Carbon
Letters
Vol. 8, No. 4 December 2007 pp. 285-291
Carbon Material from Natural Sources as an Anode in Lithium
Secondary Battery
Sunil Bhardwaj ♠, Maheshwar Sharon , T. Ishihara , Sandesh Jayabhaye ,
Rakesh Afre , T. Soga and Madhuri Sharon
1,
1
3
3
2
1
4
1
Nanotechnology Research Center, Birla College, Kalyan 421304, Maharashtra, India
Department of Applied Chemistry, Faculty of Engineering, Oita University, Oita 870-1192, Japan
3
Graduate School of Engineering, Nagoya Institute of Technology, Nagoya. Japan
4
Monad Nanotech Private Ltd., A702, Bhawani Towers, Adishankaracharya Marg, Powai, Mumbai - 400 076, India
♠e-mail: [email protected]
(Received June 14, 2007; Accepted October 24, 2007)
2
Abstract
Carbon materials of various morphologies were synthesized by pyrolysis of Soap-nut seeds (Sapindus mukorossi), Jack
Fruit seeds (Artocarpus heterophyllus), Date-seeds (Phoenix dactylifera), Neem seeds (Azadirachta indica), Tea leaves
(Ehretia microphylla), Bamboo stem (Bambusa bambus) and Coconut fiber (Cocos nucifera), without using any catalyst.
Carbon materials thus formed were characterized by SEM XRD and Raman. Carbon thus synthesized varied in size (in µm)
but all showed highly porous morphology. These carbon materials were utilized as the anode in Lithium secondary battery.
Amongst the various precursors, carbon fibers obtained from Soap-nut seeds (Sapindus mukorossi) and Bamboo stem
(Bambusa bambus), even after 100th cycles, showed the highest capacity of 130.29 mAh/g and 92.74 mAh/g respectively.
Morphology, surface areas and porosity of carbon materials obtained from these precursors were analyzed to provide
interpretation for their capacity to intercalate lithium. From the Raman studies it is concluded that graphitic nature of carbon
materials assist in the intercalation of lithium. Size of cavity (or pore size of channels type structure) present in carbon
materials were found to facilitate the intercalation of lithium.
Keywords :
Carbon materials, Lithium battery carbon, Lithium intercalation, carbon from oil seeds, carbon from fibrous plant
1. Introduction
The application of lithium battery as a portable energy
source is increasing very fast [1] especially because it can
provide as high as 3 V cell potential. Moreover, cost of
electrode is another consideration which needs to be lowered
by developing cheaper materials. Lithium metal as anode, in
conjunction with an organic electrolyte results in nonuniform formation of a passive film on the anode surface [24] causing dendrite growth of lithium metal. Lithium alloys
e.g. LiSn becomes very large during insertion/deinsertion of
lithium [5]. Hence, scientists are looking for some other
material like graphite etc which could be used as an anode in
aqueous solution.
It has been observed that electrochemical intercalation of
lithium in graphite [6] generates considerable negative
potential close to that of lithium and is less reactive and
easily reversible. Therefore, graphite like materials are
expected to facilitate intercalation and deintercalation in
lithium battery (Fig. 1). Because they can either accept or
donate electrons. As a result various attempts have been
made to use carbon nanotubes (CNT) as the anode in lithium
ion battery [3,7-9].
The intercalation of Li-ion with carbon depends upon
various factors such as pore size (either the size of the
diameter of the CNT or pore size of channels present in
fibrous carbon materials), density, surface area, and
activation of CNM etc [10]. The capacity of graphitized
carbon is theoretically calculated to be 372 mAh/g (LiC6).
One of the major disadvantage with carbon nanotubes in
comparison with either graphite or coke as a raw material is
the high cost of production [11].
Carbon nanomaterials have been synthesized mainly from
precursors which are derived from either fossil materials or
petroleum based products. These fossil materials are destined to deplete. Therefore, search for precursors which are
derived from plants especially the waste part or non-edible
part of plants was thought of. Sharon et al. [12] have been
making efforts to find out suitable plant derived precursors
to synthesize carbon nano-materials. They have also [3, 1316] studied the intercalation of lithium with carbon nanobeads synthesized from camphor. As an extension of their
work, precursors from various other plant based materials
like oil seeds, fibrous stems like baggas, cotton etc. were
selected to synthesize carbon materials and examine whether
they could give better results than what was obtained with
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Sunil Bhardwaj et al. / Carbon Letters Vol. 8, No. 4 (2007) 285-291
A schematic diagram to show how the intercalation takes
place in the host lattice (e.g. graphite) and charging/discharging
process in lithium battery.
