Journal of New Materials for Electrochemical Systems 6, 75-80 (2003)
© J. New. Mat. Electrochem. Systems
Comparison of the Electrochemichal Performance of Carbon Produced
from Sepiolite with Different Surface Characteristics
Giselle Sandía, Humberto Joachinb, Wenquan Lub, Jai Prakashb, and Giuseppe Tassarac
b Dep.
aChemistry Division, Argonne National Laboratory, 9700 South Cass Ave, Argonne, IL 60439;
of Chemical and Environmental Engineering, Illinois Institute of Technology, 10 W. 33rd Street, Chicago, IL 60616;
cTechnologie Riqualificazione e Microseparazioone Materiali, Viale Venezia 170, 25123 Brescia- IT
(Received April 15, 2002; received revised form October 4, 2002)
Abstract: A pure carbon-based material with applications in electrochemical processes was synthesized using different fractions of sepiolite clay.
The produced carbon was initially characterized to determine its purity and surface properties including surface area, pore volume, and pore
diameter. The extent of the purity was assessed from thermogravimetric analysis (TGA). The electrochemical properties, i.e. the potential use as
electrode in Li ion batteries, were evaluated using conventional electrochemical testings such as charge/discharge and impedance spectroscopy. The
results indicated that there is a correlation of the reversible specific capacity obtained and the surface properties of the template sepiolite. It is shown
that their electrochemical performances for anode Li cells are related to their surface chemical properties rather than their BET surface area.
Keywords: carbon, sepiolite, lithium ion batteries, electrochemistry, capacity
carbon type. Lithium intercalates in layered carbons such as
graphite, and it adsorbs on the surfaces of single carbon layers
in nongraphitizable hard carbons. Lithium also appears to
reversibly bind near hydrogen atoms in carbonaceous
materials containing substantial hydrogen, which are made by
heating organic precursors to temperatures near 700°C. Each
of these three classes of materials appears suitable for use in
advanced lithium batteries.
More recently, Sandí et al.
proposed a mechanism based on the concept that carbons with
curved lattices can exhibit enhanced lithium capacity over
that of graphite. This idea was underscored by computational
studies of endohedal lithium complexes of buckminster
fullerene, C60 [2, 3]. It was found that the interior of the C60
molecule was large enough to easily accommodate two or
three lithiums. Furthermore, the curved ring structure of the
C60 molecule facilitated the close approach, 2.96 Å, of the
lithiums even in the trilithiated species. This is significantly
closer than the interlithium distance in the stage-one graphite
complex LiC6 and suggests that lithium anode capacities may
be improved over graphitic carbon by synthesizing carbons
1. INTRODUCTION
Carbon due to different allotropes (graphite, diamond,
fullerenes/nanotubes), various microtextures (more or less
ordered) owing to the degree of graphitization, a rich variety
of dimensionality from 0 to 3D, and the ability for existence
under different forms (from powders to fibers, foams, fabrics,
and composites) represents a very attractive material for
electrochemical applications, specially for the storage of
energy.
The successful utilization of a carbon host to store Li-ions in
the rechargeable negative electrode has lead to the
commercial development of Li-ion cells. Storage of Li in
carbon to form the negative electrode in Li-ion cells occurs by
different mechanisms. Danh et al. [1] proposed three
mechanisms for lithium insertion in carbonaceous materials.
The physical mechanism for this insertion depends on the
*To whom correspondence should be addressed:
[email protected]; Fax: (630) 252-9288
E-mail:
75
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G. Sandì et al. / J. New. Mat. Electrochem. Systems 6, 75-80 (2003)
with curved lattices that approximate a portion of a buckey
ball [4].
