-1100/M32). All milled
materials were sieved so that they consisted of particles
Figure 1. First cycle of constant current charge/discharge
test at 10 mA/g for the sample K-1100/ZF.
below 32 μm only. The results obtained on the above
materials were compared with those for graphite flakes
(Graphitwerk Kropfmühl AG, 99.98 wt.% C) smaller in
diameter than 32 μm (sample GK32).
The XRD measurements were carried out using Xray diffractometer (Phillips PW-1710, 35kV, 20mA) with
FeKα radiation (λ = 0.1937 nm). SEM images of examined materials were obtained using the scanning electron
microscopy (Tescan - Vega 5135).
The lithium insertion/deinsertion behaviour of samples
were examined in a two electrode coin cell (CR 2430-type )
with a lithium foil playing simultaneously the role of both
the counter and reference electrode. The electrolyte was
1M LiClO4 - EC/DEC (1:1 by weight). The working electrodes (except for K-1100/ZF in case of which electrodes
were prepared by simple cutting round pellets out of the
original plate) were prepared by mixing the sample (90
wt.%) with PVDF (10 wt.%) dissolved in cyclopentanone.
After spreading the slurry on the nickel grid current collector the electrodes were dried under vacuum at 140◦ C for
4 hours. The cells were assembled in glove box filled with
dry argon and then galvanostatically cycled between 0 V
and 2 V vs. Li/Li+ . For each cell five charge/discharge cycles were done at the rate of 10 mA/g of active substance,
followed by additional three cycles at the rate of 30 mA/g,
followed by additional three cycles at the rate of 100 mA/g.
3. RESULTS AND DISCUSSION
The composite carbon material examined in the present
work is produced in a form of hard, brittle plates having
ca. 1 mm in thickness. Although the fibers are sunk in the
hard carbon matrix, the fibrous morphology of the material can still easily be seen. To obtain the electrochemical
data on the composite as-received, the electrodes were prepared by simple cutting the round pellets out of the plates,
and then testing them in a coin cells with metallic lithium
as counter-electrodes. The galvanostatic curve for the first
New Type of Carbon/Carbon Composite as Anode Material for High Power Li-ion Cells
/ J. New Mat. Electrochem. Systems
355
Table 1. Electrochemical parameters of examined materials.
Sample
Q1ch 10
Q1dis 10
Irr.
Eff.
Qdis 30
K-1100/ZF
K-1100/ZF-R32
K-1100/ZF-M32
K-1100
K-1100-M32
GK32
[mAh×g−1 ]
99
240
248
301
335
369
[mAh×g−1 ]
93
220
204
272
293
342
[mAh×g−1 ]
6
20
44
29
42
27
[%]
94
92
82
90
87
93
[mAh×g−1 ]
30
205
174
229
214
291
Q1ch 10 :
Q1d is 10 :
Irr.:
Eff.:
Qd is 30 :
Qd is 100 :
Capacity
dropQdis 30
vs.Q1dis 10
[%]
68
7
15
16
27
15
Qdis 30
[mAh×g−1 ]
−
142
89
100
47
102
Capacity
dropQdis 100
vs.Q1dis 10
[%]
−
35
56
63
84
69
Charge capacity in the first cycle of constant current test at 10 mA/g
Discharge capacity in the first cycle of constant current test at 10 mA/g
Irreversible capacity in the first cycle defined as Q1ch 10 - Q1d is 10
Efficiency in the first cycle defined as (Q1d is 10 / Q1ch 10 )*100
Discharge capacity in the third cycle of constant current test at 30 mA/g
Discharge capacity in the third cycle of constant current test at 100 mA/g
Figure 2. SEM image for the sample K-1100/ZF-R32.
Figure 3. SEM image for the sample K-1100/ZF-M32.
cycle can be seen in Fig. 1, and the relevant numerical
data is given in Table 1 (sample K-1100/ZF). Small values of the first charge and discharge capacities (99 mAh/g
and 93 mAh/g, respectively, at 10 mA/g) allow the conclusion that not the whole of sample is accessible for lithium
intercalation, even at such a small charge/discharge rate.
The shape of the curves, which is characteristic of disordered carbons rather than of graphitic ones (the lack of any
signs of the stage structure, a relatively large hysteresis between the charge and discharge curves) indicates, that the
carbon shell of the composite is preferably accessible for
lithium insertion, whereas the graphite fiber core remains
non-intercalated. This is not surprising taking into account
that the electrode is rather thick and thus the transport
conditions for lithium ions are aggravated. Moreover, the
exceptionally small value of the irreversible capacity loss
(6 mAh/g; see Table 1) means that the surface area accessible for electrolyte penetration is very low which goes well
with the above mentioned observations.
In order to improve the electrochemical performance of
the examined material in terms of enhanced lithium intercalation conditions, it was decided to transform it to a powder form. The pulverization of sample was carried out in
two ways: (a) manually in a mortar (mild/non-destructive
pulverization; sample K-1100/ZF-R32) and (b) automatically in a mortar grinder (intensive/destructive pulverization; sample K-1100/ZF-M32). To understand the changes
in structure and morphology of the samples subjected to
mild and intensive powdering, the scanning electron microscopy images were taken and XRD measurements were
done. SEM images made for samples K-1100/ZF-R32 (Fig.
