U.S. patent application number 15/749972 was filed with the patent office on 2018-08-16 for hardly graphitizable carbonaceous material for nonaqueous electrolyte secondary batteries fully charged to be used, method for producing same, negative electrode material for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery fully charged to be used.
This patent application is currently assigned to KURARAY CO., LTD.. The applicant listed for this patent is KURARAY CO., LTD., KUREHA CORPORATION. Invention is credited to Jun-Sang CHO, Junji FUJIOKA, Hideharu IWASAKI, Taketoshi OKUNO.
Application Number | 20180233749 15/749972 |
Document ID | / |
Family ID | 57942939 |
Filed Date | 2018-08-16 |
United States Patent
Application |
20180233749 |
Kind Code |
A1 |
FUJIOKA; Junji ; et
al. |
August 16, 2018 |
HARDLY GRAPHITIZABLE CARBONACEOUS MATERIAL FOR NONAQUEOUS
ELECTROLYTE SECONDARY BATTERIES FULLY CHARGED TO BE USED, METHOD
FOR PRODUCING SAME, NEGATIVE ELECTRODE MATERIAL FOR NONAQUEOUS
ELECTROLYTE SECONDARY BATTERIES, AND NONAQUEOUS ELECTROLYTE
SECONDARY BATTERY FULLY CHARGED TO BE USED
Abstract
To provide a hardly graphitizable carbonaceous material used in
a negative electrode material for nonaqueous electrolyte secondary
batteries (for example, a lithium ion battery) having not only high
charge capacity but also high charge-discharge efficiency and being
fully charged to be used and a method for producing the same. To
provide a negative electrode material for nonaqueous electrolyte
secondary batteries comprising such a hardly graphitizable
carbonaceous material, and a nonaqueous electrolyte secondary
battery comprising such a negative electrode material for
nonaqueous electrolyte secondary batteries and being fully charged
to be used. A hardly graphitizable carbonaceous material, being a
hardly graphitizable carbonaceous material for nonaqueous
electrolyte secondary batteries fully charged to be used and having
an oxygen element content of 0.25% by mass or less.
Inventors: |
FUJIOKA; Junji;
(Kurashiki-shi, JP) ; CHO; Jun-Sang;
(Kurashiki-shi, JP) ; OKUNO; Taketoshi;
(Kurashiki-shi, JP) ; IWASAKI; Hideharu;
(Kurashiki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KURARAY CO., LTD.
KUREHA CORPORATION |
Kurashiki-shi
Chuo-ku |
|
JP
JP |
|
|
Assignee: |
KURARAY CO., LTD.
Kurashiki-shi
JP
KUREHA CORPORATION
Chuo-ku
JP
|
Family ID: |
57942939 |
Appl. No.: |
15/749972 |
Filed: |
July 20, 2016 |
PCT Filed: |
July 20, 2016 |
PCT NO: |
PCT/JP2016/071266 |
371 Date: |
February 2, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0525 20130101;
Y02P 70/50 20151101; H01M 10/058 20130101; Y02E 60/10 20130101;
H01M 4/587 20130101; Y02T 10/70 20130101; H01M 2004/021 20130101;
H01M 10/052 20130101; H01M 4/133 20130101; H01M 4/525 20130101 |
International
Class: |
H01M 4/587 20060101
H01M004/587; H01M 10/0525 20060101 H01M010/0525; H01M 10/058
20060101 H01M010/058; H01M 4/133 20060101 H01M004/133; H01M 4/525
20060101 H01M004/525 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2015 |
JP |
2015-155168 |
Claims
1: A hardly graphitizable carbonaceous material, having an oxygen
element content of 0.25% by mass or less.
2: The hardly graphitizable carbonaceous material according to
claim 1, wherein, when taken out of a nonaqueous electrolyte
secondary battery in a fully charged state, a main resonance peak
position of a chemical shift value observed by .sup.7Li
nuclear-solid state NMR analysis is downfield by more than 115 ppm
from a peak position of lithium chloride.
3: The hardly graphitizable carbonaceous material according to
claim 1, which is derived from a carbon precursor originating from
a plant.
4: The hardly graphitizable carbonaceous material according to
claim 1, further having an average face-to-face dimension d.sub.002
of the (002) face calculated from the Bragg equation by a wide
angle X-ray diffraction method of 0.36 to 0.42 nm.
5: The hardly graphitizable carbonaceous material according to
claim 1, further having a specific surface area determined by a
nitrogen adsorption BET three-point method of 1 to 20
m.sup.2/g.
6: The hardly graphitizable carbonaceous material according to
claim 1, further having a true density determined by a butanol
method of 1.40 to 1.70 g/cm.sup.3.
7: The hardly graphitizable carbonaceous material according to
claim 1, further having a potassium element content of 0.1% by mass
or less and an iron element content of 0.02% by mass or less.
8: A method for producing the hardly graphitizable carbonaceous
material according to claim 1, comprising: subjecting a carbon
precursor to an acid treatment; and calcining an acid-treated
carbon precursor under an inert gas atmosphere at a temperature of
1100.degree. C. to 1400.degree. C.
9: A negative electrode material suitable for nonaqueous
electrolyte secondary batteries, comprising the hardly
graphitizable carbonaceous material according to claim 1.
10: A nonaqueous electrolyte secondary battery, which comprises a
negative electrode material comprising a hardly graphitizable
carbonaceous material and which is fully charged to be used,
wherein the hardly graphitizable carbonaceous material has an
oxygen element content of 0.25% by mass or less.
11: The nonaqueous electrolyte secondary battery according to claim
10, wherein a main resonance peak position of a chemical shift
value of the hardly graphitizable carbonaceous material observed by
.sup.7Li nuclear-solid state NMR analysis is downfield by more than
115 ppm from a peak position of lithium chloride.
12: The nonaqueous electrolyte secondary battery according to claim
10, wherein the hardly graphitizable carbonaceous material is
derived from a carbon precursor originating from a plant.
13: The nonaqueous electrolyte secondary battery according to claim
10, wherein the hardly graphitizable carbonaceous material has an
average face-to-face dimension d.sub.002 of the (002) face
calculated from the Bragg equation by a wide angle X-ray
diffraction method of 0.36 to 0.42 nm.
14: The nonaqueous electrolyte secondary battery according to claim
10, wherein the hardly graphitizable carbonaceous material has a
specific surface area determined by a nitrogen adsorption BET
three-point method of 1 to 20 m.sup.2/g.
15: The nonaqueous electrolyte secondary battery according to claim
10, wherein the hardly graphitizable carbonaceous material has a
true density determined by a butanol method of 1.40 to 1.70
g/cm.sup.3.
16: The nonaqueous electrolyte secondary battery according to claim
10, wherein the hardly graphitizable carbonaceous material has a
potassium element content of 0.1% by mass or less and an iron
element content of 0.02% by mass or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hardly graphitizable
carbonaceous material suitable as a negative electrode material for
nonaqueous electrolyte secondary batteries (for example, a lithium
ion secondary battery) fully charged to be used and a method for
producing the same, a negative electrode material for nonaqueous
electrolyte secondary batteries, and a nonaqueous electrolyte
secondary battery fully charged to be used.
BACKGROUND ART
[0002] The lithium ion secondary battery has hitherto been widely
used for small mobile equipment such as a mobile phone and a
notebook personal computer. A hardly graphitizable carbonaceous
material has been developed as a negative electrode material for
lithium ion secondary batteries (Patent Document 1) and has also
been used therefor because the hardly graphitizable carbonaceous
material is capable of doping (charging) and dedoping (discharging)
of lithium in an amount more than 372 mAh/g being the theoretical
capacity of graphite and is also excellent in input-output
characteristics, cycle durability, and low-temperature
properties.
[0003] The hardly graphitizable carbonaceous material can be
obtained from carbon sources such as petroleum pitch, coal pitch,
phenol resins, and plants. Of these carbon sources, plants have
been attracting attention because plants are raw materials that can
be cultivated to be sustainedly stably supplied and are available
inexpensively. Moreover, satisfactory charge-discharge capacity is
expected because there are many fine pores in a carbonaceous
material obtained by calcining a carbon raw material originating
from plants (for example, Patent Document 1 and Patent Document
2).
[0004] When a lithium ion secondary battery goes through a fully
charged state into an overcharged state during charging, many
lithium ions are generated by a reaction at a positive electrode, a
negative electrode becomes difficult to hold lithium ions, metallic
lithium is precipitated on a surface of the negative electrode, and
thermal stability of the lithium ion secondary battery is lowered
as well as a decomposition reaction of a solvent is caused and the
temperature of the lithium ion secondary battery is elevated.
Accordingly, usually, for example, a current breaking device is
adopted or a small amount of an aromatic compound as an additive is
added to the electrolytic solution (for example, Patent Document 3
and Patent Document 4) to secure the safety against overcharging.
On the other hand, in order to more surely secure the safety,
charging of a lithium ion secondary battery is usually performed by
a method (constant-current constant-voltage method) in which
constant-current charging is performed (for example, at 0.5
mA/cm.sup.2) until the electrical potential of a negative electrode
terminal based on that of metallic lithium becomes a predetermined
electrical potential of 0 mV or more, constant-voltage charging is
performed after the electrical potential of the negative electrode
terminal reached the predetermined electrical potential, and
charging is completed when the current value is kept at a constant
value (for example, at 20 .mu.A) during a predetermined period of
time. In this case, practically, the lithium ion secondary battery
is not charged to be in a fully charged state even when there is
still space to hold lithium ions in the negative electrode.
[0005] On the other hand, in recent years, there is a growing
concern for the environment, the development of a lithium ion
secondary battery in the field of on-vehicle batteries has been
advanced, and such a battery has been put into practical use.
Therefore, a hardly graphitizable carbonaceous material with which
a lithium ion battery having not only higher charge capacity but
also higher charge-discharge efficiency can be produced has been
desired.
PRIOR ART DOCUMENTS
Patent Documents
[0006] Patent Document 1: JP-A-H9-161801
[0007] Patent Document 2: JP-A-H10-21919
[0008] Patent Document 3: JP-A-2015-88354
[0009] Patent Document 4: JP-A-2001-15155
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0010] One object of the present invention is to provide a hardly
graphitizable carbonaceous material used in a negative electrode
material for nonaqueous electrolyte secondary batteries (for
example, a lithium ion battery) having not only high charge
capacity but also high charge-discharge efficiency and being fully
charged to be used and a method for producing the same. Another
object of the present invention is to provide a negative electrode
material for nonaqueous electrolyte secondary batteries comprising
such a hardly graphitizable carbonaceous material, and a nonaqueous
electrolyte secondary battery comprising such a negative electrode
material for nonaqueous electrolyte secondary batteries and being
fully charged to be used.
Solutions to the Problems
[0011] As a result of extensive researches, the present inventors
have found out that the above-mentioned problems can be solved by
using a hardly graphitizable carbonaceous material having an oxygen
element content within a specific range in a negative electrode
material for nonaqueous electrolyte secondary batteries fully
charged to be used, and thus, the present invention has been
completed.
[0012] That is, the present invention includes the following
preferred embodiments.
[0013] [1] A hardly graphitizable carbonaceous material being a
hardly graphitizable carbonaceous material for nonaqueous
electrolyte secondary batteries fully charged to be used and having
an oxygen element content of 0.25% by mass or less.
[0014] [2] The hardly graphitizable carbonaceous material according
to [1] mentioned above, wherein, when being taken out of a
nonaqueous electrolyte secondary battery in a fully charged state,
a main resonance peak position of a chemical shift value observed
by .sup.7Li nuclear-solid state NMR analysis is downfield by more
than 115 ppm from a peak position of lithium chloride.
[0015] [3] The hardly graphitizable carbonaceous material according
to [1] or [2] mentioned above, being derived from a carbon
precursor originating from plants.
[0016] [4] The hardly graphitizable carbonaceous material according
to any one of [1] to [3] mentioned above, further having an average
face-to-face dimension d.sub.002 of the (002) face calculated from
the Bragg equation by a wide angle X-ray diffraction method of 0.36
to 0.42 nm.
[0017] [5] The hardly graphitizable carbonaceous material according
to any one of [1] to [4] mentioned above, further having a specific
surface area determined by a nitrogen adsorption BET three-point
method of 1 to 20 m.sup.2/g.
[0018] [6] The hardly graphitizable carbonaceous material according
to any one of [1] to [5] mentioned above, further having a true
density determined by a butanol method of 1.40 to 1.70
g/cm.sup.3.
[0019] [7] The hardly graphitizable carbonaceous material according
to any one of [1] to [6] mentioned above, further having a
potassium element content of 0.1% by mass or less and an iron
element content of 0.02% by mass or less.
[0020] [8] A method for producing the hardly graphitizable
carbonaceous material according to any one of [1] to [7] mentioned
above, comprising a step of subjecting a carbon precursor to an
acid treatment and a step of calcining an acid-treated carbon
precursor under an inert gas atmosphere at 1100.degree. C. to
1400.degree. C.
[0021] [9] A negative electrode material for nonaqueous electrolyte
secondary batteries, comprising the hardly graphitizable
carbonaceous material according to any one of [1] to [7] mentioned
above.
[0022] [10] A nonaqueous electrolyte secondary battery, which
comprises a negative electrode material for nonaqueous electrolyte
secondary batteries comprising a hardly graphitizable carbonaceous
material and which is fully charged to be used, wherein the hardly
graphitizable carbonaceous material has an oxygen element content
of 0.25% by mass or less.
[0023] [11] The nonaqueous electrolyte secondary battery according
to [10] mentioned above, wherein a main resonance peak position of
a chemical shift value of the hardly graphitizable carbonaceous
material observed by .sup.7Li nuclear-solid state NMR analysis is
downfield by more than 115 ppm from a peak position of lithium
chloride.
[0024] [12] The nonaqueous electrolyte secondary battery according
to [10] or [11] mentioned above, wherein the hardly graphitizable
carbonaceous material is derived from a carbon precursor
originating from plants.
