U.S. patent application number 13/940287 was filed with the patent office on 2014-01-23 for secondary battery.
The applicant listed for this patent is SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Nobuhiro Inoue, Junpei Momo, Tamae Moriwaka, Teppei Oguni, Ryota Tajima, Shunpei YAMAZAKI.
Application Number | 20140023920 13/940287 |
Document ID | / |
Family ID | 49946799 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140023920 |
Kind Code |
A1 |
YAMAZAKI; Shunpei ; et
al. |
January 23, 2014 |
SECONDARY BATTERY
Abstract
A secondary battery in which graphite that is an active material
can occlude and release lithium efficiently is provided. Further, a
highly reliable secondary battery in which the amount of lithium
inserted and extracted into/from graphite that is an active
material is prevented from varying is provided. The secondary
battery includes a negative electrode including a current collector
and graphite provided over the current collector, and a positive
electrode. The graphite includes a plurality of graphene layers.
Surfaces of the plurality of graphene layers are provided
substantially along the direction of an electric field generated
between the positive electrode and the negative electrode.
Inventors: |
YAMAZAKI; Shunpei; (Tokyo,
JP) ; Oguni; Teppei; (Atsugi, JP) ; Moriwaka;
Tamae; (Isehara, JP) ; Momo; Junpei;
(Sagamihara, JP) ; Tajima; Ryota; (Isehara,
JP) ; Inoue; Nobuhiro; (Atsugi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMICONDUCTOR ENERGY LABORATORY CO., LTD. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
49946799 |
Appl. No.: |
13/940287 |
Filed: |
July 12, 2013 |
Current U.S.
Class: |
429/211 |
Current CPC
Class: |
H01M 4/366 20130101;
H01M 4/133 20130101; Y02E 60/10 20130101; H01M 4/587 20130101 |
Class at
Publication: |
429/211 |
International
Class: |
H01M 4/133 20060101
H01M004/133 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 20, 2012 |
JP |
2012-161489 |
Claims
1. A secondary battery comprising: a negative electrode comprising
a current collector provided with graphite, the graphite comprising
a plurality of graphene layers; and a positive electrode, wherein
surfaces of the plurality of graphene layers are provided
substantially in parallel to a direction of an electric field
generated between the positive electrode and the negative
electrode.
2. The secondary battery according to claim 1, wherein edges of the
plurality of graphene layers are each terminated by one or more of
--O--Si, --O--P, --O-M, --Si, --P, and -M, and wherein M is a metal
element.
3. The secondary battery according to claim 1, wherein edges of the
plurality of graphene layers each have a structure of one or more
of C--O--Si, C--O--P, C--O-M, C--Si, C--P, and C-M, and wherein M
is a metal element.
4. The secondary battery according to claim 1, wherein edges of the
plurality of graphene layers are in contact with the current
collector.
5. The secondary battery according to claim 1, wherein the graphite
is a highly oriented pyrolytic graphite.
6. An electronic device comprising the secondary battery according
to claim 1.
7. A secondary battery comprising: a negative electrode comprising
a current collector provided with graphite, the graphite comprising
a first plurality of graphene layers and a second plurality of
graphene layers; and a positive electrode, wherein surfaces of the
first plurality of graphene layers are provided substantially in
parallel to a direction of an electric field generated between the
positive electrode and the negative electrode, wherein surfaces of
the second plurality of graphene layers are provided substantially
in parallel to the direction of the electric field generated
between the positive electrode and the negative electrode, and
wherein a normal direction of the first plurality of graphene
layers and a normal direction of the second plurality of graphene
layers are different from each other.
8. The secondary battery according to claim 7, wherein edges of the
first plurality of graphene layers are each terminated by one or
more of --O--Si, --O--P, --O-M, --Si, --P, and -M, and wherein M is
a metal element.
9. The secondary battery according to claim 7, wherein edges of the
first plurality of graphene layers each have a structure of one or
more of C--O--Si, C--O--P, C--O-M, C--Si, C--P, and C-M, and
wherein M is a metal element.
10. The secondary battery according to claim 7, wherein edges of
the first plurality of graphene layers and edges of the second
plurality of graphene layers are in contact with the current
collector.
11. The secondary battery according to claim 7, wherein the
graphite is a highly oriented pyrolytic graphite.
12. An electronic device comprising the secondary battery according
to claim 7.
13. A secondary battery comprising: a negative electrode comprising
a current collector provided with graphite, the graphite comprising
a first plurality of graphene layers and a second plurality of
graphene layers; and a positive electrode, wherein surfaces of the
first plurality of graphene layers are provided substantially in
parallel to a direction of an electric field generated between the
positive electrode and the negative electrode, wherein surfaces of
the second plurality of graphene layers are provided substantially
in parallel to the direction of the electric field generated
between the positive electrode and the negative electrode, and
wherein the second plurality of graphene layers is stacked over the
first plurality of graphene layers.
14. The secondary battery according to claim 13, wherein a normal
direction of the first plurality of graphene layers and a normal
direction of the second plurality of graphene layers are parallel
to each other.
15. The secondary battery according to claim 13, wherein a normal
direction of the first plurality of graphene layers and a normal
direction of the second plurality of graphene layers are orthogonal
to each other.
16. The secondary battery according to claim 13, wherein edges of
the first plurality of graphene layers are each terminated by one
or more of --O--Si, --O--P, --O-M, --Si, --P, and -M, and wherein M
is a metal element.
17. The secondary battery according to claim 13, wherein edges of
the first plurality of graphene layers each have a structure of one
or more of C--O--Si, C--O--P, C--O-M, C--Si, C--P, and C-M, and
wherein M is a metal element.
18. The secondary battery according to claim 13, wherein edges of
the first plurality of graphene layers are in contact with the
current collector.
19. The secondary battery according to claim 13, wherein the
graphite is a highly oriented pyrolytic graphite.
20. An electronic device comprising the secondary battery according
to claim 13.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a secondary battery.
[0003] 2. Description of the Related Art
[0004] In recent years, portable electronic devices such as cell
phones, smartphones, electronic book (e-book) readers, and portable
game machines have come into wide use. Being used as power sources
for driving these devices, nonaqueous secondary batteries typified
by lithium secondary batteries have been researched and developed
actively. Secondary batteries are of growing importance in a
variety of uses; for example, hybrid electric vehicles and electric
vehicles receive attention because of an increased interest in
global environmental problems and an oil resources problem.
[0005] For a negative electrode of a lithium secondary battery, a
typical example of secondary batteries, graphite that is
crystalline carbon has been widely used because of its high
theoretical capacity of 372 mAh/g, high efficiency, and excellent
cycle performance.
[0006] Graphite is a layered compound in which a plurality of
graphene layers is stacked in parallel to each other by van der
Waals forces. When a secondary battery using such a graphite
material for a negative electrode active material is charged,
lithium is inserted between the plurality of graphene layers to
form a lithium-graphite intercalation compound, and lithium is
occluded (intercalated) between the graphene layers. This
suppresses formation of a dendrite which presents problems when
metallic lithium is used as an active material. On the other hand,
lithium is released (deintercalated) when the secondary battery is
discharged. In such a manner, charge/discharge reaction of the
secondary battery occurs.
[0007] A surface of the graphite material includes a plane parallel
to the graphene layer (also referred to as a basal plane) and a
plane where edges of a plurality of graphene layers are provided
(also referred to as an edge plane). In the basal plane, one
surface of the outmost layer of the graphene layers which compose
graphite is exposed. In the edge plane, the edges of the plurality
of graphene layers are exposed. At the time of charging and
discharging the secondary battery, lithium is inserted and
extracted into/from the edge plane of the graphite material and not
inserted and extracted into/from the basal plane.
[0008] In a secondary battery using a commercially available
graphite electrode, for example, graphite powder is used as the
graphite material as described in Patent Document 1.
REFERENCE
[0009] [Patent Document 1] Japanese Published Patent Application
No. 2007-103382
SUMMARY OF THE INVENTION
[0010] However, graphite powder is mixed with a binder and a
conductive additive to form a negative electrode active material
layer, and edge planes of the graphite powder face various
directions. For this reason, lithium moving in the direction of an
electric field generated between positive and negative electrodes
cannot be intercalated between graphene layers of graphite
efficiently, which suppresses output characteristics of the
secondary battery.
[0011] Since the graphite powder has the edge planes facing various
directions as described above, the amount of inserted and extracted
lithium varies, leading to deterioration of the negative electrode,
which is a problem in the reliability of the secondary battery.
[0012] The volume ratio or weight ratio of graphite in the negative
electrode active material layer is decreased because the graphite
powder is used by being mixed with the binder, the conductive
additive, and the like as described above; thus, active material
density of the negative electrode active material layer is
decreased. Further, in the negative electrode active material
layer, the graphite powder needs to be impregnated with an
electrolyte solution.