2) and K-1100/ZF-M32 (Fig. 3) reveal that the composite powdered in a mortar grinder is much more efficiently
pulverized as compared to the sample powdered manually.
In the latter case relatively large blocks of carbon are still
visible as well as fragments of fibers having maintained to
a large extent their original fibrous morphology. In contrast with this, SEM image for sample K-1100/ZF-M32
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Table 2. Crystallographic parameters of examined materials.
Sample
d [nm]
LC [nm]
K-1100/ZF-M32
0.3380
43
K-1100/ZF-R32
0.3385
58
K-1100-M32
0.3373
68
K-1100.
0.3365
62
d:
Interlayer distance
LC : Crystallite dimension along crystallographic c-axis
Figure 5. XRD pattern for the sample K-1100/ZF-R32.
Figure 4. Enlarged SEM image of a single graphite fibre
in the sample K-1100/ZF-M32.
reveals that this product is much more fragmented, with
poor features of the original morphology. Fig. 4 presents
SEM image made under higher magnification for one fiber
selected from the composite. XRD patterns for both samples indicate that together with changes in morphology a
significant change in the crystal structure takes place (Table 2 and Figs. 5 and 6). As can be seen for these figures,
the peaks arising for the graphite phase are predominant
on the diffraction patterns. Moreover, it is noteworthy that
the d002 interlayer spacings calculated based on the (002)
graphite peak for both the milled composites and fibers
are close to each other and similar to that for the original
fibers (Table 2). The broadening of the diffraction peaks
allows the assessment of the structural disorder which was
created in the composites due to their milling. The crystallite dimension along c-axis (Lc ) was calculated on the
basis of the Scherer equation:
LC (002) =
0.89λ
B002 cos(θ002 )
where λ is the wave length of FeKα radiation, B002 is the
width at half-maximum of the (002) diffraction peak and
θ002 is the corresponding diffraction angle. In all case the
(002) peaks were asymmetric in shape, and it was possible to extract two sub-peaks from the experimental profiles
Figure 6. XRD pattern for the sample K-1100/ZF-M32.
by fitting. However, for the simplicity, the total width was
taken for the determination of the crystallite size. Such values of Lc , although do not possess clear physical meaning,
reflect the existence of the unorganized phase in the samples. The crystallite size for the sample K-1100/ZF-M32
(43 nm) is smaller in comparison to sample K-1100/ZFR32 (58 nm) which means that the crystal structure of
the former sample was destroyed during milling (see Table 2). Intensive milling changes significantly the lithium
intercalation behaviour (see Figs. 7 and 8 and Table 1).
At the current density of 10 mA/g the reversible capacity for sample K-1100/ZF-M32 (204 mAh/g) is lowered
in relation to sample K-1100/ZF-R32 (220 mAh/g). A
probable reason for this is that prolonged powdering in
a mill introduces some disorder to the sample structure.
It is well known that the presence of turbostratic structure in graphite makes the lithium uptake in the prepared
electrodes lower. Simultaneously, the irreversible capacity
rises as the effect of greater pulverisation of the sample be-
New Type of Carbon/Carbon Composite as Anode Material for High Power Li-ion Cells
/ J. New Mat. Electrochem. Systems
357
Figure 7. First cycle of constant current charge/discharge
test at 10 mA/g for the sample K-1100/ZF-R32.
Figure 8. First cycle of constant current charge/discharge
test at 10 mA/g for the sample K-1100/ZF-M32.
cause of increased surface area (44 mAh/g and 20 mAh/g,
respectively). On comparing the galvanostatic curves one
can notice that the shape of curve for sample K-1100/ZFR32 is distinctly more “graphitic” in character. On the
curves there exist voltage plateaux characteristic of the
stage transformation. The hysteresis between charge and
discharge curves of the cycle is very small. In the case of
sample K-1100/ZF-M32 this hysteresis is markedly larger.
These observations support the conclusion that too excessive milling destroys the materials in terms of crystal structure and electrochemical performance. On the other hand,
mild powdering enhances greatly the material performance
as compared to the original composite plates (compare
data for samples K-1100/ZF and K-1100/ZF-R32 in Table
1), which can be ascribed to the improvement of transport
conditions for lithium ions.
Very interesting observations arise from comparing the
behavior of the studied materials under higher current
rates. It is not surprising that at 30 mA/g and 100 mA/g
all the materials exhibit dramatically smaller capacities
(Table 1). The most pronounced capacity drop is observed
for the original composite (sample K-1100/ZF). On comparing the figures for mildly and intensively powdered composite it is particularly striking that the first one shows
much better properties at higher rates in relation to the
second one (35% and 56% capacity drop between 10 mA/g
and 100 mA/g, respectively). For better understanding of
this phenomenon, additional electrochemical experiments
were done. The graphite fibers used for preparing the composite materials were electrochemically tested both in the
unchanged form (only cut down to ca. 1 mm long pieces;
sample K-1100) and intensively powdered in a grinder using identical conditions as for the sample K-1100/ZF-M32
(the product is denoted K-1100-M32). Finally, typical
flaky graphite (d < 32 μm) was tested (sample GK32). It
appears from data presented in Table 1 that both graphite
fibers and graphite flakes are characterized by similar and
relatively large capacity drop between 10 mA/g and 100
mA/g (63 % and 69 % respectively). Intensively milled
graphite fibers exhibit even larger capacity drop (84 %).
