U.S. patent application number 17/064724 was filed with the patent office on 2021-10-14 for positive electrode active material including lithium cobaltate coated with lithium titanate and magnesium oxide.
This patent application is currently assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. The applicant listed for this patent is SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Takahiro KAWAKAMI, Mayumi MIKAMI, Yohei MOMMA, Teruaki OCHIAI, Masahiro TAKAHASHI, Ayae TSURUTA.
Application Number | 20210320290 17/064724 |
Document ID | / |
Family ID | 1000005866211 |
Filed Date | 2021-10-14 |
United States Patent
Application |
20210320290 |
Kind Code |
A9 |
OCHIAI; Teruaki ; et
al. |
October 14, 2021 |
POSITIVE ELECTRODE ACTIVE MATERIAL INCLUDING LITHIUM COBALTATE
COATED WITH LITHIUM TITANATE AND MAGNESIUM OXIDE
Abstract
A positive electrode active material which can improve cycle
characteristics of a secondary battery is provided. Two kinds of
regions are provided in a superficial portion of a positive
electrode active material such as lithium cobaltate which has a
layered rock-salt crystal structure. The inner region is a
non-stoichiometric compound containing a transition metal such as
titanium, and the outer region is a compound of representative
elements such as magnesium oxide. The two kinds of regions each
have a rock-salt crystal structure. The inner layered rock-salt
crystal structure and the two kinds of regions in the superficial
portion are topotaxy; thus, a change of the crystal structure of
the positive electrode active material generated by charging and
discharging can be effectively suppressed. In addition, since the
outer coating layer in contact with an electrolyte solution is the
compound of representative elements which is chemically stable, the
secondary battery having excellent cycle characteristics can be
obtained.
Inventors: |
OCHIAI; Teruaki; (Atsugi,
JP) ; KAWAKAMI; Takahiro; (Atsugi, JP) ;
MIKAMI; Mayumi; (Atsugi, JP) ; MOMMA; Yohei;
(Isehara, JP) ; TAKAHASHI; Masahiro; (Atsugi,
JP) ; TSURUTA; Ayae; (Atsugi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMICONDUCTOR ENERGY LABORATORY CO., LTD. |
Atsugi-shi |
|
JP |
|
|
Assignee: |
SEMICONDUCTOR ENERGY LABORATORY
CO., LTD.
ATSUGI-SHI
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20210020910 A1 |
January 21, 2021 |
|
|
Family ID: |
1000005866211 |
Appl. No.: |
17/064724 |
Filed: |
October 7, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
16885350 |
May 28, 2020 |
|
|
|
17064724 |
|
|
|
|
15638449 |
Jun 30, 2017 |
10741828 |
|
|
16885350 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0525 20130101;
H01M 4/134 20130101; H01M 4/525 20130101; H01M 2004/028 20130101;
H01M 4/131 20130101; H01M 4/366 20130101 |
International
Class: |
H01M 4/134 20060101
H01M004/134; H01M 4/131 20060101 H01M004/131; H01M 4/525 20060101
H01M004/525; H01M 4/36 20060101 H01M004/36; H01M 10/0525 20060101
H01M010/0525 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2016 |
JP |
2016-133143 |
Jul 6, 2016 |
JP |
2016-133997 |
Jan 11, 2017 |
JP |
2017-002831 |
Feb 22, 2017 |
JP |
2017-030693 |
Apr 21, 2017 |
JP |
2017-084321 |
Jun 19, 2017 |
JP |
2017-119272 |
Claims
1. A lithium-ion secondary battery comprising: a positive electrode
active material comprising a transition metal, wherein the positive
electrode active material is configured to diffuse lithium in a
region from a depth of 0.5 nm to a depth of 50 nm from a surface of
the positive electrode active material, and wherein the positive
electrode active material is configured to suppress dissolution of
the transition metal from the positive electrode active
material.
2. The lithium-ion secondary battery according to claim 1, wherein
the positive electrode active material is configured to suppress
release of oxygen from the positive electrode active material.
3. A lithium-ion secondary battery comprising: a positive electrode
active material comprising lithium, cobalt, and oxygen, wherein the
positive electrode active material is configured to diffuse lithium
in a region from a depth of 0.5 nm to a depth of 50 nm from a
surface of the positive electrode active material, and wherein the
positive electrode active material is configured to suppress
dissolution of cobalt from the positive electrode active
material.
4. The lithium-ion secondary battery according to claim 3, wherein
the positive electrode active material is configured to suppress
release of oxygen from the positive electrode active material.
5. A lithium-ion secondary battery comprising: a positive electrode
active material comprising lithium cobaltate, wherein the positive
electrode active material is configured to diffuse lithium in a
region from a depth of 0.5 nm to a depth of 50 nm from a surface of
the positive electrode active material, and wherein the positive
electrode active material is configured to suppress dissolution of
cobalt from the positive electrode active material.
6. The lithium-ion secondary battery according to claim 5, wherein
the positive electrode active material is configured to suppress
release of oxygen from the positive electrode active material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 16/885,350, filed May 28, 2020, now pending, which is a
continuation of U.S. application Ser. No. 15/638,449, filed Jun.
30, 2017, now U.S. Pat. No. 10,741,828, which claims the benefit of
foreign priority applications filed in Japan as Serial No.
2016-133143 on Jul. 5, 2016, Serial No. 2016-133997 on Jul. 6,
2016, Serial No. 2017-002831 on Jan. 11, 2017, Serial No.
2017-030693 on Feb. 22, 2017, Serial No. 2017-084321 on Apr. 21,
2017, and Serial No. 2017-119272 on Jun. 19, 2017, all of which are
incorporated by reference.
TECHNICAL FIELD
[0002] One embodiment of the present invention relates to an
object, a method, or a manufacturing method. The present invention
relates to a process, a machine, manufacture, or a composition of
matter. One embodiment of the present invention relates to a
semiconductor device, a display device, a light-emitting device, a
power storage device, a lighting device, an electronic device, or a
manufacturing method thereof. In particular, one embodiment of the
present invention relates to a positive electrode active material
that can be used in a secondary battery, a secondary battery, and
an electronic device including a secondary battery.
[0003] In this specification, the power storage device is a
collective term describing elements and devices having a power
storage function. For example, a storage battery such as a
lithium-ion secondary battery (also referred to as secondary
battery), a lithium-ion capacitor, and an electric double layer
capacitor are included in the category of the power storage
device.
[0004] Note that electronic devices in this specification mean all
devices including power storage devices, and electro-optical
devices including power storage devices, information terminal
devices including power storage devices, and the like are all
electronic devices.
BACKGROUND ART
[0005] In recent years, a variety of power storage devices such as
lithium-ion secondary batteries, lithium-ion capacitors, and air
batteries have been actively developed. In particular, demand for
lithium-ion secondary batteries with high output and high energy
density has rapidly grown with the development of the semiconductor
industry, for portable information terminals such as mobile phones,
smartphones, tablets, and laptop computers; portable music players;
digital cameras; medical equipment; next-generation clean energy
vehicles such as hybrid electric vehicles (HEV), electric vehicles
(EV), and plug-in hybrid electric vehicles (PHEV); and the like.
The lithium-ion secondary batteries are essential as rechargeable
energy supply sources for today's information society.
[0006] The performance required for lithium-ion secondary batteries
includes increased energy density, improved cycle performance, safe
operation under a variety of environments, and longer-term
reliability.
[0007] Thus, improvement of a positive electrode active material
has been studied to increase the cycle performance and the capacity
of the lithium ion secondary battery (Patent Document 1 and Patent
Document 2).
REFERENCE
Patent Documents
[0008] [Patent Document 1] Japanese Published Patent Application
No. 2012-018914 [0009] [Patent Document 2] Japanese Published
Patent Application No. 2015-201432
DISCLOSURE OF INVENTION
[0010] Development of lithium ion secondary batteries and positive
electrode active materials used therein is susceptible to
improvement in terms of charge and discharge characteristics, cycle
characteristics, reliability, safety, cost, and the like.
[0011] An object of one embodiment of the present invention is to
provide a positive electrode active material which suppresses a
reduction in capacity due to charge and discharge cycles when used
in a lithium ion secondary battery. Another object of one
embodiment of the present invention is to provide a high-capacity
secondary battery. Another object of one embodiment of the present
invention is to provide a secondary battery with excellent charge
and discharge characteristics. Another object of one embodiment of
the present invention is to provide a highly safe or highly
reliable secondary battery.
[0012] Another object of one embodiment of the present invention is
to provide a novel material, a novel active material particle, a
novel secondary battery, or a formation method thereof.
[0013] Note that the descriptions of these objects do not disturb
the existence of other objects. In one embodiment of the present
invention, there is no need to achieve all the objects. Other
objects can be derived from the description of the specification,
the drawings, and the claims.
[0014] To achieve the above objects, in one embodiment of the
present invention, two kinds of regions which are different from a
region inside the positive electrode active material are provided
in a superficial portion of the positive electrode active material.
It is preferable that the inner region contain a non-stoichiometric
compound and the outer region contain a stoichiometric
compound.
[0015] In addition, it is preferable that the inner region contain
titanium and the outer region contain magnesium. Furthermore, these
two kinds of regions may overlap.
[0016] In addition, it is preferable that the inner region be
formed through a coating process such as a sol-gel method and the
outer region be formed by segregation due to heating.
[0017] One embodiment of the present invention is a positive
electrode active material including a first region, a second
region, and a third region. The first region is present inside the
positive electrode active material. The second region and the third
region are present in a superficial portion of the positive
electrode active material. The third region is present in a region
closer to a surface of the positive electrode active material than
the second region is. The first region contains an oxide of lithium
and a first transition metal and has a layered rock-salt crystal
structure. The second region contains a non-stoichiometric compound
containing an oxide of a second transition metal and the
non-stoichiometric compound has a rock-salt crystal structure. The
third region contains a compound of representative elements and the
compound of representative elements has a rock-salt crystal
structure.
[0018] In the above structure, it is preferable that the first
transition metal be cobalt, the second transition metal be
titanium, and the compound of representative elements be magnesium
oxide.
[0019] In the above structure, the third region may contain
fluorine. Furthermore, the second region and the third region may
each contain cobalt.
[0020] In the above structure, it is preferable that crystal
orientations of the first region and the second region be partly
aligned with each other and crystal orientations of the second
region and the third region be partly aligned with each other.
[0021] In the above structure, a degree of a mismatch between a
(1-1-4) plane of the layered rock-salt crystal structure in the
first region or a plane orthogonal to the (1-1-4) plane and a {100}
plane of the rock-salt crystal structure in the second region is
preferably less than or equal to 0.12, and a degree of a mismatch
between the {100} plane of the rock-salt crystal structure in the
second region and a {100} plane of the rock-salt crystal structure
in the third region is preferably less than or equal to 0.12.
[0022] Another embodiment of the present invention is a positive
electrode active material containing lithium, titanium, cobalt,
magnesium, oxygen, and fluorine. When the concentration of cobalt
which is present in a superficial portion of the positive electrode
active material and is measured by X-ray photoelectron spectroscopy
is 1, the concentration of titanium is greater than or equal to
0.05 and less than or equal to 0.4, the concentration of magnesium
is greater than or equal to 0.4 and less than or equal to 1.5, and
the concentration of fluorine is greater than or equal to 0.05 and
less than or equal to 1.5.
[0023] Another embodiment of the present invention is a method for
forming a positive electrode active material including: a step of
mixing a source of lithium, a source of cobalt, a source of
magnesium, and a source of fluorine; a step of heating the mixture
of the source of lithium, the source of cobalt, the source of
magnesium, and the source of fluorine at 800.degree. C. or higher
and 1100.degree. C. or lower for 2 hours or longer and 20 hours or
shorter to obtain particles containing lithium, cobalt, magnesium,
oxygen, and fluorine; a step of dissolving titanium alkoxide into
alcohol; a step of mixing the particles containing lithium, cobalt,
magnesium, oxygen, and fluorine into the alcohol solution of the
titanium alkoxide and stirring the mixed solution in an atmosphere
containing water vapor; a step of collecting precipitate from the
mixed solution; and a step of heating the collected precipitate at
500.degree. C. or higher and 1200.degree. C. or lower in an
atmosphere containing oxygen under a condition where a retention
time is 50 hours or shorter.
[0024] In the above formation method, a ratio of the number of
atoms of lithium in the source of lithium to the number of atoms of
cobalt in the source of cobalt is preferably greater than or equal
to 1.00 and less than 1.07.
[0025] In the above formation method, a ratio between the number of
atoms of magnesium in the source of magnesium and the number of
atoms of fluorine in the source of fluorine is preferably Mg:F=1:x
(1.5.ltoreq.x.ltoreq.4).
[0026] In the above formation method, the number of atoms of
magnesium in the source of magnesium is preferably greater than or
equal to 0.5 atomic % and less than or equal to 1.5 atomic % of the
number of atoms of cobalt in the source of cobalt.
[0027] In the above formation method, lithium carbonate, cobalt
oxide, magnesium oxide, and lithium fluoride can be used as a
source of lithium, a source of cobalt, a source of magnesium, and a
source of fluorine, respectively.
[0028] When the surface of the positive electrode active material
is covered with a coating film to protect the above crystal
structure, a decrease in capacity due to charge and discharge
cycles can be suppressed. As the coating film covering the surface
of the positive electrode active material, a coating film
containing carbon (a film containing a graphene compound) or a
coating film containing lithium or a decomposition product of an
electrolyte solution is used.
[0029] In particular, powder in which the surface of the positive
electrode active material is coated with graphene oxide using a
spray dry apparatus is preferably obtained. The spray dry apparatus
is a manufacturing apparatus using a spray dry method by which a
dispersion medium is removed by supplying a hot wind to a
suspension.
[0030] When charge and discharge cycles are repeated, deformation
of the particles of the positive electrode active materials, such
as cracking or breaking, might occur. It is said that such
deformation makes a new surface of the positive electrode active
material exposed, and the surface is in contact with an electrolyte
solution to cause a decomposition reaction or the like, so that the
cycle characteristics and the charge and discharge characteristics
of the secondary battery are degraded.
[0031] Thus, a coating film is preferably provided to prevent the
deformation of the particles of the positive electrode active
materials, such as cracking or breaking.
[0032] However, when suspension is formed and stirred by a rotary
and revolutionary mixer to coat the surface of the positive
electrode active material whose weight per unit volume is large
with graphene oxide whose weight is relatively small, coating is
insufficient.
[0033] Thus, to coat the surfaces of the particles of the positive
electrode active materials with the graphene oxide, a method in
which the graphene oxide and a polar solvent (such as water) are
mixed and ultrasonic treatment is performed, the particles of the
positive electrode active materials are mixed therein to prepare
the suspension, and dried powder is produced with a spray dry
apparatus is preferably used. The dried powder produced in this
manner is referred to as a composite in some cases.
[0034] The size of one drop of spray liquid sprayed from a nozzle
of the spray dry apparatus depends on a nozzle diameter.
[0035] When the particle diameter is smaller than the nozzle
diameter, there are a plurality of particles in one drop of the
spray liquid sprayed from the nozzle. When the surface of the
particle after drying under the condition where the largest
particle size is smaller than the nozzle diameter is observed,
there are some portions where the surface is coated with the
graphene oxide; however, the coating is insufficient.
[0036] The nozzle diameter of the spray dry apparatus is preferably
substantially equal to the largest particle size of the active
material because the coverage of the active material can be
improved. Moreover, the largest particle size of the positive
electrode active material is preferably adjusted to be
substantially equal to the nozzle diameter in forming the positive
electrode active material.
[0037] Since the graphene oxide is well dispersed into water, the
suspension of water and the graphene oxide can be formed by
stirring using ultrasonic waves. The positive electrode active
material is added to the suspension, and the suspension is sprayed
with the spray dry apparatus, whereby powder in which the surface
of the positive electrode active material is coated with the
graphene oxide can be obtained.
[0038] Note that the suspension becomes more acidic as the amount
of graphene oxide is increased. Thus, part of the surface of the
positive electrode active material (e.g., LiCoO.sub.2 containing Mg
and F) might be etched. Then, a hydrogen ion exponent (pH) of the
suspension before being sprayed is preferably adjusted to be close
to approximately pH7, that is, close to neutral, or higher than or
equal to pH8, that is, alkaline. For the pH adjustment, a LiOH
aqueous solution is preferably used. For example, in the case where
LiCoO.sub.2 is used for the positive electrode active material and
only pure water is used as the dispersion medium of the suspension,
the surface of the positive electrode active material may be
damaged. Thus, a mixed solution of ethanol and water is used as the
dispersion medium of the suspension, whereby damage to the surface
of the active material may be reduced.
[0039] The suspension is formed in the above manner, whereby the
positive electrode active material whose surface is coated with the
graphene oxide can be prepared efficiently. When the surface is
coated with the graphene oxide, the deformation of the particles of
the positive electrode active materials, such as cracking or
breaking can be prevented. Moreover, even if the positive electrode
active material whose surface is coated with the graphene oxide is
exposed to the air after the formation, the change of properties or
degradation can be suppressed. Here, "after the formation" refers
to a period from the termination of the formation of the positive
electrode active material to the start of the fabrication of the
secondary battery containing the positive electrode active material
and includes the storing, the transporting, and the like of the
positive electrode active material. In addition, when the coating
film is formed, the positive electrode active material and the
electrolyte solution can be prevented from being in direct contact
with each other to react; thus, the secondary battery using the
coating film has high reliability.
[0040] For the spray dry method, a known apparatus can be utilized,
for example, a countercurrent pressure-nozzle-type spray dry
apparatus and a counter-cocurrent pressure-nozzle-type spray dry
apparatus can be utilized.
[0041] Note that the graphene oxide coating the surface of the
active material may be reduced when used in the secondary battery.
The reduced graphene oxide is referred to as "RGO" in some cases.
In RGO, in some cases, part of oxygen atoms remains in a state of
oxygen or atomic group containing oxygen that is bonded to carbon.
For example, RGO includes a functional group, e.g., an epoxy group,
a carbonyl group such as a carboxyl group, or a hydroxyl group.
[0042] Another embodiment of the present invention is a secondary
battery which includes a positive electrode containing the
above-described positive electrode active material or the
above-described positive electrode active material coated with a
coating film and a negative electrode.
[0043] The secondary battery can have any of a variety of shapes to
fit the form of the device to be used, for example, a cylindrical
shape, a rectangular shape, a coin-type shape, and a laminated
(flat plate) shape.
[0044] According to one embodiment of the present invention, a
positive electrode active material which suppresses a reduction in
capacity due to charge and discharge cycles when used in a lithium
ion secondary battery is provided. In addition, a secondary battery
with excellent charge and discharge characteristics is provided. In
addition, a highly safe or highly reliable secondary battery is
provided. In addition, a novel material, a novel active material
particle, a novel secondary battery, or a formation method thereof
is provided.
BRIEF DESCRIPTION OF DRAWINGS
[0045] In the accompanying drawings:
[0046] FIGS. 1A to 1C show examples of a positive electrode active
material;
[0047] FIGS. 2A and 2B illustrate crystal structures of a positive
electrode active material;
[0048] FIG. 3 illustrates crystal structures of a positive
electrode active material;
[0049] FIGS. 4A-1, 4A-2, 4A-3, 4B, 4C, 4D-1, and 4D-2 show a
sol-gel method;
[0050] FIGS. 5A to 5C illustrate a segregation model of elements
contained in a positive electrode active material;
[0051] FIGS. 6A to 6D illustrate a segregation model of elements
contained in a positive electrode active material;
[0052] FIGS. 7A and 7B are cross-sectional views of an active
material layer containing a graphene compound as a conductive
additive;
[0053] FIGS. 8A to 8C illustrate a method for charging a secondary
battery;
[0054] FIGS. 9A to 9D illustrate a method for charging a secondary
battery;
[0055] FIG. 10 illustrates a method for discharging a secondary
battery;
[0056] FIGS. 11A to 11C illustrate a coin-type secondary
battery;
[0057] FIGS. 12A to 12D illustrate a cylindrical secondary
battery;
[0058] FIGS. 13A and 13B illustrate an example of a secondary
battery;
[0059] FIGS. 14A-1, 14A-2, 14B-1, and 14B-2 illustrate examples of
secondary batteries;
[0060] FIGS. 15A and 15B illustrate examples of secondary
batteries;
[0061] FIG. 16 illustrates an example of a secondary battery;
[0062] FIGS. 17A to 17C illustrate a laminated secondary
battery;
[0063] FIGS. 18A and 18B illustrate a laminated secondary
battery;
[0064] FIG. 19 is an external view of a secondary battery;
[0065] FIG. 20 is an external view of a secondary battery;
[0066] FIGS. 21A to 21C illustrate a formation method of a
secondary battery;
[0067] FIGS. 22A, 22B1, 22B2, 22C, and 22D illustrate a bendable
secondary battery;
[0068] FIGS. 23A and 23B illustrate a bendable secondary
battery;
[0069] FIGS. 24A to 24H illustrate examples of electronic
devices;
[0070] FIGS. 25A to 25C illustrate an example of an electronic
device;
[0071] FIG. 26 illustrates examples of electronic devices;
[0072] FIGS. 27A to 27C illustrate examples of electronic
devices;
[0073] FIG. 28 is a transmission electron microscope image of a
positive electrode active material in Example 1;
[0074] FIGS. 29A1, 29A2, 29B1, 29B2, 29C1, and 29C2 are FFT images
of transmission electron microscope images of a positive electrode
active material in Example 1;
[0075] FIGS. 30A1, 30A2, 30B1, 30B2, 30C1, and 30C2 are element
mapping images of a positive electrode active material in Example
1;
[0076] FIGS. 31A1, 31A2, 31B1, 31B2, 31C1, and 31C2 are element
mapping images of a positive electrode active material of a
comparative example in Example 1;
[0077] FIG. 32 is a graph showing TEM-EDX line analysis results of
a positive electrode active material in Example 1;
[0078] FIG. 33 is a graph showing charge and discharge
characteristics of a secondary battery in Example 1;
[0079] FIG. 34 is a graph showing charge and discharge
characteristics of a secondary battery of a comparative example in
Example 1;
[0080] FIG. 35 is a graph showing cycle characteristics of a
secondary battery in Example 1;
[0081] FIG. 36 is a graph showing cycle characteristics of a
secondary battery in Example 1;
[0082] FIGS. 37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2, 37E1, and
37E2 are TEM-EDX plane analysis images of a comparative example in
Example 2;
[0083] FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1, 38D2, 38E1, and
38E2 are TEM-EDX plane analysis images of a positive electrode
active material in Example 2;
[0084] FIGS. 39A, 39B1, 39B2, 39C1, 39C2, 39D1, 39D2, 39E1, and
39E2 are TEM-EDX plane analysis images of a comparative example in
Example 2;
[0085] FIGS. 40A, 40B1, 40B2, 40C1, 40C2, 40D1, 40D2, 40E1, and
40E2 are TEM-EDX plane analysis images of a positive electrode
active material in Example 2;
[0086] FIGS. 41A and 41B are each a graph showing EDX point
analysis results in Example 2;
[0087] FIGS. 42A and 42B are each a graph showing EDX point
analysis results in Example 2;
[0088] FIG. 43 is a graph showing rate characteristics of a
secondary battery in Example 2;
[0089] FIG. 44 is a graph showing temperature characteristics of a
secondary battery in Example 2;
[0090] FIG. 45 is a graph showing cycle characteristics of a
secondary battery in Example 2;
[0091] FIGS. 46A and 46B are each a graph showing XPS analysis
results of a positive electrode active material in Example 3;
[0092] FIGS. 47A and 47B are each a graph showing cycle
characteristics of a secondary battery containing a positive
electrode active material in Example 3;
[0093] FIG. 48 is a graph showing cycle characteristics of a
secondary battery containing a positive electrode active material
in Example 3;
[0094] FIG. 49 is a graph showing cycle characteristics of a
secondary battery containing a positive electrode active material
in Example 3;
[0095] FIGS. 50A to 50C are each a graph showing charge and
discharge characteristics of a secondary battery containing a
positive electrode active material in Example 3;
[0096] FIGS. 51A to 51C are SEM images of a positive electrode
active material in Example 4;
[0097] FIGS. 52A-1, 52A-2, 52B-1, 52B-2, 52C-1, and 52C-2 are
SEM-EDX images of a positive electrode active material in Example
4;
[0098] FIG. 53 is a process flow chart of Example 5;
[0099] FIG. 54 illustrates a spray dry apparatus in Example 5;
[0100] FIG. 55 is a TEM image showing one embodiment of the present
invention in Example 5;
[0101] FIG. 56 is a SEM image showing one embodiment of the present
invention in Example 5;
[0102] FIG. 57 is a SEM image showing a comparative example in
Example 5; and
[0103] FIGS. 58A and 58B are cross-sectional views of an active
material layer containing a graphene compound as a conductive
additive in Example 5.