[0013] Furthermore, on the edge plane including the edges of the
plurality of graphene layers, a film called a solid electrolyte
interphase is formed at the time of initial charging and lithium is
consumed due to the formation of the film, which generates
irreversible capacity. In the case where graphite powder obtained
by pulverizing graphite into fine particles is particularly used as
an active material, quantity of electricity consumed on the
formation of the film is increased because of a large specific
surface area, resulting in an increase in irreversible capacity. In
the case where a plurality of dangling bonds is formed in the edges
of the graphene layers, irreversible capacity is further
increased.
[0014] This film has ion conductivity which allows passage of
lithium ions. When the film has electron conductivity, the film and
an electrolyte solution cause decomposition reaction which
facilitates the formation of the film, resulting in a decrease in
discharge capacity and deterioration of a negative electrode.
[0015] In view of the above problems, an object of one embodiment
of the present invention is to provide a secondary battery in which
lithium can be inserted and extracted into/from graphite that is an
active material efficiently.
[0016] An object of one embodiment of the present invention is to
provide a highly reliable secondary battery in which the amount of
lithium inserted and extracted into/from graphite that is an active
material is prevented from varying.
[0017] An object of one embodiment of the present invention is to
provide a secondary battery having high output and high capacity by
including an active material layer whose active material density is
high.
[0018] Further, an object of one embodiment of the present
invention is to provide a secondary battery in which an active
material layer does not need to be impregnated with an electrolyte
solution and lithium can be inserted and extracted into/from a
surface of the active material layer efficiently.
[0019] Furthermore, an object of one embodiment of the present
invention is to provide a secondary battery including a highly
reliable negative electrode in which defects in edges of a
plurality of graphene layers included in an edge plane are
reduced.
[0020] One embodiment of the present invention is a secondary
battery including a negative electrode which includes a current
collector and graphite provided over the current collector, and a
positive electrode. The graphite includes a plurality of graphene
layers. Surfaces of the plurality of graphene layers are provided
substantially along the direction of an electric field generated
between the positive electrode and the negative electrode.
[0021] One embodiment of the present invention is a secondary
battery including a negative electrode which includes a current
collector and graphite provided over the current collector, and a
positive electrode. The graphite includes a plurality of graphene
layers. Surfaces of the plurality of graphene layers are provided
substantially in parallel to the direction of an electric field
generated between the positive electrode and the negative
electrode.
[0022] One embodiment of the present invention is a secondary
battery including a negative electrode which includes a current
collector and a plurality of graphite and provided over the current
collector and a positive electrode. Each of the plurality of
graphite includes a plurality of graphene layers. Surfaces of the
plurality of graphene layers are provided substantially along the
direction of an electric field generated between the positive
electrode and the negative electrode.
[0023] One embodiment of the present invention is a secondary
battery including a negative electrode which includes a current
collector and a plurality of graphite and provided over the current
collector and a positive electrode. Each of the plurality of
graphite includes a plurality of graphene layers. Surfaces of the
plurality of graphene layers are provided substantially in parallel
to the direction of an electric field generated between the
positive electrode and the negative electrode.
[0024] Here, graphite is a layered compound in which a plurality of
graphene layers is stacked in parallel to each other by van der
Waals forces. Further, the graphene layer is a sheet composed of a
hexagonal net pattern of a one-atom thick layer of carbon formed by
carbon atoms which are covalently bonded to each other to form
sp.sup.2 hybrid orbitals and tricoordinate with each other at an
angle of 120.degree. in a surface. Note that defects or functional
groups may be partly included in the graphene layer.
[0025] Graphite occurs naturally in nature (this is referred to as
natural graphite). Graphite is classified as vein graphite, flake
graphite, amorphous graphite, or the like according to its shape.
On the other hand, graphite can be artificially formed although the
resulting graphite is of inferior crystallinity in general. For
example, pyrolytic graphite is subjected to heat treatment at a
high temperature around 3000.degree. C., whereby graphite can be
obtained. This is referred to as artificial graphite. As artificial
graphite, mesophase spherules, pitch based carbon fibers, pitch
cokes, or the like can be given.
[0026] In one embodiment of the present invention, a plurality of
graphene layers included in such graphite has surfaces provided
substantially along or in parallel to the direction of an electric
field generated between positive and negative electrodes. Since the
surfaces of the plurality of graphene layers are stacked in
parallel to each other, a gap between the graphene layers where
lithium is intercalated is aligned with the direction of entry of
lithium by providing the graphene layers substantially along the
direction of the electric field; thus, lithium can be efficiently
occluded between the graphene layers.
[0027] Further, one embodiment of the present invention is a
secondary battery in which edges of the graphene layers are each
terminated by one or more of --O--Si, --O--P, O-M (M is a metal),
--Si, --P, and -M (M is a metal).
[0028] One embodiment of the present invention is a secondary
battery in which edges of the graphene layers each have a structure
of one or more of C--O--Si, C--O--P, C--O-M (M is a metal), C--Si,
C--P, and C-M (M is a metal).
[0029] Further, each of the edges of the plurality of graphene
layers can be chemically stable by terminating each of dangling
bonds formed in the edges of the graphene layers by one or more of
--O--Si, --O--P, --O-M (M is a metal), --Si, --P, and -M (M is a
metal).
[0030] One embodiment of the present invention makes it possible to
provide a secondary battery in which lithium can be inserted and
extracted into/from graphite that is an active material
efficiently.
[0031] One embodiment of the present invention makes it possible to
provide a highly reliable secondary battery in which the amount of
lithium inserted and extracted into/from graphite that is an active
material is prevented from varying.
[0032] One embodiment of the present invention makes it possible to
provide a secondary battery having high output and high capacity by
including an active material layer whose active material density is
high.
[0033] One embodiment of the present invention makes it possible to
provide a secondary battery including a highly reliable negative
electrode in which defects in edges of a plurality of graphene
layers included in an edge plane are reduced.
[0034] One embodiment of the present invention makes it possible to
provide a secondary battery in which an active material layer does
not need to be impregnated with an electrolyte solution and lithium
can be inserted and extracted into/from a surface of the active
material layer efficiently.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIGS. 1A to 1C illustrate a negative electrode.
[0036] FIGS. 2A and 2B illustrate insertion of lithium into a
negative electrode.
[0037] FIGS. 3A and 3B illustrate negative electrodes.
[0038] FIGS. 4A and 4B illustrate a negative electrode.
[0039] FIGS. 5A and 5B illustrate a negative electrode.
[0040] FIG. 6 illustrates a negative electrode.
[0041] FIGS. 7A and 7B illustrate negative electrodes.
[0042] FIGS. 8A to 8C illustrate termination of an edge of a
graphene layer.
[0043] FIGS. 9A and 9B illustrate a coin-type secondary battery and
a laminated secondary battery.
[0044] FIGS. 10A and 10B illustrate a cylindrical secondary
battery.
[0045] FIG. 11 illustrates electronic devices.
[0046] FIGS. 12A to 12C illustrate an electronic device.
[0047] FIGS. 13A and 13B illustrate an electronic device.
[0048] FIGS. 14A and 14B illustrate cells used in CV
measurement.
[0049] FIGS. 15A and 15B show results of CV measurement.
DETAILED DESCRIPTION OF THE INVENTION
[0050] Embodiments and an example are described below with
reference to drawings. However, the embodiments and the example can
be implemented with various modes. It will be readily appreciated
by those skilled in the art that modes and details 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 and the example.
Embodiment 1
[0051] In this embodiment, a secondary battery of one embodiment of
the present invention is described with reference to FIGS. 1A to 1C
and FIGS. 2A and 2B.
[0052] FIG. 1A is a perspective view of a negative electrode. A
negative electrode 100 has a structure in which an active material
layer 102 is provided over a current collector 101. Note that
although the active material layer 102 is provided on one surface
of the current collector 101 in FIG. 1A, the active material layer
102 may be provided on both surfaces of the current collector
101.
[0053] The current collector 101 can be formed using a material
which has high conductivity and is not alloyed with carrier ions
such as lithium ions, e.g., a metal typified by stainless steel,
gold, platinum, zinc, iron, copper, aluminum, titanium, or
tantalum, or an alloy thereof. Alternatively, the current collector
101 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. Further alternatively, the
current collector 101 may be formed using a metal element which
forms silicide by reacting with silicon. Examples of the metal
element which forms silicide by reacting with silicon include
zirconium, titanium, hafnium, vanadium, niobium, tantalum,
chromium, molybdenum, tungsten, cobalt, nickel, and the like. The
current collector 101 can have a foil-like shape, a plate-like
shape (sheet-like shape), a net-like shape, a cylindrical shape, a
coil shape, a punching-metal shape, an expanded-metal shape, or the
like, as appropriate. The current collector 101 preferably has a
thickness of more than or equal to 10 .mu.m and less than or equal
to 30 .mu.m.
[0054] As illustrated in FIG. 1A, the active material layer 102 is
provided over the current collector 101. In one embodiment of the
present invention, graphite that is a crystalline carbon material
is used as the active material layer 102. As graphite, natural
graphite such as vein graphite, flake graphite, or amorphous
graphite, or artificial graphite such as mesophase spherules,
pitch-based carbon fibers, pitch cokes, kish graphite, or highly
oriented pyrolytic graphite (HOPG) can be used.
