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Carbon materials Structure, texture and intercalation
SOLID STATE ELSEWER
Solid State Ionics 86-88
IONICS
(1996) 833-839
Invited paper
Carbon materials Structure, texture and intercalation Michio Inagaki Facula of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan
Abstract Carbon materials are known to have wide ranges of structure, texture and properties, and, as a consequence of this diversity, to have wide applications. Into these carbon materials, various atoms, ions and molecules have been intercalated, resulting in certain modification of properties, and new fields of science, technology and applications have been created. In the present paper, the diversity in carbon materials is explained by emphasizing the wide range of texture in nanometre scale, which is based on the preferred orientation of hexagonal carbon layers. The intercalation reactions into these carbon materials are discussed in relation to their structure and texture. Keywords:
Carbon; Graphite; Structure; Texture; Intercalation;
Preferred
1. Introduction The monolithic materials in which carbon atoms are the main constituents are called carbon materials (or simply carbons). When the graphite structure is predominant in carbon materials and when the materials have experienced high temperatures (above 2500°C) even though graphite structure is not developed, conventionally they are called graphite materials (or simply graphites). With heat treatment at high temperatures, the structure of carbon materials is known to approach more or less that of graphite. Graphite electrodes for iron refining has supported the development in modern technologies after the Industrial Revolution. Carbon blacks are important materials for rubbers and led to their huge usage for automobiles. Activated carbons are important as adsorbents. In electrochemical fields, carbon materials have been used in different parts, 0167.2738/96/$15.00 Copyright PZZ SO167-2738(96)00337-2
01996
orientation
such as electrodes in various cells and batteries, and conductive components in various electrodes. In the present paper, the wide range of structure and texture in carbon materials are reviewed, with special attention on texture in the nanometre scale (hereafter nanotexture) because of their determinative role on the development in structure and properties of carbon materials. In relation to electrochemical applications, intercalation reactions into carbon materials are discussed referring to their structure and texture.
2. Nanotextures
in carbon materials
It has been pointed out that one of the characteristics of carbon materials is the variety of their textures in different scales [ 11. All carbon materials formed from precursors by heating up to about 1300-1500°C
Elsevier Science B.V. All rights reserved
834
M. Inagaki I Solid State tonics 86-88
under atmospheric and reduced pressures have been shown by electron microscopy to consist of small domains of graphite-like hexagonal layers of carbon atoms with the diameter of I- 1.5 nm and parallel stacking of two or three layers, which have been named basic structural units (BSUs). Each carbon material has different ways of aggregation of these domains, the author proposing to call them textures in nanometre scale (nanotextures), and to classify them on the basis of the scheme and degree of preferred orientation of BSUs [2,3], as shown in Fig. 1. The first scheme is the orientation of the carbon layers along a reference plane. The extreme case, perfect orientation, in this scheme is the single crystal of graphite which can be found in the flakes of natural graphite and kish graphite. Highly orientated pyrolytic graphite (HOPG), which is prepared by high-temperature annealing of deposited pyrolytic carbon under pressure, is a typical example of synthetic graphite having a high degree of plane orientation. Extended layer structures are recognised under high-fidelity scanning electron microscopy, by the cross-section perpendicular to the reference plane as shown in Fig. 2a. In these HOPGs, however, the u-axis of each structural units graphite layers are found, through electron channelling contrast on its cleaved surface (Fig. 2b), to be randomly distributed in parallel to the reference plane. A gradual improvement in preferred orientation along the reference plane is observed in a series of pyrolytic carbons heat-treated at different temperatures. A similar
PLANE ORIENTATION reference
D
plane -
Concentric
POINT ORIENTATlON
Radial
Fig. 1. Nanotextures
in carbon materials
[2].