BEST MODE FOR CARRYING OUT THE INVENTION
[0104] Embodiments of the present invention will be described below
in detail with reference to the drawings. Note that the present
invention is not limited to the description below, and it is easily
understood by those skilled in the art that modes and details of
the present invention can be modified in various ways. In addition,
the present invention should not be construed as being limited to
the description in the embodiments given below.
[0105] Note that in drawings used in this specification, the sizes,
thicknesses, and the like of components such as a positive
electrode, a negative electrode, an active material layer, a
separator, and an exterior body are exaggerated for simplicity in
some cases. Therefore, the sizes of the components are not limited
to the sizes in the drawings and relative sizes between the
components.
[0106] Note that in structures of the present invention described
in this specification and the like, the same portions or portions
having similar functions are denoted by common reference numerals
in different drawings, and descriptions thereof are not repeated.
Further, the same hatching pattern is applied to portions having
similar functions, and the portions are not especially denoted by
reference numerals in some cases.
[0107] In this specification and the like, the Miller index is used
for the expression of crystal planes and orientations. In the
crystallography, a superscript bar is placed over a number in the
expression using the Miller index; however, in this specification
and the like, crystal planes and orientations are expressed by
placing a minus sign (-) at the front of a number instead of
placing the bar over a number because of expression limitations.
Furthermore, an individual direction which shows an orientation in
crystal is denoted by "[ ]", a set direction which shows all of the
equivalent orientations is denoted by "< >", an individual
direction which shows a crystal plane is denoted by "( )", and a
set plane having equivalent symmetry is denoted by "{ }". In the
drawings, the crystal planes and orientations are expressed by a
number with a bar placed thereover, which is an original
crystallographic expression. Note that 1 .ANG. is 10.sup.-10 m.
[0108] In this specification and the like, segregation refers to a
phenomenon in which, in a solid made of a plurality of elements
(e.g., A, B, and C), a certain element (for example, B) is
non-uniformly distributed.
[0109] In this specification and the like, a layered rock-salt
crystal structure included in a composite oxide containing lithium
and a transition metal refers to a crystal structure in which a
rock-salt ion arrangement where cations and anions are alternately
arranged is included and the lithium and the transition metal are
regularly arranged to form a two-dimensional plane, so that lithium
can be two-dimensionally diffused. Note that a defect such as a
cation or anion vacancy can exist. In the layered rock-salt crystal
structure, strictly, a lattice of a rock-salt crystal is distorted
in some cases.
[0110] In this specification and the like, a rock-salt crystal
structure refers to a structure in which cations and anions are
alternately arranged. Note that a cation or anion vacancy may
exist.
[0111] Anions of a layered rock-salt crystal and anions of a
rock-salt crystal each form a cubic closest packed structure
(face-centered cubic lattice structure). When a layered rock-salt
crystal and a rock-salt crystal are in contact with each other,
there is a crystal plane at which directions of cubic closest
packed structures formed of anions are aligned with each other. A
space group of the layered rock-salt crystal is R-3m, which is
different from a space group Fm-3m of a general rock-salt crystal
and a space group Fd-3m of a rock-salt crystal having the simplest
symmetry; thus, the Miller index of the crystal plane satisfying
the above conditions in the layered rock-salt crystal is different
from that in the rock-salt crystal. In this specification, in the
layered rock-salt crystal and the rock-salt crystal, a state where
the directions of the cubic closest packed structures formed of
anions are aligned with each other is referred to as a state where
crystal orientations are substantially aligned with each other.
[0112] Whether the crystal orientations in two regions are aligned
with each other or not can be judged by a transmission electron
microscope (TEM) image, a scanning transmission electron microscope
(STEM) image, a high-angle annular dark field scanning transmission
electron microscopy (HAADF-STEM) image, an annular bright-field
scan transmission electron microscopy (ABF-STEM) image, and the
like. X-ray diffraction, electron diffraction, neutron diffraction,
and the like can be used for judging. In the TEM image and the
like, alignment of cations and anions can be observed as repetition
of bright lines and dark lines. When the orientations of cubic
closest packed structures of the layered rock-salt crystal and the
rock-salt crystal are aligned with each other, a state where an
angle between the repetition of bright lines and dark lines in the
layered rock-salt crystal and the repetition of bright lines and
dark lines in the rock-salt crystal is less than or equal to
5.degree., preferably less than or equal to 2.5.degree. is
observed. Note that, in the TEM image and the like, a light element
such as oxygen or fluorine is not clearly observed in some cases;
however, in such a case, alignment of orientations can be judged by
arrangement of metal elements.
[0113] Furthermore, in this specification and the like, a state
where structures of two-dimensional interfaces have similarity is
referred to as "epitaxy". Crystal growth in which structures of
two-dimensional interfaces have similarity is referred to as
"epitaxial growth". In addition, a state where three-dimensional
structures have similarity or orientations are crystallographically
the same is referred to as "topotaxy". Thus, in the case of
topotaxy, when part of a cross section is observed, orientations of
crystals in two regions (e.g., a region serving as a base and a
region formed through growth) are substantially aligned with each
other.
Embodiment 1
[Structure of Positive Electrode Active Material]
[0114] First, a positive electrode active material 100, which is
one embodiment of the present invention, is described with
reference to FIGS. 1A to 1C. The positive electrode active material
100 refers to a substance containing a transition metal which can
receive and release lithium ion electrochemically. As illustrated
in FIG. 1A, the positive electrode active material 100 includes a
first region 101 inside and a second region 102 and a third region
103 in a superficial portion.
[0115] As illustrated in FIG. 1B, the second region 102 does not
necessarily cover the entire first region 101. Similarly, the third
region 103 does not necessarily cover the entire second region 102.
In addition, the third region 103 may be present in contact with
the first region 101.
[0116] The thicknesses of the second region 102 and the third
region 103 may each differ depending on the positions.
[0117] Furthermore, the third region 103 may be present inside the
positive electrode active material 100. For example, in the case
where the first region 101 is a polycrystal, the third region 103
may be present in the vicinity of a grain boundary. Furthermore,
the third region 103 may be present in a portion which includes
crystal defects or a crack portion in the positive electrode active
material 100 or in the vicinity thereof. In FIG. 1B, parts of grain
boundaries are shown by dotted lines. In this specification and the
like, crystal defects refer to defects which can be observed from a
TEM image and the like, that is, a structure in which another
element enters crystal, a cavity, and the like. In addition, a
crack portion refers to a crack or a break formed in a particle
like a crack portion 106 illustrated in FIG. 1C, for example.
[0118] Similarly, as illustrated in FIG. 1B, the second region 102
may be present inside the positive electrode active material 100.
For example, in the case where the first region 101 is a
polycrystal, the second region 102 may be present in the vicinity
of a grain boundary. Furthermore, the second region 102 may be
present in a portion which includes crystal defects or a crack
portion in the positive electrode active material 100 or in the
vicinity thereof. Moreover, the third region 103 and the second
region 102 inside the positive electrode active material 100 may
overlap.
<First Region 101>
[0119] The first region 101 contains a composite oxide of lithium
and a first transition metal. In other words, the first region 101
contains lithium, a first transition metal, and oxygen.
[0120] The composite oxide of lithium and a first transition metal
preferably has a layered rock-salt crystal structure.
[0121] As the first transition metal, only cobalt may be used,
cobalt and manganese may be used, or cobalt, manganese, and nickel
may be used.
[0122] That is, the first region can include lithium cobalt oxide,
lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide
in which manganese is substituted for part of cobalt, lithium
nickel-manganese-cobalt oxide, or the like. In addition to the
transition metal, the first region 101 may include a metal other
than the transition metal, such as aluminum.
[0123] The first region 101 serves as a region which particularly
contributes to a charge and discharge reaction in the positive
electrode active material 100. To increase capacity of a secondary
battery containing the positive electrode active material 100, the
volume of the first region 101 is preferably larger than those of
the second region and the third region.
[0124] A material having a layered rock-salt crystal structure has
features such as high discharge capacity and low resistance due to
lithium that can be diffused two-dimensionally, which is preferably
used for the first region 101. In addition, in the case where the
first region 101 has a layered rock-salt crystal structure, a
segregation of a representative element such as magnesium, which is
described later, tends to occur unexpectedly.
[0125] Note that the first region 101 may be a single crystal or a
polycrystal. For example, the first region 101 may be a polycrystal
in which an average crystallite size is greater than or equal to
280 nm and less than or equal to 630 nm. In the case of a
polycrystal, a grain boundary can be observed from the TEM or the
like in some cases. In addition, the average of crystal grain sizes
can be calculated from the half width of XRD.
[0126] The polycrystal has a clear crystal structure; thus, a
two-dimensional diffusion path of lithium ions can be sufficiently
ensured. In addition, a polycrystal is easily produced as compared
with a single crystal; thus, a polycrystal is preferably used for
the first region 101.
[0127] Furthermore, the entire first region 101 does not
necessarily have a layered rock-salt crystal structure. For
example, part of the first region 101 may be amorphous or has
another crystal structure.
<Second Region 102>
[0128] The second region 102 contains an oxide of a second
transition metal. In other words, the second region 102 contains a
second transition metal and oxygen.
[0129] As the second transition metal, a non-stoichiometric metal
is preferably used. In other words, the second region 102
preferably includes a non-stoichiometric compound. For example, as
the second transition metal, at least one of titanium, vanadium,
manganese, iron, chromium, niobium, cobalt, zinc, zirconium,
nickel, and the like can be used. Note that the second transition
metal is preferably an element different from that of the first
transition metal.
[0130] In this specification and the like, a non-stoichiometric
metal refers to a metal that can have a plurality of valences. In
addition, a non-stoichiometric compound refers to a compound of a
metal that can have a plurality of valences and another
element.
[0131] The second region 102 preferably has a rock-salt crystal
structure.
[0132] The second region 102 serves as a buffer region which
connects the first region 101 to the third region 103 which is
described later. In the non-stoichiometric compound, an interatomic
distance can be changed in accordance with a change in valence of a
metal contained in the non-stoichiometric compound. In addition, in
the non-stoichiometric compound, a cation or anion vacancy and
dislocation (what is called Magneli phase) are often formed. Thus,
the second region 102, which serves as a buffer region, can absorb
a strain generated between the first region 101 and the third
region 103.
[0133] Furthermore, the second region 102 may contain lithium in
addition to the second transition metal and oxygen. For example,
lithium titanate or lithium manganite may be contained. Moreover,
the second region 102 may contain a representative element
contained in the third region 103 which is described later. The
second region 102 that contains an element contained in the first
region 101, such as lithium, and an element contained in the third
region 103 is preferable because the second region 102 serves as a
buffer region.
[0134] That is, the second region 102 can contain lithium titanate,
titanium oxide, vanadium oxide, manganese oxide, iron oxide, copper
oxide, chromium oxide, niobium oxide, cobalt oxide, zinc oxide, or
the like.
[0135] In addition, the second region 102 may contain the first
transition metal. For example, the second transition metal may be
present in part of a first transition metal site of the composite
oxide including the first transition metal.
[0136] For example, in the case where the second transition metal
is titanium, titanium may be present as titanium oxide (TiO.sub.2)
or lithium titanate (LiTiO.sub.2) in the second region 102.
Alternatively, in the second region 102, titanium may be
substituted for part of the first transition metal site of the
composite oxide of lithium and the first transition metal.
[0137] Moreover, the second region 102 may contain fluorine.
[0138] The second region 102 preferably has a crystal structure
which is the same as that of the third region 103 which is
described later. In this case, orientations of crystals of the
second region 102 and the third region 103 are likely to be aligned
with each other.
[0139] The second region 102 preferably has a rock-salt crystal
structure; however, the entire second region 102 does not
necessarily have a rock-salt crystal structure. For example, the
second region 102 may have another crystal structure such as a
spinel crystal structure, an olivine crystal structure, a corundum
crystal structure, or a rutile crystal structure.
[0140] Furthermore, a crystal structure may have a strain as long
as a structure where six oxygen atoms are adjacent to cations is
kept. In addition, a cation vacancy may be present in part of the
second region 102.
[0141] Moreover, part of the second region 102 may be
amorphous.
[0142] When the thickness of the second region 102 is too small,
the function as the buffer region is degraded; however, when the
thickness of the second region 102 is too large, the capacity might
be decreased. Thus, the second region 102 is preferably provided in
a range from the surface of the positive electrode active material
100 to a depth of 20 nm, preferably a depth of 10 nm, in a depth
direction. The second transition metal may have a concentration
gradient.
<Third Region 103>
[0143] The third region 103 contains a compound of representative
elements. A compound of representative elements is a stoichiometric
compound. As the compound of representative elements, a compound
made of representative elements which are electrochemically stable
is preferable, and at least one of magnesium oxide, calcium oxide,
beryllium oxide, lithium fluoride, and sodium fluoride can be used,
for example.
[0144] The third region 103 is in contact with an electrolyte
solution when the positive electrode active material 100 is used in
a secondary battery. Thus, for the third region 103, a material
which is hardly changed electrochemically in the process of
charging and discharging and is not easily transformed by contact
with the electrolyte solution is preferably used. The compound of
representative elements which is a stoichiometric compound and
electrochemically stable is preferably used for the third region
103. The positive electrode active material 100 includes the third
region 103 in a superficial portion to improve stability in
charging and discharging of the secondary battery. Here, a state
where stability of a secondary battery is high refers to a state
where the crystal structure of the composite oxide of lithium and
the first transition metal contained in the first region 101 is
more stable. Alternatively, it refers to a state where a change in
capacity of the secondary battery is small even if charging and
discharging are repeated or a state where a change in valence of a
metal contained in the positive electrode active material 100 is
suppressed even after charging and discharging are repeated.
[0145] The third region 103 may contain fluorine. In the case where
the third region 103 contains fluorine, fluorine may be substituted
for some anions in the compound of the representative elements.
[0146] Fluorine is substituted for some anions in the compound of
the representative elements, whereby diffusion properties of
lithium can be improved. Thus, the third region 103 is less likely
to prevent charging and discharging. In addition, when fluorine is
present in a superficial portion of a positive electrode active
material particle, corrosion resistance against a hydrofluoric acid
generated by decomposition of an electrolyte solution is increased
in some cases.
[0147] Moreover, the third region 103 may include lithium, the
first transition metal, and the second transition metal.
[0148] The compound of the representative elements contained in the
third region 103 preferably has a rock-salt crystal structure. When
the third region 103 has a rock-salt crystal structure,
orientations of crystals are likely to be aligned with those of the
second region 102. The orientations of crystals of the first region
101, the second region 102, and the third region 103 are
substantially aligned with each other, whereby the second region
102 and the third region 103 can serve as a more stable coating
layer.
[0149] However, the entire third region 103 does not necessarily
have a rock-salt crystal structure. For example, the third region
103 may have another crystal structure such as a spinel crystal
structure, an olivine crystal structure, a corundum crystal
structure, or a rutile crystal structure.
[0150] Furthermore, a crystal structure may have a strain as long
as a structure where six oxygen atoms are adjacent to cations is
kept. In addition, a cation vacancy may be present in part of the
third region 103.
[0151] Moreover, part of the third region 103 may be amorphous.
[0152] When the thickness of the third region 103 is too small, the
function of increasing stability in charging and discharging is
degraded; however, when the thickness of the third region 103 is
too large, the capacity might be decreased. Thus, the thickness of
the third region 103 is preferably greater than or equal to 0.5 nm
and less than or equal to 50 nm, further preferably greater than or
equal to 0.5 nm and less than or equal to 2 nm.
[0153] In the case where the third region 103 contains fluorine,
fluorine is preferably present in a bonding state other than
magnesium fluoride (MgF.sub.2), lithium fluoride (LiF), and cobalt
fluoride (CoF.sub.2). Specifically, when an XPS analysis is
performed on the vicinity of the surface of the positive electrode
active material 100, a peak position of bonding energy with
fluorine is preferably higher than or equal to 682 eV and lower
than or equal to 685 eV, further preferably approximately 684.3 eV.
The bonding energy does not correspond to those of MgF.sub.2, LiF,
and CoF.sub.2.
[0154] In this specification and the like, a peak position of
bonding energy with an element in an XPS analysis refers to a value
of bonding energy at which the maximum intensity of an energy
spectrum is obtained in a range corresponding to bonding energy of
the element.
[0155] In general, when charging and discharging are repeated, a
side reaction occurs in a positive electrode active material, for
example, a first transition metal such as manganese, cobalt, or
nickel is dissolved in an electrolyte solution, oxygen is released,
and a crystal structure becomes unstable, so that the positive
electrode active material deteriorates. However, the positive
electrode active material 100 of one embodiment of the present
invention includes the second region 102 serving as a buffer region
and the third region 103 which is electrochemically stable. Thus,
the dissolution of the first transition metal can be effectively
suppressed, and the crystal structure of the composite oxide of
lithium and the transition metal contained in the first region 101
can be more stable. As a result, the cycle characteristics of the
secondary battery including the positive electrode active material
100 can be significantly improved. In addition, in the case where
charging and discharging are performed at a voltage higher than 4.3
V (vs. Li/Li.sup.+), in particular, a high voltage of 4.5 V (vs.
Li/Li.sup.+) or more, the structure of one embodiment of the
present invention is significantly effective.
<Heteroepitaxial Growth and Topotaxy>
[0156] The second region 102 is preferably formed by
heteroepitaxial growth from the first region 101. Furthermore, the
third region 103 is preferably formed by heteroepitaxial growth
from the second region 102. A region formed by heteroepitaxial
growth becomes topotaxy which has crystal orientations
substantially three-dimensionally aligned with those of a region
serving as a base. Thus, the first region 101, the second region
102, and the third region 103 can be topotaxy.
[0157] When the crystal orientations of the first region 101, the
second region 102, and the third region 103 are substantially
aligned with each other, the second region 102 and the third region
103 serve as a coating layer which has a stable bond with the first
region 101. As a result, the positive electrode active material 100
including a strong coating layer can be provided.
[0158] Since the second region 102 and the third region 103 have a
stable bond with the first region 101, when the positive electrode
active material 100 is used for the secondary battery, a change of
the crystal structure in the first region 101 which is caused by
charging and discharging can be effectively suppressed. Even when
lithium is released from the first region 101 due to charging, the
coating layer having a stable bond can suppress release of cobalt
and oxygen from the first region 101. Furthermore, a chemically
stable material can be used for a region in contact with the
electrolyte solution. Thus, a secondary battery having excellent
cycle characteristics can be provided.
<Degree of Mismatch Between Regions>
[0159] To perform heteroepitaxial growth, the degree of a mismatch
between crystals in a region serving as a base and crystals on
which crystal growth is performed is important.
[0160] In this specification and the like, the degree of a mismatch
f is defined by the following Formula 1. The average of the nearest
neighbor distances between oxygen and cations of the crystals in
the region serving as a base is represented by a, and the average
of the natural nearest neighbor distances between anions and
cations of the crystals on which crystal growth is performed is
represented by b.
[ Formula .times. .times. 1 ] .times. f = b - a a ( Formula .times.
.times. 1 ) ##EQU00001##
[0161] To perform heteroepitaxial growth, the degree of a mismatch
f between crystals in a region serving as a base and crystals on
which crystal growth is performed needs to be less than or equal to
0.12. To perform more stable heteroepitaxial growth to form a
layered shape, the degree of a mismatch f is preferably less than
or equal to 0.08, further preferably less than or equal to
0.04.
[0162] Thus, materials of the first region 101 and the second
region 102 are preferably selected so that the degree of a mismatch
f between the layered rock-salt crystal structure in the first
region 101 and the rock-salt crystal structure in the second region
102 is less than or equal to 0.12.
[0163] Furthermore, materials of the second region 102 and the
third region 103 are preferably selected so that the degree of a
mismatch f between the rock-salt crystal structure in the second
region 102 and the rock-salt crystal structure in the third region
103 is less than or equal to 0.12.
[0164] Examples of materials and crystal planes of the first region
101, the second region 102, and the third region 103 are shown
below, which satisfy the above-described conditions: the degree of
a mismatch f between the layered rock-salt crystal structure in the
first region 101 and the rock-salt crystal structure in the second
region 102 is less than or equal to 0.12; and the degree of a
mismatch f between the rock-salt crystal structure in the second
region 102 and the rock-salt crystal structure in the third region
103 is less than or equal to 0.12.
EXAMPLE 1
Lithium Cobaltate, Lithium Titanate, and Magnesium Oxide
[0165] First, FIGS. 2A and 2B and FIG. 3 show an example in which
the first transition metal is cobalt, the first region 101 contains
lithium cobaltate having a layered rock-salt crystal structure, the
second transition metal is titanium, the second region 102 contains
lithium titanate having a rock-salt crystal structure, and the
compound of the representative elements in the third region 103 is
magnesium oxide having a rock-salt crystal structure.
[0166] FIG. 2A illustrates a model of a layered rock-salt crystal
structure (a space group R-3mH) of lithium cobaltate (LiCoO.sub.2),
a model of a rock-salt crystal structure (a space group Fd-3mZ) of
lithium titanate (LiTiO.sub.2), and a model of a rock-salt crystal
structure (a space group Fd-3mZ) of magnesium oxide. FIG. 2A
illustrates models each of which is seen from the b-axis
direction.
[0167] From FIG. 2A, it is not seen that the layered rock-salt
crystals and the rock-salt crystals make topotaxy. Then, here, the
layered rock-salt crystals are seen from a different direction
(e.g., a direction indicated by an arrow in FIG. 2A). FIG. 2B
illustrates a model of the layered rock-salt crystals seen from the
<1-1-4> plane direction and models of the rock-salt crystals
seen from the <100> plane direction.
[0168] As illustrated in FIG. 2B, when the layered rock-salt
crystals are seen from the <1-1-4> plane direction, atomic
arrangement of the layered rock-salt crystals is highly similar to
those of the rock-salt crystals seen from the <100> plane
direction. In addition, the nearest neighbor distances between
metal and oxygen have similar values. For example, in the layered
rock-salt lithium cobaltate, a distance between Li and O is 2.089
.ANG. and a distance between Co and O is 1.925 .ANG.. In the
rock-salt lithium titanate, a distance between Li and O is 2.138
.ANG. and a distance between Ti and O is 2.051 .ANG.. In the
rock-salt magnesium oxide, a distance between Mg and O is 2.106
.ANG..
[0169] Then, with reference to FIG. 3, the degree of a mismatch
between the regions when the (1-1-4) crystal plane of the layered
rock-salt crystal and the {100} crystal plane of the rock-salt
crystal are in contact with each other is described.
[0170] As illustrated in FIG. 3, a distance between metals through
oxygen in a (1-1-4) crystal plane 101p (1-1-4) of lithium cobaltate
having the layered rock-salt crystal structure in the first region
101 is 4.01 .ANG.. Furthermore, a distance between metals through
oxygen in a {100} crystal plane 102p {100} of lithium titanate
having the rock-salt crystal structure in the second region 102 is
4.19 .ANG.. Thus, the degree of a mismatch f between the crystal
plane 101p (1-1-4) and the crystal plane 102p {100} is 0.04.
[0171] In addition, a distance between metals through oxygen in a
{100} crystal plane 103p {100} of magnesium oxide having the
rock-salt crystal structure in the third region 103 is 4.21 .ANG..
Thus, the degree of a mismatch f between the crystal plane 102p
{100} and the crystal plane 103p {100} is 0.02.
[0172] In this manner, the degree of a mismatch between the first
region 101 and the second region 102 and the degree of a mismatch
between the second region 102 and the third region 103 are
sufficiently small; thus, the first region 101, the second region
102, and the third region 103 can be topotaxy.
[0173] Although not illustrated in FIG. 3, if the crystal plane
101p (1-1-4) in the first region 101 and the crystal plane 103p
{100} in the third region 103 are in contact with each other, the
degree of a mismatch f is 0.05. That is, owing to the second region
102, the degree of a mismatch can be small. Moreover, with the
second region 102 that is a non-stoichiometric transition metal
oxide, the first region 101, the second region 102, and the third
region 103 can be more stable topotaxy. Thus, the second region 102
and the third region 103 can serve as a coating layer having a
stable bond with the first region 101.
[0174] In this embodiment, the (1-1-4) plane of the layered
rock-salt crystal and the {100} plane of the rock-salt crystal are
in contact with each other; however, one embodiment of the present
invention is not limited thereto as long as crystal planes which
can be topotaxy are in contact with each other.
EXAMPLE 2
Lithium Cobaltate, Manganese Oxide, and Calcium Oxide
[0175] Next, an example in which the first transition metal is
cobalt, the first region 101 contains lithium cobaltate having a
layered rock-salt crystal structure, the second transition metal is
manganese, the second region 102 contains manganese oxide having a
rock-salt crystal structure, and the compound of the representative
elements in the third region 103 is calcium oxide having a
rock-salt crystal structure is shown.