Fig. 1.
Schematic diagram showing the apparatus for the preparation of carbon materials by pyrolysis of natural precursors.
Fig. 2.
carbon nano beads prepared from camphor.
We have examined 30 different plant based precursors for
this purpose, but in this paper, we report the result of lithium
intercalation by carbon materials obtained from the pyrolysis
of some selected oil seeds and stems: Soap-nut seeds
(Sapindus mukorossi), Jack Fruit seeds (Artocarpus
heterophyllus), Date-seeds (Phoenix dactylifera ), Neem
seeds (Azadirachta indica), Tea leaves (ehretia microphylla),
Bamboo stem (Bambusa bambus) and Coconut fiber (Cocos
nucifera), because carbon materials obtained from these
precursors showed better capacity for Lithium intercalation,
as compared to results obtained with other precursors.
2. Experimental:
2.1. Preparation of Carbon
Soap-nut seeds (Sapindus mukorossi), Jack Fruit seeds
(Artocarpus heterophyllus), Date-seeds (Phoenix dactylifera ),
Neem seeds (Azadirachta indica), Tea leaves (ehretia
microphylla), Bamboo stem (Bambusa bambus) and Coconut
fiber (Cocos nucifera) were washed with water and dried in
oven for three hours at 100oC. Known weight of precursor
was kept in a quartz boat and inserted into a quartz tube kept
in the furnace (Fig. 2). Furnace was thoroughly closed and
flushed with Nitrogen gas for 15 min to flush out the air
from the quartz tube. Through out the pyrolysis as well as
during cooling the furnace, the flow of Nitrogen was
maintained at 50 cc/min.
The furnace was heated to 850oC and maintained at that
SEM micrographs of carbon materials obtained by the
pyrolysis of various plant based precursors.
Fig. 3.
temperature for 3 h to complete the process of pyrolysis.
After cooling the furnace, carbon material was taken out and
powdered using motor pestle and were weighed to find out
the % yield. Carbon was then purified by soaking in 3M
HNO3 for 3 h followed by several washes with distilled
water and finally dried at 100oC. Purified carbon materials
were characterized and used as anode in lithium battery.
2.2. SEM characterization
SEM (FEI quanta 200) of carbon materials obtained by the
pyrolysis was examined (Fig. 3). SEM micrograph showed
presence of pores and cavity. The size of cavity or pore size
of channels present in carbon materials was also calculated
from SEM micrographs (Table 1).
SEM micrograph of carbon from Tea leaves appears like
bee hives filled with small pebbles of about 1 µm diameter.
The magnified view of SEM micrograph suggests that each
pebble is made of clusters of carbon nano beads of about
Carbon Material from Natural Sources as an Anode in Lithium Secondary Battery
287
IG/ID Ratio, Pore Size and Surface Area Obtained with Carbon Materials Obtained from Various Precursors
Pore size
Surface Area
Carbon material obtained
Capacity (mAh/g)
IG/ID
ratio
(µm)
(m2/g)
From following plant’s precursors
after 100th
Tea (Ehretia microphylla)
133.76
0.999
1.25
96.21
Coconut fiber (Cocos nucifera)
137.5
1.007
5-9
55.59
Bamboo(Bambusa bambus)
192.74
1.025
8.15
49.17
Soap-nut seed (Sapindus mukorossi),
130.29
1.025
8.25
114.03
Jack fruit seed (Artocarpus heterophyllus)
115.90
0.998
114.05
Date-seed (Phoenix dactylifera)
115.31
0.990
18.25
29.22
Neem seed (Azadirachta indica)
110.71
0.998
12.25
14.25
Table 1.
500 nm diameter.
Carbon from coconut fiber is heavily porous in nature and
appears like a strainer. The pores of carbon have varying
sizes ranging from 5-9 µm diameter. Carbon from bamboo
has structure composed of bundles of channels lined together
of approximately 10 µm diameter.
Soap-nut seeds yielded porous carbon block which looked
like cotton balls. It had disorganized pores all over the
surface. Size of pores is around 8 µm diameter.
Carbon fibers obtained from jack fruit seed also has cotton
ball like appearance with disorganized pores all over on the
surface having similar pore size as that of the soap-nut
carbon.