(1996) 833-839
Fig. 2. Nanotexture in highly oriented pyrolytic graphite (courtesy of Dr. A. Yoshida). (a) SEM of the cross-section, (b) electron channelling contrast of the surface.
development in orientation of layers was observed in cokes with the increase in heat treatment temperature (HTT), accompanied by the growth of layer diameter and stacking number. However, the development of graphite structure is somewhat restrained in these coke particles because of the incorporation of axial orientation scheme into plane one in so-called needle-like coke. The second scheme is the axial orientation of layers (Fig. l), where two cases must be differentiated in the cross-section perpendicular to the reference axis, coaxial and radial. All of the fibrous carbon materials reported have this scheme of orientation. The extreme case of coaxial orientation is the graphite whisker. In so-called vapour-grown carbon fibres, the gradual improvement in coaxial orientation was observed by lattice fringe technique under transmission electron microscope. The vapourgrown carbon fibres heat-treated at high temperatures are closest to perfect orientation in axial scheme and
hf. lnagaki
I Solid Sfare Ionics 86-88
tion is random arrangement of BSUs in carbon materials, an example being the glass-like carbons prepared from various precursors. In Fig. 4, lattice fringe micrographs of the carbon prepared from a sugar are shown. In the low-temperature sugar carbon, BSUs are very small and oriented in random (Fig. 4a). After the heat-treatment at temperatures as high as 28OO”C, however, the growth of layers are found to be limited, as shown in Fig. 4b, and it appears to make closed shells which are connected with each other through the wall of small carbon layers. A structural model of these carbons after heat treatment at high temperatures is shown in Fig. 4c [4]. This model explains the existence of large amounts of closed pores in glass-like carbon materials.
the growth of hexagonal layers results in the polygonization of the cross-section of the fibres. In carbon fibres, different mesophase-pitch-based nanotextures of BSUs in their cross-sections (called radial with wedge, zig-zag radial, concentric and random) were prepared. The third scheme is the orientation of BSUs around a reference point (point orientation), in which two cases have to be differentiated, radial and concentric (Fig. 1). Most of spherical carbon particles have this nanotexture. All carbon blacks are known to have a statistically concentric arrangement of BSUs in their particles, in small-size ones the arrangement being close to random. In large-size carbon blacks called thermal black, polygonization of particles was observed after high temperature heat treatment, as a consequence of marked concentric point orientation and the growth of carbon layers. The spherical particles were obtained in pitches, so-called mesophase spheres (or mesocarbon microbeads), and also by pressure carbonization of a mixture of polyethylene wirh a small amount of polyvinylchloride, called carbon spherules. These particles were found to have a radial arrangement of BSUs. In Fig. 3, the arrangement of BSUs in these three kinds of carbon particles are shown schematically. In both mesophase spheres and carbon spherules, the BSUs orient radially at their surfaces and, as a consequence, there are poles. The nanotexture in these two particles are different at their central parts, in the former BSUs arranging in parallel with each other but in the latter perfectly radial. Another extreme case opposite to perfect orienta-
(a)
-1Opm
3. Structure
development
in carbon materials
Structure of carbon materials has been pointed out to depend strongly on precursors and heat treatment conditions (temperature and pressure). For the characterization of structure of various carbon materials, X-ray diffraction technique has been widely employed and given important information. The average interlayer spacing between neighbouring hexagonal carbon layers do,,, crystallite sizes along the c and a axes Lc and L,, and probability of graphitic AB stacking between adjacent layers P, have been commonly used as fundamental structure parameters. The measurement of magnetoresistance (Aplp) on carbon materials has been pointed out to give also useful information on their structure and
(b)
l-
835
(1996) 833-839
2flm
(4
30-
500 nm
Fig. 3. Arrangement of basic structural units (nanotexture) in spherical particles. (a) Mesophase sphere, (b) carbon spherule, (c) carbon black. A short bar reveals either hexagonal carbon layer or a stack of layers (basic structural unit).
