U.S. patent application number 15/643887 was filed with the patent office on 2017-10-26 for amorphous carbon particles.
The applicant listed for this patent is JFE CHEMICAL CORPORATION. Invention is credited to Ryuta HAGA, Makiko IJIRI, Katsuhiro NAGAYAMA, Tetsuo SHIODE.
Application Number | 20170309895 15/643887 |
Document ID | / |
Family ID | 48612186 |
Filed Date | 2017-10-26 |
United States Patent
Application |
20170309895 |
Kind Code |
A1 |
IJIRI; Makiko ; et
al. |
October 26, 2017 |
AMORPHOUS CARBON PARTICLES
Abstract
A method for producing amorphous carbon particles comprising
includes adding and mixing graphite particles into a precursor of
amorphous carbon and then cross-linking the precursor of amorphous
carbon to obtain a first cross-linked product, or cross-linking a
precursor of amorphous carbon and then adding and mixing graphite
particles into the cross-linked precursor of amorphous carbon to
obtain a second cross-linked product. Infusibility is imparted to
the first or second cross-linked product to obtain an infusibilized
product to which infusibility has been imparted. The infusibilized
product is baked to obtain amorphous carbon particles. The
amorphous carbon particles include the graphite particles and
amorphous carbon which embeds the graphite particles.
Inventors: |
IJIRI; Makiko; (Tokyo,
JP) ; HAGA; Ryuta; (Tokyo, JP) ; SHIODE;
Tetsuo; (Tokyo, JP) ; NAGAYAMA; Katsuhiro;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE CHEMICAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
48612186 |
Appl. No.: |
15/643887 |
Filed: |
July 7, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14304660 |
Jun 13, 2014 |
9735421 |
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15643887 |
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PCT/JP2012/007940 |
Dec 12, 2012 |
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14304660 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2220/20 20130101;
H01M 10/0525 20130101; H01M 4/043 20130101; Y02P 70/54 20151101;
H01M 4/133 20130101; Y02E 60/10 20130101; Y02E 60/122 20130101;
H01M 4/1393 20130101; H01M 4/583 20130101; Y02P 70/50 20151101;
H01M 4/587 20130101; C01B 32/05 20170801; H01M 4/0471 20130101 |
International
Class: |
H01M 4/1393 20100101
H01M004/1393; H01M 4/04 20060101 H01M004/04; H01M 4/133 20100101
H01M004/133; H01M 4/587 20100101 H01M004/587; H01M 4/583 20100101
H01M004/583; H01M 4/04 20060101 H01M004/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 2011 |
JP |
2011-275910 |
Oct 24, 2012 |
JP |
2012-234919 |
Claims
1. Amorphous carbon particles comprising: graphite particles; and
amorphous carbon embedding the graphite particles.
2. The amorphous carbon particles according to claim 1, wherein a
specific surface of the amorphous carbon particles is 10 m.sup.2/g
or less.
3. The amorphous carbon particles according to claim 1, wherein a
specific surface of the amorphous carbon particles is 0.5 to 8
m.sup.2/g.
4. The amorphous carbon particles according to claim 1, wherein a
specific surface of the amorphous carbon particles is 1.0 to 4
m.sup.2/g.
5. The amorphous carbon particles according to claim 1, wherein a
specific surface of the amorphous carbon particles is 1.2 to 3.5
m.sup.2/g.
6. The amorphous carbon particles according to claim 1, wherein a
true specific gravity of the amorphous carbon particles is from
1.600 to 1.700 according to JIS R7222:1997.
7. The amorphous carbon particles according to claim 1, wherein a
true specific gravity of the amorphous carbon particles is from
1.600 to 1.690 according to JIS R7222:1997.
8. The amorphous carbon particles according to claim 1, wherein a
true specific gravity of the amorphous carbon particles is from
1.620 to 1.685 according to JIS R7222:1997.
9. The amorphous carbon particles according to claim 1, wherein a
content of the graphite particles in the amorphous carbon particles
is from 1% to 50% by mass.
10. The amorphous carbon particles according to claim 1, wherein a
content of the graphite particles in the amorphous carbon particles
is from 5% to 20% by mass.
11. The amorphous carbon particles according to claim 1, wherein a
content of the graphite particles in the amorphous carbon particles
is from 10% to 20% by mass.
12. The amorphous carbon particles according to claim 1, wherein an
average particle size of the graphite particles in the amorphous
carbon particles is from 1 to 25 .mu.m.
13. The amorphous carbon particles according to claim 1, wherein an
average particle size of the amorphous carbon particles is from 2
to 15 .mu.m.
14. The amorphous carbon particles according to claim 1, wherein an
average particle size of the graphite particles in the amorphous
carbon particles is from 3 to 15 .mu.m.
15. The amorphous carbon particles according to claim 1, wherein
the amorphous carbon particles are produced by a method comprising:
adding and mixing an addition amount of graphite particles into a
precursor of amorphous carbon and then cross-linking the precursor
of amorphous carbon to obtain a first cross-linked product, wherein
the precursor is in a fluid state and the graphite particles are
added to the precursor with stirring, or cross-linking a precursor
of amorphous carbon and then adding and mixing graphite particles
into the cross-linked precursor of amorphous carbon to obtain a
second cross-linked product, wherein the cross-linked precursor is
in a fluid state and the graphite particles are added to the
cross-linked precursor with stirring; subjecting the first or
second cross-linked product to mechanochemical treatment to obtain
a mechanochemically treated product; imparting infusibility to the
mechanochemically treated product to obtain an infusibilized
product; and baking the infusibilized product to obtain amorphous
carbon particles, the amorphous carbon particles including the
graphite particles and amorphous carbon embedding the graphite
particles.
16. The amorphous carbon particles according to claim 15, wherein
the addition amount of the graphite particles is from 1% to 50% by
mass based on an amount of the precursor of amorphous carbon.
17. The amorphous carbon particles according to claim 15, wherein
an average particle size of the graphite particles in the amorphous
carbon particles is from 1 to 25 .mu.m.
18. The amorphous carbon particles according to claim 15, wherein a
true specific gravity of the amorphous carbon particles is from
1.600 to 1.700 according to JIS R7222:1997.
19. The amorphous carbon particles according to claim 15, wherein a
content of the graphite particles in the amorphous carbon particles
is from 1% to 50% by mass.
20. The amorphous carbon particles according to claim 15, wherein a
specific surface of the amorphous carbon particles is 10 m.sup.2/g
or less.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation application of
International Application No. PCT/JP2012/007940, filed Dec. 12,
2012, which claims priority to Japanese Patent Application No.
2011-275910, filed Dec. 16, 2011 and to Japanese Patent Application
No. 2012-234919, filed Oct. 24, 2012. The contents of these
applications are incorporated herein by reference in their
entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to a method for producing
amorphous carbon particles, amorphous carbon particles, a negative
electrode material for a lithium ion secondary battery, and a
lithium ion secondary battery.
Discussion of the Background
[0003] Conventionally, mainly from the viewpoint of price and
weight reduction, nickel metal hydride batteries have been used as
rechargeable batteries for hybrid cars. In order to further reduce
weight, the use of lithium ion secondary batteries which have a
high voltage and a high energy density has been anticipated.
[0004] Regarding batteries for cars driven by batteries only, such
as electric cars, in order to ensure a sufficient distance per
charge, the use of materials having a high energy density, and
graphite-based materials in negative electrodes, has been widely
studied.
[0005] On the other hand, in a system in which the volume to be
loaded in a car is small and it is necessary to regenerate energy
during deceleration by braking, such as in a battery for a hybrid
car, there has been a demand for batteries having high
charge/discharge specific output density, and use of amorphous
carbon particles, a representative example of which is hard carbon,
has been studied. Background art relating to the amorphous carbon
particles is described in, for example, Japanese Unexamined Patent
Application Publication No. 3-252053, Japanese Unexamined Patent
Application Publication No. 6-89721, Japanese Unexamined Patent
Application Publication No. 8-115723, and Japanese Unexamined
Patent Application Publication No. 9-153359.