[0176] Also in this case, as in FIGS. 2A and 2B and FIG. 3, when
the layered rock-salt crystals are seen from the <1-1-4>
plane direction, atomic arrangement of the layered rock-salt
crystals in the first region 101 is highly similar to those of the
rock-salt crystals in the second region 102 and the third region
103 seen from the <100> plane direction.
[0177] The degree of a mismatch between the regions when the
(1-1-4) crystal plane of the layered rock-salt crystal and the
{100} crystal plane of the rock-salt crystal are in contact with
each other is described. A distance between metals through oxygen
in a crystal plane (1-1-4) of lithium cobaltate having the layered
rock-salt crystal structure in the first region 101 is 4.01 .ANG..
Furthermore, a distance between metals through oxygen in a crystal
plane {100} of manganese oxide having the rock-salt crystal
structure in the second region 102 is 4.45 .ANG.. Thus, the degree
of a mismatch f between the crystal plane (1-1-4) in the first
region 101 and the crystal plane {100} in the second region 102 is
0.11.
[0178] In addition, a distance between metals through oxygen in a
crystal plane {100} of calcium oxide having the rock-salt crystal
structure in the third region 103 is 4.82 .ANG.. Thus, the degree
of a mismatch f between the crystal plane {100} in the second
region 102 and the crystal plane {100} in the third region 103 is
0.08.
[0179] In this manner, the degree of a mismatch between the first
region 101 and the second region 102 and the degree of a mismatch
between the second region 102 and the third region 103 are
sufficiently small; thus, the first region 101, the second region
102, and the third region 103 can be topotaxy.
[0180] If the crystal plane (1-1-4) in the first region 101 and the
crystal plane {100} in the third region 103 are in contact with
each other, the degree of a mismatch f is 0.20; thus, it is
difficult to perform the heteroepitaxial growth. That is, owing to
the second region 102, the heteroepitaxial growth from the first
region to the third region can be performed. Thus, the second
region 102 and the third region 103 can serve as a coating layer
having a stable bond with the first region 101.
EXAMPLE 3
Lithium Nickel-Manganese-Cobalt Oxide, Manganese Oxide, and Calcium
Oxide
[0181] Next, an example in which the first transition metals are
nickel, manganese, and cobalt, the first region 101 contains
lithium nickel-manganese-cobalt oxide
(LiNi.sub.0.33Co.sub.0.33Mn.sub.0.33O.sub.2) having a layered
rock-salt crystal structure, the second transition metal is
manganese, the second region 102 contains manganese oxide having a
rock-salt crystal structure, and the compound of the representative
elements in the third region 103 is calcium oxide having a
rock-salt crystal structure is shown.
[0182] Also in this case, as illustrated in FIGS. 2A and 2B and
FIG. 3, when the layered rock-salt crystals are seen from the
<1-1-46> plane direction, atomic arrangement of the layered
rock-salt crystals is highly similar to those of the rock-salt
crystals seen from the <100> plane direction. The degree of a
mismatch between the regions when the (1-1-4) crystal plane of the
layered rock-salt crystal and the {100} crystal plane of the
rock-salt crystal are in contact with each other is described.
[0183] A distance between metals through oxygen in a crystal plane
(1-1-4) of lithium nickel-manganese-cobalt oxide having the layered
rock-salt crystal structure in the first region 101 is 4.07 .ANG..
Furthermore, a distance between metals through oxygen in a crystal
plane {100} of manganese oxide having the rock-salt crystal
structure in the second region 102 is 4.45 .ANG.. Thus, the degree
of a mismatch f between the crystal plane (1-1-4) in the first
region 101 and the crystal plane {100} in the second region 102 is
0.09.
[0184] In addition, a distance between metals through oxygen in a
crystal plane {100} of calcium oxide having the rock-salt crystal
structure in the third region 103 is 4.82 .ANG.. Thus, the degree
of a mismatch f between the crystal plane {100} in the second
region 102 and the crystal plane {100} in the third region 103 is
0.08.
[0185] In this manner, the degree of a mismatch between the first
region 101 and the second region 102 and the degree of a mismatch
between the second region 102 and the third region 103 are
sufficiently small; thus, the first region 101, the second region
102, and the third region 103 can be topotaxy.
[0186] If the crystal plane (1-1-4) in the first region 101 and the
crystal plane {100} in the third region 103 are in contact with
each other, the degree of a mismatch f is 0.18; thus, it is
difficult to perform the heteroepitaxial growth. That is, the
second region 102 is provided, whereby the heteroepitaxial growth
from the first region to the third region can be performed. Thus,
the second region 102 and the third region 103 can serve as a
coating layer having a stable bond with the first region 101.
<Boundaries Between Regions>
[0187] As described above, the first region 101, the second region
102, and the third region 103 have different compositions. The
element contained in each region has a concentration gradient in
some cases. For example, the second transition metal in the second
region 102 may have a concentration gradient. In addition, the
third region 103 may have a concentration gradient of a
representative element because a representative element preferably
segregates in the third region 103 as described later. Thus, the
boundaries between the regions are not clear in some cases.
[0188] The difference of compositions of the first region 101, the
second region 102, and the third region 103 can be observed using a
TEM image, a STEM image, fast Fourier transform (FFT) analysis,
energy dispersive X-ray spectrometry (EDX), an analysis in the
depth direction by time-of-flight secondary ion mass spectrometry
(ToF-SIMS), X-ray photoelectron spectroscopy (XPS), Auger electron
spectroscopy, thermal desorption spectroscopy (TDS), or the
like.
[0189] For example, in the TEM image and the STEM image, difference
of constituent elements is observed as difference of brightness;
thus, difference of constituent elements of the first region 101,
the second region 102, and the third region 103 can be observed.
Furthermore, also in plane analysis of EDX (e.g., element mapping),
it can be observed that the first region 101, the second region
102, and the third region 103 contain different elements.
[0190] By line analysis of EDX and analysis in the depth direction
using ToF-SIMS, a peak of concentration of each element contained
in the first region 101, the second region 102, and the third
region 103 can be detected.
[0191] However, clear boundaries between the first region 101, the
second region 102, and the third region 103 are not necessarily
observed by the analyses.
[0192] In this specification and the like, the third region 103
that is present in a superficial portion of the positive electrode
active material 100 refers to a region from the surface of the
positive electrode active material 100 to a region where a
concentration of a representative element such as magnesium which
is detected by analysis in the depth direction is 1/5 of a peak. As
the analysis in the depth direction, the line analysis of EDX,
analysis in the depth direction using ToF-SIMS, or the like, which
is described above, can be used.
[0193] Furthermore, a peak of a concentration of a representative
element is preferably present in a region from the surface of the
positive electrode active material 100 to a depth of 3 nm toward
the center, further preferably to a depth of 1 nm, and still
further preferably to a depth of 0.5 nm.
[0194] Although the depth at which the concentration of the
representative element becomes 1/5 of the peak is different
depending on the manufacturing method, in the case of a
manufacturing method described later, the depth is approximately 2
nm to 5 nm from the surface of the positive electrode active
material.
[0195] The third region 103 that is present inside the first region
101 in the vicinity of a grain boundary, a crystal defect, or the
like also refers to a region where a concentration of a
representative element which is detected by analysis in the depth
direction is higher than or equal to 1/5 of a peak.
[0196] A distribution of fluorine in the positive electrode active
material 100 preferably overlaps with a distribution of the
representative element. Thus, fluorine also has a concentration
gradient, and a peak of a concentration of fluorine is preferably
present in a region from the surface of the positive electrode
active material 100 to a depth of 3 nm toward the center, further
preferably to a depth of 1 nm, and still further preferably to a
depth of 0.5 nm.
[0197] In this specification and the like, the second region 102
that is present in a superficial portion of the positive electrode
active material 100 refers to a region where a concentration of the
second transition metal which is detected by analysis in the depth
direction is higher than or equal to 1/2 of a peak. The second
region 102 that is present inside the first region 101 in the
vicinity of a grain boundary, a crystal defect, or the like also
refers to a region where a concentration of the second transition
metal which is detected by analysis in the depth direction is
higher than or equal to 1/2 of a peak. As the analysis method, the
line analysis of EDX, analysis in the depth direction using
ToF-SIMS, or the like, which is described above, can be used.
[0198] Thus, the third region 103 and the second region 102 overlap
in some cases. Note that the third region 103 is preferably present
in a region closer to the surface of the positive electrode active
material particle than the second region 102 is. In addition, the
peak of the concentration of the representative element is
preferably present in a region closer to the surface of the
positive electrode active material particle than the peak of the
concentration of the second transition metal is.
[0199] The peak of the second transition metal is preferably
present in a region from a depth of 0.2 nm or more to a depth of 10
nm or less from the surface of the positive electrode active
material 100 toward the center, further preferably in a region from
a depth of 0.5 nm or more to a depth of 3 nm or less.
[0200] The measurement range of the XPS is from the surface of the
particle of the positive electrode active material 100 to a region
at a depth of approximately 5 nm. Thus, a concentration of an
element present at a depth of approximately 5 nm from the surface
can be analyzed quantitatively. Thus, the concentration of elements
in the third region 103 and the second region 102 present at a
depth of approximately 5 nm from the surface can be analyzed
quantitatively.
[0201] When the surface of the positive electrode active material
100 is subjected to the XPS analysis and the concentration of the
first transition metal is defined as 1, a relative value of the
concentration of the second transition metal is preferably greater
than or equal to 0.05 and less than or equal to 0.4, further
preferably greater than or equal to 0.1 and less than or equal to
0.3. In addition, a relative value of the concentration of the
representative element is preferably greater than or equal to 0.4
and less than or equal to 1.5, further preferably greater than or
equal to 0.45 and less than or equal to 1.00. Furthermore, a
relative value of the concentration of fluorine is preferably
greater than or equal to 0.05 and less than or equal to 1.5,
further preferably greater than or equal to 0.3 and less than or
equal to 1.00.
[0202] Note that, as described above, elements contained in the
first region 101, the second region 102, and the third region 103
may each have a concentration gradient; thus, the first region 101
may contain the element in the second region 102 or the third
region 103, such as fluorine. Similarly, the third region 103 may
contain the element in the first region 101 or the second region
102. In addition, the first region 101, the second region 102, and
the third region 103 may each contain another element such as
carbon, sulfur, silicon, sodium, calcium, chlorine, or
zirconium.
[Particle Diameter]
[0203] If the particle diameter of the positive electrode active
material 100 is too large, diffusion of lithium is difficult,
whereas if the particle diameter is too small, it is difficult to
maintain a crystal structure described later. Thus, D50 (also
referred to as a median diameter) is preferably 5 .mu.m or more and
100 .mu.m or less, and further preferably 10 .mu.m or more and 70
.mu.m or less. In the case where the coating film is formed on the
surface of the positive electrode active material 100 by a spray
dry apparatus in a later step, it is preferable that the nozzle
diameter and the maximum particle diameter of the positive
electrode active material 100 be substantially the same. When the
particle diameter is less than 5 .mu.m and a spray dry apparatus
having a nozzle diameter of 20 .mu.m is used, secondary particles
are covered collectively, which leads to a decrease in
coverage.
[0204] To increase the density of the positive electrode active
material layer, it is effective to mix large particles (the longest
portion is approximately 20 .mu.m or more and 40 .mu.m or less) and
small particles (the longest portion is approximately 1 .mu.m) and
embed spaces between the large particles with the small particles.
Thus, there may be two peaks of particle size distribution.
[0205] The particle size of the positive electrode active material
is influenced not only by the particle sizes of starting materials
but also by a ratio between lithium and the first transition metal
(hereinafter expressed as a ratio of Li to the first transition
metal) which are contained in the starting material.
[0206] In the case where the particle size of the starting material
is small, the grain growth needs to be performed at the time of
baking so that the grain size of the positive electrode active
material is in the above-described preferred range.
[0207] To promote the grain growth at the time of baking, it is
effective to make the ratio of Li to the first transition metal of
the starting material larger than 1, that is, to make the amount of
lithium a little larger. For example, when the ratio of Li to the
first transition metal is approximately 1.06, a positive electrode
active material in which D50 is larger than or equal to 15 .mu.m is
easily obtained. Note that, as described later, lithium may be lost
to the outside of a system in the formation process of the positive
electrode active material; thus, the ratio between lithium and the
first transition metal of the obtained positive electrode active
material does not agree with the ratio between lithium and the
first transition metal of the starting material in some cases.
[0208] However, if the amount of lithium is too large to make the
particle size be in the preferred range, the capacity retention
rate of a secondary battery containing the positive electrode
active material might be decreased.
[0209] Then, the present inventors found that with the second
region 102 containing the second transition metal in the
superficial portion, the particle size can be in the preferred
range by control of the ratio of Li to the first transition metal
and a positive electrode active material having high capacity
retention rate can be formed.
[0210] In the positive electrode active material of one embodiment
of the present invention including a region containing the second
transition metal in the superficial portion, the ratio of Li to the
first transition metal in the starting material is preferably
greater than or equal to 1.00 and less than or equal to 1.07,
further preferably greater than or equal to 1.03 and less than or
equal to 1.06.
[Formation of Second Region]
[0211] The second region 102 can be formed by coating particles of
the composite oxide of lithium and the first transition metal with
a material containing the second transition metal.
[0212] As the coating method of the material containing the second
transition metal, a liquid phase method such as a sol-gel method, a
solid phase method, a sputtering method, an evaporation method, a
chemical vapor deposition (CVD) method, a pulsed laser deposition
(PLD) method, or the like can be used. In this embodiment, the case
where the sol-gel method which can be performed with a uniform
coverage under an atmospheric pressure is used is described.
<Sol-Gel Method>
[0213] A method for forming a material containing the second
transition metal using a sol-gel method is described with reference
to FIGS. 4A-1, 4A-2, 4A-3, 4B, 4C, 4D-1, and 4D-2.
[0214] First, an alkoxide of the second transition metal is
dissolved in alcohol.
[0215] FIG. 4A-1 shows a general formula of the alkoxide of the
second transition metal. In the formula of FIG. 4A-1, M2 indicates
the alkoxide of the second transition metal. R represents an alkyl
group having 1 to 18 carbon atoms or a substituted or unsubstituted
aryl group having 6 to 13 carbon atoms. Although FIG. 4A-1 shows
the general formula in which the second transition metal has a
valence of 4, one embodiment of the present invention is not
limited thereto. The second transition metal may have a valence of
2, a valence of 3, a valence of 5, a valence of 6, or a valence of
7. In this case, the alkoxide of the second transition metal
includes an alkoxy group corresponding to the valence of the second
transition metal.
[0216] FIG. 4A-2 shows a general formula of the titanium alkoxide
used when titanium is used as the second transition metal. R in
FIG. 4A-2 represents an alkyl group having 1 to 18 carbon atoms or
a substituted or unsubstituted aryl group having 6 to 13 carbon
atoms.
[0217] As the titanium alkoxide, tetramethoxytitanium,
tetraethoxytitanium, tetra-n-propoxytitanium,
tetra-i-propoxytitanium (also referred to as tetraisopropyl
orthotitanate, titanium (IV) isopropoxide, titanium
tetraisopropoxide (IV), TTIP, and the like),
tetra-n-butoxytitanium, tetra-i-butoxytitanium,
tetra-sec-butoxytitanium, tetra-t-butoxytitanium, or the like can
be used.
[0218] FIG. 4A-3 shows a chemical formula of titanium (IV)
isopropoxide (TTIP) described in a formation method below, which is
a kind of titanium alkoxide.
[0219] As a solvent in which the alkoxide of the second transition
metal is dissolved, an alcohol such as methanol, ethanol, propanol,
2-propanol, butanol, or 2-butanol is preferably used.
[0220] Next, particles of composite oxide of lithium, a transition
metal, magnesium, and fluorine are mixed into the alcohol solution
of the alkoxide of the second transition metal and stirred in an
atmosphere containing water vapor.
[0221] When the solution is put in an atmosphere containing
H.sub.2O, hydrolysis of water and an alkoxide of the second
transition metal occurs as in FIG. 4B. Then, as in FIG. 4C,
dehydration condensation occurs between the products of FIG. 4B.
When the hydrolysis of FIG. 4B and the condensation reaction of
FIG. 4C occur repeatedly, a sol of an oxide of the second
transition metal is generated. This reaction also occurs on a
particle 110 of the composite oxide as in FIGS. 4D-1 and 4D-2, and
a layer containing the second transition metal is formed on the
surface of the particle 110.
[0222] After that, the particle 110 is collected, and the alcohol
is vaporized. The details of the formation method are described
later.
[0223] Note that in this embodiment, an example in which the
particles of the composite oxide of lithium, the first transition
metal, the representative element, and fluorine are coated with the
material containing the second transition metal before the
particles are applied to a positive electrode current collector is
described; however, one embodiment of the present invention is not
limited thereto. After the positive electrode active material layer
including the particles of the composite oxide of lithium, the
first transition metal, the representative element, and fluorine is
formed on the positive electrode current collector, the positive
electrode current collector and the positive electrode active
material layer may be both soaked into an alkoxide solution of the
second transition metal to be coated with the material containing
the second transition metal.
[Segregation of Third Region]
[0224] The third region 103 can be formed by a sputtering method, a
solid phase method, a liquid phase method such as a sol-gel method,
or the like. However, the present inventors found that when a
source of a representative element such as magnesium and a source
of fluorine are mixed with a material of the first region 101 and
then the mixture is heated, the representative element is
segregated on a superficial portion of the positive electrode
active material particle to form the third region 103. In addition,
they found that with the third region 103 formed in this manner,
the positive electrode active material 100 has excellent cycle
characteristics.
[0225] In the case where the third region 103 is formed through the
heating as described above, the heating is preferably performed
after the particle of the composite oxide is coated with the
material containing the second transition metal. This is because
even after the particle is coated with the material containing the
second transition metal, the representative element such as
magnesium is unexpectedly segregated on the surface of the particle
when the heating is performed.
[0226] Segregation models of the representative element are
described with reference to FIGS. 5A to 5C and FIGS. 6A to 6D. It
is probable that the segregation model of the representative
element such as magnesium is slightly different in accordance with
the ratio between lithium and the first transition metal contained
in a starting material. Then, a segregation model in which the
ratio of Li to the first transition metal in the starting material
is less than 1.03, that is, the amount of lithium is small, is
described with reference to FIGS. 5A to 5C. In addition, a
segregation model in which the ratio of Li to the first transition
metal in the starting material is greater than or equal to 1.03,
that is, the amount of lithium is large, is described with
reference to FIGS. 6A to 6D. In these segregation models in FIGS.
5A to 5C and FIGS. 6A to 6D, the first transition metal is cobalt,
the second transition metal is titanium, and the representative
element is magnesium.
[0227] FIG. 5A illustrates a model diagram of the vicinity of the
surface of the particle 110 of the composite oxide containing
lithium, cobalt, magnesium, and fluorine, which is formed at a
ratio of Li to Co in the starting material of less than 1.03. A
region 111 in the drawings contains lithium, cobalt, magnesium, and
fluorine, and lithium cobaltate (LiCoO.sub.2) is a main component
of the region 111. Lithium cobaltate has a layered rock-salt
structure.
[0228] It is generally known that, at the time of synthesizing
particles of the composite oxide containing lithium, cobalt,
magnesium, and fluorine, lithium partly moves outside a system (a
particle on which lithium is formed). This is because lithium is
volatilized at the time of baking, lithium is eluted to a solvent
at the time of mixing the starting material, and the like. Thus,
the ratio of Li to Co in the particle 110 of the composite oxide
containing lithium, cobalt, magnesium, and fluorine becomes smaller
than the ratio of Li to Co in the starting material in some
cases.
[0229] When the ratio of Li to Co in the starting material is less
than 1.03, on the surface of the particle 110, lithium is released
from the lithium cobaltate and cobalt oxide is easily generated.
Thus, as illustrated in FIG. 5A, the surface of the particle 110 of
the composite oxide is covered with a cobalt oxide (CoO) layer 114
in some cases.
[0230] The cobalt oxide has a rock-salt crystal structure. Thus, in
the particle 110 in FIG. 5A, the cobalt oxide layer 114 having a
rock-salt crystal structure is provided over and in contact with
the region 111 containing lithium cobaltate having a layered
rock-salt crystal structure in some cases.
[0231] The particle 110 is coated with a material containing
titanium by a sol-gel method or the like. FIG. 5B illustrates a
state where the particle 110 is coated with a layer 112 containing
titanium by a sol-gel method. At the stage of FIG. 5B, the layer
112 containing titanium is a gel of titanium oxide; thus, the
crystallinity is low.
[0232] Next, the particle 110 coated with the layer 112 containing
titanium is heated. Although the details of the heating conditions
are described later, for example, FIG. 5C illustrates a state where
the particle 110 is heated in an oxygen atmosphere at 800.degree.
C. for two hours to form the positive electrode active material
100, which is one embodiment of the present invention. By heating,
titanium in the layer 112 containing titanium is diffused into the
inside of the particle 110. At the same time, magnesium and
fluorine contained in the region 111 are segregated on the surface
of the particle 110.
[0233] As described above, on the surface of the particle 110,
cobalt oxide having a rock-salt structure is present. In addition,
magnesium oxide also has a rock-salt crystal structure. Thus, it is
probable that magnesium is more stable in the state of being
present as magnesium oxide on the surface of the particle 110 as
compared with the state of being present inside the particle 110.
That could be why magnesium is segregated on the surface of the
particle 110 when the particle 110 is heated.
[0234] Moreover, it is considered that fluorine contained in the
starting material promotes the segregation of magnesium.
[0235] Fluorine has higher electronegativity than oxygen. Thus, it
is probable that even in a stable compound such as magnesium oxide,
when fluorine is added, uneven charge distribution occurs and a
bond between magnesium and oxygen is weakened. Furthermore, it is
probable that fluorine is substituted for oxygen in the magnesium
oxide, whereby magnesium easily moves around the substituted
fluorine.
[0236] Moreover, this can also be described from a phenomenon in
which a melting point of a mixture decreases. When magnesium oxide
(melting point: 2852.degree. C.) and lithium fluoride (melting
point: 848.degree. C.) are added at the same time, the melting
point of the magnesium oxide is lowered. It is considered that the
melting point is lowered, whereby magnesium easily moves at the
time of heating, and the segregation of magnesium easily
occurs.
[0237] Lastly, the third region 103 becomes a solid solution of
cobalt oxide and magnesium oxide which has a rock-salt crystal
structure. Furthermore, fluorine is probably substituted for part
of oxygen contained in the cobalt oxide and the magnesium
oxide.
[0238] Cobalt sites of lithium cobaltate are substituted for part
of the diffused titanium and lithium titanate is substituted for
another part of the diffused titanium. The second region 102 after
the heating contains lithium titanate having a rock-salt crystal
structure.
[0239] The first region 101 after the heating contains lithium
cobaltate having a layered rock-salt crystal structure.
[0240] Next, the case where the ratio of Li to Co in the starting
material is larger than or equal to 1.03 is described with
reference to FIGS. 6A to 6D. FIG. 6A illustrates a model diagram of
the vicinity of the surface of the particle 120 of the composite
oxide containing lithium, cobalt, magnesium, and fluorine, which is
formed at a ratio of Li to Co in the starting material of greater
than or equal to 1.03. A region 121 in the drawings contains
lithium, cobalt, magnesium, and fluorine.
[0241] Since the particle 120 in FIG. 6A contains a sufficient
amount of lithium, even when lithium is released from the particle
120 at the time of baking the particle 120 of the composite oxide
of lithium, cobalt, magnesium, and fluorine or the like, lithium is
diffused from the inside of the particle 120 to the surface thereof
to compensate; as a result, a cobalt oxide layer is not easily
formed on the surface.
[0242] FIG. 6B illustrates a state where the particle 120 in FIG.
6A is coated with a layer 122 containing titanium by a sol-gel
method. At the stage of FIG. 6B, the layer 122 containing titanium
is a gel of titanium oxide; thus, the crystallinity is low.
[0243] FIG. 6C illustrates the state where the particle 120 coated
with the layer 122 containing titanium in FIG. 6B starts to be
heated. By heating, titanium in the layer 122 containing titanium
is diffused into the inside of the particle 110. The diffused
titanium is bonded with lithium contained in the region 121 to
become lithium titanate, and a layer 125 containing the lithium
titanate is formed.
[0244] Since lithium is bonded with titanium to form lithium
titanate, the amount of lithium is relatively insufficient at the
surface of the particle 120. Thus, it is probable that, as
illustrated in FIG. 6C, a cobalt oxide layer 124 is temporarily
formed on the surface of the particle 120.
