Method of producing a graphite intercalation compound

A method of producing a graphite intercalation compound by intercalating a substance into graphite between layers thereof, characterized in that graphite particles are subjected to electrolysis in an electrolytic solution containing a substance capable of intruding into the interlayer spacings of the graphite while applying a load to the graphite particles in at least one direction to press all the graphite particles to the surface of an anode. According to the method, there can be obtained the desired porduct having a uniform and high quality.

This invention relates to a novel method of producing a graphite 
intercalation compound. More particularly, the present invention is 
concerned with a method of producing a graphite intercalation compound by 
intercalating a substance into graphite between the layers thereof, 
characterized in that the graphite particles are subjected to electrolysis 
in an electrolytic solution containing a substance capable of intruding 
into the interlayer spacings of the graphite while applying pressure to 
the graphite particles in at least one direction to press all the graphite 
particles against the surface of an anode. 
In general, graphite is a hexagonal system crystal of a packing structure 
in which hexagonal network faces, each formed by covalent bonding of each 
carbon atom with its adjacent carbon atoms, are stacked. The bonding 
between the layers which are stacked in a direction perpendicular to the 
hexagonal network face is very weak. Therefore, a graphite intercalation 
compound can be obtained by intercalating a substance capable of intruding 
into the interlayer spacings of graphite (hereinafter often referred to as 
"intruding substance") into graphite between the layers thereof. 
Usually, the graphite intercalation compound is classified in three groups 
of compounds, namely, a non-conductive graphite intercalation compound, a 
conductive graphite intercalation compound and a residue compound. The 
conductive graphite intercalation compound is further classified in two 
groups of compounds, namely, an electrolytic intercalation compound and a 
non-electrolytic intercalation compound. 
In preparing an electrolytic intercalation compound, an auxiliary means is 
used to cause a reaction of the intruding substance with the graphite to 
be promoted, because the intruding substance to be intercalated into the 
interlayer spacings of the graphite does not react by its own chemical 
nature with the graphite. As the auxiliary means, an external battery, as 
an external power source can be used. For example, Gerhart Henning has 
obtained graphite bisulfate, which is an electrolytic intercalation 
compound produced by electrolytic oxidation of the graphite employed as 
the anode in concentrated sulfuric acid (see The Journal of Chemical 
Physics, Vol. 19, No. 7, July 1951, pp. 922-929). 
Further, an intercalation compound which has an identical structure with 
that of the electrolytic intercalation compound can be obtained by the use 
of an appropriate oxidizer, instead of the external battery, as an 
oxidation promotor together with an intruding substance. For example, as 
disclosed in U.S. Pat. No. 3,404,061, graphite bisulfate or graphite 
nitrate can be obtained by dipping graphite particles in an oxidative 
mixture of concentrated sulfuric acid, as the intruding substance, and 
concentrated nitric acid, a nitrate, chromic acid, potassium chromate, 
potassium dichromate, a chlorate, perchloric acid and/or the like as an 
oxidizing agent. Also, fuming nitric acid can be utilized, as a material 
capable of functioning both as the intruding substance and the oxidizing 
agent (in this instance, nitrogen dioxide is supposed to act as the 
oxidizing agent), or graphite particles can be dipped in an oxidative 
mixture of concentrated nitric acid, as the intruding substance, and 
potassium chlorate, potassium permanganate and/or the like, as the 
oxidizing agent. 
When the graphite intercalation compound, as mentioned above, is heated at 
a high temperature, for example, at 600.degree.-1,300.degree. C., the 
graphite intercalation compound is expanded in a direction perpendicular 
to the faces of the layers of graphite, that is, in a direction of c-axis 
to obtain an expanded graphite having extremely low bulk density. The 
expanded graphite has excellent characteristics inherent of graphite, such 
as high thermal resistance, lubricity, chemical resistance and the like. 
Further, the expanded graphite can be easily formed into a shaped product 
having good flexibility by subjecting it alone or together with a suitable 
binder, such as a phenolic resin, to compression molding. Therefore, the 
expanded graphite is very useful as a raw material for producing sealing 
articles, such as gaskets and packings. 