[0006] Amorphous carbon particles have a low true specific gravity
compared with graphite particles (i.e., when measured in accordance
with a method for measuring true specific gravity using butanol,
the true specific gravity of a graphite-based material is about
2.23, while the true specific gravity of common hard carbon is low
at 1.5 to 1.6 although depending on the material), and amorphous
carbon particles are hard particles. Therefore, it is difficult to
improve the electrode density, and the electrode press formability
may degrade in some cases.
[0007] Furthermore, amorphous carbon particles tend to have low
conductivity compared with graphite particles. Improvement in
conductivity is believed to be an effective measure to improve
charge/discharge, input/output characteristics per second in which
electron conductivity mainly dominates.
[0008] Furthermore, in the process of fabricating an electrode
using amorphous carbon particles, it is also conceivable to add
afterwards graphite particles having good conductivity, as a
conductive auxiliary material. However, for example, in the case
where an electrolyte solution containing propylene carbonate which
has excellent low-temperature characteristics and which does not
solidify even in cold climates is used, reaction (decomposition
reaction) occurs between graphite particles and the electrolyte
solution, and charging is not performed, adversely affecting the
battery performance.
SUMMARY OF THE INVENTION
[0009] According to one aspect of the present invention, a method
for producing amorphous carbon particles comprising includes adding
and mixing graphite particles into a precursor of amorphous carbon
and then cross-linking the precursor of amorphous carbon to obtain
a first cross-linked product, or cross-linking a precursor of
amorphous carbon and then adding and mixing graphite particles into
the cross-linked precursor of amorphous carbon to obtain a second
cross-linked product. Infusibility is imparted to the first or
second cross-linked product to obtain an infusibilized product to
which infusibility has been imparted. The infusibilized product is
baked to obtain amorphous carbon particles. The amorphous carbon
particles include the graphite particles and amorphous carbon which
embeds the graphite particles.
[0010] According to another aspect of the present invention, a
method for producing amorphous carbon particles includes adding and
mixing graphite particles into a precursor of amorphous carbon and
then cross-linking the precursor of amorphous carbon to obtain a
first cross-linked product, or cross-linking a precursor of
amorphous carbon and then adding and mixing graphite particles into
the cross-linked precursor of amorphous carbon to obtain a second
cross-linked product. The first or second cross-linked product is
subjected to mechanochemical treatment to obtain a
mechanochemically treated product. Infusibility is imparted to the
mechanochemically treated product to obtain an infusibilized
product. The infusibilized product is baked to obtain amorphous
carbon particles. The amorphous carbon particles include the
graphite particles and amorphous carbon which embeds the graphite
particles.
[0011] According to further aspect of the present invention, a
method for producing amorphous carbon particles includes adding and
mixing graphite particles into a precursor of amorphous carbon and
then cross-linking the precursor of amorphous carbon to obtain a
first cross-linked product, or cross-linking a precursor of
amorphous carbon and then adding and mixing graphite particles into
the cross-linked precursor of amorphous carbon to obtain a second
cross-linked product. Infusibility is imparted to the first or
second cross-linked product to obtain an infusibilized product. The
infusibilized product is subjected to mechanochemical treatment to
obtain a mechanochemically treated product. The mechanochemically
treated product is baked to obtain amorphous carbon particles. The
amorphous carbon particles include the graphite particles and
amorphous carbon which embeds the graphite particles.
[0012] According to further aspect of the present invention,
amorphous carbon particles include graphite particles, and
amorphous carbon which embeds the graphite particles.
[0013] According to further aspect of the present invention, a
negative electrode material for a lithium ion secondary battery
includes the amorphous carbon particles.
[0014] According to further aspect of the present invention, a
lithium ion secondary battery includes a negative electrode
material which includes the amorphous carbon particles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings.
[0016] FIG. 1 is a cross-sectional view showing a coin-type
secondary battery for evaluation.
[0017] FIG. 2 is a polarization microphotograph showing a
cross-section of amorphous carbon particles.
DESCRIPTION OF THE EMBODIMENTS
[0018] Embodiments of the present invention provide the following
(1) to (11).
[0019] (1) A method for producing amorphous carbon particles
including a step of adding and mixing graphite particles into a
precursor of amorphous carbon, and then performing cross-linking
treatment to obtain a first cross-linked product, or a step of
subjecting a precursor of amorphous carbon to cross-linking
treatment, and then adding and mixing graphite particles thereinto
to obtain a second cross-linked product; an infusibility-imparting
treatment step of subjecting the first or second cross-linked
product to infusibility-imparting treatment to obtain a product
subjected to infusibility-imparting treatment (also referred to as
"an infusibilized product"); and a step of baking the product
subjected to infusibility-imparting treatment to obtain amorphous
carbon particles which contain the graphite particles.
[0020] (2) A method for producing amorphous carbon particles
including a step of adding and mixing graphite particles into a
precursor of amorphous carbon, and then performing cross-linking
treatment to obtain a first cross-linked product, or a step of
subjecting a precursor of amorphous carbon to cross-linking
treatment, and then adding and mixing graphite particles thereinto
to obtain a second cross-linked product; a mechanochemical
treatment step of subjecting the first or second cross-linked
product to mechanochemical treatment to obtain a mechanochemically
treated product; an infusibility-imparting treatment step of
subjecting the mechanochemically treated product to
infusibility-imparting treatment to obtain a product subjected to
infusibility-imparting treatment; and a step of baking the product
subjected to infusibility-imparting treatment to obtain amorphous
carbon particles which contain the graphite particles.
[0021] (3) A method for producing amorphous carbon particles
including a step of adding and mixing graphite particles into a
precursor of amorphous carbon, and then performing cross-linking
treatment to obtain a first cross-linked product, or a step of
subjecting a precursor of amorphous carbon to cross-linking
treatment, and then adding and mixing graphite particles thereinto
to obtain a second cross-linked product; an infusibility-imparting
treatment step of subjecting the first or second cross-linked
product to infusibility-imparting treatment to obtain a product
subjected to infusibility-imparting treatment; a mechanochemical
treatment step of subjecting the product subjected to
infusibility-imparting treatment to mechanochemical treatment to
obtain a mechanochemically treated product; and a step of baking
the mechanochemically treated product to obtain amorphous carbon
particles which contain the graphite particles.
[0022] (4) The method for producing amorphous carbon particles
according to any one of (1) to (3), in which the addition amount of
the graphite particles is 1% to 50% by mass relative to the
precursor of amorphous carbon.
[0023] (5) The method for producing amorphous carbon particles
according to any one of (1) to (4), in which the average particle
size of the graphite particles is 1 to 25 .mu.m.
[0024] (6) Amorphous carbon particles characterized by containing
graphite particles.
[0025] (7) The amorphous carbon particles according to (6),
characterized in that the true specific gravity of the amorphous
carbon particles is 1.600 to 1.700.
[0026] (8) The amorphous carbon particles according to (6) or (7),
in which the content of the graphite particles is 1% to 50% by
mass.
[0027] (9) The amorphous carbon particles according to any one of
(6) to (8), in which the average particle size of the graphite
particles is 1 to 25 .mu.m.
[0028] (10) A negative electrode material for a lithium ion
secondary battery including the amorphous carbon particles
according to any one of (6) to (9).
[0029] (11) A lithium ion secondary battery in which the amorphous
carbon particles according to any one of (6) to (9) are used as a
negative electrode material.
[0030] According to the embodiment of the present invention, it is
possible to obtain amorphous carbon particles having excellent
press formability and conductivity while suppressing reactivity
with an electrolyte solution. The embodiments will now be described
with reference to the accompanying drawings, wherein like reference
numerals designate corresponding or identical elements throughout
the various drawings.