In our laboratories carbonaceous materials with enhanced
lithium capacity have been derived from ethylene or
propylene upon incorporation in the vapor phase in the
channels of sepiolite, taking advantage of the Brønsted
acidity in the channels to polymerize olefins [5]. Sepiolite is a
phyllosilicate clay insofar as it contains a continuous twodimensional tetrahedral silicate sheet. However, it differs from
other clays in that it lacks a continuous octahedral sheet
structure. Instead, its structure can be considered to contain
ribbons of 2:1 phyllosilicate structure, with each ribbon
linked to the next by inversion of SiO4 tetrahedra along a set
of Si-O-Si bonds. In this framework, rectangular channels run
parallel to the x-axis between opposing 2:1 ribbons, which
results in a fibrous morphology with channels running
parallel to the fiber length. Channels are 3.7 x 10.6 Å in
sepiolite (they are 3.7 x 6.4 Å in palygorskite). Individual
fibers generally range from about 100 Å to 4-5 microns in
length, 100-300 Å width, and 50-100 Å thickness. Inside the
channels are protons, coordinated water, a small number of
exchangeable cations, and zeolitic water.
this paper, a comparison of the electrochemical performance
of carbon anodes derived from different particle size and
surface area sepiolite samples is presented.
2. EXPERIMENTAL
2.1. Sepiolite samples
The sepiolite samples were obtained fromTechnology for
Requalification and Microseparation of Materials (TRM),
Viale Venezia 170, 25123 Brescia- Italy. Five types of
sepiolite samples were used as carbon templates: “raw
material”, “fraction 1”, “fraction 2”, “fraction 3”, and
“fraction 4”.
Raw material is the type of sepiolite that they receive and
processes in their plant. The plant has a pneumatic conveyor,
which can micronize and divide an input powder into two to
four fractions through a special mechanic system. The
mechanical systems is composed of a crashing micronizer, 2
cyclonic separators (each of them provided with a decantation
valve at the bottom), one powder decantation cyclone, and
one sleeve air-filter. All these elements are put on line. Figure
2 shows a picture of the home-made apparatus for the
separation.
Figure 1 shows a bright field TEM of the resulting carbon
after the clay has been removed. Carbon fibers (1-1.5 microns
long) are obtained whose orientation and shape resemble that
of the original clay. The SAED pattern of the carbon fibers
shows diffuse rings typical of amorphous carbon; no
diffraction spots were observed.
Figure 2. TRM home-made apparatus used for the separation of the sepiolite fractions.
Figure 1. TEM of a carbon sample derived from sepiolite raw material. A JEOL
100CXII Transmission Electron Microscope operating at 100kV was used.
Aurbach et al. [6] and Fong et al. [7] suggested that low
surface area carbons are favorable for practical applications,
since the amount of lithium consumed in the formation of the
passivating layer that contributed to the irreversible capacity
was proportional to the surface area of the carbon. However,
Nasrin et al. [8] showed that porous high surface area carbons
proved to be excellent candidates for lithium ion batteries. In
The separation is obtained by difference of granulometric size
and/or density of the particles (in this case only granulometric
size, since the material was homogeneous). Fraction 1 is
composed by the heaviest and most dense particles decanted
by the first separator, fraction 2 or intermediate decanted by
the second separator, fraction 3 or light decanted by the
cyclone, and fraction 4 or extra-light decanted through the
air-filter. It is then assumed that particles of fraction 1 have a
larger average granulometric size than fraction 2, while
fraction 3 and 4 will have smaller granulometric size.
2.2. Carbon synthesis
Ethylene and propylene (AGA, 99.95%) were loaded in the
sepiolite samples and pyrolyzed in the gas phase in one step.
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Comparison of the Electrochemichal Performance of Carbon / J. New. Mat. Electrochem. Systems 6, 75-80 (2003)
A three-zone furnace was used. Quartz boats containing
sepiolite were placed within a quartz tube. The tube was
initially flushed with nitrogen for about 3 hours. The gas was
then switched to propylene or ethylene and the gas flow was
kept about 5 cm3/min. The temperature of the oven was
gradually increased from room temperature (about 5 °C/min)
to 700 °C. The oven was then held at that target temperature
for 4 hours.
previously dissolved in1 methyl-2pyrrolidinone (NMP) with
carbonaceous material (92 % carbon and 8% PVDF). The
slurry was applied to a copper foil (current collector) using a
Gardner coater.
During the application, copper foil was
maintained flat on a vacuum plate. Upon completion of the
coating, copper was dried overnight under vacuum at
approximately 100 °C.
The clay from the loaded/pyrolyzed sepiolite sample was
removed using HF, previously cooled at 0 °C to passivate the
exothermic reaction. The resulting slurry was stirred for about
one hour. It was then rinsed to neutral pH and refluxed with
concentrated HCl for 2 hours. The sample was washed with
distilled water until the pH was > 5 to ensure that there was no
acid left. The resultant carbon was oven dried overnight at
120 °C.