[0025] [13] The nonaqueous electrolyte secondary battery according
to any one of [10] to [12] mentioned above, wherein the hardly
graphitizable carbonaceous material has an average face-to-face
dimension d.sub.002 of the (002) face calculated from the Bragg
equation by a wide angle X-ray diffraction method of 0.36 to 0.42
nm.
[0026] [14] The nonaqueous electrolyte secondary battery according
to any one of [10] to [13] mentioned above, wherein the hardly
graphitizable carbonaceous material has a specific surface area
determined by a nitrogen adsorption BET three-point method of 1 to
20 m.sup.2/g.
[0027] [15] The nonaqueous electrolyte secondary battery according
to any one of [10] to [14] mentioned above, wherein the hardly
graphitizable carbonaceous material has a true density determined
by a butanol method of 1.40 to 1.70 g/cm.sup.3.
[0028] [16] The nonaqueous electrolyte secondary battery according
to any one of [10] to [15] mentioned above, wherein the hardly
graphitizable carbonaceous material has a potassium element content
of 0.1% by mass or less and an iron element content of 0.02% by
mass or less.
Effects of the Invention
[0029] When the hardly graphitizable carbonaceous material
according to the present invention is used to produce a nonaqueous
electrolyte secondary battery fully charged to be used, such a
nonaqueous electrolyte secondary battery has not only extremely
high charge capacity but also extremely high charge-discharge
efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 shows an .sup.7Li nuclear-solid state NMR spectrum of
the hardly graphitizable carbonaceous material prepared in Example
1.
[0031] FIG. 2 shows an .sup.7Li nuclear-solid state NMR spectrum of
the carbonaceous material prepared in Comparative Example 2.
EMBODIMENTS OF THE INVENTION
<Hardly Graphitizable Carbonaceous Material>
[0032] The hardly graphitizable carbonaceous material according to
the present invention is a hardly graphitizable carbonaceous
material for nonaqueous electrolyte secondary batteries fully
charged to be used and has an oxygen element content of 0.25% by
mass or less.
[0033] In the present specification, "a nonaqueous electrolyte
secondary battery being fully charged to be used" refers to a
nonaqueous electrolyte secondary battery being assembled with the
use of a negative electrode comprising a hardly graphitizable
carbonaceous material and a positive electrode comprising lithium,
and allowing the negative electrode to be charged (doped) with
lithium until just before precipitation of metallic lithium is
confirmed by .sup.7Li nuclear-solid state NMR analysis; and usually
means a nonaqueous electrolyte secondary battery being charged at a
constant current value so as to have a charge capacity within the
range of 580 to 700 mAh/g per unit mass of the negative electrode
active material. Accordingly, this has a meaning completely
different from completing the charging by a conventional
constant-current constant-voltage method described above to use a
conventional nonaqueous electrolyte secondary battery. In this
connection, from the viewpoint of keeping the battery capacity and
charge-discharge efficiency high, the charge capacity of a negative
electrode at the time when the negative electrode is charged with
lithium until just before precipitation of metallic lithium is
confirmed by .sup.7Li nuclear-solid state NMR analysis is
preferably set to 85 to 98%, further preferably set to 88 to 95%,
and especially preferably set to 90 to 92% relative to a charge
capacity at the time when lithium precipitation is confirmed.
<Oxygen Element Content>
[0034] It is good to make the oxygen element content of the hardly
graphitizable carbonaceous material according to the present
invention smaller. An analysis value thereof obtained by elemental
analysis is usually 0.25% by mass or less and preferably 0.24% by
mass or less. It is further preferred that the hardly graphitizable
carbonaceous material contains substantially no oxygen element. In
this context, containing substantially no oxygen element means
having an oxygen element content equal to or less than 10.sup.-6%
by mass which is a detection limit of the elemental analysis method
(inert gas fusion-thermal conductivity method) described below.
When the oxygen element content is the above-mentioned value or
less, the lowering in utilization efficiency of lithium ions, which
is caused because lithium ions are consumed by a reaction of the
lithium ion with oxygen, and the lowering in utilization efficiency
of lithium ions, which is caused because moisture in the air is
induced by oxygen and water is adsorbed by the hardly graphitizable
carbonaceous material and hardly desorbed therefrom, can be
suppressed.
[0035] A method of adjusting the oxygen element content to the
above-mentioned value or less is not limited at all. For example,
by subjecting a carbon precursor originating from plants to an acid
treatment at a predetermined temperature, then, mixing an
acid-treated carbon precursor with a volatile organic substance,
and calcining the mixture under an inert gas atmosphere at a
temperature of 1100.degree. C. to 1400.degree. C., the oxygen
element content can be adjusted to the above-mentioned value or
less. The details of the measurement of the oxygen element content
are as described in EXAMPLES.
<Main Resonance Peak Position of Chemical Shift Value>
[0036] When the hardly graphitizable carbonaceous material
according to the present invention is taken out of a nonaqueous
electrolyte secondary battery charged (doped) with lithium until a
fully charged state thereof is attained, a main resonance peak
position of a chemical shift value observed by subjecting the
hardly graphitizable carbonaceous material to .sup.7Li
nuclear-solid state NMR analysis is preferably downfield by more
than 115 ppm, more preferably downfield by more than 118 ppm, and
especially preferably downfield by more than 120 ppm, from a peak
position of lithium chloride. The details of the .sup.7Li
nuclear-solid state NMR analysis are as described in EXAMPLES.
[0037] A battery prepared with a hardly graphitizable carbonaceous
material in which a main resonance peak position of a chemical
shift value is downfield by more than 115 ppm from a peak position
of lithium chloride, that is, the above-mentioned main resonance
peak position is observed at the lower magnetic field side by more
than 115 ppm, means a battery prepared with the hardly
graphitizable carbonaceous material having a large storage amount
of clustered lithium atoms, that is, a battery having a high charge
capacity. The clustered lithium atoms occluded in a hardly
graphitizable carbonaceous material are reversible lithium atoms
that can be discharged (dedoped) therefrom, and a battery prepared
with the hardly graphitizable carbonaceous material having a large
storage amount of clustered lithium atoms means a battery having
high charge-discharge efficiency calculated from "the discharge
capacity/the charge capacity".
[0038] Having high charge-discharge efficiency means having little
loss of lithium in the negative electrode caused by a side reaction
during the charge-discharge and the like. When a battery has little
loss of lithium in the negative electrode, it becomes unnecessary
to complement lithium for the negative electrode by using an excess
amount of materials for the positive electrode, and the battery
becomes advantageous from an aspect of capacity per volume of the
battery or cost of the battery.
[0039] Even by the use of a carbonaceous material having a main
resonance peak position of a chemical shift value which is
downfield by less than 115 ppm from a peak position of lithium
chloride, unlike the hardly graphitizable carbonaceous material
according to the present invention, it is quite difficult to
achieve both extremely high charge capacity and extremely high
charge-discharge efficiency because the storage amount of clustered
lithium atoms thereof is small.
<Carbon Precursor>
[0040] For example, as described in JP-A-H9-161801 (the Patent
Document 1) and JP-A-H10-21919 (the Patent Document 2), the hardly
graphitizable carbonaceous material according to the present
invention is derived from a phenol resin, a furan resin, pitch,
tar, a carbon precursor originating from plants, or the like.
[0041] The hardly graphitizable carbonaceous material according to
the present invention is preferably derived from a carbon precursor
originating from plants. In the present invention, "a carbon
precursor originating from plants" means a substance before
carbonization originating from plants or a substance after
carbonization originating from plants (char derived from plants). A
plant as a raw material (hereinafter, sometimes referred to as "a
plant raw material") is not particularly limited. For example,
coconut shell, coffee beans, tea leaves, sugarcane, fruits (for
example, mandarine oranges and bananas), straws, rice husks, a
broad-leaved tree, a needle-leaved tree and bamboo can be
exemplified. These exemplified plants include wastes after provided
for its original purpose (for example, used tea leaves) and a
portion of the plant raw material (for example, banana peels and
mandarine orange peels). These plants can be used singly or in
combination of two or more thereof. Of these plants, coconut shell,
which is easily available abundantly, is preferred.
[0042] The coconut shell is not particularly limited. Examples
thereof can include coconut shells of palm coconut (oil palm), coco
palm, Salak, and sea coconut. These coconut shells can be used
singly or in combination. Coconut shells of coco palm and palm
coconut, which are utilized as foodstuffs, raw material of a
detergent, raw material of a biodiesel fuel oil, and the like and
are biomass wastes generated in large quantities, are especially
preferred.
[0043] A method of carbonizing the plant raw material, that is, a
method of producing the char derived from plants, is not
particularly limited. The method can be performed, for example, by
subjecting the plant raw material to a heat treatment under an
inert gas atmosphere at 300.degree. C. or more (hereinafter,
sometimes referred to as "temporary calcination").
[0044] Moreover, the plant raw material in the form of char (for
example, coconut shell char) is also available.
<Average Face-to-Face Dimension d.sub.002>
[0045] The average face-to-face dimension d.sub.002 of the (002)
face calculated from the Bragg equation by a wide angle X-ray
diffraction method of the hardly graphitizable carbonaceous
material according to the present invention preferably falls within
the range of 0.36 nm to 0.42 nm, more preferably falls within the
range of 0.38 nm to 0.40 nm, and especially preferably falls within
the range of 0.381 nm to 0.389 nm. When the average face-to-face
dimension d.sub.002 of the (002) face falls within the
above-mentioned range, the lowering in input-output characteristics
of a lithium ion battery caused by an electrical resistance made
large at the time when lithium ions are inserted into the
carbonaceous material or an electrical resistance made large at the
time of output can be suppressed. Moreover, the lowering in
stability of a battery material due to repeated expansion and
shrinkage of the hardly graphitizable carbonaceous material can be
suppressed. Furthermore, the lowering in effective capacity per
volume caused by a volume of the hardly graphitizable carbonaceous
material made large while a diffusion resistance of the lithium ion
is made small can be avoided. In order to adjust the average
face-to-face dimension within the above-mentioned range, for
example, a carbon precursor giving a hardly graphitizable
carbonaceous material may be calcined at a calcination temperature
within the range of 1100 to 1400.degree. C. Moreover, a method in
which a carbon precursor is mixed with a thermally-decomposable
resin such as polystyrene to be calcined can also be adopted. In
this context, the details of the measurement of the average
face-to-face dimension d.sub.002 are as described in EXAMPLES.
<Specific Surface Area>
[0046] The specific surface area determined by a nitrogen
adsorption BET three-point method of the hardly graphitizable
carbonaceous material according to the present invention preferably
falls within the range of 1 to 20 m.sup.2/g, more preferably falls
within the range of 1.2 to 10 m.sup.2/g, and especially preferably
falls within the range of 1.4 to 9.5 m.sup.2/g. When the specific
surface area falls within the above-mentioned range, the number of
micropores in the hardly graphitizable carbonaceous material can be
sufficiently reduced by a calcination step described below, the
moisture-absorption characteristics of the hardly graphitizable
carbonaceous material can be sufficiently lowered, and in a
nonaqueous electrolyte secondary battery produced with the use of
the hardly graphitizable carbonaceous material, a lowering in
utilization efficiency of lithium ions can be suppressed. The
specific surface area can be adjusted by controlling the
temperature in a demineralization step described below. In this
context, the details of the measurement of the specific surface
area by a nitrogen adsorption BET three-point method are as
described in EXAMPLES.
<True Density .rho..sub.Bt>
[0047] From the viewpoint of making the capacity per mass of the
battery high, the true density by a butanol method of the hardly
graphitizable carbonaceous material according to the present
invention preferably falls within the range of 1.40 to 1.70
g/cm.sup.3, more preferably falls within the range of 1.42 to 1.65
g/cm.sup.3, and especially preferably falls within the range of
1.44 to 1.60 g/cm.sup.3. The true density within the
above-mentioned range can be attained, for example, by setting the
calcination step temperature at the time of producing a hardly
graphitizable carbonaceous material from the plant raw material to
1100 to 1400.degree. C. In this context, the details of the
measurement of the true density .rho..sub.Bt are as described in
EXAMPLES.
<Potassium Element Content and Iron Element Content>
[0048] From the viewpoint of making the dedoping capacity large and
the viewpoint of making the nondedoping capacity small, the
potassium element content of the hardly graphitizable carbonaceous
material according to the present invention is preferably 0.1% by
mass or less, more preferably 0.05% by mass or less, and further
preferably 0.03% by mass or less. It is especially preferred that
the hardly graphitizable carbonaceous material contains
substantially no potassium element. Moreover, from the viewpoint of
making the dedoping capacity large and the viewpoint of making the
nondedoping capacity small, the iron element content of the hardly
graphitizable carbonaceous material according to the present
invention is preferably 0.02% by mass or less, more preferably
0.015% by mass or less, and further preferably 0.01% by mass or
less. It is especially preferred that the hardly graphitizable
carbonaceous material contains substantially no iron element. In
this context, containing substantially no potassium element or
substantially no iron element means having a potassium element
content or an iron element content equal to or less than the
detection limit value in the X-ray fluorescence analysis (for
example, analysis using the "LAB CENTER XRF-1700" available from
SHIMADZU CORPORATION) described below. When the potassium element
content and the iron element content are equal to or less than the
above-mentioned values, respectively, in a nonaqueous electrolyte
secondary battery prepared with the hardly graphitizable
carbonaceous material, a sufficient dedoping capacity and a
satisfactory nondedoping capacity can be attained. Furthermore, a
safety problem of the nonaqueous electrolyte secondary battery due
to a short circuit at the time when these metallic elements are
eluted into the electrolyte to be precipitated again can be
avoided. The details of the measurement of the potassium element
content and iron element content are as described in EXAMPLES.
<Moisture Content>
[0049] The moisture content of the hardly graphitizable
carbonaceous material according to the present invention is
preferably 10000 ppm or less, more preferably 9000 ppm or less, and
especially preferably 8000 ppm or less. The smaller the moisture
content, the amount of water that adsorbs to the hardly
graphitizable carbonaceous material is reduced and the number of
lithium ions that adsorb to the hardly graphitizable carbonaceous
material is increased, which is preferable. Moreover, the smaller
the moisture content, self-discharge caused by a reaction of
lithium ions with adsorbed water can be reduced, which is
preferable. The moisture content of the hardly graphitizable
carbonaceous material can be reduced, for example, by reducing the
number of oxygen atoms contained in a hardly graphitizable
carbonaceous material. The moisture content of the hardly
graphitizable carbonaceous material can be measured, for example,
by the use of a Karl Fischer moisture meter or the like. The
details of the measurement of the moisture content are as described
in EXAMPLES.