[0031] [Method for Producing Amorphous Carbon Particles]
[0032] The method for producing amorphous carbon particles
according to the embodiment of the present invention (hereinafter,
may be simply referred to as the "production method of the
embodiment of the present invention") roughly includes a step of
adding and mixing graphite particles into a precursor of amorphous
carbon, and then performing cross-linking treatment to obtain a
first cross-linked product, or a step of subjecting the precursor
of amorphous carbon to cross-linking treatment and then adding and
mixing the graphite particles thereinto to obtain a second
cross-linked product; and a step of subjecting the first or second
cross-linked product to infusibility-imparting treatment, and then
performing baking to obtain amorphous carbon particles which
contain the graphite particles. Furthermore, mechanochemical
treatment may be performed before or after the
infusibility-imparting treatment.
[0033] The production method of the embodiment of the present
invention will be described in detail below.
[0034] [Precursor of Amorphous Carbon]
[0035] The precursor of amorphous carbon used in the embodiment of
the present invention is not particularly limited, and any known
precursor can be used. Examples thereof include pitches, such as a
coal-based pitch and a petroleum-based pitch; resins, such as a
phenolic resin and a furan resin; mixtures of a pitch and a resin;
and the like. Above all, from the viewpoint of economics and the
like, a pitch, such as a coal-based pitch or a petroleum-based
pitch, is preferable. Specific examples of the coal-based pitch
include coal-tar pitch, coal-liquefied pitch, and the like.
[0036] In the case where a pitch is used, from the viewpoint of
increasing the battery capacity, the quinoline insoluble (QI)
content is preferably 0% to 2% by mass, although not particularly
limited.
[0037] [Graphite Particles]
[0038] The graphite particles used in the embodiment of the present
invention are not particularly limited. Examples thereof include
natural graphite; artificial graphite; composite particles in which
these graphites are coated with amorphous carbon; and the like.
Furthermore, examples of natural graphite include scaly graphite,
flake graphite, spheroidal graphite, earthy graphite, and the like.
Above all, flake graphite is preferable. Examples of artificial
graphite include a product obtained by graphitizing mesocarbon
microbeads, a product obtained by pulverizing and graphitizing a
bulk mesophase, a product obtained by graphitizing mesophase carbon
fibers, a product obtained by graphitizing highly crystalline
needle coke, and the like. Artificial graphite made from mesocarbon
microbeads can be produced, for example, by a method described in
Japanese Patent No. 3866452.
[0039] The average particle size of the graphite particles, which
depends on the particle size of final amorphous carbon particles,
is preferably 1 to 25 .mu.m, and more preferably 3 to 15 .mu.m.
When the particle size of the graphite particles is excessively
small, mixing becomes difficult, and the graphite particles are
unlikely to be contained. When the particle size is excessively
large, the probability of exposure of edge faces of the graphite
particles to the outside increases. When the particle size is in
the range described above, the graphite particles are likely to be
contained, and the possibility of exposure is reduced, thus
enhancing the advantage of the embodiment of the present
invention.
[0040] Note that, in the embodiment of the present invention, the
average particle size of the graphite particles is measured by a
laser diffraction particle size analyzer.
[0041] [Adding/Mixing]
[0042] According to an embodiment of the production method of the
present invention, first, graphite particles are added and mixed
into a precursor of amorphous carbon (hereinafter, may be simply
referred to as the "precursor"). The method therefor is not
particularly limited. For example, a method may be used in which,
using an autoclave equipped with a stirrer, the precursor is heated
to a fluid state, then the graphite particles are gradually added
thereinto under stirring, and stirring is continued until uniform
mixing is achieved.
[0043] In this case, the addition amount of the graphite particles,
which depends on the shape and crystallinity of the graphite
particles, is preferably, 1% to 50% by mass, and more preferably 5%
to 20% by mass, relative to the precursor. When the addition amount
is excessively large, the probability that all of the graphite
particles can be contained decreases. When the addition amount is
excessively small, it may be difficult to obtain the effect of
improving conductivity. When the addition amount is in the range
described above, almost all of the graphite particles can be
contained, thus enhancing the effect of improving conductivity.
[0044] [Cross-Linking Treatment]
[0045] Next, by performing cross-linking treatment, a cross-linked
product (first cross-linked product) is obtained. Examples of the
method for performing cross-linking treatment include a method
using an air-blowing reaction; a dry method using an oxidizing gas
(air, oxygen); a wet method using an aqueous solution of nitric
acid, sulfuric acid, hypochlorous acid, a mixed acid, or the like;
and others. Above all, a method using an air-blowing reaction is
preferable.
[0046] The air-blowing reaction is a reaction in which, for
example, by blowing an oxidizing gas (e.g., air, oxygen, ozone, or
a mixture thereof) into the reaction mixture under heating, the
softening point is increased. In accordance with the air-blowing
reaction, for example, it is possible to obtain a cross-linked
product (e.g., air-blown pitch) having a high softening point of
200.degree. C. or higher.
[0047] Japanese Unexamined Patent Application Publication No.
9-153359 describes that the air-blowing reaction is a reaction in a
liquid-phase state, and it is known that incorporation of oxygen
atoms into a carbon material hardly occurs compared with a
cross-linking treatment in a solid-phase state.
[0048] Furthermore, in the air-blowing reaction, a reaction mainly
consisting of an oxidative dehydration reaction proceeds, and
polymerization proceeds by biphenyl-type cross-linking. By the
subsequent imparting of infusibility and baking (which will be
described later), it is considered that carbon particles which have
a non-oriented, three-dimensional structure with the cross-linked
portions being dominant and with many voids in which lithium is
absorbed remaining are obtained.
[0049] The conditions for the air-blowing reaction are not
particularly limited. When the temperature is excessively high,
mesophases generate, and when the temperature is low, the reaction
rate decreases. For these reasons, the reaction temperature is
preferably 280.degree. C. to 420.degree. C., and more preferably
320.degree. C. to 380.degree. C. Furthermore, the blowing amount of
the oxidizing gas is, in the case of air, preferably 0.5 to 15
L/min, and more preferably 1.0 to 10 L/min, for 1,000 g of pitch.
The reaction pressure is not particularly limited, and may be a
normal pressure, a reduced pressure, or an increased pressure.
[0050] Furthermore, according to another embodiment of the
production method of the present invention, a cross-linked product
(second cross-linked product) may be obtained by, first, subjecting
a precursor of amorphous carbon to cross-linking treatment, and
then adding and mixing graphite particles thereinto. As the methods
for cross-linking treatment and adding and mixing, the same methods
as those described above may be used. Furthermore, the second
cross-linked product may be further subjected to cross-linking
treatment.
[0051] Hereinafter, the first cross-linked product and the second
cross-linked product may be collectively simply referred to as the
"cross-linked product".
[0052] The softening point of the cross-linked product thus
obtained is preferably 200.degree. C. to 400.degree. C., and more
preferably 250.degree. C. to 350.degree. C., from the viewpoint of
ease of infusibility-imparting treatment.
[0053] [Pulverization]
[0054] The resulting cross-linked product is preferably pulverized
to adjust the particle size. The pulverization method is not
particularly limited, and any known method can be used. The average
particle size after pulverization is, for example, preferably 1 to
50 .mu.m, and more preferably 2 to 15 .mu.m. Furthermore, such
pulverization may be performed on the product subjected to
infusibility-imparting treatment, which will be described
later.
[0055] Note that, in the embodiment of the present invention, the
average particle size after pulverization is measured by a laser
diffraction particle size analyzer.
[0056] [Infusibility-Imparting Treatment]
[0057] Next, the appropriately pulverized cross-linked product is
subjected to infusibility-imparting treatment to obtain a product
subjected to infusibility-imparting treatment. The
infusibility-imparting treatment is a kind of cross-linking
treatment (oxidation treatment) performed in a solid-phase state.
Oxygen is incorporated into the structure of the cross-linked
product, and since cross-linking further proceeds, the product
becomes difficult to melt at a high temperature.
[0058] The method for infusibility-imparting treatment is not
particularly limited. Examples thereof include a dry method using
an oxidizing gas (air, oxygen); a wet method using an aqueous
solution of nitric acid, sulfuric acid, hypochlorous acid, a mixed
acid, or the like; and others. Above all, a dry method using an
oxidizing gas is preferable.