In the conventional method of producing the electrolytic graphite 
intercalation compound by electrolytic oxidation of graphite, the 
diffusion resistance of the intruding substance is large due to the 
insufficient mutual contact of the graphite particles and, hence, the 
reaction velocity is reduced and the reaction tends to be non-uniform, 
thus causing the production of a product of uniform quality to be 
difficult. For this reason, there is commercially adopted a method of 
producing the same kind of graphite intercalation compound as that of the 
electrolytic graphite intercalation compound, which method comprises 
treating graphite with an oxidative medium. However, the above-mentioned 
commercially adopted method has various drawbacks. For example, a large 
amount of concentrated acid and, in some cases, a poisonous 
metal-containing oxidizing agent is used in the method. Therefore, there 
is a danger to the workers. Further, there is a problem of selection of a 
material for the apparatus to be used in manufacturing the product. Still 
further, a large amount of alkali is required to neutralize the waste acid 
discharged from the process. Still further, there is a possibility of 
causing environmental pollution due to acidic gas (SO.sub.2 and/or 
sulfuric acid mist), nitrogen oxides (NO.sub.x) and chromium. It goes 
without saying that there is required a large amount of expenditure for 
disposing the poisonous materials and preventing environmental pollution. 
In addition, there is a trouble that when the product and the electrolyte 
are separated by means of a centrifugal separator, the centrifugation 
cannot be carried out before the concentrated acid is diluted due to the 
properties of the material used for making the centrifugal separator. 
Accordingly, it is an object of the present invention to provide a method 
of producing a graphite intercalation compound having uniform and high 
quality, which method also eliminates the troubles and problems of the 
conventional methods. 
It is another object of the present invention to provide a method of 
producing a graphite intercalation compound, characterized in that 
graphite is electrolyzed in an electrolytic solution having a 
comparatively low concentration so that the operation can be easily 
carried out without encountering any troubles inevitably accompanying the 
conventional method, and that at least part of the used electrolytic 
solution and the washings obtained in the process of water-washing of the 
product can be recycled for re-use as a part of the electrolytic solution 
so that the method is economic and material-saving.

In one and a principal aspect of the present invention, there is provided a 
method of producing a graphite intercalation compound comprising 
electrolyzing (using an electrolytic cell including a cathode, an anode 
and an anode chamber adapted to accomodate therein graphite particles) 
graphite particles in an electrolytic solution containing a substance 
capable of intruding into the interlayer spacings of graphite thereby to 
intercalate said substance into the interlayer spacings of graphite, 
characterized in that graphite particles are subject to electrolysis while 
applying pressure by means of a load to the graphite particles accomodated 
in the anode chamber in at least one direction to press all the graphite 
particles against the surface of the anode. In another aspect of the 
present invention, there is provided a method of the character described 
above, characterized in that at least part of a used electrolytic solution 
obtained by the electrolysis and a washing obtained by water-washing the 
graphite intercalation compound obtained by the electrolysis is recycled, 
after a substance capable of intruding into the interlayer spacings of 
graphite is replenished to adjust the concentration of the used 
electrolytic solution and the washing if required, for re-use as an 
electrolytic solution. 
The essential feature of the present invention resides in that the graphite 
particles accomodated in the anode chamber are electrolyzed while applying 
pressure by means of a load to the graphite particles accomodated in the 
anode chamber in at least one direction to press all the graphite 
particles against the surface of the anode. Accordingly, the mutual 
contact of the graphite particles is improved, and not only does it become 
easy to treat the waste acid but also the operation can be carried out at 
low cost. In addition, there can be eliminated the problems of 
environmental pollution due to acidic gas, NO.sub.x and chromium. Thus, 
many advantages can be attained by the method of the present invention. 
A detailed explanation of the method of producing a graphite intercalation 
compound according to this invention will be made, referring to FIGS. 1 
and 2. The direction in which pressure by means of a load is applied to 
the graphite particles may be any direction in so far as all the graphite 
particles are pressed against the surface of anode. In order to give an 
illustrative explanation, two forms of the apparatus or electrolytic cell 
are shown in FIGS. 1 and 2. 