[0245] FIG. 6D illustrates the state where the state of FIG. 6C is
sufficiently heated to form the positive electrode active material
100, which is one embodiment of the present invention. It is
considered that, since the cobalt oxide layer 124 having a
rock-salt crystal structure is present on the surface, magnesium is
more stable in the state of being present as magnesium oxide on the
surface of the particle 120 as compared with the state of being
present inside the particle 120. As in the case of FIGS. 5A to 5C,
fluorine promotes the segregation of magnesium.
[0246] Thus, as illustrated in FIG. 6D, magnesium and fluorine
contained in the region 121 are segregated on the surface to be the
third region 103 with the cobalt oxide.
[0247] In this manner, the positive electrode active material 100,
which includes the third region 103 containing magnesium oxide and
cobalt oxide, the second region 102 containing lithium titanate,
and the first region 101 containing lithium cobaltate, is
formed.
[0248] Note that in the case where the representative element is
segregated by heating, when the composite oxide containing lithium
and the first transition metal included in the first region 101 is
a polycrystal or has crystal defects, the representative element
can be segregated not only in the superficial portion but also in
the vicinity of a grain boundary of the composite oxide containing
lithium and the first transition metal or in the vicinity of
crystal defects thereof. The representative element segregated in
the vicinity of a grain boundary or in the vicinity of crystal
defects can contribute to further improvement in stability of the
crystal structure of the composite oxide containing lithium and the
first transition metal included in the first region 101.
[0249] When the composite oxide containing lithium and the first
transition metal included in the first region 101 includes a crack
portion, the representative element is also segregated in the crack
portion by heating. In addition, not only the representative
element but also the second transition metal can be segregated. The
crack portion is in contact with the electrolyte solution like the
surface of the particle. Thus, the representative element and the
second transition metal are segregated in the crack portion, and
the third region 103 and the second region 102 are generated,
whereby a chemically stable material can be used for the region in
contact with the electrolyte solution. As a result, a secondary
battery having excellent cycle characteristics can be provided.
[0250] The ratio between a representative element (T) and fluorine
(F) in a starting material is preferably in a range of T:F=1:x
(1.5.ltoreq.x.ltoreq.4) (atomic ratio) because the segregation of
the representative element effectively occurs. Further preferably,
the ratio between T and F is approximately 1:2 (atomic ratio).
[0251] Since the third region 103 formed by segregation is formed
by epitaxial growth, orientations of crystals in the second region
102 and the third region 103 are partly and substantially aligned
with each other in some cases. That is, the second region 102 and
the third region 103 become topotaxy in some cases. When the
orientations of crystals in the second region 102 and the third
region 103 are substantially aligned with each other, these regions
can serve as a more favorable coating layer.
[0252] However, not all of the representative elements such as
magnesium which is added as a starting material need not be
segregated in the third region 103. For example, the first region
101 may contain a small amount of representative element such as
magnesium.
<Fourth Region 104>
[0253] In addition, as illustrated in FIG. 1C, the positive
electrode active material 100 may include a fourth region 104 on
the third region 103. Furthermore, when the positive electrode
active material 100 contains a defect such as a crack portion 106,
the fourth region 104 may be present to embed the defect such as
the crack portion 106.
[0254] The fourth region 104 contains some elements contained in
the second region 102 and the third region 103. For example, the
fourth region 104 contains the second transition metal and the
representative element.
[0255] The fourth region 104 may have a projection, a stripe shape,
or a layered shape. The fourth region 104 is formed using the
second transition metal and the representative element not
contained in the second region 102 or the third region 103 of the
second transition metals and the representative elements contained
in the starting material and the like. That is, with the fourth
region 104, the amount of the second transition metal and the
representative element contained in the second region 102 and the
third region 103 can be kept in an appropriate range and the
crystal structures of the second region 102 and the third region
103 can be stabilized in some cases. Moreover, with the fourth
region 104, the defect such as the crack portion 106 included in
the positive electrode active material 100 can be repaired.
[0256] The presence of the fourth region 104 and the shape of the
fourth region 104 can be observed by a scanning electron microscope
(SEM) or the like. Elements contained in the fourth region 104 can
be analyzed by SEM-EDX or the like.
[Method for Forming Positive Electrode Active Material]
[0257] Next, an example of a method for forming the positive
electrode active material 100, which is one embodiment of the
present invention, is described.
<Step 11: Preparation of Starting Materials>
[0258] First, starting materials are prepared. From the starting
materials prepared in this process, the first region 101 and the
third region 103 are formed finally.
[0259] As materials of lithium and the first transition metal
contained in the first region 101, a source of lithium and a source
of the first transition metal are prepared. In addition, as
materials of the compound of the representative elements contained
in the third region 103, a source of the representative element is
prepared.
[0260] In addition to these sources, a source of fluorine is
preferably prepared. Fluorine used for the materials has an effect
of segregating the representative elements contained in the third
region 103 on the surface of the positive electrode active material
100 in a later step.
[0261] As the source of lithium, for example, lithium carbonate and
lithium fluoride can be used. As the source of the first transition
metal, for example, an oxide of the first transition metal can be
used. As the source of the representative element, for example, an
oxide of the representative element contained in the third region
and fluoride of the representative element contained in the third
region can be used.
[0262] As the source of fluorine, for example, lithium fluoride and
fluoride of the representative element contained in the third
region can be used. That is, lithium fluoride can be used as either
the source of lithium or the source of fluorine.
[0263] The amount of fluorine contained in the source of fluorine
is preferably 1.0 time to 4 times (atomic ratio), further
preferably 1.5 times to 3 times (atomic ratio) the amount of
representative element contained in the source of the
representative element.
<Step 12: Mixing of Starting Materials>
[0264] Next, the source of lithium, the source of the first
transition metal, and the source of the representative element are
mixed. In addition, the source of fluorine is preferably added. For
example, a ball mill and a bead mill can be used for the
mixing.
<Step 13: First Heating>
[0265] Next, the materials mixed in Step 12 are heated. In this
step, the heating is referred to as baking or first heating in some
cases. The heating is preferably performed at higher than or equal
to 800.degree. C. and lower than or equal to 1100.degree. C.,
further preferably at higher than or equal to 900.degree. C. and
lower than or equal to 1000.degree. C. The heating time is
preferably greater than or equal to 2 hours and less than or equal
to 20 hours. The baking is preferably performed in a dried
atmosphere such as dry air. In the dried atmosphere, for example,
the dew point is preferably lower than or equal to -50.degree. C.,
further preferably lower than or equal to -100.degree. C. In this
embodiment, the heating is performed at 1000.degree. C. for 10
hours, the temperature rising rate is 200.degree. C./h, and dry air
whose dew point is -109.degree. C. flows at 10 L/min. After that,
the heated materials are cooled to room temperature.
[0266] By the heating in Step 13, the composite oxide of lithium
and the first transition metal having a layered rock-salt crystal
structure can be synthesized. At this time, the representative
element and fluorine contained in the starting materials form a
solid solution in the composite oxide. However, some representative
elements have been already segregated on the surface of the
composite oxide in some cases.
[0267] In addition, as the starting materials, particles of the
composite oxide containing lithium, cobalt, fluorine, and magnesium
which are synthesized in advance may be used. In this case, Step 12
and Step 13 can be omitted. For example, lithium cobalt oxide
particles (C-20F, produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.)
can be used as one of the starting materials. The lithium cobalt
oxide particle has a diameter of approximately 20 .mu.m and
contains fluorine, magnesium, calcium, sodium, silicon, sulfur, and
phosphorus in a region which can be analyzed by XPS from the
surface.
<Step 14: Coating with Second Transition Metal>
[0268] Next, the composite oxide of lithium and the first
transition metal is cooled to room temperature. Then, the surface
of the composite oxide particle of lithium and the first transition
metal is coated with a material containing the second transition
metal. In the formation method example, a sol-gel method is
used.
[0269] First, the alkoxide of the second transition metal which is
dissolved in alcohol and the composite oxide particles of lithium
and the first transition metal are mixed.
[0270] For example, in the case where titanium is used as the
second transition metal, TTIP can be used as the alkoxide of the
second transition metal. As alcohol, isopropanol can be used, for
example.
[0271] Next, the above mixed solution is stirred in an atmosphere
containing water vapor. The stirring can be performed with a
magnetic stirrer, for example The stirring time is not limited as
long as water and TTIP in an atmosphere cause hydrolysis and
polycondensation reaction. For example, the stirring can be
performed at 25.degree. C. and a humidity of 90% RH (Relative
Humidity) for 4 hours.
[0272] As described above, when water and TTIP in an atmosphere are
reacted, a sol-gel reaction can proceed more slowly as compared
with the case where liquid water is added. Alternatively, when
titanium alkoxide and water are reacted at room temperature, a
sol-gel reaction can proceed more slowly as compared with the case
where heating is performed at a temperature higher than the boiling
point of alcohol which is a solvent, for example. A sol-gel
reaction proceeds slowly, whereby a high-quality coating layer
containing titanium with a uniform thickness can be formed.
[0273] After the above process, precipitate is collected from the
mixed solution. As the collection method, filtration,
centrifugation, evaporation and drying, or the like can be used. In
this embodiment, filtration is used. For the filtration, a paper
filter is used, and the residue is washed by alcohol which is the
same as the solvent in which titanium alkoxide is dissolved.
[0274] Then, the collected residue is dried. In this embodiment,
vacuum drying is performed at 70.degree. C. for one hour.
<Step 15: Second Heating>
[0275] Next, the composite oxide particle coated with the material
containing the second transition metal, which is formed in Step 14,
is heated. This step is referred to as second heating in some
cases. In the heating, the retention time within a specified
temperature range is preferably shorter than or equal to 50 hours,
further preferably longer than or equal to 2 hours and shorter than
or equal to 10 hours, still further preferably longer than or equal
to 1 hour and shorter than or equal to 3 hours. If the heating time
is too short, there is concern that the segregation of the
representative elements does not occur; however, if the heating
time is too long, there is concern that the favorable second region
102 is not formed because diffusion of the second transition metal
proceeds too much.
[0276] The specified temperature is preferably higher than or equal
to 500.degree. C. and lower than or equal to 1200.degree. C.,
further preferably higher than or equal to 800.degree. C. and lower
than or equal to 1000.degree. C. If the specified temperature is
too low, there is concern that the segregation of the
representative elements and the second transition metal does not
occur. However, if the specified temperature is too high, there is
concern that the first transition metal in the composite oxide
particle is reduced to decompose the composite oxide particle, that
a layered structure of lithium and the first transition metal in
the composite oxide particle cannot be kept, and the like.
[0277] In this embodiment, the specified temperature is 800.degree.
C. and kept for 2 hours, the temperature rising rate is 200.degree.
C./h, and the flow rate of dry air is 10 L/min.
[0278] By the heating in Step 15, the composite oxide of lithium
and the first transition metal and the oxide of the second
transition metal covering the composite oxide become topotaxy. In
other words, the first region 101 and the second region 102 become
topotaxy.
[0279] By the heating in Step 15, the representative elements which
form a solid solution inside the composite oxide particle of
lithium and the first transition metal are unevenly distributed on
the surface to form a solid solution, that is, the representative
elements are segregated, the compound of the representative
elements is formed, and the third region 103 is formed. At this
time, the compound of the representative elements is formed by
heteroepitaxial growth from the second region 102. That is, the
second region 102 and the third region 103 become topotaxy.
[0280] Since the second region 102 and the third region 103 contain
crystals whose orientations are substantially aligned with each
other and have a stable bond with the first region 101, when the
positive electrode active material 100 is used for the secondary
battery, a change of the crystal structure in the first region 101
which is caused by charging and discharging can be effectively
suppressed. Even when lithium is released from the first region 101
due to charging, the superficial portion having a stable bond can
suppress release of oxygen and the first transition metal such as
cobalt from the first region 101. Furthermore, a chemically stable
material can be used for a region in contact with the electrolyte
solution. Thus, a secondary battery having excellent cycle
characteristics can be provided.
[0281] Note that the entire first region 101 and second region 102
does not need to become topotaxy as long as part of the first
region 101 and second region 102 becomes topotaxy. Furthermore, the
entire second region 102 and third region 103 does not need to
become topotaxy as long as part of the second region 102 and third
region 103 becomes topotaxy.
[0282] In the case where the compound of the representative
elements contained in the third region contains oxygen, the heating
in Step 15 is preferably performed in an atmosphere containing
oxygen. Heating in an atmosphere containing oxygen promotes the
formation of the third region 103.
[0283] Furthermore, fluorine contained in the starting materials
promotes the segregation of the representative elements.
[0284] In this manner, in the method for forming the positive
electrode active material of one embodiment of the present
invention, after the elements forming the second region 102 are
coated, heating is performed to form the third region 103, and two
kinds of regions can be formed on the surface of the positive
electrode active material 100. That is, in general, two coating
steps are necessary for providing two kinds of regions in a
superficial portion; however, in the method for forming the
positive electrode active material of one embodiment of the present
invention, only one coating step (sol-gel process) is needed, which
is a formation method with high productivity.
<Step 16: Cooling>
[0285] Next, the particles heated in Step 15 are cooled to room
temperature. The time of decreasing temperature is preferably long
because topotaxy is easily generated. For example, the time of
decreasing temperature from retention temperature to room
temperature is preferably the same as the time of increasing
temperature or longer, specifically longer than or equal to 10
hours and shorter than or equal to 50 hours.
<Step 17: Collecting>
[0286] Next, the cooled particles are collected. Moreover, the
particles are preferably made to pass through a sieve. Through the
above process, the positive electrode active material 100 including
the first region 101, the second region 102, and the third region
103 can be formed.
[0287] This embodiment can be implemented in appropriate
combination with any of the other embodiments.
Embodiment 2
[0288] In this embodiment, examples of materials which can be used
for a secondary battery containing the positive electrode active
material 100 described in the above embodiment are described. In
this embodiment, a secondary battery in which a positive electrode,
a negative electrode, and an electrolyte solution are wrapped in an
exterior body is described as an example.
[Positive Electrode]
[0289] The positive electrode includes a positive electrode active
material layer and a positive electrode current collector.
<Positive Electrode Active Material Layer>
[0290] The positive electrode active material layer contains at
least a positive electrode active material. The positive electrode
active material layer may contain, in addition to the positive
electrode active material, other materials such as a coating film
of the active material surface, a conductive additive, and a
binder.
[0291] As the positive electrode active material, the positive
electrode active material 100 described in the above embodiment can
be used. When the above-described positive electrode active
material 100 is used, a secondary battery with high capacity and
excellent cycle characteristics can be obtained.
[0292] Examples of the conductive additive include a carbon
material, a metal material, and a conductive ceramic material.
Alternatively, a fiber material may be used as the conductive
additive. The content of the conductive additive with respect to
the total amount of the active material layer is preferably greater
than or equal to 1 wt % and less than or equal to 10 wt %, more
preferably greater than or equal to 1 wt % and less than or equal
to 5 wt %.
[0293] A network for electric conduction can be formed in the
active material layer by the conductive additive. The conductive
additive also allows maintaining of a path for electric conduction
between the positive electrode active material particles. The
addition of the conductive additive to the active material layer
increases the electric conductivity of the active material
layer.
[0294] Examples of the conductive additive include natural
graphite, artificial graphite such as mesocarbon microbeads, and
carbon fiber. Examples of carbon fiber include mesophase
pitch-based carbon fiber, isotropic pitch-based carbon fiber,
carbon nanofiber, and carbon nanotube. Carbon nanotube can be
formed by, for example, a vapor deposition method. Other examples
of the conductive additive include carbon materials such as carbon
black (e.g., acetylene black (AB)), graphite (black lead)
particles, graphene, and fullerene. Alternatively, metal powder or
metal fibers of copper, nickel, aluminum, silver, gold, or the
like, a conductive ceramic material, or the like can be used.
[0295] Alternatively, a graphene compound may be used as the
conductive additive.
[0296] A graphene compound has excellent electrical characteristics
of high conductivity and excellent physical properties of high
flexibility and high mechanical strength. Furthermore, a graphene
compound has a planar shape. A graphene compound enables
low-resistance surface contact. Furthermore, a graphene compound
has extremely high conductivity even with a small thickness in some
cases and thus allows a conductive path to be formed in an active
material layer efficiently even with a small amount. For this
reason, it is preferable to use a graphene compound as the
conductive additive because the area where the active material and
the conductive additive are in contact with each other can be
increased. It is preferable to form the graphene compound serving
as a conductive additive as a coating film to cover the entire
surface of the active material with a spray dry apparatus, in which
case the electrical resistance may be reduced. Here, it is
particularly preferable to use, for example, graphene, multilayer
graphene, or RGO as a graphene compound. Note that RGO refers to a
compound obtained by reducing graphene oxide (GO), for example
[0297] In the case where an active material with a small particle
diameter (e.g., 1 .mu.m or less) is used, the specific surface area
of the active material is large and thus more conductive paths for
the active material particles are needed. Thus, the amount of
conductive additive tends to increase and the supported amount of
active material tends to decrease relatively. When the supported
amount of active material decreases, the capacity of the secondary
battery also decreases. In such a case, a graphene compound that
can efficiently form a conductive path even in a small amount is
particularly preferably used as the conductive additive because the
supported amount of active material does not decrease.
[0298] A cross-sectional structure example of an active material
layer 200 containing a graphene compound as a conductive additive
is described below.
[0299] FIG. 7A shows a longitudinal cross-sectional view of the
active material layer 200. The active material layer 200 includes
positive electrode active material particles 100, a graphene
compound 201 serving as a conductive additive, and a binder (not
illustrated). Here, graphene or multilayer graphene may be used as
the graphene compound 201, for example The graphene compound 201
preferably has a sheet-like shape. The graphene compound 201 may
have a sheet-like shape formed of a plurality of sheets of
multilayer graphene and/or a plurality of sheets of graphene that
partly overlap with each other.
[0300] The longitudinal cross section of the active material layer
200 in FIG. 7B shows substantially uniform dispersion of the
sheet-like graphene compounds 201 in the active material layer 200.
The graphene compounds 201 are schematically shown by thick lines
in FIG. 7B but are actually thin films each having a thickness
corresponding to the thickness of a single layer or a multi-layer
of carbon molecules. The plurality of graphene compounds 201 are
formed in such a way as to partly coat or adhere to the surfaces of
the plurality of positive electrode active material particles 100,
so that the graphene compounds 201 make surface contact with the
positive electrode active material particles 100.
[0301] Here, the plurality of graphene compounds are bonded to each
other to form a net-like graphene compound sheet (hereinafter
referred to as a graphene compound net or a graphene net). The
graphene net covering the active material can function as a binder
for bonding active materials. The amount of a binder can thus be
reduced, or the binder does not have to be used. This can increase
the proportion of the active material in the electrode volume or
weight. That is to say, the capacity of the secondary battery can
be increased.
[0302] Here, it is preferable to perform reduction after a layer to
be the active material layer 200 is formed in such a manner that
graphene oxide is used as the graphene compound 201 and mixed with
an active material. When graphene oxide with extremely high
dispersibility in a polar solvent is used for the formation of the
graphene compounds 201, the graphene compounds 201 can be
substantially uniformly dispersed in the active material layer 200.
The solvent is removed by volatilization from a dispersion medium
in which graphene oxide is uniformly dispersed, and the graphene
oxide is reduced; hence, the graphene compounds 201 remaining in
the active material layer 200 partly overlap with each other and
are dispersed such that surface contact is made, thereby forming a
three-dimensional conduction path. Note that graphene oxide can be
reduced either by heat treatment or with the use of a reducing
agent, for example
[0303] Unlike a conductive additive in the form of particles, such
as acetylene black, which makes point contact with an active
material, the graphene compound 201 is capable of making
low-resistance surface contact; accordingly, the electrical
conduction between the positive electrode active material particles
100 and the graphene compounds 201 can be improved with a smaller
amount of the graphene compound 201 than that of a normal
conductive additive. This increases the proportion of the positive
electrode active material particles 100 in the active material
layer 200, resulting in increased discharge capacity of the
secondary battery.
[0304] Alternatively, the graphene compound may cover the entire
surface of the active material in advance with a spray dry
apparatus. After that, at the time of forming the positive
electrode active material layer, a graphene compound can be further
added to make the conductive path between the active materials more
favorable.
[0305] As the binder, a rubber material such as styrene-butadiene
rubber (SBR), styrene-isoprene-styrene rubber,
acrylonitrile-butadiene rubber, butadiene rubber, or
ethylene-propylene-diene copolymer can be used, for example.
Alternatively, fluororubber can be used as the binder.
[0306] For the binder, for example, water-soluble polymers are
preferably used. As the water-soluble polymers, a polysaccharide
and the like can be used. As the polysaccharide, a cellulose
derivative such as carboxymethyl cellulose (CMC), methyl cellulose,
ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or
regenerated cellulose, starch, or the like can be used. It is more
preferred that such water-soluble polymers be used in combination
with any of the above rubber materials.
[0307] Alternatively, as the binder, a material such as
polystyrene, poly(methyl acrylate), poly(methyl methacrylate)
(PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene
oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride,
polytetrafluoroethylene, polyethylene, polypropylene,
polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene
fluoride (PVdF), polyacrylonitrile (PAN), ethylene-propylene-diene
polymer, polyvinyl acetate, or nitrocellulose is preferably
used.
[0308] A plurality of the above materials may be used in
combination for the binder.
[0309] For example, a material having a significant viscosity
modifying effect and another material may be used in combination.
For example, a rubber material or the like has high adhesion or
high elasticity but may have difficulty in viscosity modification
when mixed in a solvent. In such a case, a rubber material or the
like is preferably mixed with a material having a significant
viscosity modifying effect, for example As a material having a
significant viscosity modifying effect, for example, a
water-soluble polymer is preferably used. An example of a
water-soluble polymer having an especially significant viscosity
modifying effect is the above-mentioned polysaccharide; for
example, a cellulose derivative such as carboxymethyl cellulose
(CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose,
diacetyl cellulose, or regenerated cellulose, or starch can be
used.
[0310] Note that a cellulose derivative such as carboxymethyl
cellulose obtains a higher solubility when converted into a salt
such as a sodium salt or an ammonium salt of carboxymethyl
cellulose, and accordingly, easily exerts an effect as a viscosity
modifier. The high solubility can also increase the dispersibility
of an active material and other components in the formation of
slurry for an electrode. In this specification, cellulose and a
cellulose derivative used as a binder of an electrode include salts
thereof.
[0311] The water-soluble polymers stabilize viscosity by being
dissolved in water and allow stable dispersion of the active
material and another material combined as a binder such as
styrene-butadiene rubber in an aqueous solution. Furthermore, a
water-soluble polymer is expected to be easily and stably adsorbed
to an active material surface because it has a functional group.
Many cellulose derivatives such as carboxymethyl cellulose have
functional groups such as a hydroxyl group and a carboxyl group.
Because of functional groups, polymers are expected to interact
with each other and cover an active material surface in a large
area.
[0312] In the case where the binder covering or being in contact
with the active material surface forms a film, the film is expected
to serve as a passivation film to suppress the decomposition of the
electrolyte solution. Here, the passivation film refers to a film
without electric conductivity or a film with extremely low electric
conductivity, and can inhibit the decomposition of an electrolyte
solution at a potential at which a battery reaction occurs in the
case where the passivation film is formed on the active material
surface, for example. It is preferred that the passivation film can
conduct lithium ions while suppressing electric conduction.
<Positive Electrode Current Collector>
[0313] The positive electrode current collector can be formed using
a material that has high conductivity, such as a metal like
stainless steel, gold, platinum, aluminum, or titanium, or an alloy
thereof. It is preferred that a material used for the positive
electrode current collector not dissolve at the potential of the
positive electrode. Alternatively, the positive electrode current
collector can be formed using an aluminum alloy to which an element
that improves heat resistance, such as silicon, titanium,
neodymium, scandium, or molybdenum, is added. Still alternatively,
a metal element that forms silicide by reacting with silicon can be
used. Examples of the metal element that forms silicide by reacting
with silicon include zirconium, titanium, hafnium, vanadium,
niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and
nickel. The current collector can have any of various shapes
including a foil-like shape, a plate-like shape (sheet-like shape),
a net-like shape, a punching-metal shape, and an expanded-metal
shape. The current collector preferably has a thickness of 5 .mu.m
to 30 .mu.m.
[Negative Electrode]
[0314] The negative electrode includes a negative electrode active
material layer and a negative electrode current collector. The
negative electrode active material layer may contain a conductive
additive and a binder.
<Negative Electrode Active Material>
[0315] As a negative electrode active material, for example, an
alloy-based material or a carbon-based material can be used.
