U.S. patent application number 15/275824 was filed with the patent office on 2017-01-12 for composite graphite particles for nonaqueous secondary battery negative electrode, active material for nonaqueous secondary battery negative electrode, and nonaqueous secondary battery.
This patent application is currently assigned to Mitsubishi Chemical Corporation. The applicant listed for this patent is Mitsubishi Chemical Corporation. Invention is credited to Satoshi Akasaka, Nobuyuki Ishiwatari, Junko Kagimoto, Shingo Morokuma, Akio UEDA.
Application Number | 20170012280 15/275824 |
Document ID | / |
Family ID | 54195630 |
Filed Date | 2017-01-12 |
United States Patent
Application |
20170012280 |
Kind Code |
A1 |
UEDA; Akio ; et al. |
January 12, 2017 |
COMPOSITE GRAPHITE PARTICLES FOR NONAQUEOUS SECONDARY BATTERY
NEGATIVE ELECTRODE, ACTIVE MATERIAL FOR NONAQUEOUS SECONDARY
BATTERY NEGATIVE ELECTRODE, AND NONAQUEOUS SECONDARY BATTERY
Abstract
An object of the present invention is to provide composite
graphite particles for a nonaqueous secondary battery negative
electrode, wherein metal particles capable of alloying with Li can
be internally present with favorable dispersibility. The present
invention relates to composite graphite particles for a nonaqueous
secondary battery negative electrode containing graphite (A) and
metal particles (B) capable of alloying with Li, wherein the degree
of dispersion of the metal particles (B) in the composite graphite
particles is 0.78 or more.
Inventors: |
UEDA; Akio; (Inashiki-gun,
JP) ; Akasaka; Satoshi; (Inashiki-gun, JP) ;
Ishiwatari; Nobuyuki; (Inashiki-gun, JP) ; Kagimoto;
Junko; (Chiyoda-ku, JP) ; Morokuma; Shingo;
(Inashiki-gun, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mitsubishi Chemical Corporation |
Chiyoda-ku |
|
JP |
|
|
Assignee: |
Mitsubishi Chemical
Corporation
Chiyoda-ku
JP
|
Family ID: |
54195630 |
Appl. No.: |
15/275824 |
Filed: |
September 26, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP2015/059287 |
Mar 26, 2015 |
|
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15275824 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0566 20130101;
H01M 4/38 20130101; H01M 4/625 20130101; Y02T 10/70 20130101; H01M
4/133 20130101; H01M 4/587 20130101; H01M 4/134 20130101; H01M
10/0525 20130101; H01M 4/364 20130101; H01M 4/62 20130101; Y02E
60/10 20130101; H01M 4/622 20130101; H01M 4/386 20130101 |
International
Class: |
H01M 4/36 20060101
H01M004/36; H01M 4/38 20060101 H01M004/38; H01M 4/134 20060101
H01M004/134; H01M 4/587 20060101 H01M004/587; H01M 10/0525 20060101
H01M010/0525; H01M 4/133 20060101 H01M004/133 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2014 |
JP |
2014-064534 |
Claims
1. Composite graphite particles for a nonaqueous secondary battery
negative electrode, comprising graphite and metal particles capable
of alloying with Li, wherein the degree of dispersion of the metal
particles in the composite graphite particles is 0.78 or more;
wherein the degree of dispersion is defined by the following
measurement method: when a lattice is drawn in the form of a grid
having a length of 2 .mu.m per side (however, length per side A/10
.mu.m in the case the length of the long axis <20 .mu.m) for
each of the images of the scanning electron microscope (SEM) of the
cross-sections of 10 composite graphite particles satisfying the
following condition: |0.5.times.(A+B)-R|.ltoreq.3, wherein, A
represents the length of the long axis (.mu.m), B represents the
length of the short axis (.mu.m), and R represents the mean
particle diameter d50 (.mu.m), the number of squares in the lattice
that contain composite graphite particles are defined as x, and the
number of squares in the lattice containing composite graphite
particles that also contain metal particles are defined as y, then
the values of y/x for any 5 particles are calculated, and the
average value thereof is defined as the degree of dispersion.
2. The composite graphite particles for a nonaqueous secondary
battery negative electrode according to claim 1, wherein the tapped
density of the composite graphite particles for a negative
electrode is 0.8 g/cm.sup.3 or more.
3. The composite graphite particles for a nonaqueous secondary
battery negative electrode according to claim 1, wherein the metal
particles are contained at 1% by weight to 30% by weight.
4. The composite graphite particles for a nonaqueous secondary
battery negative electrode according to claim 1, wherein the
specific surface area as determined by the BET method is 0.1
m.sup.2/g to 20 m.sup.2/g.
5. An active material for a nonaqueous secondary battery negative
electrode comprising the composite graphite particles for a
nonaqueous secondary battery negative electrode according to claim
1, and one or more types of materials selected from the group
consisting of natural graphite, artificial graphite, carbonaceous
material-coated graphite, resin-coated graphite and amorphous
carbon.
6. A nonaqueous secondary battery provided with a positive
electrode and negative electrode, capable of occluding and
releasing metal ions, and an electrolytic solution; wherein, the
negative electrode is provided with a current collector and a
negative electrode active material formed on the current collector,
and the negative electrode active material contains the active
material for a nonaqueous secondary battery negative electrode
according to claim 1.
7. Composite graphite particles comprising graphite and at least
one kind of metal particles suitable for alloying with Li; wherein
the degree of dispersion of the metal particles in the composite
graphite particles is 0.78 or more.
8. The composite graphite particles of claim 7, wherein said at
least one kind of metal particles is selected from the group
consisting of Fe, Co, Sb, Bi, Pb, Ni, Ag, Si, Sn, Al, Zr, Cr, V,
Mn, Nb, Mo, Cu, Zn, Ge, As, In, Ti and W; or a metal compound
thereof.
9. A nonaqueous secondary battery that comprises a negative
electrode comprising the composite graphite particles of claim 7.
Description
TECHNICAL FIELD
[0001] The present invention relates to composite graphite
particles for a nonaqueous secondary battery negative electrode, to
an active material for a nonaqueous secondary battery negative
electrode that uses those particles, and to a nonaqueous secondary
battery provided with a negative electrode containing this negative
electrode active material.
BACKGROUND ART
[0002] The demand for high-capacity secondary batteries is growing
accompanying the reduced size of electronic devices. Nonaqueous
secondary batteries, and particularly lithium ion secondary
batteries, which demonstrate high energy density in comparison with
nickel-cadmium batteries or nickel-hydrogen batteries, are
attracting particular attention. Lithium ion secondary batteries,
which are composed of positive and negative electrodes capable of
occluding and releasing lithium ions and a nonaqueous electrolytic
solution obtained by dissolving a lithium salt such as LiPF.sub.6
or LiBF.sub.4, have been deployed not only in conventional laptop
personal computers, mobile communications devices, portable cameras
and handheld game consoles, but also in power tools and electric
automobiles, thus resulting in a growing need for higher
capacities, faster charging and discharging characteristics and
higher cycling characteristics in lithium ion secondary batteries
accompanying their deployment in such applications.
[0003] Although various types of materials have been proposed for
the negative electrode material of these batteries, natural
graphite, artificial graphite obtained by graphitization of coke
and the like, graphitized mesophase pitch and graphitic carbon
materials such as graphitized carbon fiber are used at present due
to their high capacity and the superior flatness of their discharge
potential.
[0004] In recent years, studies have been conducted on the
application of materials having high theoretical capacity, and
particularly metal particle negative electrodes, with the aim of
further increasing capacity.
[0005] For example, Patent Documents 1 and 2 propose methods for
producing composite Si-graphite particles by firing a mixture of
fine particles of an Si compound, graphite and a carbonaceous
precursor in the form of pitch and the like.
[0006] Patent Document 3 proposes composite Si-graphite particles
obtained by compounding Si fine particles with a carbonaceous
material so that the Si fine particles are unevenly distributed on
the surface of spherical natural graphite.
[0007] Patent Document 4 proposes composite graphite particles
having as main components thereof metal capable of alloying with
Li, flake graphite and a carbonaceous material, wherein the metal
is held between a plurality of flake graphite layers, and
specifically discloses composite Si-graphite particles.
[0008] Patent Document 5 proposes composite graphite particles
composed of a granulated body obtained by crushing and granulating
a mixture of a graphite raw material and metal powder in a
high-speed airflow, wherein a portion of the graphite serving as
raw material is crushed and the graphite raw material and its
crushed product aggregate to form a laminated structure in which
metal powder is present in a dispersed state internally and on the
surface thereof, and specifically discloses composite Si-graphite
particles.
[0009] Patent Document 6 discloses composite Si-graphite particles
composed of roughly spherical particles having carbon
microprotrusions on the surface thereof obtained by granulating and
spheroidizing a mixture of void forming agents selected from vein
and/or flake natural graphite, fine particles of an Si compound,
carbon black, polyvinyl alcohol, polyethylene glycol,
polycarbosilane, polyacrylic acid and cellulose-based polymers, and
impregnating and coating the resulting spherical granulation
product with a mixture of a carbon precursor and carbon black
followed by firing.
[0010] Patent Document 7 discloses composite Si-graphite--particles
of a form in which Si particles are sandwiched between flake
graphite, and are obtained by applying compressive force and
shearing force to a mixture of Si particles, flake graphite and a
solid non-graphitic carbon raw material at a temperature equal to
or higher than the softening point of the non-graphitic carbon raw
material to prepare intermediate composite particles followed by
heat treatment.
[0011] Patent Document 8 discloses composite Si-graphite particles
having a structure in which graphite is folded by mixing Si
particles and flake graphite and subjecting to spheroidizing
treatment.
PRIOR ART DOCUMENTS
Patent Documents
[0012] Patent Document 1: Japanese Unexamined Patent Publication
No. 2003-223892
[0013] Patent Document 2: Japanese Unexamined Patent Publication
No. 2012-043546
[0014] Patent Document 3: Japanese Unexamined Patent Publication
No. 2012-124116
[0015] Patent Document 4: Japanese Unexamined Patent Publication
No. 2005-243508
[0016] Patent Document 5: Japanese Unexamined Patent Publication
No. 2008-027897
[0017] Patent Document 6: Japanese Unexamined Patent Publication
No. 2008-186732
[0018] Patent Document 7: International Publication No. WO
2013/141104
[0019] Patent Document 8: International Publication No. WO
2014/046144
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0020] However, according to studies conducted by the inventors of
the present invention, in the art described in Patent Document 1,
since composite Si-graphite particles obtained by compounding
graphite and Si compound particles with a carbonaceous material
exhibit weak bindability of the carbonaceous material responsible
for binding the composite Si-graphite particles, the composite
Si-graphite particles collapse due to volume expansion of the Si
compound particles accompanying charging and discharging, thereby
preventing this art from reaching the level of practical
application due to problems such as cycle deterioration
attributable to interruptions in conductive paths and the like.
[0021] In the art described in Patent Document 2, although a
structure is proposed (by defining the range of the Raman R value)
in which the surfaces of Si compound particles and the surfaces of
flake graphite particles are covered by amorphous carbon as a
result of adequately stirring and mixing composite Si-graphite
particles containing Si compound particles, flake graphite
particles and a carbonaceous material derived from coal tar pitch
prior to compounding (firing), since the bindability of compounding
is weak in this case as well, the composite Si-graphite particles
collapse due to volume expansion of the Si compound particles
accompanying charging and discharging, thereby preventing this art
from reaching the level of practical application due to problems
such as cycle deterioration attributable to interruptions in
conductive paths and the like.
[0022] In the art described in Patent Document 3, since Si compound
particles localize on particle surfaces, the Si compound particles
dissociate from the graphite surface due to volume expansion of the
Si compound particles accompanying charging and discharging,
thereby preventing this art from reaching the level of practical
application due to problems such as cycle deterioration
attributable to interruptions in conductive paths and the like.
[0023] In the art of Patent Documents 4 to 8, although it is
possible to granulate flake graphite or arrange metal particles
within particles during spheroidization, according to studies
conducted by the inventors of the present invention, even if
composite graphite particles are produced using the methods
described in these patent documents, the metal particles present
within the composite particles tend to aggregate, and were
determined to not satisfy the battery properties targeted by the
inventors of the present invention. Moreover, the efficiency at
which metal particles are contained in the composite Si-graphite
particles is low and leaves room for improvement.
[0024] With the foregoing in view, an object of the present
invention is to provide composite graphite particles for a
nonaqueous secondary battery negative electrode without allowing
metal particles capable of alloying with Li to aggregate within the
composite graphite particles, or in other words, allowing the metal
particles to be present in a highly dispersed state. As a result
thereof, a nonaqueous secondary battery is provided that has high
capacity, high charge-discharge efficiency and superior discharge
characteristics.
Means for Solving the Problems
[0025] As a result of conducting extensive studies to solve the
aforementioned problems, the inventors of the present invention
found that a nonaqueous secondary battery having high capacity,
high charge-discharge efficiency and superior discharge
characteristics can be obtained by applying to the negative
electrode material of a nonaqueous secondary battery composite
graphite particles for a nonaqueous secondary battery negative
electrode (C) (to also be referred to as "composite graphite
particles (C)") containing graphite (A) and metal particles capable
of alloying with Li (B) (to also be referred to as "metal particles
(B)"), the composite graphite particles (C) having a special
characteristic to be subsequently described when observed with a
scanning electron microscope (SEM).
[0026] When the composite graphite particles (C) of the present
invention are observed by SEM, the degree of dispersion of the
metal particles (B) in the composite graphite particles (C) as
calculated according to the measurement method indicated below is
0.78 or more.
[0027] Although the detailed mechanism behind the usefulness of the
aforementioned composite graphite particles (C) as a negative
electrode material of a nonaqueous secondary battery is not
understood, as a result of enclosing the metal particles (B) in the
composite graphite particles (C), the possibility of the metal
particles (B) making direct contact with the electrolytic solution
decreases in comparison with a negative electrode material of the
same capacity, namely containing the same amount of metal particles
(B). Consequently, the irreversible loss of Li ions attributable to
the reaction between the metal particles (B) and the nonaqueous
electrolytic solution is reduced, or in other words,
charge-discharge efficiency is improved.
[0028] In addition, since the composite graphite particles (C) of
the present invention have the metal particles (B) dispersed within
composite particles, even when compared with commonly known
granulated composite graphite particles having the metal particles
(B) enclosed therein or composite particles in which a larger
amount of the metal particles (B) are added to the outside of
graphite particles than to the inside of the particles, localized
expansion and contraction (relaxation) are absorbed by the metal
particles (B) present within the particles, thereby resulting in
decreased likelihood of the occurrence of collapse of the composite
graphite particles (C) caused by volume expansion and the
subsequent interruption of conductive paths. As a result, high
charge-discharge efficiency and superior discharge characteristics
are thought to be demonstrated.
[0029] Namely, the gist of the present invention lies in <1>
to <6> indicated below.
[0030] <1> Composite graphite particles for a nonaqueous
secondary battery negative electrode comprising graphite and metal
particles capable of alloying with Li, wherein the degree of
dispersion of the metal particles in the composite graphite
particles as calculated according to the measurement method
indicated below is 0.78 or more.