[0059] As the treatment temperature of the infusibility-imparting
treatment, preferably, a temperature equal to or lower than the
softening point of the cross-linked product is selected.
Furthermore, in the case where batch processing is performed, the
heating rate is preferably 5.degree. C./hr to 100.degree. C./hr,
and more preferably 10.degree. C./hr to 50.degree. C./hr, from the
viewpoint of further preventing fusion.
[0060] The other treatment conditions of the infusibility-imparting
treatment are not particularly limited. For example, the blowing
amount of the oxidizing gas, as compressed air, is preferably 1.0
to 20 L/min, and more preferably 2.0 to 10 L/min. The reaction
pressure is not particularly limited, and may be a normal pressure,
a reduced pressure, or an increased pressure.
[0061] The oxygen amount in the product subjected to
infusibility-imparting treatment obtained by the
infusibility-imparting treatment is preferably 3% to 20% by mass,
and more preferably 5% to 15% by mass, from the viewpoint of
preventing fusion during baking.
[0062] [Baking]
[0063] After the infusibility-imparting treatment, by baking the
product subjected to infusibility-imparting treatment in a reduced
pressure or in an inert gas atmosphere, such as nitrogen, carbon
particles are obtained. In this case, the heating rate is
preferably 50.degree. C./hr to 150.degree. C./hr, and more
preferably 80.degree. C./hr to 120.degree. C./hr. Furthermore, the
final temperature (baking temperature) is preferably 1,000.degree.
C. to 1,300.degree. C., and more preferably 1,000.degree. C. to
1,200.degree. C.
[0064] Furthermore, in the embodiment of the present invention, the
cross-linked product or the product subjected to
infusibility-imparting treatment may be subjected to
mechanochemical treatment. Thereby, since particles rub together,
the particles obtained after baking have a rounded shape with
corners being removed. Thus, the electrode density is improved, and
the press formability can be enhanced.
[0065] The mechanochemical treatment refers to a treatment in which
compressive force and shear force are applied at the same time to
particles. Although the compressive force and shear force applied
are larger than those applied in general stirring, these mechanical
stresses are desirably applied to the surfaces of particles without
breaking the particles.
[0066] The apparatus used in the mechanochemical treatment may be
any apparatus that can apply compressive force and shear force at
the same time to particles. Examples of the apparatus that can be
used include a pressure kneader, a mixer such as a twin-roll mixer,
a rotary ball mill, a hybridization system (manufactured by Nara
Machinery Co., Ltd.), a Mechano Micros (manufactured by Nara
Machinery Co., Ltd.), and a mechanofusion system (manufactured by
Hosokawa Micron Corporation).
[0067] When the intensity of shear force, compressive force, and
the like of the mechanochemical treatment is low, surface treatment
is not performed sufficiently, and the effect of improving press
formability is low. Therefore, the intensity is adjusted in
accordance with the conditions, such as the average particle size
and the corner shape. Furthermore, when the intensity of the
treatment is excessively high, formation of new surfaces and
generation of fine powder due to breakage of particles may cause
adverse effects, such as degradation of battery characteristics.
Therefore, a moderate intensity is selected.
[0068] [Amorphous Carbon Particles]
[0069] The amorphous carbon particles of the embodiment of the
present invention are obtained by the production method of the
embodiment of the present invention described above or the like,
and are amorphous carbon particles containing graphite particles.
FIG. 2 is a polarization microphotograph showing a cross-section of
amorphous carbon particles of the embodiment of the present
invention. The amorphous carbon particles were embedded in a resin,
a cross-section was polished, and then this polarization
microphotograph was taken. In the photograph shown in FIG. 2,
whitish gray portions (a) represent amorphous carbon particles, and
white needle-like portions (b) represent graphite particles
(flaky). A black portion surrounding the whitish gray portions (a)
represents the resin in which the amorphous carbon particles are
embedded.
[0070] In the amorphous carbon particles of the embodiment of the
present invention, by embedded graphite particles which have a
lower resistance and a higher true specific gravity than amorphous
carbon, it is possible to decrease the resistance of particles as a
whole to improve conductivity, and it is possible to increase the
true specific gravity of particles as a whole to improve the
electrode density and to enhance press formability.
[0071] Furthermore, at the same time, by embedded graphite
particles so as not to be exposed to the outer surface, it is
possible to suppress reaction with an electrolyte solution.
Accordingly, for example, it is possible to use an electrolyte
solution containing propylene carbonate which has excellent
low-temperature characteristics and which does not solidify even in
low temperature, and it is possible to suppress adverse effects on
the battery performance, such as poor durability and a decrease in
conducting paths due to expansion and contraction.
[0072] Such amorphous carbon particles of the embodiment of the
present invention cannot be obtained in the case where graphite
particles are added and mixed into a product subjected to
infusibility-imparting treatment which is obtained by cross-linking
a raw material before being subjected to cross-linking treatment
without containing graphite particles therein, and then performing
infusibility-imparting treatment.
[0073] In the amorphous carbon particles of the embodiment of the
present invention, the content of the graphite particles is
preferably 1% to 50% by mass, and more preferably 5% to 20% by
mass, because of a higher effect of improving conductivity.
[0074] The average particle size of the amorphous carbon particles
of the embodiment of the present invention, which depends on the
characteristics of the battery used, is preferably 1 to 25 and more
preferably 2 to 15 .mu.m, from the viewpoint of improving
input/output characteristics. The average particle size can be
adjusted to such an extent that embedded graphite particles are not
markedly exposed.
[0075] Note that, in the embodiment of the present invention, the
average particle size of the amorphous carbon particles is measured
by a laser diffraction particle size analyzer.
[0076] The specific surface of the amorphous carbon particles of
the embodiment of the present invention is preferably 10 m.sup.2/g
or less, more preferably 0.5 to 8 m.sup.2/g, still more preferably
1.0 to 4 m.sup.2/g, and most preferably 1.2 to 3.5 m.sup.2/g from
the viewpoint of suppressing reactivity with an electrolyte
solution.
[0077] Note that, in the embodiment of the present invention, the
specific surface is determined by the BET method using adsorption
of nitrogen gas.
[0078] The amorphous carbon particles of the embodiment of the
present invention preferably have an average lattice spacing
d.sub.002 of (002) plane determined by X-ray diffraction
(hereinafter, may be simply referred to as the "average lattice
spacing d.sub.002") of 0.350 nm or more from the viewpoint of
excellent discharge capacity and cycle life.
[0079] Note that, in the embodiment of the present invention, the
average lattice spacing d002 is determined by a method in which,
using a CuK.alpha. ray as an X-ray and high-purity silicon as a
standard substance, the diffraction peak of the (002) plane of
amorphous carbon particles is measured, and calculation is
performed from the position of the peak. The calculation method is
in accordance with the Gakushin method (measurement method
established by the 17th committee of Japan Society for the
Promotion of Science), and specifically, the method described in
"Carbon fibers" [Sugio Otani, pp. 733-742 (March 1986),
Kindaihenshu-sha].
[0080] The true specific gravity of the amorphous carbon particles
of the embodiment of the present invention is preferably 1.600
g/cm.sup.3 or more, more preferably 1.600 to 1.700 g/cm.sup.3,
still more preferably 1.600 to 1.690 g/cm.sup.3, and most
preferably 1.620 to 1.685 g/cm.sup.3, because the electrode density
further improves as the true specific gravity value increases.
[0081] Note that, in the embodiment of the present invention, the
true specific gravity is determined by a liquid phase substitution
method by a pycnometer, using butanol, in accordance with JIS R
7222:1997 (Test methods for physical properties of graphite
materials).
[0082] Next, description will be made on a lithium ion secondary
battery in which amorphous carbon particles of the embodiment of
the present invention are used as a negative electrode material
(hereinafter, may also be referred to as the "lithium ion secondary
battery of the embodiment of the present invention").