[0316] For the negative electrode active material, an element which
enables charge-discharge reactions by an alloying reaction and a
dealloying reaction with lithium can be used. For example, a
material containing at least one of silicon, tin, gallium,
aluminum, germanium, lead, antimony, bismuth, silver, zinc,
cadmium, indium, and the like can be used. Such elements have
higher capacity than carbon. In particular, silicon has a
significantly high theoretical capacity of 4200 mAh/g. For this
reason, silicon is preferably used as the negative electrode active
material. Alternatively, a compound containing any of the above
elements may be used. Examples of the compound 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, and SbSn.
Here, an element that enables charge-discharge reactions by an
alloying reaction and a dealloying reaction with lithium, a
compound containing the element, and the like may be referred to as
an alloy-based material.
[0317] In this specification and the like, SiO refers, for example,
to silicon monoxide. SiO can alternatively be expressed as
SiO.sub.x. Here, x preferably has an approximate value of 1. For
example, x is preferably 0.2 or more and 1.5 or less, more
preferably 0.3 or more and 1.2 or less.
[0318] As the carbon-based material, graphite, graphitizing carbon
(soft carbon), non-graphitizing carbon (hard carbon), a carbon
nanotube, graphene, carbon black, and the like can be used.
[0319] Examples of graphite include artificial graphite and natural
graphite. Examples of artificial graphite include meso-carbon
microbeads (MCMB), coke-based artificial graphite, and pitch-based
artificial graphite. As artificial graphite, spherical graphite
having a spherical shape can be used. For example, MCMB is
preferably used because it may have a spherical shape. Moreover,
MCMB may preferably be used because it can relatively easily have a
small surface area. Examples of natural graphite include flake
graphite and spherical natural graphite.
[0320] Graphite has a low potential substantially equal to that of
a lithium metal (higher than or equal to 0.05 V and lower than or
equal to 0.3 V vs. Li/Li.sup.+) when lithium ions are intercalated
into the graphite (while a lithium-graphite intercalation compound
is formed). For this reason, a lithium-ion secondary battery can
have a high operating voltage. In addition, graphite is preferred
because of its advantages such as a relatively high capacity per
unit volume, relatively small volume expansion, low cost, and
higher level of safety than that of a lithium metal.
[0321] Alternatively, for the negative electrode active material,
an oxide such as titanium dioxide (TiO.sub.2), lithium titanium
oxide (Li.sub.4Ti.sub.5O.sub.12), lithium-graphite intercalation
compound (Li.sub.xC.sub.6), niobium pentoxide (Nb.sub.2O.sub.5),
tungsten oxide (WO.sub.2), or molybdenum oxide (MoO.sub.2) can be
used.
[0322] Still alternatively, for the negative electrode active
material, Li.sub.3-xM.sub.xN (M.dbd.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
and 1890 mAh/cm.sup.3).
[0323] A nitride containing lithium and a transition metal is
preferably used, in which case lithium ions are contained 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 contain lithium
ions, such as V.sub.2O.sub.5 or Cr.sub.3O.sub.8. In the case of
using a material containing lithium ions as a positive electrode
active material, the nitride containing lithium and a transition
metal can be used for the negative electrode active material by
extracting the lithium ions contained in the positive electrode
active material in advance.
[0324] Alternatively, a material which causes a conversion reaction
can be used for the negative electrode active material; for
example, a transition metal oxide which does not form an alloy with
lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron
oxide (FeO), may be used. Other examples of the material which
causes a 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.
[0325] For the conductive additive and the binder that can be
included in the negative electrode active material layer, materials
similar to those of the conductive additive and the binder that can
be included in the positive electrode active material layer can be
used.
<Negative Electrode Current Collector>
[0326] For the negative electrode current collector, a material
similar to that of the positive electrode current collector can be
used. Note that a material which is not alloyed with a carrier ion
such as lithium is preferably used for the negative electrode
current collector.
[Electrolyte Solution]
[0327] The electrolyte solution contains a solvent and an
electrolyte. As a solvent of 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, ethyl acetate, methyl propionate,
ethyl propionate, propyl propionate, 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.
[0328] Alternatively, when one or more kinds of ionic liquids (room
temperature molten salts) which have features of non-flammability
and non-volatility is used as a solvent of the electrolyte
solution, a secondary battery can be prevented from exploding or
catching fire even when the secondary battery internally shorts out
or the internal temperature increases owing to overcharging or the
like. An ionic liquid contains a cation and an anion. The ionic
liquid contains an organic cation and an anion. Examples of the
organic cation used for the electrolyte solution include aliphatic
onium cations such as a quaternary ammonium cation, a tertiary
sulfonium cation, and a quaternary phosphonium cation, and aromatic
cations such as an imidazolium cation and a pyridinium cation.
Examples of the anion used for the electrolyte solution include a
monovalent amide-based anion, a monovalent methide-based anion, a
fluorosulfonate anion, a perfluoroalkylsulfonate anion, a
tetrafluoroborate anion, a perfluoroalkylborate anion, a
hexafluorophosphate anion, and a perfluoroalkylphosphate anion.
[0329] 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.
[0330] The electrolyte solution used for a secondary battery is
preferably highly purified and contains a small amount of dust
particles and elements other than the constituent elements of the
electrolyte solution (hereinafter also simply referred to as
impurities). Specifically, the weight ratio of impurities to the
electrolyte solution is less than or equal to 1%, preferably less
than or equal to 0.1%, and further preferably less than or equal to
0.01%.
[0331] Furthermore, an additive agent such as vinylene carbonate,
propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene
carbonate (FEC), LiBOB, or a dinitrile compound such as
succinonitrile or adiponitrile may be added to the electrolyte
solution. The concentration of a material to be added with respect
to the whole solvent is, for example, higher than or equal to 0.1
wt % and lower than or equal to 5 wt %.
[0332] Alternatively, a gelled electrolyte obtained in such a
manner that a polymer is swelled with an electrolyte solution may
be used.
[0333] When a polymer gel electrolyte is used, safety against
liquid leakage and the like is improved. Furthermore, a secondary
battery can be thinner and more lightweight.
[0334] As a polymer that undergoes gelation, a silicone gel, an
acrylic gel, an acrylonitrile gel, a polyethylene oxide-based gel,
a polypropylene oxide-based gel, a fluorine-based polymer gel, or
the like can be used.
[0335] Examples of the polymer include a polymer having a
polyalkylene oxide structure, such as polyethylene oxide (PEO);
PVDF; polyacrylonitrile; and a copolymer containing any of them.
For example, PVDF-HFP, which is a copolymer of PVDF and
hexafluoropropylene (HFP) can be used. The formed polymer may be
porous.
[0336] Instead of the electrolyte solution, a solid electrolyte
including an inorganic material such as a sulfide-based inorganic
material or an oxide-based inorganic material, or a solid
electrolyte including a high-molecular material such as a
polyethylene oxide (PEO)-based high-molecular material may
alternatively be used. When the solid electrolyte is used, a
separator and a spacer are not necessary. Furthermore, since the
battery can be entirely solidified, there is no possibility of
liquid leakage to increase the safety of the battery
dramatically.
[Separator]
[0337] The secondary battery preferably includes a separator. As
the separator, for example, paper; nonwoven fabric; glass fiber;
ceramics; or synthetic fiber using nylon (polyamide), vinylon
(polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or
polyurethane can be used. The separator is preferably formed to
have an envelope-like shape to wrap one of the positive electrode
and the negative electrode.
[0338] The separator may have a multilayer structure. For example,
an organic material film such as polypropylene or polyethylene can
be coated with a ceramic-based material, a fluorine-based material,
a polyamide-based material, a mixture thereof, or the like.
Examples of the ceramic-based material include aluminum oxide
particles and silicon oxide particles. Examples of the
fluorine-based material include PVDF and a polytetrafluoroethylene.
Examples of the polyamide-based material include nylon and aramid
(meta-based aramid and para-based aramid).
[0339] Deterioration of the separator in charging and discharging
at high voltage can be suppressed and thus the reliability of the
secondary battery can be improved because oxidation resistance is
improved when the separator is coated with the ceramic-based
material. In addition, when the separator is coated with the
fluorine-based material, the separator is easily brought into close
contact with an electrode, resulting in high output
characteristics. When the separator is coated with the
polyamide-based material, in particular, aramid, the safety of the
secondary battery is improved because heat resistance is
improved.
[0340] For example, both surfaces of a polypropylene film may be
coated with a mixed material of aluminum oxide and aramid.
Alternatively, a surface of the polypropylene film in contact with
the positive electrode may be coated with the mixed material of
aluminum oxide and aramid, and a surface of the polypropylene film
in contact with the negative electrode may be coated with the
fluorine-based material.
[0341] With the use of a separator having a multilayer structure,
the capacity of the secondary battery per volume can be increased
because the safety of the secondary battery can be maintained even
when the total thickness of the separator is small.
[Exterior Body]
[0342] For an exterior body included in the secondary battery, a
metal material such as aluminum and a resin material can be used,
for example An exterior body in the form of a film can also be
used. As the film, for example, a 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.
[Charging and Discharging Methods]
[0343] The secondary battery can be charged and discharged in the
following manner, for example.
<<CC Charge>>
[0344] First, CC charge, which is one of charging methods, is
described. CC charge is a charging method in which a constant
current is made to flow to a secondary battery in the whole
charging period and charge is terminated when the voltage reaches a
predetermined voltage. The secondary battery is assumed to be an
equivalent circuit with internal resistance R and secondary battery
capacitance C as illustrated in FIG. 8A. In that case, a secondary
battery voltage V.sub.B is the sum of a voltage V.sub.R applied to
the internal resistance R and a voltage V.sub.C applied to the
secondary battery capacitance C.
[0345] While the CC charge is performed, a switch is on as
illustrated in FIG. 8A, so that a constant current I flows to the
secondary battery. During the period, the current I is constant;
thus, according to the Ohm's law (V.sub.R=R.times.I), the voltage
V.sub.R applied to the internal resistance R is also constant. In
contrast, the voltage V.sub.C applied to the secondary battery
capacitance C increases over time. Accordingly, the secondary
battery voltage V.sub.B increases over time.
[0346] When the secondary battery voltage V.sub.B reaches a
predetermined voltage, e.g., 4.3 V, the charge is terminated. On
termination of the CC charge, the switch is turned off as
illustrated in FIG. 8B, and the current I becomes 0. Thus, the
voltage V.sub.R applied to the internal resistance R becomes 0 V.
Consequently, the secondary battery voltage V.sub.B is decreased by
the lost voltage drop in the internal resistance R.
[0347] FIG. 8C shows an example of the secondary battery voltage
V.sub.B and charging current during a period in which the CC charge
is performed and after the CC charge is terminated. The secondary
battery voltage V.sub.B increases while the CC charge is performed,
and slightly decreases after the CC charge is terminated.
<<CCCV Charge>>
[0348] Next, CCCV charge, which is a charging method different from
the above-described method, is described. CCCV charge is a charging
method in which CC charge is performed until the voltage reaches a
predetermined voltage and then constant voltage (CV) charge is
performed until the amount of current flow becomes small,
specifically, a termination current value.
[0349] While the CC charge is performed, a switch of a constant
current power source is on and a switch of a constant voltage power
source is off as illustrated in FIG. 9A, so that the constant
current I flows to the secondary battery. During the period, the
current I is constant; thus, according to the Ohm's law
(V.sub.R=R.times.I), the voltage V.sub.R applied to the internal
resistance R is also constant. In contrast, the voltage V.sub.C
applied to the secondary battery capacitance C increases over time.
Accordingly, the secondary battery voltage V.sub.B increases over
time.
[0350] When the secondary battery voltage V.sub.B reaches a
predetermined voltage, e.g., 4.3 V, switching is performed from the
CC charge to the CV charge. While the CV charge is performed, the
switch of the constant voltage power source is on and the switch of
the constant current power source is off as illustrated in FIG. 9B;
thus, the secondary battery voltage V.sub.B is constant. In
contrast, the voltage V.sub.C applied to the secondary battery
capacitance C increases over time. Since V.sub.B=V.sub.R+V.sub.C is
satisfied, the voltage V.sub.R applied to the internal resistance R
decreases over time. As the voltage V.sub.R applied to the internal
resistance R decreases, the current I flowing to the secondary
battery also decreases according to the Ohm's law
(V.sub.R=R.times.I).
[0351] When the current I flowing to the secondary battery becomes
a predetermined current, e.g., approximately 0.01 C, charge is
terminated. On termination of the CCCV charge, all the switches are
turned off as illustrated in FIG. 9C, so that the current I becomes
0. Thus, the voltage V.sub.R applied to the internal resistance R
becomes 0 V. However, the voltage V.sub.R applied to the internal
resistance R becomes sufficiently small by the CV charge; thus,
even when a voltage drop no longer occurs in the internal
resistance R, the secondary battery voltage V.sub.B hardly
decreases.
[0352] FIG. 9D shows an example of the secondary battery voltage
V.sub.B and charging current while the CCCV charge is performed and
after the CCCV charge is terminated. Even after the CCCV charge is
terminated, the secondary battery voltage V.sub.B hardly
decreases.
<<CC Discharge>>
[0353] Next, CC discharge, which is one of discharging methods, is
described. CC discharge is a discharging method in which a constant
current is made to flow from the secondary battery in the whole
discharging period, and discharge is terminated when the secondary
battery voltage V.sub.B reaches a predetermined voltage, e.g., 2.5
V.
[0354] FIG. 10 shows an example of the secondary battery voltage
V.sub.B and discharging current while the CC discharge is
performed. As discharge proceeds, the secondary battery voltage
V.sub.B decreases.
[0355] Next, a discharge rate and a charge rate will be described.
The discharge rate refers to the relative ratio of discharging
current to battery capacity and is expressed in a unit C. A current
of approximately 1 C in a battery with a rated capacity X (Ah) is X
A. The case where discharge is performed at a current of 2X A is
rephrased as follows: discharge is performed at 2 C. The case where
discharge is performed at a current of X/5 A is rephrased as
follows: discharge is performed at 0.2 C. Similarly, the case where
charging is performed at a current of 2X A is rephrased as follows:
charging is performed at 2 C, and the case where charging is
performed at a current of X/5 A is rephrased as follows: charging
is performed at 0.2 C.
Embodiment 3
[0356] In this embodiment, examples of a shape of a secondary
battery containing the positive electrode active material 100
described in the above embodiment are described. For the materials
used for the secondary battery described in this embodiment, the
description of the above embodiment can be referred to.
[Coin-Type Secondary Battery]
[0357] First, an example of a coin-type secondary battery is
described. FIG. 11A is an external view of a coin-type
(single-layer flat type) secondary battery, and FIG. 11B is a
cross-sectional view thereof.
[0358] In a coin-type secondary battery 300, a positive electrode
can 301 doubling as a positive electrode terminal and a negative
electrode can 302 doubling as a negative electrode terminal are
insulated from each other and sealed by a gasket 303 made of
polypropylene or the like. A positive electrode 304 includes a
positive electrode current collector 305 and a positive electrode
active material layer 306 provided in contact with the positive
electrode current collector 305. A negative electrode 307 includes
a negative electrode current collector 308 and a negative electrode
active material layer 309 provided in contact with the negative
electrode current collector 308.
[0359] Note that only one surface of each of the positive electrode
304 and the negative electrode 307 used for the coin-type secondary
battery 300 is provided with an active material layer.
[0360] For the positive electrode can 301 and the negative
electrode can 302, a metal having a corrosion-resistant property to
an electrolyte solution, such as nickel, aluminum, or titanium, an
alloy of such a metal, or an alloy of such a metal and another
metal (e.g., stainless steel) can be used. Alternatively, the
positive electrode can 301 and the negative electrode can 302 are
preferably covered with nickel, aluminum, or the like in order to
prevent corrosion due to the electrolyte solution. The positive
electrode can 301 and the negative electrode can 302 are
electrically connected to the positive electrode 304 and the
negative electrode 307, respectively.
[0361] The negative electrode 307, the positive electrode 304, and
the separator 310 are immersed in the electrolyte solution. Then,
as illustrated in FIG. 11B, 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 located therebetween. In
such a manner, the coin-type secondary battery 300 can be
manufactured.
[0362] When the positive electrode active material described in the
above embodiment is used in the positive electrode 304, the
coin-type secondary battery 300 with high capacity and excellent
cycle characteristics can be obtained.
[0363] Here, a current flow in charging a secondary battery is
described with reference to FIG. 11C. When a secondary battery
using lithium is regarded as a closed circuit, lithium ions
transfer and a current flows in the same direction. Note that in
the secondary battery using lithium, an anode and a cathode change
places in charge and discharge, and an oxidation reaction and a
reduction reaction occur on the corresponding sides; hence, an
electrode with a high reaction potential is called a positive
electrode and an electrode with a low reaction potential is called
a negative electrode. For this reason, in this specification, the
positive electrode is referred to as a "positive electrode" or a
"plus electrode" and the negative electrode is referred to as a
"negative electrode" or a "minus electrode" in all the cases where
charge is performed, discharge is performed, a reverse pulse
current is supplied, and a charging current is supplied. The use of
the terms "anode" and "cathode" related to an oxidation reaction
and a reduction reaction might cause confusion because the anode
and the cathode change places at the time of charging and
discharging. Thus, the terms "anode" and "cathode" are not used in
this specification. If the term "anode" or "cathode" is used, it
should be mentioned that the anode or the cathode is which of the
one at the time of charging or the one at the time of discharging
and corresponds to which of a positive (plus) electrode or a
negative (minus) electrode.
[0364] Two terminals in FIG. 11C are connected to a charger, and
the coin-type secondary battery 300 is charged. As the charge of
the coin-type secondary battery 300 proceeds, a potential
difference between electrodes increases.
[Cylindrical Secondary Battery]
[0365] Next, an example of a cylindrical secondary battery will be
described with reference to FIGS. 12A to 12D. A cylindrical
secondary battery 600 includes, as illustrated in FIG. 12A, a
positive electrode cap (battery lid) 601 on the top surface and a
battery can (outer can) 602 on the side and bottom surfaces. The
positive electrode cap and the battery can (outer can) 602 are
insulated from each other by a gasket (insulating packing) 610.
[0366] FIG. 12B is a schematic cross-sectional view of the
cylindrical secondary battery. Inside the battery can 602 having a
hollow cylindrical shape, a battery element in which a strip-like
positive electrode 604 and a strip-like negative electrode 606 are
wound with a strip-like separator 605 located therebetween is
provided. Although not illustrated, the battery element is wound
around a center pin. 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-resistant property to an electrolyte solution,
such as nickel, aluminum, or titanium, an alloy of such a metal, or
an alloy of such a metal and another metal (e.g., stainless steel)
can be used. Alternatively, the battery can 602 is preferably
covered with nickel, aluminum, or the like in order to prevent
corrosion due to the electrolyte solution. Inside the battery can
602, the battery element in which the positive electrode, the
negative electrode, and the separator are wound is provided between
a pair of insulating plates 608 and 609 that face each other.
Furthermore, a nonaqueous electrolyte solution (not illustrated) is
injected inside the battery can 602 provided with the battery
element. As the nonaqueous electrolyte solution, a nonaqueous
electrolyte solution that is similar to that of the coin-type
secondary battery can be used.
[0367] Since the positive electrode and the negative electrode of
the cylindrical secondary battery are wound, active materials are
preferably 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. Both the
positive electrode terminal 603 and the negative electrode terminal
607 can be formed using a metal material such as aluminum The
positive electrode terminal 603 and the negative electrode terminal
607 are resistance-welded to a safety valve mechanism 612 and the
bottom of the battery can 602, respectively. 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 exceeds a
predetermined threshold value. The PTC element 611, which serves as
a thermally sensitive resistor whose resistance increases as
temperature rises, limits the amount of current by increasing the
resistance, in order to prevent abnormal heat generation. Barium
titanate (BaTiO.sub.3)-based semiconductor ceramic can be used for
the PTC element.
[0368] Alternatively, as illustrated in FIG. 12C, a plurality of
cylindrical secondary batteries 600 may be sandwiched between a
conductive plate 613 and a conductive plate 614 to form a module
615. The plurality of cylindrical secondary batteries 600 may be
connected in parallel, connected in series, or connected in series
after being connected in parallel. With the module 615 including
the plurality of cylindrical secondary batteries 600, large
electric power can be extracted.
[0369] FIG. 12D is a top view of the module 615. The conductive
plate 613 is shown by a dotted line for clarity of the drawing. As
illustrated in FIG. 12D, the module 615 may include a wiring 616
which electrically connects the plurality of cylindrical secondary
batteries 600 to each other. It is possible to provide the
conductive plate 613 over the wiring 616 to overlap with each
other. In addition, a temperature control device 617 may be
provided between the plurality of cylindrical secondary batteries
600. When the cylindrical secondary batteries 600 are overheated,
the temperature control device 617 can cool them, and when the
cylindrical secondary batteries 600 are cooled too much, the
temperature control device 617 can heat them. Thus, the performance
of the module 615 is not easily influenced by the outside air
temperature.
[0370] When the positive electrode active material described in the
above embodiment is used in the positive electrode 604, the
cylindrical secondary battery 600 with high capacity and excellent
cycle characteristics can be obtained.
[Structural Examples of Secondary Battery]
[0371] Other structural examples of secondary batteries will be
described with reference to FIGS. 13A and 13B, FIGS. 14A-1, 14A-2,
14B-1, and 14B-2, FIGS. 15A and 15B, and FIG. 16.
[0372] FIGS. 13A and 13B are external views of a secondary battery.
The secondary battery includes a circuit board 900 and a secondary
battery 913. A label 910 is attached to the secondary battery 913.
As shown in FIG. 13B, the secondary battery further includes a
terminal 951, a terminal 952, an antenna 914, and an antenna
915.
[0373] The circuit board 900 includes terminals 911 and a circuit
912. The terminals 911 are connected to the terminals 951 and 952,
the antennas 914 and 915, and the circuit 912. Note that a
plurality of terminals 911 serving as a control signal input
terminal, a power supply terminal, and the like may be
provided.
[0374] The circuit 912 may be provided on the rear surface of the
circuit board 900. The shape of each of the antennas 914 and 915 is
not limited to a coil shape and may be a linear shape or a plate
shape. Furthermore, a planar antenna, an aperture antenna, a
traveling-wave antenna, an EH antenna, a magnetic-field antenna, a
dielectric antenna, or the like may be used. Alternatively, the
antenna 914 or the antenna 915 may be a flat-plate conductor. The
flat-plate conductor can serve as one of conductors for electric
field coupling. That is, the antenna 914 or the antenna 915 can
serve as one of two conductors of a capacitor. Thus, electric power
can be transmitted and received not only by an electromagnetic
field or a magnetic field but also by an electric field.
[0375] The line width of the antenna 914 is preferably larger than
that of the antenna 915. This makes it possible to increase the
amount of electric power received by the antenna 914.
[0376] The secondary battery includes a layer 916 between the
secondary battery 913 and the antennas 914 and 915. The layer 916
has a function of blocking an electromagnetic field from the
secondary battery 913, for example. As the layer 916, for example,
a magnetic body can be used.
[0377] Note that the structure of the secondary battery is not
limited to that shown in FIGS. 13A and 13B.
[0378] For example, as shown in FIGS. 14A-1 and 14A-2, two opposite
surfaces of the secondary battery 913 in FIGS. 13A and 13B may be
provided with respective antennas. FIG. 14A-1 is an external view
showing one side of the opposite surfaces, and FIG. 14A-2 is an
external view showing the other side of the opposite surfaces. For
portions similar to those in FIGS. 13A and 13B, a description of
the secondary battery illustrated in FIGS. 13A and 13B can be
referred to as appropriate.
[0379] As illustrated in FIG. 14A-1, the antenna 914 is provided on
one of the opposite surfaces of the secondary battery 913 with the
layer 916 located therebetween, and as illustrated in FIG. 14A-2,
an antenna 918 is provided on the other of the opposite surfaces of
the secondary battery 913 with a layer 917 located therebetween.
The layer 917 has a function of blocking an electromagnetic field
from the secondary battery 913, for example As the layer 917, for
example, a magnetic body can be used.
[0380] With the above structure, both of the antennas 914 and 918
can be increased in size. The antenna 918 has a function of
communicating data with an external device, for example An antenna
with a shape that can be applied to the antenna 914, for example,
can be used as the antenna 918. As a system for communication using
the antenna 918 between the secondary battery and another device, a
response method that can be used between the secondary battery and
another device, such as NFC, can be employed.
[0381] Alternatively, as illustrated in FIG. 14B-1, the secondary
battery 913 in FIGS. 13A and 13B may be provided with a display
device 920. The display device 920 is electrically connected to the
terminal 911. It is possible that the label 910 is not provided in
a portion where the display device 920 is provided. For portions
similar to those in FIGS. 13A and 13B, a description of the
secondary battery illustrated in FIGS. 13A and 13B can be referred
to as appropriate.