[0055] FIG. 1B is an enlarged view of part of the negative
electrode 100 in FIG. 1A. FIG. 1C is a cross-sectional view of the
negative electrode 100 in the thickness direction. A plurality of
graphene layers 103 is included in the graphite provided as the
active material layer 102. Graphite is a layered compound in which
the plurality of graphene layers 103 is stacked in parallel to each
other by van der Waals forces. In one embodiment of the present
invention, surfaces of the plurality of graphene layers 103 are
provided substantially along or in parallel to the direction of an
electric field generated between a positive electrode and the
negative electrode as described later. In other words, the
direction of this electric field is substantially parallel to the
shortest possible line of straight lines between the positive
electrode and the negative electrode. Note that "a surface of a
graphene layer is substantially along or in parallel to the
direction of an electric field" means that the angle between the
direction of the electric field and the surface of the graphene
layer ranges from -20.degree. to 20.degree., for example.
[0056] Note that in FIG. 1C, a schematic diagram of one embodiment
of the present invention, a surface formed by edges of the
plurality of graphene layers 103 (a surface of an aggregate of the
edges) as a surface of the active material layer 102 is parallel to
a top surface of the current collector 101 and perpendicular to the
direction of the electric field. Note that the surface of the
current collector 101 is not necessarily flat, and has unevenness
in some cases. It is sufficient that surfaces of the plurality of
graphene layers are substantially parallel to the direction of the
electric field even when the surface of the current collector 101
has unevenness. In the edge plane, positions of the edges of the
graphene layers 103 are not necessarily aligned. The edge plane may
have a step-like form, and the surfaces of the plurality of
graphene layers may be at least substantially parallel to the
direction of the electric field.
[0057] Lithium can be inserted into the active material layer 102
efficiently by providing the graphene layers 103 included in the
graphite substantially along or in parallel to the direction of the
electric field in such a manner. Since every point of the surface
of the active material layer 102 includes the edge of the graphene
layer 103, lithium is uniformly inserted between the graphene
layers, so that there is no variation in the concentration of
lithium in the whole area of the active material layer. Thus, the
reliability of the negative electrode can be improved. In addition,
the active material layer 102 illustrated in FIGS. 1A to 1C is a
single crystal film in which the plurality of graphene layers 103
is provided in the same direction. Accordingly, the active material
layer 102 includes graphite only, and therefore can be used for a
negative electrode with high density in a secondary battery.
[0058] FIG. 2A is a schematic diagram of a secondary battery of one
embodiment of the present invention including the negative
electrode 100 illustrated in FIG. 1C. A positive electrode 150 is
provided to face the negative electrode 100. The positive electrode
150 includes a current collector 151 and an active material layer
152 provided over the current collector 151 (under the current
collector 151 in FIG. 2A). A porous separator 153 which is shown by
a dotted frame is provided between the positive electrode 150 and
the negative electrode 100, and an electrolyte solution (not
illustrated) fills pores of the separator 153. When the secondary
battery is charged, a voltage is applied from the positive
electrode 150 to the negative electrode 100 and an electric field
154 is generated. Accordingly, lithium in an electrolyte contained
in the electrolyte solution, such as LiPF.sub.6, moves. After
reaching the negative electrode 100, lithium is inserted into a gap
between the graphene layers 103. Lithium is occluded into the
active material layer 102 by forming a lithium-graphite
intercalation compound in the gap between the plurality of graphene
layers 103.
[0059] FIG. 2B is a schematic diagram of electric potential
distribution between positive and negative electrodes. There is a
gradient of the electric potential from a positive electrode 160 to
a negative electrode 164. A potential difference between the
positive electrode 160 and the negative electrode 164 is a cell
voltage. Although the electric potential changes little in a region
between the positive electrode 160 and the negative electrode 164
which is filled with an electrolyte solution 162, an electric
double layer 161 and an electric double layer 163 are formed at the
interface between the electrolyte solution 162 and the positive
electrode 160 and the interface between the electrolyte solution
162 and the negative electrode 164, respectively, and the potential
difference is generated. Consequently, a strong electric field is
generated particularly at the interface between the electrode and
the electrolyte solution.
[0060] The plurality of graphene layers 103 provided substantially
along or in parallel to such an electric field allows efficient
insertion and extraction of lithium.
[0061] Further, as illustrated in FIG. 2B, electric potential
distribution is seen inside the positive electrode 160 and the
negative electrode 164 in some cases. A potential difference
generated inside an electrode is due to ohmic loss or the like. In
that case, lithium which is inserted into the active material layer
102 further moves along the direction of the electric field in the
electrode, and moves into a deep portion of the gap between the
graphene layers by diffusing and being influenced by the electric
field. At this time, the surfaces of the plurality of graphene
layers are provided substantially along or in parallel to the
direction of the electric field, which promotes intercalation of
lithium.
[0062] The above-described structure of the negative electrode 100
can be obtained by forming HOPG or kish graphite over the current
collector 101 with the direction of its edge plane adjusted as
appropriate, for example.
[0063] Next, the above-described structure of a negative electrode
including graphite in which a plurality of graphene layers is
provided substantially along or in parallel to an electric field is
described with reference to FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS.
5A and 5B, and FIG. 6.
[0064] FIGS. 1A to 1C illustrate the graphite in a film form in
which the graphene layers are oriented over the current collector
101. This is an example of single crystal graphite. On the other
hand, FIGS. 3A and 3B each illustrate the negative electrode 100 in
which as the active material layer 102, a plurality of active
materials is provided over the current collector 101. In FIG. 3A,
as the active material layer 102, two active materials 102a and
102b are provided over the current collector 101, for example. The
number of active materials may be more than two. In the example in
FIG. 3A, when a direction connecting a positive electrode and the
negative electrode is a Z axis, surfaces of the graphene layers 103
in the two active materials 102a and 102b are substantially
parallel to the Z-axis direction, and the surfaces of the graphene
layers in different active materials are provided in the same
direction (i.e., in parallel to each other).
[0065] In FIG. 3B, the active material layer 102 includes a
plurality of active materials 102c, 102d, and 102e. Surfaces of the
graphene layers 103 included in different active materials are
provided in different directions. That is, when a direction
connecting the positive electrode and the negative electrode is the
Z axis, although the surfaces of the graphene layers 103 are
substantially parallel to the Z-axis direction, the surfaces are
not necessarily parallel to an X-axis direction and a Y-axis
direction, which are orthogonal to the Z axis-direction.
[0066] As described above, a plurality of active materials may be
provided over the current collector 101. This makes it possible to
fabricate a negative electrode showing the same effect even when it
is difficult to form single crystal graphite in a large size. In
the case of providing a plurality of active materials over the
current collector 101, the plurality of active materials may be
dotted at random as illustrated in FIG. 3B, or may be spread all
over the current collector 101 with their directions controlled. In
any case, it is important to provide the active materials so that
the surfaces of the graphene layers are substantially along or
parallel to the direction of the electric field.
[0067] Note that although not illustrated, when a plurality of
active materials is provided over the current collector 101, a
conductive additive or a binder may be provided to fill spaces
between the plurality of active materials. In the case of providing
the conductive additive, an electron conducting path is formed
between the active materials, so that electric potentials are
uniform at any point in the negative electrode 100, which enables
variations in deterioration of the negative electrode to be
reduced. In the case of providing the binder, separation of the
active material caused by expansion and contraction of the active
material due to charging and discharging of a secondary battery can
be suppressed, leading to an improvement in the reliability of the
negative electrode.
[0068] As the conductive additive, carbon particles such as
acetylene black particles, ketjen black particles, or carbon
nanofibers can be used. Further, graphene obtained by reducing
graphene oxide, which is separated from graphite by a synthesis
method such as a Hummers method, through heat treatment,
electrochemical treatment, chemical treatment, or the like can also
be used as the conductive additive.
[0069] As the binder, instead of polyvinylidene fluoride (PVDF) as
a typical one, polyimide, polytetrafluoroethylene, polyvinyl
chloride, an ethylene-propylene-diene polymer, butadiene rubber,
styrene-butadiene rubber, butyl rubber, acrylonitrile-butadiene
rubber, fluorine rubber, polyvinyl acetate, polymethyl
methacrylate, polyethylene, polypropylene, nitrocellulose, or the
like can be used.
[0070] Next, examples of a negative electrode with a structure
including a stack of active materials each of which is the active
material illustrated in FIG. 1B are described with reference to
FIGS. 4A and 4B, FIGS. 5A and 5B, and FIG. 6.
[0071] In a negative electrode 200 illustrated in FIGS. 4A and 4B,
an active material layer 202 provided over a current collector 201
includes a stack of an active material 202a and an active material
202b. The active material 202a is graphite including a plurality of
graphene layers 203a and the active material 202b is graphite
including a plurality of graphene layers 203b. Surfaces of the
plurality of graphene layers 203a included in the active material
202a and surfaces of the plurality of graphene layers 203b included
in the active material 202b are provided substantially along or in
parallel to the direction of the electric field. In the negative
electrode 200, the surfaces of the plurality of graphene layers
203a included in the active material 202a and the surfaces of the
plurality of graphene layers 203b included in the active material
202b face the same direction (the X-axis direction in FIG. 4A).