[0083] [Lithium Ion Secondary Battery]
[0084] A lithium ion secondary battery generally includes, as main
battery elements, a negative electrode, a positive electrode, and a
nonaqueous electrolyte solution. The positive and negative
electrodes are each composed of a substance (as a layered
compound), a compound, or a cluster capable of absorbing lithium
ions. In the charging/discharging process, lithium ions move in and
out of the electrodes between layers. It has a battery mechanism in
which lithium ions are doped into the negative electrode during
charging and dedoped from the negative electrode during
discharging.
[0085] The lithium ion secondary battery of the embodiment of the
present invention is not particularly limited except that the
amorphous carbon particles of the embodiment of the present
invention are used as a negative electrode material. Regarding the
other battery elements, battery elements generally used for lithium
ion secondary batteries are used.
[0086] [Negative Electrode]
[0087] A method for producing a negative electrode from the
amorphous carbon particles of the embodiment of the present
invention is not particularly limited, and a common production
method can be employed. In the production process of the negative
electrode, a negative electrode mixture prepared by adding a binder
to the amorphous carbon particles of the embodiment of the present
invention can be used. Binders having chemical stability and
electrochemical stability against electrolytes are preferably used
as the binder. The binder is usually preferably used in an amount
of about 1% to 20% by mass relative to the total amount of the
negative electrode mixture. As the binder, polyvinylidene fluoride,
carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), or
the like can be used. Furthermore, a carbon material other than the
amorphous carbon particles of the embodiment of the present
invention or a graphite material may be added as an active
material. Furthermore, as a conductive agent, for example, carbon
black, carbon fibers, or the like may be added.
[0088] By mixing the amorphous carbon particles of the embodiment
of the present invention and the binder, a negative electrode
mixture coating material in a paste form is prepared, and the
negative electrode mixture coating material is usually applied to
one surface or both surfaces of a current collector to form a
negative electrode mixture layer. In the preparation of the coating
material, a common solvent may be used. The shape of the current
collector used in the negative electrode is not particularly
limited, and for example, the current collector may be foil-shaped;
net-like, such as mesh or expanded metal; or the like. Examples of
the current collector include copper, stainless steel, nickel, and
the like.
[0089] [Positive Electrode]
[0090] As the material for the positive electrode (positive
electrode active material), a material capable of doping/dedoping a
sufficient amount of lithium ions is preferably selected. Examples
of such a positive electrode active material include transition
metal oxides, transition metal chalcogenides, vanadium oxides,
lithium-containing compounds thereof, Chevrel compounds represented
by general formula M.sub.xMo.sub.6S.sub.8-y (where X is a numerical
value satisfying 0.ltoreq.X.ltoreq.4, Y is a numerical value
satisfying 0.ltoreq.Y.ltoreq.1, and M represents a metal, such as a
transition metal), activated carbon, activated carbon fibers, and
the like. These may be used alone or in combination of two or more.
For example, a carbonate, such as lithium carbonate, may be added
into the positive electrode.
[0091] A lithium-containing transition metal oxide is a composite
oxide of lithium and a transition metal, and may be a solid
solution of lithium and two or more transition metals. A
lithium-containing transition metal oxide is specifically
represented by LiM(1).sub.1-pM(2).sub.pO.sub.2 (where P is a
numerical value satisfying 0.ltoreq.P.ltoreq.1, and M(1) and M(2)
each represent at least one transition metal element), or
LiM(1).sub.2-qM(2).sub.qO.sub.4 (where Q is a numerical value
satisfying 0.ltoreq.Q.ltoreq.1, and M(1) and M(2) each represent at
least one transition metal element). Examples of the transition
metal element represented by M include Co, Ni, Mn, Cr, Ti, V, Fe,
Zn, Al, In, Sn, and the like, and Co, Fe, Mn, Ti, Cr, V, and Al are
preferable.
[0092] Such a lithium-containing transition metal oxide can be
obtained, for example, using oxides or salts of Li and a transition
metal as starting materials, by mixing these starting materials
according to the composition, and baking in an oxygen atmosphere in
a temperature range of 600.degree. C. to 1,000.degree. C. The
starting materials are not limited to oxides or salts, and
synthesis is also possible using hydroxides or the like.
[0093] As the method for producing a positive electrode using such
a positive electrode material, for example, a positive electrode
mixture coating material in a paste form including the positive
electrode material, a binder, and a conductive agent is applied to
one surface or both surfaces of a current collector to form a
positive electrode mixture layer. As the binder, the same binder
exemplified in the negative electrode can be used. As the
conductive agent, for example, a particulate carbon material, a
fibrous carbon material, graphite, or carbon black can be used. The
shape of the current collector is not particularly limited, and the
same shape as that for the negative electrode can be used. As the
material for the current collector, aluminum, nickel, a stainless
steel foil, or the like can be usually used.
[0094] In the process of producing the negative electrode and the
positive electrode, various additives, such as known conductive
agents and binders, may be appropriately used.
[0095] [Electrolyte]
[0096] As the electrolyte, a common nonaqueous electrolyte
containing a lithium salt, such as LiPF.sub.6 or LiBF.sub.4, as an
electrolyte salt is used.
[0097] The nonaqueous electrolyte may be a liquid-state nonaqueous
electrolyte solution or a polymer electrolyte, such as a solid
electrolyte or gel electrolyte.
[0098] In the case where a liquid-state nonaqueous electrolyte
solution is used, as the nonaqueous solvent, an aprotic organic
solvent, such as ethylene carbonate, propylene carbonate, or
dimethyl carbonate, can be used.
[0099] In the case where a polymer electrolyte is used, a matrix
polymer gelled by a plasticizer (nonaqueous electrolyte solution)
is included. Examples of the matrix polymer include ether polymers,
such as polyethylene oxide or cross-linked product thereof,
polymethacrylate-based polymers, polyacrylate-based polymers, and
fluorine-based polymers, such as polyvinylidene fluoride and
vinylidene fluoride-hexafluoropropylene copolymers. These may be
used alone or as a mixture of two or more. Above all, from the
viewpoint of oxidation-reduction stability and the like,
fluorine-based polymers are preferable.
[0100] As the electrolyte salt and the nonaqueous solvent
constituting the plasticizer (nonaqueous electrolyte solution)
contained in the polymer electrolyte, those which can be used for a
liquid-state electrolyte solution can be used.
[0101] In the lithium ion secondary battery of the embodiment of
the present invention, usually, a separator, such as a microporous
body of polypropylene or polyethylene or a multilayer structure
thereof; a nonwoven fabric; or the like, is used. It is also
possible to use a gel electrolyte. In this case, for example, a
negative electrode containing the amorphous carbon particles of the
embodiment of the present invention, a gel electrode, and a
positive electrode are stacked in that order and the resulting
stacked body is placed in a battery case.
[0102] The lithium ion secondary battery of the embodiment of the
present invention may have any structure. The shape and
configuration thereof are not particularly limited and, for
example, may be selected arbitrarily from cylindrical type, square
type, and coin type.
EXAMPLES
[0103] The present invention will be described specifically below
with reference to examples. However, it is to be understood that
the present invention is not limited thereto.
Example 1
[0104] First, 1,000 g of coal-tar pitch (actual carbon ratio: 60%
by mass, quinoline insoluble (QI): 0.1% by mass) was placed in an
autoclave equipped with an anchor stirrer, and heated to a fluid
state. Then, 30 g of powder of natural graphite (average particle
size: 4 .mu.m), as graphite particles, was gradually added
thereinto under stirring, and stirring was continued until uniform
mixing was achieved.
[0105] After stirring, under a nitrogen stream, the mixture was
heated to 320.degree. C. in the autoclave, then compressed air was
blown into the pitch while being circulated at 2 L/min, and heating
was performed at 320.degree. C. for five hours. Thus, the mixture
was subjected to cross-linking treatment by the air-blowing
reaction. Then, cooling was performed to room temperature and the
content was taken out. The oxygen amount in the resulting content
is shown in Table 1 below.
[0106] Next, the resulting content was pulverized using an
atomizer, and the average particle size was adjusted to 12 .mu.m.