Coin cells (2032) were prepared in a glove box under Helium
gas atmosphere by punching a 9/16 inch laminate and lithium
metal. The electrolyte was 1 M LiPF6 in EC/DEC (1:1). A
Celgard 3501 was used as separator. An Arbin Cycler was
used to apply a C/10 current density on the cell. In order to
measure the dynamic impedance behavior, 30 seconds
interruption was applied during charge and discharge
processes and Area Specific Impedance (ASI) was calculated
for the tests.
2.3. Characterization of the produced carbons
The BET surface area was obtained by using a Micromeritics
ASAP 2010 Surface Area Analyzer. Approximately 0.1 grams
of the carbonaceous material was placed in a sample tube and
degas at 120 °C for at least 12 hours. The data was then
collected at a relative pressure of 0.05 to 0.2, where
monolayer coverage of nitrogen molecules is assumed to be
complete.
X-ray powder diffraction (XRD) patterns of sepiolite,
sepiolite/organic composite and carbons were determined
using a Rigaku Miniflex, with Cu Kα radiation and a NaI
detector at a scan rate of 0.5° 2θ/min.
2.4.2. Electrochemical characterization
3. RESULTS AND DISCUSSION
3.1. Physical characterization
Figure 3 shows the N2 isotherms of the carbon samples. As
indicated by Giles et al. [9], the hysteresis loop observed in all
the samples is due to mesopores present. When the isotherm
does not flatten completely at the highest values of p/p0, this
indicates that there are large pores which have not been
completely filled.
500
Volume adsorbed (cm3/g STP)
Volume adsorbed (cm3/g STP)
500
Thermal gravimetric analysis (TGA) of the sepiolite samples
was carried out on an EXSTAR 6000 simultaneous DTA-TGA
instrument using a nitrogen flow of 100 mL/min at a scan rate
of 10 °C/min. The carbon samples were measured using an
oxygen flow of 100 mL/min at a scan rate of 5 °C/min from
room temperature to 300 °C and then changing the scan rate
to 1 °C/min to 800 °C.
a
400
300
200
100
0
0.0
0.2
0.4
0.6
0.8
2.4.1. Preparation of slurry and carbon films
Carbon films (laminates) were prepared using copper foil and
slurries of carbon mixed with a binder. The slurry was
prepared by mixing polyvinylidene fluoride (PVDF)
200
100
0
0.0
1.0
0.2
0.4
0.8
1.0
0.6
0.8
1.0
500
c
300
200
100
0
0.0
0.6
P/P 0
Volume adsorbed (cm3/g STP)
Volume adsorbed (cm3/g STP)
400
0.2
0.4
0.6
0.8
400
d
300
200
100
0
0.0
1.0
0.2
0.4
P/P0
P/P 0
500
Volume adsorbed (cm3/g STP)
2.4. Determination of the electrochemical properties
300
P/P0
500
Transmission electron microscopy (TEM) was performed in a
JEOL 100CXII Transmission Electron Microscope operating
at 100kV. Approximately 0.01 g of the powder sample was
placed into a vial containing about 10 ml of methanol. After
sonicating for 30 seconds, copper grids with “holey” carbon
films were then dipped into the resulting slurry. The Cu grids
were allowed to dry for 2 hours in a vacuum oven at 100 °C.
Once dry, the grids were inserted into non-tilt holders and
loaded into the instrument. Only regions overhanging holes
in the carbon grid were used. Scale markers placed on the
micrographs are accurate to within three percent.
b
400
400
e
300
200
100
0
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Figure 3: N2 isotherms obtained for the carbon samples derived from sepiolite.
78
G. Sandì et al. / J. New. Mat. Electrochem. Systems 6, 75-80 (2003)
Table 1: Surface area of the sepiolite samples and the templated carbons.