[0382] The display device 920 can display, for example, an image
showing whether charge is being carried out, an image showing the
amount of stored power, or the like. As the display device 920,
electronic paper, a liquid crystal display device, an
electroluminescent (EL) display device, or the like can be used.
For example, the use of electronic paper can reduce power
consumption of the display device 920.
[0383] Alternatively, as illustrated in FIG. 14B-2, the secondary
battery 913 illustrated in FIGS. 13A and 13B may be provided with a
sensor 921. The sensor 921 is electrically connected to the
terminal 911 via a terminal 922. For portions similar to those in
FIGS. 13A and 13B, a description of the secondary battery
illustrated in FIGS. 13A and 13B can be referred to as
appropriate.
[0384] The sensor 921 has a function of measuring, for example,
displacement, position, speed, acceleration, angular velocity,
rotational frequency, distance, light, liquid, magnetism,
temperature, chemical substance, sound, time, hardness, electric
field, electric current, voltage, electric power, radiation, flow
rate, humidity, gradient, oscillation, odor, or infrared rays. With
the sensor 921, for example, data on an environment (e.g.,
temperature) where the secondary battery is placed can be
determined and stored in a memory inside the circuit 912.
[0385] Furthermore, structural examples of the secondary battery
913 will be described with reference to FIGS. 15A and 15B and FIG.
16.
[0386] The secondary battery 913 illustrated in FIG. 15A includes a
wound body 950 provided with the terminals 951 and 952 inside a
housing 930. The wound body 950 is soaked in an electrolyte
solution inside the housing 930. The terminal 952 is in contact
with the housing 930. An insulator or the like inhibits contact
between the terminal 951 and the housing 930. Note that in FIG.
15A, the housing 930 divided into two pieces is illustrated for
convenience; however, in the actual structure, the wound body 950
is covered with the housing 930 and the terminals 951 and 952
extend to the outside of the housing 930. For the housing 930, a
metal material (such as aluminum) or a resin material can be
used.
[0387] Note that as illustrated in FIG. 15B, the housing 930 in
FIG. 15A may be formed using a plurality of materials. For example,
in the secondary battery 913 in FIG. 15B, a housing 930a and a
housing 930b are bonded to each other, and the wound body 950 is
provided in a region surrounded by the housing 930a and the housing
930b.
[0388] For the housing 930a, an insulating material such as an
organic resin can be used. In particular, when a material such as
an organic resin is used for the side on which an antenna is
formed, blocking of an electric field from the secondary battery
913 can be inhibited. When an electric field is not significantly
blocked by the housing 930a, an antenna such as the antennas 914
and 915 may be provided inside the housing 930a. For the housing
930b, a metal material can be used, for example
[0389] FIG. 16 illustrates the structure of the wound body 950. The
wound body 950 includes a negative electrode 931, a positive
electrode 932, and separators 933. The wound body 950 is obtained
by winding a sheet of a stack in which the negative electrode 931
overlaps with the positive electrode 932 with the separator 933
provided therebetween. Note that a plurality of stacks each
including the negative electrode 931, the positive electrode 932,
and the separator 933 may be stacked.
[0390] The negative electrode 931 is connected to the terminal 911
in FIGS. 13A and 13B via one of the terminals 951 and 952. The
positive electrode 932 is connected to the terminal 911 in FIGS.
13A and 13B via the other of the terminals 951 and 952.
[0391] When the positive electrode active material described in the
above embodiment is used in the positive electrode 932, the
secondary battery 913 with high capacity and excellent cycle
characteristics can be obtained.
[Laminated Secondary Battery]
[0392] Next, an example of a laminated secondary battery will be
described with reference to FIGS. 17A to 17C, FIG. 18A and 18B,
FIG. 19, FIG. 20, FIGS. 21A to 21C, FIGS. 22A, 22B1, 22B2, 22C, and
22D, and FIGS. 23A and 23B. When the laminated secondary battery
has flexibility and is used in an electronic device at least part
of which is flexible, the secondary battery can be bent as the
electronic device is bent.
[0393] A laminated secondary battery 980 is described with
reference to FIGS. 17A to 17C. The laminated secondary battery 980
includes a wound body 993 illustrated in FIG. 17A. The wound body
993 includes a negative electrode 994, a positive electrode 995,
and a separator 996. The wound body 993 is, like the wound body 950
illustrated in FIG. 16, obtained by winding a sheet of a stack in
which the negative electrode 994 overlaps with the positive
electrode 995 with the separator 996 therebetween.
[0394] Note that the number of stacks each including the negative
electrode 994, the positive electrode 995, and the separator 996
may be determined as appropriate depending on capacity and an
element volume which are required. The negative electrode 994 is
connected to a negative electrode current collector (not
illustrated) via one of a lead electrode 997 and a lead electrode
998. The positive electrode 995 is connected to a positive
electrode current collector (not illustrated) via the other of the
lead electrode 997 and the lead electrode 998.
[0395] As illustrated in FIG. 17B, the wound body 993 is packed in
a space formed by bonding a film 981 and a film 982 having a
depressed portion that serve as exterior bodies by
thermocompression bonding or the like, whereby the laminated
secondary battery 980 can be formed as illustrated in FIG. 17C. The
wound body 993 includes the lead electrode 997 and the lead
electrode 998, and is soaked in an electrolyte solution inside a
space surrounded by the film 981 and the film 982 having a
depressed portion.
[0396] For the film 981 and the film 982 having a depressed
portion, a metal material such as aluminum or a resin material can
be used, for example With the use of a resin material for the film
981 and the film 982 having a depressed portion, the film 981 and
the film 982 having a depressed portion can be changed in their
forms when external force is applied; thus, a flexible secondary
battery can be fabricated.
[0397] Although FIGS. 17B and 17C illustrate an example where a
space is formed by two films, the wound body 993 may be placed in a
space formed by bending one film
[0398] When the positive electrode active material described in the
above embodiment is used in the positive electrode 995, the
laminated secondary battery 980 with high capacity and excellent
cycle characteristics can be obtained.
[0399] In FIGS. 17A to 17C, an example in which the laminated
secondary battery 980 includes a wound body in a space formed by
films serving as exterior bodies is described; however, as
illustrated in FIGS. 18A and 18B, a secondary battery may include a
plurality of strip-shaped positive electrodes, a plurality of
strip-shaped separators, and a plurality of strip-shaped negative
electrodes in a space formed by films serving as exterior bodies,
for example.
[0400] A laminated secondary battery 500 illustrated in FIG. 18A
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 provided between the
positive electrode 503 and the negative electrode 506 in the
exterior body 509. The exterior body 509 is filled with the
electrolyte solution 508. The electrolyte solution described in
Embodiment 2 can be used for the electrolyte solution 508.
[0401] In the laminated secondary battery 500 illustrated in FIG.
18A, the positive electrode current collector 501 and the negative
electrode current collector 504 also serve as terminals for an
electrical contact with an external portion. For this reason, the
positive electrode current collector 501 and the negative electrode
current collector 504 may be arranged so as to be partly exposed to
the outside of the exterior body 509. Alternatively, a lead
electrode and the positive electrode current collector 501 or the
negative electrode current collector 504 may be bonded to each
other by ultrasonic welding, and instead of the positive electrode
current collector 501 and the negative electrode current collector
504, the lead electrode may be exposed to the outside of the
exterior body 509.
[0402] As the exterior body 509 of the laminated secondary battery
500, for example, a laminate film having a three-layer structure
can be employed 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 over the metal thin
film as the outer surface of the exterior body.
[0403] FIG. 18B illustrates an example of a cross-sectional
structure of the laminated secondary battery 500. Although FIG. 18A
illustrates an example including only two current collectors for
simplicity, an actual battery includes a plurality of electrode
layers.
[0404] The example in FIG. 18B includes 16 electrode layers. The
laminated secondary battery 500 has flexibility even though
including 16 electrode layers. FIG. 18B illustrates a structure
including 8 layers of negative electrode current collectors 504 and
8 layers of positive electrode current collectors 501, i.e., 16
layers in total. Note that FIG. 18B illustrates a cross section of
the lead portion of the negative electrode, and the 8 negative
electrode current collectors 504 are bonded to each other by
ultrasonic welding. It is needless to say that the number of
electrode layers is not limited to 16, and may be more than 16 or
less than 16. With a large number of electrode layers, the
secondary battery can have high capacity. In contrast, with a small
number of electrode layers, the secondary battery can have small
thickness and high flexibility.
[0405] FIGS. 19 and 20 each illustrate an example of the external
view of the laminated secondary battery 500. In FIGS. 19 and 20,
the positive electrode 503, the negative electrode 506, the
separator 507, the exterior body 509, a positive electrode lead
electrode 510, and a negative electrode lead electrode 511 are
included.
[0406] FIG. 21A illustrates external views of the positive
electrode 503 and the negative electrode 506. The positive
electrode 503 includes the positive electrode current collector
501, and the positive electrode active material layer 502 is formed
on a surface of the positive electrode current collector 501. The
positive electrode 503 also includes a region where the positive
electrode current collector 501 is partly exposed (hereinafter
referred to as a tab region). The negative electrode 506 includes
the negative electrode current collector 504, and the negative
electrode active material layer 505 is formed on a surface of the
negative electrode current collector 504. The negative electrode
506 also includes a region where the negative electrode current
collector 504 is partly exposed, that is, a tab region. The areas
and the shapes of the tab regions included in the positive
electrode and the negative electrode are not limited to those
illustrated in FIG. 21A.
[Method for Manufacturing Laminated Secondary Battery]
[0407] Here, an example of a method for manufacturing the laminated
secondary battery whose external view is illustrated in FIG. 19
will be described with reference to FIGS. 21B and 21C.
[0408] First, the negative electrode 506, the separator 507, and
the positive electrode 503 are stacked. FIG. 21B illustrates a
stack including the negative electrode 506, the separator 507, and
the positive electrode 503. The secondary battery described here as
an example includes 5 negative electrodes and 4 positive
electrodes. Next, the tab regions of the positive electrodes 503
are bonded to each other, and the tab region of the positive
electrode on the outermost surface and the positive electrode lead
electrode 510 are bonded to each other. The bonding can be
performed by ultrasonic welding, for example. In a similar manner,
the tab regions of the negative electrodes 506 are bonded to each
other, and the negative electrode lead electrode 511 is bonded to
the tab region of the negative electrode on the outermost
surface.
[0409] After that, the negative electrode 506, the separator 507,
and the positive electrode 503 are placed over the exterior body
509.
[0410] Subsequently, the exterior body 509 is folded along a dashed
line as illustrated in FIG. 21C. Then, the outer edge of the
exterior body 509 is bonded. The bonding can be performed by
thermocompression bonding, for example At this time, a part (or one
side) of the exterior body 509 is left unbonded (to provide an
inlet) so that the electrolyte solution 508 can be introduced
later.
[0411] Next, the electrolyte solution 508 is introduced into the
exterior body 509 from the inlet of the exterior body 509. The
electrolyte solution 508 is preferably introduced in a reduced
pressure atmosphere or in an inert gas atmosphere. Lastly, the
inlet is bonded. In the above manner, the laminated secondary
battery 500 can be manufactured.
[0412] When the positive electrode active material described in the
above embodiment is used in the positive electrode 503, the
laminated secondary battery 500 with high capacity and excellent
cycle characteristics can be obtained.
[Bendable Secondary Battery]
[0413] Next, an example of a bendable secondary battery is
described with reference to FIGS. 22A, 22B1, 22B2, 22C and 22D and
FIGS. 23A and 23B.
[0414] FIG. 22A is a schematic top view of a bendable secondary
battery 50. FIGS. 22B1, 22B2, and 22C are schematic cross-sectional
views taken along cutting line C1-C2, cutting line C3-C4, and
cutting line A1-A2, respectively, in FIG. 22A. The battery 50
includes an exterior body 51 and a positive electrode 11a, and a
negative electrode 11b held in the exterior body 51. A lead 12a
electrically connected to the positive electrode 11a and a lead 12b
electrically connected to the negative electrode 11b are extended
to the outside of the exterior body 51. In addition to the positive
electrode 11a and the negative electrode 11b, an electrolyte
solution (not illustrated) is enclosed in a region surrounded by
the exterior body 51.
[0415] FIGS. 23A and 23B illustrate the positive electrode 11a and
the negative electrode 11b included in the battery 50. FIG. 23A is
a perspective view illustrating the stacking order of the positive
electrode 11a, the negative electrode 11b, and the separator 14.
FIG. 23B is a perspective view illustrating the lead 12a and the
lead 12b in addition to the positive electrode 11a and the negative
electrode 11b.
[0416] As illustrated in FIG. 23A, the battery 50 includes a
plurality of strip-shaped positive electrodes 11a, a plurality of
strip-shaped negative electrodes 11b, and a plurality of separators
14. The positive electrode 11a and the negative electrode 11b each
include a projected tab portion and a portion other than the tab. A
positive electrode active material layer is formed on one surface
of the positive electrode 11a other than the tab portion, and a
negative electrode active material layer is formed on one surface
of the negative electrode 11b other than the tab portion.
[0417] The positive electrodes 11a and the negative electrodes 11b
are stacked so that surfaces of the positive electrodes 11a on each
of which the positive electrode active material layer is not formed
are in contact with each other and that surfaces of the negative
electrodes 11b on each of which the negative electrode active
material layer is not formed are in contact with each other.
[0418] Furthermore, the separator 14 is provided between the
surface of the positive electrode 11a on which the positive
electrode active material is formed and the surface of the negative
electrode 11b on which the negative electrode active material is
formed. In FIG. 23A, the separator 14 is shown by a dotted line for
easy viewing.
[0419] In addition, as illustrated in FIG. 23B, the plurality of
positive electrodes 11a are electrically connected to the lead 12a
in a bonding portion 15a. The plurality of negative electrodes 11b
are electrically connected to the lead 12b in a bonding portion
15b.
[0420] Next, the exterior body 51 is described with reference to
FIGS. 22B1, 22B2, 22C, and 22D.
[0421] The exterior body 51 has a film-like shape and is folded in
half with the positive electrodes 11a and the negative electrodes
11b between facing portions of the exterior body 51. The exterior
body 51 includes a folded portion 61, a pair of seal portions 62,
and a seal portion 63. The pair of seal portions 62 is provided
with the positive electrodes 11a and the negative electrodes 11b
positioned therebetween and thus can also be referred to as side
seals. The seal portion 63 has portions overlapping with the lead
12a and the lead 12b and can also be referred to as a top seal.
[0422] Part of the exterior body 51 that overlaps with the positive
electrodes 11a and the negative electrodes 11b preferably has a
wave shape in which crest lines 71 and trough lines 72 are
alternately arranged. The seal portions 62 and the seal portion 63
of the exterior body 51 are preferably flat.
[0423] FIG. 22B1 shows a cross section cut along the part
overlapping with the crest line 71. FIG. 22B2 shows a cross section
cut along the part overlapping with the trough line 72. FIGS. 22B1
and 22B2 correspond to cross sections of the battery 50, the
positive electrodes 11a, and the negative electrodes 11b in the
width direction.
[0424] The distance between an end portion of the negative
electrode 11b in the width direction and the seal portion 62 is
referred to as a distance La. When the battery 50 changes in shape,
for example, is bent, the positive electrode 11a and the negative
electrode 11b change in shape such that the positions thereof are
shifted from each other in the length direction as described later.
At the time, if the distance La is too short, the exterior body 51
and the positive electrode 11a and the negative electrode 11b are
rubbed hard against each other, so that the exterior body 51 is
damaged in some cases. In particular, when a metal film of the
exterior body 51 is exposed, there is concern that the metal film
is corroded by the electrolyte solution. Thus, the distance La is
preferably set as long as possible. However, if the distance La is
too long, the volume of the battery 50 is increased.
[0425] The distance La between the end portion of the negative
electrode 11b and the seal portion 62 is preferably increased as
the total thickness of the stacked positive electrodes 11a and
negative electrodes 11b is increased.
[0426] Specifically, when the total thickness of the stacked
positive electrodes 11a and negative electrodes 11b and the
separators 214 is referred to as a thickness t, the distance La is
preferably 0.8 times or more and 3.0 times or less, further
preferably 0.9 times or more and 2.5 times or less, still further
preferably 1.0 times or more and 2.0 times or less as large as the
thickness t. When the distance La is in the above-described range,
a compact battery which is highly reliable for bending can be
obtained.
[0427] Furthermore, when a distance between the pair of seal
portions 62 is referred to as a distance Lb, it is preferable that
the distance Lb be sufficiently longer than a width Wb of the
negative electrode 11b. In this case, even when the positive
electrode 11a and the negative electrode 11b come into contact with
the exterior body 51 by change in the shape of the battery 50 such
as repeated bending, the position of part of the positive electrode
11a and the negative electrode 11b can be shifted in the width
direction; thus, the positive and negative electrodes 11a and 11b
and the exterior body 51 can be effectively prevented from being
rubbed against each other.
[0428] For example, the difference between the distance Lb (i.e.,
the distance between the pair of seal portions 62) and the width Wb
of the negative electrode 11b is preferably 1.6 times or more and
6.0 times or less, further preferably 1.8 times or more and 5.0
times or less, still further preferably 2.0 times or more and 4.0
times or less as large as the total thickness t of the positive
electrode 11a and the negative electrode 11b.
[0429] In other words, the distance Lb, the width Wb, and the
thickness t preferably satisfy the relation of the following
Formula 2.
[ Formula .times. .times. 2 ] .times. L .times. b - W .times. b 2
.times. t .gtoreq. a ( Formula .times. .times. 2 ) ##EQU00002##
[0430] In the formula, a is 0.8 or more and 3.0 or less, preferably
0.9 or more and 2.5 or less, further preferably 1.0 or more and 2.0
or less.
[0431] FIG. 22C illustrates a cross section including the lead 12a
and corresponds to a cross section of the battery 50, the positive
electrode 11a, and the negative electrode 11b in the length
direction. As illustrated in FIG. 22C, a space 73 is preferably
provided between end portions of the positive electrode 11a and the
negative electrode 11b in the length direction and the exterior
body 51 in the folded portion 61.
[0432] FIG. 22D is a schematic cross-sectional view of the battery
50 in a state of being bent. FIG. 22D corresponds to a cross
section along cutting line B1-B2 in FIG. 22A.
[0433] When the battery 50 is bent, a part of the exterior body 51
positioned on the outer side in bending is unbent and the other
part positioned on the inner side changes its shape as it shrinks.
More specifically, the part of the exterior body 51 positioned on
the outer side in bending changes its shape such that the wave
amplitude becomes smaller and the length of the wave period becomes
larger. In contrast, the part of the exterior body 51 positioned on
the inner side in bending changes its shape such that the wave
amplitude becomes larger and the length of the wave period becomes
smaller. When the exterior body 51 changes its shape in this
manner, stress applied to the exterior body 51 due to bending is
relieved, so that a material itself that forms the exterior body 51
does not need to expand and contract. As a result, the battery 50
can be bent with weak force without damage to the exterior body
51.
[0434] Furthermore, as illustrated in FIG. 22D, when the battery 50
is bent, the positions of the positive electrode 11a and the
negative electrode 11b are shifted relatively. At this time, ends
of the stacked positive electrodes 11a and negative electrodes 11b
on the seal portion 63 side are fixed by the fixing member 17.
Thus, the plurality of positive electrodes 11a and the plurality of
negative electrodes 11b are more shifted at a position closer to
the folded portion 61. Therefore, stress applied to the positive
electrode 11a and the negative electrode 11b is relieved, and the
positive electrode 11a and the negative electrode 11b themselves do
not need to expand and contract. As a result, the battery 50 can be
bent without damage to the positive electrode 11a and the negative
electrode 11b.
[0435] Furthermore, the space 73 is provided between the end
portions of the positive and negative electrodes 11a and 11b and
the exterior body 51, whereby the relative positions of the
positive electrode 11a and the negative electrode 11b can be
shifted while the end portions of the positive electrode 11a and
the negative electrode 11b located on an inner side when the
battery 50 is bent do not contact the exterior body 51.
[0436] In the battery 50 illustrated in FIGS. 22A, 22B1, 22B2, 22C
and 22D and FIGS. 23A and 23B, the exterior body, the positive
electrode 11a, and the negative electrode 11b are less likely to be
damaged and the battery characteristics are less likely to
deteriorate even when the battery 50 is repeatedly bent and unbent.
When the positive electrode active material described in the above
embodiment is used for the positive electrode 11a included in the
battery 50, a battery with more excellent cycle characteristics can
be obtained.
Embodiment 4
[0437] In this embodiment, examples of electronic devices including
the secondary battery of one embodiment of the present invention
are described.
[0438] First, FIGS. 24A to 24G show examples of electronic devices
including the bendable secondary battery described in Embodiment 3.
Examples of an electronic device including a flexible secondary
battery include television sets (also referred to as televisions or
television receivers), monitors of computers or the like, digital
cameras or digital video cameras, digital photo frames, mobile
phones (also referred to as cellular phones or mobile phone
devices), portable game machines, portable information terminals,
audio reproducing devices, and large game machines such as pachinko
machines.
[0439] In addition, a flexible secondary battery can be
incorporated along a curved inside/outside wall surface of a house
or a building or a curved interior/exterior surface of an
automobile.
[0440] FIG. 24A illustrates an example of a mobile phone. A mobile
phone 7400 is provided with a display portion 7402 incorporated in
a housing 7401, an operation button 7403, an external connection
port 7404, a speaker 7405, a microphone 7406, and the like. Note
that the mobile phone 7400 includes a secondary battery 7407. When
the secondary battery of one embodiment of the present invention is
used as the secondary battery 7407, a lightweight mobile phone with
a long lifetime can be provided.
[0441] FIG. 24B illustrates the mobile phone 7400 that is bent.
When the whole mobile phone 7400 is curved by external force, the
secondary battery 7407 included in the mobile phone 7400 is also
curved. FIG. 24C illustrates the curved secondary battery 7407. The
secondary battery 7407 is a thin storage battery. The secondary
battery 7407 is curved and fixed. Note that the secondary battery
7407 includes a lead electrode 7408 electrically connected to a
current collector 7409. The current collector 7409 is, for example,
copper foil, and partly alloyed with gallium; thus, adhesion
between the current collector 7409 and an active material layer in
contact with the current collector 7409 is improved and the
secondary battery 7407 can have high reliability even in a state of
being bent.
[0442] FIG. 24D illustrates an example of a bangle display device.
A portable display device 7100 includes a housing 7101, a display
portion 7102, an operation button 7103, and a secondary battery
7104. FIG. 24E illustrates the bent secondary battery 7104. When
the curved secondary battery 7104 is on a user's arm, the housing
changes its form and the curvature of a part or the whole of the
secondary battery 7104 is changed. Note that the radius of
curvature of a curve at a point refers to the radius of the
circular arc that best approximates the curve at that point. The
reciprocal of the radius of curvature is curvature. Specifically,
part or the whole of the housing or the main surface of the
secondary battery 7104 is changed in the range of radius of
curvature from 40 mm to 150 mm. When the radius of curvature at the
main surface of the secondary battery 7104 is greater than or equal
to 40 mm and less than or equal to 150 mm, the reliability can be
kept high. When the secondary battery of one embodiment of the
present invention is used as the secondary battery 7104, a
lightweight portable display device with a long lifetime can be
provided.
[0443] FIG. 24F illustrates an example of a watch-type portable
information terminal. A portable information terminal 7200 includes
a housing 7201, a display portion 7202, a band 7203, a buckle 7204,
an operation button 7205, an input output terminal 7206, and the
like.
[0444] The portable information terminal 7200 is capable of
executing a variety of applications such as mobile phone calls,
e-mailing, viewing and editing texts, music reproduction, Internet
communication, and a computer game.
[0445] The display surface of the display portion 7202 is curved,
and images can be displayed on the curved display surface. In
addition, the display portion 7202 includes a touch sensor, and
operation can be performed by touching the screen with a finger, a
stylus, or the like. For example, by touching an icon 7207
displayed on the display portion 7202, application can be
started.
[0446] With the operation button 7205, a variety of functions such
as time setting, power on/off, on/off of wireless communication,
setting and cancellation of a silent mode, and setting and
cancellation of a power saving mode can be performed. For example,
the functions of the operation button 7205 can be set freely by
setting the operation system incorporated in the portable
information terminal 7200.
[0447] The portable information terminal 7200 can employ near field
communication that is a communication method based on an existing
communication standard. For example, mutual communication between
the portable information terminal 7200 and a headset capable of
wireless communication can be performed, and thus hands-free
calling is possible.