[0031] (Measurement Method)
[0032] When a lattice is drawn in the form of a grid having a
length of 2 .mu.m per side (however, length per side A/10 .mu.m in
the case the length of the long axis <20 .mu.m) for each of the
images of the scanning electron microscope (SEM) of the
cross-sections of 10 composite graphite particles satisfying the
following condition:
|0.5.times.(A+B)-R|.ltoreq.3
(wherein, A represents the length of the long axis (.mu.m), B
represents the length of the short axis (.mu.m), and R represents
the mean particle diameter d50 (.mu.m)), the number of squares in
the lattice that contain composite graphite particles are defined
as x, and the number of squares in the lattice containing composite
graphite particles that also contain metal particles are defined as
y, then the values of y/x for any 5 particles are calculated, and
the average value thereof is defined as the degree of
dispersion.
[0033] <2> The composite graphite particles for a nonaqueous
secondary battery negative electrode described in <1> above,
wherein the tapped density of the composite graphite particles for
a negative electrode is 0.8 g/cm.sup.3 or more.
[0034] <3> The composite graphite particles for a nonaqueous
secondary battery negative electrode described in <1> or
<2> above, wherein the metal particles are contained at 1% by
weight to 30% by weight.
[0035] <4> The composite graphite particles for a nonaqueous
secondary battery negative electrode described in any one of
<1> to <3> above, wherein the specific surface area as
determined by the BET method is 0.1 m.sup.2/g to 20 m.sup.2/g.
[0036] <5> An active material for a nonaqueous secondary
battery negative electrode comprising the composite graphite
particles for a nonaqueous secondary battery negative electrode
described in any one of <1> to <4> above, and one or
more types of materials selected from the group consisting of
natural graphite, artificial graphite, carbonaceous material-coated
graphite, resin-coated graphite and amorphous carbon.
[0037] <6> A nonaqueous secondary battery provided with a
positive electrode and negative electrode, capable of occluding and
releasing metal ions, and an electrolytic solution, wherein the
negative electrode is provided with a current collector and a
negative electrode active material formed on the current collector,
and the negative electrode active material contains the active
material for a nonaqueous secondary battery negative electrode
described in any one of <1> to <5> above.
Effects of the Invention
[0038] As a result of using the composite graphite particles (C)
for a nonaqueous secondary battery negative electrode according to
the present invention as a negative electrode active material of a
negative electrode for a nonaqueous secondary battery, a nonaqueous
secondary battery can be provided that demonstrates high capacity,
high charge-discharge efficiency and superior discharge
characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a schematic diagram of a cross-sectional image of
composite graphite particles (C) indicating one example of a method
for measuring the degree of dispersion of metal particles (B) in
the composite graphite particles (C).
[0040] FIG. 2 is a scanning electron microscope (SEM) image of a
cross-section of composite graphite particles of Example 1.
[0041] FIG. 3 is a scanning electron microscope (SEM) image of a
cross-section of composite graphite particles of Comparative
Example 2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0042] The following provides a detailed description of the
contents of the present invention. Furthermore, explanations of
constituents of the present invention described below are merely
examples (representative examples) of embodiments of the present
invention, and the present invention is not limited to these
embodiments provided the gist thereof is not exceeded.
[0043] The composite graphite particles (C) for a nonaqueous
secondary battery negative electrode of the present invention are
composite graphite particles (C) for a nonaqueous secondary battery
negative electrode that contain graphite (A) and metal particles
(B) capable of alloying with Li, wherein the degree of dispersion
of the metal particles (B) in the composite graphite particles (C)
when viewing a cross-section of the composite graphite particles
(C) with a scanning electron microscope as calculated according a
specific measurement method to be subsequently described is 0.78 or
more.
[0044] These composite graphite particles (C) are preferably
produced by mixing at least the graphite (A) and the metal
particles (B) capable of alloying with Li followed by spheroidizing
treatment.
[0045] <Graphite (A)>
[0046] Although one example of the graphite (A), which is one
constituent component of the composite graphite particles (C) of
the present invention, is indicated below, there are no particular
limitations on the graphite (A), conventionally known graphite and
commercially available products may be used, and the graphite (A)
may be fabricated by any production method.
[0047] (Types of Graphite (A))
[0048] The graphite (A) can be obtained by carrying out removal of
impurities, crushing, screening and classification treatment as
necessary on flake, bulk or plate natural graphite or flake, bulk
or plate artificial graphite produced by heating, for example,
petroleum coke, coal pitch coke, coal-based needle coke or
mesophase pitch to 2500.degree. C. or higher.
[0049] Among these, natural graphite is classified into flake
graphite, crystalline (vein) graphite or amorphous graphite (refer
to section on Graphite in "Compilation of Particulate Process
Technologies", Industrial Technology Center, 1974) and "Handbook of
Carbon, Graphite, Diamond and Fullerenes", Noyes Publications).
[0050] Since vein graphite has the highest degree of graphitization
at 100% followed by flake graphite as 99.9%, these types of
graphite are used preferably.
[0051] Natural graphite in the form of flake graphite is produced
in countries such as Madagascar, China, Brazil, Ukraine and Canada,
while vein graphite is mainly produced in Sri Lanka. The main
production sites of amorphous graphite include the Korea peninsula,
China and Mexico.
[0052] Among these types of natural graphite, since flake graphite
and vein graphite offer the advantages of high degrees of
graphitization and low levels of impurities, they can be used
preferably in the present invention.
[0053] Examples of visual techniques for confirming that graphite
is flake graphite include observing particle surfaces with a
scanning electron microscope, and after having embedding particles
in resin to prepare thin resin sections and then cutting out
particle cross-sections, or after having prepared a coated film
cross-section from a coated film composed of particles using a
cross-section polisher and cutting out particle cross-sections, the
particle cross-sections are observed with a scanning electron
microscope.
[0054] Flake graphite and vein graphite consist of natural
graphite, for which purity has been enhanced so as to demonstrate
crystals having crystallinity completely similar to that of
graphite, and artificially formed graphite, and is preferably
natural graphite from the viewpoint of being industrially
inexpensive.
[0055] (Physical Properties of Graphite (A))
[0056] Physical properties of the graphite (A) in the present
invention are indicated below. Furthermore, although there are no
particular limitations on measurement methods used in the present
invention, they comply with the measurement methods described in
the examples unless there are special circumstances.
[0057] (1) Volume Mean Particle Diameter (d50) of Graphite (A)
[0058] The volume mean particle diameter (d50) of the graphite (A)
prior to compounding with the metal particles (B) (to also be
referred to as "d50" in the present invention) is normally 1 .mu.m
to 50 .mu.m, preferably 2 .mu.m to 40 .mu.m and more preferably 5
.mu.m to 30 .mu.m. If the volume mean particle diameter (d50) is
within these ranges, composite graphite particles (C) can be
produced that are embedded with the metal particles (B). In
addition, if the volume mean particle diameter (d50) of the
graphite (A) is excessively large, the particle diameter of the
composite graphite particles (C) embedded with the metal particles
(B) becomes large, thereby resulting in the formation of streaks
and surface irregularities caused by large particles in a step for
coating an electrode material mixed with the composite graphite
particles (C) in the form of a slurry by adding a binder, water or
organic solvent. If the volume mean particle diameter is
excessively small, the specific surface area of the composite
graphite particles (C) increases, resulting in the risk of an
increase in side reactions with the electrolytic solution.
[0059] Here, volume mean particle diameter (d50) refers to the
volume-based median diameter as measured by laser diffraction
scattering particle size distribution measurement.
[0060] (2) Mean Aspect Ratio of Graphite (A)
[0061] Mean aspect ratio, which is the ratio of the length of the
long axis to the length of the short axis of the graphite (A) prior
to compounding with the metal particles (B), is normally 1 to 50,
preferably 3 to 30 and more preferably 5 to 20. If mean aspect
ratio is within these ranges, the metal particles (B) within the
composite graphite particles (C) can be arranged with favorable
dispersibility.
[0062] Furthermore, in the present invention, aspect ratio refers
to the ratio of the length of the long axis to the length of the
short axis of particles, and since the minimum value thereof is 1,
the lower limit of aspect ratio is normally 1. Aspect ratio is
measured by capturing electron micrographs of target particles,
selecting 20 particles within a randomly selected region, defining
the longest axis of each particle as .alpha. (.mu.m) and the
shortest axis as .beta. (.mu.m), determining the ratio
.alpha./.beta., and taking the mean value of .alpha./.beta. of the
20 particles to be the aspect ratio.
[0063] (3) Tapped Density of Graphite (A)
[0064] The tapped density of graphite (A) prior to compounding with
the metal particles (B) is normally 0.1 g/cm.sup.3 to 1.0
g/cm.sup.3, preferably 0.13 g/cm.sup.3 to 0.8 g/cm.sup.3 and more
preferably 0.15 g/cm.sup.3 to 0.6 g/cm.sup.3. If the tapped density
of the graphite (A) is within the aforementioned ranges, the metal
particles (B) can be dispersed at a high degree of dispersion in
the composite graphite particles (C).
[0065] In the present invention, tapped density is defined as the
density determined by dropping a raw material carbon material
through a sieve having an opening size of 300 .mu.m into a
cylindrical tapping cell having a diameter of 1.6 cm and volume
capacity of 20 cm.sup.3 using a powder density tester and
completely filling the cell, followed by tapping 1000 times at a
stroke length of 10 mm and determining density from the volume and
sample weight at that time.
[0066] (4) Specific Surface Area of Graphite (A) Determined
According to BET Method
[0067] Specific surface area of the graphite (A) prior to
compounding with the metal particles (B) as determined according to
the BET method is normally 1 m.sup.2/g to 40 m.sup.2/g, preferably
2 m.sup.2/g to 35 m.sup.2/g, and more preferably 3 m.sup.2/g to 30
m.sup.2/g. Specific surface area of the graphite (A) according to
the BET method is reflected in the specific surface area of the
composite graphite particles (C). Consequently, by making the
specific surface area of the graphite (A) to be 40 m.sup.2/g or
less, decreases in battery capacity caused by irreversible
increases in capacity when using the composite graphite particles
(C) in an active material for a nonaqueous secondary battery
negative electrode can be prevented.
[0068] In the present invention, specific surface area is measured
according to the BET multipoint method using nitrogen gas
adsorption.
[0069] (5) Interplanar Spacing of 002 Plane (d002) and Lc of
Graphite (A)
[0070] Interplanar space of the 002 plane (d002) of the graphite
(A) as determined by wide-angle X-ray diffraction is normally 0.337
nm or less. On the other hand, since the theoretical value of the
interplanar spacing of the 002 plane of graphite is 0.335 nm, the
interplanar spacing of the 002 plane of graphite is normally 0.335
nm or more.
[0071] In addition, crystallite size (Lc) of the graphite (A) in
the direction of the c axis as determined by wide-angle X-ray
diffraction is 90 nm or more and preferably 95 nm or more.
[0072] If the interplanar spacing of the 002 plane (d002) is 0.337
nm or less, the graphite (A) demonstrates high crystallinity,
allowing the obtaining of high-capacity composite graphite
particles (C). In addition, in the case Lc is 90 nm or more as
well, the graphite (A) demonstrates high crystallinity, allowing
the obtaining of a high-capacity negative electrode material that
uses the composite graphite particles (C) containing the graphite
(A).
[0073] (6) True Density of Graphite (A)
[0074] True density of the graphite (A) prior to compounding with
the metal particles (B) is normally 2.1 g/cm.sup.3 or more,
preferably 2.15 g/cm.sup.3 or more and more preferably 2.2
g/cm.sup.3 or more. Graphite that demonstrates high crystallinity
having a true density of 2.1 g/cm.sup.3 or more allows the
obtaining of high-capacity composite graphite particles (C) having
low irreversible capacity.
[0075] (7) Lengths of Long and Short Axes of Particles of Graphite
(A)
[0076] The length of the long axis of the graphite (A) prior to
compounding with the metal particles (B) is preferably 40 .mu.m or
less, more preferably 35 .mu.m or less and even more preferably 30
.mu.m or less, and preferably 3 .mu.m or more, more preferably 5
.mu.m or more and even more preferably 8 .mu.m or more.
[0077] In addition, the length of the short axis of the graphite
(A) is normally 0.9 .mu.m to 5 .mu.m and preferably 1.5 .mu.m to 4
.mu.m. If the lengths of the long and short axes of the graphite
(A) are within the aforementioned ranges, the degree of dispersion
of the metal particles (B) in the composite graphite particles (C)
improves.
[0078] (8) Oil Absorption of Graphite (A)
[0079] The amount of oil absorbed by the graphite (A) prior to
compounding with the metal particles (B) when using dibutyl
phthalate (DBP) is preferably 50 ml/100 g or more and more
preferably 80 ml/100 g or more, and preferably 200 ml/100 g or less
and more preferably 200 ml/100 g or less. Although there are no
problems when oil absorption is excessively low, if oil absorption
is excessively high, when the graphite (A) and metal particles (B)
are mixed in a slurry and subjected to shearing force, a larger
amount of solvent is required thereby resulting in the risk of poor
productivity.
[0080] Furthermore, characteristics of the graphite (A) after
having been compounded with the metal particles (B) are preferably
within the range of the characteristics of the graphite (A) prior
to compounding with the metal particles (B).
[0081] <Metal Particles (B) Capable of Alloying with Li>
[0082] In the composite graphite particles (C) of the present
invention, the metal particles (B) capable of alloying with Li are
mainly embedded within the composite graphite particles (C)
although a portion thereof may be present on the surface of the
composite graphite particles (C).
[0083] (Types of Metal Particles (B) Capable of Alloying with
Li)
[0084] Although conventionally known metal particles can be used
for the metal particles (B) capable of alloying with Li, a metal or
compound thereof selected from the group consisting of Fe, Co, Sb,
Bi, Pb, Ni, Ag, Si, Sn, Al, Zr, Cr, V, Mn, Nb, Mo, Cu, Zn, Ge, As,
In, Ti and W is preferable from the viewpoints of capacity and
cycle life. In addition, an alloy composed of two or more types of
metals may be used, or the metal particles may be alloy particles
formed from two or more types of metal elements. Among these, a
metal or compound thereof selected from the group consisting of Si,
Sn, As, Sb, Al, Zn and W is preferable.
[0085] In addition, although the metal particles (B) can be in the
form of single crystals, polycrystals or in amorphous form
regardless of the crystalline state thereof, they are preferably in
the form of polycrystals from the viewpoints of facilitating
reduced particle diameter and being able to anticipate higher
capacity and high C-rate characteristics.
[0086] Examples of metal compounds include metal oxides, metal
nitrides and metal carbides. In addition, metal compounds composed
of two or more types of metals may also be used.
[0087] Si and/or Si compounds are preferable form the metal
particles (B) from the viewpoint of high capacity. In the present
description, Si and/or Si compounds are collectively referred to as
Si compounds.