836
M. Inaguki
I Soltd
State
Fig. 4. Nanotexmre in glass-like carbon (a sugar coke) (courtesy of Mme. A. Oberlin). (a) Lattice fringe after 1300”C-treated, (b) lattice fringe after ZXOO”C-treated, (c) structural model [4)
Ionics
86-H
(1996)
8.73-839
on representative carbon materials. The development of three-dimensional gtaphitic structure evaluated by P, in Fig. 5a) is markedly depressed in the minute spherical particles (as small as 30 nm m diameter) of a carbon black (channel black), but comparable in the pyrolytic carbon and two cokes. However, the growth of crystallite size L< (002) is quite different in these faw carbon materials, as shown in Fig. Sb) which has been understood to depend strongly on the size of local orientation of BSUs in the precursors. In the pyrolytic carbon, the extension of local orientation in a plane orientation scheme is much larger than in other precursors, and consequently a large ,L< (002) value is attained. in the needle-like coke, local orientation of BSUs in a plane orientation is reasonably supposed to be in roughly micrometer order, smaller than pyrolytic carbons, but larger than regular cokes and much larger than carbon blacks with small particle size and point orientation scheme. In the fibers with axial orientation, the growth of graphltic structure was found to depend strongly on their diameters. In spherical particles with point orientation, the graphitization was found to be strongly constrained. Fig. 6 shows clearly the effect of particle size on graphitization, where the size of hexagonal layers L, in the spherical carbon particles after the heat treatment above 2800°C is plotted agamst the size of the particle. In this figure, the value of glass-like carbon with random orientation locates on an extrapolation of the relation on the spherjcai particles, which is reasonable if the pores consist of small carbon layers are assumed to be primarily particles of the glass-like carbon (see Fig. 4~). Pronounced effect of nanotexture an the growth of graphctic stacking and the crystalhte in carbon materials has been discussed on the relations among these structural parameters P,, dnD2, L<, Lu and magnetoresistance [5,6].
4. Intercalation texture [3]. Its absolute value is very sensitive to the imperfections in the graphite structure and SD is a good parameter for characterization of the structure of carbon materials, particularly highly graphitized ones. In Fig. 5, the structure parameters, P, and t, (002) are plotted against heat treatment temperature (HTT)
into carbon materials
4.1. Criteria of host carbons for intercalation reaction By using a wide range of carbon materials with different structures and textures, the intercalation reactions of sulphuric acid and of sodium-tetrahy-
M. Inagaki
I Solid State lonics 86-88
(1996)
837
833-839
1.0
Heat treatment temperature
(c)
Heat treatment temperature
Fig. 5. Changes of structure parameters with heat treatment temperature. (a) Degree of graphitization L< (002). A: Pyrolytic carbon, 0: needle-like coke, 0: regular coke, V: channel black.
(“C)
P,, (b) crystallite
size along c-axis,
Particle size (nm) Fig. 6. Dependence
of size of hexagonal
drofuran complexes (Na-THF) were studied. The criteria of host carbon materials for the intercalation reactions were discussed. Intercalation reaction at room temperature was found to be strongly governed by the structure of host carbon material, as shown in Fig. 7a and b for H,SO, and Na-THF, respectively [7-91. In these figures, the open marks stand for the formation of stage- 1 structure, well-defined from the appearance of sharp X-ray diffraction lines with high indices, and the host carbons shown by closed marks gave a broad X-ray diffraction peak with a little longer spacing that the original (may be called random staging). For these two intercalates of acceptor- and donor-types the criteria for the formation of stage-l structure is located at a P, of about 0.2 and LC (002) of 20-30 nm.
layers, L,, on particle size of spherical
particles
The difference in intercalation reaction was possible to be discussed in relation to the electronic band structure in host carbons evaluated by magnetoresistance measurement at room temperature. All host carbon shown by using open marks in these figures had positive values of (Aplp) at room temperature where the intercalation reactions were carried out, suggesting the coexistence of two carriers, electrons and holes. In these host carbons, therefore, the charge transfer either from or to hexagonal carbon layers occurs easily and completely, which leads to the formation of stage-l structure. On the other hand, most of the host carbons shown by closed marks in the figures were shown to have (Aplp) values of either near zero or negative, in which the predominant carriers were positive holes, except some electrons excited at room temperature.
838
M. Inaguki I Solid Stute Ionics 86-88
l
0
0
I
I
02
I
I
criteria for the intercalation.