Referring to FIG. 1, the graphite particles 1 are accomodated in an anode 
chamber defined by two liquid-permeable partition plates 6 (for example, 
glass filter) which are capable of resisting a load applied to the 
graphite particles. One of the partition plates 6 is closely attached to a 
porous anode plate 3 which allows the electrolytic solution to pass 
through. In FIG. 1, the electrolytic cell further comprises a cathode 
plate 4, an O-ring 7, a clamping member 8, and a double packing 9. The 
electrolytic cell body is divided into two portions so that a lead wire 
can be provided between the double packing 9. However, the electrolytic 
cell body is not required to be divided into two portions provided that 
the lead wire can be brought out through the side wall. A weight 5 is 
placed in such a manner that the load is applied in the direction 
perpendicular to the surface of the anode plate 3 and over the bulk of the 
graphite particles through the upper partition plate 6 and the anode plate 
closely attached thereto, so that all the graphite particles are pressed 
against the lower surface of the anode plate 3. The electrolytic solution 
2 is introduced as an anolyte from an inlet shown by an arrow 10 and 
discharged as a catholyte from an outlet shown by another arrow 11. 
Referring to FIG. 2, graphite particles 12 are accomodated, in the form of 
a bulk, in an anode chamber provided with an anode cylinder 14 and defined 
by a liquid-permeable cylindrical diaphragm 17, which is capable of 
resisting a load to be applied to the graphite particles, and a pressing 
plate formed at the lower end of a cylindrical support 18 which is 
provided for supporting the bed of a weight 16. A pressing plate 18' is 
arranged so that a liquid permeable cloth 19 is held between two plates 
provided with a plurality of through-holes and then secured by a retaining 
means 21. The electrolytic cell shown in FIG. 2 is provided with a cathode 
cylinder 15 of a net structure and an O-ring 20. In the electrolytic cell, 
a weight 16 serves to apply a load in the direction parallel to the 
surface of the anode cylinder 14 and then on the bulk of the graphite 
particles through the pressing plate 18' formed at the lower end of the 
cylindrical support 18. Consequently, all the graphite particles are 
pressed against the surface of the anode cylinder. The electrolytic 
solution 13 is introduced from an inlet shown by an arrow 22 as an anolyte 
and discharged from an outlet shown by another arrow 23. As described, the 
electrolytic oxidation of the graphite is carried out while pressing all 
the graphite particles against the surface of the anode. In the 
electrolytic oxidation, the lower limit of the load is usually 10 
g/cm.sup.2, preferably 30 g/cm.sup.2, though it varies according to the 
specific gravity of the electrolytic solution and the particle size of the 
graphite particles. The upper limit of the load is usually 5 Kg/cm.sup.2, 
and preferably 100 g/cm.sup.2. When a load of more than 5 Kg/cm.sup.2 is 
applied to the graphite particles, there is not any unfavorable effect on 
the formation of a graphite intercalation compound, but there is required 
an increased resistivity of an apparatus to the load, and therefore, too 
heavy a load is not economical. In a factory, a load is usually produced 
utilizing air pressure or hydraulic pressure. 
In the present invention, the particle size of the graphite particles is 
not critical, but may generally be 20-150 mesh (Tyler). 
As Examples of the intruding substance, there can be mentioned BF.sub.3 
(CH.sub.3 COOH).sub.2, CF.sub.3 COOH, H.sub.2 F.sub.2, H.sub.3 PO.sub.4, 
H.sub.3 AsO.sub.4, HClO.sub.4, HNO.sub.3 and the like. However, H.sub.2 
F.sub.2 and HClO.sub.4 are generally not so preferable due to generation 
of gas when the graphite intercalation compound is heated to obtain an 
expanded graphite. However, they still may be useful for graphite 
intercalation compounds used for other purposes. Since the concentration 
of the graphite intercalation compound in the final product is 
equilibrated with the concentration of the electrolytic solution, it is 
preferred that the concentration of the intruding substance-containing 
electrolytic solution is high. The higher the concentration of the 
electrolytic solution, the more smooth the reaction. However, it is to be 
noted that a high concentration such as 95% or more, which is required in 
the method of producing a graphite intercalation compound by the treatment 
of graphite with chemicals, is not necessary for the method of the present 
invention. The lower limit of the concentration of the intruding substance 
in the electrolytic solution varies depending on the kind of the intruding 
substance, but may generally be 1-3 mols/liter, preferably 3 mols/liter or 
more. The most preferred intruding substances are sulfuric acid and nitric 
acid. In the case of sulfuric acid, the concentration of the electrolytic 
solution is 30% by weight or more, preferably 50% by weight or more. In 
the case of nitric acid, the concentration of the electrolytic solution is 
20% by weight or more, preferably 30% by weight or more. By the use of the 
above-mentioned concentrations of the electrolytic solution, it is 
possible to obtain graphite hydrogensulfate or graphite nitrate having a 
uniform and high quality. 