[0088] Specific examples of Si compounds include those represented
by the general formulas Si, SiOx, SiNx and SiCx. In addition, Si
compounds may be doped from a portion of the surface to the inside
thereof by forming bonds between Si and elements such as C, N, P or
F. Although the general formula SiOx is obtained by using raw
materials consisting of silicon dioxide (SiO.sub.2) and metal
silicon (Si), the value of x is normally such that 0<x<2,
preferably 0.05 to 1.8, more preferably 0.1 to 1.6 and even more
preferably 0.15 to 1.4. If the value of x is within these ranges,
irreversible capacity caused by bonding between Li and oxygen can
be reduced while simultaneously increasing capacity.
[0089] Bonding between the metal particles (B) and the
aforementioned atoms can be analyzed by methods such as X-ray
photoelectron spectroscopy (XPS), infrared spectroscopy (IR) or
X-ray absorption fine structure analysis (XAFS).
[0090] The aforementioned sputtering treatment or a method
consisting of mixing in a compound having the aforementioned atoms
during mechanical energy treatment or high temperature treatment
can be used to form bonds between the metal particles (B) and the
aforementioned atoms. In addition, the metal particles (B)
containing the aforementioned atoms can also be used as a raw
material.
[0091] Impurities other than those previously described may be
present within the Si compound or on the surface thereof.
[0092] (Physical Properties of Metal Particles (B) Capable of
Alloying with Li)
[0093] Although there are no particular limitations on the metal
particles (B) capable of alloying with Li in the present invention
provided they are able to alloy with Li, the metal particles (B)
preferably demonstrate the physical properties indicated below.
[0094] Furthermore, although there are no particular limitations on
the measurement methods used in the present invention, they comply
with the measurement methods described in the examples unless there
are special circumstances.
[0095] (1) Volume Mean Particle Diameter (d50) of Metal Particles
(B)
[0096] The volume mean particle diameter (d50) of the metal
particles (B) in the composite graphite particles (C) is normally
0.005 .mu.m or more, preferably 0.01 .mu.m or more, more preferably
0.02 .mu.m or more and more preferably 0.03 .mu.m or more, and
normally 10 .mu.m or less, preferably 9 .mu.m or less and more
preferably 8 .mu.m or less. If the volume mean particle diameter
(d50) is within these ranges, volume expansion accompanying
charging and discharging is reduced and favorable cycling
characteristics can be obtained while maintaining charge-discharge
capacity.
[0097] Si and SiOx have a large theoretical capacity in comparison
with graphite, and amorphous Si or nano-sized Si crystals
facilitate the migration of alkaline ions such as lithium ions,
thereby allowing the obtaining of high capacity. Polycrystalline Si
is preferably used to obtain the aforementioned ranges from the
viewpoints of facilitating reductions in particle diameter and
demonstrating superior cycling characteristics and C-rate
characteristics.
[0098] (2) Specific Surface Area of Metal Particles (B) Determined
According to BET Method
[0099] Specific surface area of metal particles (B) in the
composite graphite particles (C) as determined according to the BET
method is normally 0.5 m.sup.2/g to 120 m.sup.2/g and preferably 1
m.sup.2/g to 100 m.sup.2/g. If specific surface area of the metal
particles capable of alloying with Li as determined according to
the BET method is within the aforementioned ranges,
charge-discharge efficiency and discharge capacity of the resulting
battery are high, migration of Li ions is rapid during high-speed
charging and discharging, and rate characteristics are superior,
thereby making this preferable.
[0100] (3) Oxygen Content of Metal Particles (B)
[0101] Although there are no particular limitations thereon, the
oxygen content of the metal particles (B) in the composite graphite
particles (C) is normally 0.01% by weight to 50% by weight and
preferably 0.05% by weight to 30% by weight. Although the
distribution of oxygen within the particles may be near the
surface, within the particles or uniformly within the particles, it
is preferably present near the surface in particular. If the oxygen
content of the metal particles is within the aforementioned ranges,
in addition to demonstrating superior initial efficiency, volume
expansion accompanying charging and discharging is inhibited and
cycling characteristics are superior due to the strong bonding
between Si and O, thereby making this preferable.
[0102] (4) Purity of Metal Particles (B)
[0103] Although there are no particular limitations thereon, purity
of the metal particles (B) in the composite graphite particles (C)
is preferably 70% or more, more preferably 80% or more and even
more preferably 90% or more. If purity is within the aforementioned
ranges, capacity can be obtained that is close to the theoretical
capacity of the metal particles (B). In addition, although there
are no particular limitations on the composition of impurities
present in the metal particles (B), the content of substances that
cause side reactions in a nonaqueous secondary battery is
preferably low.
[0104] (5) Crystallite Size of Metal Particles (B)
[0105] Although there are no particular limitations on crystallite
size in the case the metal particles (B) within the composite
graphite particles (C) are crystals, crystallite size of the (111)
plane as calculated by X-ray diffraction (XRD) is normally 0.05 nm
or more and preferably 1 nm or more, and normally 100 nm or less
and preferably 50 nm or less. If the crystallite size of the metal
particles is within the aforementioned ranges, the reaction between
Si and Li ions proceeds rapidly, there is little susceptibility to
the occurrence of dissociation of metal particles due to the
formation of cracks since Li concentration in the metal particles
is uniform, cycling characteristics are superior, and both
input-output and rate characteristics are superior, thereby making
this preferable.
[0106] Furthermore, characteristics of the metal particles (B)
prior to compounding with the graphite (A) are preferably within
the ranges of the characteristics of the metal particles (B) within
the composite graphite particles (C) as described above.
[0107] (Production Method of Metal Particles (B) Capable of
Alloying with Li)
[0108] (1) Previous Production Methods
[0109] The bottom-up approach is known to be one method for
currently producing fine particles of the metal particles (B). This
bottom-up approach consists of temporarily decomposing a material
to the atomic or molecular level by reacting or evaporating
followed by re-solidifying. Examples of vapor phase methods include
plasma-enhanced vapor deposition that utilizes a chemical reaction
(see, for example, Japanese Unexamined Patent Publication No.
H6-279015) or are melting consisting of physically evaporating a
material (see, for example, Japanese Unexamined Patent Publication
No. 2005-097654). Moreover, known examples of liquid phase methods
include methods capable of defining a dispersed state in a liquid
phase such as the co-precipitation method, reverse micelle method
or hot soap method (see, for example, Japanese Unexamined Patent
Publication No. 2003-515459). In addition, fine particles can also
be synthesized using the top-down approach in which large particles
are crushed into fine particles (see, for example, Japanese
Unexamined Patent Publication No. H7-88391). An example of the
aforementioned top-down approach is a method using a wet bead mill
that carries out crushing using beads for the media (see, for
example, Japanese Unexamined Patent Publication No.
2009-235263).
[0110] Although either the bottom-up approach or top-down approach
can be used to produce the fine particles (B), producing the fine
particles (B) using the top-down approach is preferable in the case
of considering industrial productivity.
[0111] (2) Production Method of Metal Particles (B) Using Top-Down
Approach
[0112] The metal particles (B) capable of alloying with Li are
preferably used after selecting and combining crushers used
according to the size of the raw materials and the target particle
diameter. For example, a media mill (ball mill), vibration mill,
pulverizer or jet mill and the like is used for the dry crusher if
the target diameter is about 5 .mu.m, and is preferably used
together with coarse crusheres as necessary according to the raw
material size. In the case the submicron range or smaller is the
target particle diameter, a wet bead mill is preferable. Although
examples of methods used by the crusher to separate the beads and
media include gap separation, screen separation and centrifugal
separation, any of these methods can be used provided they enable
beads to be separated that are required for achieving the target
particle diameter. In addition, since there are cases in which
crushing is not completed by only passing particles through the
crusher once, a system that allows circulation of solvent is
preferable. Although there are no particular limitations thereon,
examples include the Starmill LMZ Nanogetter HFM manufactured by
Ashizawa Finetech Ltd., Pico Mill PCM manufactured by Asada Iron
Works Co., Ltd., UVM Alpha Mill AM manufactured by Aimex Corp., and
Ultra Apex Mill UAM manufactured by Kotobuki Kogyo Co., Ltd.
[0113] (3) Slurry Composition
[0114] Raw Material of Metal Particles (B) Capable of Alloying with
Li
[0115] Commercially available metal particles may be used for the
metal particles (B) provided they satisfy the characteristics of
the present invention.
[0116] In addition, although there are no particular limitations on
the production method, metal particles produced according to the
method described in Japanese Patent No. 3952118, for example, can
be used for the metal particles (B). In the case of producing SiOx,
for example, after mixing silicon dioxide powder and silicon metal
powder at a specific ratio and filling this mixture into a reactor,
SiOx gas is generated by heating to 1000.degree. C. or higher and
holding at that temperature either at normal pressure or after
reducing pressure to a specific pressure followed by cooling and
precipitating the gas to obtain particles represented by the
general formula SiOx (sputtering treatment). The precipitate can be
used in the form of particles by applying mechanical energy
treatment.
[0117] Although conventionally known raw materials can be used for
the raw material of the metal particles (B), from the viewpoints of
capacity and cycle life, a metal or compound thereof selected from
the group consisting of Fe, Co, Sb, Bi, Pb, Ni, Ag, Si, Sn, Al, Zr,
Cr, V, Mn, Nb, Mo, Cu, Zn, Ge, As, In, Ti and W is preferable. In
addition, an alloy composed of two or more types of metals may be
used, and the metal particles may be alloy particles formed from
two or more types of metal elements. Among these, a metal or
compound thereof selected from the group consisting of Si, Sn, As,
Sb, Al, Zn and W is preferable. Furthermore, the raw material of
the metal particles can be used in the form of single crystals,
polycrystals or amorphous form regardless of the crystalline state
thereof. Although there are no particular limitations thereon, in
the case of an Si compound, for example, crushed ingots having
purity of any of metal grade (3N), solar cell grade (6N) or
semiconductor grade (9N) may be used, and either single crystal or
polycrystalline ingots may be used (see, for example, Japanese
Unexamined Patent Publication No. 2012-206923).
[0118] Dispersion Solvent
[0119] Although there are no particular limitations on the
dispersion solvent used, that having favorable wettability with the
raw material of the metal particles (B) is preferable from the
viewpoint of improving dispersibility of the metal particles (B) in
the composite graphite particles (C). In addition, a slurry used
with a wet bead mill has favorable dispersibility, and that having
low viscosity is preferable from the viewpoint of productivity.
Moreover, a trace amount of a dispersant (surfactant) may be added
to further increase wettability and dispersibility. A dispersant
having no or extremely little reactivity with the metal particles
(B) is preferably suitably selected for the dispersant.
[0120] The use of a nonpolar solvent or aprotic polar solvent is
preferable for inhibiting reactions between solvent and
newly-formed surfaces of the metal particles (B) following
crushing, and that having an aromatic ring is particularly
preferable. Although there are no particular limitations on the
type of nonpolar compound having an aromatic ring, that which
exhibits extremely little reactivity with the raw material of the
metal particles (B) is more preferable. Examples thereof include
liquid aromatic compounds such as benzene, toluene, xylene, cumene
or methylnaphthalene, alicyclic hydrocarbons such as cyclohexane,
methylcyclohexane, methylcyclohexene or bicyclohexyl, and residual
oils of petrochemicals and coalchemicals in the manner of light oil
or heavy oil. Among the aforementioned solvents, xylene is
preferable from the viewpoint of crushing efficiency due to its low
solvent viscosity and ease of drying.
[0121] In addition, solvents that dissolve not only water but also
organic solvents are preferable for the aprotic polar solvent,
examples of which include glycol-based solvents not having a
hydroxyl group such as N-methyl-2-pyrrolidone (NMP),
.gamma.-butyrolactone (GBL), N,N-dimethylformamide (DMF) or
propylene glycol monoethyl ether acetate (PGMEA).
N,N-dimethylformamide (DMF) is preferable from the viewpoint of
favorable dispersibility with Si compounds, its low boiling point
and its low viscosity.
[0122] In the case of crushing with a protic polar solvent, it is
preferably used by inhibiting decomposition gas.
[0123] The proportion of dispersion medium in the metal particles
(B) and dispersion medium is normally 50% by weight or more and
preferably 60% by weight or more and normally 90% by weight or less
and preferably 80% by weight or less.
[0124] If the proportion of dispersion medium is excessively high,
costs tend to increase when considering drying, and container size
also becomes larger resulting in increased susceptibility to
unevenness. Conversely, if the mixed proportion of the dispersion
medium is excessively low, slurry viscosity increases due to a
decrease in average distance between particles and slurry
dispersibility tends to become poor.
[0125] Types of Dispersants
[0126] A dispersant may be added to the aforementioned dispersion
medium when producing the metal particles (B). The dispersant is
only required to be able to dissolve in each dispersion medium, and
may be a low molecular weight surfactant that improves wettability.
Examples thereof include Noigen TDS30, Noigen EN, Noigen ET65,
Noigen ET115, Noigen ES-149D, DKS-NL-DASH400, DKS-NL-DASH408,
Amiradine C1802, Dianol CDE, Plysurf A208B, Plysurf A208F, Catiogen
ESL-9, Noigen TDS100, Noigen XL40, Noigen XL80, Noigen TDX50,
Noigen LF60, Noigen EA87, Noigen EA167, Sorgen 30V, Noigen ET69,
Noigen ET149, Noigen ES99D, DKSNL15, DKSNL50, Eban 410 (Dai-ichi
Kogyo Seiyaku Co., Ltd.), Nopcosperse 092, SN Dispersant 9228, SN
Sperse 70 (San Nopco Ltd.), PW36, DA375, KS806, KS873, 1831, 1850,
DA1401, 1860 (Kusumoto Chemicals, Ltd.), Arquad 22-80, Armin 8D,
Armin CD, Armin M20, Duomin CD, Duomin T, Esomin T/12, Erimin 0/12
and Armin OD. In addition, a high molecular weight dispersant is
more preferable in which the target system is able to realize a
dispersion system due to steric hindrance effects as particle
diameter decreases. Examples thereof include Ajisper PA111, Ajisper
PB821, Ajisper PB822, Ajisper PB881, Ajisper PN411 (Ajinomoto Co.,
Inc.), Anti-Terra U100, Byk-LPN6919, Disperbyk-102, Disperbyk-103,
Disperbyk-111, Dispebyk-118, Disperbyk-167, Disperbyk-180,
Disperbyk-2000, Disperbyk-2001, Disperbyk-2009, Dipserbyk-2013,
Disperbyk-2022, Disperbyk-2050, Disperbyk-2152, Disperbyk-2155,
Disperbyk-2164, Disperplast-1142, Disperplast-1148,
Disperplast-1150 (Byk Chemie GmbH), Hinoact KF-100, Hinoact
KF-1500, Hinoact T-6000, Hinoact T-8000, Hinoact T-8000E, Hinoact
T-9100 (Kawaken Fine Chemicals Co., Ltd.), DA-703-50, DA375,
DA1200, SPD200, SPD201, SPD201, DA325 and DN900 (Kusumoto
Chemicals, Ltd.).
[0127] <Other Materials>
[0128] The composite graphite particles (C) for a nonaqueous
secondary battery negative electrode of the present invention may
contain materials other than the graphite (A) and the metal
particles (B).