I
I
0.4
Degree of gnphitnation
Fig. 7. Structural
(1996) 833-839
0.6
0
02
P,
(a) Sulphuric
0.4
0.6
Degree of graphitization PI
acid, (b) Na-THF
From these carbons, the charge transfer to the acceptor H,SO, is supposed not to be completed and so defined stage structure is not possible to be established. The charge transfer from Na-THF complexes to carbon layers is possible, but limited to the concentration of holes and results in random staging. The electrochemical intercalation of lithium into various materials has been studied extensively in order to develop new anode material and understand its reaction. Very similar discussion on structural criteria was proposed [lo], though another mechanism for the insertion of lithium into carbon materials with low degree of structure development had to be supposed. 4.2. Effect of nanotexture of curbon on intercalation and de-intercalation of sulphuric acid A marked and interesting effect of nanotexture of host carbon materials on the intercalation by chemical oxidation and subsequent de-intercalation by electrochemical reduction was observed on sulphuric acid [ll]. The intercalation of sulphuric acid by chemical oxidation of host carbons is shown to be exactly the same as the one by electrochemical intercalation. Therefore, it corresponds to charging by chemical means, chemical charging, and as a consequence galvanostatic discharging is possible [ 12,131. In Fig. 8, the discharge curve with a constant current of 4 mA on a mesophase-pitch-based carbon fibres heat-treated at 3000°C is compared with that
complex.
0: Stage-l
structure,
-.\._,
0
100 Electric
0: random stage structure.
Mesophase-path-basedcarbonfiber ofthe radial texture wth wedge
300
200 quantity
600
(mAh/g-carbon)
Fig. 8. Discharge curves with a constant sulphuric acid with oxidant.
current
of 4 mA in
on natural graphite flakes, the former having a radial arrangement of straight hexagonal carbon layers and the latter the flakes with high degree of plane orientation of layers. Though the discharge capacity for natural graphite is a little smaller than the theoretical value, the discharge capacity for the mesophase-pitch-based carbon fibres is extremely large. This was explained by a competition between charging due to chemical oxidation by the oxidant nitric acid in the electrolyte and discharging due to electrochemical reduction. To have a high rate of chemical oxidation, it is essential that the edge surface of carbon layers where the intercalation occurs has to be exposed to sulphuric acid at high percentages, being realized in the present carbon fibres. Using other carbon fibres, vapour-grown carbon fibres with concentric axial orientation, for example, such a high discharge
M. lnagaki
I Solid State lonics 86-88
capacity was not obtained. If small current density (0.4 mA) was selected, an increase in potential discharge was observed on the present mesophasepitch-based carbon fibres, suggesting an overcoming of chemical oxidation to galvanostatic reduction.
References [II M. Inagaki
and Y. Hishiyama, New Carbon Materials, (Ghoudo Publ. Co., Tokyo, 1994) p. 9. PI M. Inagaki, Tanso 122 (1985) 114. [31 Y. Hishiyama, Y. Kaburdgi and M. Inagaki, in: Chemistry and Physics of Carbon, Vol. 23, ed. P.A. Thrower (Marcel Dekker, New York, 1991) p. 1.
(1996)
833-839
839
[41 M. Shiraishi, in: Introduction to Carbon Materials, Revised Version (Carbon Sot. Japan, Tokyo, 1984) p. 33. [Sl N. Iwashita and M. Inagaki. Carbon 31 (1993) 1107. [61 N. Iwashita, Ph.D. Thesis. Hokkaido Univ. (1992). [71 M. Inagaki and N. Iwashita, Mol. Cryst. Liq. Cryst. 244 (1994) 89. WI M. Inagaki, 0. Tanaike and N. Iwashita, Synth. Met. 73 (1995) 83. L91 M. Inagaki and 0. Tanaike, Synth. Met. 24 ( 1995) 77. [101 M. Inagaki, Tanso 170 (1995) 298. [Ill N. Iwashita and M. Inagaki, Synth. Met. 34 (1989) 139; M. Inagaki. N. Iwashita and E. Kouno, Carbon 28 (1990) 49. 1121 M. Inagaki and Iwashita, Solid State tonics 70171 (1994) 425. 1131 M. Inagaki and N. Iwashita, J. Power Sources 52 (1994) 69.