[0448] Moreover, the portable information terminal 7200 includes
the input output terminal 7206, and data can be directly
transmitted to and received from another information terminal via a
connector. In addition, charging via the input output terminal 7206
is possible. Note that the charging operation may be performed by
wireless power feeding without using the input output terminal
7206.
[0449] The display portion 7202 of the portable information
terminal 7200 includes the secondary battery of one embodiment of
the present invention. When the secondary battery of one embodiment
of the present invention is used, a lightweight portable
information terminal with a long lifetime can be provided. For
example, the secondary battery 7104 illustrated in FIG. 24E that is
in the state of being curved can be provided in the housing 7201.
Alternatively, the secondary battery 7104 illustrated in FIG. 24E
can be provided in the band 7203 such that it can be curved.
[0450] A portable information terminal 7200 preferably includes a
sensor. As the sensor, for example a human body sensor such as a
fingerprint sensor, a pulse sensor, or a temperature sensor, a
touch sensor, a pressure sensitive sensor, an acceleration sensor,
or the like is preferably mounted.
[0451] FIG. 24G illustrates an example of an armband display
device. A display device 7300 includes a display portion 7304 and
the secondary battery of one embodiment of the present invention.
The display device 7300 can include a touch sensor in the display
portion 7304 and can serve as a portable information terminal.
[0452] The display surface of the display portion 7304 is bent, and
images can be displayed on the bent display surface. A display
state of the display device 7300 can be changed by, for example,
near field communication, which is a communication method based on
an existing communication standard.
[0453] The display device 7300 includes an input output terminal,
and data can be directly transmitted to and received from another
information terminal via a connector. In addition, charging via the
input output terminal is possible. Note that the charging operation
may be performed by wireless power feeding without using the input
output terminal.
[0454] When the secondary battery of one embodiment of the present
invention is used as the secondary battery included in the display
device 7300, a lightweight display device with a long lifetime can
be provided.
[0455] In addition, FIG. 24H, FIGS. 25A to 25C, and FIG. 26 show
examples of electronic devices including the secondary battery with
excellent cycle characteristics described in the above
embodiment.
[0456] When the secondary battery of one embodiment of the present
invention is used as a secondary battery of a daily electronic
device, a lightweight product with a long lifetime can be provided.
As the daily electronic devices, an electric toothbrush, an
electric shaver, electric beauty equipment, and the like are given.
As secondary batteries of these products, in consideration of
handling ease for users, small and lightweight stick type secondary
batteries with high capacity are desired.
[0457] FIG. 24H is a perspective view of a device which is called a
vaporizer. In FIG. 24H, a vaporizer 7500 includes an atomizer 7501
including a heating element, a secondary battery 7504 supplying
power to the atomizer, and a cartridge 7502 including a liquid
supply bottle, a sensor, and the like. To improve safety, a
protection circuit which prevents overcharge and overdischarge of
the secondary battery 7504 may be electrically connected to the
secondary battery 7504. The secondary battery 7504 in FIG. 24H
includes an output terminal for connecting to a charger. When the
vaporizer 7500 is held by a user, the secondary battery 7504
becomes a tip portion; thus, it is preferable that the secondary
battery 7504 have a short total length and be lightweight. With the
secondary battery of one embodiment of the present invention which
has high capacity and excellent cycle characteristics, the small
and lightweight vaporizer 7500 which can be used for a long time
for a long period can be provided.
[0458] Next, FIGS. 25A and 25B illustrate an example of a foldable
tablet terminal A tablet terminal 9600 illustrated in FIGS. 25A and
25B includes a housing 9630a, a housing 9630b, a movable portion
9640 connecting the housings 9630a and 9630b, a display portion
9631 including a display portion 9631a and a display portion 9631b,
a display mode changing switch 9626, a power switch 9627, a power
saving mode changing switch 9625, a fastener 9629, and an operation
switch 9628. A flexible panel is used for the display portion 9631,
whereby a tablet terminal with a larger display portion can be
provided. FIG. 25A illustrates the tablet terminal 9600 that is
opened, and FIG. 25B illustrates the tablet terminal 9600 that is
closed.
[0459] The tablet terminal 9600 includes a power storage unit 9635
inside the housings 9630a and 9630b. The power storage unit 9635 is
provided across the housings 9630a and 9630b, passing through the
movable portion 9640.
[0460] Part of the display portion 9631a can be a touch panel
region and data can be input when a displayed operation key is
touched. Although a structure in which a half region in the display
portion 9631a has only a display function and the other half region
has a touch panel function is shown as an example, the display
portion 9631a is not limited to the structure. The whole region in
the display portion 9631a may have a touch panel function. For
example, the display portion 9631a can display keyboard buttons in
the whole region to be a touch panel, and the display portion 9631b
can be used as a display screen.
[0461] Like the display portion 9631a, part of the display portion
9631b can be a touch panel region. A switching button 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 9631b.
[0462] Touch input can be performed in the touch panel region and
the touch panel region at the same time.
[0463] The display mode switch 9626 can switch the display between
a portrait mode and a landscape mode, and between monochrome
display and color display, for example The power saving mode
changing switch 9625 can control display luminance in accordance
with the amount of external light in use of the tablet terminal
9600, which is measured with an optical sensor incorporated in the
tablet terminal 9600. Another detection device including a sensor
for detecting inclination, such as a gyroscope sensor or an
acceleration sensor, may be incorporated in the tablet terminal, in
addition to the optical sensor.
[0464] Although the display portion 9631a and the display portion
9631b have the same area in FIG. 25A, one embodiment of the present
invention is not limited to this example. The display portion 9631a
and the display portion 9631b may have different areas or different
display quality. For example, one display panel may be capable of
higher-definition display than the other display panel.
[0465] The tablet terminal is closed in FIG. 25B. The tablet
terminal includes the housing 9630, a solar cell 9633, and a charge
and discharge control circuit 9634 including a DCDC converter 9636.
The power storage unit of one embodiment of the present invention
is used as the power storage unit 9635.
[0466] The tablet terminal 9600 can be folded such that the
housings 9630a and 9630b overlap with each other when not in use.
Thus, the display portions 9631a and 9631b can be protected, which
increases the durability of the tablet terminal 9600. With the
power storage unit 9635 including the secondary battery of one
embodiment of the present invention which has high capacity and
excellent cycle characteristics, the tablet terminal 9600 which can
be used for a long time for a long period can be provided.
[0467] The tablet terminal illustrated in FIGS. 25A and 25B 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, or the time 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.
[0468] The solar cell 9633, which is attached on the surface of the
tablet terminal, supplies electric power to a touch panel, a
display portion, an image signal processor, and the like. Note that
the solar cell 9633 can be provided on one or both surfaces of the
housing 9630 and the power storage unit 9635 can be charged
efficiently. The use of a lithium-ion battery as the power storage
unit 9635 brings an advantage such as reduction in size.
[0469] The structure and operation of the charge and discharge
control circuit 9634 illustrated in FIG. 25B will be described with
reference to a block diagram in FIG. 25C. The solar cell 9633, the
power storage unit 9635, the DC-DC converter 9636, a converter
9637, switches SW1 to SW3, and the display portion 9631 are
illustrated in FIG. 25C, and the power storage unit 9635, the DC-DC
converter 9636, the converter 9637, and the switches SW1 to SW3
correspond to the charge and discharge control circuit 9634 in FIG.
25B.
[0470] First, an example of the operation in the case where power
is generated by the solar cell 9633 using external light is
described. The voltage of electric power generated by the solar
cell is raised or lowered by the DCDC converter 9636 to a voltage
for charging the power storage unit 9635. When the power from the
solar cell 9633 is used for the operation of the display portion
9631, the switch SW1 is turned on and the voltage of the power is
raised or lowered by the converter 9637 to a voltage needed for
operating the display portion 9631. When display on the display
portion 9631 is not performed, the switch SW1 is turned off and the
switch SW2 is turned on, so that the power storage unit 9635 can be
charged.
[0471] Note that the solar cell 9633 is described as an example of
a power generation means; however, one embodiment of the present
invention is not limited to this example The power storage unit
9635 may be charged using another power generation means such as a
piezoelectric element or a thermoelectric conversion element
(Peltier element). For example, the power storage unit 9635 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.
[0472] FIG. 26 illustrates other examples of electronic devices. In
FIG. 26, a display device 8000 is an example of an electronic
device including a secondary battery 8004 of one embodiment of the
present invention. Specifically, the display device 8000
corresponds to a display device for TV broadcast reception and
includes a housing 8001, a display portion 8002, speaker portions
8003, the secondary battery 8004, and the like. The secondary
battery 8004 of one embodiment of the present invention is provided
in the housing 8001. The display device 8000 can receive electric
power from a commercial power supply. Alternatively, the display
device 8000 can use electric power stored in the secondary battery
8004. Thus, the display device 8000 can operate with the use of the
secondary battery 8004 of one embodiment of the present invention
as an uninterruptible power supply even when electric power cannot
be supplied from a commercial power supply due to power failure or
the like.
[0473] 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 8002.
[0474] Note that the display device includes, in its category, all
of information display devices for personal computers,
advertisement displays, and the like other than TV broadcast
reception.
[0475] In FIG. 26, an installation lighting device 8100 is an
example of an electronic device using a secondary battery 8103 of
one embodiment of the present invention. Specifically, the lighting
device 8100 includes a housing 8101, a light source 8102, the
secondary battery 8103, and the like. Although FIG. 26 illustrates
the case where the secondary battery 8103 is provided in a ceiling
8104 on which the housing 8101 and the light source 8102 are
installed, the secondary battery 8103 may be provided in the
housing 8101. The lighting device 8100 can receive electric power
from a commercial power supply. Alternatively, the lighting device
8100 can use electric power stored in the secondary battery 8103.
Thus, the lighting device 8100 can operate with the use of the
secondary battery 8103 of one embodiment of the present invention
as an uninterruptible power supply even when electric power cannot
be supplied from a commercial power supply due to power failure or
the like.
[0476] Note that although the installation lighting device 8100
provided in the ceiling 8104 is illustrated in FIG. 26 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 8105, a floor 8106, a window 8107, or the
like other than the ceiling 8104. Alternatively, the secondary
battery can be used in a tabletop lighting device or the like.
[0477] As the light source 8102, 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 or an organic EL
element are given as examples of the artificial light source.
[0478] In FIG. 26, an air conditioner including an indoor unit 8200
and an outdoor unit 8204 is an example of an electronic device
including a secondary battery 8203 of one embodiment of the present
invention. Specifically, the indoor unit 8200 includes a housing
8201, an air outlet 8202, the secondary battery 8203, and the like.
Although FIG. 26 illustrates the case where the secondary battery
8203 is provided in the indoor unit 8200, the secondary battery
8203 may be provided in the outdoor unit 8204. Alternatively, the
secondary batteries 8203 may be provided in both the indoor unit
8200 and the outdoor unit 8204. The air conditioner can receive
electric power from a commercial power supply. Alternatively, the
air conditioner can use electric power stored in the secondary
battery 8203. Particularly in the case where the secondary
batteries 8203 are provided in both the indoor unit 8200 and the
outdoor unit 8204, the air conditioner can operate with the use of
the secondary battery 8203 of one embodiment of the present
invention as an uninterruptible power supply even when electric
power cannot be supplied from a commercial power supply due to
power failure or the like.
[0479] Note that although the split-type air conditioner including
the indoor unit and the outdoor unit is illustrated in FIG. 26 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.
[0480] In FIG. 26, an electric refrigerator-freezer 8300 is an
example of an electronic device using a secondary battery 8304 of
one embodiment of the present invention. Specifically, the electric
refrigerator-freezer 8300 includes a housing 8301, a refrigerator
door 8302, a freezer door 8303, the secondary battery 8304, and the
like. The secondary battery 8304 is provided in the housing 8301 in
FIG. 26. The electric refrigerator-freezer 8300 can receive
electric power from a commercial power supply. Alternatively, the
electric refrigerator-freezer 8300 can use electric power stored in
the secondary battery 8304. Thus, the electric refrigerator-freezer
8300 can operate with the use of the secondary battery 8304 of one
embodiment of the present invention as an uninterruptible power
supply even when electric power cannot be supplied from a
commercial power supply due to power failure or the like.
[0481] Note that among the electronic devices described above, a
high-frequency heating apparatus 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.
[0482] 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 8300, power can be
stored in the secondary battery 8304 in night time when the
temperature is low and the refrigerator door 8302 and the freezer
door 8303 are not often opened and closed. On the other hand, in
daytime when the temperature is high and the refrigerator door 8302
and the freezer door 8303 are frequently opened and closed, the
secondary battery 8304 is used as an auxiliary power source; thus,
the usage rate of power in daytime can be reduced.
[0483] According to one embodiment of the present invention, the
secondary battery can have excellent cycle characteristics and
improve reliability. Furthermore, in accordance with one embodiment
of the present invention, a secondary battery with high capacity
can be obtained; thus, the secondary battery itself can be made
more compact and lightweight as a result of improved
characteristics of the secondary battery. Thus, the secondary
battery of one embodiment of the present invention is used in the
electronic device described in this embodiment, whereby a more
lightweight electronic device with a longer lifetime can be
obtained. This embodiment can be implemented in appropriate
combination with any of the other embodiments.
Embodiment 5
[0484] In this embodiment, examples of vehicles including the
secondary battery of one embodiment of the present invention are
described.
[0485] The use of secondary batteries in vehicles enables
production of next-generation clean energy vehicles such as hybrid
electric vehicles (HEVs), electric vehicles (EVs), and plug-in
hybrid electric vehicles (PHEVs).
[0486] FIGS. 27A to 27C each illustrate an example of a vehicle
using the secondary battery of one embodiment of the present
invention. An automobile 8400 illustrated in FIG. 27A is an
electric vehicle that runs on the power of an electric motor.
Alternatively, the automobile 8400 is a hybrid electric vehicle
capable of driving appropriately using either an electric motor or
an engine. One embodiment of the present invention can provide a
high-mileage vehicle. The automobile 8400 includes the secondary
battery. As the secondary battery, the modules of the secondary
batteries illustrated in FIGS. 12C and 12D may be arranged to be
used in a floor portion in the automobile. Alternatively, a battery
pack in which a plurality of secondary batteries each of which is
illustrated in FIGS. 17A to 17C are combined may be placed in a
floor portion in the automobile. The secondary battery is used not
only for driving an electric motor 8406, but also for supplying
electric power to a light-emitting device such as a headlight 8401
or a room light (not illustrated).
[0487] The secondary battery can also supply electric power to a
display device of a speedometer, a tachometer, or the like included
in the automobile 8400. Furthermore, the secondary battery can
supply electric power to a semiconductor device included in the
automobile 8400, such as a navigation system.
[0488] FIG. 27B illustrates an automobile 8500 including the
secondary battery. The automobile 8500 can be charged when the
secondary battery is supplied with electric power through external
charging equipment by a plug-in system, a contactless power feeding
system, or the like. In FIG. 27B, secondary batteries 8024 and 8025
included in the automobile 8500 is charged with the use of a
ground-based charging apparatus 8021 through a cable 8022. In
charging, a given method such as CHAdeMO (registered trademark) or
Combined Charging System may be employed as a charging method, the
standard of a connector, or the like as appropriate. The
ground-based charging apparatus 8021 may be a charging station
provided in a commerce facility or a power source in a house. With
the use of a plug-in technique, the secondary battery 8024 included
in the automobile 8500 can be charged by being supplied with
electric power from the outside, for example The charging can be
performed by converting AC electric power into DC electric power
through a converter such as an AC-DC converter.
[0489] Furthermore, although not illustrated, the vehicle may
include a power receiving device so that it can be charged by being
supplied with electric power from an above-ground power
transmitting device in a contactless manner. In the case of the
contactless power feeding system, by fitting a power transmitting
device in a road or an exterior wall, charging can be performed not
only when the electric vehicle is stopped but also when driven. In
addition, the contactless power feeding system may be utilized to
perform transmission and reception of electric power between
vehicles. A solar cell may be provided in the exterior of the
automobile to charge the secondary battery when the automobile
stops or moves. To supply electric power in such a contactless
manner, an electromagnetic induction method or a magnetic resonance
method can be used.
[0490] FIG. 27C shows an example of a motorcycle using the
secondary battery of one embodiment of the present invention. A
motor scooter 8600 illustrated in FIG. 27C includes a secondary
battery 8602, side mirrors 8601, and indicators 8603. The secondary
battery 8602 can supply electric power to the indicators 8603.
[0491] Furthermore, in the motor scooter 8600 illustrated in FIG.
27C, the secondary battery 8602 can be held in a storage unit under
seat 8604. The secondary battery 8602 can be held in the storage
unit under seat 8604 even with a small size. The secondary battery
8602 is detachable; thus, the secondary battery 8602 is carried
indoors when it is charged, and is stored before the motorcycle is
driven.
[0492] In accordance with one embodiment of the present invention,
the secondary battery can have improved cycle characteristics and
the capacity of the secondary battery can be increased. Thus, the
secondary battery itself can be made more compact and lightweight.
The compact and lightweight secondary battery contributes to a
reduction in the weight of a vehicle, and thus increases the
driving radius. Furthermore, the secondary battery included in the
vehicle can be used as a power source for supplying electric power
to products other than the vehicle. In such a case, the use of a
commercial power source can be avoided at peak time of electric
power demand, for example. If the use of a commercial power source
can be avoided at peak time of electric power demand, the avoidance
can contribute to energy saving and a reduction in carbon dioxide
emissions. Moreover, if the cycle characteristics are excellent,
the secondary battery can be used for a long period; thus, the use
amount of rare metals such as cobalt can be reduced.
[0493] This embodiment can be implemented in appropriate
combination with the other embodiments.
EXAMPLE 1
[0494] In this example, the positive electrode active materials
which are embodiments of the present invention are formed, and the
observation results of the positive electrode active materials by
STEM, the results of TEM images subjected to fast Fourier
transform, and the analysis results obtained by energy dispersive
X-ray spectroscopy (EDX) are described. In addition, the evaluation
results of characteristics of secondary batteries containing the
positive electrode active materials are described.
[Formation of Positive Electrode Active Material]
<<Sample 01>>
[0495] In this example, a positive electrode active material of
Sample 01, which contains lithium cobaltate as a composite oxide of
lithium and a first transition metal contained in a first region,
lithium titanate as an oxide of a second transition metal contained
in a second region, and magnesium oxide as an oxide of a
representative element contained in a third region, was formed.
[0496] In this example, lithium cobalt oxide particles (C-20F,
produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.) were used as a
starting material. Thus, in this example, Step 12 and Step 13
described in Embodiment 1 were omitted. Note that the
above-described lithium cobalt oxide particles each have a particle
diameter of approximately 20 .mu.m, and contain fluorine,
magnesium, calcium, sodium, silicon, sulfur, and phosphorus in a
region which can be analyzed by XPS.
[0497] Next, as Step 14, the lithium cobalt oxide particles
containing magnesium and fluorine were coated with a material
containing titanium by a sol-gel method. Specifically, TTIP was
dissolved in isopropanol, and an isopropanol solution of TTIP was
formed. Then, the lithium cobalt oxide particles were mixed into
the solution so that TTIP to lithium cobalt oxide containing
magnesium and fluorine was 0.01 ml/g.
[0498] The above mixed solution was stirred with a magnetic stirrer
for four hours, at 25.degree. C., and at a humidity of 90% RH.
Through the process, water in an atmosphere and TTIP caused
hydrolysis and polycondensation reaction, and a layer containing
titanium was formed on the surface of the lithium cobalt oxide
particle containing magnesium and fluorine.
[0499] The mixed solution which had been subjected to the above
process was filtered to collect the residue. As a filter for
filtration, Kiriyama filter paper (No. 4) was used.
[0500] The collected residue was dried in a vacuum at 70.degree. C.
for one hour.
[0501] Next, the lithium cobalt oxide particles coated with the
material containing titanium was heated. With a muffle furnace, the
heating was performed under the following conditions: the flow rate
of dry air was 10 L/min; the temperature was 800.degree. C. (the
temperature rising rate was 200.degree. C./h); and the retention
time was two hours. The dew point of the dry air is preferably
lower than or equal to -109.degree. C.
[0502] Then, the heated particles were cooled to room temperature.
The time of decreasing temperature from the retention temperature
to room temperature was 10 hours to 15 hours. After that, crushing
treatment was performed. In the crushing treatment, the particles
were made to pass through a sieve. The sieve has an aperture width
of 53 .mu.m.
[0503] Lastly, the cooled particles were collected, and the
positive electrode active material of Sample 01 was obtained.
<<Sample 02>>
[0504] Sample 02 was formed as a comparative example by heating
lithium cobalt oxide particles containing magnesium and fluorine
without being coated with a material containing titanium.
[0505] Lithium cobalt oxide particles produced by NIPPON CHEMICAL
INDUSTRIAL CO., LTD. (product name: C-20F) were used as the lithium
cobalt oxide particles containing magnesium and fluorine.
[0506] The lithium cobalt oxide particles containing magnesium and
fluorine were heated. The heating was performed under the following
conditions: the temperature was 800.degree. C. (the temperature
rising rate was 200.degree. C./h); the retention time was two
hours; and the flow rate of oxygen was 10 L/min.
[0507] The heated particles were cooled and made to pass through a
sieve like Sample 01 to obtain a positive electrode active material
of Sample 02.
[0508] It is probable that Sample 02 is a positive electrode active
material which contains lithium cobalt oxide inside and includes a
region containing magnesium in a superficial portion.
<<Sample 03>>
[0509] Sample 03 was formed as a comparative example in the
following manner: a region containing titanium was formed in
lithium cobalt oxide particles which do not contain magnesium by a
sol-gel method and then the lithium cobalt oxide particles were
heated.
[0510] Lithium cobalt oxide particles produced by NIPPON CHEMICAL
INDUSTRIAL CO., LTD. (product name: C-10N) were used. In the
lithium cobalt oxide particles, magnesium is not detected and
fluorine is detected at approximately 1 atomic % by XPS.
[0511] A region containing titanium was formed by a sol-gel method
in the lithium cobalt oxide particles, and the lithium cobalt oxide
particles were dried, heated, cooled, and made to pass through a
sieve, like Sample 01. The obtained lithium cobalt oxide particles
were used as a positive electrode active material of Sample 03.
[0512] It is probable that Sample 03 is a positive electrode active
material which contains lithium cobalt oxide inside and includes a
region containing titanium in a superficial portion.
<<Sample 04>>
[0513] For Sample 04, as a comparative example, lithium cobalt
oxide particles were used as it is without being heated.
[0514] Lithium cobalt oxide particles produced by NIPPON CHEMICAL
INDUSTRIAL CO., LTD. (product name: C-10N) were used.
[0515] Sample 04 is a positive electrode active material which does
not have a coating layer.
<<Sample 05>>
[0516] For Sample 05, as a comparative example, lithium cobalt
oxide particles containing magnesium and fluorine were used as it
is without being heated.
[0517] Lithium cobalt oxide particles produced by NIPPON CHEMICAL
INDUSTRIAL CO., LTD. (product name: C-20F) were used as the lithium
cobalt oxide particles containing magnesium and fluorine. That is,
Sample 05 was used as the same as the starting material of Sample
01.
[0518] Table 1 shows the conditions of Sample 01 to Sample 05.
TABLE-US-00001 TABLE 1 Conditions Sample 01 LiCoO.sub.2 + Mg + F,
coated with a material containing Ti, heated Sample 02 LiCoO.sub.2
+ Mg + F, heated Sample 03 LiCoO.sub.2, coated with a material
containing Ti, heated Sample 04 LiCoO.sub.2, not heated Sample 05
LiCoO.sub.2 + Mg + F, not heated
[STEM]
[0519] The obtained positive electrode active material of Sample 01
was observed by an electron microscope (JEM-ARM200F, manufactured
by JEOL Ltd.) under the condition where the acceleration voltage
was 200 kV. FIG. 28 shows the obtained electron microscope image.
As shown in FIG. 28, the positive electrode active material
probably includes three different regions: the first region 101;
the second region 102; and the third region 103. The third region
103 is observed as a region brighter than the first region 101 and
the second region 102. Furthermore, crystal orientations of the
first region 101 and the second region 102 are partly aligned with
each other, and crystal orientations of the second region 102 and
the third region 103 are partly aligned with each other.
[STEM-FFT]
[0520] FIG. 29A1 shows a fast Fourier transform (FFT) image of a
region 103FFT in the STEM image of FIG. 28. In FIG. 29A2, a center
point O of FIG. 29A1 is shown by a cross, and bright points A, B,
and C are each surrounded by a circle. Similarly, FIG. 29B1 shows
an FFT image of a region 102FFT. In FIG. 29B2, a center point O of
FIG. 29B1 is shown by a cross, and bright points A, B, and C are
each surrounded by a circle. In addition, FIG. 29C1 shows an FFT
image of a region 101FFT. In FIG. 29C2, a center point O of FIG.