Infusibility-imparting treatment was performed in which the
resulting powder was placed in a rotary furnace, heating was
performed at a heating rate of 20.degree. C./hr while circulating
compressed air at 2 L/min, and the powder was held at 250.degree.
C. for three hours. After a product subjected to
infusibility-imparting treatment was obtained, it was left to cool.
The oxygen amount in the resulting product subjected to
infusibility-imparting treatment is shown in Table 1 below.
[0107] Next, 100 g of the resulting product subjected to
infusibility-imparting treatment was placed in a graphite container
with a lid, under a nitrogen stream, heating was performed at a
heating rate of 100.degree. C./hr, and baking was performed at
1,150.degree. C. for two hours. Thereby, carbon powder was
obtained.
Examples 2 and 3
[0108] In Examples 2 and 3, carbon powder was obtained as in
Example 1 except that the coal-tar pitch and graphite particles
used were changed as shown in Table 1 below. The artificial
graphite used in Example 3 was produced as described below.
[0109] (Production Method of Artificial Graphite)
[0110] By subjecting coal-tar pitch, as a raw material, to heat
treatment at 450.degree. C., mesocarbon microbeads were generated.
Six times equivalent of tar middle oil serving as a solvent was
added to the coal-tar pitch, and filtration was performed with a
filter under a pressure of 2.0 (kg/cm.sup.2) to separate mesocarbon
microbeads. Next, the resulting mesocarbon microbeads were dried
under normal pressure at 120.degree. C. The dried product was
heated in a cylindrical kiln in a nitrogen atmosphere at
350.degree. C. for one hour, and then naturally cooled to room
temperature to obtain a heat-treated product. Next, the
heat-treated product was crushed by an atomizer, and then coarse
powder (150 .mu.m or more) was removed. The heat-treated mesocarbon
microbeads were charged into the cylindrical kiln again, and heated
in a nitrogen atmosphere at 320.degree. C. for one hour. The
resulting mesocarbon microbeads were further baked at 1,000.degree.
C. to obtain a baked product. The resulting baked product was
pulverized. Next, graphitization was performed at 3,000.degree. C.
During the graphitization, particles did not fuse together, and the
average particle size after the graphitization was 4 .mu.m.
Example 4
[0111] 1,000 g of coal-tar pitch (actual carbon ratio: 60% by mass,
quinoline insoluble (QI): 0.1% by mass) was placed in an autoclave
equipped with an anchor stirrer and, under a nitrogen stream,
heated to 320.degree. C. Then, compressed air was blown into the
pitch while being circulated at 2 L/min, and heating was performed
at 320.degree. C. for five hours. Thus, cross-linking treatment by
the air-blowing reaction was performed. At this stage, a paste
obtained by mixing 30 g of powder of natural graphite (average
particle size: 4 .mu.m) and a small amount of coal-tar pitch, as
graphite particles, was gradually added thereto under stirring, and
stirring was continued until uniform mixing was achieved.
[0112] Then, cooling was performed to room temperature and the
content was taken out. The oxygen amount in the resulting content
is shown in Table 1 below.
[0113] Next, the resulting content was pulverized using an
atomizer, and the average particle size was adjusted to 12 .mu.m.
Infusibility-imparting treatment was performed in which the
resulting powder was placed in a rotary furnace, heating was
performed at a heating rate of 20.degree. C./hr while circulating
compressed air at 2 L/min, and the powder was held at 250.degree.
C. for three hours. After a product subjected to
infusibility-imparting treatment was obtained, it was left to cool.
The oxygen amount in the resulting product subjected to
infusibility-imparting treatment is shown in Table 1 below.
[0114] Next, 100 g of the resulting product subjected to
infusibility-imparting treatment was placed in a graphite container
with a lid, under a nitrogen stream, heating was performed at a
heating rate of 100.degree. C./hr, and baking was performed at
1,150.degree. C. for two hours. Thereby, carbon powder was
obtained.
Example 5
[0115] As in Example 1, 1,000 g of coal-tar pitch (actual carbon
ratio: 60% by mass, quinoline insoluble (QI): 0.1% by mass) was
placed in an autoclave equipped with an anchor stirrer, and heated
to a fluid state. Then, 90 g of powder of natural graphite (average
particle size: 4 .mu.m), as graphite particles, was gradually added
thereinto under stirring, and stirring was continued until uniform
mixing was achieved.
[0116] After stirring, under a nitrogen stream, the mixture was
heated to 320.degree. C. in the autoclave, then compressed air was
blown into the pitch while being circulated at 2 L/min, and heating
was performed at 320.degree. C. for five hours. Thus, the mixture
was subjected to cross-linking treatment by the air-blowing
reaction. Then, cooling was performed to room temperature and the
content was taken out. The oxygen amount in the resulting content
is shown in Table 1 below.
[0117] Next, the resulting content was pulverized using an
atomizer, and the average particle size was adjusted to 12 .mu.m.
The resulting powder was placed in a mechanofusion system
manufactured by Hosokawa Micron Corporation and treated at a
peripheral speed of 15 m/s for 15 minutes. Infusibility-imparting
treatment was performed in which the treated powder was placed in a
rotary furnace, heating was performed at a heating rate of
20.degree. C./hr while circulating compressed air at 2 L/min, and
the powder was held at 250.degree. C. for three hours. After a
product subjected to infusibility-imparting treatment was obtained,
it was left to cool. The oxygen amount in the resulting product
subjected to infusibility-imparting treatment is shown in Table 1
below.
[0118] Next, 100 g of the resulting product subjected to
infusibility-imparting treatment was placed in a graphite container
with a lid, under a nitrogen stream, heating was performed at a
heating rate of 100.degree. C./hr, and baking was performed at
1,150.degree. C. for two hours. Thereby, carbon powder was
obtained.
Example 6
[0119] As in Example 1, 1,000 g of coal-tar pitch (actual carbon
ratio: 60% by mass, quinoline insoluble (QI): 0.1% by mass) was
placed in an autoclave equipped with an anchor stirrer, and heated
to a fluid state. Then, 90 g of powder of natural graphite (average
particle size: 4 .mu.m), as graphite particles, was gradually added
thereinto under stirring, and stirring was continued until uniform
mixing was achieved.
[0120] After stirring, under a nitrogen stream, the mixture was
heated to 320.degree. C. in the autoclave, then compressed air was
blown into the pitch while being circulated at 2 L/min, and heating
was performed at 320.degree. C. for five hours. Thus, the mixture
was subjected to cross-linking treatment by the air-blowing
reaction. Then, cooling was performed to room temperature and the
content was taken out. The oxygen amount in the resulting content
is shown in Table 1 below.
[0121] Next, the resulting content was pulverized using an
atomizer, and the average particle size was adjusted to 12
.mu.m.
[0122] Infusibility-imparting treatment was performed in which the
resulting powder was placed in a rotary furnace, heating was
performed at a heating rate of 20.degree. C./hr while circulating
compressed air at 2 L/min, and the powder was held at 250.degree.
C. for three hours. After a product subjected to
infusibility-imparting treatment was obtained, it was left to cool.
The oxygen amount in the resulting product subjected to
infusibility-imparting treatment is shown in Table 1 below.
[0123] Next, the resulting product subjected to
infusibility-imparting treatment was placed in a mechanofusion
system manufactured by Hosokawa Micron Corporation and treated at a
peripheral speed of 18 m/s for 15 minutes. The treated powder was
placed in a graphite container with a lid, under a nitrogen stream,
heating was performed at a heating rate of 100.degree. C./hr, and
baking was performed at 1,150.degree. C. for two hours. Thereby,
carbon powder was obtained.
Example 7
[0124] 1,000 g of coal-tar pitch (actual carbon ratio: 60% by mass,
quinoline insoluble (QI): 0.1% by mass) was placed in an autoclave
equipped with an anchor stirrer and, under a nitrogen stream,
heated to 320.degree. C. Then, compressed air was blown into the
pitch while being circulated at 2 L/min, and heating was performed
at 320.degree. C. for five hours. Thus, cross-linking treatment by
the air-blowing reaction was performed. At this stage, a paste
obtained by mixing 30 g of powder of natural graphite (average
particle size: 4 .mu.m) and a small amount of coal-tar pitch, as
graphite particles, was gradually added thereto under stirring, and
stirring was continued until uniform mixing was achieved.