Carbon from date-seeds is like fossilized porous rock with
large pores (18.25 µm). Carbon from neem seed appears like
rectangular block with some cavities on the surface. Details
of the pore sizes of these carbon materials, as calculated
from the SEM micrograph are given in Table 1.
2.3. Surface area measurement
Adsorption of methylene blue in liquid phase by natural
solids: activated carbon, charcoal, graphite, and silica for
specific surface area determination has been widely adopted
[17, 18]. In absence of BET surface area measurement unit,
it was decided to measure the surface area by methylene
blue technique.
Various concentrations (ranging from 0.1 × 10− 5 to 1 × 10− 5
M) of Methylene blue (Lobe Chemie) were prepared in
distilled water and their absorbance at its λmax (670 nm) were
recorded with a spectrophotometer (Systronic Double Beam
Spectrophotometer 2203). These optical densities were
plotted against concentration of methylene blue. This graph
was used as the standard graph for the calculation of
methylene blue present in the measuring samples. The molar
extinction coefficient was calculated which lies in the range
of (3.9 − 9.5) × 10− 4. This is similar to the reported value by
Bergman and O’Konski [17].
0.025 g of carbon powder was added to 30 cc of methylene blue solution of known concentration (5 × 10− 5 M). The
mixture was stirred until the carbon got submerged in the
dye solution. This mixture was kept at room temperature for
24 h and shaken periodically. The optical density of the
supernatant liquid was determined. The concentration of
methylene blue adsorbed by the carbon nanomaterial was
calculated from the difference between the methylene blue
concentration before and after adsorption onto the carbon by
using the standard graph. From this difference in concentration of Methylene blue the surface area of carbon was
calculated by using equation-1.
(1)
S(surface area, km2/kg) = NgaMBN10− 20/M
where Ng, aMB, N and M are number of molecule of
methylene blue adsorbed at the surface of carbon (i.e., Ng =
Nm * M), surface area of one molecule of methylene blue
(which is 197.2 Å2), Avagador’s number (6.02 × 1023 mole− 1)
and molecular weight of methylene blue (373.9 g mol− 1)
respectively. Nm is the number of moles of methylene blue
per gram of carbon required to form a monolayer. 10− 20 is
the conversion factor to get km2/kg.
Results of the surface area of carbon materials are shown
in table-1. Surface area calculated with methylene blue may
not be as accurate as the BET measurement, because the
adsorption of methylene blue over the carbon material may
be due to either of the scheme as shown in Fig. 4. If shape of
the methylene blue is like cylindrical type, surface area will
depend upon the orientation of methylene molecule with
A schematic description of possibilities of methylene
blue adsorbing on the carbon materials. If methylene molecule
takes the shape of cylindrical type, then it can get adsorbed by
either attaching as A or B. If spherical then it has one possibility
as C.
Fig. 4.
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Raman spectra of carbon materials prepared from various plant based precursors showing their relative intensities for
G- and D-bands.
Fig. 5.
carbon materials. If adsorbed like “A” (Fig. 4) then the
calculated surface area will be larger than if it is adsorbed as
“B”. However if it is spherical “C”, then it will give more or
less same result as one would obtain with BET technique.
Assuming the molecule of methylene blue to be spherical,
Chongrak Kaewprasit et al [18] observed that the surface
area of cotton fiber calculated by methylene blue technique
was 32 times more than what observed with BET measurements. Whether the same result would be obtained with
carbon fiber needs to be checked.
Results of surface area (table-1) suggest that surface area
of carbon material is highest for carbon material obtained
from Jackfruit seed (114.05 m2/g) and Soap-nut seed (114 03
m2/g).
2.4. Raman spectroscopic studies
Raman spectra (JASCO, NRS-2100) of each carbon
materials were taken and the results are shown in Fig. 5.
Raman spectra of all compounds show intense peaks for D-
and G- bands. It appears that D- band and G-bands for all
carbon materials measured have shifted more towards higher
wave number as compared to spectra of diamond or graphite
(not shown in figure). The IG/ID ratio of carbon materials
(Table 1) obtained from Soap-nut seed and Bamboo fiber
show highest value suggesting that it contains more graphitic
carbon than disordered carbon atoms. Intensities of Raman
spectra observed with carbon materials obtained from these
precursors follows: Date seed > Soap-nut seed > Coconut
fiber > Bamboo stem > Neem seed > Tea leaves > Jackfruit
seed. It is interesting to note that peak heights of G and D
bands for carbon obtained with Soap-nut seed, Coconut fiber
and Date seeds are higher than other carbon materials and
amongst these three, carbon from Date seed shows maximum peak height (Fig. 5).
2.5. XRD studies of carbon nanomaterials
XRD of carbon materials synthesized from Tea leaves,
Soap-nut seeds, Jackfruit seed and Neem seeds are shown in
figure 6. XRD spectrum shows the presence of (002) and
(100) plane of graphite. Carbon derived Soap-nut seed
showed sharpest peak suggesting that they are more
crystalline compared to carbon materials obtained from Tea
leaves and Neem seed. XRD of carbon from Jack fruit seed
suggested presence of some amorphous carbon also.
Whereas XRD of carbon from Coconut fiber, Date-seeds,
Bamboo stems (not shown here), showed no peaks and even
the intensities of spectrum was also very less.
2.6. Electrochemical measurements
Each purified carbon sample was mixed with ethylene-
XRD of carbon nanomaterials synthesized by pyrolysis of different plant based precursors: Tea leaves, Ritha, Jackfruit and
Neem seed.
Fig. 6.
Carbon Material from Natural Sources as an Anode in Lithium Secondary Battery
289
First Charging (A) and discharging (B) curve obtained
with carbon nanomaterials obtained from different plant based
precursors.
Fig. 7.
propylene dimethyl-monomer (EPDM) in cyclohexane and
then pressed into a stainless-steel grid on the stainless-steel
plate as an anode. A constant dc current was applied across
the anode and a Li-metal counter electrode to measure the
extent of Li-intercalation capacity of the anode. Ethylene
carbonate (EC): dimethyl carbonate DMQ (1 : 20 solution),
dissolved in LiPF6 (1 wt. %), were used as an electrolyte.
The charge-discharge characteristics were measured between
0 and 1.5 V (Fig. 7). The results of charging/discharging
with 100 cycles are shown in Fig. 8.
From the results of charging/discharging curves (Fig. 8), it
appears that the highest capacity of 850 mAh/g could be
obtained with carbon obtained from Jackfruit. While carbon
obtained from Coconut fiber and Soap-nut seeds could give
the highest capacity of 800 mAh/g and 650 mAh/g, respectively. All other carbon materials gave much lower capacitance.
Though the carbon materials obtained from Coconut fiber
and Soap-nut seed showed higher values of capacitance in
their first cycles, but it was important to know whether these
materials can sustain several charging/discharging cycles and
whether they could give same amount of the capacitance after
several cycles. Therefore, these materials were tried for 100
charging/discharging cycles. Results are shown in Fig. 8.
Capacity of Li-ion cell for various charging and discharging process. (A) Represents charging process and (B) represents discharging process.
Fig. 8.
From Fig. 8, it is observed that except carbon obtained
from Soap-nut seeds and Bamboo all others showed rapid
decrease in the capacitance just after 5th cycles. The sustainable capacitance after 100th cycles of charging and
discharging, obtained with all these carbon materials are
shown in Table 1.
3. Results and Discussion:
3.1. Lithium Intercalation
Carbon nanomaterials obtained from Soap-nut seeds
(Sapindus Mukorossi), Jack Fruit seeds (Artocarpus
heterophyllus), Date-seeds (Phoenix acaulis), Neem seeds
(Azadirachta indica), Tea leaves (ehretia microphylla),
Bamboo stem (Bambusa bambus) and Coconut fiber (Cocos
nucifera) were examined for their application in lithium
battery. Results of charging/discharging up to 1.5 V and
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number of reversible cycle are shown in Fig. 7 and 8.
The charging of battery is shown by curve A and that of
discharging behavior is shown by curve B. The slops were
calculated for charging and discharging curves. It appears
that in general, rate of charging is slower than the rate of
discharging. The reversibility of the battery was studied by
measuring the change in capacity versus the number of cycle
under the constant current of 1 mA. At 100th cycles the
magnitude of the capacitance available from each of the
carbon materials is calculated and the values are shown in
the table-1. The examination of these values suggests that
carbon materials obtained from the Soap-nut seeds gives the
highest capacitance of 130.29 mAh/g. The next highest
capacitance i.e. 92.74 mAh/g is obtained with carbon from
Bamboo stem The efficiency of charging/discharging process even at 100th cycles was found to be around 96%. This
indicates that the cycle stability of Bamboo and Soap-nut
seeds in the Li-ion intercalation process is relatively high, as
compared to the other carbon material.
It is interesting to note that the electrochemical properties
of multi walled carbon nano tubes reported from bamboo
also showed a good electrochemical properties [19].
In order to understand these behaviors of carbon materials,
a graph was plotted between the ratio of intensities of
Raman spectra at G- band and D- band (i.e. IG/ID) and the
capacitance obtained after 100th cycles (Fig. 9). From the
figure 9A it appears that as the magnitude of the IG/ID ratio
increases the capacitance of the battery with the carbon
materials increases. This suggests that when the carbon
materials contain more graphitic carbon, intercalation of
lithium is favored resulting into giving higher capacitance.
The examination of Raman spectra (Fig. 5) suggests that
almost all carbon materials show a large shift in the G-peak
from 1575 cm− 1 (as obtained with HOPG) to wards higher
side. Therefore another graph was plotted between the
difference of peak position of the carbon nanomaterials used
from this work and position of the G-peak obtained with
HOPG measured under the identical condition (Fig. 9B).
This graph shows that as soon as this difference increases
more than 20 cm− 1, there is a sharp decrease in the
capacitance of the battery, suggesting that the capacitance of
the battery (or the efficiency of intercalation and deintercalation of lithium) largely depends upon the graphitic
nature of the carbon materials. This also means that the
contribution of D-band in the carbon materials should be as
low as possible to give higher capacitance.
Though XRD of carbon from Bamboo did not show any
peak, but it showed a capacitance of 92.74 mAh/g. This
suggests that capacity of lithium battery does not entirely
depend upon the graphitic nature of carbon. Some other
factors like size of pores and/or surface area also plays a
role.
For this purpose, a graph was plotted between the surface
area of the carbon materials and the capacitance of the
(A) Graph plotted between the intensities obtained at Gband and D-band (i.e. IG/ID), versus the capacitance obtained
after 100th cycles with carbon materials obtained from these precursors (B) Graph showing the relationship between the difference in the G-band peak position of carbon material and G-band
peak position obtained with HOPG measured under the identical condition and (C) Graph drawn between the capacitance
obtained after the 100th cycles and pore size of the carbon nano
materials used for the battery.
Fig. 9.
battery obtained after 100th cycles. Unfortunately there
seems to have no specific trend between the capacitance and
the surface area of the carbon nano material (graph not
shown here). The reasons could be either there is no such
relationship between the surface area and the capacitance or
the surface area determined by the methylene blue technique
does not give the actual surface area of the material. In
absence of facilities to measure the BET surface area, we are
not in a position to clarify this point.
In our next attempt a graph was plotted between the pore
size (as this was obtained from SEM micrographs) and the
capacitance after 100th cycles (Fig. 9C). This graph shows a
maxima at around 8.1 nm, suggesting that pore size of the
material should be around 8 nm. Considering the size of
lithium Vander waals radius (0.41 nm), the ratio of pore size
(8.1 nm diameter) to the diameter of Van der Waals diameter
comes to 9.88. This suggests that pore size (or the cavity) of
the carbon nanomaterial should be about 10 times bigger
than the diameter of the lithium ion. Smaller or larger than
10 nm would also not be preferred (Fig. 9C). Carbon
nanomaterials obtained with Soap-nut seed seems to meet
these requirements and hence gives highest capacitance. It
can be concluded that the capacity of lithium battery
depends upon factors like pore size (i.e., the size of cavity)
and graphitic nature of the carbon.
Carbon Material from Natural Sources as an Anode in Lithium Secondary Battery
4. Conclusion
The analysis of pore size, and capacity of battery suggests
that carbon materials should posses certain specific pore size
to give an optimum capacitance for a lithium battery.
Moreover, carbon materials should possess more graphitic
structure and less of D-band to give optimum capacitance.
Under these conditions, amongst carbon materials prepared
from Soap-nut seeds (Sapindus mukorossi), Jack Fruit seeds
(Artocarpus heterophyllus), Date-seeds (Phoenix dactylifera),
Neem seeds (Azadirachta indica), Tea leaves (Ehretia
microphylla), Bamboo stem (Bambusa bambus) and Coconut
fiber (Cocos nucifera), by the pyrolysis, Soap-nut seeds meet
these requirements and gives the best capacitance value of
130 mAh/g even after 100th cycles of charging/discharging
and maintaining the capacitance through out (Fig. 8). Efforts
are being directed towards improving the porosity, pore size
and graphitic nature of carbon material obtained from Soapnut seeds, so as to get better capacitance
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