The electric current density of the anode is suitably up to 500 mA/cm.sup.2 
and the electrolysis must be carried out at an anode electric current 
density of less than that mentioned-above. In order to obtain an 
intercalation compound with a high current efficiency, the current density 
is suitably 50 mA/cm.sup.2 or less. Room temperature is suitable as an 
electrolytic reaction temperature. According to the kind of intruding 
substance used, however, control of the temperature is necessary to reduce 
volatilization of the intruding substance in the electrolytic solution 
because the temperature rises with the progress of the electrolysis. In 
order to prevent the rise of the temperature and to continue the 
electrolysis at a constant temperature, it is desirable to circulate the 
electrolytic solution through a cooler by means of a circulation pump. 
A more desirable intercalation compound having a uniform quality can be 
obtained if an alternating current of 0.1-100 Hz is applied after 
completion of the electrolytic oxidation, thereby increasing the effect of 
the present invention. 
In another aspect of the present invention, at least part of the used 
electrolytic solution and the washing obtained from the washing step of 
the graphite intercalation compound, is recycled for re-use. The used 
electrolytic solution and/or the washings (which is obtained in the 
earlier stage of the washing step and still includes the intruding 
substance at high concentration) are desirably re-used as the electrolytic 
solution by replenishing them with the intruding substance to adjust the 
concentration of the used electrolytic solution and the washings to the 
predetermined value, and therefore the utilization efficiency of the 
intruding substance can be increased. This recycling not only eliminates 
the necessity of a large amount of a neutralizing material, but also 
prevents environmental pollution, leading to great advantages. The 
concentration of the intruding substance in the washings for recycling is 
determined according to the production amount of the intercalation 
compound and the manufacturing cost therefor. Further, it is to be noted 
that an electrolytic solution having a comparatively low concentration can 
be used in the method of the present invention so that it is possible to 
recycle and re-use the used electrolytic solution and/or the washings. In 
the aforementioned conventional method in which graphite is treated with a 
chemical by dipping, a treating liquid should have a high concentration 
and, therefore, recycling for re-use of the used electrolytic solution 
and/or washings is difficult to achieve. 
As described, according to the method of the present invention, the 
graphite intercalation compound having homogeneity can be advantageously 
obtained, overcoming difficulties inevitably accompanying the conventional 
dipping method. 
The method according to the present invention has such an advantage that 
there may be used an electrolytic solution having a low concentration of 
the intruding substance when compared with that of the treating solution 
to be used in the conventional dipping method, so that it is possible to 
recycle and re-use the used electrolytic solution and the washings. 
Further, in the present invention, it is not required to additionally use 
strong oxidizing agents which cause various problems, and the reaction 
velocity can be electrically maintained at a predetermined value without 
regard to the concentration of the electrolytic solution and temperature 
while providing the graphite intercalation compound having a uniform and 
high quality. Furthermore, it should be noted that in the method of the 
present invention there is no need of such a complicated operation that 
the anode chamber is rotated. 
The present invention will be illustrated with reference to the following 
Examples which should not be construed as limiting the scope of the 
present invention. 
EXAMPLE 1 
Using an electrolytic cell as shown in FIG. 1, 100 g of Madagascar-produced 
graphite particles (bulk density of 0.65 g/cm.sup.3) having a particle 
size of 42 to 80 mesh (Tyler) and a peak of the particle size distribution 
at 60 mesh were accommodated in a cylindrical anode chamber having an 
inner diameter of about 10 cm and provided with two glass filter-made 
partition plates. By means of a liquid-permeable pressing plate composed 
of a platinum-made perforated circular anode plate and one of the 
above-mentioned two partition plates, a load of 30 g/cm.sup.2 was applied 
to the graphite particles. A cathode chamber was provided with a 
platinum-made cathode plate. A 50% aqueous sulfuric acid solution as an 
electrolytic solution was introduced into the electrolytic cell. 