<Production Method of Hardly Graphitizable Carbonaceous
Material>
[0050] A method for producing the hardly graphitizable carbonaceous
material according to the present invention comprises a step of
subjecting a carbon precursor (for example, a carbon precursor
originating from plants) to an acid treatment and a step of
calcining an acid-treated carbon precursor under an inert gas
atmosphere at 1100.degree. C. to 1400.degree. C.
<Carbon Precursor>
[0051] As described above, "a carbon precursor" refers to a phenol
resin, a furan resin, pitch, tar, a carbon precursor originating
from plants, or the like. In the present invention, it is preferred
that the carbon precursor is a carbon precursor originating from
plants.
[0052] As described above, "a carbon precursor originating from
plants" means a substance before carbonization originating from
plants or a substance after carbonization originating from plants
(char derived from plants). A plant as a raw material (a plant raw
material) is not particularly limited. Such plants exemplified
above can be used singly or in combination of two or more thereof.
Of these, coconut shell, which is easily available abundantly, is
preferred.
[0053] The coconut shell is not particularly limited. Such coconut
shells exemplified above can be used singly or in combination.
Coconut shells of coco palm and palm coconut, which are utilized as
foodstuffs, raw material of a detergent, raw material of a
biodiesel fuel oil, and the like and are biomass wastes generated
in large quantities, are especially preferred.
[0054] A method of carbonizing the plant raw material, that is, a
method of producing the char derived from plants, is not
particularly limited. For example, the method can be performed by
subjecting the plant raw material to a heat treatment under an
inert gas atmosphere at 300.degree. C. or more (hereinafter,
sometimes referred to as "temporary calcination").
[0055] Moreover, the plant raw material in the form of char (for
example, coconut shell char) is also available.
[0056] In general, the plant raw material contains alkali metal
elements (for example, potassium, sodium), alkaline earth metal
elements (for example, magnesium, calcium), transition metal
elements (for example, iron, copper), non-metallic elements (for
example, phosphorus), and the like in large amounts. When a hardly
graphitizable carbonaceous material containing such metallic
elements and non-metallic elements in large amounts is used as a
negative electrode material, these elements sometimes can give an
undesirable influence on electrochemical characteristics or safety
of the nonaqueous electrolyte secondary battery.
<Acid Treatment>
[0057] Accordingly, a method for producing the hardly graphitizable
carbonaceous material according to the present invention comprises
a step of subjecting a carbon precursor (for example, a carbon
precursor originating from plants) to an acid treatment. In this
context, hereinafter, subjecting a carbon precursor (for example, a
carbon precursor originating from plants) to an acid treatment to
lower the content of a metallic element and/or a non-metallic
element in the carbon precursor also refers to demineralizing a
carbon precursor.
[0058] A method for the acid treatment, that is, a method for the
demineralization, is not particularly limited. For example, a
method of extracting a metallic component in a carbon precursor
with the use of acidic water containing mineral acids such as
hydrochloric acid and sulfuric acid, organic acids such as acetic
acid and formic acid, and the like to demineralize the carbon
precursor (liquid phase demineralization), a method of exposing a
carbon precursor to a high-temperature vapor phase containing a
halogen compound such as hydrogen chloride to demineralize the
carbon precursor (vapor phase demineralization), and the like can
be adopted.
<Liquid Phase Demineralization>
[0059] In the liquid phase demineralization, it is preferred that a
carbon precursor (for example, a carbon precursor originating from
plants) is immersed in an aqueous organic acid solution to elute
alkali metal elements, alkaline earth metal elements, and/or
non-metallic elements to be removed from the carbon precursor into
the aqueous organic acid solution.
[0060] When a carbon precursor (for example, a carbon precursor
originating from plants) containing these metallic elements is
carbonized, a necessary carbonaceous component is sometimes
decomposed at the time of carbonization. Moreover, a carbon
precursor containing a non-metallic element is not preferred
because the non-metallic element such as phosphorus is liable to be
oxidized to make the degree of oxidation on the surface of a
carbonized product vary and to make characteristics of the
carbonized product significantly vary. Furthermore, when a carbon
precursor is carbonized and then subjected to a liquid phase
demineralization treatment, phosphorus, calcium, and magnesium
sometimes fail to be sufficiently removed. Moreover, the required
time for the liquid phase demineralization and the remaining amount
of a metallic element and/or a non-metallic element in a carbonized
product after liquid phase demineralization greatly vary depending
on the content of a metallic element and/or a non-metallic element
in a carbonized product before liquid phase demineralization.
Accordingly, it is preferred that a metallic element and/or a
non-metallic element in a carbon precursor is sufficiently removed
before carbonization to lower the content thereof. That is, it is
preferred that, in liquid phase demineralization, a substance (for
example, a substance originating from plants) before carbonization
is used as "a carbon precursor (for example, a carbon precursor
originating from plants)".
[0061] It is preferred that the organic acid used in liquid phase
demineralization does not contain any element acting as an impurity
source such as phosphorus, sulfur, and a halogen. In the case where
the organic acid does not contain any element such as phosphorus,
sulfur, and a halogen, such an organic acid is advantageous because
a carbonized product that can be suitably used as the carbon
material is obtained even when a water washing process after liquid
phase demineralization is omitted and a carbon precursor allowing
an organic acid to remain therein is carbonized. Moreover, such an
organic acid is advantageous because a waste liquid treatment for a
waste liquid of the organic acid after use can be relatively easily
performed without using a special apparatus.
[0062] Examples of the organic acid include saturated carboxylic
acids such as formic acid, acetic acid, propionic acid, oxalic
acid, tartaric acid, and citric acid, unsaturated carboxylic acids
such as acrylic acid, methacrylic acid, maleic acid, and fumaric
acid, and aromatic carboxylic acids such as benzoic acid, phthalic
acid, and naphthoic acid. From the viewpoints of availability,
corrosion due to the degree of acidity, and influence on human
bodies, acetic acid, oxalic acid, and citric acid are
preferred.
[0063] In the present invention, from the viewpoints of solubility
of a metallic compound to be eluted, waste disposal treatment,
environmental suitability, and the like, an organic acid is mixed
with an aqueous medium to be used as an aqueous organic acid
solution. Examples of the aqueous medium include water, a mixture
of water and a water-soluble organic solvent, and the like.
Examples of the water-soluble organic solvent include alcohols such
as methanol, ethanol, propylene glycol, and ethylene glycol.
[0064] The concentration of an acid in the aqueous organic acid
solution is not particularly limited. The acid concentration of an
aqueous organic acid solution can be adjusted depending on the kind
of the acid used to prepare the aqueous organic acid solution. In
the present invention, an aqueous organic acid solution with an
acid concentration falling within the range of usually 0.001% by
mass to 20% by mass, more preferably 0.01% by mass to 18% by mass,
and especially preferably 0.02% by mass to 15% by mass, on the
basis of the whole amount of the aqueous organic acid solution is
usually used. It is possible to perform a liquid phase
demineralization treatment within a practical time period as long
as the acid concentration falls within the above-mentioned range
because an appropriate elution rate of a metallic element and/or a
non-metallic element can be attained. Moreover, since the residual
amount of the acid in a carbon precursor becomes small, the
influence on a product in a subsequent process also becomes
small.
[0065] The pH of an aqueous organic acid solution is preferably 3.5
or less and more preferably 3 or less. In the case where the pH of
an aqueous organic acid solution is not more than the
above-mentioned value, the dissolution rate of a metallic element
and/or a non-metallic element eluted into the aqueous organic acid
solution is not lowered and the removal of a metallic element
and/or a non-metallic element can be effectively performed.
[0066] The liquid temperature of an aqueous organic acid solution
at the time of immersing a carbon precursor therein is not
particularly limited. The liquid temperature thereof falls within
the range of preferably 45.degree. C. to 120.degree. C., more
preferably 50.degree. C. to 110.degree. C., and especially
preferably 60.degree. C. to 100.degree. C. Such an aqueous organic
acid solution is preferred because decomposition of the acid used
is suppressed and an elution rate of a metallic element enabling a
liquid phase demineralization treatment within a practical time
period to be performed is attained as long as the liquid
temperature of the aqueous organic acid solution at the time of
immersing a carbon precursor therein falls within the
above-mentioned range. Moreover, such an aqueous organic acid
solution is preferred because a liquid phase demineralization
treatment can be performed without using a special apparatus.
[0067] The time period during which a carbon precursor is immersed
in an aqueous organic acid solution can be appropriately adjusted
depending on the acid used. In the present invention, from the
viewpoints of economy and demineralization efficiency, the
immersion time falls within the range of usually 1 to 100 hours,
preferably 2 to 80 hours, and more preferably 2.5 to 50 hours.
[0068] The proportion of the mass of a carbon precursor to be
immersed in an aqueous organic acid solution to the mass of the
aqueous organic acid solution can be appropriately adjusted
depending on the kind of the aqueous organic acid solution used,
the concentration, the temperature, and the like and falls within
the range of usually 0.1% by mass to 200% by mass, preferably 1% by
mass to 150% by mass, and more preferably 1.5% by mass to 120% by
mass. Such a proportion is preferred because a metallic element
and/or a non-metallic element eluted into an aqueous organic acid
solution hardly precipitate from the aqueous organic acid solution
and reattachment thereof to a carbon precursor is suppressed as
long as the proportion falls within the above-mentioned range.
Moreover, such a proportion is preferred in the point of an
economic aspect because the volume efficiency becomes appropriate
as long as the proportion falls within the above-mentioned
range.
[0069] The atmosphere under which a liquid phase demineralization
treatment is performed is not particularly limited and may vary
depending on the method used for the immersion. In the present
invention, a liquid phase demineralization treatment is usually
performed under an air atmosphere.
[0070] A series of these operations can be repeated preferably one
time to five times and more preferably two times to four times to
perform the liquid phase demineralization.
[0071] In the present invention, after liquid phase
demineralization, a washing step and/or a drying step can be
performed as necessary.
<Vapor Phase Demineralization>
[0072] In the vapor phase demineralization, it is preferred that a
carbon precursor (for example, a carbon precursor originating from
plants) is subjected to a heat treatment in a vapor phase
containing a halogen compound. When the vapor phase
demineralization is accompanied by a sudden thermal decomposition
reaction of a carbon precursor at the time of the heat treatment,
sometimes, the vapor phase demineralization efficiency can be
lowered due to generation of thermally decomposed components, the
inside of a heat treatment apparatus can be contaminated by the
thermally decomposed components generated, and the thermally
decomposed components can disturb safe operation. From these
viewpoints, it is preferred that a substance (for example, a
substance originating from plants) after carbonization is used as
"a carbon precursor (for example, a carbon precursor originating
from plants)".
[0073] The halogen compound used in a vapor phase demineralization
treatment is not particularly restricted. For example, fluorine,
chlorine, bromine, iodine, hydrogen fluoride, hydrogen chloride,
hydrogen bromide, iodine bromide, chlorine fluoride (ClF), iodine
chloride (ICl), iodine bromide (IBr), bromine chloride (BrCl), and
a mixture thereof can be used. Compounds that generate these
halogen compounds by thermal decomposition or a mixture thereof can
also be used. From the viewpoints of supply stability and stability
of the halogen compound used, it is preferred that hydrogen
chloride is used.
[0074] In a vapor phase demineralization treatment, a halogen
compound and an inert gas may be mixed to be used. The inert gas is
not particularly restricted as long as the inert gas does not react
with a carbon component constituting a carbon precursor (for
example, a carbon precursor originating from plants). For example,
nitrogen, helium, argon, krypton, or a mixed gas thereof can be
used. From the viewpoints of supply stability and economy, it is
preferred that nitrogen is used.
[0075] In a vapor phase demineralization treatment, the mixing
ratio between a halogen compound and an inert gas is not
particularly limited as long as sufficient demineralization can be
attained. For example, from the viewpoints of safety, economy, and
residual properties on the carbon component, the volume ratio of a
halogen compound to an inert gas preferably falls within the range
of 0.01 to 10.0% by volume, more preferably falls within the range
of 0.05 to 8.0% by volume, and especially preferably falls within
the range of 0.1 to 5.0% by volume.
[0076] Although the treatment temperature for a vapor phase
demineralization treatment may vary depending on the kind of a
carbon precursor (for example, a carbon precursor originating from
plants) to be demineralized, from the viewpoint of attaining a
desired oxygen element content and specific surface area, the vapor
phase demineralization treatment can be performed at for example
500 to 950.degree. C., preferably 600 to 940.degree. C., more
preferably 650 to 940.degree. C., and especially preferably 850 to
930.degree. C. When the demineralization temperature falls within
the above-mentioned range, satisfactory demineralization efficiency
can be attained to sufficiently demineralize the carbon precursor
and activation by a halogen compound can be avoided.
[0077] The time period for the vapor phase demineralization is not
particularly restricted. From the viewpoints of economic efficiency
in reaction facilities and structure-preserving properties of a
carbonaceous component, the time period for example falls within
the range of 5 to 300 minutes, preferably falls within the range of
10 to 200 minutes, and more preferably falls within the range of 20
to 150 minutes.
[0078] The particle diameter of a carbon precursor (for example, a
carbon precursor originating from plants) to be subjected to vapor
phase demineralization is not particularly limited. The lower limit
of an average value of particle diameters is preferably 100 .mu.m
or more, more preferably 300 .mu.m or more, and especially
preferably 500 .mu.m or more because it may become difficult for
the vapor phase containing potassium and the like which are removed
from a carbon precursor and the carbon precursor to be separated
from each other in the case where the particle diameter is too
small. Moreover, from the viewpoint of the fluidity in a mixed gas
stream, the upper limit of an average value of particle diameters
is preferably 10000 .mu.m or less, more preferably 8000 .mu.m or
less, and especially preferably 5000 .mu.m or less. In this
context, the details of the measurement of the particle diameter
are as described in EXAMPLES.