Solid State Ionics 86-88
IONICS
(1996) 833-839
Invited paper
Carbon materials Structure, texture and intercalation Michio Inagaki Facula of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan
Abstract Carbon materials are known to have wide ranges of structure, texture and properties, and, as a consequence of this diversity, to have wide applications. Into these carbon materials, various atoms, ions and molecules have been intercalated, resulting in certain modification of properties, and new fields of science, technology and applications have been created. In the present paper, the diversity in carbon materials is explained by emphasizing the wide range of texture in nanometre scale, which is based on the preferred orientation of hexagonal carbon layers. The intercalation reactions into these carbon materials are discussed in relation to their structure and texture. Keywords:
Carbon; Graphite; Structure; Texture; Intercalation;
Preferred
1. Introduction The monolithic materials in which carbon atoms are the main constituents are called carbon materials (or simply carbons). When the graphite structure is predominant in carbon materials and when the materials have experienced high temperatures (above 2500°C) even though graphite structure is not developed, conventionally they are called graphite materials (or simply graphites). With heat treatment at high temperatures, the structure of carbon materials is known to approach more or less that of graphite. Graphite electrodes for iron refining has supported the development in modern technologies after the Industrial Revolution. Carbon blacks are important materials for rubbers and led to their huge usage for automobiles. Activated carbons are important as adsorbents. In electrochemical fields, carbon materials have been used in different parts, 0167.2738/96/$15.00 Copyright PZZ SO167-2738(96)00337-2
01996
orientation
such as electrodes in various cells and batteries, and conductive components in various electrodes. In the present paper, the wide range of structure and texture in carbon materials are reviewed, with special attention on texture in the nanometre scale (hereafter nanotexture) because of their determinative role on the development in structure and properties of carbon materials. In relation to electrochemical applications, intercalation reactions into carbon materials are discussed referring to their structure and texture.
2. Nanotextures
in carbon materials
It has been pointed out that one of the characteristics of carbon materials is the variety of their textures in different scales [ 11. All carbon materials formed from precursors by heating up to about 1300-1500°C
Elsevier Science B.V. All rights reserved
834
M. Inagaki I Solid State tonics 86-88
under atmospheric and reduced pressures have been shown by electron microscopy to consist of small domains of graphite-like hexagonal layers of carbon atoms with the diameter of I- 1.5 nm and parallel stacking of two or three layers, which have been named basic structural units (BSUs). Each carbon material has different ways of aggregation of these domains, the author proposing to call them textures in nanometre scale (nanotextures), and to classify them on the basis of the scheme and degree of preferred orientation of BSUs [2,3], as shown in Fig. 1. The first scheme is the orientation of the carbon layers along a reference plane. The extreme case, perfect orientation, in this scheme is the single crystal of graphite which can be found in the flakes of natural graphite and kish graphite. Highly orientated pyrolytic graphite (HOPG), which is prepared by high-temperature annealing of deposited pyrolytic carbon under pressure, is a typical example of synthetic graphite having a high degree of plane orientation. Extended layer structures are recognised under high-fidelity scanning electron microscopy, by the cross-section perpendicular to the reference plane as shown in Fig. 2a. In these HOPGs, however, the u-axis of each structural units graphite layers are found, through electron channelling contrast on its cleaved surface (Fig. 2b), to be randomly distributed in parallel to the reference plane. A gradual improvement in preferred orientation along the reference plane is observed in a series of pyrolytic carbons heat-treated at different temperatures. A similar
PLANE ORIENTATION reference
D
plane -
Concentric
POINT ORIENTATlON
Radial
Fig. 1. Nanotextures
in carbon materials
[2].
