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:
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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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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.
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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.
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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)
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2.4. Determination of the electrochemical properties
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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.
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Figure 3: N2 isotherms obtained for the carbon samples derived from sepiolite.
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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
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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.
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% Weight
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10 Raw material, 487 °C first fraction, 490 °C second fraction, 497 °C third fraction, 501 °C fourth fraction, 510 °C
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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.
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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
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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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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.
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Figure 7. Area specific impedance plot (ASI) of the carbon samples prepared from the different sepiolite fractions.
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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.
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Figure 6. Electrochemical Performance of carbon samples derived from different fractions of sepiolite.
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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
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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
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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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