[0079] An apparatus used in the vapor phase demineralization is not
particularly limited as long as the apparatus enables the vapor
phase containing a halogen compound to be heated and stirred
together with a carbon precursor (for example, a carbon precursor
originating from plants). For example, a fluidized bed or the like
can be used and a continuous or batch-wise fluid layer distribution
system can be adopted. The supply volume (fluidization quantity) of
the vapor phase is also not particularly limited. From the
viewpoint of the fluidity in a mixed gas stream, the vapor phase is
supplied at a rate of preferably 1 mL/minute or more, more
preferably 5 mL/minute or more, and especially preferably 10
mL/minute or more, per 1 g of a carbon precursor (for example, a
carbon precursor originating from plants).
[0080] In the vapor phase demineralization, it is preferred that,
after a heat treatment in an inert gas atmosphere containing a
halogen compound (hereinafter, sometimes referred to as "a halogen
heat treatment"), a heat treatment in an atmosphere containing no
halogen compound (hereinafter, sometimes referred to as "a vapor
phase deacidification treatment") is further performed. Since the
halogen heat treatment causes halogen elements to be contained in a
carbon precursor (for example, a carbon precursor originating from
plants), it is preferred that halogen elements contained in the
carbon precursor is removed by a vapor phase deacidification
treatment. Specifically, as the vapor phase deacidification
treatment, a heat treatment in an inert gas atmosphere containing
no halogen compound is performed at a temperature falling within
the range of usually 500.degree. C. to 940.degree. C., preferably
600 to 940.degree. C., more preferably 650 to 940.degree. C., and
especially preferably 850 to 930.degree. C. It is preferred that
this heat treatment is performed at a temperature equal to or
higher than the treatment temperature for the preceding halogen
heat treatment. For example, after the halogen heat treatment,
supply of the halogen compound can be cut off to perform a heat
treatment as the vapor phase deacidification treatment. Moreover,
the time period for the vapor phase deacidification treatment is
also not particularly limited. The time period preferably falls
within the range of 5 minutes to 300 minutes, more preferably falls
within the range of 10 minutes to 200 minutes, and especially
preferably falls within the range of 10 minutes to 100 minutes.
[0081] The acid treatment in the present invention is a treatment
in which potassium, iron, and the like contained in a carbon
precursor (for example, a carbon precursor originating from plants)
are removed (a carbon precursor is demineralized). With regard to
the carbon precursor to be subjected to an acid treatment, the
potassium element content is reduced by the acid treatment
preferably to 0.1% by mass or less, more preferably to 0.05% by
mass or less, and further preferably to 0.03% by mass or less. The
potassium element content is reduced especially preferably to such
a degree that the hardly graphitizable carbonaceous material
contains substantially no potassium element. Moreover, with regard
to the carbon precursor to be subjected to an acid treatment, the
iron element content is reduced by the acid treatment preferably to
0.02% by mass or less, more preferably to 0.015% by mass or less,
and further preferably to 0.01% by mass or less. The iron element
content is reduced especially preferably to such a degree that the
hardly graphitizable carbonaceous material contains substantially
no iron element. In this context, containing substantially no
potassium element or substantially no iron element means having a
potassium element content or an iron element content equal to or
less than the detection limit value in the X-ray fluorescence
analysis (for example, analysis using the "LAB CENTER XRF-1700"
available from SHIMADZU CORPORATION) described below. As described
above, when the potassium element content and the iron element
content are equal to or less than the above-mentioned values,
respectively, a sufficient dedoping capacity and a satisfactory
nondedoping capacity can be attained and a safety problem of the
nonaqueous electrolyte secondary battery can be avoided. The
details of the measurement of the potassium element content and
iron element content are as described in EXAMPLES.
[0082] In the acid treatment in the present invention, a part of
carbon components is removed while the carbon precursor is
demineralized. Specifically, a part of carbon components is removed
by the elution in the case of liquid phase demineralization and a
part of carbon components is removed by the activation action of
chlorine in the case of vapor phase demineralization. A space from
which a carbon component is removed plays a role of a storage site
for clustered lithium atoms after a calcination step described
below.
[0083] In the present invention, the acid treatment is performed at
least one time. The same or different acids may be used to perform
the acid treatment two times or more.
[0084] A carbon precursor (for example, a carbon precursor
originating from plants) to be subjected to an acid treatment is a
substance (for example, a substance originating from plants) before
carbonization or a substance (for example, a substance originating
from plants) after carbonization. In the case of performing an acid
treatment by liquid phase demineralization, from the viewpoint of
an increase in the elution amount of carbon components, that is, an
increase in the number of lithium storage sites, it is preferred
that a raw material (for example, a plant raw material) before
carbonization itself is subjected to a liquid phase
demineralization treatment as an acid treatment.
[0085] In the case where a carbon precursor after the acid
treatment is a carbon precursor not subjected to a carbonization
treatment yet, that is, the case where a carbon precursor after the
acid treatment is a carbon precursor prepared by subjecting a raw
material (for example, a plant raw material) before carbonization
to the acid treatment, subsequently, the carbon precursor is
subjected to a carbonization treatment. As described above, the
carbonizing method is not particularly limited. For example, an
acid-treated raw material (for example, an acid-treated plant raw
material) before carbonization can be subjected to a heat treatment
under an inert gas atmosphere at 300.degree. C. or more (temporary
calcination) to be carbonized.
[0086] A carbon precursor (for example, a carbon precursor
originating from plants) may be pulverized and classified as
necessary and the average particle diameter thereof may be
adjusted. It is preferred that a pulverization step and a
classification step is performed after an acid treatment.
<Pulverization>
[0087] In a pulverization step, it is preferred that a carbon
precursor (for example, a carbon precursor originating from plants)
is pulverized so as to have an average particle diameter after a
calcination step falling within, for example, the range of 3 to 30
.mu.m from the viewpoint of coating properties at the time of
preparing an electrode. That is, the hardly graphitizable
carbonaceous material according to the present invention is
adjusted so as to have an average particle diameter (Dv.sub.50)
falling within, for example, the range of 3 to 30 .mu.m. When the
average particle diameter of the hardly graphitizable carbonaceous
material is 3 .mu.m or more, a tendency that the amount of fine
powder increases, the specific surface area increases, the
reactivity with an electrolytic solution is heightened, the
irreversible capacity being a capacity which is not dischargeable
in spite of being charged into a battery increases, and the portion
of useless capacity in a positive electrode increases can be
suppressed. Moreover, at the time of using the resulting hardly
graphitizable carbonaceous material to produce a negative
electrode, voids to be formed between particles of the carbonaceous
material can be sufficiently secured and satisfactory transfer of
lithium ions in an electrolytic solution can be secured. The
average particle diameter (Dv.sub.50) of the carbonaceous material
of the present invention is preferably 3 .mu.m or more, more
preferably 4 .mu.m or more, and especially preferably 5 .mu.m or
more. On the other hand, when the average particle diameter is 30
.mu.m or less, such a carbonaceous material is preferred because
the mean free path of lithium ions diffusing into the inside of a
particle is shortened and the quick charge-discharge is possible.
Furthermore, in a lithium ion secondary battery, enlarging the
electrode area is of importance for enhancing the input-output
characteristics, and for this reason, the coating thickness of an
active material onto a current collecting plate is required to be
thinned at the time of the electrode preparation. In order to make
the coating thickness thin, the active material is required to have
a small particle diameter. From such viewpoints, the average
particle diameter is preferably 30 .mu.m or less, more preferably
19 .mu.m or less, further preferably 17 .mu.m or less, still
further preferably 16 .mu.m or less, and especially preferably 15
.mu.m or less.
[0088] In this connection, depending on the conditions of the final
calcination described below, a carbon precursor (for example, a
carbon precursor originating from plants) is shrunk by 0 to 20% or
so. Therefore, in order to make the average particle diameter after
calcination fall within the range of 3 to 30 .mu.m, it is preferred
that the average particle diameter of a carbon precursor is
adjusted so as to be an average particle diameter larger by 0 to
20% or so than the desired average particle diameter after
calcination. Accordingly, it is preferred that the pulverization is
performed so as to make the average particle diameter after
pulverization fall within the range of preferably 3 to 36 .mu.m,
more preferably 3 to 22.8 .mu.m, further preferably 3 to 20.4
.mu.m, still further preferably 3 to 19.2 .mu.m, and especially
preferably 3 to 18 .mu.m.
[0089] Since the carbon precursor does not melt even when subjected
to the calcination step described below, the pulverization step is
not particularly limited in the sequence of steps. In view of the
recovery (yield) of a carbon precursor in an acid treatment, it is
preferred that the pulverization step is performed after an acid
treatment and it is preferred that the pulverization step is
performed before a calcination step from the viewpoint of
sufficiently reducing the specific surface area of the carbonaceous
material. However, the pulverization step may also be performed
before an acid treatment or after a calcination step, and these
cases are not eliminated.
[0090] A pulverizer used in the pulverization step is not
particularly limited. For example, a jet mill, a ball mill, a
hammer mill, a rod mill, or the like can be used. From the
viewpoint of having little generation of fine powder, a jet mill
equipped with a classifying function is preferred. In the case of
using a ball mill, a hammer mill, a rod mill, or the like,
classification can be performed after a pulverization step to
remove fine powder.
<Classification>
[0091] A classification step enables the average particle diameter
of the carbonaceous material to be more accurately adjusted. For
example, it is possible to remove particles with a particle
diameter of 1 .mu.m or less.
[0092] In the case of removing particles with a particle diameter
of 1 .mu.m or less by classification, it is preferred that the
hardly graphitizable carbonaceous material according to the present
invention has a content of particles with a particle diameter of 1
.mu.m or less of 3% by volume or less. Although no particular
restriction is put on the removal of particles with a particle
diameter of 1 .mu.m or less as long as the removal is performed
after pulverization, it is preferred that pulverization and
classification is simultaneously performed. In the hardly
graphitizable carbonaceous material according to the present
invention, from the viewpoints of lowering the specific surface
area and lowering the irreversible capacity, the content of
particles with a particle diameter of 1 .mu.m or less is preferably
3% by volume or less, more preferably 2.5% by volume or less, and
especially preferably 2.0% by volume or less.
[0093] The classifying method is not particularly restricted.
Examples thereof can include classification with a sieve, wet
classification, and dry classification. Examples of a wet
classifier can include a classifier utilizing the principle of
gravity classification, inertial classification, hydraulic
classification, centrifugal classification, or the like. Examples
of a dry classifier can include a classifier utilizing the
principle of sedimentary classification, mechanical classification,
centrifugal classification, or the like.
[0094] A pulverization step and a classification step may be
performed with the use of one apparatus. For example, a jet mill
equipped with a dry classifying function can be used to perform a
pulverization step and a classification step. Furthermore, an
apparatus equipped with a pulverizer and a classifier, both of
which are constituted independently respectively, may be used. In
this case, pulverization and classification can be sequentially
performed, and moreover, pulverization and classification can be
discontinuously performed.
<Calcination>
[0095] After being pulverized and classified depending on the
situation, a carbon precursor which has been subjected to an acid
treatment and a carbonization treatment can be calcined to produce
the hardly graphitizable carbonaceous material according to the
present invention. A calcination step is a step of elevating the
atmosphere temperature from room temperature to a predetermined
calcination temperature, and then, performing calcination at the
calcination temperature. The carbon precursor (a) may be calcined
at a temperature of 1100 to 1400.degree. C. (final calcination) or
the carbon precursor (b) may be calcined at a temperature of 350 to
less than 1100.degree. C. (preliminary calcination), and then,
further calcined at a temperature of 1100 to 1400.degree. C. (final
calcination). Hereinafter, an example of each of the procedure of
preliminary calcination and the procedure of final calcination will
be described in this order.
<Preliminary Calcination>
[0096] For example, a carbon precursor which has been subjected to
an acid treatment and a carbonization treatment can be calcined at
a temperature of 350 to less than 1100.degree. C. to perform a
preliminary calcination step in a method for producing the hardly
graphitizable carbonaceous material according to the present
invention. By performing preliminary calcination to remove volatile
components (for example, CO.sub.2, CO, CH.sub.4, H.sub.2, and the
like) and a tar constituent, the generation amounts thereof in
final calcination can be decreased to reduce the load of a
calciner. The preliminary calcination temperature usually falls
within the range of 350 to less than 1100.degree. C. and preferably
falls within the range of 400 to less than 1100.degree. C. The
preliminary calcination can be performed according to a usual
procedure for preliminary calcination. Specifically, the
preliminary calcination can be performed in an inert gas atmosphere
and examples of the inert gas can include nitrogen, argon, or the
like. Moreover, the preliminary calcination can also be performed
under reduced pressure, and for example, the preliminary
calcination can be performed under a pressure of 10 KPa or less.
The time period for the preliminary calcination is not particularly
limited, usually falls within the range of 0.5 to 10 hours, and
preferably falls within the range of 1 to 5 hours.
[0097] In this connection, in the case of performing the
preliminary calcination in a method for producing the hardly
graphitizable carbonaceous material according to the present
invention, it is considered that a phenomenon in which a carbon
precursor is coated with a tar constituent and a hydrocarbon-based
gas occurs in the preliminary calcination step. It is considered
that this carbonaceous coating film causes the specific surface
area of a hardly graphitizable carbonaceous material to be
favorably decreased.
<Final Calcination>
[0098] A final calcination step in a method for producing the
hardly graphitizable carbonaceous material according to the present
invention can be performed according to a usual procedure for final
calcination, and after the final calcination, a hardly
graphitizable carbonaceous material is obtained.
[0099] The final calcination temperature usually falls within the
range of 1100 to 1400.degree. C., preferably falls within the range
of 1200 to 1380.degree. C., and more preferably falls within the
range of 1250 to 1350.degree. C. The final calcination can be
performed in an inert gas atmosphere and examples of the inert gas
can include nitrogen, argon, or the like. Moreover, it is also
possible to perform the final calcination in an inert gas
atmosphere containing a halogen gas. Furthermore, it is also
possible to perform the final calcination under reduced pressure,
for example, under a pressure of 10 KPa or less. The time period
for the final calcination is not particularly limited, and the time
period falls within the range of for example 0.05 to 10 hours,
preferably 0.05 to 8 hours, and more preferably 0.05 to 6
hours.