(1996) 833-839
Fig. 2. Nanotexture in highly oriented pyrolytic graphite (courtesy of Dr. A. Yoshida). (a) SEM of the cross-section, (b) electron channelling contrast of the surface.
development in orientation of layers was observed in cokes with the increase in heat treatment temperature (HTT), accompanied by the growth of layer diameter and stacking number. However, the development of graphite structure is somewhat restrained in these coke particles because of the incorporation of axial orientation scheme into plane one in so-called needle-like coke. The second scheme is the axial orientation of layers (Fig. l), where two cases must be differentiated in the cross-section perpendicular to the reference axis, coaxial and radial. All of the fibrous carbon materials reported have this scheme of orientation. The extreme case of coaxial orientation is the graphite whisker. In so-called vapour-grown carbon fibres, the gradual improvement in coaxial orientation was observed by lattice fringe technique under transmission electron microscope. The vapourgrown carbon fibres heat-treated at high temperatures are closest to perfect orientation in axial scheme and
hf. lnagaki
I Solid Sfare Ionics 86-88
tion is random arrangement of BSUs in carbon materials, an example being the glass-like carbons prepared from various precursors. In Fig. 4, lattice fringe micrographs of the carbon prepared from a sugar are shown. In the low-temperature sugar carbon, BSUs are very small and oriented in random (Fig. 4a). After the heat-treatment at temperatures as high as 28OO”C, however, the growth of layers are found to be limited, as shown in Fig. 4b, and it appears to make closed shells which are connected with each other through the wall of small carbon layers. A structural model of these carbons after heat treatment at high temperatures is shown in Fig. 4c [4]. This model explains the existence of large amounts of closed pores in glass-like carbon materials.
the growth of hexagonal layers results in the polygonization of the cross-section of the fibres. In carbon fibres, different mesophase-pitch-based nanotextures of BSUs in their cross-sections (called radial with wedge, zig-zag radial, concentric and random) were prepared. The third scheme is the orientation of BSUs around a reference point (point orientation), in which two cases have to be differentiated, radial and concentric (Fig. 1). Most of spherical carbon particles have this nanotexture. All carbon blacks are known to have a statistically concentric arrangement of BSUs in their particles, in small-size ones the arrangement being close to random. In large-size carbon blacks called thermal black, polygonization of particles was observed after high temperature heat treatment, as a consequence of marked concentric point orientation and the growth of carbon layers. The spherical particles were obtained in pitches, so-called mesophase spheres (or mesocarbon microbeads), and also by pressure carbonization of a mixture of polyethylene wirh a small amount of polyvinylchloride, called carbon spherules. These particles were found to have a radial arrangement of BSUs. In Fig. 3, the arrangement of BSUs in these three kinds of carbon particles are shown schematically. In both mesophase spheres and carbon spherules, the BSUs orient radially at their surfaces and, as a consequence, there are poles. The nanotexture in these two particles are different at their central parts, in the former BSUs arranging in parallel with each other but in the latter perfectly radial. Another extreme case opposite to perfect orienta-
(a)
-1Opm
3. Structure
development
in carbon materials
Structure of carbon materials has been pointed out to depend strongly on precursors and heat treatment conditions (temperature and pressure). For the characterization of structure of various carbon materials, X-ray diffraction technique has been widely employed and given important information. The average interlayer spacing between neighbouring hexagonal carbon layers do,,, crystallite sizes along the c and a axes Lc and L,, and probability of graphitic AB stacking between adjacent layers P, have been commonly used as fundamental structure parameters. The measurement of magnetoresistance (Aplp) on carbon materials has been pointed out to give also useful information on their structure and
(b)
l-
835
(1996) 833-839
2flm
(4
30-
500 nm
Fig. 3. Arrangement of basic structural units (nanotexture) in spherical particles. (a) Mesophase sphere, (b) carbon spherule, (c) carbon black. A short bar reveals either hexagonal carbon layer or a stack of layers (basic structural unit).