[0129] (Carbon Fine Particles)
[0130] The composite graphite particles (C) of the present
invention may also contain carbon fine particles for improving
electrical conductivity.
[0131] Volume Mean Particle Diameter (d50)
[0132] The volume mean particle diameter of the carbon fine
particles is normally 0.01 .mu.m to 10 .mu.m, preferably 0.05 .mu.m
or more, more preferably 0.07 .mu.m or more and even more
preferably 0.1 .mu.m or more, and preferably 8 .mu.m or less, more
preferably 5 .mu.m or less and even more preferably 1 .mu.m or
less.
[0133] In the case the carbon fine particles have a secondary
structure resulting from the congregation and aggregation of
primary particles, although there are no particular limitations on
the type or other properties thereof provided the primary particle
diameter is 3 nm to 500 nm, the primary particle diameter is
preferably 3 nm or more, more preferably 15 nm or more, even more
preferably 30 nm or more and particularly preferably 40 nm or more,
and preferably 500 nm or less, more preferably 200 nm or less, even
more preferably 100 nm or less and particularly preferably 70 nm or
less. The primary particle diameter of carbon fine particles can be
measured by observing with an electron microscope such as an SEM or
by measuring with a laser diffraction particle size distribution
analyzer.
[0134] Types of Carbon Fine Particles
[0135] There are no particular limitations on the shape of the
carbon fine particles, and may have a granular shape, spherical
shape, linear shape, needle shape, fibrous shape, plate-like shape
or flake-like shape.
[0136] More specifically, although there are no particular
limitations on the carbon fine particles, examples thereof includes
substances having a nanostructure such as coal fine powder, vapor
phase carbon powder, carbon black, Ketjen black, fullerene, carbon
nanofibers, carbon nanotubes or carbon nanowalls. Carbon black is
particularly preferable among them. The use of carbon black
improves input-output characteristics at low temperatures while
also simultaneously offering the advantages of being inexpensive
and readily available.
[0137] (Carbon Precursor)
[0138] In addition, the composite graphite particles (C) of the
present invention may also be mixed with a carbon precursor for
inhibiting reactions between the metal particles (B) and nonaqueous
electrolytic solution.
[0139] Reactions between the metal particles (B) and nonaqueous
electrolytic solution can be inhibited as a result of the carbon
precursor covering the periphery of the metal particles (B).
[0140] Mixing with the carbon precursor may be carried out prior to
firing the composite particles or may be carried out by dissolving
in a crushed slurry containing the metal particles (B) provided it
is soluble therein.
[0141] Types of Carbon Precursors
[0142] Carbon materials described in the following (.alpha.) and/or
(.beta.) are preferably used for the aforementioned carbon
precursor.
[0143] (.alpha.) Carbonizable organic matter selected from the
group consisting of coal-based heavy oil, direct flow heavy oil,
cracked petroleum heavy oil, aromatic hydrocarbons, N ring
compounds, S ring compounds, polyphenylene, synthetic organic
polymers, natural polymers, thermoplastic resins and thermosetting
resins
[0144] (.beta.) Product of dissolving carbonizable organic matter
in low molecular weight organic solvents
[0145] Coal tar pitch ranging from soft pitch to hard pitch or dry
distillation liquefied oil is preferable for the aforementioned
coal-based heavy oil. Atmospheric residue or vacuum residue is
preferable for the aforementioned direct flow heavy oil. Ethylene
tar produced as a by-product during cracking of crude oil, naphtha
and the like is preferable for the aforementioned cracked petroleum
heavy oil. Acenaphthylene, decacyclene, anthracene and phenanthrene
are preferable for the aforementioned aromatic hydrocarbons.
Phenazine and acridine are preferable for the aforementioned N ring
compounds. Thiophene and bithiophene are preferable for the
aforementioned S ring compounds. Biphenyl or terphenyl is
preferable for the aforementioned polyphenylene. Polyvinyl
chloride, polyvinyl alcohol, polyvinyl butyral, insoluble treatment
products thereof, nitrogen-containing polymers such as
polyacrylonitrile, polypyrrole, polyallylamine, polyvinylamine,
polyethyleneimine, urethane resin and urea resin, polythiophene,
polystyrene and polymethacrylic acid are preferable for the
aforementioned synthetic organic polymers. Polysaccharides such as
cellulose, lignin, mannan, polygalacturonic acid, chitosan or
saccharose are preferable for the aforementioned natural polymers.
Polyphenylene sulfide and polyphenylene oxide are preferable for
the aforementioned thermoplastic resins. Furfuryl alcohol resins,
phenol-formaldehyde resins and imide resins are preferable for the
aforementioned thermosetting resins.
[0146] In addition, the carbonizable organic matter may also be a
carbide of a solution obtained by dissolving in a low molecular
weight organic solvent such as benzene, toluene, xylene, quinoline
or n-hexane. One type of this carbonizable organic matter may be
used alone or two or more types may be used in an arbitrary
combination.
[0147] X-Ray Parameters of Carbon Materials Obtained by Firing
Carbon Precursor
[0148] The d value of interplanar spacing of the (002) plane as
determined by wide-angle X-ray diffraction of a powder of a carbon
material obtained by firing the carbon precursor is normally 0.340
nm or more and preferably 0.342 nm or more. In addition, the d
value is normally less than 0.380 nm, preferably 0.370 nm or less
and more preferably 0.360 nm or less. An excessively large d002
value results in low crystallinity and cycling characteristics tend
to decrease, while if the d002 value is excessively small, it is
difficult to obtain the effect of compounding the carbon
material.
[0149] In addition, the crystallite size (Lc(002)) of a carbon
material as determined by X-ray diffraction according to the
Gakushin method of a powder of a carbon material obtained by firing
the carbon precursor is normally 5 nm or more, preferably 10 nm or
more and more preferably 20 nm or more. In addition, crystallite
size (Lc(200)) is normally 300 .mu.m or less, preferably 200 nm or
less and more preferably 100 nm or less. If crystallite size is
excessively large, cycling characteristics tend to decrease, while
if crystallite size is excessively small, charge-discharge
reactivity decreases, resulting the risk of increases in generated
gas during high-temperature storage and a decrease in large-current
charge-discharge characteristics.
[0150] (Resin Serving as Void Forming Material)
[0151] In addition, the composite graphite particles (C) of the
present invention may also be mixed with a resin serving as a void
forming material in order to alleviate destruction of the composite
graphite particles (C) caused by expansion and contraction of the
metal particles (B). Furthermore, although the resin serving as a
void forming material as referred to in the present description is
included in some of the aforementioned carbon precursors,
coal-based heavy oil, direct flow heavy oil and cracked petroleum
heavy oil are not included.
[0152] Molecular Weight
[0153] Although there are no particular limitations thereon, the
weight average molecular weight of the resin serving as a void
forming material is normally 500 or more, preferably 1000 or more,
more preferably 1500 or more, even more preferably 2000 or more and
particularly preferably 2500 or more. On the other hand, the weight
average molecular weight is normally 1,000,000 or less, preferably
500,000 or less, more preferably 300,000 or less, even more
preferably 100,000 or less, particularly preferably 50,000 or less
and most preferably 10,000 or less. In the case the molecular
weight is excessively low, specific surface area increases
resulting in a tendency for charge-discharge efficiency to decrease
when contained in particles, while in the case the molecular weight
is excessively high, viscosity increases, which tends to make
uniform mixing and dispersion difficult.
[0154] Firing Yield
[0155] The firing yield of the resin serving as a void forming
material is normally 0.1% or more, preferably 1% or more, more
preferably 5% or more and even more preferably 10% or more, while
normally less than 20%, preferably 18% or less, more preferably 16%
or less and even more preferably 14% or less. In the case the
firing yield is excessively high, voids are not formed and the
alleviating action accompanying expansion and contraction of the
metal particles (B) tends to decrease.
[0156] Decomposition Temperature
[0157] The decomposition temperature of the void forming material
is normally 30.degree. C. or higher, preferably 50.degree. C. or
higher, more preferably 100.degree. C. or higher and even more
preferably 150.degree. C. or higher, while normally 500.degree. C.
or lower, preferably 400.degree. C. or lower, more preferably
300.degree. C. or lower and even more preferably 200.degree. C. or
lower. In the case the decomposition temperature is excessively
low, there is the risk of the void forming material easily
decomposing, while in the case the decomposition temperature is
excessively high, it becomes difficult for the void forming
material to dissolve in solvent, thereby resulting in the risk of
making it difficult to disperse the void forming material
uniformly.
[0158] Types of Resins
[0159] Although there are no particular limitations thereon,
examples of resins that can be used for the void forming material
include polyvinyl alcohol, polyethylene glycol, polycarbosilane,
polyacrylic acid and cellulose-based polymers, and polyvinyl
alcohol and polyethylene glycol can be used particularly preferably
from the viewpoint of low remaining carbon on firing and having a
comparatively low decomposition temperature.
[0160] (Other Resins)
[0161] In addition, the metal particles (B) may also contain a
resin so as to cover the surface of the metal particles (B). Silane
coupling agent-derived resins that are able to bind to silanol
groups (see, for example, Japanese Unexamined Patent Publication
No. 2006-196338) and resins having functional groups demonstrating
high affinity for the metal particles (B) are preferable.
[0162] <Composite Graphite Particles (C) for Nonaqueous
Secondary Battery Negative Electrode>
[0163] The composite graphite particles (C) of the present
invention contain graphite (A) and metal particles (B) capable of
alloying with Li, and when cross-sections of the composite graphite
particles (C) are observed with a scanning electron microscope
(SEM), the degree of dispersion of the aforementioned metal
particles (B) in the composite graphite particles (C) as calculated
according to the measurement method to be subsequently described is
0.78 or more.
[0164] Degree of Dispersion of Metal Particles (B) within Composite
Graphite Particles (C)
[0165] The degree of dispersion of the metal particles (B) in the
composite graphite particles (C) as measured according to the
method used to measure the composite graphite particles (C) of the
present invention indicated below is 0.78 or more, preferably 0.8
or more, more preferably 0.85 or more, even more preferably 0.87 or
more, particularly preferably 0.89 or more and most preferably 0.92
or more. In addition, the maximum degree of dispersion is 1. A
higher value within the aforementioned ranges indicates that the
metal particles (B) are uniformly dispersed in the composite
graphite particles (C), and when used to form a negative electrode,
particle destruction caused by local expansion within the particles
can be prevented, reductions in charge-discharge efficiency in each
cycle can be inhibited, and rate characteristics tend to be
superior.
[0166] The degree of dispersion of the metal particles (B) in the
composite graphite particles (C) of the present invention is
calculated in the manner indicated below. It is necessary to
observed particle cross-sections of the composite graphite
particles (C) in order to calculate degree of dispersion. Although
there are no particular limitations on the method used to observe
particle cross-sections, particle cross-sections can be observed by
an observation method consisting of preparing a polar plate of the
composite graphite particles (C), a coated film of the composite
graphite particles (C), or resin thin sections by embedding the
composite graphite particles (C) in resin and the like, followed by
cutting the prepared specimens with a focused ion beam (FIB) or by
ion milling, cutting out particle cross-sections and then observing
the particle cross-sections with a scanning electron microscope
(SEM).
[0167] The accelerating voltage when observing cross-sections of
primary particles of the composite graphite particles (C) with a
scanning electron microscope (SEM) is normally preferably 1 kV or
more, more preferably 2 kV or more and even more preferably 3 kV or
more, and normally 10 kV or less, more preferably 8 kV or less and
even more preferably 5 kV or less. If accelerating voltage is
within these ranges, it becomes easy to distinguish between
graphite particles and Si compounds due to differences in reflected
secondary electron images present in the resulting SEM images. In
addition, the imaging magnification factor is normally 500.times.
or more, more preferably 1000.times. or more and even more
preferably 2000.times. or more, and normally 10000.times. or less.
If the magnification factor is within the aforementioned ranges, it
becomes possible to acquire an image of an entire primary particle
of the composite graphite particles (C). Resolution is 200 dpi
(ppi) or more and preferably 256 dpi (ppi) or more. In addition,
the images are preferably evaluated at 800 pixels or more. Although
elements of the graphite (A) and metal particles (B) may be
identified by energy-dispersive X-ray spectroscopy (EDX) and
wavelength-dispersive X-ray spectroscopy (WDX) while observing
images, since, for example, the electron conductivity of Si
compounds is normally not favorable, they tend to appear white
particularly in the case of reflected electron images.
Consequently, they can be easily distinguished from graphite (A)
and amorphous carbon. In the case graphite (A) and metal particles
(B) are easily distinguished, particles having a white reflection
in reflected electron images and the like are defined as metal
particles (B).
[0168] Furthermore, since there is the risk of the boundaries
between metal particles (B) and graphite (A) becoming ambiguous in
cases of low resolution during EDX (EDS) or WDX, mapping is
preferably carried out at a high resolution roughly equal to that
during SEM when carrying out analyses by mapping images.
[0169] Degree of Dispersion Measurement Method and Conditions
[0170] The method used to measure degree of dispersion is defined
as satisfying conditions 1 to 4 indicated below.
[0171] (Condition 1)
[0172] 100 or more composite graphite particles (C) are randomly
selected in an acquired image (and preferably, a polar plate). At
this time, the particles are demarcated in contour units of the
composite graphite particles (C). Although there are no particular
limitations on the method used to demarcate the particles, any
image processing software may be used provided background and the
distinction between light and dark are well-defined, although the
boundaries may be defined manually in cases in which distinction is
difficult. Although the particles may be approximated with
polygons, they are preferably minimally approximated with hectogons
or better to prevent the particles from becoming coarse. The center
of gravity (centroid) is then defined with a single, arbitrary
extracted composite graphite particle (C). First, the particle
demarcated from this boundary is approximated with squares.
Although the size of the squares is not particularly specified, it
preferably corresponds to an actual dimension of 5 nm or less.
Two-dimensional coordinates are then determined on the image. The
coordinates at the center of the squares is defined. The squares
are then numbered from 1 to N assuming the squares to have the same
weight. The coordinates of the center of gravity of the composite
particle are then determined from the following equation 1.
[ Equation 1 ] .fwdarw. r G = 1 N .fwdarw. r i N ( 1 )
##EQU00001##
[0173] Here, r.sub.i represents the coordinates of the ith square
and r.sub.G represents the coordinates of the center of gravity.
The procedure for determining the center of gravity may be carried
out with any imaging software, and the demarcated grid may be
determined with the following equation provided each center of
gravity can be defined with an arbitrary figure.
[ Equation 2 ] .fwdarw. r G = 1 N A i .fwdarw. r i 1 N A i ( 2 )
##EQU00002##
[0174] Here, A.sub.i represents the area of the ith figure and
r.sub.i represents the coordinates of the center of gravity
(centroid) of the ith figure.
[0175] Next, the longest line segment among the arbitrary line
segments demarcated by the aforementioned established boundary
according to the determined center of gravity is defined as the
long axis. In addition, the line segment that is perpendicular to
the long axis among the arbitrary line segments demarcated by the
aforementioned established boundary according to the center of
gravity is defined as the short axis.