Consequently, the directions of gaps between the plurality of
graphene layers in the active material 202a and the directions of
gaps between the plurality of graphene layers in the active
material 202b are oriented. The graphene layers 203a and 203b
provided in such a manner enable lithium to be inserted into a gap
between the graphene layers efficiently and to be further inserted
into a deep portion of the gap.
[0072] Although the two active materials are stacked in FIGS. 4A
and 4B, the number of stacked active materials is not limited to
two and may be three or more. The thicknesses of the active
materials may each be determined depending on a depth at which
lithium can be inserted into the active material. For example, the
total thickness of a stack of the active materials (i.e., the
thickness of the active material layer) is preferably more than or
equal to 5 .mu.m and less than or equal to 1 mm The active
materials stacked in such a manner enable the thickness of the
active material layer to be increased. In graphite, a diffusion
rate of lithium is higher than that in other active materials;
therefore, the active material layer can operate as a negative
electrode.
[0073] In a negative electrode 300 illustrated in FIGS. 5A and 5B,
which has a stack of active materials as in the negative electrode
200 illustrated in FIGS. 4A and 4B, the active materials have
different arrangements of graphene layers. That is, an active
material layer 302 of the negative electrode 300 includes an active
material 302a provided over a current collector 301 and an active
material 302b stacked over the active material 302a. Surfaces of a
plurality of graphene layers 303a included in the active material
302a and surfaces of a plurality of graphene layers 303b included
in the active material 302b are both substantially parallel to the
Z-axis direction (the direction of the electric field), and the
surfaces of the graphene layers 303a and the surfaces of the
graphene layers 303b are provided orthogonal to the X-axis
direction and the Y-axis direction, respectively.
[0074] In such a stacked structure, the surfaces of the graphene
layers included in each of the active materials are substantially
parallel to the direction of the electric field, which enables
lithium to be inserted into the active material efficiently. Note
that although the surfaces of the graphene layers included in one
of the active materials are orthogonal to the surfaces of the
graphene layers included in the other active material in FIGS. 5A
and 5B, the arrangements of the surfaces of the graphene layers are
not limited thereto, and the active materials may be stacked so
that the surfaces of the graphene layers make a given angle with
each other.
[0075] FIG. 6 illustrates a negative electrode 350 including active
materials the number of which is larger than that of those in the
negative electrode 300 illustrated in FIGS. 5A and 5B. That is, the
negative electrode 350 includes an active material layer 352 over a
current collector 351; the active material layer 352 includes
active materials 352a, 352b, 352c, and 352d which are stacked.
Surfaces of pluralities of graphene layers 353a and 353c which are
included in the active materials 352a and 352c, respectively, and
surfaces of pluralities of graphene layers 353b and 353d which are
included in the active materials 352b and 352d, respectively are
both substantially parallel to the Z-axis direction (the direction
of the electric field). The surfaces of the pluralities of graphene
layers 353a and 353c are provided orthogonal to the X-axis
direction and the surfaces of the pluralities of graphene layers
353b and 353d are provided orthogonal to the Y-axis direction.
[0076] In such a stacked structure, the surfaces of the graphene
layers included in each of the active materials are substantially
parallel to the direction of the electric field, which enables
lithium to be inserted into the active material efficiently. Note
that although the surfaces of the graphene layers which are
included in the active materials in contact with each other in a
vertical direction are provided orthogonal to each other in FIG. 6,
the arrangements of the surfaces of the graphene layers are not
limited thereto, and the active materials may be stacked so that
the surfaces of the graphene layers make a given angle with each
other.
[0077] FIGS. 7A and 7B each illustrate a negative electrode in
which cylindrical graphite is used for an active material. FIG. 7A
illustrates a negative electrode 370 in which carbon fibers each
having an onion-like structure are used for active materials 372
over a current collector 371. In the onion-like structure, edges of
a plurality of graphene layers form concentric circles around the
central axis of a cylinder. The edges of the plurality of graphene
layers are positioned at a top surface in each of the cylinders.
The cylindrical active materials 372 are provided so that the
central axes thereof are orthogonal to a surface of the current
collector 371. FIG. 7B illustrates a negative electrode 380 in
which carbon nanotubes are used for active materials 382 provided
over a current collector 381. A carbon nanotube is a carbon
material that is a coaxial tube formed of a single layer or a
multilayer of graphene. Thus, surfaces of the graphene layers
formed as coaxial tubes can be provided substantially along or in
parallel to the direction of the electric field by providing the
carbon nanotubes so that the central axes thereof are orthogonal to
a surface of the current collector 381. Note that the densities of
the active materials 372 and 382 can be adjusted as appropriate
depending on the specification of a secondary battery to be
manufactured.
[0078] As described above, surfaces of a plurality of graphene
layers included in an active material are provided substantially
along or in parallel to the direction of an electric field, which
enables graphite that is the active material to occlude and release
lithium efficiently. Further, it is possible to prevent the amount
of lithium inserted and extracted into/from graphite that is the
active material from varying. Furthermore, with the use of an
active material layer whose active material density is high, a
secondary battery with high output and high capacity can be
manufactured.
[0079] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 2
[0080] In this embodiment, termination of edges of a plurality of
graphene layers included in graphite that is crystalline carbon is
described with reference to FIGS. 8A to 8C.
[0081] FIG. 8A illustrates the negative electrode 100 in FIG. 1B
described in Embodiment 1. The current collector 101 is illustrated
at the back of the drawing and the active material layer 102 is
illustrated at the front for convenience. The active material layer
102 includes the plurality of graphene layers 103, and surfaces of
the plurality of graphene layers are stacked in parallel to each
other.
[0082] There are carbon atoms having one or more dangling bonds in
edges of the plurality of graphene layers 103 exposed to an
electrolyte solution. The dangling bond can be regarded as a defect
in the edge of the graphene layer. To deactivate and stabilize the
graphene layer having such a dangling bond, the defect is repaired
by terminating the dangling bond. Specifically, one or more of
groups such as --O--Si, --O--P, --O-M (M is a metal), --Si, --P,
and -M (M is a metal) are bonded to terminate the dangling
bond.
[0083] When the group contains a metal (M), examples of the metal
include aluminum (Al), titanium (Ti), vanadium (V), chromium (Cr),
manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu),
zinc (Zn), niobium (Nb), molybdenum (Mo), silver (Ag), cadmium
(Cd), indium (In), tin (Sn), barium (Ba), hafnium (Hf), tantalum
(Ta), tungsten (W), iridium (Ir), platinum (Pt), gold (Au), lead
(Pb), lanthanum (La), selenium (Ce), palladium (Pr), an alloy of
any of these metals, and the like.
[0084] By terminating the edge of the graphene layer, a structure
of one or more of C--O--Si, C--O--P, C--O-M (M is a metal), C--Si,
C--P, and C-M (M is a metal) are formed in the edge of the graphene
layer.
[0085] FIGS. 8B and 8C illustrate examples in which the edges of
the graphene layers 103 are terminated. FIG. 8B illustrates a state
where an edge of the one graphene layer 103 is terminated by
--O--Si. As illustrated in the drawing, the edge of the graphene
layer can be terminated by one or more of the above-described
groups such as --O--Si, --O--P, --O-M (M is a metal), --Si, --P,
and -M (M is a metal). By terminating dangling bonds so that
adjacent carbon atoms or carbon atoms close to each other are
connected in the edge of the one graphene layer as in FIG. 8B, the
edge of the graphene layer 103 can be stabilized.
[0086] FIG. 8C illustrates a state where edges of the plurality of
graphene layers 103 stacked with each other are terminated by
--O--Si--O--. As illustrated in the drawing, the edges of the
graphene layers 103 can be terminated by one or more of the
above-described groups such as --O--Si, --O--P, --O-M (M is a
metal), --Si, --P, and -M (M is a metal). By terminating dangling
bonds so that adjacent carbon atoms or carbon atoms close to each
other are connected as in FIG. 8C, the edges of the graphene layers
103 can be stabilized.
[0087] The structure of one or more of C--O--Si, C--O--P, C--O-M (M
is a metal), C--Si, C--P, and C-M (M is a metal), which is formed
in the edge of the graphene layer by terminating the dangling bonds
in the edge of the graphene layer as described above, is formed to
have at least a thickness similar to that of a one-atom thick layer
or a several-atom thick layer. However, the structure of one or
more of C--O--Si, C--O--P, C--O-M (M is a metal), C--Si, C--P, and
C-M (M is a metal) may be formed to have a thickness larger than
that of the one-atom thick layer or the several-atom thick
layer.
[0088] An edge plane can be chemically stabilized by terminating
the dangling bonds in each of the edges of the plurality of
graphene layers and two-dimensionally terminating the dangling
bonds in the edges of the plurality of graphene layers. A structure
formed by modification of --O--Si or the like, such as C--O--Si,
functions as a protective film, which makes it possible to prevent
deterioration of graphite due to repeated charge and discharge.
[0089] Since a group such as --O--Si has low electric conductivity,
decomposition reaction of an electrolyte solution at an interface
between the electrolyte solution and an electrode can be prevented.
Thus, a film is prevented from being excessively formed, resulting
in a decrease in irreversible capacity.
[0090] In addition, when the edges of the plurality of graphene
layers 103 which are stacked as illustrated in FIG. 8C are
terminated by --O--Si--O-- or the like, the distance between the
graphene layers 103 bonded to --O--Si--O-- is less likely to
increase. For this reason, it is probable that solvated lithium
ions are difficult to be inserted into a gap between the graphene
layers. Thus, it is probable that solvated lithium ions are
desolvated in a film formed outside a layer of the structure formed
by termination, such as C--O--Si, and only lithium ions are
inserted into the gap between the graphene layers. This makes it
possible to prevent deterioration of graphite.
[0091] The edge of the graphene layer can be terminated as
described above by a sol-gel method, a plating method such as an
electroless plating method, a sputtering method, a CVD method, or
the like.
[0092] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 3
[0093] In this embodiment, a variety of structures of secondary
batteries described in Embodiments 1 and 2 are described with
reference to FIGS. 9A and 9B and FIGS. 10A and 10B.
(Coin-Type Secondary Battery)
[0094] FIG. 9A is an external view of a coin-type (single-layer
flat type) secondary battery, part of which also illustrates a
cross-sectional view of part of the coin-type secondary
battery.
[0095] In a coin-type secondary battery 450, a positive electrode
can 451 serving also as a positive electrode terminal and a
negative electrode can 452 serving also as a negative electrode
terminal are insulated and sealed with a gasket 453 formed of
polypropylene or the like. A positive electrode 454 includes a
positive electrode current collector 455 and a positive electrode
active material layer 456 which is provided to be in contact with
the positive electrode current collector 455. A negative electrode
457 is formed of a negative electrode current collector 458 and a
negative electrode active material layer 459 which is provided to
be in contact with the negative electrode current collector 458. A
separator 460 and an electrolyte solution (not illustrated) are
included between the positive electrode active material layer 456
and the negative electrode active material layer 459.
[0096] As the negative electrode 457, any of the negative
electrodes 100, 200, 300, and 350 described in the above
embodiments is used.
[0097] As the positive electrode 454, any of a variety of known
positive electrodes can be used. For example, the positive
electrode 454 may include the positive electrode current collector
455 and the positive electrode active material layer 456 provided
thereover.
[0098] The positive electrode current collector 455 can be formed
using a highly conductive material such as a metal typified by
stainless steel, gold, platinum, zinc, iron, copper, aluminum, or
titanium, or an alloy thereof. Note that the positive electrode
current collector 455 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. Further,
the positive electrode current collector 455 may be formed using a
metal element which forms silicide by reacting with silicon.
Examples of the metal element which forms silicide by reacting with
silicon include zirconium, titanium, hafnium, vanadium, niobium,
tantalum, chromium, molybdenum, tungsten, cobalt, nickel, and the
like. The positive electrode current collector 455 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.
[0099] As a positive electrode active material used for the
positive electrode active material layer, a material into/from
which lithium ions can be inserted and extracted 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 given.
[0100] As the lithium-containing composite oxide with an olivine
crystal structure, a composite oxide represented by a general
formula LiMPO.sub.4 (M is one or more of Fe(II), Mn(II), Co(II),
and Ni(II)) can be given. Typical examples of the general formula
LiMPO.sub.4 include 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.ltoreq.i<1), and
the like.
[0101] LiFePO.sub.4 is particularly preferable because it meets
requirements with balance for a positive electrode active material,
such as safety, stability, high capacity density, high potential,
and the existence of lithium ions that can be extracted in initial
oxidation (charging).
[0102] Examples of the lithium-containing composite oxide with a
layered rock-salt crystal structure include lithium cobalt oxide
(LiCoO.sub.2); LiNiO.sub.2; LiMnO.sub.2; Li.sub.2MnO.sub.3; an
NiCo-based lithium-containing composite oxide (a general formula
thereof is LiNi.sub.xCo.sub.1-xO.sub.2 (0<x<1)) such as
LiNi.sub.0.8Co.sub.2.0O.sub.2; an NiMn-based lithium-containing
composite oxide (a general formula thereof is
LiNi.sub.xMn.sub.1-xO.sub.2 (0<x<1)) such as
LiNi.sub.0.5Mn.sub.0.5O.sub.2; and an NiMnCo-based
lithium-containing composite oxide (also referred to as NMC, and a
general formula thereof is 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. Moreover,
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 can
be given.
[0103] LiCoO.sub.2 is particularly preferable because it has high
capacity, is more stable in the air than LiNiO.sub.2, and is more
thermally stable than LiNiO.sub.2, for example.
[0104] Examples of the lithium-containing composite oxide with a
spinel crystal structure include LiMn.sub.2O.sub.4,
Li.sub.1+xMn.sub.2-xO.sub.4, Li(MnAl).sub.2O.sub.4,
LiMn.sub.1.5Ni.sub.0.5O.sub.4, and the like.
[0105] A lithium-containing composite oxide with a spinel crystal
structure including manganese, such as LiMn.sub.2O.sub.4, is
preferably mixed with a small amount of lithium nickel oxide (e.g.,
LiNiO.sub.2 or LiNi.sub.1-xMO.sub.2 (M=Co, Al, or the like)), in
which case elution of manganese is suppressed, for example.
[0106] As the positive electrode active material, a composite oxide
represented by a general formula Li(.sub.2-j)MSiO.sub.4 (M is one
or more of Fe(II), Co(II), and Ni(II); 0.ltoreq.j.ltoreq.2) can be
used. Typical examples of the general formula
Li(.sub.2-j)MSiO.sub.4 include 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.ltoreq.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.nM.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.
[0107] Further, as the positive electrode active material, a
nasicon compound represented by a general formula
A.sub.xM.sub.2(XO.sub.4).sub.3 (A=Li, Na, or Mg; M=Fe, Mn, Ti, V,
Nb, or Al; and X.dbd.S, P, Mo, W, As, or Si) can be used. Examples
of the nasicon compound include 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. Furthermore, as the positive electrode active material, a
compound represented by a general formula Li.sub.2MPO.sub.4F,
Li.sub.2MP.sub.2O.sub.7, or Li.sub.5MO.sub.4 (M=Fe or Mn);
perovskite fluoride such as NaF.sub.3 or FeF.sub.3; metal
chalcogenide such as TiS.sub.2 or MoS.sub.2 (sulfide, selenide, or
telluride); a lithium-containing composite oxide with an inverse
spinel crystal structure such as LiMVO.sub.4; a vanadium oxide
based material (e.g., V.sub.2O.sub.5, V.sub.6O.sub.13, and
LiV.sub.3O.sub.8); a manganese oxide based material; an organic
sulfur based material; or the like can be used.
[0108] When carrier ions are alkali metal ions other than lithium
ions, such as alkaline-earth metal ions, beryllium ions, or
magnesium ions, an alkali metal (e.g., sodium or potassium), an
alkaline-earth metal (e.g., calcium, strontium, or barium),
beryllium, or magnesium may be used as the positive electrode
active material layer 456, instead of lithium in the lithium
compound and the lithium-containing composite oxide.
[0109] The positive electrode active material layer 456 is formed
over the positive electrode current collector 455 by a coating
method or a physical vapor deposition method (e.g., a sputtering
method), whereby the positive electrode 454 can be formed. In the
case where a coating method is employed, the positive electrode
active material layer 456 is formed in such a manner that a paste
in which a conductive additive (e.g., acetylene black (AB)), a
binder (e.g., polyvinylidene fluoride (PVDF))), and the like are
mixed with any of the above materials of the positive electrode
active material layer 456 is applied to the positive electrode
current collector 455 and dried. In this case, the positive
electrode active material layer 456 is preferably molded by
applying pressure as needed.
[0110] Note that as the conductive additive, an electron-conductive
material can be used as long as it does not chemically change in
the secondary battery. For example, a carbon-based material such as
graphite or carbon fibers; a metal material such as copper, nickel,
aluminum, or silver; and powder, fiber, and the like of mixtures
thereof can be given. Further, graphene may be used instead of
these conductive additives. For example, a polar solvent to which a
positive electrode active material, a binder, and graphene oxide
are added is mixed, and the mixture is subjected to heat treatment
or the like to reduce graphene oxide; thus, a positive electrode
active material layer containing graphene can be formed. An
electron conducting path connecting the positive electrode active
materials is formed in the positive electrode active material layer
including graphene in such a manner; thus, the positive electrode
active material layer can have high electron conductivity, which is
similar to the negative electrode of one embodiment of the present
invention.
[0111] As the binder, instead of polyvinylidene fluoride (PVDF) as
a typical one, polyimide, polytetrafluoroethylene, polyvinyl
chloride, ethylene-propylene-diene polymer, butadiene rubber,
styrene-butadiene rubber, butyl rubber, acrylonitrile-butadiene
rubber, fluorine rubber, polyvinyl acetate, polymethyl
methacrylate, polyethylene, polypropylene, nitrocellulose or the
like can be used.
[0112] The positive electrode active material layer 456 is not
necessarily formed on and in direct contact with the positive
electrode current collector 455. Between the positive electrode
current collector 455 and the positive electrode active material
layer 456, any of the following functional layers may be formed
using a conductive material such as a metal: an adhesive layer for
the purpose of improving adhesiveness between the positive
electrode current collector 455 and the positive electrode active
material layer 456, a planarization layer for reducing unevenness
of the surface of the positive electrode current collector 455, a
heat radiation layer for radiating heat, and a stress relaxation
layer for reducing stress on the positive electrode current
collector 455 or the positive electrode active material layer 456.
Further, to have these functions, treatment for modifying a state
of a surface may be performed on the surface of the positive
electrode current collector 455.
[0113] Next, as the separator 460, a porous insulator such as
cellulose (paper), polypropylene (PP), polyethylene (PE),
polybutene, nylon, polyester, polysulfone, polyacrylonitrile,
polyvinylidene fluoride, or tetrafluoroethylene can be used.
Further, nonwoven fabric of a glass fiber or the like, or a
diaphragm in which a glass fiber and a polymer fiber are mixed may
also be used.
[0114] As a solvent for the electrolyte solution, an aprotic
organic solvent is preferably used. For example, one of ethylene
carbonate (EC), propylene carbonate (PC), butylene carbonate,
chloroethylene carbonate, vinylene carbonate,
.gamma.-butyrolactone, .gamma.-valerolactone, dimethyl carbonate
(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC),
methyl formate, methyl acetate, methyl butyrate, 1,3-dioxane,
1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl
ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran,
sulfolane, and sultone can be used, or two or more of these
solvents can be used in an appropriate combination in an
appropriate ratio.
[0115] As an electrolyte dissolved in the above-described solvent,
one of lithium salts such as LiPF.sub.6, LiClO.sub.4, LiAsF.sub.6,
LiBF.sub.4, LiAlCl.sub.4, LiSCN, LiBr, LiI, Li.sub.2SO.sub.4,
Li.sub.2B.sub.10Cl.sub.10, Li.sub.2B.sub.12Cl.sub.12,
LiCF.sub.3SO.sub.3, LiC.sub.4F.sub.9SO.sub.3,
LiC(CF.sub.3SO.sub.2).sub.3, LiC(C.sub.2F.sub.5SO.sub.2).sub.3,
LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.4F.sub.9SO.sub.2)(CF.sub.3SO.sub.2), and
LiN(C.sub.2F.sub.5SO.sub.2).sub.2 can be used, or two or more of
these lithium salts can be used in an appropriate combination in an
appropriate ratio.
[0116] For the positive electrode can 451 and the negative
electrode can 452, a metal having a corrosion-resistant property to
a liquid such as an electrolytic solution in charging and
discharging a 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 451 and the negative electrode can 452 are electrically
connected to the positive electrode 454 and the negative electrode
457, respectively.
[0117] The negative electrode 457, the positive electrode 454, and
the separator 460 are immersed in the electrolyte solution. Then,
as illustrated in FIG. 9A, the positive electrode can 451, the
positive electrode 454, the separator 460, the negative electrode
457, and the negative electrode can 452 are stacked in this order
with the positive electrode can 451 positioned at the bottom, and
the positive electrode can 451 and the negative electrode can 452
are subjected to pressure bonding with the gasket 453 interposed
therebetween. In such a manner, the coin-type secondary battery 450
is manufactured.
(Laminated Secondary Battery)
[0118] Next, an example of a laminated secondary battery is
described with reference to FIG. 9B. In FIG. 9B, a structure inside
the laminated secondary battery is partly exposed for
convenience.
[0119] A laminated secondary battery 470 illustrated in FIG. 9B
includes a positive electrode 473 including a positive electrode
current collector 471 and a positive electrode active material
layer 472, a negative electrode 476 including a negative electrode
current collector 474 and a negative electrode active material
layer 475, a separator 477, an electrolyte solution (not
illustrated), and an exterior body 478. The separator 477 is placed
between the positive electrode 473 and the negative electrode 476
provided in the exterior body 478. The exterior body 478 is filled
with the electrolyte solution. Although the one positive electrode
473, the one negative electrode 476, and the one separator 477 are
used in FIG. 9B, the secondary battery may have a stacked-layer
structure in which positive electrodes, negative electrodes, and
separators are alternately stacked.
[0120] As the negative electrode 476, any of the negative
electrodes 100, 200, 300, and 350 described in the above
embodiments is used.
[0121] For the electrolyte solution, an electrolyte and a solvent
which are similar to those in the above-described coin-type
secondary battery can be used.
[0122] In the laminated secondary battery 470 illustrated in FIG.
9B, the positive electrode current collector 471 and the negative
electrode current collector 474 also serve as terminals (tabs) for
an electrical contact with the outside. For this reason, the
positive electrode current collector 471 and the negative electrode
current collector 474 are provided so that part of the positive
electrode current collector 471 and part of the negative electrode
current collector 474 are exposed outside the exterior body
478.
[0123] As the exterior body 478 in the laminated secondary battery
470, for example, a laminate film having a three-layer structure in
which 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-based resin, a polyester-based 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.
(Cylindrical Secondary Battery)
[0124] Next, an example of a cylindrical secondary battery is
described with reference to FIGS. 10A and 10B. As illustrated in
FIG. 10A, a cylindrical secondary battery 480 includes a positive
electrode cap (battery cap) 481 on a top surface and a battery can
(outer can) 482 on the side surface and bottom surface. The
positive electrode cap 481 and the battery can 482 are insulated
from each other by a gasket 490 (insulating packing).
[0125] FIG. 10B is a diagram schematically illustrating a cross
section of the cylindrical secondary battery. In the battery can
482 with a hollow cylindrical shape, a battery element is provided
in which a strip-like positive electrode 484 and a strip-like
negative electrode 486 are wound with a separator 485 provided
therebetween. Although not illustrated, the battery element is
wound around a center pin as a center. One end of the battery can
482 is close and the other end thereof is open.
[0126] As the negative electrode 486, any of the negative
electrodes 100, 200, 300, and 350 described in the above
embodiments is used.
[0127] For the battery can 482, a metal having a
corrosion-resistant property to a liquid such as an electrolytic
solution in charging and discharging a 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 482, the battery element in which the
positive electrode, the negative electrode, and the separator are
wound is provided between a pair of insulating plates 488 and 489
which face each other.
[0128] An electrolyte solution (not illustrated) is injected inside
the battery can 482 in which the battery element is provided. For
the electrolyte solution, an electrolyte and a solvent which are
similar to those in the above-described coin-type secondary battery
and laminated secondary battery can be used.
[0129] Since the positive electrode 484 and the negative electrode
486 of the cylindrical secondary battery are wound, active
materials are formed on both sides of the current collectors. A
positive electrode terminal (positive electrode current collecting
lead) 483 is connected to the positive electrode 484, and a
negative electrode terminal (negative electrode current collecting
lead) 487 is connected to the negative electrode 486. A metal
material such as aluminum can be used for both the positive
electrode terminal 483 and the negative electrode terminal 487. The
positive electrode terminal 483 and the negative electrode terminal
487 are resistance-welded to a safety valve mechanism 492 and the
bottom of the battery can 482, respectively. The safety valve
mechanism 492 is electrically connected to the positive electrode
cap 481 through a positive temperature coefficient (PTC) element
491. In the case where an internal pressure of the battery is
increased to exceed a predetermined threshold value, the safety
valve mechanism 492 electrically disconnects the positive electrode
cap 481 and the positive electrode 484. The PTC element 491 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.
[0130] Note that in this embodiment, the coin-type secondary
battery, the laminated secondary battery, and the cylindrical
secondary battery are given as examples of the secondary battery;
however, any of secondary batteries with other various shapes, such
as a sealed secondary battery and a square 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 rolled may be employed.
[0131] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 4
[0132] The secondary battery of one embodiment of the present
invention can be used as a power source for a variety of electronic
devices which can operate with electric power.
[0133] Specific examples of electronic devices each using the
secondary battery of one embodiment of the present invention are as
follows: display devices such as televisions and monitors, lighting
devices, desktop personal computers and laptop personal computers,
word processors, image reproduction devices which reproduce still
images and moving images stored in recording media such as digital
versatile discs (DVDs), portable compact disc (CD) players, radio
receivers, tape recorders, headphone stereos, stereos, remote
controls, table clocks, wall clocks, cordless phone handsets,
transceivers, cell phones, car phones, portable game machines,
passometers, calculators, portable information terminals,
electronic notebooks, e-book readers, electronic translators, audio
input devices, cameras such as video cameras and digital still
cameras, toys, electric shavers, electric toothbrushes,
high-frequency heating devices such as microwave ovens, electric
rice cookers, electric washing machines, electric vacuum cleaners,
water heaters, electric fans, hair dryers, air-conditioning systems
such as humidifiers, dehumidifiers, and air conditioners,
dishwashers, dish dryers, clothes dryers, futon dryers, electric
refrigerators, electric freezers, electric refrigerator-freezers,
freezers for preserving DNA, flashlights, electric power tools,
smoke sensors, and medical equipment such as hearing aids, cardiac
pacemakers, and dialyzers. The examples also include industrial
equipment such as guide lights, traffic lights, meters such as gas
meters and water meters, belt conveyors, elevators, escalators,
industrial robots, radio relay stations, cell phone base stations,
power storage systems, and power storage devices for leveling the
amount of power supply and smart grid. In addition, moving objects
driven by motors using electric power from a secondary battery are
also included in the category of electronic devices. Examples of
the moving objects include 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 caterpillars are substituted for wheels of these vehicles,
motorized bicycles including motor-assisted bicycles, motorcycles,
electric wheelchairs, golf carts, boats, ships, submarines,
aircrafts such as fixed wing aircrafts and rotorcrafts, rockets,
artificial satellites, space probes, planetary probes, and
spacecrafts.
[0134] In the above electronic devices, the secondary battery of
one embodiment of the present invention can be used as a main power
source for supplying enough power for almost the whole power
consumption. Alternatively, in the above electronic devices, the
secondary battery of one embodiment of the present invention can be
used as an uninterruptible power source which can supply power to
the electronic devices when the supply of power from the main power
source or a commercial power source is stopped. Still
alternatively, in the above electronic devices, the secondary
battery of one embodiment of the present invention can be used as
an auxiliary power source for supplying power to the electronic
devices at the same time as the power supply from the main power
source or a commercial power source.
[0135] FIG. 11 illustrates specific structures of the electronic
devices. In FIG. 11, a display device 500 is an example of an
electronic device including a secondary battery 504 of one
embodiment of the present invention. Specifically, the display
device 500 corresponds to a display device for TV broadcast
reception and includes a housing 501, a display portion 502,
speaker portions 503, the secondary battery 504, and the like. The
secondary battery 504 of one embodiment of the present invention is
provided in the housing 501. The display device 500 can receive
power from a commercial power source. Alternatively, the display
device 500 can use power stored in the secondary battery 504. Thus,
the display device 500 can be operated with the use of the
secondary battery 504 of one embodiment of the present invention as
an uninterruptible power source even when power cannot be supplied
from a commercial power source due to power failure or the
like.
[0136] 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
electrophoretic display device, a digital micromirror device (DMD),
a plasma display panel (PDP), or a field emission display (FED) can
be used for the display portion 502.
[0137] Note that the display device includes, in its category, all
of information display devices for personal computers,
advertisement displays, and the like in addition to TV broadcast
reception.
[0138] In FIG. 11, an installation lighting device 510 is an
example of an electronic device using a secondary battery 513 of
one embodiment of the present invention. Specifically, the
installation lighting device 510 includes a housing 511, a light
source 512, the secondary battery 513, and the like. Although FIG.
11 illustrates the case where the secondary battery 513 is provided
in a ceiling 514 on which the housing 511 and the light source 512
are installed, the secondary battery 513 may be provided in the
housing 511. The installation lighting device 510 can receive power
from a commercial power source. Alternatively, the installation
lighting device 510 can use power stored in the secondary battery
513. Thus, the installation lighting device 510 can be operated
with the use of the secondary battery 513 of one embodiment of the
present invention as an uninterruptible power source even when
power cannot be supplied from a commercial power source due to
power failure or the like.
[0139] Note that although the installation lighting device 510
provided in the ceiling 514 is illustrated in FIG. 11 as an
example, the secondary battery of one embodiment of the present
invention can be used as an installation lighting device provided
in, for example, a wall 515, a floor 516, a window 517, or the like
other than the ceiling 514. Alternatively, the secondary battery
can be used in a tabletop lighting device or the like.
[0140] As the light source 512, 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 a light-emitting element such as an LED and an organic EL
element are given as examples of the artificial light source.
[0141] In FIG. 11, an air conditioner including an indoor unit 520
and an outdoor unit 524 is an example of an electronic device using
a secondary battery 523 of one embodiment of the present invention.
Specifically, the indoor unit 520 includes a housing 521, an air
outlet 522, the secondary battery 523, and the like. Although FIG.
11 illustrates the case where the secondary battery 523 is provided
in the indoor unit 520, the secondary battery 523 may be provided
in the outdoor unit 524. Alternatively, the secondary battery 523
may be provided in both the indoor unit 520 and the outdoor unit
524. The air conditioner can receive power from a commercial power
source. Alternatively, the air conditioner can use power stored in
the secondary battery 523. Particularly in the case where the
secondary batteries 523 are provided in both the indoor unit 520
and the outdoor unit 524, the air conditioner can be operated with
the use of the secondary battery 523 of one embodiment of the
present invention as an uninterruptible power source even when
power cannot be supplied from a commercial power source due to
power failure or the like.
[0142] Note that although the split-type air conditioner including
the indoor unit and the outdoor unit is illustrated in FIG. 11 as
an example, the 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.
[0143] In FIG. 11, an electric refrigerator-freezer 530 is an
example of an electronic device using a secondary battery 534 of
one embodiment of the present invention. Specifically, the electric
refrigerator-freezer 530 includes a housing 531, a door for a
refrigerator 532, a door for a freezer 533, the secondary battery
534, and the like. The secondary battery 534 is provided inside the
housing 531 in FIG. 11. The electric refrigerator-freezer 530 can
receive power from a commercial power source. Alternatively, the
electric refrigerator-freezer 530 can use power stored in the
secondary battery 534. Thus, the electric refrigerator-freezer 530
can be operated with the use of the secondary battery 534 of one
embodiment of the present invention as an uninterruptible power
source even when power cannot be supplied from a commercial power
source due to power failure or the like.
[0144] Note that among the electronic devices described above, a
high-frequency heating device such as a microwave oven and an
electronic device such as an electric rice cooker require high
power in a short time. The tripping of a circuit breaker of a
commercial power source in use of electronic devices can be
prevented by using the secondary battery of one embodiment of the
present invention as an auxiliary power source for supplying power
which cannot be supplied enough by a commercial power source.
[0145] In addition, in a time period when electronic devices are
not used, particularly when the proportion of the amount of power
which is actually used to the total amount of power which can be
supplied from a commercial power source (such a proportion referred
to as a usage rate of power) is low, power can be stored in the
secondary battery, whereby the usage rate of power can be reduced
in a time period when the electronic devices are used. For example,
in the case of the electric refrigerator-freezer 530, power can be
stored in the secondary battery 534 in nighttime when the
temperature is low and the door for a refrigerator 532 and the door
for a freezer 533 are not often opened and closed. On the other
hand, in daytime when the temperature is high and the door for a
refrigerator 532 and the door for a freezer 533 are frequently
opened and closed, the secondary battery 534 is used as an
auxiliary power source; thus, the usage rate of power in daytime
can be reduced.
[0146] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 5
[0147] Next, a portable information terminal which is an example of
a portable electronic device is described with reference to FIGS.
12A to 12C.
[0148] FIGS. 12A and 12B illustrate a tablet terminal 600 that can
be folded. FIG. 12A illustrates the tablet terminal 600 in the
state of being unfolded. The tablet terminal 600 includes a housing
601, a display portion 602a, a display portion 602b, a switch 603
for switching display modes, a power switch 604, a switch 605 for
switching to power-saving mode, and an operation switch 607.
[0149] Part of the display portion 602a can be a touch panel region
608a and data can be input when a displayed operation key 609 is
touched. Note that FIG. 12A illustrates, as an example, that half
of the area of the display portion 602a has only a display function
and the other half of the area has a touch panel function. However,
the structure of the display portion 602a is not limited to this,
and all the area of the display portion 602a may have a touch panel
function. For example, all the area of the display portion 602a can
display keyboard buttons and serve as a touch panel while the
display portion 602b can be used as a display screen.
[0150] Like the display portion 602a, part of the display portion
602b can be a touch panel region 608b. When a finger, a stylus, or
the like touches the place where a button 610 for switching to
keyboard display is displayed in the touch panel, keyboard buttons
can be displayed on the display portion 602b.
[0151] Touch input can be performed on the touch panel regions 608a
and 608b at the same time.
[0152] The switch 603 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. With the
switch 605 for switching to power-saving mode, the luminance of
display can be optimized depending on the amount of external light
at the time when the tablet terminal is in use, which is detected
with an optical sensor incorporated in the tablet terminal. The
tablet terminal may include another detection device such as a
sensor for detecting orientation (e.g., a gyroscope or an
acceleration sensor) in addition to the optical sensor.
[0153] Although the display area of the display portion 602a is the
same as that of the display portion 602b in FIG. 12A, one
embodiment of the present invention is not particularly limited
thereto. The display area of the display portion 602a may be
different from that of the display portion 602b, and further, the
display quality of the display portion 602a may be different from
that of the display portion 602b. For example, one of them may be a
display panel that can display higher-definition images than the
other.
[0154] FIG. 12B illustrates the tablet terminal 600 in the state of
being closed. The tablet terminal 600 includes the housing 601, a
solar cell 611, a charge and discharge control circuit 650, a
battery 651, and a DCDC converter 652. Note that FIG. 12B
illustrates an example in which the charge and discharge control
circuit 650 includes the battery 651 and the DCDC converter 652,
and the battery 651 includes the secondary battery of one
embodiment of the present invention.
[0155] Since the tablet terminal 600 can be folded, the housing 601
can be closed when the tablet terminal 600 is not in use. Thus, the
display portions 602a and 602b can be protected, thereby providing
the tablet terminal 600 with excellent endurance and excellent
reliability for long-term use.
[0156] The tablet terminal illustrated in FIGS. 12A and 12B can
also have a function of displaying various 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 data
displayed on the display portion by touch input, a function of
controlling processing by various kinds of software (programs), and
the like.
[0157] The solar cell 611, which is attached on the surface of the
tablet terminal 600, supplies power to the touch panel, the display
portion, a video signal processor, and the like. Note that the
solar cell 611 is preferably provided on one or both surfaces of
the housing 601, in which case the battery 651 can be charged
efficiently.
[0158] The structure and operation of the charge and discharge
control circuit 650 illustrated in FIG. 12B are described with
reference to a block diagram in FIG. 12C. The solar cell 611, the
battery 651, the DCDC converter 652, a converter 653, switches SW1
to SW3, and the display portion 602 are illustrated in FIG. 12C,
and the battery 651, the DCDC converter 652, the converter 653, and
the switches SW1 to SW3 correspond to the charge and discharge
control circuit 650 illustrated in FIG. 12B.
[0159] First, an example of the operation in the case where power
is generated by the solar cell 611 using external light is
described. The voltage of power generated by the solar cell 611 is
raised or lowered by the DCDC converter 652 so that the power has a
voltage for charging the battery 651. Then, when the power from the
solar cell 611 is used for the operation of the display portion
602, the switch SW1 is turned on and the voltage of the power is
raised or lowered by the converter 653 so as to be a voltage needed
for the display portion 602. In addition, when display on the
display portion 602 is not performed, the switch SW1 may be turned
off and the switch SW2 may be turned on so that the battery 651 is
charged.
[0160] Here, the solar cell 611 is described as an example of a
power generation means; however, there is no particular limitation
on the power generation means, and the battery 651 may be charged
with another power generation means such as a piezoelectric element
or a thermoelectric conversion element (Peltier element). For
example, the battery 651 may be charged with a non-contact power
transmission module that transmits and receives power wirelessly
(without contact) to charge the battery or with a combination of
other charging means.
[0161] It is needless to say that one embodiment of the present
invention is not limited to the electronic device illustrated in
FIGS. 12A to 12C as long as the electronic device is equipped with
the secondary battery of one embodiment of the present invention
which is described in any of the above embodiments.
Embodiment 6
[0162] An example of the moving object which is an example of the
electronic devices is described with reference to FIGS. 13A and
13B.
[0163] The 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.
[0164] FIGS. 13A and 13B illustrate an example of an electric
vehicle. An electric vehicle 660 is equipped with a battery 661.
The output of power of the battery 661 is adjusted by a control
circuit 662 and the power is supplied to a driving device 663. The
control circuit 662 is controlled by a processing unit 664
including a ROM, a RAM, a CPU, or the like which is not
illustrated.
[0165] The driving device 663 includes a DC motor or an AC motor
either alone or in combination with an internal-combustion engine.
The processing unit 664 outputs a control signal to the control
circuit 662 based on input data such as data on operation (e.g.,
acceleration, deceleration, or stop) by a driver of the electric
vehicle 660 or data on driving the electric vehicle 660 (e.g., data
on an upgrade or a downgrade, or data on a load on a driving
wheel). The control circuit 662 adjusts the electric energy
supplied from the battery 661 in accordance with the control signal
of the processing unit 664 to control the output of the driving
device 663. In the case where the AC motor is mounted, although not
illustrated, an inverter which converts direct current into
alternate current is also incorporated.
[0166] The battery 661 can be charged by external electric power
supply using a plug-in technique. For example, the battery 661 is
charged by a commercial power source through a power plug. The
battery 661 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 secondary
battery of one embodiment of the present invention as the battery
661 can contribute to an increase in the capacity of the battery,
so that convenience can be improved. When the battery 661 itself
can be made compact and lightweight with improved characteristics
of the battery 661, the vehicle can be made lightweight, leading to
an increase in fuel efficiency.
[0167] Note that it is needless to say that one embodiment of the
present invention is not limited to the electronic devices
described above as long as the secondary battery of one embodiment
of the present invention is included.
[0168] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Example 1
[0169] In this example, results of examining the correlation
between a direction of an electric field and a direction of
surfaces of graphene layers included in graphite are described.
[0170] Specifically, in each of the case where surfaces of a
plurality of graphene layers included in graphite used as an active
material of a negative electrode are provided substantially along
or in parallel to the direction of an electric field (i.e., the
case where an edge plane is the uppermost surface of the negative
electrode) and the case where surfaces of graphene layers are
provided substantially perpendicular to the direction of the
electric field (i.e., the case where a basal plane is the uppermost
surface of the negative electrode), the degree of lithium insertion
and extraction was measured by cyclic voltammetry (hereinafter
referred to as CV measurement).
[0171] A coin cell was used in the CV measurement. A piece of
graphite to be examined was used as a working electrode; metallic
lithium, a counter electrode; a mixed solution of an ethylene
carbonate (EC) solution and diethyl carbonate (DEC) (volume ratio
of 1:1) in which 1M lithium perchlorate (LiClO.sub.4) was
dissolved, an electrolyte solution; and polypropylene (PP), a
separator. The measurement was performed at a scanning rate of 1.0
mV/sec in a scan range from 0.01 V to 2.5 V (vs. Li/Li.sup.+) for 3
cycles.
[0172] Here, as the piece of graphite to be examined, HOPG 701 and
HOPG 706 each of which was cut into a rectangular parallelepiped
(1.7 mm.times.1.7 mm.times.10 mm) with a dicing machine to have a
weight of approximately 0.05 g were used. As illustrated in FIGS.
14A and 14B, the HOPG 701 was set in a coin cell 700 so that a
basal plane thereof was exposed as a top surface (see FIG. 14A),
and the HOPG 706 was set in a coin cell 705 so that an edge plane
thereof was exposed as a top surface (see FIG. 14B). The separator
and the metallic lithium were provided over each of the HOPG 701
and the HOPG 706. The former is referred to as a cell A and the
latter, a cell B.
[0173] The results of the CV measurement are shown in FIGS. 15A and
15B. FIG. 15A shows the results of the CV measurement of the cell
A. The horizontal axis represents an electric potential (V vs.
Li/Li.sup.+) and the vertical axis represents current density
(mA/g). In the graph, the dotted lines show the results in the
first cycle; the solid lines, the results in the second cycle; and
the thick lines, the results in the third cycle. In all of the
cycles, the current density varied from -2 V to 2 V (vs.
Li/Li.sup.+) in the scan range, and the peak of the current
density, which was observed when the electric potential was around
1.1 V in the forward scan, was not high. The measurement results
did not depend on the number of cycles.
[0174] On the other hand, in the results of the CV measurement of
the cell B in FIG. 15B, current density higher than that in the
results of the CV measurement of the cell A was observed. In
particular, marked peaks were observed when the electric potential
was around 0.7 V (vs. Li/Li.sup.+) in the forward scan. The peak
values increased as the number of cycles increased from the first
cycle (dotted line) to the second cycle (solid line) and the third
cycle (thick line).
[0175] As illustrated in FIG. 14A, in the cell A, the HOPG 701
which is the active material in the shape of the rectangular
parallelepiped was set so that a basal plane 702 thereof faces
upward (faces the counter electrode). Thus, there were two edge
planes 703 on the side surfaces of the HOPG 701, and therefore the
area of the edge planes which were exposed to the electrolyte
solution was larger than the area of the basal plane which was
exposed to the electrolyte solution. The surfaces of the plurality
of graphene layers included in the HOPG 701 were not provided in
parallel to the direction of the electric field. The results of the
CV measurement of the cell A showed the low current density, that
is, low reactivity of the electrode.
[0176] On the other hand, as illustrated in FIG. 14B, in the cell
B, the HOPG 706 which is the active material in the shape of the
rectangular parallelepiped was set so that the edge plane 708
thereof faces upward (faces the counter electrode). Thus, there
were two basal planes 707 on the side surfaces of the HOPG 706, and
therefore the area of the edge plane which was exposed to the
electrolyte solution was smaller than the area of the basal planes
which were exposed to the electrolyte solution in the HOPG 706. The
surfaces of the plurality of graphene layers included in the HOPG
706 were provided substantially along or in parallel to the
direction of the electric field. The results of the CV measurement
of the cell B showed the high current density, that is, high
reactivity of the electrode.
[0177] These results suggest that surfaces of a plurality of
graphene layers provided substantially along or in parallel to the
direction of an electric field enable an electrode to have high
reactivity, that is, lithium is inserted and extracted
efficiently.
[0178] This application is based on Japanese Patent Application
serial no. 2012-161489 filed with Japan Patent Office on Jul. 20,
2012, the entire contents of which are hereby incorporated by
reference.
* * * * *