[0125] Then, cooling was performed to room temperature and the
content was taken out. The oxygen amount in the resulting content
is shown in Table 1 below.
[0126] Next, the resulting content was pulverized using an
atomizer, and the average particle size was adjusted to 12 .mu.m.
The resulting powder was placed in a mechanofusion system
manufactured by Hosokawa Micron Corporation and treated at a
peripheral speed of 15 m/s for 15 minutes. Infusibility-imparting
treatment was performed in which the treated powder was placed in a
rotary furnace, heating was performed at a heating rate of
20.degree. C./hr while circulating compressed air at 2 L/min, and
the powder was held at 250.degree. C. for three hours. After a
product subjected to infusibility-imparting treatment was obtained,
it was left to cool. The oxygen amount in the resulting product
subjected to infusibility-imparting treatment is shown in Table 1
below.
[0127] Next, 100 g of the resulting product subjected to
infusibility-imparting treatment was placed in a graphite container
with a lid, under a nitrogen stream, heating was performed at a
heating rate of 100.degree. C./hr, and baking was performed at
1,150.degree. C. for two hours. Thereby, carbon powder was
obtained.
Example 8
[0128] 1,000 g of coal-tar pitch (actual carbon ratio: 60% by mass,
quinoline insoluble (QI): 0.1% by mass) was placed in an autoclave
equipped with an anchor stirrer and, under a nitrogen stream,
heated to 320.degree. C. Then, compressed air was blown into the
pitch while being circulated at 2 L/min, and heating was performed
at 320.degree. C. for five hours. Thus, cross-linking treatment by
the air-blowing reaction was performed. At this stage, a paste
obtained by mixing 30 g of powder of natural graphite (average
particle size: 4 .mu.m) and a small amount of coal-tar pitch, as
graphite particles, was gradually added thereto under stirring, and
stirring was continued until uniform mixing was achieved.
[0129] Then, cooling was performed to room temperature and the
content was taken out. The oxygen amount in the resulting content
is shown in Table 1 below.
[0130] Next, the resulting content was pulverized using an
atomizer, and the average particle size was adjusted to 12 .mu.m.
Infusibility-imparting treatment was performed in which the
resulting powder was placed in a rotary furnace, heating was
performed at a heating rate of 20.degree. C./hr while circulating
compressed air at 2 L/min, and the powder was held at 250.degree.
C. for three hours. After a product subjected to
infusibility-imparting treatment was obtained, it was left to cool.
The oxygen amount in the resulting product subjected to
infusibility-imparting treatment is shown in Table 1 below.
[0131] Next, the resulting product subjected to
infusibility-imparting treatment was placed in a mechanofusion
system manufactured by Hosokawa Micron Corporation and treated at a
peripheral speed of 18 m/s for 15 minutes. The treated powder was
placed in a graphite container with a lid, under a nitrogen stream,
heating was performed at a heating rate of 100.degree. C./hr, and
baking was performed at 1,150.degree. C. for two hours. Thereby,
carbon powder was obtained.
Comparative Examples 1 and 2
[0132] In Comparative Examples 1 and 2, carbon powder was obtained
as in Examples 1 and 2, respectively, except that graphite
particles were not added to coal-tar pitch.
Comparative Example 3
[0133] In Comparative Example 3, carbon powder was obtained as in
Comparative Example 1. However, when a negative electrode mixture
paste was prepared, which will be described later, powder obtained
by adding 5 parts by mass of natural graphite (average particle
size: 5 .mu.m) to 100 parts by mass of the carbon powder
(hereinafter, simply referred to as the "carbon powder") was used
as a negative electrode material.
[0134] <Evaluation>
[0135] (Evaluation of Carbon Powder after Baking)
[0136] In each of Examples and Comparative Examples, the carbon
powder obtained by the baking was subjected to measurement of the
average particle size (units of measure: .mu.m), specific surface
(units of measure: m.sup.2/g), and true specific gravity (units of
measure: g/cm.sup.2) by the methods described above. The results
thereof are shown in Table 1 below.
[0137] Next, using carbon powders obtained in Examples and
Comparative Examples as a negative electrode material, coin-type
secondary batteries (refer to FIG. 1) were fabricated, and various
evaluations were performed. The results are shown in Table 2
below.
[0138] (Preparation of Negative Electrode Mixture Paste)
[0139] First, a negative electrode mixture paste was prepared using
the resulting carbon powder as a negative electrode material.
Specifically, carbon powder (95 parts by mass) and 12%
N-methylpyrrolidinone solution of polyvinylidene fluoride (5 parts
by mass in solid content) were placed in a planetary mixer, and
stirring was performed at 100 rpm for 15 minutes.
N-methylpyrrolidinone was further added thereto to adjust the solid
content ratio to 60%, and stirring was continued for 15 minutes.
Thereby, the negative electrode mixture paste was prepared.
[0140] (Fabrication of Working Electrode (Negative Electrode))
[0141] The negative electrode mixture paste thus prepared was
applied onto a copper foil so as to obtain a uniform thickness.
Furthermore, the copper foil was placed in a fan dryer, and the
solvent was volatilized at 100.degree. C. to form a negative
electrode mixture layer. Next, the negative electrode mixture layer
was pressed with a roller press and die-cut to a circular shape
with a diameter of 15.5 mm. Thereby, a working electrode (negative
electrode) including the negative electrode mixture layer closely
attached to the current collector made of a copper foil was
fabricated. Before evaluations were performed, drying was performed
in vacuum at 100.degree. C. for 8 hours or more.
[0142] (Electrode Press Formability (Electrode Density))
[0143] The working electrode thus fabricated was sandwiched between
mirror plates having a certain area, and a pressure of 250 MPa was
applied thereto for 20 seconds using a hand press. Then, the
electrode density (units of measure: g/cm.sup.3) was determined.
The electrode density was determined by measuring the mass and
thickness of the negative electrode mixture layer, followed by
calculation. The press formability can be evaluated to be higher
when the electrode density is higher.
[0144] (Conductivity of Electrode (Volume Resistivity))
[0145] The negative electrode mixture paste prepared as described
above was applied to a film made of polyethylene terephthalate
(manufactured by Toray Industries, Inc.), and pressing was
performed at a pressure of 250 MPa. Three points of the film were
measured using a resistance measuring device (manufactured by
Mitsubishi Chemical Corporation), and the volume resistivity (units
of measure: .OMEGA.cm) was determined from the average value. The
conductivity can be evaluated to be higher when the volume
resistivity is lower.
[0146] (Preparation of Electrolyte Solution)
[0147] Electrolyte solution A: A nonaqueous electrolyte solution
was prepared by dissolving LiPF.sub.6 at a concentration of 1
mol/dm.sup.3 in a mixed solvent obtained by mixing ethylene
carbonate (33% by volume) and methyl ethyl carbonate (67% by
volume).
[0148] Electrolyte solution B: A nonaqueous electrolyte solution
was prepared by dissolving LiPF.sub.6 at a concentration of 1
mol/dm.sup.3 in propylene carbonate.
[0149] (Fabrication of Evaluation Battery)
[0150] Next, using the working electrode (negative electrode)
fabricated as described above, a coin-type secondary battery for
evaluation (may also be simply referred to as the "evaluation
battery") shown in FIG. 1 was fabricated. FIG. 1 is a
cross-sectional view showing the coin-type secondary battery for
evaluation.
[0151] First, a lithium metal foil was pressed against a nickel net
and die-cut into a circular shape with a diameter of 15.5 mm.
Thereby, a disk-shaped counter electrode 4 formed of a lithium foil
closely attached to a current collector 7a formed of a nickel net
was fabricated.