Thereafter, while circulating the electrolytic solution by means of a 
circulation pump and flowing a current constantly having a current density 
of 40 mA/cm.sup.2 for 7 hours, the graphite particles were subjected to 
electrolysis. In this instance, while the voltage was 2.3 V at the initial 
stage, at the final stage the voltage was elevated to 4.4 V, the electric 
resistance increased from 1.15.OMEGA. to 2.20.OMEGA. and the temperature 
of the electrolytic solution increased from 11.degree. C. to 38.5.degree. 
C. 
After the electrolytic oxidation, the product was sufficiently washed with 
water and dried at 95.degree. C..+-.5.degree. C. The graphite bisulfate 
thus produced had a bulk density of 0.35 g/cm.sup.3. When the product was 
heated at 1,000.degree. C. for 1 minute, there was obtained an expanded 
graphite having a bulk density of 0.004 g/cm.sup.3. This means that the 
expansion rate was 163 times. 
EXAMPLE 2 
Using an electrolytic cell as shown in FIG. 2, 100 g of Madagascar-produced 
graphite particles (bulk density of 0.65 g/cm.sup.3) having a particle 
size of 42 to 80 mesh and a peak of the particle size distribution at 60 
mesh were accomodated in a cylindrical anode chamber having an inner 
diameter of about 10 cm and provided with a titanium-made anode plate. By 
means of a liquid-permeable pressing plate which is airtightly, vertically 
movable along the periphery of an unglazed cermic-made partition cylinder 
having therein a titanium-made perforated cylindrical cathode, a load of 
60 g/cm.sup.2 was applied to the graphite particles. A 30% aqueous nitric 
acid solution as an electrolytic solution was introduced into the 
electrolytic cell. While circulating the electrolytic solution by means of 
a circulation pump so that the temperature of the electrolytic solution 
was maintained at 25.degree. C. during the flowing of current, current 
constantly having a current density of 50 mA/cm.sup.2 was flowed for 6 
hours, thereby to electrolytically oxidize the graphite particles. In the 
course of the electrolysis, the voltage changed from 2.1 V to 4.5 V and 
the electric resistance changed from 1.1.OMEGA. to 2.15.OMEGA.. 
After the electrolytic oxidation, the product was sufficiently washed with 
water and air-dried. The graphite nitrate thus produced had a bulk density 
of 0.4 g/cm.sup.3. When the product was heated at 1,000.degree. C. for 1 
minute, there was obtained an expanded graphite having a bulk density of 
0.006 g/cm.sup.3. This means that the expansion rate was 108 times. 
EXAMPLE 3 
Using Madagascar-produced graphite particles (bulk density of 0.65 
g/cm.sup.3) having a particle size of 42 to 80 mesh and a peak of the 
particle size distribution at 60 mesh, the electrolytic oxidation was 
conducted in the same manner as in Example 1. After completion of the 
electrolytic oxidation, the electrolytic solution was separation-recovered 
from the product by means of a centrifugal separator. The resulting 
graphite intercalation compound was sufficiently washed with water and 
dried at 90.degree. C., whereupon the bulk density was measured. The 
product was heated at 1,000.degree. C. for 1 minute to obtain an expanded 
graphite. The bulk density of the expanded graphite was measured. The 
expansion rate was obtained from the bulk density ratio. 
Using the used electrolytic solution recovered by the centrifugation, the 
electrolytic oxidation of graphite particles was conducted in the same 
manner as in Example 1. Similar procedures were repeated five times. The 
concentration of the electrolytic solution (i.e. concentration of sulfuric 
acid) was measured, after every electrolytic oxidation, by titration with 
a 1 N aqueous sodium hydroxide solution. The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Concentration 
Bulk density 
Run of electrolytic After heat- 
Expansion rate, 
No. solution, wt % 
Product expansion 
times 
______________________________________ 
1 50.0 0.36 0.004 163 
2 49.4 0.38 0.004 163 
3 48.9 0.40 0.005 130 
4 48.5 0.39 0.004 163 
5 47.9 0.41 0.005 130 
6 47.4 0.41 0.005 130 
______________________________________ 
As is apparent from Table 1, according to the present invention, even if an 
intruding substance is replenished, the electrolytic solution used in the 
method of this invention can be used repeatedly to give graphite bisulfate 
without any substantial change in bulk density and expansion rate.