Sepiolite sample
Surface area
(m2/g)
Carbon sample
Surface area
(m2/g)
Total pore volume
(cm3/g)
Average pore
diameter (Å)
Raw material
214
From raw material
245
0.0922
15.0
Fraction 1
198
From Fraction 1
207
0.0752
14.6
Fraction 2
218
From Fraction 2
214
0.0722
13.6
Fraction 3
225
From Fraction 3
212
0.0788
14.9
Fraction 4
193
From Fraction 4
119
0.0451
15.2
100
Table 1 shows the BET surface area of the sepiolite samples
and the carbons derived from them. The surface area of both
the sepiolite and the corresponding carbon is very similar
(around 200 m2/g). An increase in surface area of the carbon
samples is observed as the fraction number increases. This
result was expected based on the separation process, that is, as
the particles become smaller, the surface area should increase.
An exception is the fourth fraction, where both the surface
area of the sepiolite sample and the carbon is lower than
expected. The pore volume of the carbon sample derived
from the fourth fraction (calculated by using the BJH method
[10, 11]), is about 40% smaller than the other carbon samples,
leading to a smaller surface area, as found by BET.
a)
% Weight
80
40
20
0
0
200
400
Temperature, °C
b)
DTA,:W/mg
20
10
Raw material, 487 °C
first fraction, 490 °C
second fraction, 497 °C
third fraction, 501 °C
fourth fraction, 510 °C
0
-10
0
200
400
Temperature, °C
600
Figure 4. a) Thermal gravimetric analysis of the carbon samples derived from the
sepiolite fractions. The samples were heated under oxygen at 5 °C/min from RT to
300 °C, then at 1 °C/min from 300 to 600 °C, and finally at 5 °C/min to 800 °C. b)
Differential thermal analysis of the same samples. The change in slope is due to
changes in the temperature program.
d002
Relative Intensity
3.2. Determination of the electrochemical properties
Due to the high surface area of this carbon, it was not
anticipated to observe any staging, but rather, a voltage
profile similar to that of disordered carbons, where the lithium
insertion occurs close to 0.8 V vs. Li. Since these carbons
contain pores with an estimated pore size of 15 Å, lithium
ions can easily diffuse through this matrix and the process is
reversible. In fact, Fandrois et al. [13] indicated that in high
surface area with closed pores, trapping of lithium on the
closed pores is the main source of irreversibility. However, if
the pores are open, the lithium ions should diffuse easily and
the irreversible component is reduced, if not eliminated.
600
30
Figure 4 shows the thermal gravimetric analysis of the
produced carbons. Only one weight loss curve was observed
on all the samples (Fig. 4a), indicating that there were no
impurities present. Figure 4b shows the differential thermal
analysis of all the samples. The combustion of the samples
starts at about 400 °C and ends at about 530 °C. The change
in the slope at about 320 °C is an artifact due to the change in
heating rate.
Figure 5 shows X-ray powder diffractions of the carbonaceous
materials. The broad d002 peak at 3.52 Å is an indication of
the disordered nature of the material. Sandí et al. [12] have
found that the lithium uptake by disordered carbons is greater
than for organized systems. It is then expected to obtain
specific capacities larger than those of graphitic materials.
Raw material
first fraction
second fraction
third fraction
fourth fraction
60
3.42 Å
D
Raw material
First fraction
Second fraction
Third fraction
Fourth fraction
10
20
30
40
Angle 22
θ
Figure 5. X-ray powder diffraction of the carbon samples. The d002 peak is broad
indicating the non-graphitic character of the samples.
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Comparison of the Electrochemichal Performance of Carbon / J. New. Mat. Electrochem. Systems 6, 75-80 (2003)
Figure 6 shows the cycle profiles for all the carbon samples
derived from the different fractions of sepiolite. All the plots
in the figure have the same scale for easy of comparison. The
highest discharge capacity on the first cycle was obtained
from the carbon derived of the raw material. However, this
sample showed the highest irreversibility. As the fraction
number increases, the specific capacity decreases and the
irreversible capacity also decreases. At the end of the 10th
cycle, the specific capacity stabilizes on all the samples and
the value is close to 350 mAh/g at a C/10 current rate.
the processes represents a semi-crystalline structure which
occurs after lithium ion is fully intercalated.