[0100] In this connection, in the case of not performing
preliminary calcination in a method for producing the hardly
graphitizable carbonaceous material according to the present
invention, it is considered that a phenomenon in which a carbon
precursor is coated with a tar constituent and a hydrocarbon-based
gas occurs in the final calcination step.
[0101] In the present invention, at the time of calcining a carbon
precursor, the carbon precursor can be mixed with a volatile
organic substance to be calcined. By being mixed with a volatile
organic substance to be calcined, a hardly graphitizable
carbonaceous material obtained from the carbon precursor can have a
specific surface area more suitable for a negative electrode
material for lithium ion secondary batteries.
<Volatile Organic Substance>
[0102] Although a volatile organic substance that can be used in
the present invention is not particularly limited as long as the
volatile organic substance is solid at an ambient temperature and
has a residual carbon ratio of less than 5% by mass on the basis of
the mass of a volatile organic substance before ashing in the case
of being ashed at 800.degree. C. A volatile organic substance, from
which volatile substances (for example, a hydrocarbon-based gas and
a tar constituent) enabling the specific surface area of a hardly
graphitizable carbonaceous material produced from the carbon
precursor to be decreased are generated, is preferred. Although the
content of volatile substances enabling the specific surface area
to be decreased in a volatile organic substance is not particularly
limited, the content thereof usually falls within the range of 1 to
20% by mass and preferably falls within the range of 3 to 15% by
mass on the basis of the mass of the volatile organic substance. In
this connection, in the present specification, the ambient
temperature refers to 25.degree. C.
[0103] Examples of the volatile organic substance can include a
thermoplastic resin and a low molecular weight organic compound.
More specifically, examples of the thermoplastic resin can include
polystyrene, polyethylene, polypropylene, poly(meth)acrylic acid, a
poly(meth)acrylic acid ester, or the like, and examples of the low
molecular weight organic compound can include toluene, xylene,
mesitylene, styrene, naphthalene, phenanthrene, anthracene, pyrene,
or the like. From the viewpoint that the surface of a carbon
precursor is not oxidatively activated in the case of being allowed
to volatilize and thermally decomposed at a calcination
temperature, polystyrene, polyethylene, or polypropylene is
preferred as the thermoplastic resin and naphthalene, phenanthrene,
anthracene, or pyrene is preferred as the low molecular weight
organic compound. From the viewpoint of being preferable in view of
restriction of safety because of the low volatility under an
ordinary temperature condition, it is further preferred that
naphthalene, phenanthrene, anthracene, or pyrene is used.
[0104] The residual carbon ratio of the sample can be measured by
quantitatively determining the carbon content of an
intensively-heating residue obtained after a sample is intensively
heated in an inert gas atmosphere. A sample being intensively
heated means about 1 g (the accurately weighed mass is defined as
W.sub.1 (g)) of a volatile organic substance being placed in a
crucible, and the temperature of the crucible being elevated at a
rate of 10.degree. C./minute to 800.degree. C. in an electric
furnace with a nitrogen flow at a rate of 20 liters per 1 minute,
and then, maintained for 1 hour at 800.degree. C. A residue thus
obtained corresponds to an intensively-heating residue, and the
mass thereof is defined as W.sub.2 (g).
[0105] Then, the intensively-heating residue is analyzed for the
elemental analysis in accordance with a method stipulated in JIS
M8819 to be measured for the mass proportion P.sub.1 (%) of carbon.
The residual carbon ratio P.sub.2 (%) is calculated from the
following equation.
P.sub.2=P.sub.1.times.W.sub.2/W.sub.1 [Mathematical 1]
[0106] In the case where a carbon precursor and a volatile organic
substance are mixed to be calcined, the carbon precursor and the
volatile organic substance are mixed preferably at a mass ratio of
97:3 to 40:60. This mixing ratio more preferably falls within the
range of 95:5 to 60:40 and especially preferably falls within the
range of 93:7 to 80:20. By making the mixing ratio fall within the
above-mentioned range, while the specific surface area of a hardly
graphitizable carbonaceous material can be sufficiently decreased,
the wastefull consumption of the volatile organic substance due to
the saturation of the effect of decreasing the specific surface
area can be avoided.
[0107] The mixing of a carbon precursor and a volatile organic
substance may be performed in a stage before or after pulverization
of the carbon precursor. In the case where a carbon precursor
before pulverization is mixed with a volatile organic substance,
the carbon precursor and the volatile organic substance can be
simultaneously weighed and fed into a pulverizing apparatus to
simultaneously perform pulverization and mixing. It is also
preferred that a carbon precursor after pulverization is mixed with
a volatile organic substance. As a mixing method in this case, any
mixing method may be adopted as long as the two are uniformly
mixed.
[0108] It is preferred that a volatile organic substance is mixed
in a particulate form, but the shape of particles and the particle
diameter are not particularly limited. From the viewpoint of
uniformly dispersing a volatile organic substance in a mixture of a
pulverized product, of a carbon precursor and the volatile organic
substance, the average particle diameter of the volatile organic
substance preferably falls within the range of 0.1 to 2000 .mu.m,
more preferably falls within the range of 1 to 1000 .mu.m, and
especially preferably falls within the range of 2 to 600 .mu.m.
[0109] The mixture of a carbon precursor and a volatile organic
substance may contain an additional ingredient other than the
carbon precursor and the volatile organic substance as long as
effects on the hardly graphitizable carbonaceous material according
to the present invention are exerted, that is, as long as the
specific surface area of the hardly graphitizable carbonaceous
material is decreased. For example, the mixture can further contain
natural graphite, artificial graphite, a metal-based material, an
alloy-based material, or an oxide-based material. The content of
the additional ingredient is not particularly limited and is
preferably 50 parts by mass or less, more preferably 30 parts by
mass or less, further preferably 20 parts by mass or less, and
especially preferably 10 parts by mass or less, relative to 100
parts by mass of the mixture of a carbon precursor and a volatile
organic substance.
<Negative Electrode Material for Nonaqueous Electrolyte
Secondary Batteries>
[0110] The negative electrode material for nonaqueous electrolyte
secondary batteries according to the present invention comprises
the hardly graphitizable carbonaceous material according to the
present invention.
<Production of anode Electrode>
[0111] By adding a binding agent (binder) to the hardly
graphitizable carbonaceous material, further adding a suitable
amount of a suitable solvent thereto, kneading the contents to
prepare an electrode mixture, then, applying the electrode mixture
onto a current collecting plate composed of a metal plate and the
like, drying the electrode mixture, and subjecting the dried
electrode mixture to pressure forming, an anode electrode material
containing the hardly graphitizable carbonaceous material according
to the present invention can be produced. By the use of the hardly
graphitizable carbonaceous material according to the present
invention, in particular, without the need for adding a conductive
additive, an electrode having high electrical conductivity can be
produced. However, in order to impart the electrode with higher
electrical conductivity, at the time of preparing an electrode
mixture, a conductive additive can also be added to the electrode
mixture, as necessary.
[0112] As the conductive additive, conductive carbon black, a
vapor-grown carbon fiber (VGCF), a nanotube, and the like can be
used. Although the addition amount of a conductive additive varies
with the kind of the conductive additive used, the addition amount
thereof preferably falls within the range of 0.5 to 10% by mass (in
this context, the equation: the active material (hardly
graphitizable carbonaceous material) amount+the binding agent
amount+the conductive additive amount=100% by mass holds), more
preferably falls within the range of 0.5 to 7% by mass, and
especially preferably falls within the range of 0.5 to 5% by mass.
By making the addition amount of a conductive additive fall within
the above-mentioned range, without deteriorating the dispersion
state of the conductive additive in an electrode mixture, expected
high-level electrical conductivity can be attained.
[0113] No particular limitation is put on the binding agent added
as long as the binding agent does not react with an electrolytic
solution. Examples thereof can include PVDF (polyvinylidene
fluoride), polytetrafluoroethylene, a mixture of SBR
(styrene-butadiene-rubber) and CMC (carboxymethyl cellulose), and
the like. Of these, PVDF is preferred because PVDF adhered to the
active material surface hardly impedes the transfer of lithium ions
and satisfactory input-output characteristics are attained.
Although the addition amount of a binding agent varies with the
kind of the binding agent used, with regard to a PVDF-based binding
agent, the addition amount thereof preferably falls within the
range of 3 to 13% by mass and more preferably falls within the
range of 3 to 10% by mass on the basis of the whole mass of the
hardly graphitizable carbonaceous material, the binding agent, and
the conductive additive. By making the addition amount of a binding
agent fall within the above-mentioned range, problems that the
electrical resistance of the resulting electrode becomes large, the
internal resistance of a battery becomes large, the battery
characteristics are lowered, and the electrical connection between
two anode material particles and between an anode material particle
and a current collecting plate becomes insufficient can be
avoided.
[0114] For the purpose of dissolving PVDF in a solvent to prepare
slurry, as the solvent, a polar solvent such as N-methylpyrrolidone
(NMP) may be preferably used. Moreover, for the purpose of
preparing an aqueous emulsion of SBR and the like or an aqueous
solution of CMC and the like, as the solvent, water may be
preferably used. As the binding agent to be blended with water as a
solvent, a mixture of plural binding agents such as a mixture of
SBR and CMC is often used. The addition amount of a solvent
preferably falls within the range of 0.5 to 5% by mass and more
preferably falls within the range of 1 to 4% by mass on the basis
of the whole mass of the binding agent used.
[0115] Although, basically, electrode active material layers are
respectively formed on both faces of a current collecting plate, as
necessary, one electrode active material layer may be formed only
on one face thereof. The thickness of the active material layer
(per one face) preferably falls within the range of 10 to 80 .mu.m,
more preferably falls within the range of 20 to 75 .mu.m, and
especially preferably falls within the range of 20 to 60 .mu.m. By
making the thickness fall within the above-mentioned range, while
highly enhanced capacity can be realized because the volume of a
current collecting plate, a separator, and the like can be reduced,
high input-output characteristics can be attained because a wide
electrode area opposed to the counter electrode can be secured.
<Nonaqueous electrolyte secondary Battery>
[0116] The nonaqueous electrolyte secondary battery according to
the present invention comprises the negative electrode material for
nonaqueous electrolyte secondary batteries according to the present
invention. A nonaqueous electrolyte secondary battery which is
produced with a negative electrode material for nonaqueous
electrolyte secondary batteries comprising the hardly graphitizable
carbonaceous material according to the present invention and which
is fully charged to be used shows not only high charge capacity but
also high charge-discharge efficiency.
[0117] In the case of using the hardly graphitizable carbonaceous
material according to the present invention to form a negative
electrode material for nonaqueous electrolyte secondary batteries,
no particular limitation is put on other constituent materials for
a battery such as a positive electrode material, a separator, and
an electrolytic solution. It is possible to use various materials
which are used in or proposed for a conventional nonaqueous solvent
secondary battery.
[0118] For example, as the positive electrode material, layered
oxide-based composite metal chalcogen compounds (represented by
LiMO.sub.2, wherein M represents a metallic element: for example,
LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.2, or
LiNi.sub.xCo.sub.yMo.sub.zO.sub.2 (wherein, x, y, and z each
represent a composition ratio)), olivine-based composite metal
chalcogen compounds (represented by LiMPO.sub.4, wherein M
represents a metallic element: for example, LiFePO.sub.4 or the
like), and spinel-based composite metal chalcogen compounds
(represented by LiM.sub.2O.sub.4, wherein M represents a metallic
element: for example, LiMn.sub.2O.sub.4 or the like) are preferred.
These chalcogen compounds may be mixed as necessary. These positive
electrode materials, together with an appropriate binder and a
carbon material that imparts electrical conductivity to the
electrode, are molded and formed into a layer on an electrically
conductive current collecting member to form a positive
electrode.
[0119] A nonaqueous solvent type electrolytic solution combined
with these positive and negative electrodes to be used is generally
formed by dissolving an electrolyte in a nonaqueous solvent. As the
nonaqueous solvent, for example, one of organic solvents such as
propylene carbonate, ethylene carbonate, dimethyl carbonate,
diethyl carbonate, dimethoxyethane, diethoxyethane,
.gamma.-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran,
sulfolane, and 1,3-dioxolane can be used alone or two or more
thereof can be used in combination. Moreover, as the electrolyte,
LiClO.sub.4, LiPF.sub.6, LiBF.sub.4, LiCF.sub.3SO.sub.3,
LiAsF.sub.6, LiCl, LiBr, LiB(C.sub.6H.sub.5).sub.4,
LiN(SO.sub.3CF.sub.3).sub.2, or the like may be used.
[0120] In general, a nonaqueous electrolyte secondary battery is
produced by opposing the positive electrode and the negative
electrode which are formed as described above, as necessary with a
liquid-permeable separator, composed of nonwoven fabric and other
porous materials, interposed therebetween, and by immersing them in
an electrolytic solution. As the separator, a permeable separator,
composed of nonwoven fabric and other porous materials, generally
used in a secondary battery can be used. Alternatively, instead of
the separator or together with the separator, a solid electrolyte
composed of a polymer gel impregnated with an electrolytic solution
can be used.
[0121] In the present specification, "a nonaqueous electrolyte
secondary battery fully charged to be used" refers to a nonaqueous
electrolyte secondary battery obtained by assembling the nonaqueous
electrolyte secondary battery according to the present invention,
and allowing its negative electrode to be charged (doped) with
lithium until just before precipitation of metallic lithium is
confirmed by .sup.7Li nuclear-solid state NMR analysis; and usually
means a nonaqueous electrolyte secondary battery charged at a
constant current value so as to have a charge capacity within the
range of 580 to 700 mAbig per mass of the negative electrode active
material.
<Oxygen Element Content>
[0122] In the nonaqueous electrolyte secondary battery according to
the present invention, it is good to make the oxygen element
content of the hardly graphitizable carbonaceous material smaller.
An analysis value thereof obtained by elemental analysis is usually
0.25% by mass or less and preferably 0.24% by mass or less. It is
further preferred that the hardly graphitizable carbonaceous
material contains substantially no oxygen element. In this context,
containing substantially no oxygen element means having an oxygen
element content equal to or less than 10.sup.-6% by mass which is a
detection limit of the elemental analysis method (inert gas
fusion-thermal conductivity method) described below. As described
above, when the oxygen element content is the above-mentioned value
or less, the lowering in utilization efficiency of lithium ions can
be suppressed.
[0123] Moreover, as described above, a method of adjusting the
oxygen element content is not limited at all. For example, by
subjecting a carbon precursor originating from plants to an acid
treatment at a predetermined temperature, then, mixing an
acid-treated carbon precursor with a volatile organic substance,
and calcining the mixture under an inert gas atmosphere at a
temperature of 1100.degree. C. to 1400.degree. C., the oxygen
element content can be adjusted. The details of the measurement of
the oxygen element content are as described in EXAMPLES.
<Main Resonance Peak Position of Chemical Shift Value>
[0124] When a hardly graphitizable carbonaceous material in the
nonaqueous electrolyte secondary battery according to the present
invention is taken out of the nonaqueous electrolyte secondary
battery charged (doped) with lithium until a fully charged state
thereof is attained, a main resonance peak position of a chemical
shift value observed by subjecting the hardly graphitizable
carbonaceous material to .sup.7Li nuclear-solid state NMR analysis
is preferably downfield by more than 115 ppm, more preferably
downfield by more than 118 ppm, and especially preferably downfield
by more than 120 ppm, from a peak position of lithium chloride. The
details of the .sup.7Li nuclear-solid state NMR analysis are as
described in EXAMPLES.
[0125] As described above, a battery prepared with a hardly
graphitizable carbonaceous material in which a main resonance peak
position of a chemical shift value is downfield by more than 115
ppm from a peak position of lithium chloride means a battery having
a high charge capacity and having high charge-discharge efficiency
calculated from "the discharge capacity/the charge capacity". Since
the charge-discharge efficiency is high, the battery becomes
advantageous from an aspect of capacity per volume of the battery
or cost of the battery. By the use of a carbonaceous material
having a main resonance peak position of a chemical shift value
which is downfield by less than 115 ppm from a peak position of
lithium chloride, unlike the hardly graphitizable carbonaceous
material according to the present invention, it is very difficult
to achieve both extremely high charge capacity and extremely high
charge-discharge efficiency.
<Carbon Precursor>
[0126] As described above, the hardly graphitizable carbonaceous
material in the nonaqueous electrolyte secondary battery according
to the present invention is derived from a phenol resin, a furan
resin, pitch, tar, a carbon precursor originating from plants, or
the like.
[0127] The hardly graphitizable carbonaceous material is preferably
derived from a carbon precursor originating from plants. In the
present invention, "a carbon precursor originating from plants"
means a substance before carbonization originating from plants or a
substance after carbonization originating from plants (char derived
from plants). As described above, the plant raw material is not
particularly limited. Such plants described above can be used
singly or in combination of two or more thereof. Of these, coconut
shell, which is easily available abundantly, is preferred.
[0128] The coconut shell is not particularly limited. The coconut
shells exemplified above can be used singly or in combination.
Coconut shells of coco palm and palm coconut, which are utilized as
foodstuffs, raw material of a detergent, raw material of a
biodiesel fuel oil, and the like and are biomass wastes generated
in large quantities, are especially preferred.
[0129] A method of carbonizing the plant raw material, that is, a
method of producing the char derived from plants, is not
particularly limited. The method can be performed, for example, by
subjecting the plant raw material to a heat treatment under an
inert gas atmosphere at 300.degree. C. or more (hereinafter,
sometimes referred to as "temporary calcination").
[0130] Moreover, the plant raw material in the form of char (for
example, coconut shell char) is also available.
<Average Face-to-Face Dimension d.sub.002>
[0131] The average face-to-face dimension d.sub.002 of the (002)
face calculated from the Bragg equation by a wide angle X-ray
diffraction method of the hardly graphitizable carbonaceous
material in the nonaqueous electrolyte secondary battery according
to the present invention preferably falls within the range of 0.36
nm to 0.42 nm, more preferably falls within the range of 0.38 nm to
0.40 nm, and especially preferably falls within the range of 0.381
nm to 0.389 nm. As described above, when the average face-to-face
dimension d.sub.002 of the (002) face falls within the
above-mentioned range, the lowering in input-output characteristics
of a lithium ion battery can be suppressed, the lowering in
stability of a battery material can be suppressed, and the lowering
in effective capacity per volume can be avoided. In order to adjust
the average face-to-face dimension within the above-mentioned
range, for example, a carbon precursor giving a hardly
graphitizable carbonaceous material may be calcined at a
calcination temperature within the range of 1100 to 1400.degree. C.
Moreover, a method in which a carbon precursor is mixed with a
thermally-decomposable resin such as polystyrene to be calcined can
also be adopted. The details of the measurement of the average
face-to-face dimension d.sub.002 are as described in EXAMPLES.
<Specific Surface Area>
[0132] The specific surface area determined by a nitrogen
adsorption BET three-point method of the hardly graphitizable
carbonaceous material in the nonaqueous electrolyte secondary
battery according to the present invention preferably falls within
the range of 1 to 20 m.sup.2/g, more preferably falls within the
range of 1.2 to 10 m.sup.2/g, and especially preferably falls
within the range of 1.4 to 9.5 m.sup.2/g. As described above, when
the specific surface area falls within the above-mentioned range,
the number of micropores in the hardly graphitizable carbonaceous
material can be sufficiently reduced by a calcination step
described below, the moisture-absorption characteristics of the
hardly graphitizable carbonaceous material can be sufficiently
lowered, and the lowering in utilization efficiency of lithium ions
can be suppressed. The specific surface area can be adjusted by
controlling the temperature in a demineralization step described
below. The details of the measurement of the specific surface area
by a nitrogen adsorption BET three-point method are as described in
EXAMPLES.
<True Density .rho..sub.Bt>
[0133] From the viewpoint of making the capacity per mass of the
battery high, the true density by a butanol method of the hardly
graphitizable carbonaceous material in the nonaqueous electrolyte
secondary battery according to the present invention preferably
falls within the range of 1.40 to 1.70 g/cm.sup.3, more preferably
falls within the range of 1.42 to 1.65 g/cm.sup.3, and especially
preferably falls within the range of 1.44 to 1.60 g/cm.sup.3. The
true density within the above-mentioned range can be attained, for
example, by setting the calcination step temperature to 1100 to
1400.degree. C. The details of the measurement of the true density
.rho..sub.Bt are as described in EXAMPLES.
<Potassium Element Content and Iron Element Content>
[0134] As described above, the potassium element content of the
hardly graphitizable carbonaceous material in the nonaqueous
electrolyte secondary battery according to the present invention is
preferably 0.1% by mass or less, more preferably 0.05% by mass or
less, and further preferably 0.03% by mass or less, and it is
especially preferred that the hardly graphitizable carbonaceous
material contains substantially no potassium element. Moreover, as
described above, the iron element content of the hardly
graphitizable carbonaceous material in the nonaqueous electrolyte
secondary battery according to the present invention is preferably
0.02% by mass or less, more preferably 0.015% by mass or less, and
further preferably 0.01% by mass or less, and it is especially
preferred that the hardly graphitizable carbonaceous material
contains substantially no iron element. In this context, containing
substantially no potassium element or substantially no iron element
means having a potassium element content or an iron element content
equal to or less than the detection limit value in the X-ray
fluorescence analysis (for example, analysis using the "LAB CENTER
XRF-1700" available from SHIMADZU CORPORATION) described below. As
described above, when the potassium element content and the iron
element content are equal to or less than the above-mentioned
values, respectively, a sufficient dedoping capacity and a
satisfactory nondedoping capacity can be attained and a safety
problem of the nonaqueous electrolyte secondary battery can be
avoided. The details of the measurement of the potassium element
content and iron element content are as described in EXAMPLES.
<Moisture Content>
[0135] The moisture content of the hardly graphitizable
carbonaceous material in the nonaqueous electrolyte secondary
battery according to the present invention is preferably 10000 ppm
or less, more preferably 9000 ppm or less, and especially
preferably 8000 ppm or less. As described above, the smaller the
moisture content, the number of lithium ions that adsorb to the
hardly graphitizable carbonaceous material is increased and
self-discharge caused by a reaction of lithium ions with adsorbed
water can be reduced, which is preferable. The moisture content of
the hardly graphitizable carbonaceous material can be reduced, for
example, by reducing the number of oxygen atoms contained in a
hardly graphitizable carbonaceous material. The moisture content of
the hardly graphitizable carbonaceous material can be measured, for
example, with the use of a Karl Fischer moisture meter or the like.
The details of the measurement of the moisture content are as
described in EXAMPLES.
EXAMPLES
[0136] Hereinafter, the present invention will be described in
detail by reference to examples, but these should not be construed
to limit the scope of the present invention. In this connection,
measuring methods for physical property values of a hardly
graphitizable carbonaceous material will be described below, but
physical property values, including values in examples, described
in the present specification are based on values determined by the
following methods.
<Elemental Analysis>
[0137] With the use of "Oxygen/Nitrogen/Hydrogen Analyzer EMGA-930"
available from HORIBA, Ltd., elemental analysis was performed.
[0138] In this apparatus, as detection methods, an Oxygen: inert
gas fusion-Non Dispersive Infrared Ray absorption method (NDIR), a
Nitrogen: inert gas fusion-Thermal Conductivity Detection method
(TCD), and a Hydrogen: inert gas fusion-Non Dispersive Infrared Ray
absorption method (NDIR) were adopted. The calibration was
performed by the use of an (Oxygen/Nitrogen) Ni capsule, TiH.sub.2
(H standard sample), and SS-3 (N, O standard sample). The water
content of a sample was measured at 250.degree. C. for about 10
minutes as a pretreatment, and then, 20 mg of the sample was placed
in the Ni capsule, subjected to a degasification for 30 seconds in
the elemental analyzer, and analyzed for the elemental analysis. In
a test, three specimens were analyzed and an average value thereof
was defined as an analysis value.
<.sup.7Li Nuclear-Solid State NMR Analysis>
[0139] A negative electrode portion containing a hardly
graphitizable carbonaceous material fully doped with lithium ions
was taken out of a cell in a fully charged state, and the whole
negative electrode portion from which the electrolyte was wiped off
was placed into a sample tube for NMR. With the use of "Nuclear
magnetic resonance apparatus AVANCE 300" available from Bruker
Corporation, .sup.7Li nuclear-solid state NMR analysis was
performed. At the time of measurement, lithium chloride was adopted
as a reference material and the peak position thereof was set to 0
ppm.
<Measurement of Average Face-to-Face Dimension d.sub.002 of
(002) Face>
[0140] With the use of "MiniFlex II" available from Rigaku
Corporation, hardly graphitizable carbonaceous material powder was
placed into a sample holder and a CuK.alpha. ray which was made
monochromatic by a Ni filter was adopted as the radiation source to
obtain an X-ray diffraction pattern. A peak position in the
diffraction pattern was determined by a centroid method (a method
in which a centroid position of the diffraction pattern is
determined and a 2.theta. value corresponding thereto is calculated
to determine a peak position) and corrected by the use of a
diffraction peak of the (111) face of high purity silicon powder
for standard reference materials. The wavelength of the CuK.alpha.
ray was defined as 0.15418 nm to calculate d.sub.002 according to
the Bragg's formula mentioned below.
[ Mathematical 2 ] d 002 = .lamda. 2 sin .theta. ( Bragg ` s
formula ) ##EQU00001##
<Measurement of Specific Surface Area by Nitrogen Adsorption BET
Three-Point Method>
[0141] An approximate equation derived from the BET equation is
mentioned below.
p/[v(p.sub.0-p)]=(1/v.sub.mc)+[(c-1)/v.sub.mc](p/p.sub.0)
[Mathematical 3]
[0142] With the use of the above-mentioned approximate equation,
v.sub.m was determined by a three-point method utilizing nitrogen
adsorption at a liquid nitrogen temperature to calculate the
specific surface area of a sample according to the following
equation.
[ Mathematical 4 ] Specific surface area = ( v m Na 22400 ) .times.
10 - 18 ##EQU00002##
[0143] In this context, v.sub.m represent's an adsorption amount
(cm.sup.3/g) required for a monomolecular layer to be formed on the
sample surface, v represents an actually measured adsorption amount
(cm.sup.3/g), p.sub.0 represents a saturated vapor pressure, p
represents an absolute pressure, c represents a constant
(reflecting the heat of adsorption), N represents Avogadro's number
6.022.times.10.sup.23, and a (nm.sup.2) represents an area occupied
by molecules of the adsorbate on the sample surface (molecular
sectional area at the monolayer).
[0144] Specifically, with the use of "BELL Sorb Mini" available
from MicrotracBEL Corp., the amount of nitrogen adsorbed to a
hardly graphitizable carbonaceous material at a liquid nitrogen
temperature was measured in the following way. A pulverized hardly
graphitizable carbonaceous material with a particle diameter of
about 5 to 50 .mu.m was placed in a sample tube, the internal
pressure of the sample tube in a state of being cooled to
-196.degree. C. was once reduced, and then, the hardly
graphitizable carbonaceous material was made to adsorb nitrogen
(purity of 99.999%) at a desired relative pressure. The amount of
nitrogen adsorbed to a sample when the adsorption equilibrium is
attained at each desired relative pressure was defined as the
adsorption gas amount v.
<Measurement of True Density .rho..sub.Bt by Butanol
Method>
[0145] According to a method stipulated in JIS R 7212, the true
density was measured by a butanol method. A specific gravity bottle
with a side tube having an internal volume of about 40 mL was
accurately weighed for the mass (m.sub.1). Next, a sample was
placed into the bottle so that the sample flattened all over a
bottom part of the bottle and the thickness of the sample becomes
about 10 mm, after which the specific gravity bottle was accurately
weighed for the mass (m.sub.2). To this, 1-butanol was carefully
added so that a depth from the bottom to the liquid level of
1-butanol became 20 mm or so. Next, weak vibrations were applied to
the specific gravity bottle and it was confirmed that no more large
air bubble was generated, after which the specific gravity bottle
was placed in a vacuum desiccator and the inside of the vacuum
desiccator was gradually evacuated to a degree of 2.0 to 2.7 kPa.
After the degree of evacuation was kept for 20 minutes or more and
the generation of air bubbles stopped, the specific gravity bottle
was taken out of the vacuum desiccator. The specific gravity bottle
was filled with 1-butanol, stoppered, and immersed for 15 minutes
or more in a constant-temperature water bath (the temperature was
adjusted to 30.+-.0.03.degree. C.) to make the liquid level of
1-butanol coincide with the marked line. Next, the specific gravity
bottle was taken out thereof, the outside surface was thoroughly
wiped, and the contents were cooled to room temperature, after
which the specific gravity bottle was accurately weighed for the
mass (m.sub.4). Next, the identical specific gravity bottle was
filled only with 1-butanol and immersed in a constant-temperature
water bath in the same manner as above to make the liquid level of
1-butanol coincide with the marked line, after which the specific
gravity bottle was weighed for the mass (m.sub.3). Moreover, the
identical specific gravity bottle was filled only with distilled
water prepared by being boiled just before use to remove dissolved
gas and immersed in a constant-temperature water bath in the same
manner as above to make the liquid level of distilled water
coincide with the marked line, after which the specific gravity
bottle was weighed for the mass (m.sub.5).
[0146] The true density .rho..sub.Bt was calculated according to
the following equation.
[ Mathematical 5 ] .rho. Bt = m 2 - m 1 m 2 - m 1 - ( m 4 - m 3 )
.times. m 3 - m 1 m 5 - m 1 d ##EQU00003##
[0147] In this context, d represents the specific gravity (0.9946)
of water at 30.degree. C.
<Measurement of Metallic Element Content>
[0148] The potassium element content and the iron element content
were measured in the following manner. Carbon samples containing
predetermined amounts of the potassium element and the iron element
were prepared beforehand, and with the use of an X-ray fluorescence
spectroscopic analyzer, a calibration curve showing the
relationship between the intensity of the potassium K.alpha. ray
and the potassium element content and a calibration curve showing
the relationship between the intensity of the iron K.alpha. ray and
the iron element content were prepared. Then, a sample was measured
for the intensities of the potassium K.alpha. ray and the iron
K.alpha. ray in X-ray fluorescence analysis to determine the
potassium element content and the iron element content by the use
of the previously prepared calibration curves. With the use of "LAB
CENTER XRF-1700" available from SHIMADZU CORPORATION, the X-ray
fluorescence analysis was performed under the following condition.
A holder for the upper part irradiation system was used and the
measurement area of the sample was set within the circumference of
a diameter of 20 mm. With regard to the installation of a sample to
be measured, 0.5 g of the sample to be measured was placed in a
polyethylene-made vessel with an inner diameter of 25 mm, the back
side of the vessel was supported with a plankton net, and the
measurement surface was covered with a polypropylene-made film to
perform the measurement. Conditions of an X-ray source were set to
40 kV and 60 mA. With regard to potassium, LiF (200) was used as
the dispersive crystal, a gas flow type proportional counter was
used as a detector, and the sample was measured at a scanning speed
of 8.degree./minute within the range of 90 to 140.degree. as the
value of 2.theta.. With regard to iron, LiF (200) was used as the
dispersive crystal, a scintillation counter was used as a detector,
and the sample was measured at a scanning speed of 8.degree./minute
within the range of 56 to 60.degree. as the value of 2.theta..
<Measurement of Moisture Content>
[0149] Ten grams of a pulverized hardly graphitizable carbonaceous
material with a particle diameter of about 5 to 50 .mu.m was placed
in a sample tube, pre-dried for 2 hours at 120.degree. C. under a
reduced pressure of 133 Pa, transferred to a glass-made petri dish
of 50 mm.PHI., and allowed to stand in a constant-temperature and
constant-humidity chamber at a temperature of 25.degree. C. and a
humidity of 50% for a predetermined period of time. One gram of the
sample was taken out, and with the use of the Karl Fischer Moisture
Meter (available from Mitsubishi Chemical Analytech Co., Ltd.), the
sample was heated to 250.degree. C. and measured for the water
content under a stream of nitrogen.
Example 1 (Liquid Phase Demineralization, Half-Cell Evaluation)
<Preparation of Carbon Precursor>
[0150] In 150 g of a 7.4% by mass aqueous citric acid solution, 100
g of coconut shell chips of about 5 mm square from Mindanao Island
in the Philippines was immersed. The coconut shell in the solution
was heated to 80.degree. C., heated for 4 hours, cooled to room
temperature, and subjected to filtration to remove the filtrate. A
series of this operation was performed five times to perform
demineralization. The demineralized coconut shell was dried for 24
hours at 80.degree. C. under a vacuum of 1 Torr. In a crucible, 20
g of the coconut shell chips thus demineralized was placed. With
the use of the KTF1100 Furnace (inner diameter of 70 mm.PHI.)
available from Koyo Thermo Systems Co., Ltd., under a stream of
nitrogen with an oxygen content of 15 ppm at a flow rate of 3
L/minute (0.012 meter/second), the temperature of the crucible was
elevated to 500.degree. C. at 10.degree. C./minute, maintained for
60 minutes, and then, cooled over a period of 6 hours. The crucible
was taken out thereof at 50.degree. C. or less to obtain a
carbonized product.
[0151] The obtained carbonized product was coarsely pulverized so
as to have an average particle diameter of 10 .mu.m using a ball
mill, and then, pulverized using a compact jet mill (available from
SEISHIN ENTERPRISE Co., Ltd., Co-Jet system .alpha.-mk III) and
classified to obtain a carbon precursor with an average particle
diameter of 9.0 .mu.m.
<Preparation of Hardly Graphitizable Carbonaceous
Material>
[0152] Nine point one grams of the carbon precursor prepared as
above and 0.9 g of polystyrene (available from SEKISUI PLASTICS
CO., Ltd., the average particle diameter of 400 .mu.m, the residual
carbon ratio of 1.2%) were mixed. In a graphite-made sheath (100 mm
long, 100 mm wide, and 50 mm high), 10 g of this mixture was
placed. The temperature of the graphite-made sheath was elevated to
1270.degree. C. at a temperature increasing rate of 60.degree. C.
per minute under a stream of nitrogen at a flow rate of 5 L per
minute in a high-speed temperature rising furnace available from
MOTOYAMA K.K., and then, maintained for 11 minutes, and allowed to
spontaneously cool. After the drop of the internal temperature of
the furnace to 200.degree. C. or less was confirmed, a hardly
graphitizable carbonaceous material was taken out of the furnace.
The amount of the recovered hardly graphitizable carbonaceous
material was determined to be 7.3 g, and the recovery rate from the
carbon precursor was determined to be 80%.
<Half-Cell Evaluation>
[0153] Ninety four parts by mass of the hardly graphitizable
carbonaceous material prepared as above, 6 parts by mass of PVDF
(polyvinylidene fluoride), and 90 parts by mass of NMP
(N-methylpyrrolidone) were mixed to obtain slurry. The obtained
slurry was applied onto a sheet of copper foil with a thickness of
14 .mu.m, dried, and then, pressed to obtain an electrode with a
thickness of 60 to 70 .mu.m. The obtained electrode was determined
to have a density of 0.9 to 1.1 g/cm.sup.3.
[0154] The electrode corresponding to a negative electrode was used
as a working electrode and metallic lithium was used as a counter
electrode and a reference electrode. Ethylene carbonate and
methylethyl carbonate were mixed at a volume ratio of 3:7, and the
obtained mixture was used as a solvent. In this solvent, LiPF.sub.6
was dissolved at a concentration of 1 mol/L, and the obtained
solution was used as an electrolyte. A sheet of glass fiber
nonwoven fabric was used as a separator. In a glove box, a coin
cell was prepared under an argon atmosphere.
[0155] With the use of a charge-discharge testing device ("TOSCAT"
available from TOYO SYSTEM CO., LTD.), an anode half cell of the
above-mentioned constitution was subjected to a charge-discharge
test. Lithium doping was performed at a rate of 30 mA/g relative to
the mass of the active material until a predetermined capacity (580
to 700 mAh/g), at which no metallic lithium was precipitated, was
attained, and the doping was completed. The capacity (mAh/g)
attained at this time was defined as the charge capacity. Then,
dedoping was performed at a rate of 30 mA/g relative to the mass of
the active material until the potential becomes 1.5 V relative to
the lithium potential, and the capacity discharged at this time was
defined as the discharge capacity. The percentage of the discharge
capacity/the charge capacity was defined as the charge-discharge
efficiency (initial charge-discharge efficiency) and defined as an
index of the utilization efficiency of lithium ions in a battery.
In this context, "a predetermined capacity at which no metallic
lithium was precipitated" refers to an upper limit charge capacity
(mAh/g) at which no precipitation of metallic lithium was observed
by Li-NMR.
[0156] The hardly graphitizable carbonaceous material was subjected
to .sup.7Li nuclear-solid state NMR analysis, and measured for the
oxygen element content, the average face-to-face dimension
d.sub.002 of the (002) face, the specific surface area, the true
density, the potassium element content and iron element content,
and the moisture content. The results are collected in Table 1 and
the .sup.7Li nuclear-solid state NMR spectrum is shown in FIG.
1.
Example 2 (Vapor Phase Demineralization, Half-Cell Evaluation)
<Preparation of Carbon Precursor>
[0157] Coconut shell was crushed and dry-distilled at 500.degree.
C. to obtain coconut shell char with a particle diameter of 2.36 to
0.85 mm. While nitrogen gas containing hydrogen chloride gas in a
content of 1% by volume was supplied at a flow rate of 10 L/minute
to 100 g of the coconut shell char, the coconut shell char was
subjected to a halogen heat treatment for 30 minutes at 870.degree.
C. Afterward, only the supply of hydrogen chloride gas was stopped.
While nitrogen gas was supplied at a flow rate of 10 L/minute
thereto, the coconut shell char was subjected to a vapor phase
deacidification treatment for 30 minutes at 900.degree. C. to
obtain a carbon precursor.
[0158] The obtained carbon precursor was coarsely pulverized so as
to have an average particle diameter of 10 .mu.m using a ball mill,
and then, pulverized using a compact jet mill (available from
SEISHIN ENTERPRISE Co., Ltd., Co-Jet system .alpha.-mk III), and
classified to obtain a carbon precursor with an average particle
diameter of 9.6 .mu.m.
<Preparation of Hardly Graphitizable Carbonaceous
Material>
[0159] Nine point one grams of the carbon precursor prepared as
above and 0.9 g of polystyrene (available from SEKISUI PLASTICS
CO., Ltd., the average particle diameter of 400 .mu.m, the residual
carbon ratio of 1.2%) were mixed. In a graphite-made sheath (100 mm
long, 100 mm wide, and 50 mm high), 10 g of this mixture was
placed. The temperature of the graphite-made sheath was elevated to
1320.degree. C. at a temperature increasing rate of 60.degree. C.
per minute under a stream of nitrogen at a flow rate of 5 L per
minute in a high-speed temperature rising furnace available from
MOTOYAMA K.K., and then, maintained for 11 minutes, and allowed to
spontaneously cool. After the drop of the internal temperature of
the furnace to 200.degree. C. or less was confirmed, a hardly
graphitizable carbonaceous material was taken out of the furnace.
The amount of the recovered hardly graphitizable carbonaceous
material was determined to be 8.1 g, and the recovery rate from the
carbon precursor was determined to be 89%.
<Half-Cell Evaluation>
[0160] Ninety four parts by mass of the hardly graphitizable
carbonaceous material prepared as above, 6 parts by mass of PVDF
(polyvinylidene fluoride), and 90 parts by mass of NMP
(N-methylpyrrolidone) were mixed to obtain slurry. The obtained
slurry was applied onto a sheet of copper foil with a thickness of
14 .mu.m, dried, and then, pressed to obtain an electrode with a
thickness of 60 to 70 .mu.m. The obtained electrode was determined
to have a density of 0.9 to 1.1 g/cm.sup.3.
[0161] The electrode corresponding to a negative electrode was used
as a working electrode, and metallic lithium was used as a counter
electrode and a reference electrode. Ethylene carbonate and
methylethyl carbonate were mixed at a volume ratio of 3:7, and the
obtained mixture was used as a solvent. LiPF.sub.6 was dissolved at
a concentration of 1 mol/L in this solvent, and the obtained
solution was used as an electrolyte. A sheet of glass fiber
nonwoven fabric was used as a separator. In a glove box, a coin
cell was prepared under an argon atmosphere.
[0162] With the use of a charge-discharge testing device ("TOSCAT"
available from TOYO SYSTEM CO., LTD.), an anode half cell of the
above-mentioned constitution was subjected to a charge-discharge
test. Lithium doping was performed at a rate of 30 mA/g relative to
the mass of the active material until a predetermined capacity (580
to 700 mAh/g), at which no metallic lithium was precipitated, was
attained, and the doping was completed. The capacity (mAh/g)
attained at this time was defined as the charge capacity. Then,
dedoping was performed at a rate of 30 mA/g relative to the mass of
the active material until the potential becomes 1.5 V relative to
the lithium potential, and the capacity discharged at this time was
defined as the discharge capacity. The percentage of the discharge
capacity/the charge capacity was defined as the charge-discharge
efficiency (initial charge-discharge efficiency) and defined as an
index of the utilization efficiency of lithium ions in a battery.
In this context, "a predetermined capacity at which no metallic
lithium was precipitated" refers to an upper limit charge capacity
(mAh/g) at which no precipitation of metallic lithium was observed
by Li-NMR.
[0163] The hardly graphitizable carbonaceous material was subjected
to .sup.7Li nuclear-solid state NMR analysis, and measured for the
oxygen element content, the average face-to-face dimension
d.sub.002 of the (002) face, the specific surface area, the true
density, the potassium element content and iron element content,
and the moisture content.
Example 3 (Vapor Phase Demineralization, Full-Cell Evaluation)
[0164] A hardly graphitizable carbonaceous material and a negative
electrode which are similar to those in Example 2 were used to
perform the following full-cell evaluation.
<Full-Cell Evaluation>
[0165] A negative electrode similar to that in Example 2 was
used.
Preparation of Positive Electrode
[0166] Ninety two parts by mass of a ternary oxide
(LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2) as a positive electrode
active material, 3 parts by mass of PVDF (polyvinylidene fluoride),
5 parts by mass of acetylene black, and 120 parts by mass of NMP
(N-methylpyrrolidone) were mixed to obtain slurry. The obtained
slurry was applied onto a sheet of aluminum foil with a thickness
of 20 .mu.m, dried, and then, pressed to obtain an electrode with a
thickness of 75 to 85 .mu.m. The obtained electrode was determined
to have a density of 2.4 to 2.6 g/cm.sup.3. This electrode sheet
was punched into a disk-like shape with a diameter of 14 mm to
obtain a positive electrode plate.
Preparation of Cathode Half Cell
[0167] The obtained electrode was used as a positive electrode and
metallic lithium was used as a counter electrode and a reference
electrode. Ethylene carbonate and methylethyl carbonate were mixed
at a volume ratio of 3:7, and the obtained mixture was used as a
solvent. LiPF.sub.6 was dissolved at a concentration of 1 mol/L in
this solvent, and the obtained solution was used as an electrolyte.
A sheet of glass fiber nonwoven fabric was used as a separator. In
a glove box, a coin cell was prepared under an argon
atmosphere.
[0168] With the use of a charge-discharge testing device ("TOSCAT"
available from TOYO SYSTEM CO., LTD.), a cathode half cell of the
above-mentioned constitution was subjected to a charge-discharge
test. Lithium dedoping from the positive electrode was performed at
a rate of 15 mA/g relative to the mass of the active material until
the potential becomes 4.2 V relative to the lithium potential, and
the capacity attained at this time was defined as the charge
capacity. Then, lithium doping to the positive electrode was
performed at a rate of 15 mA/g relative to the mass of the active
material until the potential becomes 3.0 V relative to the lithium
potential, and the capacity attained at this time was defined as
the discharge capacity. The charge capacity attained and the
discharge capacity attained were determined to be 174 mAh/g and 154
mAh/g, respectively, and the charge-discharge efficiency (initial
charge-discharge efficiency) calculated as the percentage of the
discharge capacity/the charge capacity was determined to be
88.5%.
Preparation of Full Cell and Evaluation
[0169] A negative electrode mixture-applied face and a positive
electrode mixture-applied face were opposed to each other with a
separator composed of glass fiber nonwoven fabric interposed
therebetween so that the positive electrode (with a diameter of 14
mm) did not protrude from the negative electrode face area with a
diameter of 15 mm obtained in Example 2. On this occasion, a ratio
(anode capacity/cathode capacity) of an anode charge capacity (mAh)
per opposing area to a cathode charge capacity (mAh) per opposing
area was adjusted to be 1. Ethylene carbonate and methylethyl
carbonate were mixed at a volume ratio of 3=7, and the obtained
mixture was used as a solvent. LiPF.sub.6 was dissolved at a
concentration of 1 mol/L in this solvent, and the obtained solution
was used as an electrolyte. In a glove box, a coin cell was
prepared under an argon atmosphere.
Charge-Discharge Test
[0170] With the use of a charge-discharge testing device ("TOSCAT"
available from TOYO SYSTEM CO., LTD.), a coin cell (full cell) of
the above-mentioned constitution was subjected to a
charge-discharge test. Charging was performed at a rate of 30 mA/g
relative to the mass of the negative electrode active material
until the potential becomes 4.2 V relative to the lithium
potential, and the capacity attained at this time was defined as
the charge capacity. Then, discharging was performed at a rate of
30 mA/g relative to the mass of the negative electrode active
material until the potential becomes 2.0 V relative to the lithium
potential, and the capacity attained at this time was defined as
the discharge capacity.
[0171] The hardly graphitizable carbonaceous material was subjected
to .sup.7Li nuclear-solid state NMR analysis, and measured for the
oxygen element content, the average face-to-face dimension
d.sub.002 of the (002) face, the specific surface area, the true
density, the potassium element content and iron element content,
and the moisture content. The results are collected in Table 1.
Comparative Example 1 (Half-Cell Evaluation)
<Preparation of Carbon Precursor>
[0172] In a crucible, 100 g of coconut shell chips of about 5 mm
square from Mindanao Island in the Philippines was placed. With the
use of the KTF1100 Furnace (inner diameter of 70 mm.PHI.) available
from Koyo Thermo Systems Co., Ltd., under a stream of nitrogen with
an oxygen content of 15 ppm at a flow rate of 3 L/minute (0.012
meter/second), the temperature of the crucible was elevated to
500.degree. C. at 10.degree. C./minute, maintained for 60 minutes,
and then, cooled over a period of 6 hours. The crucible was taken
out thereof at 50.degree. C. or less to obtain a carbonized
product.
[0173] The obtained carbonized product was coarsely pulverized so
as to have an average particle diameter of 10 .mu.m using a ball
mill, and then, pulverized using a compact jet mill (available from
SEISHIN ENTERPRISE Co., Ltd., Co-Jet system .alpha.-mk III) and
classified to obtain a carbon precursor with an average particle
diameter of 9.0 .mu.m.
[0174] The operation, in which 20 g of the carbon precursor thus
obtained was immersed for 1 hour in 100 g of a 35% by mass aqueous
hydrochloric acid solution and then washed for 1 hour with water at
80.degree. C., was performed two times to perform demineralization.
The demineralized coconut shell was dried for 24 hours at
80.degree. C. under a vacuum of 1 Torr.
<Preparation of Hardly Graphitizable Carbonaceous
Material>
[0175] Nine point one grams of the carbon precursor prepared as
above and 0.9 g of polystyrene (available from SEKISUI PLASTICS
CO., Ltd., the average particle diameter of 400 .mu.m, the residual
carbon ratio of 1.2%) were mixed. In a graphite-made sheath (100 mm
long, 100 mm wide, and 50 mm high), 10 g of this mixture was
placed. The temperature of the graphite-made sheath was elevated to
1270.degree. C. at a temperature increasing rate of 60.degree. C.
per minute under a stream of nitrogen at a flow rate of 5 L per
minute in a high-speed temperature rising furnace available from
MOTOYAMA K.K., and then, maintained for 11 minutes, and allowed to
spontaneously cool. After the drop of the internal temperature of
the furnace to 200.degree. C. or less was confirmed, a hardly
graphitizable carbonaceous material was taken out of the furnace.
The amount of the recovered hardly graphitizable carbonaceous
material was determined to be 7.6 g and the recovery rate from the
carbon precursor was determined to be 84%.
<Half-Cell Evaluation>
[0176] Ninety four parts by mass of the hardly graphitizable
carbonaceous material prepared as above, 6 parts by mass of PVDF
(polyvinylidene fluoride), and 90 parts by mass of NMP
(N-methylpyrrolidone) were mixed to obtain slurry. The obtained
slurry was applied onto a sheet of copper foil with a thickness of
14 .mu.m, dried, and then, pressed to obtain an electrode with a
thickness of 60 to 70 .mu.m. The obtained electrode was determined
to have a density of 0.9 to 1.1 g/cm.sup.3.
[0177] The electrode corresponding to a negative electrode was used
as a working electrode, and metallic lithium was used as a counter
electrode and a reference electrode. Ethylene carbonate and
methylethyl carbonate were mixed at a volume ratio of 3:7, and the
obtained mixture was used as a solvent. LiPF.sub.6 was dissolved at
a concentration of 1 mol/L in this solvent, and the obtained
solution was used as an electrolyte. A sheet of glass fiber
nonwoven fabric was used as a separator. In a glove box, a coin
cell was prepared under an argon atmosphere.
[0178] With the use of a charge-discharge testing device ("TOSCAT"
available from TOYO SYSTEM CO., LTD.), an anode half cell of the
above-mentioned constitution was subjected to a charge-discharge
test. Lithium doping was performed at a rate of 30 mA/g relative to
the mass of the active material until a predetermined capacity (450
to 600 mAh/g), at which no metallic lithium was precipitated, was
attained, and the doping was completed. The capacity (mAh/g)
attained at this time was defined as the charge capacity. Then,
dedoping was performed at a rate of 30 mA/g relative to the mass of
the active material until the potential becomes 1.5 V relative to
the lithium potential, and the capacity discharged at this time was
defined as the discharge capacity. The percentage of the discharge
capacity/the charge capacity was defined as the charge-discharge
efficiency (initial charge-discharge efficiency) and defined as an
index of the utilization efficiency of lithium ions in a battery.
In this context, "a predetermined capacity at which no metallic
lithium was precipitated" refers to an upper limit charge capacity
(mAh/g) at which no precipitation of metallic lithium was observed
by Li-NMR.
[0179] The hardly graphitizable carbonaceous material was subjected
to .sup.7Li nuclear-solid state NMR analysis, and measured for the
oxygen element content, the average face-to-face dimension
d.sub.002 of the (002) face, the specific surface area, the true
density, the potassium element content and iron element content,
and the moisture content.
Comparative Example 2 (Half-Cell Evaluation)
<Half-Cell Evaluation>
[0180] Ninety four parts by mass of CARBOTRON PJ available from
KUREHA CORPORATION, 6 parts by mass of PVDF (polyvinylidene
fluoride), and 90 parts by mass of NMP (N-methylpyrrolidone) were
mixed to obtain slurry. The obtained slurry was applied onto a
sheet of copper foil with a thickness of 14 .mu.m, dried, and then,
pressed to obtain an electrode with a thickness of 60 to 70 .mu.m.
The obtained electrode was determined to have a density of 0.9 to
1.1 g/cm.sup.3.
[0181] The electrode corresponding to a negative electrode was used
as a working electrode, and metallic lithium was used as a counter
electrode and a reference electrode. Ethylene carbonate and
methylethyl carbonate were mixed at a volume ratio of 3:7, and the
obtained mixture was used as a solvent. LiPF.sub.6 was dissolved at
a concentration of 1 mol/L in this solvent, and the obtained
solution was used as an electrolyte. A sheet of glass fiber
nonwoven fabric was used as a separator. In a glove box, a coin
cell was prepared under an argon atmosphere.
[0182] With the use of a charge-discharge testing device ("TOSCAT"
available from TOYO SYSTEM CO., LTD.), an anode half cell of the
above-mentioned constitution was subjected to a charge-discharge
test. Lithium doping was performed at a rate of 30 mA/g relative to
the mass of the active material until a predetermined capacity (450
to 600 mAh/g), at which no metallic lithium was precipitated, was
attained, and the doping was completed. The capacity (mAh/g)
attained at this time was defined as the charge capacity. Then,
dedoping was performed at a rate of 30 mA/g relative to the mass of
the active material until the potential becomes 1.5 V relative to
the lithium potential, and the capacity discharged at this time was
defined as the discharge capacity. The percentage of the discharge
capacity/the charge capacity was defined as the charge-discharge
efficiency (initial charge-discharge efficiency) and defined as an
index of the utilization efficiency of lithium ions in a battery.
In this context, "a predetermined capacity at which no metallic
lithium was precipitated" refers to an upper limit charge capacity
(mAh/g) at which no precipitation of metallic lithium was observed
by Li-NMR.
[0183] The hardly graphitizable carbonaceous material was subjected
to .sup.7Li nuclear-solid state NMR analysis, and measured for the
oxygen element content, the average face-to-face dimension
d.sub.002 of the (002) face, the specific surface area, the true
density, the potassium element content and iron element content,
and the moisture content. The results are collected in Table 1 and
the .sup.7Li nuclear-solid state NMR spectrum is shown in FIG.
2.
TABLE-US-00001 TABLE 1 Oxygen Average element face-to-face Specific
content dimension surface True Acid treatment Carboni- Acid
treatment Calci- (% by d.sub.002 area density Raw materials
(Demineralization) zation (Demineralization) nation mass) (nm)
(m.sup.2/g) (g/cm.sup.3) Example 1 Coconut shell Liquid phase
~500.degree. C. -- 1270.degree. C. 0.21 0.387 5.1 1.50 (Citric
acid) 2 Coconut shell -- ~500.degree. C. Vapor phase 1320.degree.
C. 0.11 0.389 3.8 1.47 (870.degree. C.) 3 Coconut shell --
~500.degree. C. Vapor phase 1320.degree. C. 0.11 0.389 3.8 1.47
(870.degree. C.) Comparative 1 Coconut shell -- ~500.degree. C. 35%
by mass 1270.degree. C. 0.31 0.388 13 1.48 Example HCl 2 CARBOTRON
-- -- -- -- 0.34 0.381 4.8 1.52 PJ Main Potassium Iron resonance
element element peak position Charge- content content Moisture of
chemical Dis- Irreversible discharge (% by (% by content shift
value Charging charging capacity efficiency mass) mass) (ppm) (ppm)
(mAh/g) (mAh/g) (mAh/g) (%) Example 1 0.0023 0.0020 7200 123 600
500 100 83.3 2 0.0020 0.0018 3100 126 589 491 98 83.4 3 0.0020
0.0018 3100 122 586 488 98 83.3 Comparative 1 0.3200 0.0240 12400
107 560 461 99 82.3 Example 2 0.0010 0.0012 10200 105 520 427 93
82.1
INDUSTRIAL APPLICABILITY
[0184] A nonaqueous electrolyte secondary battery comprising the
hardly graphitizable carbonaceous material according to the present
invention and being fully charged to be used has not only extremely
high charge capacity but also extremely high charge-discharge
efficiency. Accordingly, the nonaqueous electrolyte secondary
battery can be used especially in the field of on-vehicle batteries
for a vehicle such as a hybrid vehicle (HEY), an electric vehicle
(EV), and the like.
* * * * *