836
M. Inaguki
I Soltd
State
Fig. 4. Nanotexmre in glass-like carbon (a sugar coke) (courtesy of Mme. A. Oberlin). (a) Lattice fringe after 1300”C-treated, (b) lattice fringe after ZXOO”C-treated, (c) structural model [4)
Ionics
86-H
(1996)
8.73-839
on representative carbon materials. The development of three-dimensional gtaphitic structure evaluated by P, in Fig. 5a) is markedly depressed in the minute spherical particles (as small as 30 nm m diameter) of a carbon black (channel black), but comparable in the pyrolytic carbon and two cokes. However, the growth of crystallite size L< (002) is quite different in these faw carbon materials, as shown in Fig. Sb) which has been understood to depend strongly on the size of local orientation of BSUs in the precursors. In the pyrolytic carbon, the extension of local orientation in a plane orientation scheme is much larger than in other precursors, and consequently a large ,L< (002) value is attained. in the needle-like coke, local orientation of BSUs in a plane orientation is reasonably supposed to be in roughly micrometer order, smaller than pyrolytic carbons, but larger than regular cokes and much larger than carbon blacks with small particle size and point orientation scheme. In the fibers with axial orientation, the growth of graphltic structure was found to depend strongly on their diameters. In spherical particles with point orientation, the graphitization was found to be strongly constrained. Fig. 6 shows clearly the effect of particle size on graphitization, where the size of hexagonal layers L, in the spherical carbon particles after the heat treatment above 2800°C is plotted agamst the size of the particle. In this figure, the value of glass-like carbon with random orientation locates on an extrapolation of the relation on the spherjcai particles, which is reasonable if the pores consist of small carbon layers are assumed to be primarily particles of the glass-like carbon (see Fig. 4~). Pronounced effect of nanotexture an the growth of graphctic stacking and the crystalhte in carbon materials has been discussed on the relations among these structural parameters P,, dnD2, L<, Lu and magnetoresistance [5,6].
4. Intercalation texture [3]. Its absolute value is very sensitive to the imperfections in the graphite structure and SD is a good parameter for characterization of the structure of carbon materials, particularly highly graphitized ones. In Fig. 5, the structure parameters, P, and t, (002) are plotted against heat treatment temperature (HTT)
into carbon materials
4.1. Criteria of host carbons for intercalation reaction By using a wide range of carbon materials with different structures and textures, the intercalation reactions of sulphuric acid and of sodium-tetrahy-
M. Inagaki
I Solid State lonics 86-88
(1996)
837
833-839
1.0
Heat treatment temperature
(c)
Heat treatment temperature
Fig. 5. Changes of structure parameters with heat treatment temperature. (a) Degree of graphitization L< (002). A: Pyrolytic carbon, 0: needle-like coke, 0: regular coke, V: channel black.
(“C)
P,, (b) crystallite
size along c-axis,
Particle size (nm) Fig. 6. Dependence
of size of hexagonal
drofuran complexes (Na-THF) were studied. The criteria of host carbon materials for the intercalation reactions were discussed. Intercalation reaction at room temperature was found to be strongly governed by the structure of host carbon material, as shown in Fig. 7a and b for H,SO, and Na-THF, respectively [7-91. In these figures, the open marks stand for the formation of stage- 1 structure, well-defined from the appearance of sharp X-ray diffraction lines with high indices, and the host carbons shown by closed marks gave a broad X-ray diffraction peak with a little longer spacing that the original (may be called random staging). For these two intercalates of acceptor- and donor-types the criteria for the formation of stage-l structure is located at a P, of about 0.2 and LC (002) of 20-30 nm.
layers, L,, on particle size of spherical
particles
The difference in intercalation reaction was possible to be discussed in relation to the electronic band structure in host carbons evaluated by magnetoresistance measurement at room temperature. All host carbon shown by using open marks in these figures had positive values of (Aplp) at room temperature where the intercalation reactions were carried out, suggesting the coexistence of two carriers, electrons and holes. In these host carbons, therefore, the charge transfer either from or to hexagonal carbon layers occurs easily and completely, which leads to the formation of stage-l structure. On the other hand, most of the host carbons shown by closed marks in the figures were shown to have (Aplp) values of either near zero or negative, in which the predominant carriers were positive holes, except some electrons excited at room temperature.
838
M. Inaguki I Solid Stute Ionics 86-88
l
0
0
I
I
02
I
I
criteria for the intercalation.