[0176] (Condition 2)
[0177] The 100 or more composite graphite particles (C) randomly
selected according to Condition 1 are subjected to processing in
the manner described above to determine the center of gravity, long
axis and short axis, and 10 particles that satisfy the following
condition are randomly selected.
|0.5.times.(A+B)-R|.ltoreq.3
Here, A represents the length of the long axis (.mu.m), B
represents the length of the short axis (.mu.m), and R represents
the mean particle diameter d50 (.mu.m).
[0178] (Condition 3)
[0179] A grid-like lattice is drawn for the 10 composite graphite
particles (C) selected according to Condition 2. The lattice is
placed so as to be parallel with the long axis of the particles (C)
and so that one intersection of the lattice aligns with the center
of gravity. The squares of the lattice are in the shape of squares,
those for which the length of the long axis A of the particles (C)
is longer than 20 .mu.m (namely, in the case A.gtoreq.20 .mu.m)
have a length on one side of 2 .mu.m, and those that are smaller
than this (namely, in the case A<20 .mu.m) have a length on one
side equal to one-tenth the length of the long axis A (namely, A/10
.mu.m). Among the lattice squares, the number of squares in the
lattice that contain a composite graphite particle (C) is
designated as x. The area of the composite particle (C) relative to
the area of the square is 50% or less are defined as squares in the
lattice that do not contain the composite graphite particle (C).
The number of squares in the lattice that contain a composite
graphite particle (C) and also contain a metal particle (B) is
designated as y. The values of y/x of five random particles among
the 10 particles selected according to Condition 2 for which the
lattice is drawn are respectively calculated, and the average value
thereof is defined as the degree of dispersion (see FIG. 1).
[0180] (Condition 4)
[0181] Furthermore, particles not composed of graphite (A) and/or
metal particles (B) are excluded from the target particles.
Moreover, composite graphite particles (C) that are obviously
fragmented or torn are also excluded since they are not suitable as
evaluation targets of the composite graphite particles (C). In
addition, in order to avoid selecting peculiar particles, degree of
dispersion is judged to be unmeasurable in the case less than 5 of
the 100 composite graphite particles (C) contained in the image
satisfy the aforementioned extraction conditions.
[0182] (Preferable Conditions)
[0183] More preferable conditions for selecting the particles
extracted according to Condition 3 preferably consist of selecting
those particles in which the relationship between the long axis A
and short axis B satisfies the equation indicated below.
|(A/B of particles extracted according to Condition 3)/(average
value of (A/B) of all particles extracted according to Condition
1)-1|.ltoreq.0.5
[0184] In addition, although there are no particular limitations on
the range over which the particles extracted according to Condition
1 are selected, they are preferably extracted from a range of a
plurality of images measuring 100 .mu.m.times.150 .mu.m and more
preferably from one or more images, while preferably extracted from
a range of 10 images or less. In the case of being unable to
extract 100 or more particles within the aforementioned ranges,
particles are preferably selected by making contrivances to the
method used to prepare the polar plate.
[0185] Although target composite graphite particles (C) that
satisfy these Conditions 1 to 4 and satisfy the degree of
dispersion of the present invention are only normally required to
be present at 5% or more as the number of particles in a
cross-section over a range of 200 .mu.m.times.200 .mu.m
cross-section of a negative electrode, the number of particles is
normally 30%, more preferably 50%, even more preferably 90% or more
and particularly preferably 99% or more based on the total number
of target composite graphite particles (C).
[0186] (Physical Properties of Composite Graphite Particles
(C))
[0187] Although there are no particular limitations on the
composite graphite particles (C) provided the degree of dispersion
of the metal particles (B) in the composite graphite particles (C)
is 0.78 or more as calculated according to the aforementioned
measurement method when observing cross-sections of the composite
graphite particles with a scanning electron microscope (SEM), the
composite graphite particles (C) preferably have the properties
indicated below.
[0188] (1) Tapped Density of Composite Graphite Particles (C)
[0189] The tapped density of the composite graphite particles (C)
of the present invention is normally 0.8 g/cm.sup.3 or more,
preferably 0.85 g/cm.sup.3 or more, more preferably 0.88 g/cm.sup.3
or more, and even more preferably 0.9 g/cm.sup.3 or more. On the
other hand, the tapped density is normally 1.5 g/cm.sup.3 or less,
preferably 1.4 g/cm.sup.3 or less and more preferably 1.35
g/cm.sup.3 or less.
[0190] The demonstrating of a large value for tapped density by the
composite graphite particles (C) of the present invention is an
indicator that the composite graphite particles (C) exhibit a
spherical shape. A smaller tapped density is an indicator that the
composite graphite particles (C) do not have a sufficiently
spherical shape. If tapped density is excessively low, adequate
continuous voids are unable to be secured in the electrode and
mobility of Li ions with electrolytic solution retained in the
voids decreases, thereby resulting in the risk of a decrease in
rapid charge-discharge characteristics.
[0191] Furthermore, in the case the metal particles (B) are
aggregated, compounding with the graphite (A) occurs non-uniformly
starting at the aggregated particles, and as a result thereof,
there is the possibility of a decrease in particle fillabiity, or
in other words, a decrease in tapped density. However, since the
metal particles (B) are uniformly dispersed in the composite
graphite particles (C) of the present invention and maintain
favorable binding with graphite, they are thought to be particles
having high tapped density.
[0192] (2) Interplanar Spacing d Value of (002) Plane of Composite
Graphite Particles (C)
[0193] The interplanar spacing value d of the (002) plane of the
composite graphite particles (C) of the present invention as
determined by wide-angle X-ray diffraction is normally 0.337 nm or
less, while on the other hand, since the theoretical value of the
interplanar spacing value d of the 002 plane of graphite is 0.335,
the interplanar spacing value d of the 002 plane of graphite is
normally 0.335 nm or more. In addition, the Lc value of the
graphite (A) as determined by wide-angle X-ray diffraction is 90 nm
or more and preferably 95 nm or more. The interplanar spacing d
value of the (002) plane and Lc value of the as determined by
wide-angle X-ray diffraction being within the aforementioned ranges
indicates that the composite graphite particles (C) can serve as a
material of a high-capacity electrode.
[0194] (3) Volume Mean Particle Diameter (d50) of Composite
Graphite Particles (C)
[0195] The volume mean particle diameter (d50) of the composite
graphite particles (C) of the present invention is preferably 40
.mu.m or less and more preferably 30 .mu.m or less, and normally 1
.mu.m or more, preferably 4 .mu.m or more and more preferably 6
.mu.m or more. If the volume mean particle diameter d50 is
excessively large, problems such as streaking occur during coating,
while if the volume mean particle diameter d50 is excessively
small, a larger amount of binder is required, resistance increases
and high current density charge-discharge characteristics tend to
decrease.
[0196] (4) Raman R Value of Composite Graphite Particles (C)
[0197] The ratio of peak intensity in the vicinity of 1360
cm.sup.-1 to the peak intensity in the vicinity of 1580 cm.sup.-1
in an argon ion laser Raman spectrum of the composite graphite
particles (C) of the present invention in the form of the Raman R
value is normally 0.05 to 0.4 and preferably 0.1 to 0.35. If the
Raman R value is within these ranges, crystallinity of the surface
of the composite graphite particles (C) is orderly and high
capacity can be expected.
[0198] (5) Specific Surface Area of Composite Graphite Particles
(C) Determined According to BET Method
[0199] Specific surface area of the composite graphite particles
(C) of the present invention as determined according to the BET
method is normally 20 m.sup.2/g or less, preferably 15 m.sup.2/g or
less, more preferably 10 m.sup.2/g or less, even more preferably
8.5 m.sup.2/g or less and still more preferably 8 m.sup.2/g or
less, and normally 0.1 m.sup.2/g or more, preferably 1 m.sup.2/g or
more, more preferably 3 m.sup.2/g or more, even more preferably 5
m.sup.2/g or more and particularly preferably 6.5 m.sup.2/g or
more. If the specific surface area is excessively large, since
those locations where the composite graphite particles (C) contact
the nonaqueous electrolytic solution increase when using as an
active material for a negative electrode, reactivity increases, the
amount of gas generated becomes excessively large, and it tends to
be difficult to obtain a preferable battery. If specific surface
area is excessively small, the acceptability of lithium ions during
charging tends to become poor in the case of using as an active
material for a negative electrode.
[0200] (6) Content of Metal Particles (B) in Composite Graphite
Particles (C)
[0201] The content of the metal particles (B) in the composite
graphite particles (C) of the present invention based on the
composite graphite particles (C) is normally 0.5% by weight or
more, preferably 1% by weight or more, more preferably 1.5% by
weight or more and even more preferably 2% by weight or more. In
addition, the content of the metal particles (B) is normally 99% by
weight or less, preferably 70% by weight or less, more preferably
50% by weight or less, even more preferably 30% by weight or less
and particularly preferably 25% by weight or less. If the content
of the metal particles (B) is within these ranges, it becomes
possible to obtain adequate capacity, thereby making this
preferable. Furthermore, measurement of the content of the metal
particles (B) in the composite graphite particles (C) is carried
out using the method to be subsequently described.
[0202] (7) Internal Void Fraction of Composite Graphite Particles
(C)
[0203] The internal void fraction of the composite graphite
particles (C) of the present invention based on the graphite (A) of
the composite graphite particles (C) is normally 1% or more,
preferably 3% or more, more preferably 5% or more and even more
preferably 7% or more. In addition, the internal void fraction is
normally less than 50%, preferably 40% or less, more preferably 30%
or less and even more preferably 20% or less. If the internal void
fraction is excessively low, the composite graphite particles (C)
tend to be destroyed when the metal particles (B) expand. In
addition, these voids may be filled with a substance such as
amorphous carbon, graphite material or resin so as to alleviate
expansion and contraction of the metal particles (B) capable of
alloying with Li. When confirming void fraction by SEM using
cross-sections of the composite graphite particles (C), void
fraction can be calculated using the following equation (2) on 20
random particles.
Equation (2)
Area of voids within composite graphite particles (C)/area of voids
of graphite (A)+metal particles (B)+composite graphite particles
(C) (2)
[0204] <Production Method of Composite Graphite Particles (C)
for Nonaqueous Secondary Battery Negative Electrode>
[0205] Although there are no particular limitations on the method
used to produce the composite graphite particles (C) for a
nonaqueous secondary battery negative electrode in the present
invention provided the method allows the obtaining of composite
graphite particles (C) having the aforementioned properties, an
example thereof consists of stirring the graphite (A), having a
comparatively small particle diameter such that the value of d50 is
20 .mu.m or less, and the metal particles (B) after adjusting the
concentration such that suitable shearing stress is applied in the
slurry to prepare a slurry containing an aggregate of the graphite
(A) and the metal particles (B), and drying this aggregate slurry
so that 5% or more of solvent remains therein, followed by carrying
out spheroidizing treatment with the solvent residue still
contained in the slurry to efficiently embed the metal particles
(B) in the composite graphite particles (C) in a highly dispersed
state. Here, components other than the graphite (A) and the metal
particles (B) may also be mixed in simultaneously.
[0206] More specifically, the production method preferably
comprises the following step 1, step 2 and step 3.
[0207] Step 1: Step for obtaining a mixture at least containing
graphite (A) and metal particles (B).
[0208] Step 2: Step for obtaining an aggregate of the graphite (A)
and the metal particles (B).
[0209] Step 3: Step for applying mechanical energy to the aggregate
of step 2 and subjecting to spheroidizing treatment.
[0210] The following provides a detailed explanation of the
production method of the present invention.
[0211] (Step 1: Step for Obtaining Mixture Containing at Least
Graphite (A) and Metal Particles (B))
[0212] Although examples of the state of the mixture obtained in
this step include that in the form of granules, a solid, a block or
a slurry, the mixture is preferably in the form of a block from the
viewpoint of ease of handling.
[0213] The mixing ratio of the metal particles (B) to the total
amount of the graphite (A) and the metal particles (B) is normally
1% by weight or more, preferably 3% by weight or more, more
preferably 5% by weight or more and even more preferably 7% by
weight or more. In addition, the mixing ratio is normally 95% by
weight or less, preferably 90% by weight or less, more preferably
80% by weight or less and even more preferably 70% by weight or
less. If the mixing ratio is within these ranges, adequate capacity
can be obtained, thereby making this preferable. The graphite (A)
used in this step is normally 1 .mu.m to 50 .mu.m, preferably 2
.mu.m to 40 .mu.m and more preferably 5 .mu.m to 30 .mu.m. If
within these ranges, composite graphite particles (C) can be
produced that are embedded with the metal particles (B), and
composite particles can be obtained in which the metal particles
(B) are highly dispersed therein.
[0214] In addition, in the present step, carbon fine particles may
also be mixed in to improve electrical conductivity of the
composite graphite particles (C), a carbon precursor may be mixed
in to inhibit a reaction between the metal particles (B) and the
nonaqueous electrolytic solution, or a void forming material in the
form of a resin and the like may be mixed in to alleviate
destruction of composite graphite particles caused by expansion and
contraction of the metal particles (B).
[0215] In the case of mixing in other materials other than the
graphite (A) and metal particles (B), the mixing ratio of other
materials to the total amount of the graphite (A), the metal
particles (B) and the other materials is normally 0.1% by weight or
more, preferably 0.3% by weight or more, more preferably 0.5% by
weight or more and even more preferably 0.7% by weight or more. In
addition, the mixing ratio is normally 30% by weight or less,
preferably 28% by weight or less, more preferably 26% by weight or
less and even more preferably 25% by weight or less. If the mixing
ratio is within these ranges, adequate capacity can be obtained,
thereby making this preferable.
[0216] In the present step, there are no particular limitations on
the method used to mix the graphite (A), the metal particles (B)
and the other materials provided a mixture is obtained that at
least contains the graphite (A) and the metal particles (B) capable
of alloying with Li.
[0217] The mixing method may consist of simultaneously adding the
graphite (A), the metal particles (B) capable of alloying with Li
and the other materials followed by mixing, or sequentially adding
each of these components separately followed by mixing.
[0218] An example of a preferable method for obtaining the mixture
consists of using wet metal particles (B) and mixing with the
graphite (A) so as not to allow the metal particles (B) to dry.
[0219] Metal particles (B) obtained directly by producing the metal
particles (B) using a wet method, metal particles (B) produced
using a dry method may be dispersed in a dispersion medium prior to
mixing with the graphite (A), or metal particles (B) mixed with
other materials dissolved in a solvent and the like to wet the
metal particles (B) may be used for the wet metal particles
(B).
[0220] The metal particles (B) that have been wetted in this manner
can be uniformly dispersed during mixing since aggregation of the
metal particles (B) is inhibited, and metal particles (B) are
easily immobilized on the surface of the graphite (A), thereby
making this preferable.