A very poor rate capability of graphite fibers (both for the
original and intensively milled) in comparison to the composites, especially those mildly powdered, suggests that
one should search for a factor responsible for a surprisingly
good rate performance of these composites. At this stage
of investigations it might be assumed that the shell carbon
produced by carbonization of phenol resin has so good high
rate capability that even together with such a poor component as fibers elevates the performance of the resultant
composite. However, this hypothesis might be controversial, because there is no direct evidence for such excellent
characteristics of phenol resin carbon itself. Besides, the
mentioned characteristics would have to be extraordinarily
good to balance and overcompensate the influence of the
fibers. This view, however, does not exclude the case that
the carbon shell adds certain positive value to the overall
rate performance of the composite since hard carbons are
known to have better abilities to work under higher loads
than graphites.
Another possible explanation is that, apart from a positive contribution of the carbon shell, also graphite fiber
core undergoes certain positive changes during heat treatment of the composite. These changes should account for
the enhanced transport conditions in these fibers. Indeed,
after carbonization the fiber surface loses its smoothness
and carbon fibers become brittle in contrast to flexible
fibers used for the preparation of the composites, which
is an observation of a decisive importance. This is due to
phenomena occurring at the fiber/matrix boundary due to
thermal shrinkage of sample resulting in mechanically created imperfections. Such a view is consistent with reports
evidencing that the stress-induced graphitization occurs
anomalously in the fiber/matrix interface at high temperature [8,9]. As can be seen in Figure 4, fissures and cracks
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J.M. Skowroński et al. / J. New Mat. Electrochem. Systems
are present on the fiber surface. Such structural imperfections may provide paths of fast ionic transport inside
the fiber. The destruction of the core fiber component
upon creation of the composite can be regarded as a very
particular kind of mechanical modification of these fibers.
This beneficial effect is reduced by intensive milling. This
effect is clearly noticed from comparison of the behavior
of pure untreated fibers (sample K-1100) and intensively
milled composite (sample K-1100-M32).
The enhancement of the rate capabilities has already
been reported by some authors for C/C composites obtained by carbonization technique. For example, Liu et al.
[2] studied lithium insertion/deinsertion behavior of a material composed of flaky natural graphite as the core and
epoxy resin-derived carbon as the shell. They subjected
their material to constant current charging/discharging
regimes with current densities rising from 0.50 mA/cm2
up to a maximum of 1.80 mA/cm2 (which means a 3.6-fold
increase) and found that the composite loses markedly less
of its reversible capacity upon shift to larger loads (60%
capacity drop accompanies a 3.6-fold current density increase) as compared to the natural graphite (80% capacity
drop in the same conditions). In the present work only 35%
capacity drop is observed upon a 10-fold current density
increase. If one assumes that the essential reason for the
enhancement of rate capability observed for carbonizationtype C/C composites is mechanical stress inherent to the
carbonization procedure, which is claimed in this work,
then better result presented here for fibers-based composite in comparison with flakes-based composite presented
by Liu et al. can be explained by the contribution of radial texture of fiber. Such an orientation of graphite fibers
might promote the formation of cracks along the fiber axis.
This effect is less pronounced for graphite flakes having
plane texture.
From the above considerations one can derive two general remarks. The first one is that the advantage of vapor deposition-type C/C composites comes from the fact
that they exhibit decreased irreversible capacities, whereas
the advantage of carbonization-type C/C composites arises
mainly from their enhanced rate capabilities. The second
remark is that the characteristic of the vapor depositiontype C/C composites is connected directly with the existence of unorganized carbon layer that screens the edge
planes of the graphite core, whereas the uniqueness of the
carbonization-type C/C composites results from their thermal history rather than from the actual phase configuration. It seems that this distinction between the two categories of C/C composites has not been yet exhaustively
recognized.
4. CONCLUSION
Synthesis of carbon/carbon composite creates new material, the properties of which are not simply a sum of the
properties of the individual components. In the present
work a new anode material was presented, produced on
the basis of the innovatory carbon/carbon composite hav-
ing graphite fiber as the core and phenol resin-base hard
carbon as the shell. At high current loads this material
exhibits exceptionally good capacities, which makes it a
promising anode for high power Li-ion batteries. The results obtained showed that the loss of discharge capacity
for the C/C composite-based anode due to ten times increased current density is even twice lower as compared
to that of flaky graphite-type anode. The excellent rate
characteristics of the C/C composite anodes were derived
from the existence of fast ionic diffusion paths in the fibers,
which is likely the consequence of mechanical stress exerted
on the fibers during carbonization.
5. ACKNOWLEDGEMENT
Financial support for this work from the State Committee for Scientific Research of Poland (KBN Grant No. 3
T09B 068 19) is gratefully acknowledged.
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