I
I
0.4
Degree of gnphitnation
Fig. 7. Structural
(1996) 833-839
0.6
0
02
P,
(a) Sulphuric
0.4
0.6
Degree of graphitization PI
acid, (b) Na-THF
From these carbons, the charge transfer to the acceptor H,SO, is supposed not to be completed and so defined stage structure is not possible to be established. The charge transfer from Na-THF complexes to carbon layers is possible, but limited to the concentration of holes and results in random staging. The electrochemical intercalation of lithium into various materials has been studied extensively in order to develop new anode material and understand its reaction. Very similar discussion on structural criteria was proposed [lo], though another mechanism for the insertion of lithium into carbon materials with low degree of structure development had to be supposed. 4.2. Effect of nanotexture of curbon on intercalation and de-intercalation of sulphuric acid A marked and interesting effect of nanotexture of host carbon materials on the intercalation by chemical oxidation and subsequent de-intercalation by electrochemical reduction was observed on sulphuric acid [ll]. The intercalation of sulphuric acid by chemical oxidation of host carbons is shown to be exactly the same as the one by electrochemical intercalation. Therefore, it corresponds to charging by chemical means, chemical charging, and as a consequence galvanostatic discharging is possible [ 12,131. In Fig. 8, the discharge curve with a constant current of 4 mA on a mesophase-pitch-based carbon fibres heat-treated at 3000°C is compared with that
complex.
0: Stage-l
structure,
-.\._,
0
100 Electric
0: random stage structure.
Mesophase-path-basedcarbonfiber ofthe radial texture wth wedge
300
200 quantity
600
(mAh/g-carbon)
Fig. 8. Discharge curves with a constant sulphuric acid with oxidant.
current
of 4 mA in
on natural graphite flakes, the former having a radial arrangement of straight hexagonal carbon layers and the latter the flakes with high degree of plane orientation of layers. Though the discharge capacity for natural graphite is a little smaller than the theoretical value, the discharge capacity for the mesophase-pitch-based carbon fibres is extremely large. This was explained by a competition between charging due to chemical oxidation by the oxidant nitric acid in the electrolyte and discharging due to electrochemical reduction. To have a high rate of chemical oxidation, it is essential that the edge surface of carbon layers where the intercalation occurs has to be exposed to sulphuric acid at high percentages, being realized in the present carbon fibres. Using other carbon fibres, vapour-grown carbon fibres with concentric axial orientation, for example, such a high discharge
M. lnagaki
I Solid State lonics 86-88
capacity was not obtained. If small current density (0.4 mA) was selected, an increase in potential discharge was observed on the present mesophasepitch-based carbon fibres, suggesting an overcoming of chemical oxidation to galvanostatic reduction.
References [II M. Inagaki
and Y. Hishiyama, New Carbon Materials, (Ghoudo Publ. Co., Tokyo, 1994) p. 9. PI M. Inagaki, Tanso 122 (1985) 114. [31 Y. Hishiyama, Y. Kaburdgi and M. Inagaki, in: Chemistry and Physics of Carbon, Vol. 23, ed. P.A. Thrower (Marcel Dekker, New York, 1991) p. 1.
(1996)
833-839
839
[41 M. Shiraishi, in: Introduction to Carbon Materials, Revised Version (Carbon Sot. Japan, Tokyo, 1984) p. 33. [Sl N. Iwashita and M. Inagaki. Carbon 31 (1993) 1107. [61 N. Iwashita, Ph.D. Thesis. Hokkaido Univ. (1992). [71 M. Inagaki and N. Iwashita, Mol. Cryst. Liq. Cryst. 244 (1994) 89. WI M. Inagaki, 0. Tanaike and N. Iwashita, Synth. Met. 73 (1995) 83. L91 M. Inagaki and 0. Tanaike, Synth. Met. 24 ( 1995) 77. [101 M. Inagaki, Tanso 170 (1995) 298. [Ill N. Iwashita and M. Inagaki, Synth. Met. 34 (1989) 139; M. Inagaki. N. Iwashita and E. Kouno, Carbon 28 (1990) 49. 1121 M. Inagaki and Iwashita, Solid State tonics 70171 (1994) 425. 1131 M. Inagaki and N. Iwashita, J. Power Sources 52 (1994) 69.