[0221] In the present description, in the case of mixing the metal
particles (B) in the form of a slurry when mixing the metal
particles (B) into the graphite (A), the solid content of the metal
particles (B) in the slurry is normally 10% by weight or more,
preferably 15% by weight or more and more preferably 20% by weight
or more, and normally 90% by weight or less, preferably 85% by
weight or less and more preferably 80% by weight or less. If the
proportion of this solid content is excessively high, the slurry
loses fluidity and the metal particles (B) tend to be difficult to
disperse in the graphite (A), while if the proportion of the solid
content is excessively low, handling tends to become difficult in
this step.
[0222] In addition, after mixing the graphite (A) and the metal
particles (B), the mixture is stirred so as to suitably apply
shearing stress to the slurry, and from the viewpoint of
facilitating the formation of an aggregate, removing solvent from
the mixed slurry or adding the dispersion medium used when
wet-crushing the metal particles (B) during mixing makes it
possible to control the solid content.
[0223] The non-volatile (N V) ratio at the time of mixing the
graphite (A) and the metal particles (B) when applying shearing
stress is preferably 30% or more and more preferably 40% or more.
In addition, the non-volatile (NV) ratio is preferably 70% or less
and more preferably 60% or less.
[0224] In a slurry having an NV ratio within the aforementioned
ranges, the slurry is preferably in a state in which it does not
flow when leveled, and if the NV ratio is excessively high,
excessive shearing stress ends up being applied to the graphite
(A), which tends to destroy the graphite (A) or prevent the
formation of aggregates of the graphite (A) and the metal particles
(B). In addition, if the NV ratio is excessively low, the metal
particles (B) undergo migration when removing the solvent, which
tends to lower the degree of dispersion of the composite graphite
particles (C).
[0225] (Step 2: Step for Obtaining Aggregates of Graphite (A) and
Metal Particles (B))
[0226] In the present invention following step 1, aggregates of the
graphite (A) and the metal particles (B) are preferably formed by
evaporating off and drying the dispersion medium using an
evaporater, a dryer and the like.
[0227] Alternatively, aggregates of the graphite (A) and metal
particles (B) are preferably formed as a result of suitably
applying shearing stress to the slurry by mixing while evaporating
the dispersion medium and heating in a high-speed stirrer directly
without adding a surplus of dispersion medium. Furthermore, the
aggregates are preferably put into a wet state after drying. The
content of solvent residue in aggregates of the graphite (A) and
metal particles (B) based on the total weight of the graphite (A)
and metal particles (B) is normally 1% by weight or more,
preferably 2% by weight or more and more preferably 3% by weight or
more. In addition, the content of solvent residue is normally 40%
by weight or less, preferably 30% by weight or less and more
preferably 20% by weight or less. If within these ranges, composite
particles can be obtained in which the Si compound is highly
dispersed. In addition, the amount of solvent residue may be
adjusted to a suitable amount by suitably adding dispersion medium
after drying.
[0228] Furthermore, there are no particular limitations on the time
at which the other materials are mixed in order to obtain the
mixture and aggregates, and for example, other materials may be
added when mixing the graphite (A) and metal particles (B), the
other materials may be added to wet metal particles (B) or a slurry
of metal particles (B), or the other materials may be added when
wet-crushing the metal particles (B). Although the state when
mixing the other materials may be a powder or a solution obtained
by dissolving in a solvent, a solution is preferable from the
viewpoint of allowing the other materials to be uniformly
dispersed.
[0229] Among these other materials, the carbon precursor and resin
serving as a void forming material not only fulfill the role of
immobilizing the metal particles (B) on the graphite (A), but are
also thought to fulfill the role of preventing dissociation of the
metal particles (B) from the graphite (A) during the
spheroidization step. As a result, destruction of the composite
graphite particles (C) can be inhibited.
[0230] Among the mixtures previously described, a more preferable
mixture is obtained by mixing the graphite (A), the metal particles
(B) and a resin, and from the viewpoint of being able to uniformly
disperse the graphite (A), the metal particles (B) and the resin
for inhibiting dissociation of the metal particles (B) in the
mixture, mixing a slurry of the metal particles (B) with a resin
for inhibiting dissociation of the metal particles (B) dissolved in
a solvent followed by mixing the graphite (A) therein is more
preferable for the combination of steps for obtaining this mixture.
In addition, at this time, another material in the form of a carbon
precursor may be further mixed in from the viewpoint of being able
to inhibit reactivity between the metal particles (B) and the
electrolytic solution, or a void forming material in the form of a
resin may be mixed in to alleviate destruction of the composite
graphite particles (C) caused by expansion and contraction of the
metal particles (B).
[0231] Although mixing is normally carried out at normal pressure,
it can also be carried out under reduced pressure or added pressure
as desired. Mixing can also be carried out by a batch process or a
continuous process. In either case, mixing efficiency can be
improved by combining the use of a device suitable for coarse
mixing and a device suitable for fine mixing. In addition, a device
may also be used that carries out mixing and immobilization
(drying) simultaneously. Although drying can normally be carried
out under reduced pressure or added pressure, drying is preferably
carried out under reduced pressure.
[0232] Drying time is normally 5 minutes or more, preferably 10
minutes or more, more preferably 20 minutes or more and even more
preferably 30 minutes or more, and normally 2 hours or less,
preferably 1.5 hours or less and even more preferably 1 hour or
less. If drying time is excessively long, it leads to increased
costs, while if drying time is excessively short, uniform drying
tends to become difficult.
[0233] Although varying according to the solvent, drying
temperature is preferably the amount of time that enables drying to
be realized in the times indicated above.
[0234] In addition, the drying temperature is also preferably equal
to or lower than the temperature at which the resin serving as
another material does not undergo degeneration.
[0235] Examples of devices used for a batch type of mixing device
include mixers employing a structure in which two frames rotate
while revolving, devices employing a structure in which a single
blade carries out stirring and dispersing in a tank in the manner
of a high-speed, high-shear mixer in the form of a dissolver or
high-viscosity butterfly mixer, so-called kneader-type devices that
have a structure in which a sigma-type or other stirring blade
rotates along the lateral surface of a semi-cylindrical mixing
tank, a type of device using biaxial or triaxial stirring blades,
and a so-called bead mill type of device having a rotating disk and
a dispersion medium in a container.
[0236] In addition, a device employing a structure having a
container in which paddles rotated by a shaft are housed therein,
in which the sides of the container substantially follow the
outermost line of the paddle rotation, and are preferably formed
into a long catamaran shape, and a large number of pairs of paddles
are arranged in the axial direction of the shaft so as to as to
bite into mutually opposing sides while able to slide over the
sides (such as the KRC Reactor or SC Processor manufactured by
Kurimoto, Ltd., the Model TEM manufactured by Toshiba Machine
Selmac Co., Ltd., or the Model TEX-K manufactured by Japan Steel
Works Ltd.), or an (exothermic) device employing a structure having
a single internal shaft and a container having a plurality of
spade-shaped or serrated paddles attached to the shaft arranged
while changing the phase thereof, in which the inside walls thereof
substantially follow the outermost line of the paddle rotation, and
are preferably formed into a cylindrical shape (such as the Lodige
Mixer manufactured by Lodige GmbH, the Flow Share Mixer
manufactured by Pacific Machinery & Engineering Co., Ltd., or
the DT Dryer manufactured by Tsukishima Kikai Co., Ltd.). A
pipeline mixer, continuous bead mill and the like may be used when
carrying out mixing with a continuous process. In addition, a
uniform mixture can also be obtained with a means such as
ultrasonic dispersion. Among the aforementioned mixing devices, a
device of a type employing biaxial or triaxial stirring blades is
used preferably from the viewpoint of ease of adjusting shearing
stress during mixing. In addition, a powder step consisting of
crushing, shredding or classification may also be applied to the
mixture obtained in this step.
[0237] Although there are no particular limitations on the device
used for crushing and shredding, examples of coarse crushers
include shearing mills, jaw crushers, impact crushers and cone
crushers, examples of intermediate crushers include roll crushers
and hammer mills, and examples of fine crushers include ball mills,
vibration mills, pin mills, mixing mills and jet mills.
[0238] Although there are no particular limitations on the device
used for classification treatment, in the case of, for example, dry
screening, a rotary sieve, shaking sieve, gyrating sieve or
vibrating sieve can be used, in the case of dry air classification,
a gravity classifier, inertial classifier or centrifugal classifier
(such as a classifier or cyclone) can be used, or a wet sieve,
mechanical wet classifier, hydraulic classifier, settling
classifier or centrifugal wet classifier and the like can be
used.
[0239] (Step 3: Step for Carrying Out Spheroidizing Treatment by
Applying Mechanical Energy to Aggregate of Step 2)
[0240] As a result of going through step 3, aggregates containing
the graphite (A) and the metal particles (B) can be further
compounded, resulting in the attainment of high tapped density and
a high degree of dispersion, thereby making this preferable.
[0241] In other words, although an example of a production method
for obtaining the composite graphite particles (C) of the present
invention consists of carrying out spheroidizing treatment on
aggregates containing the metal particles (B) on the surface of the
graphite (A) obtained in the aforementioned step 2 (to also be
simply referred to as aggregates in the present description), in
the present invention in particular, production conditions as
subsequently described are preferably suitably set so that the
metal particles (B) are made to be present in voids of the graphite
(A) within a prescribed range. As a result of carrying out
spheroidizing treatment in a state of containing solvent residue,
adhesion between the graphite (A) and metal particles (B) in the
aggregates increases, and since the metal particles (B) can be
inhibited from separating from the aggregates and aggregating,
composite graphite particles (C) tend to be obtained that have a
high degree of dispersion.
[0242] Furthermore, spheroidizing treatment specifically refers to
treatment that utilizes mechanical energy (mechanical action such
as impact compression, friction or shearing force). More
specifically, treatment using a hybridization system is preferable.
This system has a rotor having a large number of blades that apply
mechanical action such as impact compression, friction or shearing
force, a large air flow is generated by rotation of the rotor,
considerable centrifugal force is applied to the graphite (A) in
the mixture obtained in the aforementioned step 1 as a result
thereof, and the aggregates obtained in step 2 can be compounded as
a result of collisions between graphite (A) in the aggregates
obtained in the aforementioned step 2 and between the graphite (A)
present in the aggregates obtained the aforementioned step 2, the
walls and the blades.
[0243] An example of a device used for spheroidizing treatment
consists of a device that performs surface treatment by having a
rotor installed with a large number of blades housed in a casing,
and applying mechanical action such as impact compression, friction
or shearing force to graphite present in the mixture obtained in
step 1 that has been introduced therein by rotating the rotor at
high speeds. Although examples thereof include a dry ball mill, wet
bead mill, planetary ball mill, vibration ball mill, mechanofusion
system, Agromaster (Hosokawa Micron Corp.), Hybridization System,
Micros, Miralo (Nara Machinery Co., Ltd.), CF Mill (Ube Industries,
Ltd.) and Theta Composer (Tokuju Corp.), preferable examples of
devices include a dry ball mill, wet bead mill, planetary ball
mill, vibration ball mill, mechanofusion system, Agromaster
(Hosokawa Micron Corp.), Hybridization System, Micros, Miralo (Nara
Machinery Co., Ltd.), CF Mill (Ube Industries, Ltd.), Theta
Composer (Tokuju Corp.) and pulverizer. Among these, the
Hybridization System manufactured by Nara Machinery Co., Ltd. is
particularly preferable.
[0244] Furthermore, although the graphite (A) in the aggregates
obtained in the aforementioned step 2 subjected to spheroidizing
treatment may also be that which has already been subjected to a
certain spheroidizing treatment under the conditions of a
conventional method, flake graphite (A) is preferable from the
viewpoint of dispersibility. In addition, the aggregates obtained
in step 2 may also be repeatedly subjected to mechanical action by
circulating or by going through this step a plurality of times.
[0245] Although spheroidizing treatment is carried out using this
type of device, when carrying out this treatment, spheroidizing
treatment is carried out at a rotor rotating speed of normally 2000
rpm or more, preferably 4000 rpm or more, more preferably 5000 rpm
or more, even more preferably 6000 rpm or more and particularly
preferably 6500 rpm or more, and normally at 9000 rpm or less,
preferably 8000 rpm or less, more preferably 7500 rpm or less and
even more preferably 7200 rpm or less, for a duration of normally
30 seconds or more, preferably 1 minute or more, more preferably 1
minute 30 seconds or more, even more preferably 2 minutes or more
and particularly preferably 2 minutes 30 seconds or more, and
normally for 60 minutes or less, preferably 30 minutes or less,
more preferably 10 minutes or less and even more preferably 5
minutes or less.
[0246] Furthermore, if the rotor rotating speed is excessively
slow, treatment for obtaining composite particles becomes
ineffective and there is the possibility of an insufficient
increase in degree of dispersion, while if the rotating speed is
excessively fast, the effect of crushing the particles becomes more
effective than the treatment for obtaining composite particles,
resulting in the possibility of the particles collapsing and the
degree of dispersion decreasing. Moreover, if the duration of
spheroidizing treatment is excessively short, particle diameter is
unable to be made sufficiently small while achieving a high degree
of dispersion, while if the duration is excessively long, the
graphite (A) in the aggregates obtained in step 2 is crushed,
thereby resulting in the possibility of being unable to achieve the
object of the present invention.
[0247] Furthermore, classification treatment may also be carried
out on the resulting composite graphite particles (C). In the case
the resulting composite graphite particles (C) are not within the
range of the physical properties defined in the present invention,
they can be made to be within a desired range of physical
properties by repeatedly subjecting to classification treatment
(normally for 2 to 10 times and preferably for 2 to 5 times).
Although examples of classification include dry classification (air
classification, screening) and wet classification, dry
classification, and particularly air classification, is preferable
in terms of cost and productivity.
[0248] The composite graphite particles (C) can be produced
according to the production method described above.
[0249] <Carbonaceous Material-Coated Composite Graphite
Particles>
[0250] Although the composite graphite particles (C) used in the
present invention are obtained in the manner described above, the
composite graphite particles (C) preferably contain a carbonaceous
material, and a preferable specific aspect thereof consists of
composite graphite particles having a carbonaceous material at
least partially coated on the surface thereof (to be referred to as
carbonaceous material-coated composite graphite particles).
[0251] Furthermore, in the present description, although the
carbonaceous material-coated composite graphite particles are
distinguished from the composite graphite particles (C) for the
sake of convenience, the carbonaceous material-coated composite
graphite particles are interpreted as being included in the
composite graphite particles (C).
[0252] (Production Method of Carbonaceous Material-Coated Composite
Graphite Particles)
[0253] The carbonaceous material-coated composite graphite
particles can be produced by going through a step 4 following the
previously described step 3.
[0254] Step 4: Step for coating the composite graphite particles
subjected to spheroidizing treatment in step 3 with a carbonaceous
material.
[0255] The following provides a detailed explanation of step 4.
[0256] (Step 4: Step for Coating Composite Graphite Particles
Subjected to Spheroidizing Treatment in Step 3 with Carbonaceous
Material)
[0257] Carbonaceous Material
[0258] Examples of the carbonaceous material include amorphous
carbon and graphitizable carbon depending on differences in the
heating temperature in the production method thereof to be
subsequently described. Among these, amorphous carbon is preferable
from the viewpoint of acceptable of lithium ions.
[0259] More specifically, the aforementioned carbonaceous material
can be obtained by heat treating a carbon precursor thereof in the
manner described below. Carbon precursors explained in the
previously described section on other materials are preferably used
for the aforementioned carbon precursor.
[0260] Coating Treatment
[0261] In the coating treatment, a carbon precursor for obtaining a
carbonaceous material is used as a coating material for coating the
composite graphite particles obtained in the aforementioned step 2,
and coated graphite is obtained by their mixing and firing.
[0262] If the firing temperature is normally 600.degree. C. or
higher, preferably 700.degree. C. or higher and more preferably
900.degree. C. or higher, and normally 2000.degree. C. or lower,
preferably 1500.degree. C. or lower and more preferably
1200.degree. C. or lower, amorphous carbon is obtained as
carbonaceous material. On the other hand, if heat treatment is
carried out at a firing temperature of normally 2000.degree. C. or
higher, preferably 2500.degree. C. or higher and normally
3200.degree. C. or lower, graphitizable carbon is obtained as
carbonaceous material. The amorphous carbon refers to carbon having
a low degree of crystallinity, while the graphitizable carbon
refers to carbon having a high degree of crystallinity.
[0263] During coating treatment, the aforementioned composite
graphite particles (C) are used as a core material, the precursor
carbon for obtaining a carbonaceous material is used as a coating
raw material, and these are then mixed and fired to obtain
carbonaceous material-coated composite graphite particles.
[0264] Furthermore, in the case of coating other resins and the
like, the resin is preferably dissolved in a solvent and mixed with
the composite fired in step 4 followed by blending well, washing
and drying.
[0265] Mixing of Metal Particles (B) and Carbon Fine Particles
[0266] The coating layer may contain the metal particles (B) and
carbon fine particles previously explained in the section on other
materials.
[0267] Other Steps
[0268] In addition, the carbonaceous material-coated composite
graphite particles that have gone through the aforementioned steps
may undergo powder processing such as the crushing, shredding and
classification treatment described in step 1.
[0269] The carbonaceous material-coated composite graphite
particles of the present invention can be produced according to the
production method described above.
[0270] (Physical Properties of Carbonaceous Material-Coated
Composite Graphite Particles)
[0271] Although the carbonaceous material-coated composite graphite
particles demonstrate the same physical properties as those of the
previously described composite graphite particles, those preferable
physical properties of the carbonaceous material-coated composite
graphite particles that are particularly altered by coating
treatment are described below.
[0272] d Value of Interplanar Spacing of (002) Plane
[0273] The d value of interplanar spacing of the (002) plane of the
carbonaceous material-coated composite graphite particles as
determined by wide-angle X-ray diffraction is normally 0.336 nm or
more, preferably 0.337 nm or more, more preferably 0.340 nm or more
and even more preferably 0.342 nm or more. In addition, the d value
is normally less than 0.380 nm, preferably 0.370 nm or less and
more preferably 0.360 nm or less. An excessively large d002 value
results in the demonstration of low crystallinity and cycling
characteristics tend to decrease, while if the d002 value is
excessively small, it becomes difficult to obtain the effect of
compounding the carbonaceous material.
[0274] Coverage Factor
[0275] Although the carbonaceous material-coated composite graphite
particles are coated with amorphous carbon or graphitizable carbon,
coating with amorphous carbon is preferable from the viewpoint of
acceptability of lithium ions, and the coverage factor thereof is
normally 0.5% or more, preferably 1% or more, more preferably 3% or
more, even more preferably 4% or more, particularly preferably 5%
or more and most preferably 6% or more, and normally 30% or less,
preferably 25% or less, more preferably 20% or less, even more
preferably 15% or less, particularly preferably 10% or less and
most preferably 8% or less. If this content is excessively high,
the amorphous carbon portion of the negative electrode material
becomes excessively large and reversible capacity when
incorporating in a battery tends to decrease. If the content is
excessively low, in addition to amorphous carbon sites not being
uniformly coated on the graphite particles serving as cores, solid
granulation does not occur and particle diameter tends to be
excessively small during crushing after firing.
[0276] Furthermore, the content of carbide derived from organic
compounds in the ultimately obtained carbon material for an
electrode (coverage factor) can be calculated using the following
equation (3) based on the amount of raw material carbon used, the
amount of organic compound, and the residual carbon ratio measured
according to the micro method in compliance with JIS K2270-02
(2009).
Coverage factor of carbide derived from organic compound
(%)=(weight of organic compound.times.residual carbon
ratio.times.100)/{weight of raw material carbon+(weight of organic
compound.times.residual carbon ratio)} Equation (3)
[0277] <Other Mixtures>
[0278] Although the composite graphite particles (C) of the present
invention can be used alone as an active material for a nonaqueous
secondary battery negative electrode, it is preferably used for the
active material of a nonaqueous secondary battery negative
electrode by further containing one or more types of materials
selected from the group consisting of natural graphite, artificial
graphite, vapor-grown carbon fibers, electrically conductive carbon
black, carbonaceous material-coated graphite, resin-coated
graphite, amorphous carbon and materials obtained by subjecting
these materials to suitable treatment that have different forms and
physical properties from the aforementioned composite graphite
particles (C). Among these, an active material for a nonaqueous
secondary battery negative electrode containing the composite
graphite particles (C) of the present invention and one or more
types of materials selected from the group consisting of natural
graphite, artificial graphite, carbonaceous material-coated
graphite, resin-coated graphite and amorphous carbon is more
preferable.
[0279] As a result of suitably selecting and mixing carbonaceous
particles having different forms and physical properties, cycling
characteristics can be improved as a result of improving electrical
conductivity, charge acceptance can be improved, irreversible
capacity can be decreased and rollability can be improved.
[0280] The mixing ratio of the composite graphite particles (C) to
the total amount of composite graphite particles (C) and
carbonaceous particles having a form and properties different
therefrom is normally 1% by weight or more, preferably 1.5% by
weight or more, more preferably 2% by weight or more, and even more
preferably 2.5% by weight or more. In addition, the mixing ratio is
normally 99% by weight or less, preferably 95% by weight or less,
more preferably 90% by weight or less and even more preferably 85%
by weight or less.
[0281] If the amount of composite graphite particles (C) is
excessively high, volume expansion accompanying charging and
discharging increases, deterioration of capacity becomes prominent
and press load may decrease. In addition, if the amount of
composite graphite particles (C) is excessively low, adequate
capacity tends to not be obtained.
[0282] Among those carbonaceous particles having different form and
properties, highly purified flake graphite or spheroidized
graphite, for example, can be used as natural graphite.
[0283] Particles obtained by compounding coke powder or natural
graphite with a binder or particles obtained by firing and
graphitizing single graphite precursor particles while still in the
form of a powder, for example, can be used as artificial
graphite.
[0284] Particles obtained by coating a carbonaceous material
precursor onto natural graphite or artificial graphite followed by
firing, or particles obtained by coating a carbonaceous material
onto the surface of natural graphite or artificial graphite, for
example, can be used as carbonaceous material-coated graphite.
[0285] Particles obtained by coating a polymeric material onto
natural graphite or artificial graphite followed by drying, for
example, can be used as resin-coated graphite, while particles
obtained by firing bulk mesophase or particles obtained by
subjecting a carbonaceous material precursor to infusibilization
treatment followed by firing, for example, can be used as amorphous
carbon.
[0286] <Negative Electrode for Nonaqueous Secondary
Battery>
[0287] In order to fabricate a negative electrode using an active
material for a nonaqueous secondary battery negative electrode
containing the composite graphite particles (C) according to the
present invention, a mixture obtained by incorporating a binder
resin in an active material for a nonaqueous secondary battery
negative electrode is formed into a slurry with water or organic
solvent, followed by coating onto a current collector after adding
a thickener as necessary and drying.
[0288] A binder resin that is stable in nonaqueous electrolytic
solution and is non-water-soluble is preferably used for the binder
resin. Examples thereof include rubber-like polymers such as
styrene, butadiene rubber, isoprene rubber or ethylene-propylene
rubber, synthetic resins such as polyethylene, polypropylene,
polyethylene terephthalate or aromatic polyamides, thermoplastic
elastomers such as styrene-butadiene-styrene block copolymers and
hydrogenated products thereof, styrene-ethylene-butadiene-styrene
copolymers or styrene-isoprene-styrene block copolymers and
hydrogenated products thereof, soft resin-like polymers such as
syndiotactic 1,2-polybutadiene, ethylene-vinyl acetate copolymer or
copolymers of ethylene and .alpha.-olefins having 3 to 12 carbon
atoms, and fluorinated polymers such as polyvinylidene fluoride,
polypentafluorpropylene or polyhexafluoropropylene. Examples of
organic solvents include NMP and DMF.
[0289] The binder resin is used at preferably 0.1% by weight and
more preferably 0.2% by weight based on the active material for a
nonaqueous secondary battery negative electrode. By making the
ratio of binder resin to active material for a nonaqueous secondary
battery negative electrode to be 0.1% by weight or more, mutual
adhesion between active materials for a nonaqueous secondary
battery negative electrode and adhesion between the composite
graphite particles and current collector are adequate, thereby
making it possible to prevent reductions in battery capacity and
exacerbation of cycling characteristics caused by the active
material for a nonaqueous secondary battery negative electrode
dissociating from the negative electrode. The binder resin is
preferably used at 10% by weight at most and preferably at 7% by
weight or less.
[0290] Examples of thickeners added to the slurry include
water-soluble celluloses such as carboxymethyl cellulose, methyl
cellulose, hydroxyethyl cellulose or hydroxypropyl cellulose, as
well as polyvinyl alcohol and polyethylene glycol. Among these,
carboxymethyl cellulose is preferable. The thickener is used at
preferably 0.2% by weight to 10% by weight and more preferably at
0.5% by weight to 7% by weight based on the composite graphite
particles.
[0291] A material conventionally known to be able to be used in
this application is used for the negative electrode current
collector, and examples thereof include copper, copper alloys,
stainless steel, nickel, titanium and carbon. The shape of the
current collector is normally that of a sheet, and preferably has
surface irregularities on the surface thereof and preferably uses a
metal mesh or perforated metal.
[0292] After having coated a slurry of the active material for a
nonaqueous secondary battery negative electrode and the binder
resin onto the current collector followed by drying, the coated
slurry is preferably pressured to increase the density of the
negative electrode active material layer formed on the current
collector, resulting in an increase in the battery capacity per
unit volume of the negative electrode active material layer. The
density of the negative electrode active material layer is
preferably 1.2 g/cm.sup.3 or more and more preferably 1.3
g/cm.sup.3 or more, and preferably 1.9 g/cm.sup.3 or less and more
preferably 1.8 g/cm.sup.3 or less. As a result of making the
density of the negative electrode active material layer to be 1.2
g/cm.sup.3 or more, decreases in battery capacity accompanying
increases in electrode thickness can be prevented. As a result of
making the density of the negative electrode active material layer
to be 1.8 g/cm.sup.3 or less, reductions in the amount of
electrolytic solution retained in voids accompanying reductions in
inter-particle voids in the electrode, and decreases in rapid
charge-discharge characteristics attributable to decreased mobility
of alkaline ions such as lithium (Li) ions, can be prevented.
[0293] <Nonaqueous Secondary Battery>
[0294] The nonaqueous secondary battery according to the present
invention can be fabricated in accordance with ordinary methods
with the exception of using the aforementioned negative electrode.
Lithium transition metal complex oxides such as lithium cobalt
complex oxide having a basic composition represented by
LiCoO.sub.2, a lithium nickel complex oxide represented by
LiNiO.sub.2 or a lithium manganese complex oxide represented by
LiMnO.sub.2 or LiMn.sub.2O.sub.4, transition metal oxides such as
manganese dioxide, mixed complex oxides thereof as well as
TiS.sub.2, FeS.sub.2, Nb.sub.3S.sub.4, Mo.sub.3S.sub.4, CoS.sub.2,
V.sub.2O.sub.5, CrO.sub.3, V.sub.3O.sub.3, FeO.sub.2, GeO.sub.2 or
LiNi.sub.0.33Mn.sub.0.33O.sub.2 may be used for the positive
electrode material.
[0295] The positive electrode can be fabricated by forming a
mixture incorporating a binder resin in these positive electrode
materials into a slurry with a suitable solvent, and coating the
slurry onto a current collector followed by drying. Furthermore,
the slurry preferably contains an electrically conductive material
such as acetylene black or Ketjen black.
[0296] In addition, a thickener may also be contained as desired.
Examples of thickeners and binder resins that may be used include
commonly known thickeners and binders used in this application,
such as those used to fabricate a negative electrode.
[0297] The mixing ratio of the electrically conductive material
based on the positive electrode material is normally 0.5% by weight
to 20% by weight and particularly preferably 1% by weight to 15% by
weight. The mixing ratio of the thickener is preferably 0.2% by
weight to 10% by weight and more preferably 0.5% by weight to 7% by
weight. The mixing ratio of the binder resin when forming a slurry
with water is preferably 0.2% by weight to 10% by weight and more
preferably 0.5% by weight to 7% by weight. In addition, the mixing
ratio of a binder resin such as NMP when forming a slurry with an
organic solvent is preferably 0.5% by weight to 20% by weight and
more preferably 1% by weight to 15% by weight.
[0298] Aluminum, titanium, zirconium, hafnium, niobium, tantalum or
an alloy thereof may be used for the positive electrode current
collector.
[0299] Among these, aluminum, titanium, tantalum or an alloy
thereof is used preferably, and aluminum and/or an alloy thereof
are used most preferably.
[0300] An electrolytic solution obtained by dissolving various
lithium salts in a conventionally known nonaqueous solvent can be
used for the electrolytic solution. Examples of nonaqueous solvents
include cyclic carbonates such as ethylene carbonate,
fluoroethylene carbonate, propylene carbonate, butylene carbonate
or vinylene carbonate, linear carbonates such as dimethyl
carbonate, ethyl methyl carbonate or diethyl carbonate, cyclic
esters such as .gamma.-butyrolactone, cyclic ethers such as crown
ether, 2-methyltetrahydrofuran, tetrahydrofuran,
1,2-dimethyltetrahydrofuran or 1,3-dioxolane, and linear ethers
such as 1,2-dimethoxyethane. Normally several of these solvents are
used in combination. Among these, cyclic carbonates, linear
carbonates and those used in combination with other solvents are
preferable.
[0301] In addition, compounds such as vinylene carbonate, vinyl
ethylene carbonate, succinic anhydride, maleic anhydride, propane
sultone or diethylsulfone, or difluorophosphates in the manner of
lithium difluorophosphate, may also be added. Moreover, overcharge
preventive agents such as diphenyl ether or cyclohexylbenzene may
also be added.
[0302] Examples of electrolytes dissolved in nonaqueous solvent
that may be used include LiClO.sub.4, LiPF.sub.6, LiBF.sub.4,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2) and
LiC(CF.sub.3SO.sub.2).sub.3. The concentration of electrolyte in
the electrolytic solution is normally 0.5 mol/l to 2 mol/l and
preferably 0.6 mol/l to 1.5 mol/l.
[0303] A porous sheet composed of a polyolefin such as polyethylene
or polypropylene, or a non-woven fabric, is preferably used for the
separator interposed between the positive electrode and negative
electrode.
[0304] The capacity ratio of negative electrode/positive electrode
in the nonaqueous secondary battery according to the present
invention is preferably designed to be 1.01 to 1.5 and more
preferably designed to be 1.2 to 1.4.
[0305] The nonaqueous secondary battery is preferably a lithium ion
secondary battery provided with a positive electrode and negative
electrode, capable of occluding and releasing lithium ions, and an
electrolyte.
EXAMPLES
[0306] Although the following provides a more detailed explanation
of specific aspects of the present invention through examples
thereof, the present invention is not limited by these
examples.
[0307] Furthermore, measurement of the volume mean particle
diameter (d50), BET specific surface area, Si content of the
composite graphite particles and Si degree of dispersion of the
composite graphite particles in the present description was carried
out according to the methods indicated below.
[0308] Volume Mean Particle Diameter (d50)
[0309] Volume mean particle diameter (d50) was determined by adding
about 20 mg of carbon powder to 1 ml of a 2% aqueous solution of
polyoxyethylene (20) sorbitan monolaurate followed by dispersing
this in about 200 ml of ion exchange water, and measuring volume
particle size distribution of the resulting dispersion using a
laser diffraction particle size distribution analyzer (LA-920,
Horiba, Ltd.) to determine median diameter (d50). Measurement
conditions consisted of ultrasonic dispersion for 1 minute,
ultrasonic intensity of 2, circulating speed of 2 and relative
refractive index of 1.50.
[0310] BET Specific Surface Area (SA)
[0311] BET specific surface area (SA) was measured using the
Tristar 11300 manufactured by Micromeritics Japan G.K. After vacuum
drying for 1 hour at 150.degree. C., BET specific surface area was
measured according to the BET multipoint method using nitrogen gas
adsorption (by measuring 5 points within a relative pressure range
of 0.05 to 0.30).
[0312] Tapped Density of Composite Graphite Particles
[0313] Tapped density of the composite graphite particles was
determined by dropping a sample through a sieve having an opening
size of 300 .mu.m into a cylindrical tapping cell having a diameter
of 1.6 cm and volume capacity of 20 cm.sup.3 using a powder density
tester and completely filling the cell, followed by tapping 1000
times at a stroke length of 10 mm and determining density from the
volume and sample weight at that time.
[0314] Si Content of Composite Graphite Particles
[0315] Si content of the composite graphite particles was
determined by completely melting a sample (composite graphite
particles) with base followed by dissolving with water and bringing
to a constant volume, measuring with an inductively-coupled plasma
emission spectrometer (Ultima 2C, Horiba, Ltd.) and calculating the
amount of Si from a calibration curve. Subsequently, the Si content
of the composite graphite particles was calculated by dividing the
amount of Si by the weight of the composite graphite particles.
[0316] Degree of Dispersion of Si in Composite Graphite
Particles
[0317] The degree of dispersion of Si in the composite graphite
particles was measured in the manner indicated below. A polar plate
similar to the polar plate used to fabricate a battery for
evaluating performance as described below was used for the polar
plate containing composite graphite particles. First, a
cross-section of the electrode was processed using a cross-section
polisher (IB-09020CP, JEOL Ltd.). The graphite (A) and Si were
mapped using reflected electron images while observing the
processed electrode cross-section with an SEM (SU-70, Hitachi
High-Technologies Corp.). Furthermore, conditions for acquiring the
SEM cross-section consisted of an accelerating voltage of 3 kV and
magnification factor of 1000.times., and images were obtained over
a range capable of acquiring a single particle at a resolution of
256 dpi. Subsequently, 100 particles were extracted using two SEM
images measuring 150 .mu.m.times.100 .mu.m in accordance with the
measurement method and conditions for degree of dispersion
described above, and 10 particles were selected therefrom that
satisfy the aforementioned degree of dispersion measurement
conditions to calculate degree of dispersion using the y/x values
of 5 random particles.
Example 1
Preparation of Composite Graphite Particles (C)
[0318] (Step 1)
[0319] First, metal particles (B) in the form of polycrystalline Si
having a mean particle diameter d50 of 1 .mu.m were crushed to a
mean particle diameter d50 of 0.2 .mu.m with an LMZ10 (Ashizawa
Finetech Ltd.) to prepare an Si slurry (I). 300 g of this Si slurry
(I) (solid content: 40% by weight) were added to 1500 g of NMP
without allowing to dry followed by stirring using a mixing stirrer
(Dalton Co., Ltd.). Next, 1000 g of graphite (A) in the form of
natural flake graphite (mean particle diameter d50: 9 .mu.m) were
added followed by mixing using a mixing stirrer at an NV ratio of
40% by weight to obtain a slurry (II) in which Si compound
particles and graphite were uniformly dispersed therein.
[0320] (Step 2)
[0321] This slurry (II) was dried under reduced pressure at
150.degree. C. until the solvent residue became 10% by weight based
on the charged amounts of the graphite (A) and metal particles (B).
The resulting aggregates were crushed with a mill having a
hammer-shaped head (IKA Corp.).
[0322] (Step 3)
[0323] The crushed aggregate was charged into a hybridization
system (Nara Machinery Co., Ltd.) followed by subjecting to
spheroidizing treatment by circulating or retaining in a device at
a rotor rotating speed of 7000 rpm for 180 seconds to obtain
composite graphite particles (C) having Si compound particles
enclosed therein.
[0324] (Step 4)
[0325] The resulting composite graphite particles (C) containing Si
compound particles were mixed with coal-based heavy oil so as to
have a coverage factor after firing of 7.5% followed by kneading
and dispersing with a biaxial kneader. The resulting dispersion was
introduced into a firing furnace and fired for 1 hour at
1000.degree. C. in a nitrogen atmosphere. The fired block was
crushed under conditions of a rotating speed of 3000 rpm using the
aforementioned mill followed by classifying with a vibration sieve
having an opening size of 45 .mu.m to obtain composite graphite
particles (C) coated with amorphous carbon.
[0326] The mean particle diameter (d50), BET specific surface area,
Si content, Si degree of dispersion and tapped density of the
resulting composite graphite particles are described in Table 1.
Furthermore, a cross-sectional SEM image of the composite graphite
particles (C) is shown in FIG. 2.
[0327] When the cross-sectional structure was observed in the
cross-sectional SEM image, the degree of dispersion of Si in the
composite graphite particles (C) was 0.96 and Si dispersibility was
confirmed to be high.
[0328] Evaluation of Battery Characteristics
[0329] A battery evaluation (consisting of evaluations of discharge
capacity, initial charge-discharge efficiency and discharge C rate
characteristics) was carried out on the resulting composite
graphite particles (C). The results of the battery evaluation are
shown in Table 1.
[0330] (Fabrication of Battery for Performance Evaluation)
[0331] 97.5% by weight of the aforementioned composite graphite
particles (C), 1% by weight of a binder in the form of
carboxymethyl cellulose (CMC), and 1.5% by weight of a 48% by
weight aqueous dispersion of styrene-butadiene rubber (SBR) were
kneaded with a hybridizing mixer to obtain a slurry. This slurry
was coated onto rolled copper foil having a thickness of 18 .mu.m
to a basis weight of 4 mg/cm.sup.2 to 5 mg/cm.sup.2 by blade
coating followed by drying.
[0332] Subsequently, the coated copper foil was roll-pressed to a
density of the negative electrode active material layer of 1.3
g/cm.sup.3 to 1.5 g/cm.sup.3 with a 250 mm diameter roll press
equipped with a load cell followed by punching into a circular
shape having a diameter of 12.5 mm and vacuum drying for 2 hours at
110.degree. C. to obtain a negative electrode for evaluation. The
aforementioned negative electrode and Li foil serving as the
counter electrode were superimposed with a separator impregnated
with an electrolytic solution interposed there between to fabricate
a battery for charge-discharge testing. An electrolytic solution
obtained by dissolving fluoroethylene carbonate to 10% by weight
and LiPF.sub.6 to 1.2 mol/l in a mixed solvent of ethylene
carbonate and ethyl methyl carbonate (weight ratio: 3/7) was used
for the electrolytic solution.
[0333] (Discharge Capacity/Initial Charge-Discharge Efficiency)
[0334] First, the aforementioned battery was charged to 5 mV
relative to the positive electrode and negative electrode at a
current density of 0.8 mA/cm.sup.2, further charged at a constant
voltage of 5 mV until the current value reached 0.08 mA, and after
doping the negative electrode with lithium, the battery was
discharged to 1.5 V relative to the positive electrode and negative
electrode at a current density of 0.8 mA/cm.sup.2. The weight of
the negative active material was determined by subtracting the
weight of copper foil punched out to the same area as the negative
electrode from the weight of the negative electrode, and the
discharge capacity of the first cycle was divided by this weight of
the negative electrode active material to determine the initial
charge-discharge capacity per unit weight which was then used as
discharge capacity.
[0335] Next, the battery was charged and discharged under the same
conditions, and after determining the discharge capacity per unit
weight of the second cycle, initial charge-discharge efficiency was
determined using equation (4) indicated below.
Initial charge-discharge efficiency (%)={discharge capacity of
second cycle (mAh/g)/(discharge capacity of first cycle
(mAh/g)}.times.100 Equation (4)
[0336] (Discharge C Rate Characteristics)
[0337] The battery for which initial charge-discharge efficiency
was determined above was charged to 5 mV relative to the positive
electrode and negative electrode at a current density of 0.8
mA/cm.sup.2, and further charged at a constant voltage of 5 mV
until the current value reached 0.08 mA. This battery was then
discharged to 1.5 V relative to the positive electrode and negative
electrode at a current density of 0.8 mA/cm.sup.2. Next, after
charging this battery in the same manner, the battery was
discharged to a current density of 4.0 mA/cm.sup.2. Discharge C
rate was determined according to equation (5) below using each
discharge capacity.
Discharge C rate (%)=capacity during discharge at 4.0
mA/cm.sup.2/capacity during discharge at 0.8 mA/cm.sup.2.times.100
Equation (5)
Example 2
[0338] Composite particles were obtained using the same method as
Example 1 with the exception of adding 30 g of polyacrylonitrile to
inhibit dissociation of metal particles when mixing the slurry
(II), and using natural flake graphite (mean particle diameter d50:
13 .mu.m) for the graphite (A). The properties of the resulting
composite graphite particles and the results of battery evaluation
are described in Table 1.
Example 3
[0339] Composite particles were obtained using the same method as
Example 1 with the exception of using natural flake graphite (mean
particle diameter d50: 15 .mu.m) for the graphite (A). The
properties of the resulting composite graphite particles and the
results of battery evaluation are described in Table 1.
Comparative Example 1
[0340] Composite particles were obtained using the same method as
Example 1 with the exception of using a slurry obtained by
additionally drying the slurry (II) used in step 1 having graphite
uniformly dispersed therein so that solvent residue became 0.5% by
weight or less based on the charged amounts of the graphite (A) and
metal particles (B). The properties of the resulting composite
graphite particles and the results of battery evaluation are
described in Table 1.
Comparative Example 2
[0341] Composite particles were obtained using the same method as
Example 1 with the exception of changing the size of the flake
graphite used in step 1 to 45 .mu.m. The properties of the
resulting composite graphite particles and the results of battery
evaluation are described in Table 1. In addition, an SEM image of
the cross-section of the composite graphite particles is shown in
FIG. 3.
Comparative Example 3
[0342] The Si slurry (I) (solid content: 40% by weight) was dried
to obtain Si compound particles. After mixing 190 g of the Si
compound particles and 1000 g of spherical graphite particles (mean
particle diameter d50: 16 .mu.m), mixing with coal tar pitch and
firing were carried out using the same procedure as that of Example
1. The properties of the resulting composite graphite particles and
the results of battery evaluation are described in Table 1.
Comparative Example 4
[0343] Coal tar pitch was added in step 1 in an amount such that
the coverage factor after firing was 7.5 parts by weight.
Subsequently, after firing for 2 hours at 400.degree. C. in a
nitrogen atmosphere, the particles were crushed to a particle size
of 10 .mu.m. Following crushing, 200 g of the fired powder were
treated with a mechanofusion system (Hosokawa Micron Corp.) for 20
minutes at 2000 rpm. After firing at 1000.degree. C., the fired
block was crushed using the previously described mill under
conditions of a rotating speed of 3000 rpm followed by classifying
with a vibration sieve having an opening size of 45 .mu.m to obtain
composite graphite particles (C) coated with amorphous carbon. The
properties of the resulting composite graphite particles and the
results of battery evaluation are described in Table 1.
TABLE-US-00001 TABLE 1 Initial charge- Degree Dis- discharge d50 SA
TAP of dis- charge efficiency (.mu.m) (m.sup.2/g) (g/cm.sup.3)
persion C rate (%) Example 1 11 7.6 0.923 0.96 0.993 87.2 Example 2
11 9.2 0.984 0.90 0.993 85.9 Example 3 15 6.8 0.98 0.80 0.988 89.4
Comparative 10 6.2 0.875 0.75 0.982 86.8 Example 1 Comparative 20
13 1.05 0.66 0.990 86.4 Example 2 Comparative 19 8.3 1.03 0.4 0.855
85.6 Example 3 Comparative 10 6.8 0.695 0.56 0.988 86.6 Example
4
[0344] As shown in Table 1, the composite graphite particles (C) of
the present invention were confirmed to have high discharge C rate
characteristics. In addition, they were also confirmed to have high
initial charge-discharge efficiency. In particular, Examples 1 and
2 having favorable degrees of dispersion were confirmed to
demonstrate extremely high discharge C rate characteristics. In
addition, Examples 1 and 3, in which degree of dispersion and
specific surface area were within preferable ranges, were
determined to demonstrate superior initial charge-discharge
efficiency while maintaining a high C rate. This is thought to be
because, since the composite graphite particles (C) of the present
invention have Si present within the particles in a highly
dispersed state, there is little interruption of electron
conductive paths caused by local expansion, thereby resulting in
balance between discharge C rate characteristics and initial
charge-discharge efficiency at a high level.
[0345] Although a detailed explanation of the present invention has
been provided with reference to specific embodiments, it should be
clear to a person with ordinary skill in the art that various
alterations and modifications can be added without deviating from
the spirit and scope of the present invention.
INDUSTRIAL APPLICABILITY
[0346] A nonaqueous secondary battery provided with an electrode
using the composite graphite particles of the present invention has
high capacity and demonstrates improved initial charge-discharge
characteristics and charge-discharge efficiency, thereby making it
industrially useful as a resulting of being able to satisfy the
properties required of applications in cell phones, power tools and
electric automobiles in recent years.
BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS
[0347] (1) Solid line: Extracted composite graphite particle (C)
[0348] (2) Metal particle (B) [0349] (3) Center of gravity [0350]
(4) Dotted line: Long axis of extracted particle [0351] (5) Dotted
line: Short axis of extracted particle [0352] (6) Lattice having
square-shaped squares
* * * * *