650
600
Raw m aterial
550
First fraction
500
Second fraction
450
Third fractio n
400
Fourth fraction
350
300
250
2.5
2.5
First fraction
Raw material
200
2.0
2.0
Voltage (V)
Voltage (V)
150
1.5
1.0
1.5
100
50
1.0
0
0.5
0.5
100
150
200
Cap acity, mAh/g
250
300
350
0.0
0.0
0
200
400
600
800
1000
1200
1400
0
1600
200
400
600
800
1000
1200
1400
1600
Specific capacity (mAh/g)
Specific capacity (mAh/g)
2.5
Figure 7. Area specific impedance plot (ASI) of the carbon samples prepared from the
different sepiolite fractions.
2.5
Second fraction
Third fraction
2.0
2.0
1.5
1.5
Voltage (V)
Voltage (V)
50
1.0
0.5
1.0
0.5
0.0
0.0
0
200
400
600
800
1000
1200
1400
1600
0
200
Specific capacity (mAh/g)
400
600
800
1000
1200
1400
1600
Specific capacity (mAh/g)
Figure 8 represents the coulombic efficiency of all the carbon
samples as a function of cycle number. It is very clear that the
irreversible capacity happens at the first cycle for all the
samples. The recyclable capacity is shown to be about 350
mAh/g.
2.5
Fourth fraction
2.0
Voltage (V)
100
1.5
1.0
75
0.5
0.0
0
200
400
600
800
1000
1200
1400
1600
Specific capacity (mAh/g)
50
r aw m a te rial
fir st f ra ction
Figure 6. Electrochemical Performance of carbon samples derived from different
fractions of sepiolite.
25
s ec on d fractio n
th ird frac tio n
fo u rth frac tio n
0
The area specific impedance plot (ASI) of the 8th cycle is
shown in Figure 7. It is a very important property of the
electrode because it provides information on the nature and
magnitude of the electrochemical processes and masstransport limitations. The average ASI value was found to be
60 ohm cm2 for all the fractions, except for the last fraction,
where the value increases as the specific capacity increases.
This value includes charge resistance, ohmic resistance, and
part of diffusion resistance.
In each lithium ion deintercalation process, the impedance decreases initially, and
then it increases sharply at the end of de-intercalation since
lithium ion in carbon layers diminishes at the end of deintercalation. The observed higher value at the beginning of
0
4
8
12
Cycle Num ber
Figure 8. Coulombic efficiency of the carbon samples as a function of cycle number.
As shown, although carbon used for anode in this study has a
BET area higher than 200 m2/g (much higher than MCMB,
which is around 5 m2/g), the irreversible capacity loss in the
first cycle for this carbon is less than 20%, which is only
about 5% higher than what is for MCMB, and after the first
cycle the capacity is quite stable. In addition, no high
capacity loss can be observed as process progresses. This is
80
G. Sandì et al. / J. New. Mat. Electrochem. Systems 6, 75-80 (2003)
an indication that the BET surface area is not the limiting
factor in the irreversibly capacity loss. Accordingly, the
results do not support the fact that to reduce the irreversible
capacity loss, which is caused by SEI [14], carbon anode
materials with low surface area are commonly preferred.
These results are in agreement with the idea that, in the
selection of anode materials, it is important to consider the
fact that the capacity loss is not simply related to the BET
surface area, since the edge area could play a more important
role [15, 16].
Considering the cost associated in the
production of these porous carbons compared to that of
modified graphites, as well as the environmental benefits of
using waste materials, they can be considered as excellent
candidates for anodes in lithium anode cells.
4. CONCLUSION
The results of this study showed that the surface properties of
the inorganic templates are very important for obtaining
carbonaceous
materials
with
good
properties
for
electrochemical systems. These carbons promise to be
excellent candidates as anodes in lithium ion cells, although
their BET surface area are much higher than commercially
available graphitic materials.
The reversible capacity
obtained at the end of the 10th cycle for the produced carbons
was about 350 mAh/g, which is very similar to that of
MCMB. Efficiency higher than 95% was obtained for the
these carbons. Results also suggested that porosity places a
very important role in the diffusivity of lithium ions within
the carbon structure, thus reducing the irreversibility upon
cycling.
5. ACKNOWLEDGEMENT
This work was performed under the auspices of the U.S.
Department of Energy, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences and Biosciences,
under contract number W-31-109-ENG-38.
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