[0152] Next, a separator 5 was interposed and stacked between a
working electrode (negative electrode) 2 closely attached to a
current collector 7b and the counter electrode 4 closely attached
to the current collector 7a. Then, the working electrode 2 was
placed in an outer cup 1, and the counter electrode 4 was placed in
an outer can 3. The outer cup 1 and the outer can 3 were fitted to
each other, and peripheries of the outer cup 1 and the outer can 3
were caulked through an insulating gasket 6 and sealed. Thereby, an
evaluation battery was fabricated.
[0153] In the evaluation battery thus fabricated, the peripheries
of the outer cup 1 and the outer can 3 are caulked through the
insulating gasket 6 to form a sealed structure. In the sealed
structure, as shown in FIG. 1, the current collector 7a, the
counter electrode 4, the separator 5, the working electrode
(negative electrode) 2, and the current collector 7b are stacked in
that order on the inner surface of the outer can 3 toward the inner
surface of the outer cup 1.
[0154] (Charge-Discharge Test)
[0155] The evaluation battery thus fabricated was subjected to a
charge-discharge test at 25.degree. C. in the manner described
below. Note that, in this test, a process in which lithium ions
were doped into carbon powder was defined as "charging", and a
process in which lithium ions were dedoped from carbon powder was
defined as "discharging".
[0156] First, constant current charging was performed at a current
value of 0.9 mA until the circuit voltage reached 0 mV. At the
moment when the circuit voltage reached 0 mV, switching was
performed to constant voltage charging, and charging was continued
until the current value reached 20 .mu.A. From the conduction
amount during this period, the charge capacity (may also be
referred to as the "initial charge capacity") (units of measure:
mAh/g) was obtained. Then, a pause of 120 minutes was taken. Next,
constant current discharging was performed at a current value of
0.9 mA until the circuit voltage reached 1.5 V, and from the
conduction amount during this period, the discharge capacity (may
also be referred to as the "initial discharge capacity") (units of
measure: mAh/g) was obtained. This process was defined as the first
cycle.
[0157] Table 2 shows the initial discharge capacity, the initial
charge-discharge efficiency, and the rapid discharge efficiency in
the case where the electrolyte solution A was used.
[0158] (Initial Charge-Discharge Efficiency)
[0159] On the basis of the results of the charge-discharge test,
the initial charge-discharge efficiency (units of measure: %) was
obtained from the equation below in the case where the electrolyte
solution A was used and in the case where the electrolyte solution
B was used. The results are shown in Table 2.
Initial charge-discharge efficiency=(initial discharge
capacity/initial charge capacity).times.100
[0160] (Reactivity with Electrolyte Solution)
[0161] The reactivity with the electrolyte solution was evaluated
by obtaining a difference between the charge-discharge loss in the
case where the electrolyte solution A was used and the
charge-discharge loss in the case where the electrolyte solution B
was used.
[0162] In the case where the difference in charge-discharge loss
was 15 mAh/g or less, reaction with the electrolyte solution B
containing propylene carbonate was considered to be suppressed,
which was evaluated to be "good". In the case where the difference
in charge-discharge loss was more than that value, the reaction was
not considered to be suppressed, which was evaluated to be
"poor".
[0163] Note that, the charge-discharge loss (units of measure:
mAh/g) was obtained from the following equation:
Charge-discharge loss=initial charge capacity-initial discharge
capacity
[0164] (Rapid Discharge Efficiency)
[0165] Furthermore, high-speed discharging was performed in the
third cycle. Constant current charging was performed at a current
value of 0.9 mA until the circuit voltage reached 0 mV. At the
moment when the circuit voltage reached 0 mV, switching was
performed to constant voltage charging, and charging was continued
until the current value reached 20 .mu.A. Then, a pause of 120
minutes was taken. Subsequently, constant current discharging was
performed at a current value of 7.2 mA, which was eightfold higher,
until the circuit voltage reached 1,500 mV, and the discharge
capacity (may also be referred to as the "rapid discharge
capacity") (units of measure: mAh/g) was obtained, and the rapid
discharge efficiency (units of measure: %) was obtained from the
equation below. Table 2 shows the rapid discharge efficiency in the
case where the electrolyte solution A was used.
Rapid discharge efficiency=(rapid discharge capacity/initial
discharge capacity).times.100
TABLE-US-00001 TABLE 1 After infusibility- Graphite particles After
crosslinking imparting After baking Average treatment treatment
Average True Pitch Addition particle Oxygen Oxygen particle
Specific specific Additive at the QI Graphite amount size amount
amount size surface gravity time of [mass %] species [mass %]
[.mu.m] [mass %] [mass %] [.mu.m] [m.sup.2/g] [g/cm.sup.3]
evaluation Example 1 0.1 Natural 3 4 1 8.5 12 1.3 1.623 -- graphite
Example 2 0 Natural 10 4 1 7.2 10 1.7 1.672 -- graphite Example 3
0.2 Artificial 10 4 1 7.1 5 3.2 1.672 -- graphite Example 4 0.1
Natural 10 4 1 8.1 12 1.5 1.629 -- graphite Example 5 0.1 Natural
10 4 1 7.3 12 1.4 1.671 -- graphite Example 6 0.1 Natural 10 4 1 7
12 1.3 1.669 -- graphite Example 7 0.1 Natural 10 4 1 6.8 9 1.8
1.678 -- graphite Example 8 0.1 Natural 10 4 1 7.5 12 1.6 1.681 --
graphite Comparative 0.1 -- -- -- 1 9.2 10 1.3 1.610 -- Example 1
Comparative 0 -- -- -- 1 7.8 5 3.3 1.600 -- Example 2 Comparative
0.1 -- -- -- 1 9.2 5 3.3 1.670 Natural Example 3 graphite
TABLE-US-00002 TABLE 2 Reactivity Initial with electrolyte Rapid
discharge Initial solution discharge Electrode capacity
charge-discharge Difference efficiency density Volume (Electrolyte
efficiency [%] in charge- (Electrolyte (250 MPa) resistivity
solution A) Electrolyte Electrolyte discharge solution A)
[g/cm.sup.3] [.OMEGA. cm] [mAh/g] solution A solution B loss
Evaluation [%] Example 1 1.07 0.25 404 81.1 79.7 8.8 good 80.7
Example 2 1.13 0.13 398 81.2 79.2 12.4 good 83.9 Example 3 1.13
0.21 397 78.0 77.2 5.3 good 83.5 Example 4 1.09 0.26 398 80.9 79.7
7.4 good 80.2 Example 5 1.15 0.14 393 81.4 79.9 9.1 good 83.1
Example 6 1.15 0.14 399 82.0 80.2 10.9 good 83.0 Example 7 1.16
0.15 390 78.9 77.6 8.3 good 80.5 Example 8 1.18 0.13 395 80.1 79.5
3.7 good 79.9 Comparative 1.03 0.30 393 81.1 80.0 6.7 good 78.6
Example 1 Comparative 1.03 0.23 387 79.5 78.7 4.9 good 80.0 Example
2 Comparative 1.15 0.14 395 78.2 --* --* poor 83.1 Example 3 *Note:
Charging did not finish, and the loss was determined to be
.infin..
[0166] When comparison is made between Examples 1 to 8 and
Comparative Examples 1 and 2, in Examples 1 to 8, the true specific
gravity is high, the electrode density is improved, the press
formability is excellent, the volume resistivity is low, and thus
conductivity is high, compared with Comparative Examples 1 and 2 in
which graphite particles are not added.
[0167] Furthermore, when comparison is made between Examples 1 to 8
and Comparative Example 3, in Comparative Example 3, although the
electrode density and the like are relatively good, the reactivity
with electrolyte solution is evaluated to be "poor". The reason for
this is believed to be that, in Comparative Example 3, in the case
where the electrolyte solution B containing propylene carbonate is
used, graphite particles added afterwards react with the
electrolyte solution, and the battery is hardly charged.
[0168] In contrast, in Examples 1 to 8, the evaluation result of
the reactivity with electrolyte solution is "good", and even in the
case where the electrolyte solution B is used, charging and
discharging can be performed.
[0169] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *