U.S. patent application number 17/556384 was filed with the patent office on 2022-04-14 for graphene oxide, positive electrode for nonaqueous secondary battery using graphene oxide, method of manufacturing positive electrode for nonaqueous secondary battery, nonaqueous secondary battery, and electronic device.
The applicant listed for this patent is SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Tatsuya IKENUMA, Yumiko SAITO, Hiroatsu TODORIKI, Masaki YAMAKAJI, Rika YATABE, Mikio YUKAWA.
Application Number | 20220115668 17/556384 |
Document ID | / |
Family ID | |
Filed Date | 2022-04-14 |
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United States Patent
Application |
20220115668 |
Kind Code |
A1 |
TODORIKI; Hiroatsu ; et
al. |
April 14, 2022 |
GRAPHENE OXIDE, POSITIVE ELECTRODE FOR NONAQUEOUS SECONDARY BATTERY
USING GRAPHENE OXIDE, METHOD OF MANUFACTURING POSITIVE ELECTRODE
FOR NONAQUEOUS SECONDARY BATTERY, NONAQUEOUS SECONDARY BATTERY, AND
ELECTRONIC DEVICE
Abstract
A graphene oxide used as a raw material of a conductive additive
for forming an active material layer with high electron
conductivity with a small amount of a conductive additive is
provided. A positive electrode for a nonaqueous secondary battery
using the graphene oxide as a conductive additive is provided. The
graphene oxide is used as a raw material of a conductive additive
in a positive electrode for a nonaqueous secondary battery and, in
the graphene oxide, the atomic ratio of oxygen to carbon is greater
than or equal to 0.405.
Inventors: |
TODORIKI; Hiroatsu;
(Azumino, JP) ; YUKAWA; Mikio; (Atsugi, JP)
; SAITO; Yumiko; (Atsugi, JP) ; YAMAKAJI;
Masaki; (Atsugi, JP) ; YATABE; Rika; (Atsugi,
JP) ; IKENUMA; Tatsuya; (Atsugi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMICONDUCTOR ENERGY LABORATORY CO., LTD. |
Atsugi-shi |
|
JP |
|
|
Appl. No.: |
17/556384 |
Filed: |
December 20, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16986372 |
Aug 6, 2020 |
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17556384 |
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16170140 |
Oct 25, 2018 |
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16986372 |
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15075387 |
Mar 21, 2016 |
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16170140 |
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13826710 |
Mar 14, 2013 |
9293770 |
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15075387 |
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International
Class: |
H01M 4/62 20060101
H01M004/62; H01M 4/36 20060101 H01M004/36; H01M 10/052 20060101
H01M010/052; H01M 4/136 20060101 H01M004/136; H01M 4/58 20060101
H01M004/58; C01B 32/23 20060101 C01B032/23 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2012 |
JP |
2012-089346 |
May 31, 2012 |
JP |
2012-125138 |
Claims
1. (canceled)
2. A method for manufacturing a positive electrode for a nonaqueous
secondary battery comprising multilayer graphene, comprising the
step of: adding a positive electrode active material to a
dispersion medium to form a first mixture; kneading the first
mixture to form a second mixture; adding a binder to the second
mixture; applying a paste adding the binder on a positive electrode
current collector; and heating the positive electrode current
collector applying the paste at a temperature higher than or equal
to 130.degree. C. and lower than or equal to 200.degree. C.,
wherein the dispersion medium comprising carbon material, and
wherein the binder is configured to bind the positive electrode
active material and the multilayer graphene.
3. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 2, wherein graphene oxide is dispersed in the
dispersion medium before adding the positive electrode active
material, and wherein the graphene oxide is reduced by the step of
heating and the multilayer graphene is formed.
4. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 2, wherein a length of one side of the
multilayer graphene is greater or equal to 50 nm and less than or
equal to 100 .mu.m.
5. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 2, wherein a length of one side of the
multilayer graphene is larger than an average particle diameter of
the positive electrode active material.
6. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 2, wherein the binder comprises any one of
polyvinylidene fluoride, polyimide, poly-tetrafluoroethylene,
polyvinyl chloride, ethylene-propylenediene polymer,
styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluorine
rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene,
and nitrocellulose.
7. A method for manufacturing a positive electrode for a nonaqueous
secondary battery comprising multilayer graphene and a composite
oxide containing nickel, manganese, and cobalt, comprising the step
of: adding a positive electrode active material comprising the
composite oxide to a dispersion medium to form a first mixture;
kneading the first mixture to form a second mixture; adding a
binder to the second mixture; applying a paste adding the binder on
a positive electrode current collector; and heating the positive
electrode current collector applying the paste at a temperature
higher than or equal to 130.degree. C. and lower than or equal to
200.degree. C., wherein the dispersion medium comprising carbon
material, and wherein the binder is configured to bind the positive
electrode active material and the multilayer graphene.
8. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 7, wherein graphene oxide is dispersed in the
dispersion medium before adding the positive electrode active
material, and wherein the graphene oxide is reduced by the step of
heating and the multilayer graphene is formed.
9. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 7, wherein a length of one side of the
multilayer graphene is greater or equal to 50 nm and less than or
equal to 100 .mu.m.
10. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 7, wherein a length of one side of the
multilayer graphene is larger than an average particle diameter of
the positive electrode active material.
11. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 7, wherein the binder comprises any one of
polyvinylidene fluoride, polyimide, poly-tetrafluoroethylene,
polyvinyl chloride, ethylene-propylenediene polymer,
styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluorine
rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene,
and nitrocellulose.
12. A method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene and a
composite oxide containing nickel, manganese, and cobalt,
comprising the step of: adding a positive electrode active material
comprising the composite oxide to a dispersion medium to form a
first mixture; kneading the first mixture to form a second mixture;
adding a binder to the second mixture; applying a paste adding the
binder on a positive electrode current collector; and heating the
positive electrode current collector applying the paste at a
temperature higher than or equal to 130.degree. C. and lower than
or equal to 200.degree. C., wherein the dispersion medium
comprising carbon material, wherein the positive electrode active
material is added at greater than or equal to 85 wt % and less than
or equal to 93 wt % with respect to total weight of the paste.
wherein the binder is configured to bind the positive electrode
active material and the multilayer graphene.
13. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 12, wherein graphene oxide is dispersed in the
dispersion medium before adding the positive electrode active
material, and wherein the graphene oxide is reduced by the step of
heating and the multilayer graphene is formed.
14. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 12, wherein a length of one side of the
multilayer graphene is greater or equal to 50 nm and less than or
equal to 100 .mu.m.
15. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 12, wherein a length of one side of the
multilayer graphene is larger than an average particle diameter of
the positive electrode active material.
16. The method for manufacturing a positive electrode for a
nonaqueous secondary battery comprising multilayer graphene,
according to claim 12, wherein the binder comprises any one of
polyvinylidene fluoride, polyimide, poly-tetrafluoroethylene,
polyvinyl chloride, ethylene-propylenediene polymer,
styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluorine
rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene,
and nitrocellulose.
Description
TECHNICAL FIELD
[0001] The present invention relates to a graphene oxide, a
positive electrode for a nonaqueous secondary battery which uses
the graphene oxide, a method of manufacturing the positive
electrode for a nonaqueous secondary battery, a nonaqueous
secondary battery, and electronic deices.
BACKGROUND ART
[0002] With the recent rapid spread of portable electronic devices
such as cellular phones, smartphones, electronic books, and
portable game machines, secondary batteries for drive power supply
have been increasingly required to be smaller and to have higher
capacity. Nonaqueous secondary batteries typified by lithium
secondary batteries, which have advantages such as high energy
density and high capacity, have been widely used as secondary
batteries used for portable electronic devices.
[0003] A lithium secondary battery, which is one of nonaqueous
secondary batteries and widely used due to its high energy density,
includes a positive electrode including an active material such as
lithium cobalt oxide (LiCoO.sub.2) or lithium iron phosphate
(LiFePO.sub.4), a negative electrode formed of a carbon material
such as graphite which is capable of occlusion and release of
lithium ions, a nonaqueous electrolyte in which an electrolyte
formed of a lithium salt such as LiBF.sub.4 or LiPF.sub.6, and the
like is dispersed in an organic solvent such as ethylene carbonate
or diethyl carbonate, and the like. A lithium secondary battery are
charged and discharged in such a way that lithium ions in the
secondary battery are transferred between the positive electrode
and the negative electrode through the nonaqueous electrolyte and
intercalated into or deintercalated from the active materials of
the positive electrode and the negative electrode.
[0004] Into the positive electrode or the negative electrode, a
binding agent (also referred to as a binder) is mixed in order that
active materials can be bound or an active material and a current
collector can be bound. Since the binding agent is generally an
organic high molecular compound such as polyvinylidene fluoride
(PVDF) which has an insulating property, the electron conductivity
of the binding agent is extremely low. Therefore, as the ratio of
the mixed binding agent to the active material is increased, the
amount of the active material in the electrode is relatively
decreased, resulting in the lower discharge capacity of the
secondary battery.
[0005] Hence, by mixture of a conductive additive such as acetylene
black (AB) or a graphite particle, the electron conductivity
between active materials or between an active material and a
current collector can be improved. Thus, a positive electrode
active material with high electron conductivity can be provided
(see Patent Document 1).
REFERENCE
Patent Document 1: Japanese Published Patent Application No.
2002-110162
DISCLOSURE OF INVENTION
[0006] However, because acetylene black used as a conductive
additive is a high-volume particle with an average particle
diameter of several tens of nanometers to several hundreds of
nanometers, contact between acetylene black and an active material
hardly becomes surface contact and tends to be point contact.
Consequently, contact resistance between the active material and
the conductive additive is high. Further, if the amount of the
conductive additive is increased so as to increase contact points
between the active material and the conductive additive, the
proportion of the amount of the active material in the electrode
decreases, resulting in the lower discharge capacity of the
battery.
[0007] In the case where graphite particles are used as a
conductive additive, natural graphite is generally used in
consideration of cost. In this case, iron, lead, copper, or the
like contained as impurities in a graphite particle reacts with the
active material or the current collector, which might reduce the
potential or capacity of the battery.
[0008] Further, as particles of the active material become minuter,
cohesion between the particles becomes stronger, which makes
uniform dispersion in the binding agent or the conductive additive
difficult. Consequently, a portion where active material particles
are aggregated and densely present and a portion where active
material particles are not aggregated and thinly present are
locally generated. In the portion where active material particles
are aggregated and to which the conductive additive is not mixed,
the active material particles do not contribute to formation of the
discharge capacity of the battery.
[0009] Therefore, in view of the foregoing problems, an object of
one embodiment of the present invention is to provide a graphene
oxide which is a raw material of a conductive additive used for an
active material layer which achieves high electron conductivity
with a small amount of a conductive additive, Another object is to
provide, with a small amount of a conductive additive, a positive
electrode for a nonaqueous secondary battery which is highly filled
and includes a high-density positive electrode active material
layer. Another object is to provide, using the positive electrode
for a nonaqueous secondary battery, a nonaqueous secondary battery
having high capacity per electrode volume.
[0010] A positive electrode for a nonaqueous secondary battery in
accordance with one embodiment of the present invention includes a
graphene as a conductive additive included in a positive electrode
active material layer.
[0011] A graphene is a carbon material having a crystal structure
in which hexagonal skeletons of carbon are spread in a planar form
and is one atomic plane extracted from graphite crystals. Due to
its electrical, mechanical, or chemical characteristics which are
surprisingly excellent, the grapheme has been expected to be
applied to a variety of fields of, for example, field-effect
transistors with high mobility, highly sensitive sensors,
highly-efficient solar cells, and next-generation transparent
conductive films and has attracted a great deal of attention.
[0012] In this specification, the term graphene includes a
single-layer graphene and multilayer graphenes including two to
hundred layers. The single-layer graphene refers to a sheet of one
atomic layer of carbon molecules having .pi. bonds. The graphene
oxide refers to a compound formed by oxidation of such a grapheme.
Note that when a graphene oxide is reduced to form a graphene,
oxygen contained in the grapheme oxide is not entirely released and
part of oxygen remains in the graphene. When the graphene contains
oxygen, the proportion of oxygen is greater than or equal to 2
atomic % and less than or equal to 20 atomic %, preferably greater
than or equal to 3 atomic % and less than or equal to 15 atomic
%.
[0013] In the case where the graphene is multilayer graphenes
including the graphene obtained by reducing the graphene oxide, the
interlayer distance between the graphenes is greater than or equal
to 0.34 nm and less than or equal to 0.5 nm, preferably greater
than or equal to 0.38 nm and less than or equal to 0.42 nm, more
preferably greater than or equal to 0.39 nm and less than or equal
to 0.41 nm. In general graphite, the interlayer distance between
single-layer graphemes is 0.34 nm. Since the interlayer distance
between the graphenes used for the secondary battery of one
embodiment of the present invention is longer than that in general
graphite, carrier ions can easily transfer between the graphenes in
multilayer graphenes.
[0014] In a positive electrode for a nonaqueous secondary battery
in accordance with one embodiment of the present invention,
graphenes are overlapped with each other in a positive electrode
active material layer and dispersed so as to be in contact with a
plurality of positive electrode active material particles. In other
words, a network for electron conduction is formed by the graphenes
in a positive electrode active material layer. This maintains bonds
between the plurality of positive electrode active material
particles, which enables a positive electrode active material layer
with high electron conductivity to be formed.
[0015] A positive electrode active material layer to which a
graphene is added as a conductive additive can be manufactured by
the following method. First, after the graphene is dispersed into a
dispersion medium (also referred to as a solvent), a positive
electrode active material is added thereto and a mixture is
obtained by mixing. A binding agent (also referred to as a binder)
is added to this mixture and mixing is performed, so that a
positive electrode paste is formed. Lastly, after the positive
electrode paste is applied on a positive electrode current
collector, the dispersion medium is volatilized. Thus, the positive
electrode active material layer to which the graphene is added as a
conductive additive is manufactured.
[0016] In order that a network for electron conduction can be
formed in a positive electrode active material layer with use of
the graphene as a conductive additive, the graphene needs to be
uniformly dispersed in the dispersion medium. Dispersibility in a
dispersion medium directly depends on the dispersibility of the
graphene in a positive electrode active material layer. When the
dispersibility of the graphene is low, the graphene is aggregated
in the positive electrode active material layer and localized,
which prevents formation of the network. Thus the dispersibility of
the graphene used as a conductive additive in a dispersion medium
is an extremely important factor in the improvement of the electron
conductivity of the positive electrode active material layer.
[0017] By examining a positive electrode active material layer
formed in such a way that a graphene as a conductive additive was
put in a dispersion medium together with an active material and a
binding agent, the present inventors found that dispersibility was
insufficient and a network for electron conduction was not formed
in the positive electrode active material layer. The inventors
found the same results by examining a positive electrode active
material layer formed in such a way that, instead of a graphene, a
graphene formed by reduction of a graphene oxide (hereinafter,
referred to as an RGO (an abbreviation of reduced graphene oxide))
was put as a conductive additive in a dispersion medium.
[0018] In contrast, the present inventors have found that excellent
electron conductivity is achieved by formation of a network for
electron conduction in a positive electrode active material layer
obtained in such a way that, after a graphene oxide (also referred
to as a GO) as a conductive additive is put in a dispersion medium
together with an active material and a binding agent to form a
positive electrode paste, the dispersed graphene oxide is reduced
by heat treatment to form a graphene.
[0019] Thus, while dispersibility is low in a positive electrode
active material layer in which a graphene or a RGO is dispersed as
a raw material of a conductive additive, high dispersibility is
achieved with a graphene formed by reduction performed after a
graphene oxide is added to form a positive electrode paste.
[0020] Such a difference in the dispersibility in an active
material layer between the graphene or RGO and the graphene formed
by reduction performed after a positive electrode paste including a
graphene oxide is formed can be explained below as a difference in
the dispersibility in a dispersion medium.
[0021] FIG. 1A illustrates a structural formula of NMP (also
referred to as N-methylpyrrolidone, 1-methyl-2-pyrrolidone,
N-methyl-2-pyrrolidone, or the like), which is a typical dispersion
medium. An NMP 100 is a compound having a five-membered ring
structure and is one of polar solvents. As illustrated in FIG. 1A,
in the NW, oxygen is electrically negatively biased and carbon
forming a double bond with the oxygen is electrically positively
biased. A graphene, an RGO, or a graphene oxide is added to a
diluent solvent having such a polarity.
[0022] The graphene is a crystal structure body of carbon in which
hexagonal skeletons are spread in a planar form as already
described, and does not substantially include a functional group in
the structure body. Further, the RGO is formed by reduction of
functional groups originally included in the RGO by heat treatment,
and the proportion of functional groups in the structure body is as
low as about 10 wt %. Consequently, as illustrated in FIG. 1B, a
surface of a graphene or RGO 101 does not have polarity and
therefore has hydrophobicity. Therefore it is considered that,
while interaction between the NMP 100 which is a dispersion medium
and the graphene or RGO 101 is extremely weak, interaction occurs
between the graphenes or RGOs 101 to cause aggregation of the
graphenes or RGOs 101 (see FIG. 1C).
[0023] A graphene oxide 102 is a polar substance having a
functional group such as an epoxy group, a carbonyl group, a
carboxyl group, or a hydroxyl group. Oxygen in the functional group
in the graphene oxide 102 is negatively charged; hence, graphene
oxides hardly aggregate in a polar solvent but strongly interact
with the NMP 100 which is a polar solvent (see FIG. 2A). Thus, as
illustrated in. FIG. 2B, the functional group such as an epoxy
group included in the graphene oxide 102 interacts with a polar
solvent, which inhibits aggregation among graphene oxides;
consequently, the graphene oxide 102 is considered to be uniformly
dispersed in a dispersion medium (see FIG. 2B).
[0024] In view of the foregoing, in order that a network with high
electron conductivity be formed in a positive electrode active
material layer by using the graphene as a conductive additive, use
of the graphene oxide with high dispersibility in a dispersion
medium in manufacture of a positive electrode paste is very
effective. The dispersibility of the graphene oxide in a dispersion
medium is considered to depend on the quantity of functional groups
having oxygen such as an epoxy group (i.e., the degree of oxidation
of the graphene oxide).
[0025] One embodiment of the present invention is a graphene oxide
used as a raw material of a conductive additive in a positive
electrode for a nonaqueous secondary battery. In the graphene
oxide, the atomic ratio of oxygen to carbon is greater than or
equal to 0.405.
[0026] Here, the atomic ratio of oxygen to carbon is an indicator
of the degree of oxidation and represents the atomic of oxygen
which is a constituent element of the graphene oxide as a
proportion with respect to the atomic of carbon which is a
constituent element of the graphene oxide. Note that the atomic of
elements included in the graphene oxide can be measured by X-ray
photoelectron spectroscopy (XPS), for example.
[0027] The atomic ratio of oxygen to carbon in the graphene oxide
which is greater than or equal to 0.405 means that the graphene
oxide is a polar substance in which functional groups such as an
epoxy group, a carbonyl group, a carboxyl group, or a hydroxyl
group are sufficiently bonded to the graphene oxide for the high
dispersibility of the graphene oxide in a polar solvent.
[0028] The graphene oxide in which the atomic ratio of oxygen to
carbon is greater than or equal to 0.405 is dispersed in a
dispersion medium together with a positive electrode active
material and a binding agent, the mixture is mixed, the mixture is
applied on a positive electrode current collector, and heating are
performed. Thus, a positive electrode for a nonaqueous secondary
battery which includes a graphene with high dispersibility and a
network for electron conduction can be formed.
[0029] The length of one side of the graphene oxide is preferably
greater than or equal to 50 nm and less than or equal to 100 .mu.m,
more preferably greater than or equal to 800 nm and less than or
equal to 20 .mu.m.
[0030] Another embodiment of the present invention is a positive
electrode for a nonaqueous secondary battery which includes a
positive electrode active material layer including a plurality of
positive electrode active material particles, a conductive additive
including a plurality of graphenes, and a binding agent over a
positive electrode current collector. Each of the graphenes is
larger than an average particle diameter of each of the positive
electrode active material particles. Each of the graphenes is
dispersed in the positive electrode active material layer such that
the graphene makes surface contact with one or more graphenes
adjacent to the graphene. The graphenes make surface contact in
such a way as to wrap part of surfaces of the positive electrode
active material particles.
[0031] As already described, the graphene oxides are structure
bodies having functional groups including oxygen and therefore do
not aggregate and are uniformly dispersed in a polar solvent such
as NMP. The dispersed graphene oxides uniformly mix with the
plurality of positive electrode active material particles. Thus,
the graphenes, which are formed from the graphene oxide by
volatilization of the dispersion medium and reduction treatment of
the graphene oxide, are dispersed in the positive electrode active
material layer such that the graphenes make surface contact with
each other. Since the graphene has a sheet-like shape and partial
surface contact between the graphenes achieves electrical
connection, a network for electron conduction is considered to be
formed when some graphenes are viewed as one set. Further, the
surface contact between the graphenes can keep contact resistance
low, which leads to the formation of the network with high electron
conductivity.
[0032] Further, since the graphene is a sheet whose side has a
length greater than or equal to 50 nm and less than or equal to 100
.mu.m, preferably greater than or equal to 800 nm and less than or
equal to 20 .mu.m, which is larger than an average particle
diameter of the positive electrode active material particles, the
graphene in the form of a sheet can be connected to the plurality
of positive electrode active material particles. In particular,
since the graphene has a sheet-like shape, surface contact can be
made in such a way as to wrap the surfaces of the positive
electrode active material particles. Accordingly, without an
increase in the amount of conductive additive, contact resistance
between the positive electrode active material particles and the
graphenes can be reduced.
[0033] Note that as the positive electrode active material
particles, a material capable of inserting and extracting of
carrier ions, such as lithium iron phosphate, can be used.
[0034] Another embodiment of the present invention is a positive
electrode for a nonaqueous secondary battery which includes a
positive electrode active material layer including a plurality of
positive electrode active material particles, a conductive additive
including a plurality of graphenes, and a binding agent over a
positive electrode current collector. As bonding states of carbon
included in the positive electrode active material layer, the
proportion of a C.dbd.C bond is greater than or equal to 35% and
the proportion of a C--O bond is greater than or equal to 5% and
less than or equal to 20%.
[0035] Another embodiment of the present invention is a method of
manufacturing a positive electrode for a nonaqueous secondary
battery, which includes the steps of: dispersing a graphene oxide
in which the atomic ratio of oxygen to carbon is greater than or
equal to 0.405 into a dispersion medium; adding a positive
electrode active material to the dispersion medium into which the
graphene oxide is dispersed and performing mixing to form a
mixture; adding a binding agent to the mixture and performing
mixing to form a positive electrode paste; applying the positive
electrode paste on a positive electrode current collector; and
reducing the graphene oxide after or at the same time when the
dispersion medium included in the applied positive electrode paste
is volatilized, whereby a positive electrode active material layer
including the graphene is formed over the positive electrode
current collector.
[0036] The length of one side of each of the graphene oxide and the
graphene is preferably greater than or equal to 50 nm and less than
or equal to 100 .mu.m, more preferably greater than or equal to 800
nm and less than or equal to 20 .mu.m.
[0037] In the above manufacturing method, the positive electrode
paste is dried under a reducing atmosphere or reduced pressure.
This enables the dispersion medium included in the positive
electrode paste to be volatilized and the graphene oxide included
in the positive electrode paste to be reduced.
[0038] In the above manufacturing method, by further addition of a
dispersion medium at the time when the binding agent is added to
the mixture and mixing is performed, the viscosity of the positive
electrode paste can be adjusted.
[0039] The positive electrode active material is added to the
dispersion medium in which the graphene oxide with an atomic ratio
of oxygen to carbon greater than or equal to 0.405 is dispersed.
The resulting substance is mixed, so that the positive electrode
active material layer with high dispersibility of the graphene is
formed. The graphene oxide can be included at least at 2 wt % with
respect to the total weight of the positive electrode paste which
is a mixture of the positive electrode active material, the
conductive additive, and the binding agent. Further, the graphene
obtained after the positive electrode paste is applied on the
current collector and reduction is performed can be included at
least at 1 wt % with respect to the total weight of the positive
electrode active material layer. This is because the weight of the
graphene is reduced by almost half due to the reduction of the
graphene oxide.
[0040] Specifically, it is preferable that, in the state of the
positive electrode paste, the graphene oxide be added at greater
than or equal to 2 wt % and less than or equal to 10 wt %, the
positive electrode active material be added at greater than or
equal to 85 wt % and less than or equal to 93 wt %, and the binding
agent be added at greater than or equal to 1 wt % and less than or
equal to 5 wt %, with respect to the total weight of the positive
electrode paste. Further, it is preferable that, in the state of
the positive electrode active material layer obtained by applying
the positive electrode paste on the current collector and reducing
the graphene oxide, the graphene be added at greater than or equal
to 1 wt % and less than or equal to 5 wt %, the positive electrode
active material be added at greater than or equal to 90 wt % and
less than or equal to 94 wt %, and the binding agent be added at
greater than or equal to 1 wt % and less than or equal to 5 wt %,
with respect to the total weight of the positive electrode active
material layer.
[0041] After the positive electrode paste is applied on the
positive electrode current collector, oxygen is released from the
graphene oxide by drying under a reducing atmosphere or reduced
pressure, so that the positive electrode active material layer
including the graphene can be formed. Note that oxygen included in
the graphene oxide is not entirely released and may partly remain
in the graphene.
[0042] When the graphene includes oxygen, the proportion of oxygen
is greater than or equal to 2 atomic % and less than or equal to 20
atomic %, preferably greater than or equal to 3 atomic % and less
than or equal to 15 atomic %. As the proportion of oxygen is lower,
the conductivity of the graphene can be higher, so that a network
with high electron conductivity can be formed. As the proportion of
oxygen is higher, more openings serving as paths of ions can be
formed in the graphene.
[0043] By using the positive electrode formed in the
above-described manner, a negative electrode, an electrolyte
solution, and a separator, a nonaqueous secondary battery can be
manufactured.
[0044] A graphene oxide which is a raw material of a conductive
additive used for an active material layer which achieves high
electron conductivity can be provided with a small amount of a
conductive additive.
[0045] By using the graphene oxide as a raw material of a
conductive additive, a positive electrode for a nonaqueous
secondary battery including a positive electrode active material
layer which can achieve high electron conductivity can be provided
with a small amount of a conductive additive. A high-density
positive electrode for a nonaqueous secondary battery which
includes a positive electrode active material layer which is highly
filled can be provided with a small amount of a conductive
additive.
[0046] By using the positive electrode for a nonaqueous secondary
battery, a nonaqueous secondary battery having high capacity per
electrode volume can be provided.
BRIEF DESCRIPTION OF DRAWINGS
[0047] In the accompanying drawings:
[0048] FIGS. 1A to 1C each illustrate a dispersion state in a polar
solvent;
[0049] FIGS. 2A and 2B each illustrate a dispersion state in a
polar solvent;
[0050] FIGS. 3A to 3C illustrate a positive electrode;
[0051] FIG. 4 is a flow chart illustrating a method of forming a
positive electrode;
[0052] FIGS. 5A and 5B illustrate a coin-type secondary
battery;
[0053] FIGS. 6A and 6B illustrate an electrophoresis method and an
electrochemical reduction method, respectively;
[0054] FIG. 7 illustrates a laminated secondary battery;
[0055] FIGS. 8A and 8B illustrate a cylindrical secondary
battery;
[0056] FIG. 9 illustrates electronic devices;
[0057] FIGS. 10A to 10C illustrate an electronic device;
[0058] FIGS. 11A and 11B illustrate an electronic device;
[0059] FIG. 12 shows comparison between charge-discharge
characteristics;
[0060] FIGS. 13A and 13B show charge-discharge characteristics of a
cell A and a cell B;
[0061] FIGS. 14A and 14B are SEM images of a positive electrode
active material layer using a graphene oxide as a raw material of a
conductive additive;
[0062] FIG. 15 is a SEM image of a positive electrode active
material layer using a graphene oxide as a raw material of a
conductive additive;
[0063] FIGS. 16A and 16B are SEM images of a positive electrode
active material layer using a graphene oxide as a raw material of a
conductive additive;
[0064] FIGS. 17A and 17B are SEM images of a positive electrode
active material layer using a RGO as a raw material of a conductive
additive;
[0065] FIGS. 18A and 18B are SEM images of a positive electrode
active material layer using a graphene as a raw material of a
conductive additive;
[0066] FIG. 19 illustrates a positive electrode;
[0067] FIGS. 20A and 20B are SEM images of a positive electrode
active material layer using a graphene oxide as a raw material of a
conductive additive;
[0068] FIG. 21 is a SEM image of a positive electrode active
material layer using a graphene oxide as a raw material of a
conductive additive; and
[0069] FIG. 22 is a SEM image of a positive electrode active
material layer using a graphene oxide as a raw material of a
conductive additive.
BEST MODE FOR CARRYING OUT THE INVENTION
[0070] Hereinafter, embodiments will be described with reference to
the accompanying drawings. However, the embodiments can be
implemented in many different modes, and it will be readily
appreciated by those skilled in the art that modes and details
thereof can be changed in various ways without departing from the
spirit and scope of the present invention. Thus, the present
invention should not be interpreted as being limited to the
following description of the embodiments.
[0071] Note that in each drawing described in this specification,
the size, the film thickness, or the region of each component is
exaggerated for clarity in some cases. Therefore, embodiments of
the present invention are not limited to such scales in the
drawings.
Embodiment 1
[0072] In this embodiment, a positive electrode for a nonaqueous
secondary battery in accordance with one embodiment of the present
invention is described with reference to FIGS. 3A to 3C and FIG.
19. FIG. 3A is a perspective view of the positive electrode, FIG.
3B is a plan view of a positive electrode active material layer,
and FIG. 3C and FIG. 19 are longitudinal sectional views of the
positive electrode active material layer.
[0073] FIG. 3A is a perspective view of a positive electrode 200.
Although the positive electrode 200 in the shape of a rectangular
sheet is illustrated in FIG. 3A, there is no limitation on the
shape of the positive electrode 200 and any appropriate shape can
be selected. The positive electrode 200 is formed in such a manner
that a positive electrode paste is applied on a positive electrode
current collector 201 and then dried under a reducing atmosphere or
reduced pressure to form a positive electrode active material layer
202. The positive electrode active material layer 202 is formed
over only one surface of the positive electrode current collector
201 in FIG. 3A but may be formed over both surfaces of the positive
electrode current collector 201. The positive electrode active
material layer 202 is not necessarily formed over the entire
surface of the positive electrode current collector 201 and a
region that is not coated, such as a region for connection to a
positive electrode tab, is provided as appropriate.
[0074] The positive electrode current collector 201 can be formed
using a material that has high conductivity and is not alloyed with
a carrier ion of lithium or the like, such as a metal typified by
stainless steel, gold, platinum, zinc, iron, copper, aluminum, or
titanium, or an alloy thereof. The positive electrode current
collector 201 can be formed using an aluminum alloy to which an
element which improves heat resistance, such as silicon, titanium,
neodymium, scandium, or molybdenum, is added. Alternatively, a
metal element which forms silicide by reacting with silicon may be
used. Examples of the metal element which forms silicide by
reacting with silicon are zirconium, titanium, hafnium, vanadium,
niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel,
and the like. The positive electrode current collector 201 can have
a foil-like shape, a plate-like shape (sheet-like shape), a
net-like shape, a punching-metal shape, an expanded-metal shape, or
the like as appropriate. The positive electrode current collector
201 preferably has a thickness greater than or equal to 10 .mu.m
and less than or equal to 30 .mu.m.
[0075] FIGS. 3B and 3C are schematic views illustrating a top view
and a longitudinal section, respectively, of the positive electrode
active material layer 202. The positive electrode active material
layer 202 includes positive electrode active material particles
203, graphenes 204 as a conductive additive, and a binding agent
(also referred to as a binder) (not shown).
[0076] The positive electrode active material particle 203 is in
the form of particles made of secondary particles having average
particle diameter or particle diameter distribution, which is
obtained in such a way that material compounds are mixed at a
predetermined ratio and baked and the resulting baked product is
crushed, granulated, and classified by an appropriate means.
Therefore the positive electrode active material particles 203 are
schematically illustrated as spheres in FIGS. 3B and 3C but this
shape does not limit the present invention.
[0077] As the positive electrode active material particle 203, a
material into/from which lithium ions can be
intercalated/deintercalated can be used; for example, a
lithium-containing composite oxide with an olivine crystal
structure, a layered rock-salt crystal structure, or a spinel
crystal structure can be used.
[0078] An example of an olivine-type lithium-containing composite
oxide is LiMPO.sub.4 (general formula) (M is one or more of Fe(II),
Mn(II), Co(II), and Ni(II)). Typical examples of LiMPO.sub.4
(general formula) are LiFePO.sub.4, LiNiPO.sub.4, LiCoPO.sub.4,
LiMnPO.sub.4, LiFe.sub.aNi.sub.bPO.sub.4,
LiFe.sub.aCo.sub.bPO.sub.4, LiFe.sub.aMn.sub.bPO.sub.4,
LiNi.sub.aCo.sub.bPO.sub.4, LiNi.sub.aMn.sub.bPO.sub.4
(a+b.ltoreq.1, 0<a<1, and 0<b<1),
LiFe.sub.cNi.sub.dCo.sub.ePO.sub.4,
LiFe.sub.cNi.sub.dMn.sub.ePO.sub.4,
LiNi.sub.cCo.sub.dMn.sub.ePO.sub.4 (c+d+e.ltoreq.1,
0<c<1,0<d<1, and 0<e<1),
LiFe.sub.fNi.sub.gCo.sub.hMn.sub.iPO.sub.4 (F+g+h+i.ltoreq.1,
0<f<1, 0<g<1, 0<h<1, and 0<i<1), and the
like.
[0079] In particular, LiFePO.sub.4 is preferable because it
properly satisfies conditions necessary for the positive electrode
active material particle, such as safety, stability, high capacity
density, high potential, and the existence of lithium ions which
can be extracted in initial oxidation (charging).
[0080] Examples of a lithium-containing composite oxide with a
layered rock-salt crystal structure are lithium cobalt oxide
(LiCoO.sub.2), LiNiO.sub.2, LiMnO.sub.2, Li.sub.2MnO.sub.3,
NiCo-containing composite oxide (general formula:
LiNi.sub.xCo.sub.1-xO.sub.2 (0<x<1)) such as
LiNi.sub.0.8Co.sub.0.2O.sub.2, NiMn-containing composite oxide
(general formula: LiNi.sub.xMn.sub.1-xO.sub.2 (0<x<1)) such
as LiNi.sub.0.5Mn.sub.0.5O.sub.2, NiMnCo-containing composite oxide
(also referred to as NMC) (general formula:
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (x>0, y>0, x+y<1))
such as LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2,
Li(Ni.sub.0.8Co.sub.0.15Al.sub.0.05)O.sub.2,
Li.sub.2MnO.sub.3--LiMO.sub.2 (M=Co, Ni, or Mn), and the like.
[0081] In particular, LiCoO.sub.2 is preferable because of its
advantages such as high capacity and stability in the air higher
than that of LiNiO.sub.2 and thermal stability higher than that of
LiNiO.sub.2.
[0082] Examples of a lithium-containing composite oxide with a
spinel crystal structure are LiMn.sub.2O.sub.4,
Li.sub.1+xMn.sub.2-xO.sub.4, Li(MnAl).sub.2O.sub.4, and
LiMn.sub.1.5Ni.sub.0.5O.sub.4, and the like.
[0083] It is preferable to add a small amount of lithium nickel
oxide (LiNiO.sub.2 or LiNi.sub.1-xMO.sub.2 (M=Co, Al, or the like))
to lithium-containing composite oxide with a spinel crystal
structure which contains manganese such as LiMn.sub.2O.sub.4
because advantages such as minimization of the elution of manganese
and the decomposition of an electrolytic solution can be
obtained.
[0084] Alternatively, a composite oxide expressed by
Li.sub.(2-j)MSiO.sub.4 (general formula) (M is one or more of
Fe(II), Mn(II), Co(II), and Ni(II), 0.ltoreq.j.ltoreq.2) can be
used as the positive electrode active material particle. Typical
examples of Li.sub.(2-j)MSiO.sub.4 (general formula) are
Li.sub.(2-j)FeSiO.sub.4, Li.sub.(2-j)NiSiO.sub.4,
Li.sub.(2-j)CoSiO.sub.4, Li.sub.(2-j)MnSiO.sub.4,
Li.sub.(2-j)Fe.sub.kNi.sub.lSiO.sub.4,
Li.sub.(2-j)Fe.sub.kCo.sub.lSiO.sub.4,
Li.sub.(2-j)Fe.sub.kMn.sub.lSiO.sub.4,
Li.sub.(2-j)Ni.sub.kCo.sub.lSiO.sub.4,
Li.sub.(2-j)Ni.sub.kMn.sub.lSiO.sub.4 (k+l.ltoreq.1, 0<k<1,
and 0<l<1), Li.sub.(2-j)Fe.sub.mNi.sub.nCo.sub.qSiO.sub.4,
Li.sub.(2-j)Fe.sub.mNi.sub.nMn.sub.qSiO.sub.4,
Li.sub.(2-j)Ni.sub.mCo.sub.nMn.sub.qSiO.sub.4 (m+n+q.ltoreq.1,
0<m<1, 0<n<1, and 0<q<1),
Li.sub.(2-j)Fe.sub.rNi.sub.sCo.sub.tMn.sub.uSiO.sub.4
(r+s+t+u.ltoreq.1, 0<r<1, 0<s<1, 0<t<1, and
0<u<1), and the like.
[0085] Still alternatively, a nasicon compound expressed by
A.sub.xM.sub.2 (XO.sub.4).sub.3 (general formula) (A=Li, Na, or Mg,
M=Fe, Mn, Ti, V, Nb, or Al, X=S, P, Mo, W, As, or Si) can be used
as the positive electrode active material particle. Examples of the
nasicon compound are Fe.sub.2(MnO.sub.4).sub.3,
Fe.sub.2(SO.sub.4).sub.3, Li.sub.3Fe.sub.2(PO.sub.4).sub.3, and the
like. Further alternatively, a compound expressed by
Li.sub.2MPO.sub.4F, Li.sub.2MP.sub.2O.sub.7, or Li.sub.5MO.sub.4
(general formula) (M=Fe or Mn), a perovskite fluoride such as
NaF.sub.3 or FeF.sub.3, a metal chalcogenide (a sulfide, a
selenide, or a telluride) such as TiS.sub.2 or MoS.sub.2, a
lithium-containing composite oxide with an inverse spinet crystal
structure such as LiMVO.sub.4, a vanadium oxide (V.sub.2O.sub.5,
V.sub.6O.sub.13, LiV.sub.3O.sub.8, or the like), a manganese oxide,
an organic sulfur, or the like can be used as the positive
electrode active material particle.
[0086] In the case where carrier ions are alkali metal ions other
than lithium ions, alkaline-earth metal ions, beryllium ions, or
magnesium ions, the positive electrode active material particle may
contain, instead of lithium in the lithium compound and the
lithium-containing composite oxide, an alkali metal (e.g., sodium
or potassium), an alkaline-earth metal (e.g., calcium, strontium,
or barium), beryllium, or magnesium.
[0087] Further, the graphenes 204 added as a conductive additive to
the positive electrode active material layer 202 are formed by
reduction treatment of a graphene oxide in which the atomic ratio
of oxygen to carbon is greater than or equal to 0.405.
[0088] The graphene oxide in which the atomic ratio of oxygen to
carbon is greater than or equal to 0.405 can be formed by an
oxidation method called a Hummers method.
[0089] The Hummers method is as follows: a sulfuric acid solution
of potassium permanganate, a hydrogen peroxide solution, or the
like is mixed into a graphite powder to cause oxidation reaction;
thus, a dispersion liquid including a graphite oxide is formed.
Through the oxidation of carbon of graphite, functional groups such
as an epoxy group, a carbonyl group, a carboxyl group, or a
hydroxyl group are bonded in the graphite oxide. Accordingly, the
interlayer distance between a plurality of graphenes in the
graphite oxide is long as compared to the graphite, so that the
graphite oxide can be easily separated into thin pieces by
interlayer separation. Then, ultrasonic vibration is applied to the
dispersion liquid including the graphite oxide, so that the
graphite oxide whose interlayer distance is long can be cleaved to
separate a graphene oxide and to form a dispersion liquid
containing a graphene oxide. The solvent is removed from the
dispersion liquid including the graphene oxide, so that a powdery
graphene oxide can be obtained.
[0090] Here, the amount of an oxidizer such as potassium
permanganate is adjusted as appropriate so that the graphene oxide
in which the atomic ratio of oxygen to carbon is greater than or
equal to 0.405 can be formed. Specifically, the ratio of the amount
of an oxidizer to the amount of a graphite powder is increased, and
accordingly the degree of oxidation of the graphene oxide (the
atomic ratio of oxygen to carbon) can be increased. Therefore, in
accordance with the amount of the graphene oxide to be produced,
the ratio of the amount of an oxidizer to the amount of a graphite
powder which is a raw material can be determined.
[0091] For the production of the graphene oxide, the present
invention is not limited to the Hummers method using a sulfuric
acid solution of potassium permanganate; for example, the Hummers
method using nitric acid, potassium chlorate, nitric acid sodium,
or the like or a method of producing the graphene oxide other than
the Hummers method may be employed as appropriate.
[0092] The graphite oxide may be separated into thin pieces by
application of ultrasonic vibration, by irradiation with
microwaves, radio waves, or thermal plasma, or by application of
physical stress.
[0093] The formed graphene oxide includes an epoxy group, a
carbonyl group, a carboxyl group, a hydroxyl group, or the like. In
the graphene oxide, oxygen in a functional group is negatively
charged in a polar solvent typified by NMP; therefore, while
interacting with NMP, the graphene oxide repels with other graphene
oxides and is hardly aggregated. Accordingly, in a polar solvent,
graphene oxides can be easily dispersed uniformly.
[0094] The length of one side (also referred to as a flake size) of
the graphene oxide is greater than or equal to 50 nm and less than
or equal to 100 .mu.m, preferably greater than or equal to 800 nm
and less than or equal to 20 .mu.m. Particularly in the case where
the flake size is smaller than the average particle diameter of the
positive electrode active material particles 203, surface contact
with the plurality of positive electrode active material particles
203 and connection among graphenes become difficult, resulting in
difficulty in improving the electron conductivity of the positive
electrode active material layer 202.
[0095] As in the top view of the positive electrode active material
layer 202 in FIG. 3B, the plurality of positive electrode active
material particles 203 is coated with the plurality of graphenes
204. The sheet-like graphene 204 is connected to the plurality of
positive electrode active material particles 203. In particular,
since the graphenes 204 are in the form of a sheet, surface contact
can be made in such a way that the. graphenes 204 wrap part of
surfaces of the positive electrode active material particles 203.
Unlike a conductive additive in the form of particles such as
acetylene black, which makes point contact with a positive
electrode active material, the graphenes 204 are capable of surface
contact with low contact resistance; accordingly, the electron
conductivity of the positive electrode active material particles
203 and the graphenes 204 can be improved without an increase in
the amount of a conductive additive.
[0096] Further, surface contact is made between the plurality of
graphencs 204. This is because the graphene oxides with extremely
high dispersibility in a polar solvent are used for the formation
of the graphenes 204. The solvent is removed by volatilization from
a dispersion medium including the graphene oxides uniformly
dispersed and the graphene oxides are reduced to give the
graphenes; hence, the graphenes 204 remaining in the positive
electrode active material layer 202 are partly overlapped with each
other and dispersed such that surface contact is made, thereby
forming a path for electron conduction.
[0097] In the top view of the positive electrode active material
layer 202 in FIG. 3B, the graphenes 204 are not necessarily
overlapped with another graphene over a surface of the positive
electrode active material layer 202; the graphenes 204 are formed
so as to be three-dimensionally arranged, for example, so as to
enter the inside of the positive electrode active material layer
202. Further, the graphenes 204 are extremely thin films (sheets)
made of a single layer of carbon molecules or stacked layers
thereof and hence are over and in contact with part of the surfaces
of the positive electrode active material particles 203 in such a
way as to trace these surfaces. A portion of the graphenes 204
which is not in contact with the positive electrode active material
particles 203 is warped between the plurality of positive electrode
active material particles 203 and crimped or stretched.
[0098] The longitudinal section of the positive electrode active
material layer 202 shows, as illustrated in FIG. 3C, substantially
uniform dispersion of the sheet-like graphenes 204 in the positive
electrode active material layer 202. The graphenes 204 are
schematically shown as heavy lines in FIG. 3C but are actually thin
films having a thickness corresponding to the thickness of a single
layer or a multi-layer of carbon molecules. As in the top view of
the positive electrode active material layer 202, the plurality of
graphenes 204 are formed in such a way as to wrap or coat the
plurality of positive electrode active material particles 203, so
that the graphenes 204 make surface contact with the positive
electrode active material particles 203. Furthermore, the graphenes
204 are also in surface contact with each other; consequently, the
plurality of graphenes 204 forms a network for electron conduction.
FIG. 19 is a schematic enlarged view of FIG. 3C. The graphenes 204
coat the surfaces of the plurality of positive electrode active
material particles 203 in such a way as to cling to the surfaces
and the graphenes are also in contact with each other, and thus the
network is formed.
[0099] As illustrated in FIGS. 3B and 3C and FIG. 19, the plurality
of sheet-like graphenes 204 is three-dimensionally dispersed in the
positive electrode active material layer 202 and in surface contact
with each other, which forms the three-dimensional network for
electron conduction. Further, each graphene 204 coats and makes
surface contact with the plurality of positive electrode active
material particles 203. Thus, bond between the positive electrode
active material particles 203 is maintained. As described above,
the graphenes, whose raw material is the graphene oxide in which
the atomic ratio of oxygen to carbon is greater than or equal to
0.405 and which are formed by reduction performed after a paste is
formed, are employed as a conductive additive, so that the positive
electrode active material layer 202 with high electron conductivity
can be formed.
[0100] The proportion of the positive electrode active material
particles 203 in the positive electrode active material layer 202
can be increased because the added amount of the conductive
additive is not necessarily increased in order to increase contact
points between the positive electrode active material particles 203
and the graphenes 204. Accordingly, the discharge capacity of the
secondary battery can be increased.
[0101] The average particle diameter of the primary particle of the
positive electrode active material particles 203 is less than or
equal to 500 nm, preferably greater than or equal to 50 nm and less
than or equal to 500 nm. To make surface contact with the plurality
of positive electrode active material particles 203, the graphenes
204 have sides the length of each of which is greater than or equal
to 50 nm and less than or equal to 100 .mu.m, preferably greater
than or equal to 800 nm and less than or equal to 20 .mu.m.
[0102] As the binding agent (binder) included in the positive
electrode active material layer 202, instead of polyvinylidene
fluoride (PVDF) as a typical one, polyimide,
polytetrafluoroethylene, polyvinyl chloride,
ethylene-propylene-diene polymer, styrene-butadiene rubber,
acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate,
polymethyl methacrylate, polyethylene, nitrocellulose or the like
can be used.
[0103] The above-described positive electrode active material layer
202 preferably includes the positive electrode active material
particles 203 at greater than or equal to 90 wt % and less than or
equal to 94 wt %, the graphenes 204 as a conductive additive at
greater than or equal to 1 wt % and less than or equal to 5 wt %,
and the binding agent at greater than or equal to 1 wt % and less
than or equal to 5 wt % with respect to the total weight of the
positive electrode active material layer 202.
[0104] As described in this embodiment, the graphenes 204 larger
than the average particle diameter of the positive electrode active
material particles 203 are dispersed in the positive electrode
active material layer 202 such that one of the graphenes 204 makes
surface contact with one or more graphenes 204 adjacent to the one
of the graphenes 204, and the graphenes 204 make surface contact in
such a way as to wrap the surfaces of the positive electrode active
material particles 203. Consequently, with a small amount of a
conductive additive, a positive electrode for a nonaqueous
secondary battery which is highly filled and includes a
high-density positive electrode active material layer can be
provided.
[0105] This embodiment can be implemented combining with another
embodiment as appropriate.
Embodiment 2
[0106] Next, a method of forming the positive electrode 200
including a positive electrode active material layer 202 is
described with reference to FIG. 4. The method is as follows: a
positive electrode paste is formed using the positive electrode
active material, the conductive additive, the binding agent, and
the dispersion medium described above, applied on the positive
electrode current collector 201, and then dried under a reducing
atmosphere or reduced pressure.
[0107] First, NMP is prepared as the dispersion medium (Step S11),
and the graphene oxide in which the atomic ratio of oxygen to
carbon is greater than or equal to 0.405 and which is described in
Embodiment 1 is dispersed in NMP (Step S12). In the case where the
weight of the graphene oxide is less than 2 wt % with respect to
the total weight of the positive electrode paste, the conductivity
is decreased when the positive electrode active material layer 202
is formed. In the case where the weight of the graphene oxide
exceeds 10 wt %, although it depends on the diameter of the
positive electrode active material particle, the viscosity of the
positive electrode paste is increased. In addition, in a drying
step after the positive electrode paste is applied on the positive
electrode current collector 201, convection occurs in the positive
electrode paste by heating and the graphene oxide which is thin and
lightweight moves and is aggregated, whereby the positive electrode
active material layer 202 might cause a crack or might be separated
from the positive electrode current collector 201. Thus, the weight
of the graphene oxide is preferably set to 2 wt % to 10 wt % with
respect to the weight of the positive electrode paste (the total
weight of the positive electrode active material, the conductive
additive, and the binding agent). Note that the graphene oxide is
reduced by a later heat treatment step to give the graphene and the
weight is reduced by almost half, and consequently the weight ratio
in the positive electrode active material layer 202 becomes 1 wt %
to 5 wt %.
[0108] Next, lithium iron phosphate is added as the positive
electrode active material (Step S13). It is preferable to use
lithium iron phosphate with an average primary particle diameter
greater than or equal to 50 nm and less than or equal to 500 nm.
The weight of added lithium iron phosphate is preferably greater
than or equal to 85 wt % with respect to the total weight of the
positive electrode paste; for example, the weight is greater than
or equal to 85 wt % and less than or equal to 93 wt %.
[0109] Note that when lithium iron phosphate is baked, a
carbohydrate such as glucose may be mixed so that a particle of
lithium iron phosphate is coated with carbon. This treatment
improves the conductivity.
[0110] Next, a mixture of the above is kneaded (mixing is performed
in a highly viscous state), so that the aggregation of the graphene
oxide and lithium iron phosphate can be undone. Further, since the
graphene oxide has a functional group, oxygen in the functional
group is negatively charged in a polar solvent, which makes
aggregation among different graphene oxides difficult. In addition,
the graphene oxide strongly interacts with lithium iron phosphate.
Hence, the graphene oxide can be uniformly dispersed into lithium
iron phosphate.
[0111] Next, as the binding agent, PVDF is added to the mixture
(Step S14). The weight of PVDF can be determined in accordance with
the weight of the graphene oxide and lithium iron phosphate, and
PVDF is preferably added to the positive electrode paste at greater
than or equal to 1 wt % and less than or equal to 5 wt %. The
binding agent is added while the graphene oxide is uniformly
dispersed so as to make surface contact with the plurality of
positive electrode active material particles, so that the positive
electrode active material particles and the graphene oxide can be
bound to each other while the dispersion state is maintained.
Although the binding agent is not necessarily added depending on
the proportions of lithium iron phosphate and the graphene oxide,
adding the binding agent can enhance the strength of the positive
electrode.
[0112] Next, NMP is added to this mixture until predetermined
viscosity is obtained (Step S15) and mixed. Consequently, the
positive electrode paste can be formed (Step S16). Through the
above steps, the positive electrode paste in which the graphene
oxide, the positive electrode active material particles, and the
binding agent are uniformly mixed can be formed.
[0113] Next, the positive electrode paste is applied on the
positive electrode current collector 201 (Step S17).
[0114] Next, the positive electrode paste applied on the positive
electrode current collector 201 is dried (Step S18). The drying
step is performed by heating at 60.degree. C. to 170.degree. C. for
1 minute to 10 hours to vaporize NMP. There is no particular
limitation on the atmosphere.
[0115] Next, the positive electrode paste is dried under a reducing
atmosphere or reduced pressure (Step S19). By heating at a
temperature of 130 20 C. to 200.degree. C., for 10 hours to 30
hours under a reducing atmosphere or reduced pressure, NMP and
water which are left in the positive electrode paste are vaporized
and oxygen contained in the graphene oxide is desorbed. Thus, the
graphene oxide can be formed into graphene. Note that oxygen in the
graphene oxide may partly remain in the graphene without being
entirely released.
[0116] Through the above steps, the positive electrode 200
including the positive electrode active material layer 202 where
the graphenes 204 are uniformly dispersed in the positive electrode
active material particles 203 can be formed. Note that a step of
applying pressure to the positive electrode 200 may be performed
after the drying step.
[0117] As described in this embodiment, the graphene oxide can be
uniformly dispersed in positive electrode active material particles
by adding the positive electrode active material particles to a
dispersion medium in which the graphene oxide with an atomic ratio
of oxygen to carbon greater than or equal to 0.405 is dispersed and
mixed. By being added in a state where the graphene oxide is
dispersed so as to be in contact with the plurality of the positive
electrode active material particles, the binding agent can be
uniformly dispersed without hindering the contact between the
graphene oxide and the plurality of positive electrode active
material particles. With use of the positive electrode paste formed
in such a manner, a positive electrode which is highly filled with
the positive electrode active material and includes a high-density
positive electrode active material layer can be manufactured.
Further, when a battery is formed using the positive electrode, a
nonaqueous secondary battery with high capacity can be formed.
Since a state where the sheet-like graphenes are in contact with
the plurality of positive electrode active material particles can
be maintained by the binding agent, separation between the positive
electrode active material and the graphene can be suppressed; thus,
a nonaqueous secondary battery having good cycle characteristics
can be manufactured.
[0118] This embodiment can be implemented combining with another
embodiment as appropriate.
Embodiment 3
[0119] In this embodiment, a structure of a nonaqueous secondary
battery and a manufacturing method thereof will be described with
reference to FIGS. 5A and 5B and FIGS. 6A and 6B.
[0120] FIG. 5A is an external view of a coin-type (single-layer
flat type) nonaqueous secondary battery, and FIG. 5B is a
cross-sectional view thereof.
[0121] In a coin-type secondary battery 300, a positive electrode
can 301 serving also as a positive electrode terminal and a
negative electrode can 302 serving also as a negative electrode
terminal are insulated and sealed with a gasket 303 formed of
polypropylene or the like. A positive electrode 304 is formed of a
positive electrode current collector 305 and a positive electrode
active material layer 306 which is provided to be in contact with
the positive electrode current collector 305. On the other hand, a
negative electrode 307 is formed of a negative electrode current
collector 308 and a negative electrode active material layer 309
which is provided to be in contact with the negative electrode
current collector 308. A separator 310 and an electrolyte (not
illustrated) are included between the positive electrode active
material layer 306 and the negative electrode active material layer
309.
[0122] As the positive electrode 304, the positive electrode 200
described in Embodiment 1 and Embodiment 2 can be used.
[0123] The negative electrode 307 can be formed in such a manner
that the negative electrode active material layer 309 is formed
over the negative electrode current collector 308 by a CVD method,
a sputtering method, or a coating method.
[0124] For the negative electrode current collector 308, it is
possible to use a highly conductive material, for example, a metal
such as aluminum, copper, nickel, or titanium, an aluminum-nickel
alloy, or an aluminum-copper alloy. The negative electrode current
collector 308 can have a foil-like shape, a plate-like shape (a
sheet-like shape), a net-like shape, a punching-metal shape, an
expanded-metal shape, or the like as appropriate. The negative
electrode current collector 308 preferably has a thickness of
greater than or equal to 10 .mu.m and less than or equal to 30
.mu.m.
[0125] As the negative electrode active material, a material with
which lithium can be dissolved/precipitated or a material into/from
which lithium ions can be intercalated/deintercalated can be used;
for example, a lithium metal, a carbon-based material, an
alloy-based material, or the like can be used.
[0126] The lithium metal is preferable because of its low redox
potential (lower than that of the standard hydrogen electrode by
3.045 V) and high specific capacity per weight and volume (which
are 3860 mAh/g and 2062 mAh/cm.sup.3).
[0127] Examples of the carbon-based material include graphite,
graphitizing carbon (soft carbon), non-graphitizing carbon (hard
carbon), a carbon nanotube, graphene, carbon black, and the
like.
[0128] Examples of the graphite include artificial graphite such as
meso-carbon microbeads (MCMB), coke-based artificial graphite, and
pitch-based artificial graphite and natural graphite such as
spherical natural graphite.
[0129] Graphite has a low potential substantially equal to that of
a lithium metal (0.1 V to 0.3 V vs. Li/Li.sup.+) when lithium ions
are intercalated into the graphite (when a lithium-graphite
intercalation compound is generated). For this reason, a lithium
ion battery can have a high operating voltage. In addition,
graphite is preferable because of its advantages such as relatively
high capacity per volume, small volume expansion, low cost, and
greater safety than that of a lithium metal.
[0130] As the negative electrode active material, an alloy-based
material which enables charge-discharge reaction by alloying and
dealloying reaction with a lithium metal can be used. For example,
a material including at least one of Al, Si, Ge, Sn, Pb, Sb, Bi,
Ag, Zn, Cd, In, Ga, and the like can be given. Such elements have
higher capacity than carbon. In particular, silicon has a
theoretical capacity of 4200 mAh/g, which is significantly high.
For this reason, silicon is preferably used as the negative
electrode active material. Examples of the alloy-based material
using such elements include SiO, Mg.sub.2Si, Mg.sub.2Ge, SnO,
SnO.sub.2, Mg.sub.2Sn, SnS.sub.2, V.sub.2Sn.sub.3, FeSn.sub.2,
CoSn.sub.2, Ni.sub.3Sn.sub.2, Cu.sub.6Sn.sub.5, Ag.sub.3Sn,
Ag.sub.3Sb, Ni.sub.2MnSb, CeSb.sub.3, LaSn.sub.3,
La.sub.3Co.sub.2Sn.sub.7, CoSb.sub.3, InSb, SbSn, and the like.
[0131] Alternatively, as the negative electrode active material, an
oxide such as titanium dioxide (TiO.sub.2), lithium titanium oxide
(Li.sub.4Ti.sub.5O.sub.12), a lithium-graphite intercalation
compound (Li.sub.xC.sub.6), niobium pentoxide (Nb.sub.2O.sub.5),
tungsten oxide (WO.sub.2), molybdenum oxide (MoO.sub.2), or the
like can be used.
[0132] Further alternatively, as the negative electrode active
material, (M=Co, Ni, or Cu) with a Li.sub.3N structure, which is a
nitride containing lithium and a transition metal, can be used. For
example, Li.sub.2.6Co.sub.0.4N.sub.3 is preferable because of high
charge and discharge capacity (900 mAh/g).
[0133] A nitride containing lithium and a transition metal is
preferably used, in which case lithium ions are included in the
negative electrode active material, and thus the negative electrode
active material can be used in combination with a material for a
positive electrode active material which does not include lithium
ions, such as V.sub.2O.sub.5 or Cr.sub.3O.sub.8. Note that in the
case of using a material including lithium ions as the positive
electrode active material, the nitride containing lithium and a
transition metal can be used for the negative electrode active
material by extracting lithium ions in advance.
[0134] Still further alternatively, as the negative electrode
active material, a material which causes conversion reaction can be
used. For example, a transition metal oxide which does not cause
alloying reaction with lithium, such as cobalt oxide (CoO), nickel
oxide (NiO), or iron oxide (FeO), may be used. Other examples of
the material which causes conversion reaction include oxides such
as Fe.sub.2O.sub.3, CuO, Cu.sub.2O, RuO.sub.2, and Cr.sub.2O.sub.3,
sulfides such as CoS.sub.0.89, NiS, and CuS, nitrides such as
Zn.sub.3N.sub.2, Cu.sub.3N, and Ge.sub.3N.sub.4, phosphides such as
NiP.sub.2, FeP.sub.2, and CoP.sub.3, and fluorides such as
FeF.sub.3 and BiF.sub.3. Note that any of the fluorides can be used
as a positive electrode active material because of its high
potential.
[0135] The negative electrode active material layer 309 may be
formed by a coating method in the following manner: a conductive
additive or a binding agent is added to the negative electrode
active material to form a negative electrode paste; and the
negative electrode paste is applied on the negative electrode
current collector 308 and dried.
[0136] In the case where the negative electrode active material
layer 309 is formed using silicon as the negative electrode active
material, graphene is preferably formed on a surface of the
negative electrode active material layer 309. The volume of silicon
is greatly changed due to occlusion/release of carrier ions in
charge-discharge cycles, adhesion between the negative electrode
current collector 308 and the negative electrode active material
layer 309 is decreased, resulting in degradation of battery
characteristics caused by charge and discharge. In view of this,
graphene is preferably formed on a surface of the negative
electrode active material layer 309 containing silicon because even
when the volume of silicon is changed in charge-discharge cycles,
decrease in adhesion between the negative electrode current
collector 308 and the negative electrode active material layer 309
can be suppressed and degradation of battery characteristics is
reduced.
[0137] Graphene formed on the surface of the negative electrode
active material layer 309 can be formed by reducing graphene oxide
in a similar manner to that of the method of forming the positive
electrode. As the graphene oxide, the graphene oxide described in
Embodiment 1 can be used.
[0138] A method of forming graphene oxide on the negative electrode
active material layer 309 by an electrophoresis method will be
described with reference to FIG. 6A.
[0139] FIG. 6A is a cross-sectional view illustrating an
electrophoresis method. In a container 401, the dispersion liquid
in which graphene oxide is dispersed and which is described in
Embodiment 1 (hereinafter referred to as a graphene oxide
dispersion liquid 402) is contained. Further, a formation subject
403 is put in the graphene oxide dispersion liquid 402 and is used
as an anode. In addition, a conductor 404 serving as a cathode is
put in the graphene oxide dispersion liquid 402. Note that the
formation subject 403 is the negative electrode current collector
308 and the negative electrode active material layer 309 which is
formed thereon. Further, the conductor 404 may be formed using a
conductive material, for example, a metal material or an alloy
material.
[0140] By applying appropriate voltage between the anode and the
cathode, a graphene oxide layer is formed on a surface of the
formation subject 403, that is, the surface of the negative
electrode active material layer 309. This is because the graphene
oxide is negatively charged in the polar solvent as described
above, so that by applying voltage, the graphene oxide which is
negatively charged is drawn to the anode and deposited on the
formation subject 403. Negative charge of the graphene oxide is
derived from release of hydrogen ions from a substituent such as a
hydroxyl group or a carboxyl group included in the graphene oxide,
and the substituent is bonded to an object to result in
neutralization. Note that the voltage which is applied is not
necessarily constant. Further, by measuring the amount of charge
flowing between the anode and the cathode, the thickness of the
graphene oxide layer deposited on the object can be estimated.
[0141] The voltage is applied between the cathode and the anode in
the range of 0.5 V to 2.0 V, preferably 0.8 V to 1.5 V. For
example, when the voltage applied between the cathode and the anode
is set to 1 V, an oxide film which might be generated based on the
principle of anodic oxidation is not easily formed between the
formation subject and the graphene oxide layer.
[0142] When the graphene oxide with a required thickness is
obtained, the formation subject 403 is taken out of the graphene
oxide dispersion liquid 402 and dried.
[0143] In electrodeposition of the graphene oxide by an
electrophoresis method, a portion which is already coated with the
graphene oxide is scarcely stacked with an additional graphene
oxide. This is because the conductivity of the graphene oxide is
sufficiently low. On the other hand, a portion which is not coated
yet with the graphene oxide is preferentially stacked with graphene
oxide. Therefore, the graphene oxide formed on the surface of the
formation subject 403 has a uniform thickness sufficient for
practical use.
[0144] Time for performing electrophoresis (time for applying
voltage) is preferably longer than time for coating the surface of
the formation subject 403 with the graphene oxide, for example,
longer than or equal to 0.5 minutes and shorter than or equal to 30
minutes, more preferably longer than or equal to 5 minutes and
shorter than or equal to 20 minutes.
[0145] With the use of an electrophoresis method, an ionized
graphene oxide can be electrically transferred to the active
material, whereby the graphene oxide can be provided uniformly even
when the surface of the negative electrode active material layer
309 is uneven.
[0146] Next, part of oxygen is released from the formed graphene
oxide by reduction treatment. Although, as the reduction treatment,
reduction treatment by heating or the like, which is described in
Embodiment 1 using a graphene, may be performed, electrochemical
reduction treatment (hereinafter, referred to as electrochemical
reduction) will be described below.
[0147] The electrochemical reduction of the graphene oxide is
reduction utilizing electric energy, which is different from
reduction by heat treatment. As illustrated in FIG. 6B, a closed
circuit is configured using, as a conductor 407, the negative
electrode including the graphene oxide provided over the negative
electrode active material layer 309, and a potential at which the
reduction reaction of the graphene oxide occurs or a potential at
which the graphene oxide is reduced is supplied to the conductor
407, so that the graphene oxide is reduced to form graphene. Note
that in this specification, a potential at which the reduction
reaction of the graphene oxide occurs or a potential at which the
graphene oxide is reduced is referred to as the reduction
potential.
[0148] A method for reducing the graphene oxide will be
specifically described with reference to FIG. 6B. A container 405
is filled with an electrolyte solution 406, and the conductor 407
provided with the graphene oxide and a counter electrode 408 are
put in the container 405 so as to be immersed in the electrolyte
solution 406. Next, an electrochemical cell (open circuit) is
configured using at least the counter electrode 408 and the
electrolyte solution 406 besides the conductor 407 provided with
the graphene oxide, which serves as a working electrode, and the
reduction potential of the graphene oxide is supplied to the
conductor 407 (working electrode), so that the graphene oxide is
reduced to form graphene. Note that the reduction potential to be
supplied is a reduction potential in the case where the potential
of the counter electrode 408 is used as a reference potential or a
reduction potential in the case where a reference electrode is
provided in the electrochemical cell and the potential of the
reference electrode is used as a reference potential. For example,
when the counter electrode 408 and the reference electrode are each
made of lithium metal, the reduction potential to be supplied is a
reduction potential determined relative to the redox potential of
the lithium metal (vs. Li/Li.sup.+). Through this step, reduction
current flows through the electrochemical cell (closed circuit)
when the graphene oxide is reduced. Thus, to examine whether the
graphene oxide is reduced, the reduction current needs to be
checked continuously; the state where the reduction current is
below a certain value (where there is no peak corresponding to the
reduction current) is regarded as the state where the graphene
oxide is reduced (where the reduction reaction is completed).
[0149] In controlling the potential of the conductor 407, the
potential of the conductor 407 may be fixed to less than or equal
to the reduction potential of the graphene oxide or may be swept so
as to include the reduction potential of the graphene oxide.
Further, the sweeping may be periodically repeated like in cyclic
voltammetry. There is no limitation on the sweep rate of the
potential of the conductor 407. Note that the potential of the
conductor 407 may be swept either from a higher potential to a
lower potential or from a lower potential to a higher
potential.
[0150] Although the reduction potential of the graphene oxide
slightly varies depending on the structure of the graphene oxide
(e.g., the presence or absence of a functional group) and the way
to control the potential (e.g., the sweep rate), it is
approximately 2.0 V (vs. Li/Li.sup.+). Specifically, the potential
of the conductor 407 may be controlled so as to fall within the
range of 1.6 V to 2.4 V (vs. Li/Li.sup.+).
[0151] Through the above steps, the graphene can be formed over the
conductor 407. In the case where electrochemical reduction
treatment is performed, a proportion of C(sp.sup.2)-C(sp.sup.2)
double bonds is higher than that of the graphene formed by heat
treatment; therefore, the graphene having high conductivity can be
formed over the negative electrode active material layer 309.
[0152] The negative electrode active material layer 309 may be
predoped with lithium through the graphene after the graphene is
formed over the conductor 407. As a predoping method of lithium, a
lithium layer may be formed on a surface of the negative electrode
active material layer 309 by a sputtering method. Alternatively, a
lithium foil is provided on the surface of the negative electrode
active material layer 309, whereby the negative electrode active
material layer 309 can be predoped with lithium.
[0153] The separator 310 can be formed using an insulator such as
cellulose (paper), polyethylene with pores, or polypropylene with
pores.
[0154] As an electrolyte of the electrolyte solution, a material
which contains carrier ions is used. Typical examples of the
electrolyte include lithium salts such as LiClO.sub.4, LiAsF.sub.6,
LiBF.sub.4, LiPF.sub.6, and Li(C.sub.2F.sub.5SO.sub.2).sub.2N.
[0155] In the case where carrier ions are alkali metal ions other
than lithium ions, alkaline-earth metal ions, beryllium ions, or
magnesium ions, the electrolyte may contain, instead of lithium in
the lithium salts, an alkali metal (e.g., sodium or potassium), an
alkaline-earth metal (e.g., calcium, strontium, or barium),
beryllium, or magnesium.
[0156] As a solvent of the electrolyte solution, a material in
which carrier ions can transfer is used. As the solvent of the
electrolyte solution, an aprotic organic solvent is preferably
used. Typical examples of aprotic organic solvents include ethylene
carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl
carbonate (DEC), y-butyrolactone, acetonitrile, dimethoxyethane,
and tetrahydrofuran, and one or more of these materials can be
used. When a gelled polymer material is used as the solvent of the
electrolyte solution, safety against liquid leakage and the like is
improved. Further, the nonaqueous secondary battery can be thinner
and more lightweight. Typical examples of a gelled polymer material
include a silicone gel, an acrylic gel, an acrylonitrile gel,
polyethylene oxide, polypropylene oxide, and a fluorine-based
polymer. Alternatively, the use of one or more of ionic liquids
(room temperature molten salts) which are less likely to burn and
volatilize as the solvent of the electrolyte solution can prevent
the secondary battery from exploding or catching fire even when the
secondary battery internally shorts out or the internal temperature
increases due to overcharging or the like.
[0157] Instead of the electrolyte solution, a solid electrolyte
including a sulfide-based inorganic material, an oxide-based
inorganic material, or the like, or a solid electrolyte including a
polyethylene oxide (PEO)-based polymer material or the like can be
used. When the solid electrolyte is used, a separator or a spacer
is not necessary. Further, the battery can be entirely solidified;
therefore, there is no possibility of liquid leakage and thus the
safety of the battery is dramatically increased.
[0158] For the positive electrode can 301 and the negative
electrode can 302, a metal having a corrosion-resistance property
to a liquid (e.g., an electrolyte solution) in charging and
discharging the secondary battery, such as nickel, aluminum, or
titanium; an alloy of any of the metals; an alloy containing any of
the metals and another metal (e.g., stainless steel); a stack of
any of the metals; a stack including any of the metals and any of
the alloys (e.g., a stack of stainless steel and aluminum); or a
stack including any of the metals and another metal (e.g., a stack
of nickel, iron, and nickel) can be used. The positive electrode
can 301 and the positive electrode 304 are electrically connected
to each other, and the negative electrode can 302 and the negative
electrode 307 are electrically connected to each other.
[0159] The negative electrode 307, the positive electrode 304, and
the separator 310 are immersed in the electrolyte. Then, as
illustrated in FIG. 5B, the positive electrode 304, the separator
310, the negative electrode 307, and the negative electrode can 302
are stacked in this order with the positive electrode can 301
positioned at the bottom, and the positive electrode can 301 and
the negative electrode can 302 are subjected to pressure bonding
with the gasket 303 interposed therebetween. In such a manner, the
coin-type secondary battery 300 is manufactured.
[0160] Next, an example of a laminated secondary battery will be
described with reference to FIG. 7.
[0161] A laminated secondary battery 500 illustrated in FIG. 7
includes a positive electrode 503 including a positive electrode
current collector 501 and a positive electrode active material
layer 502, a negative electrode 506 including a negative electrode
current collector 504 and a negative electrode active material
layer 505, a separator 507, an electrolyte solution 508, and an
exterior body 509. The separator 507 is placed between the positive
electrode 503 and the negative electrode 506 provided in the
exterior body 509. The exterior body 509 is tilled with the
electrolyte solution 508.
[0162] In the laminated secondary battery 500 illustrated in FIG.
7, the positive electrode current collector 501 and the negative
electrode current collector 504 also function as terminals for
electrical contact with the outside. For this reason, each of the
positive electrode current collector 501 and the negative electrode
current collector 504 is arranged outside the exterior body 509 so
as to be partly exposed.
[0163] In the laminated secondary battery 500, as the exterior body
509, for example, a laminate film having a three-layer structure
where a highly flexible metal thin film of aluminum, stainless
steel, copper, nickel, or the like is provided over a film formed
of a material such as polyethylene, polypropylene, polycarbonate,
ionomer, or polyamide, and an insulating synthetic resin film of a
polyamide resin, a polyester resin, or the like is provided as the
outer surface of the exterior body over the metal thin film can be
used. With such a three-layer structure, permeation of an
electrolytic solution and a gas can be blocked and an insulating
property and resistance to the electrolytic solution can be
obtained.
[0164] Next, examples of a cylindrical secondary battery are
described with reference to FIGS. 8A and 8B. As illustrated in FIG.
8A, a cylindrical lithium secondary battery 600 includes a positive
electrode cap (battery lid) 601 on its top surface and a battery
can (exterior can) 602 on its side surface and bottom surface. The
positive electrode cap and the battery can (exterior can) 602 are
insulated from each other by a gasket (insulating gasket) 610.
[0165] FIG. 8B is a diagram schematically illustrating a cross
section of the cylindrical nonaqueous secondary battery. In the
battery can 602 with a hollow cylindrical shape, a battery element
is provided in which a strip-like positive electrode 604 and a
strip-like negative electrode 606 are wound with a separator 605
provided therebetween. Although not illustrated, the battery
element is wound around a center pin as a center. One end of the
battery can 602 is close and the other end thereof is open. For the
battery can 602, a metal having a corrosion-resistance property to
a liquid (e.g., an electrolyte solution) in charging and
discharging the secondary battery, such as nickel, aluminum, or
titanium; an alloy of any of the metals; an alloy containing any of
the metals and another metal (e.g., stainless steel); a stack of
any of the metals; a stack including any of the metals and any of
the alloys (e.g., a stack of stainless steel and aluminum); or a
stack including any of the metals and another metal (e.g., a stack
of nickel, iron, and nickel) can be used. Inside the battery can
602, the battery element in which the positive electrode, the
negative electrode, and the separator are wound is interposed
between a pair of insulating plates 608 and 609 which face each
other. Further, a non-aqueous electrolyte solution (not
illustrated) is injected inside the battery can 602 in which the
battery element is provided. A non-aqueous electrolyte solution
which is similar to that of the coin-type nonaqueous secondary or
the laminated nonaqueous secondary battery can be used.
[0166] Although the positive electrode 604 and the negative
electrode 606 can be formed in a manner similar to that of the
positive electrode and the negative electrode of the coin-type
nonaqueous secondary battery, the difference lies in that, since
the positive electrode and the negative electrode of the
cylindrical nonaqueous secondary battery are wound, active
materials are formed on both sides of the current collectors. A
positive electrode terminal (positive electrode current collecting
lead) 603 is connected to the positive electrode 604, and a
negative electrode terminal (negative electrode current collecting
lead) 607 is connected to the negative electrode 606. A metal
material such as aluminum can be used for both the positive
electrode terminal 603 and the negative electrode terminal 607. The
positive electrode terminal 603 is resistance-welded to a safety
valve mechanism 612, and the negative electrode terminal 607 is
resistance-welded to the bottom of the battery can 602. The safety
valve mechanism 612 is electrically connected to the positive
electrode cap 601 through a positive temperature coefficient (PTC)
element 611. The safety valve mechanism 612 cuts off electrical
connection between the positive electrode cap 601 and the positive
electrode 604 when the internal pressure of the battery increases
and exceeds a predetermined threshold value. The PTC element 611 is
a heat sensitive resistor whose resistance increases as temperature
rises, and controls the amount of current by increase in resistance
to prevent unusual heat generation. Barium titanate
(BaTiO.sub.3)-based semiconductor ceramic or the like can be used
for the PTC element.
[0167] Note that in this embodiment, the coin-type non-aqueous
secondary battery, the laminated nonaqueous secondary battery, and
the cylindrical non-aqueous secondary battery are given as examples
of the lithium secondary battery; however, any of non-aqueous
secondary batteries with the other various shapes, such as a
sealing-type non-aqueous secondary battery and a square-type
non-aqueous secondary battery, can be used. Further, a structure in
which a plurality of positive electrodes, a plurality of negative
electrodes, and a plurality of separators are stacked or wound may
be employed.
[0168] A positive electrode according to one embodiment of the
present invention is used as each of the positive electrodes of the
secondary batteries 300, 500, and 600 described in this embodiment.
Thus, the discharge capacity of each of the secondary batteries
300, 500, and 600 can be increased.
[0169] This embodiment can be implemented combining with another
embodiment as appropriate.
Embodiment 4
[0170] A nonaqueous secondary battery of one embodiment of the
present invention can be used for power supplies of a variety of
electrical appliances.
[0171] Specific examples of electrical appliances each utilizing
the nonaqueous secondary battery of one embodiment of the present
invention are as follows: display devices of televisions, monitors,
and the like, lighting devices, desktop personal computers and
laptop personal computers, word processors, image reproduction
devices which reproduce still images or moving images stored in
recording media such as digital versatile discs (DVDs), portable
compact disc (CD) players, radio receivers, tape recorders,
headphone stereos, stereos, clocks such as table clocks and wall
clocks, cordless phone handsets, transceivers, portable wireless
devices, cellular phones, car phones, portable game machines,
calculators, portable information terminals, electronic notebooks,
e-book readers, electronic translators, audio input devices,
cameras such as still cameras and video cameras, toy, electric
shavers, high-frequency heating appliances such as microwave ovens,
electric rice cookers, electric washing machines, electric vacuum
cleaners, water heaters, electric fans, hair dryers,
air-conditioning systems such as air conditioners, humidifiers, and
dehumidifiers, dishwashers, dish dryers, clothes dryers, futon
dryers, electric refrigerators, electric freezers, electric
refrigerator-freezers, freezers for preserving DNA, flashlights,
electric power tools such as chain saws, smoke detectors, and
medical equipment such as dialyzers. Further, industrial equipment
such as guide lights, traffic lights, belt conveyors, elevators,
escalators, industrial robots, power storage systems, and power
storage devices for leveling the amount of power supply and smart
grid can be given. In addition, moving objects driven by electric
motors using power from the nonaqueous secondary batteries are also
included in the category of electrical appliances. Examples of the
moving objects are electric vehicles (EV), hybrid electric vehicles
(HEV) which include both an internal-combustion engine and a motor,
plug-in hybrid electric vehicles (PHEV), tracked vehicles in which
caterpillar tracks are substituted for wheels of these vehicles,
motorized bicycles including motor-assisted bicycles, motorcycles,
electric wheelchairs, golf carts, boats or ships, submarines,
helicopters, aircrafts, rockets, artificial satellites, space
probes, planetary probes, and spacecrafts.
[0172] In the above electrical appliances, the nonaqueous secondary
battery of one embodiment of the present invention can be used as a
main power supply for supplying enough power for almost the whole
power consumption. Alternatively, in the above electrical
appliances, the nonaqueous secondary battery of one embodiment of
the present invention can be used as an uninterruptible power
supply which can supply power to the electrical appliances when the
supply of power from the main power supply or a commercial power
supply is stopped. Still alternatively, in the above electrical
appliances, the nonaqueous secondary battery of one embodiment of
the present invention can be used as an auxiliary power supply for
supplying power to the, electrical appliances at the same time as
the power supply from the main power supply or a commercial power
supply.
[0173] FIG. 9 illustrates specific structures of the above
electrical appliances. In FIG. 9, a display device is an example of
an electrical appliance including a nonaqueous secondary battery
704 of one embodiment of the present invention. Specifically, the
display device 700 corresponds to a display device for TV broadcast
reception and includes a housing 701, a display portion 702,
speaker portions 703, the nonaqueous secondary battery 704, and the
like. The nonaqueous secondary battery 704 of one embodiment of the
present invention is provided in the housing 701. The display
device 700 can receive power from a commercial power supply.
Alternatively, the display device 700 can use power stored in the
nonaqueous secondary battery 704. Thus, the display device 700 can
be operated with the use of the nonaqueous secondary battery 704 of
one embodiment of the present invention as an uninterruptible power
supply even when power cannot be supplied from a commercial power
supply due to power failure or the like.
[0174] For the display portion 702, a semiconductor display device
such as a liquid crystal display device, a light-emitting device in
which a light-emitting element such as an organic EL element is
provided in each pixel, an electrophoresis display device, a
digital micromirror device (DMD), a plasma display panel (PDP), or
a field emission display (FED) can be used.
[0175] Note that the display device includes, in its category, all
of information display devices for personal computers,
advertisement displays, and the like besides TV broadcast
reception.
[0176] In FIG. 9, an installation lighting device 710 is an example
of an electrical appliance including a nonaqueous secondary battery
713 of one embodiment of the present invention. Specifically, the
lighting device 710 includes a housing 711, a light source 712, the
nonaqueous secondary battery 713, and the like. Although FIG. 9
illustrates the case where the nonaqueous secondary battery 713 is
provided in a ceiling 714 on which the housing 711 and the light
source 712 are installed, the nonaqueous secondary battery 713 may
be provided in the housing 711. The lighting device 710 can receive
power from a commercial power supply. Alternatively, the lighting
device 710 can use power stored in the nonaqueous secondary battery
713. Thus, the lighting device 710 can be operated with the use of
the nonaqueous secondary battery 713 of one embodiment of the
present invention as an uninterruptible power supply even when
power cannot be supplied from a commercial power supply due to
power failure or the like.
[0177] Note that although the installation lighting device 710
provided in the ceiling 714 is illustrated in FIG. 9 as an example,
the nonaqueous secondary battery of one embodiment of the present
invention can be used as an installation lighting device provided
in, for example, a wall 715, a floor 716, a window 717, or the like
other than the ceiling 714. Alternatively, the nonaqueous secondary
battery can be used in a tabletop lighting device or the like.
[0178] As the light source 712, an artificial light source which
emits light artificially by using power can be used. Specifically,
an incandescent lamp, a discharge lamp such as a fluorescent lamp,
and light-emitting elements such as an LED or an organic EL element
are given as examples of the artificial light source.
[0179] In FIG. 9, an air conditioner including an indoor unit 720
and an outdoor unit 724 is an example of an electrical appliance
including a nonaqueous secondary battery 723 of one embodiment of
the present invention. Specifically, the indoor unit 720 includes a
housing 721, an air outlet 722, the nonaqueous secondary battery
723, and the like, Although FIG. 9 illustrates the case where the
nonaqueous secondary battery 723 is provided in the indoor unit
720, the nonaqueous secondary battery 723 may be provided in the
outdoor unit 724. Alternatively, thee nonaqueous secondary
batteries 723 may be provided in both the indoor unit 720 and the
outdoor unit 724. The air conditioner can receive power from a
commercial power supply. Alternatively, the air conditioner can use
power stored in the nonaqueous secondary battery 723. Particularly
in the case where the nonaqueous secondary batteries 723 are
provided in both the indoor unit 720 and the outdoor unit 724, the
air conditioner can be operated with the use of the nonaqueous
secondary battery 723 of one embodiment of the present invention as
an uninterruptible power supply even when power cannot be supplied
from a commercial power supply'due to power failure or the
like.
[0180] Note that although the split-type air conditioner including
the indoor unit and the outdoor unit is illustrated in FIG. 9 as an
example, the nonaqueous secondary battery of one embodiment of the
present invention can be used in an air conditioner in which the
functions of an indoor unit and an outdoor unit are integrated in
one housing.
[0181] In FIG. 9, an electric refrigerator-freezer 730 is an
example of an electrical appliance including a nonaqueous secondary
battery 734 of one embodiment of the present invention.
Specifically, the electric refrigerator-freezer 730 includes a
housing 731, a door 732 for the refrigerator, a door 733 for the
freezer, the nonaqueous secondary battery 734, and the like. The
nonaqueous secondary battery 734 is provided in the housing 731 in
FIG. 9. The electric refrigerator-freezer 730 can receive power
from a commercial power supply. Alternatively, the electric
refrigerator-freezer 730 can use power stored in the nonaqueous
secondary battery 734. Thus, the electric refrigerator-freezer 730
can be operated with the use of the nonaqueous secondary battery
734 of one embodiment of the present invention as an
uninterruptible power supply even when power cannot be supplied
from a commercial power supply due to power failure or the
like.
[0182] Note that among the electrical appliances described above, a
high-frequency heating apparatus such as a microwave oven and an
electrical appliance such as an electric rice cooker require high
power in a short time. The tripping of a breaker of a commercial
power supply in use of an electrical appliance can be prevented by
using the nonaqueous secondary battery of one embodiment of the
present invention as an auxiliary power supply for supplying power
which cannot be supplied enough by the commercial power supply.
[0183] In addition, in a time period when electrical appliances are
not used, particularly when the proportion of the amount of power
which is actually used to the total weight of power which can be
supplied from a commercial power supply source (such a proportion
referred to as a usage rate of power) is low, power can be stored
in the nonaqueous secondary battery, whereby the usage rate of
power can be reduced in a time period when the electrical
appliances are used. For example, in the case of the electric
refrigerator-freezer 730, power can be stored in the nonaqueous
secondary battery 734 in night time when the temperature is low and
the door 732 for the refrigerator and the door 733 for the freezer
are not often opened and closed. On the other hand, in daytime when
the temperature is high and the door 732 for the refrigerator and
the door 733 for the freezer are frequently opened and closed, the
nonaqueous secondary battery 734 is used as an auxiliary power
supply; thus, the usage rate of power in daytime can be
reduced.
[0184] This embodiment can be implemented combining with another
embodiment as appropriate.
Embodiment 5
[0185] Next, a portable information terminal which is an example of
the electrical appliance will be described with reference to FIGS.
10A to 10C.
[0186] FIGS. 10A and 10B illustrate a tablet terminal 800 that can
be folded. In FIG. 10A, the tablet terminal 800 is opened, and
includes a housing 801, a display portion 802a, a display portion
802b, a switch 803 for switching display modes, a power switch 804,
a switch 805 for switching to power-saving mode, and an operation
switch 807.
[0187] Part of the display portion 802a can be a touch panel region
808a and data can be input when a displayed operation key 809 is
touched. Although a structure in which a half region in the display
portion 802a has only a display function and the other half region
has a touch panel function is shown as an example, the display
portion 802a is not limited to the structure. The whole region in
the display portion 802a may have a touch panel function. For
example, keyboard buttons can be displayed on the entire display
portion 802a to be used as a touch panel, and the display portion
802b can be used as a display screen.
[0188] As in the display portion 802a, part of the display portion
802b can be a touch panel region 808b. A switching button 810 for
showing/hiding a keyboard of the touch panel is touched with a
finger, a stylus, or the like, so that keyboard buttons can be
displayed on the display portion 802b.
[0189] Touch input can be performed in the touch panel region 808a
and the touch panel region 808b at the same time.
[0190] The switch 803 for switching display modes can switch the
display between portrait mode, landscape mode, and the like, and
between monochrome display and color display, for example. The
switch 805 for switching to power-saving mode can control display
luminance to be optimal in accordance with the amount of external
light in use of the tablet terminal which is detected by an optical
sensor incorporated in the tablet terminal. Another detection
device including a sensor or the like for detecting inclination,
such as a gyroscope or an acceleration sensor, may be incorporated
in the tablet terminal, in addition to the optical sensor.
[0191] Note that FIG. 10A illustrates an example in which the
display portion 802a and the display portion 802b have the same
display area; however, without limitation thereon, one of the
display portions may be different from the other display portion in
size and display quality. For example, one display panel may be
capable of higher-definition display than the other display
panel.
[0192] The tablet terminal 800 is closed in FIG. 10B. The tablet
terminal includes the housing 801, a solar cell 811, a
charge-discharge control circuit 850, a battery 851, and a DC-DC
converter 852. In FIG. 10B, a structure including the battery 851
and the DC-DC converter 852 is illustrated as an example of the
charge-discharge control circuit 850. The nonaqueous secondary
battery described in any of the above embodiments is used as the
battery 851.
[0193] Since the tablet terminal 800 can be folded, the housing 801
can be closed when the tablet terminal is not used. As a result,
the display portion 802a and the display portion 802b can be
protected; thus, the tablet terminal 800 which has excellent
durability and excellent reliability also in terms of long-term use
can be provided.
[0194] In addition, the tablet terminal illustrated in FIGS. 10A
and 10B can have a function of displaying a variety of kinds of
data (e.g., a still image, a moving image, and a text image), a
function of displaying a calendar, a date, the time, or the like on
the display portion, a touch-input function of operating or editing
the data displayed on the display portion by touch input, a
function of controlling processing by a variety of kinds of
software (programs), and the like.
[0195] The solar cell 811 provided on a surface of the tablet
terminal can supply power to the touch panel, the display portion,
a video signal processing portion, or the like. Note that the solar
cell 811 can be preferably provided on one or both surfaces of the
housing 801, in which case the battery 851 can be charged
efficiently. When the nonaqueous secondary battery described in any
of the above embodiments is used as the battery 851, there is an
advantage such as a reduction in size.
[0196] The structure and the operation of the charge-discharge
control circuit 850 illustrated in FIG. 10B will be described with
reference to a block diagram in FIG. 10C. The solar cell 811, the
battery 851, the. DC-DC converter 852, a converter 853, switches
SW1 to SW3, and the display portion 802 are illustrated in FIG.
10C, and the battery 851, the DC-DC converter 852, the converter
853, and the switches SW1 to SW3 correspond to the charge-discharge
control circuit 850 in FIG. 10B.
[0197] First, an example of the operation in the case where power
is generated by the solar cell 811 using external light is
described. The voltage of power generated by the solar cell is
raised or lowered by the DC-DC converter 852 so that the power has
a voltage for charging the battery 851. Then, when the power from
the solar cell 811 is used for the operation of the display portion
802, the switch SW1 is turned on and the voltage of the power is
raised or lowered by the converter 853 so as to be a voltage needed
for the display portion 802. In addition, when display on the
display portion 802 is not performed, the switch SW1 is turned off
and the switch SW2 is turned on so that the battery 851 may be
charged.
[0198] Note that the solar cell 811 is described as an example of a
power generation means; however, without limitation thereon, the
battery 851 may be charged using another power generation means
such as a piezoelectric element or a thermoelectric conversion
element (Peltier element). For example, the battery 851 may be
charged with a non-contact power transmission module which is
capable of charging by transmitting and receiving power by wireless
(without contact), or another charging means may be used in
combination.
[0199] It is needless to say that one embodiment of the present
invention is not limited to the electrical appliance illustrated in
FIGS. 10A to 10C as long as the nonaqueous secondary battery
described in any of the above embodiments is included.
Embodiment 6
[0200] Further, an example of the moving object which is an example
of the electrical appliance will be described with reference to
FIGS. 11A and 11B.
[0201] The nonaqueous secondary battery described in any of the
above embodiments can be used as a control battery. The control
battery can be externally charged by electric power supply using a
plug-in technique or contactless power feeding. Note that in the
case where the moving object is an electric railway vehicle, the
electric railway vehicle can be charged by electric power supply
from an overhead cable or a conductor rail.
[0202] FIGS. 11A and 11B illustrate an example of an electric
vehicle. An electric vehicle 860 is equipped with a battery 861.
The output of the electric power of the battery 861 is adjusted by
a control circuit 862 and the electric power is supplied to a
driving device .863. The control circuit 862 is controlled by a
processing unit 864 including a ROM, a RAM, a CPU, or the like
which is not illustrated.
[0203] The driving device 863 includes a DC motor or an AC motor
either alone or in combination with an internal-combustion engine.
The processing unit 864 outputs a control signal to the control
circuit 862 based on input data such as data of operation (e.g.,
acceleration, deceleration, or stop) by a driver or data during
driving (e.g., data on an upgrade or a downgrade, or data on a load
on a driving wheel) of the electric vehicle 860, The control
circuit 862 adjusts the electric energy supplied from the battery
861 in accordance with the control signal of the processing unit
864 to control the output of the driving device 863. In the case
where the AC motor is mounted, although not illustrated, an
inverter which converts direct current into alternate current is
also incorporated.
[0204] The battery 861 can be charged by external electric power
supply using a plug-in technique. For example, the battery 861 is
charged through a power plug from a commercial power supply. The
battery 861 can be charged by converting external power into DC
constant voltage having a predetermined voltage level through a
converter such as an AC-DC converter. Providing the nonaqueous
secondary battery of one embodiment of the present invention as the
battery 861 can contribute to an increase in the capacity of the
battery, so that convenience can be improved. When the battery 861
itself can be more compact and more lightweight as a result of
improved characteristics of the battery 861, the vehicle can be
lightweight and fuel efficiency can be increased.
[0205] It is needless to say that one embodiment of the present
invention is not limited to the electronic device described above
as long as the electronic device includes the nonaqueous secondary
battery of one embodiment of the present invention.
[0206] This embodiment can be implemented combining with another
embodiment as appropriate.
EXAMPLE 1
[0207] The present invention will be specifically described below
with examples. This example shows results of formation of a
positive electrode by the method described in Embodiment 2. Note
that the present invention is not limited to the examples described
below.
[0208] Charge-discharge characteristics are compared between a cell
including a positive electrode with a conductive additive using a
graphene oxide (in which the atomic ratio of oxygen to carbon (also
referred to as O/C, or the degree of oxidation) was set to 0.547)
as a raw material and cells including positive electrodes using a
graphene and a reduced graphene oxide (RGO) whose degree of
oxidation is considered extremely low as conductive additives.
Charge-discharge characteristics of a cell including a positive
electrode using conventional acetylene black (AB) as a conductive
additive is also compared.
Fabrication of Positive Electrode including Conductive Additive
using Graphene oxide as Raw Material
[0209] A positive electrode was fabricated using the graphene oxide
in which the O/C was 0.547. The positive electrode was fabricated
in such a way that positive electrode active material (lithium iron
phosphate (LiFePO.sub.4)) particles, a binding agent
(polyvinylidene fluoride (PVDF) produced by Kureha Corporation),
and the graphene oxide as a conductive additive were mixed to form
a positive electrode paste and the positive electrode paste was
applied on a current collector (aluminum) and then was dried and
reduced. In the fabrication, the compounding ratio
(LiFePO.sub.4:conductive additive (graphene oxide):PVDF) in the
positive electrode paste was set to 93:2:5 (unit: wt %).
Fabrication of Positive Electrode using RGO as Conductive
Additive
[0210] The reduced graphene oxide (RGO) in this specification means
a graphene formed by reduction of a graphene oxide in advance and
is already reduced when dispersed into a dispersion medium.
Therefore functional groups such as an epoxy group are probably
almost eliminated by the reduction reaction. The graphene oxide
prepared by the method described in Embodiment 1 was reduced by
heat treatment in which, after held in a vacuum for 1 hour, the
graphene oxide was increased in temperature to 170.degree. C. and
held for 10 hours, so that the RGO was formed. The reduction
probably decreases functional groups such as an epoxy group on a
surface of the RGO to about 10 wt % (weight percent). This RGO was
mixed into NMP, and lithium iron phosphate and PVDF were added
thereto, so that the positive electrode paste was formed. The
positive electrode paste applied on the current collector was
heated and the dispersion medium was volatilized; consequently, the
positive electrode having the positive electrode active material
layer on the current collector was fabricated. The compounding
ratio (LiFePO.sub.4:conductive additive (RGO):PVDF) in the positive
electrode active material layer was set to 94:1:5.
Fabrication of Positive Electrodes using Graphene as Conductive
Additive
[0211] As the graphene, a product of Graphene Supermarket was used.
The graphene had a specific surface area of 600 m.sup.2/g, a flake
size of about 10 .mu.m, and a thickness less than or equal to 1 nm,
in which the O/C was 0.02. As in the above RGO, the number of
bonded functional groups is extremely smaller in the graphene than
in the graphene oxide. This graphene is heated at 170.degree. C.
for 10 hours by the same method as above to form the positive
electrodes. The following two positive electrodes were fabricated:
the positive electrode in which the compounding ratio in the active
material layer (LiFePO.sub.4:conductive additive (graphene):PVDF)
was 94:1:5 and the positive electrode in which the compounding
ratio in the active material layer was 90:5:5.
Fabrication of Positive Electrode using Acetylene Black as
Conductive Additive
[0212] As acetylene black (AB), a powdery product of Denki Kagaku
Kogyo Kabushiki Kaisha was used. The specific surface area was 68
m.sup.2/g and the average particle diameter was 35 nm. The
compounding ratio (LiFePO.sub.4:conductive additive (AB):PVDF) in
the positive electrode active material layer was set to
80:15:5.
Measurements of Electrode Conductivities
[0213] The conductivities of the positive electrode active material
layers using the graphene oxide, the graphene included at 1%, the
graphene included at 5%, and acetylene black were measured. The
measurements gave results shown in the following Table 1.
TABLE-US-00001 TABLE 1 Positive electrode active material Thickness
Denstiy Conductivity layers (.mu.m) (g/cm.sup.3) (S/cm) Including
conductive additive using 30 2.6 1.3 .times. 10.sup.-6 graphene
oxide Including conductive additive using 48 1.6 Measuring graphene
included at 1% limit Including conductive additive using 43 1.5 5.6
.times. 10.sup.-3 graphene included at 5% Including conductive
additive using 23 1.4 1.4 .times. 10.sup.-3 acetylene black
(AB)
[0214] The conductivity of the positive electrode active material
layer including the conductive additive using the graphene oxide
was the lowest: 1.3.times.10.sup.-6 S/cm. The conductivities of the
positive electrode active material layers using the graphene and
acetylene black are higher by two or more orders of magnitude.
Charge-Discharge Characteristics
[0215] The above-described positive electrodes using the graphene
formed by reduction performed after the paste including the
graphene oxide was applied on the current collector, the RGO, the
graphene, and acetylene black (AB) as conductive additives were
included in half cells, and charge-discharge characteristics of the
cells were measured. Here, for convenience, the cell using the
graphene oxide in accordance with the present invention as a raw
material of the conductive additive is referred to as a cell D, the
cell using the RGO is referred to as a cell E, the cell using the
graphene included at 1% is referred to as a cell F, the cell using
the graphene included at 5% is referred to as a cell G, and the
cell using AB is referred to as a cell H. In the measurements, the
charge rate was set to 0.2 C (0.16 C for the cell of the positive
electrode using AB (cell H)) and the discharge rate was set to 1 C
(0.82 C for the cell H).
[0216] As a result of the measurements, the cell E using the RGO as
the conductive additive and the cell F using the graphene included
at 1% as the conductive additive were not able to be charged or
discharged at all.
[0217] In contrast, battery properties of the cell G using the
graphene included at 5% as the conductive additive and the cell H
using conventional acetylene black as the conductive additive were
confirmed. Charge-discharge characteristics of the cells G and H in
addition to those of the cell D using the graphene oxide in
accordance with the present invention as a raw material of the
conductive additive are shown in FIG. 12.
[0218] FIG. 12 shows charge-discharge characteristics, in which the
horizontal axis represents discharge capacity (mAh/g) and the
vertical axis represents voltage (V). The heavy line is a curve
showing charge-discharge characteristics of the cell D using the
graphene oxide as a raw material of the conductive additive. The
thin line is a curve showing charge-discharge characteristics of
the cell G using the graphene included at 5% as the conductive
additive. The dashed line is a curve showing charge-discharge
characteristics of the cell H using acetylene black as the
conductive additive.
[0219] A discharge curve 901a and a charge curve 901b show that the
cell D exhibited good charge-discharge characteristics.
[0220] In contrast, the cell G using the graphene as the conductive
additive was found to have low discharge capacity, showing narrow
charge-discharge regions that were plateaus in a discharge curve
902a and a charge curve 902b.
[0221] Further, the cell H using acetylene black as the conductive
additive was found to have low discharge capacity, showing no
discharge regions that were plateaus in a discharge curve 903a and
a charge curve 903b.
[0222] As described above, the cells did not have good
charge-discharge characteristics with the positive electrodes using
the RGO and the graphene which had almost no functional group as
the conductive additive. In contrast, the cell had good
charge-discharge characteristics with the positive electrode formed
in such a way that the graphene oxide having functional groups
bonded by oxidation reaction was dispersed in the dispersion
medium. This may mean that, in the positive electrode active
material layer including the graphene formed by reduction performed
after the graphene oxide is dispersed in the positive electrode
paste, the graphene forms a network with high electron
conductivity. On the other hand, in the positive electrode active
material layer formed by dispersion of the RGO or the graphene
having almost no functional group in the positive electrode paste,
a network for electron conductivity is probably not sufficiently
formed. Thus, the use of the graphene oxide having a functional
group as a raw material of the conductive additive is important in
achieving high electron conductivity of the positive electrode
active material layer.
EXAMPLE 2
[0223] Next, experiments were conducted to examine the effect of
difference in the degree of oxidation of the graphene oxide (the
number of functional groups having oxygen such as an epoxy group)
on the charge-discharge characteristics of a secondary battery.
Fabrication of Positive Electrodes
[0224] First, to examine the effect of difference in the degree of
oxidation of the graphene oxide used on charge-discharge
characteristics of a secondary battery, three positive electrodes,
a sample A, a sample B, and a sample C, using graphene oxides with
different degrees of oxidation were prepared.
[0225] In this example, since graphene oxides with different
degrees of oxidation were necessary to examine the effect of
difference in the degree of oxidation of the graphene oxide on
charge-discharge characteristics of a secondary battery, graphenes
having almost no functional group were used as a raw material
without use of the graphite powder described in Embodiment 1. Such
graphenes can be oxidized to from graphene oxides with oxidizers
whose weights are made different while the weights of the graphenes
are uniform. Thus, graphene oxides with different degrees of
oxidation can be fabricated.
[0226] For the sample A, the sample B, and the sample C, graphenes
produced by Cheap Tubes, Inc. were used. The graphenes each had a
thickness of 3 nm on average. In the sample A, the sample B, and
the sample C, the weights of the graphenes were each set to 0.25 g.
The graphenes were oxidized by being mixed into 46 ml of sulfuric
acid to which 1.5 g of potassium permanganate (KMnO.sub.4), 0.5 g
of the same oxidizer, and 0.2 g of the same oxidizer were added as
an oxidizer for the sample A, the sample B, and sample C,
respectively. The oxidation treatment was performed by stirring at
room temperature for 2.5 hours. After that, pure water was added to
the mixture, the mixture was stirred for 15 minutes while being
heated, and a hydrogen peroxide solution was added thereto, so that
a yellow-brown suspension including a graphite oxide was
obtained.
[0227] The degrees of oxidation of the prepared graphene oxides for
the samples A to C were measured by X-ray photoelectron
spectroscopy (XPS). In the measurements, monochromatic light Al
(1486.6 eV) was used as an X-ray source, the measurement area was
set to 100 .mu.m diameter, and the extraction angle was set to
45.degree.. The measurement results were shown in Table 2 and Table
3.
TABLE-US-00002 TABLE 2 Sample C O N S O/C Sample A 66.7 32.5 -- 0.8
0.487 Sample B 70.7 28.6 -- 0.7 0.405 Sample C 75.3 23.4 0.8 0.4
0.311 unit: atomic %
TABLE-US-00003 TABLE 3 Sample C.dbd.C C--C, C--H C--O C.dbd.O
O.dbd.C--O Sample A 0.0 25.0 32.0 7.1 2.7 Sample B 0.0 30.4 33.1
4.7 2.5 Sample C 0.0 40.6 27.5 4.0 3.2 unit: atomic %
[0228] Table 2 shows the quantification values (unit:atomic %) of
the elements C, O, N, and S in the samples A to C and the atomic
ratio of oxygen to carbon (also referred to as O/C, or the degree
of oxidation). In the graphene oxide using 1.5 g of the oxidizer
for the sample A, the atomic of oxygen is higher than those in the
other samples and the O/C is 0.487. In the graphene oxide using 0.5
g of the oxidizer for the sample B, the O/C is 0.405 and in the
graphene oxide using 0.2 g of the sample C, the O/C is 0.311. Thus,
by adjustment of the weights of the oxidizers used for oxidation of
the graphenes, the graphene oxides with different degrees of
oxidation were able to be prepared.
[0229] Table 3 shows bonding states on surfaces in the graphene
oxides of the above sample A to C listed by state. As the O/C is
higher, the proportions of C--C, C--H, and O.dbd.C--O are lower
while the proportion of C--O is higher.
[0230] Next, using the graphene oxides prepared under the above
conditions, the positive electrodes of the samples A to C were
formed. The positive electrodes were each formed in such a way that
positive electrode active material (lithium iron phosphate
(LiFePO.sub.4)) particles, a binding agent (polyvinylidene fluoride
(PVDF) produced by Kureha Corporation), and one of the graphene
oxides, which were prepared under the above conditions, as a
conductive additive were mixed to form a positive electrode paste
and the positive electrode paste was applied on a current collector
(aluminum) and then was dried and reduced.
[0231] A method of forming lithium iron phosphate used as each of
the active materials of the samples A to C is described. Lithium
carbonate (Li.sub.2CO.sub.3), iron oxalate
(FeC.sub.2O.sub.4.2H.sub.2O), and ammonium dihydrogen phosphate
(NH.sub.4H.sub.2PO.sub.4), which were raw materials, were weighed
out such that the weight ratio therebetween was 1:2:2, and were
ground and mixed with a wet ball mill (the ball diameter was 3 mm
and acetone was used as a solvent) at 300 rpm for 2 hours. After
drying, pre-baking was performed at 350.degree. C. for 10 hours
under a nitrogen atmosphere.
[0232] Next, grinding and mixing were performed with a wet ball
mill (the ball diameter was 3 mm) at 300 rpm for 2 hours. Then,
baking was performed at 600.degree. C. for 10 hours under a
nitrogen atmosphere.
[0233] Next, NMP (produced by Tokyo Chemical Industry Co., Ltd.),
which is a polar solvent, was prepared as a dispersion medium.
After the graphene oxide was dispersed into NWP, lithium iron
phosphate was added and the mixture was kneaded. After PVDF was
added to the mixture of the graphene oxide and lithium iron
phosphate as the binding agent, NMP was further added as the
dispersion medium and mixed, whereby the positive electrode paste
was formed.
[0234] The positive electrode paste formed by the above-described
method was applied on a 20-.mu.m-thick aluminum foil which is to
form the current collector, dried in an air atmosphere at
80.degree. C. for 40 minutes, and then dried under a
reduced-pressure atmosphere at 170.degree. C. for 10 hours; thus
the graphene oxide in the positive electrode paste was reduced to
form the graphene. The compounding ratio in the positive electrode
paste was set such that the ratio of lithium iron phosphate to the
graphene oxide and PVDF was 93:2:5. This compounding ratio was
changed by reduction treatment of the graphene oxide such that the
ratio of lithium iron phosphate to the graphene and PVDF was
substantially 94:1:5 when the positive electrode active material
layer was formed. However, such a small change in compounding ratio
hardly affects estimation of the discharge capacity of the
secondary battery. Note that, in each of the samples A to C, anchor
coating was performed on a surface of the current collector in
order to eliminate the influence of interfacial resistance between
the current collector and the positive electrode active material
layer.
[0235] As described above, the positive electrodes using the three
graphene oxides that differed in the degree of oxidation as raw
materials of the conductive additives were fabricated as the
samples A to C.
Charge-Discharge Characteristics
[0236] The positive electrodes of the samples A to C which were
fabricated as above were included in half cells, and
charge-discharge characteristics of the cells (referred to as a
cell A, a cell B, and a cell C) were measured. When the
characteristics were estimated, each cell was in the form of a
coin-type cell of a CR2032 type (20 mm in diameter and 3.2 mm
high). Lithium foil was used as a negative electrode and a
25-.mu.m-thick polypropylene (PP) film was used as a separator. An
electrolyte solution to be used was formed in such a manner that
lithium hexafluorophosphate (LiPF.sub.6) was dissolved at a
concentration of 1 mol/L in a solution in which ethylene carbonate
(EC) and diethyl carbonate (DEC) were mixed at a volume ratio of
1:1. In charging, CCCV at 0.2 C was employed and the upper limit
voltage was set to 4.3 V. In discharging, CC at all the rates, 0.2
C, 1 C, 2 C, 5 C, and 10 C, was employed and the lower limit
voltage was set to 2 V.
[0237] Measurement results of the charge-discharge characteristics
of the cells A and B are shown in FIGS. 13A and 13B. FIG. 13A shows
the measurement results of the charge-discharge characteristics of
the cell A including the positive electrode of the sample A using
the graphene formed with the graphene oxide in which the O/C is
0.487 as a raw material. FIG. 13B shows the measurement results of
the charge-discharge characteristics of the cell B including the
positive electrode of the sample B using the graphene formed with
the graphene oxide in which the O/C is 0.405 as a raw material. In
each figure, the horizontal axis represents discharge capacity per
active material weight (unit: mAh/g) and the vertical axis
represents voltage (unit: volt).
[0238] As shown in FIG. 13A, the cell A exhibits good battery
properties.
[0239] As shown in FIG. 13B, the cell B also exhibits good battery
properties.
[0240] In contrast, the cell C using the positive electrode of the
sample C including the graphene formed using the graphene oxide in
which the O/C was 0.311 did not operate as a battery at all.
[0241] As described above, the cells A and B each including the
positive electrode including the conductive additive using the
graphene oxide in which the O/C, i.e., the degree of oxidation, was
greater than or equal to 0.405 as a raw material were able to
exhibit sufficient charge-discharge characteristics. In contrast,
the cell C including the positive electrode including the
conductive additive using the graphene oxide in which the O/C was
0.311 as a raw material was not able to exhibit battery properties.
Thus, in the case where the graphene oxide in which the O/C was at
least greater than or equal to 0.405 is used, functional groups
having oxygen bonded to the graphene oxide are sufficiently
included, and accordingly the graphene oxide in the dispersion
medium is uniformly dispersed. For this reason, the graphenes
formed by the reduction treatment of the graphene oxide performed
by heating the positive electrode paste are mixed with high
dispersibility in the positive electrode active material and in
surface contact with each other; consequently, the graphenes form a
network with high electron conductivity, thereby providing battery
properties.
[0242] In contrast, in the case where the positive electrode
including the conductive additive using the graphene oxide in which
the O/C is less than or equal to 0.311 as a raw material is used,
the dispersibility of the graphene oxide in the positive electrode
paste is low. Therefore the graphene formed by reduction was not
sufficiently dispersed in the positive electrode active material or
was aggregated, which failed to form a sufficient network for
electron conduction. This was probably why battery properties were
not able to be obtained.
EXAMPLE 3
[0243] To visually confirm that the use of a graphene oxide having
functional groups increases the dispersibility in the positive
electrode active material, scanning electron microscope (SEM)
observation was performed on the positive electrode active material
layer formed using the graphene oxide as a raw material of the
conductive additive. For comparison, a positive electrode active
material layer using a RGO as a conductive additive and a positive
electrode active material layer using a graphene as a conductive
additive were also subjected to SEM observation.
[0244] FIG. 14A shows a SEM image of a surface of the positive
electrode active material layer formed using the graphene oxide as
a raw material of a conductive additive. In the image, the reduced
graphene is present not only in a deep-color portion but also over
the entire region. The graphene is observed to adhere in a patchy
pattern. FIG. 14B is a magnified image of part of FIG. 14A. The
plurality of positive electrode active material particles 1001 is
observed. The positive electrode active material particles 1001 are
aggregated in batches of several or several tens of pieces.
Further, in FIG. 14B, as indicated in the dashed-line circle, for
example, the deep-color portion represents a graphene 1002. FIG. 15
is a magnified SEM image of part of FIG. 14B. The image reveals
that the graphenes 1002 spread in such a way as to coat a plurality
of positive electrode active material particles 1001 which is
aggregated. Since the graphene 1002 is thin, it makes surface
contact with the positive electrode active material particles in
such a way as to wrap them along surfaces of the positive electrode
active material particles. Part of the graphenes 1002 which is not
in contact with the positive electrode active material particles
1001 is stretched, warped, or crimped. In addition, the graphene
1002 is present not only on a surface of the active material layer
but also inside the active material layer.
[0245] FIGS. 16A and 16B, FIGS. 20A and 20B, FIG. 21, and FIG. 22
are SEM images showing cross sections of the positive electrode
active material layers formed using the graphene oxide as a raw
material of a conductive additive.
[0246] FIGS. 16A and 16B and FIG. 21 show positive electrode
material layers fabricated so that the ratio of lithium iron
phosphate (LiFePO.sub.4) to the conductive additive (graphene
oxide) and PVDF was 93:2:5 (unit: wt %). In the positive electrode
material layer shown in FIGS. 16A and 16B, PVDF (1100) produced by
Kureha Corporation was used. In the positive electrode material
layer in FIG. 21, PVDF (9100) produced by Kureha Corporation was
used. In addition, FIG. 21 is a voltage contrast image which
clearly shows the graphene oxide.
[0247] In the SEM images in FIG. 16A and FIG. 21, the plurality of
positive electrode active material particles is seen. In part of
the images, aggregated positive electrode active material particles
can also be seen. Here, white thread- or string-like portions
correspond to graphenes. Note that among the graphenes, a
multilayer graphene including fewer layers may fail to be observed
in the SEM images. Further, even graphenes observed far away from
each other may he connected through a multilayer graphenes
including fewer layers which fails to be observed by SEM. The
graphenes can be seen like a thread or a string in a gap (void)
between the plurality of positive electrode active material
particles and also adheres to the surfaces of the positive
electrode active material particles. In FIG. 16B, some of the
graphenes in the SEM image in FIG. 16A are highlighted by heavy
lines. In both FIG. 16B and FIG. 21, the graphenes 1002 are found
to be three-dimensionally dispersed in the positive electrode
active material particles in such a way as to wrap the positive
electrode active material particles 1001. The graphenes 1002 make
surface contact with the plurality of positive electrode active
material particles 1001 while being in surface contact with each
other. Thus, in the positive electrode active material layer, the
graphenes are connected to each other and forms a network for
electron conduction.
[0248] FIGS. 20A and 20B show a positive electrode material layer
fabricated so that the ratio of lithium iron phosphate to the
graphene oxide and PVDF was 94:1:5 (unit: wt %). In the SEM images
in FIGS. 20A and 20B, the plurality of positive electrode active
material particles is seen. In part of the images, aggregated
positive electrode active material particles can also be seen. As
in FIGS. 16A and 16B and FIG. 21, the graphenes can be seen like a
thread or a string in a gap (void) between the plurality of
positive electrode active material particles and also adheres to
the surfaces of the positive electrode active material particles.
In FIG. 20B, some of the graphenes in the SEM image in FIG. 20A are
highlighted by heavy lines. Also in FIGS. 20A and 20B, the
graphenes 1002 are found to be three-dimensionally dispersed in the
positive electrode active material particles in such a way as to
wrap the positive electrode active material particles 1001.
[0249] FIG. 22 shows a positive electrode material layer fabricated
so that the ratio of lithium iron phosphate to the graphene oxide
and PVDF was 94.4:0.6:5 (unit: wt %). In FIG. 22, some of the
graphenes in the SEM image are highlighted by heavy lines. As in
FIGS. 16A and 16B, FIGS. 20A and 20B, and FIG. 21, the graphenes
1002 are found to be three-dimensionally dispersed in the positive
electrode active material particles in such a way as to wrap the
positive electrode active material particles 1001. Further, the
graphenes 1002 make surface contact with the positive electrode
active material particles 1001 while being in surface contact with
each other. Even when the graphene oxide is included at 0.6 wt %,
in the positive electrode material layer, the graphenes are
connected to each other and form a network for electron
conduction.
[0250] Thus, regardless of the kind of PVDF or the proportion of
the graphene oxide, the graphene oxide in the positive electrode
material layer are similarly dispersed three-dimensionally so that
a network for electron conduction can be formed.
[0251] FIGS. 17A and 17B show SEM observation results of the
surface of the positive electrode active material layer using the
RGO as a conductive additive. In FIG. 17A, the RGO is present at
the deep-color portion which is slightly below the center of the
figure. FIG. 17B shows a magnified SEM image of this RGO. Although
the positive electrode active material particles 1003 and the RGO
1004 are in contact with each other, the RGO is seen only around
the center of the image and cannot be found to be in the other
region in FIG. 17B. In other words, the RGO has low dispersibility
and is aggregated on the surface of the positive electrode active
material layer.
[0252] FIGS. 18A and 18B show SEM observation results of the
surface of the positive electrode active material layer using the
graphene as a conductive additive. In FIG. 18A, several deep-color
points correspond to the graphenes. FIG. 18B is a magnified image
of part of FIG. 18A. Several graphenes 1006 are scattered
throughout a plurality of positive electrode active material
particles 1005. Like the RGO, the graphenes have low dispersibility
and are aggregated.
[0253] The above-described results reveal that, when the graphene
oxide is used as a raw material of a conductive additive,
dispersibility in a polar solvent is high because of the functional
groups of the graphene oxide, which enables the graphene formed by
reduction to be highly dispersed in the positive electrode active
material layer. This demonstrates that the graphene can form a
network for electron conduction in the positive electrode active
material layer, whereby a positive electrode with high electron
conductivity can be formed.
EXAMPLE 4
[0254] Next, XPS analysis was performed to check the compositions
of positive electrode active material layers of fabricated positive
electrodes in accordance with the present invention, each of which
includes the graphene formed by reduction performed after a paste
including the graphene oxide was applied on a current
collector.
[0255] The analysis was conducted for four positive electrodes (a
positive electrode GN1, a positive electrode GN2, a positive
electrode GN3, and a positive electrode GN4) formed by subjecting
the positive electrode pastes, in which the compounding ratio of
lithium iron phosphate to the graphene oxide and PVDF was 93:2:5,
to treatments under the four different conditions described
below.
Positive Electrode GN1
[0256] The positive electrode GN1 is an electrode formed by
performing no reduction treatment on the graphene oxide and washed
after being immersed in an electrolyte solution. The electrolyte
solution used for the immersion was formed in such a way that
lithium hexafluorophosphate (LiPF.sub.6) was dissolved at a
concentration of 1 mol/L in a solution in which ethylene carbonate
(EC) and diethyl carbonate (DEC) were mixed at a volume ratio of
1:1. The washing for removing a lithium salt was performed with
DEC. Note that, in the formation of the positive electrode GN1,
drying for volatilizing the dispersion medium was performed but it
was treatment at 80.degree. C. for 40 minutes in an air atmosphere
and these are not conditions that allow the graphene oxide to be
reduced.
Positive Electrode GN2
[0257] The positive electrode GN2 is an electrode formed by
subjecting the graphene oxide to electrochemical reduction
treatment and washed with DEC like the positive electrode GN1. For
the electrochemical reduction treatment, a coin cell using lithium
as a counter electrode was prepared. The graphene oxide was reduced
as follows: discharging was performed at 1 C until the reduction
potential reached 2.0 V (vs. Li/Li.sup.+) and the potential was
held at 2.0 V for 10 hours.
Positive Electrode GN3
[0258] For the positive electrode GN3, after the same
electrochemical reduction as that performed on the positive
electrode GN2 was performed, charging was performed at a current of
0.2 C until the potential reached 4.3 V and the potential was held
at 4.3 V until the current value became 0.01 C. In addition, the
positive electrode GN3 was extracted from the cell and washed with
DEC after being charged.
Positive electrode GN4
[0259] The positive electrode GN4 is an electrode formed by
subjecting the graphene oxide to heat reduction treatment. The heat
reduction treatment was performed at 170.degree. C. for 10 hours in
a reduced-pressure atmosphere. After that, the electrode was washed
after being immersed in an electrolyte solution, like the above
positive electrodes.
XPS Analysis Results
[0260] Table 4 and Table 5 show XPS analysis results of the
positive electrode active material layers of the positive
electrodes GN1 to GN4.
TABLE-US-00004 TABLE 4 Positive C--C, O.dbd.C--O CF2, electrode
C.dbd.C C--H C--O C.dbd.O CF metal-CO3 Positive 0.0 22.8 17.1 3.2
0.6 4.6 electrode GN1 Positive 24.1 18.8 8.4 1.7 1.9 4.5 electrode
GN2 Positive 27.5 15.2 8.8 3.0 2.2 3.0 electrode GN3 Positive 24.6
19.5 7.7 1.9 1.8 3.9 electrode GN4 unit: atomic %
TABLE-US-00005 TABLE 5 Positive C--C, O.dbd.C--O, CF2, electrode
C.dbd.C C--H C--O C.dbd.O CF metal-CO3 Positive 0.0 47.1 35.5 6.6
1.2 9.5 electrode GN1 Positive 40.5 31.7 14.2 2.8 3.2 7.6 electrode
GN2 Positive 46.0 25.5 14.8 5.1 3.7 5.0 electrode GN3 Positive 41.4
32.9 12.9 3.3 3.0 6.6 electrode GN4 unit: atomic %
[0261] Bonding states of carbon included in the positive electrode
active material layers of the positive electrodes GN1 to GN4 were
analyzed by waveform separation of a C1s spectrum and listed by
state in Table 4 and Table 5. Table 4 shows the proportions of the
bonding states of carbon measured by XPS analysis. Table 5 shows
the proportions of the bonding states in all the bonding
states.
[0262] As shown in Table 4 and Table 5, while the C.dbd.C bond was
not measured in the positive electrode GN1 which was not subjected
to reduction treatment, the positive electrodes GN2 to GN4 which
were subjected to reduction treatment include the C.dbd.C bond at
24.1 atomic % (40.5% of all the states), 27.5 atomic % (46.0%), and
24.6 atomic % (41.4%), respectively. Further, while the positive
electrode GN1 which was not subjected to reduction treatment
includes many C--O bonds (17.1 atomic %), the positive electrodes
GN2 to GN4 which were subjected to reduction treatment include
fewer C--O bonds (8.4 atomic %, 8.8 atomic %, and 7.7 atomic %,
respectively). Although the positive electrode active material
layers of the analyzed electrodes also include a binder, such
reduction treatment does not change the composition of the binder.
Thus, reduction treatment reduces the number of functional groups
bonded to the graphene oxide and accordingly the number of C--O
bonds decreases while the number of C.dbd.C bonds increases.
[0263] As described above, reduction treatment after application of
a positive electrode paste enabled the graphene oxide included in
the positive electrode paste to be reduced.
[0264] The bonding states of carbon included in a positive
electrode active material layer in a positive electrode formed by
such reduction treatment are as follows: the proportion of the
C.dbd.C bond is greater than or equal to 35% and the proportion of
the C--O bond is greater than or equal to 5% and less than or equal
to 20%; preferably, the proportion of the C.dbd.C bond is greater
than or equal to 40% and the proportion of the C--O bond is greater
than or equal to 10% and less than or equal to 15%.
[0265] By using such a positive electrode active material layer, a
positive electrode for a nonaqueous secondary battery which can
achieve high electron conductivity can be provided with a small
amount of a conductive additive. A high-density positive electrode
for a nonaqueous secondary battery which is highly filled can be
provided with a small amount of a conductive additive.
[0266] Note that the above XPS analysis was performed with the
positive electrode active material layers including a binder. For
comparison, results of XPS analysis performed before and after heat
reduction of a simple substance of graphene oxide in the form of
powder are shown in Table 6 and Table 7. Table 6 shows the weight
proportions of the bonding states of carbon included the graphene
oxide in the form of powder and Table 7 shows proportions of the
bonding states in all the bonding states.
TABLE-US-00006 TABLE 6 Positive O.dbd.C--O, electrode C.dbd.C C--C,
C--H C--O C.dbd.O CF CF2 Graphene oxide 0.0 26.6 25.8 10.0 3.7 0.0
After heat 40.5 20.5 7.1 2.9 1.7 0.0 reduction unit: atomic %
TABLE-US-00007 TABLE 7 Positive O.dbd.C--O, electrode C.dbd.C C--C,
C--H C--O C.dbd.O CF CF2 Graphene oxide 0.0 40.3 39.0 15.2 5.6 0.0
After heat 55.7 28.1 9.8 4.0 2.3 0.0 reduction unit: atomic %
[0267] It can be found that the heat reduction increases the number
of the C.dbd.C bonds while reducing the number of the C--O
bonds.
REFERENCE NUMERALS
[0268] 100: NMP, 101: graphene or RGO, 102: graphene oxide, 200:
positive electrode, 201: positive electrode current collector, 202:
positive electrode active material layer, 203: positive electrode
active material particle, 204: graphene, 300: secondary battery,
301: positive electrode can, 302: negative electrode can, 303:
gasket, 304: positive electrode, 305: positive electrode current
collector, 306: positive electrode active material layer, 307:
negative electrode, 308: negative electrode current collector, 309:
negative electrode active material layer, 310: separator, 401:
container, 402: graphene oxide dispersion liquid, 403: formation
subject, 404: conductor, 405: container, 406: electrolyte solution,
407: conductor, 408: counter electrode, 500: secondary battery,
501: positive electrode current collector, 502: positive electrode
active material layer, 503: positive electrode, 504: negative
electrode current collector, 505: negative electrode active
material layer, 506: negative electrode, 507: separator, 508:
electrolyte solution, 509: exterior body, 600: secondary battery,
601: positive electrode cap, 602: battery can, 603: positive
electrode terminal, 604: positive electrode, 605: separator, 606:
negative electrode, 607: negative electrode terminal, 608:
insulating plate, 609: insulating plate,. 610: gasket (insulating
packing), 611: PTC element, 612: safety valve mechanism, 700:
display device, 701: housing, 702: display portion, 703: speaker
portion, 704: nonaqueous secondary battery, 710: lighting device,
711: housing, 712: light source, 713: nonaqueous secondary battery,
714: ceiling, 715: wall, 716: floor, 717: window, 720: indoor unit,
721: housing, 722: air outlet, 723: nonaqueous secondary battery,
724: outdoor unit, 730: electric refrigerator-freezer, 731:
housing, 732: door for refrigerator, 733: door for freezer, 734:
nonaqueous secondary battery, 800: tablet terminal, 801: housing,
802: display portion, 802a: display portion, 802b: display portion,
803: switch for switching display modes, 804: power switch, 805:
switch for switching to power-saving mode, 807: operation switch,
808a: region, 808b: region, 809: operation key, 810: switching
button for showing/hiding keyboard, 811: solar cell, 850:
charge-discharge control circuit, 851: battery, 852: DCDC
converter, 853: converter, 860: electric vehicle, 861: battery,
862: control circuit, 863: driving device, 864: processing unit,
901a: discharge curve of cell D, 901b: charge curve of cell D,
902a: discharge curve of cell G, 902b: charge curve of cell G,
903a: discharge curve of cell H, 903b: charge curve of cell H,
1001: positive electrode active material, 1002: graphene, 1003:
positive electrode active material, 1004: RGO, 1005: positive
electrode active material, and 1006: graphene.
[0269] This application is based on Japanese Patent Application
serial no. 2012-089346 filed with the Japan Patent Office on April
10, 2012 and Japanese Patent Application serial no. 2012-125138
filed with the Japan Patent Office on May 31, 2012, the entire
contents of which are hereby incorporated by reference.
* * * * *