29C1 is shown by a cross, and bright points A, B, and C are each
surrounded by a circle.
[0521] In FIG. 29A2, a distance d between the bright point A and
the center point O is 0.256 nm, a distance d between the bright
point B and the center point O is 0.241 nm, and a distance d
between the bright point C and the center point O is 0.209 nm. In
addition, .angle.COA is 121.degree., .angle.COB is 52.degree., and
.angle.AOB is 69.degree.. From these results, the region 103FFT
probably contains magnesium oxide (MgO, cubic crystal).
[0522] Similarly, in FIG. 29B2, the distance d between the bright
point A and the center point O is 0.238 nm, the distance d between
the bright point B and the center point O is 0.225 nm, and the
distance d between the bright point C and the center point O is
0.198 nm. In addition, .angle.COA is 123.degree., .angle.COB is
52.degree., and .angle.AOB is 71.degree.. From these results, the
region 102FFT probably contains lithium titanate (LiTiO.sub.2,
cubic crystal).
[0523] In FIG. 29C2, the distance d between the bright point A and
the center point O is 0.240 nm, the distance d between the bright
point B and the center point O is 0.235 nm, and the distance d
between the bright point C and the center point O is 0.196 nm. In
addition, .angle.COA is 126.degree., .angle.COB is 52.degree., and
.angle.AOB is 74.degree.. From these results, the region 101FFT
probably contains lithium cobaltate (LiCoO.sub.2,
rhombohedral).
[EDX]
[0524] FIGS. 30A1, 30A2, 30B1, 30B2, 30C1, and 30C2 show a
high-angle annular dark field scanning transmission electron
microscopy (HAADF-STEM) image and element mapping images with EDX
of the positive electrode active material of Sample 01. FIG. 30A1
shows a HAADF-STEM image, FIG. 30A2 shows a mapping image of oxygen
atoms, FIG. 30B1 shows a mapping image of cobalt atoms, FIG. 30B2
shows a mapping image of fluorine atoms, FIG. 30C1 shows a mapping
image of titanium atoms, and FIG. 30C2 shows a mapping image of
magnesium atoms. Note that in EDX element mapping images in FIGS.
30A2, 30B1, 30B2, 30C1, and 30C2 and FIGS. 31A2, 31B1, 31B2, 31C1,
and 31C2, a region where the number of elements is less than or
equal to a lower limit of the detection is indicated in white, and
as the number of elements is increased, the white region becomes
black.
[0525] As shown in FIGS. 30A2 and 30B1, it is found that the oxygen
atoms and the cobalt atoms are distributed in the whole of the
positive electrode active material particle. In contrast, as shown
in FIGS. 30B2, 30C1, and 30C2, it is found that the fluorine atoms,
the titanium atoms, and the magnesium atoms are unevenly
distributed in a region close to the surface of the positive
electrode active material.
[0526] Next, FIGS. 31A1, 31A2, 31B1, 31B2, 31C1, and 31C2 show a
HAADF-STEM image and element mapping images with EDX of the
positive electrode active material of Sample 05, which is a
comparative example FIG. 31A1 shows a HAADF-STEM image, FIG. 31A2
shows a mapping image of oxygen atoms, FIG. 31B1 shows a mapping
image of cobalt atoms, FIG. 31B2 shows a mapping image of fluorine
atoms, FIG. 31C1 shows a mapping image of titanium atoms, and FIG.
31C2 shows a mapping image of magnesium atoms.
[0527] As shown in FIGS. 31B2 and 31C2, it is found that, even in
Sample 05 which is not heated, a certain amount of magnesium and
fluorine is unevenly distributed in the vicinity of the
surface.
[EDX Line Analysis]
[0528] FIG. 32 shows results of line analysis with TEM-EDX
performed on a cross section of the vicinity of the surface of the
positive electrode active material of Sample 01. FIG. 32 is a graph
showing data detected on a line connecting the outside of the
positive electrode active material of Sample 01 to the inside of
the positive electrode active material, and a distance of 0 nm
indicates the outside of the positive electrode active material and
a distance of 14 nm indicates the inside of the particle. With EDX,
the analysis region tends to be large, so that elements not only at
a center of an electron beam irradiation region but also in a
region around the center may be detected.
[0529] As shown in FIG. 32, it is found that there are peaks of
magnesium and titanium in the vicinity of the surface of the
positive electrode active material of Sample 01, the distribution
of magnesium is closer to the surface than the distribution of
titanium is. It is also found that the peak of magnesium is closer
to the surface than the peak of titanium is. In addition, it is
probable that cobalt and oxygen are present from the outermost
surface of the positive electrode active material particle.
[0530] As shown in FIG. 32, fluorine is hardly detected. This is
probably because fluorine, which is a light element, is difficult
to detect with EDX.
[0531] From the above STEM images, FFT images, element mapping
images with EDX, and EDX line analysis, it is found that Sample 01
is a positive electrode active material of one embodiment of the
present invention, which includes the first region containing
lithium cobaltate, the second region containing lithium, titanium,
cobalt, and oxygen, and the third region containing magnesium and
oxygen. It is found that, in Sample 01, part of the second region
and part of the third region overlap.
[0532] In the graph of FIG. 32, the amount of detected oxygen is
stable at a distance of 4 nm or more. Thus, in this example, the
average value O.sub.ave of the amount of detected oxygen in the
stable region is obtained, and a distance x of the measurement
point at which the measurement value closest to 0.5 O.sub.ave, the
value of 50% of the average value O.sub.ave, is obtained is assumed
to be the surface of the positive electrode active material
particle.
[0533] In this example, the average value O.sub.ave of the amount
of detected oxygen in a range from a distance of 4 nm to a distance
of 14 nm is 674.2. The x axis of the measurement point at which the
measurement value closest to 337.1, which is 50% of 674.2, is
obtained indicates a distance of 1.71 nm. Thus, in this example, a
distance of 1.71 nm in the graph of FIG. 32 is assumed to be the
surface of the positive electrode active material particle.
[0534] When the surface of the positive electrode active material
particle is set at a distance of 1.71 nm in FIG. 32, the peak of
magnesium and the peak of titanium are present at 0.72 nm and 1.00
nm, respectively, from the surface of the positive electrode active
material particle.
[0535] The concentration of magnesium is higher than or equal to
1/5 of the peak from the surface of the positive electrode active
material particle to a distance of 4.42 nm, that is, to a region at
2.71 nm from the surface. The measurement value of magnesium is
less than 1/5 of the peak at a distance of 4.57 nm or more, that
is, at a depth of 2.86 nm or more from the surface of the positive
electrode active material particle. Thus, it is found that in
Sample 01, a region from the surface to a depth of 2.71 nm is the
third region.
[0536] Furthermore, the concentration of titanium is higher than or
equal to 1/2 of the peak from a distance of 2.14 nm to a distance
of 3.42 nm. Thus, it is found that a region from a depth of 0.43 nm
to a depth of 1.71 nm from the surface of the positive electrode
active material particle is the second region.
[0537] Next, evaluation results of charge and discharge
characteristics of secondary batteries which are fabricated using
the positive electrode active materials of Sample 01 to Sample 05
formed in the above manner are described.
[Fabrication of Secondary Batteries]
[0538] CR2032 coin-type secondary batteries (with a diameter of 20
mm and a height of 3.2 mm) were fabricated using the positive
electrode active materials of Sample 01 to Sample 05 formed in the
above manner.
[0539] A positive electrode formed by applying slurry in which a
positive electrode active material (LCO), acetylene black (AB), and
polyvinylidene fluoride (PVDF) were mixed at a weight ratio of
95:2.5:2.5 to a current collector was used.
[0540] A lithium metal was used for the counter electrode.
[0541] As an electrolyte contained in an electrolyte solution, 1
mol/L lithium hexafluorophosphate (LiPF.sub.6) was used, and as the
electrolyte solution, a solution in which ethylene carbonate (EC)
and diethyl carbonate (DEC) at a volume ratio of 3:7 and vinylene
carbonate (VC) at a 2 weight % were mixed was used.
[0542] A positive electrode can and a negative electrode can were
formed of stainless steel (SUS).
[Evaluation of Charge and Discharge Characteristics]
[0543] Next, charge and discharge characteristics of the secondary
batteries of Sample 01 and Sample 05 formed in the above manner
were evaluated. The measurement temperature was 25.degree. C.
Twenty cycles of charging and discharging were performed at 4.6 V
(CCCV, 0.5 C, a cutoff current of 0.01 C) and 2.5 V (CC, 0.5 C),
respectively. Here, 1 C was set to 137 mA/g, which was a current
value per weight of the positive electrode active material.
[0544] FIG. 33 is a graph showing charge and discharge
characteristics of the secondary battery using the positive
electrode active material of Sample 01. FIG. 33 shows excellent
charge and discharge characteristics with a wide plateau. In
addition, results of 20 cycles of charging and discharging almost
overlap, which means that the cycle characteristics are
excellent.
[0545] FIG. 34 is a graph showing charge and discharge
characteristics of the secondary battery of Sample 05, which is a
comparative example. In the initial cycles, excellent charge and
discharge characteristics are shown; however, as indicated by
arrows in FIG. 34, charge capacity and discharge capacity decrease
with an increase in cycles.
[Evaluation of Cycle Characteristics]
<<Charging at 4.4 V>>
[0546] The cycle characteristics of the secondary batteries of
Sample 01 and Sample 05 charged at 4.4 V were evaluated. The
measurement temperature was 25.degree. C. The charging was
performed at 4.4 V (CCCV, 0.5 C, a cutoff current of 0.01 C), and
the discharging was performed at 2.5 V (CC, 0.5 C).
[0547] FIG. 35 is a graph showing the cycle characteristics of the
secondary batteries charged at 4.4 V. In FIG. 35, a solid line and
a dotted line indicate secondary batteries containing the positive
electrode active materials of Sample 01 and Sample 05,
respectively. As shown in FIG. 35, in the secondary battery
containing Sample 01, an energy density retention rate is 99.5%
even after 50 cycles of charging and discharging were performed,
which shows extremely excellent cycle characteristics. In the
secondary battery containing Sample 05, an energy density retention
rate is 94.3% after 50 cycles were performed.
<<Charging at 4.6 V>>
[0548] The cycle characteristics of the secondary batteries of
Sample 01 to Sample 04 charged at 4.6 V were evaluated. The
measurement temperature was 25.degree. C. The charging was
performed to 4.6 V (CCCV, 0.5 C, a cutoff current of 0.01 C), and
the discharging was performed to 2.5 V (CC, 0.5 C).
[0549] FIG. 36 is a graph showing the cycle characteristics charged
at 4.6 V. As shown in FIG. 36, in the secondary battery containing
Sample 01, which is the positive electrode active material of one
embodiment of the present invention, an energy density retention
rate is 94.1% even after 50 cycles of charging and discharging were
performed at a high voltage of 4.6 V, which shows extremely
excellent cycle characteristics. On the other hand, the secondary
batteries containing the positive electrode active materials of
Sample 02, Sample 03, and Sample 04, which are comparative
examples, are inferior to that of Sample 01, and in Sample 04, for
example, energy density retention rate is 33.2% after 50 cycles
were performed.
[0550] As described above, it is found that the positive electrode
active material with the structure of one embodiment of the present
invention can achieve an advantageous effect when charging and
discharging is performed at a voltage higher than 4.4 V.
EXAMPLE 2
[0551] In this example, the positive electrode active materials
which are embodiments of the present invention are formed, and
results of analysis which is different from that in Example 1 are
described. In addition, evaluation results of characteristics of
secondary batteries containing the positive electrode active
materials under conditions different from those in Example 1 are
described.
[0552] In this example, a positive electrode active material which
contains lithium cobaltate as a composite oxide of lithium and a
first transition metal contained in a first region, lithium
titanate as an oxide of a second transition metal contained in a
second region, and magnesium oxide as an oxide of a representative
element contained in a third region, was formed.
[Formation of Positive Electrode Active Material and Fabrication of
Secondary Battery]
<<Sample 06 and Sample 07>>
[0553] In this example, lithium cobalt oxide particles (C-20F,
produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.) were used as a
starting material.
[0554] Next, as Step 14, the lithium cobalt oxide particles were
coated with titanium oxide by a sol-gel method and dried. Step 14
was performed in a manner similar to that in Example 1 except that
mixture was performed so that TTIP to lithium cobalt oxide was
0.004 ml/g. The lithium cobalt oxide particles which are coated
with the titanium oxide and are not heated yet are referred to as
Sample 06.
[0555] Next, Sample 06, which is the lithium cobalt oxide particles
coated with the titanium oxide, was heated. With a muffle furnace,
the heating was performed at 800.degree. C. in an oxygen atmosphere
under the following conditions: the retention time was two hours;
and the flow rate of oxygen was 10 L/min.
[0556] Then, as in Example 1, the particles were cooled and
collected to obtain the positive electrode active material. The
heated positive electrode active material is referred to as Sample
07.
[TEM-EDX]
[0557] Sample 06 and Sample 07, in particular, cracks generated in
the particles and the vicinity of the cracks were subjected to
analysis with TEM-EDX.
[0558] First, results of TEM-EDX plane analysis of titanium are
shown in FIGS. 37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2, 37E1, and
37E2 and FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1, 38D2, 38E1, and
38E2.
[0559] FIGS. 37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2, 37E1, and
37E2 show TEM-EDX analysis results of Sample 06 before heating.
FIG. 37A is a cross-sectional TEM image showing the surfaces of the
particles and the crack portions. FIG. 37B1 and FIG. 37B2 show a
HAADF-STEM image and a Ti mapping image of a region including the
surface of the particle that is denoted by a circle marked with "1"
in FIG. 37A, respectively. Similarly, FIG. 37C1 and FIG. 37C2 show
a HAADF-STEM image and a Ti mapping image of a region at a depth of
approximately 20 nm from the surface in the crack portion that is
denoted by a circle marked with "2" in FIG. 37A, respectively. FIG.
37D1 and FIG. 37D2 show a HAADF-STEM image and a Ti mapping image
of a region at a depth of approximately 500 nm from the surface in
the crack portion that is denoted by a circle marked with "3" in
FIG. 37A, respectively. FIG. 37E1 and FIG. 37E2 show a HAADF-STEM
image and a Ti mapping image of a region at a depth of
approximately 1000 nm from the surface in the crack portion that is
denoted by a circle marked with "4" in FIG. 37A, respectively. Note
that in EDX element mapping images in FIGS. 37A, 37B1, 37B2, 37C1,
37C2, 37D1, 37D2, 37E1, and 37E2, FIGS. 38A, 38B1, 38B2, 38C1,
38C2, 38D1, 38D2, 38E1, and 38E2, FIGS. 39A, 39B1, 39B2, 39C1,
39C2, 39D1, 39D2, 39E1, and 39E2, and FIGS. 40A, 40B1, 40B2, 40C1,
40C2, 40D1, 40D2, 40E1, and 40E2, a region where the number of
elements is less than or equal to a lower limit of the detection is
indicated in white, and as the number of elements is increased, the
white region becomes black.
[0560] FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1, 38D2, 38E1, and
38E2 show TEM-EDX analysis results of Sample 07 after heating. FIG.
38A is a cross-sectional TEM image showing the surfaces of the
particles and the crack portions. FIG. 38B1 and FIG. 38B2 show a
HAADF-STEM image and a Ti mapping image of a region including the
surface of the particle that is denoted by a circle marked with "1"
in FIG. 38A, respectively. Similarly, FIG. 38C1 and FIG. 38C2 show
a HAADF-STEM image and a Ti mapping image of a region at a depth of
approximately 20 nm from the surface in the crack portion that is
denoted by a circle marked with "2" in FIG. 38A, respectively. FIG.
38D1 and FIG. 38D2 show a HAADF-STEM image and a Ti mapping image
of a region at a depth of approximately 500 nm from the surface in
the crack portion that is denoted by a circle marked with "3" in
FIG. 38A, respectively. FIG. 38E1 and FIG. 38E2 show a HAADF-STEM
image and a Ti mapping image of a region at a depth of
approximately 1000 nm from the surface in the crack portion that is
denoted by a circle marked with "4" in FIG. 38A, respectively.
[0561] As shown in FIGS. 37A, 37B1, 37B2, 37C1, 37C2, 37D1, 37D2,
37E1, and 37E2 and FIGS. 38A, 38B1, 38B2, 38C1, 38C2, 38D1, 38D2,
38E1, and 38E2, in Sample 06 before heating, segregation of
titanium on the surfaces of the particles is observed; however, no
segregation is observed in the crack portion. In contrast, in
Sample 07 after heating, segregation of titanium is observed both
on the surfaces of the particles and in the crack portion. That is,
it is found that titanium is segregated at the surface of the crack
portion by heating.
[0562] Next, results of TEM-EDX plane analysis of magnesium are
shown in FIGS. 39A, 39B1, 39B2, 39C1, 39C2, 39D1, 39D2, 39E1, and
39E2, and FIGS. 40A, 40B1, 40B2, 40C1, 40C2, 40D1, 40D2, 40E1, and
40E2.
[0563] FIG. 39A is a cross-sectional TEM image of Sample 06, which
is the same as FIG. 37A. FIGS. 39B1, 39C1, 39D1, and 39E1 are
HAADF-STEM images, which are the same as FIGS. 37B1, 37C1, 37D1,
and 37E1. FIG. 39B2 shows a Mg mapping image of a region which is
the same as FIG. 39B1. FIG. 39C2 shows a Mg mapping image of a
region which is the same as FIG. 39C1. FIG. 39D2 shows a Mg mapping
image of a region which is the same as FIG. 39D1. FIG. 39E2 shows a
Mg mapping image of a region which is the same as FIG. 39E1.
[0564] FIG. 40A is a cross-sectional TEM image of Sample 07, which
is the same as FIG. 38A. FIGS. 40B1, 40C1, 40D1, and 40E1 are
HAADF-STEM images, which are the same as FIGS. 38B1, 38C1, 38D1,
and 38E1. FIG. 40B2 shows a Mg mapping image of a region which is
the same as FIG. 40B1. FIG. 40C2 shows a Mg mapping image of a
region which is the same as FIG. 40C1. FIG. 40D2 shows a Mg mapping
image of a region which is the same as FIG. 40D1. FIG. 40E2 shows a
Mg mapping image of a region which is the same as FIG. 40E1.
[0565] As shown in FIGS. 39A, 39B1, 39B2, 39C1, 39C2, 39D1, 39D2,
39E1, and 39E2, and FIGS. 40A, 40B1, 40B2, 40C1, 40C2, 40D1, 40D2,
40E1, and 40E2, in Sample 06 before heating, segregation of
magnesium is not observed on the surfaces of the particles or in
the crack portion. In contrast, in Sample 07 after heating,
segregation of magnesium is observed both on the surfaces of the
particles and in the crack portion.
[0566] Next, to quantify titanium and magnesium, EDX point analysis
was performed on the regions indicated by circles marked with 1 to
6 in FIG. 37A and the regions indicated by circles marked with 1 to
6 in FIG. 38A. In each region, two points were measured.
[0567] FIGS. 41A and 41B show results of EDX point analysis in an
atomic ratio of titanium to cobalt. FIG. 41A shows results of
Sample 06 before heating. Detection points 1 to 6 in FIG. 41A
correspond to the regions indicated by circles marked with 1 to 6
in FIG. 37A. FIG. 41B shows results of Sample 07 after heating.
Detection points 1 to 6 in FIG. 41B correspond to the regions
indicated by circles marked with 1 to 6 in FIG. 38A.
[0568] As shown in FIGS. 41A and 41B, in the crack portion of
Sample 06, Ti/Co is less than or equal to 0.01 in each measurement
point; in contrast, in the crack portion of Sample 07, the amount
of titanium is increased in many points, and there are measurement
points where Ti/Co is greater than or equal to 0.05. Furthermore,
Ti/Co on the surfaces of the particles of Sample 07 is between 0.10
and 0.18.
[0569] Next, FIGS. 42A and 42B show results of EDX point analysis
in an atomic ratio of magnesium to cobalt. The detection points are
the same as those in FIGS. 41A and 41B.
[0570] As shown in FIGS. 42A and 42B, in Sample 06, Mg/Co is less
than or equal to 0.03 both on the surfaces of the particles and in
the crack portion; in contrast, in Sample 07, there are many points
where the amount of magnesium is increased both on the surfaces of
the particles and in the crack portion. Furthermore, Mg/Co on the
surfaces of the particles is between 0.15 and 0.50, and that in the
crack portion is between 0 and 0.22.
[0571] Next, CR2032 coin-type secondary batteries were fabricated
using the positive electrode active material of Sample 07 after
heating. A positive electrode formed by applying slurry in which a
positive electrode active material (LCO) of Sample 02, AB, and
polyvinylidene fluoride (PVDF) were mixed at a weight ratio of
95:3:2 to a positive electrode current collector was used. As the
positive electrode current collector, 20-.mu.m-thick aluminum foil
was used. The amount of positive electrode active material layer
containing the positive electrode active material, AB, and PVDF was
7.6 mg/cm.sup.2.
[0572] A lithium metal was used for the counter electrode.
[0573] An electrolyte solution formed in such a manner that 1 mol/L
LiPF.sub.6 was dissolved in a solution in which ethylene carbonate
(EC) and diethyl carbonate (DEC) were mixed at a volume ratio of
3:7, and vinylene carbonate (VC) was added to the solution at a 2
wt % was used.
[Initial Characteristics, Rate Characteristics]
[0574] Initial characteristics and rate characteristics of the
secondary battery using the positive electrode active material of
Sample 07 formed in the above manner were measured.
[0575] In the measurement of the initial characteristics, charging
was performed at CCCV, 0.2 C, 4.6 V, and a cutoff current of 0.05
C, and discharging was performed at CC, 0.2 C, and a cutoff voltage
of 3.0 V. Here, 1 C was set to 160 mA/g, which was a current value
per weight of the positive electrode active material. The
measurement temperature was 25.degree. C. Table 2 shows measurement
results of initial characteristics.
TABLE-US-00002 TABLE 2 Initial charge Initial discharge Initial
charge and capacity capacity discharge efficiency [mAh/g] [mAh/g]
[%] 221.2 217.5 98.3
[0576] The rate characteristics were measured after the initial
characteristics were measured. The measurement was performed by
changing a discharge rate in the following order: 0.2 C charge/0.2
C discharge; 0.2 C charge/0.5 C discharge; 0.2 C charge/1.0 C
discharge; 0.2 C charge/2.0 C discharge; 0.2 C charge/3.0 C
discharge; 0.2 C charge/4.0 C discharge; and 0.2 C charge/5.0 C
discharge. Note that the conditions other than the discharge rate
are the same as those of the measurement of the initial
characteristics. The measurement temperature was 25.degree. C.
[0577] Table 3 shows the measurement results of the initial
characteristics and the rate characteristics. In addition, FIG. 43
shows discharge curves of the rates.
TABLE-US-00003 TABLE 3 Discharge capacity Rate Discharge capacity
retention rate [C] [mAh/g] [%] 0.2 218.2 100.0 0.5 215.8 98.9 1.0
213.1 97.7 2.0 207.9 95.3 3.0 204.0 93.5 4.0 198.8 91.1 5.0 184.3
84.5
[Temperature Characteristics]
[0578] Next, a cell was fabricated under conditions similar to
those of a cell for evaluating the rate characteristics except that
the amount of positive electrode active material layer was 8.2
mg/cm.sup.2, and the temperature characteristics were measured.
Charging was performed at 25.degree. C., CCCV, 0.2 C, 4.6 V, and a
cutoff current of 0.05 C. Discharging was performed at 25.degree.
C., 0.degree. C., -10.degree. C., -20.degree. C., and 45.degree. C.
in this order, CC, 0.2 C, and a cutoff voltage of 3.0 V. FIG. 44
shows measurement results of temperature characteristics.
[Cycle Characteristics]
[0579] Next, a cell was fabricated under conditions similar to
those of the cell for measuring the temperature characteristics,
and the cycle characteristics were measured. In the measurement of
the cycle characteristics, charging was performed at CCCV, 1.0 C,
4.55 V, and a cutoff current of 0.05 C, and discharging was
performed at CC, 1.0 C, and a cutoff voltage of 3.0 V. The
measurement temperature of the cycle characteristics was 45.degree.
C. and 100 cycles were measured. The discharge capacity retention
rate after 100 cycles was 86%. FIG. 45 is a graph showing the
discharge capacity retention rate of the measured cycle
characteristics.
[0580] From the measurement results, the specific surface area of
the positive electrode active material of Sample 07 was 0.13
m.sup.2/g.
[0581] Furthermore, from the measurement results of particle size
distribution of the positive electrode active material of Sample
07, the average particle diameter was 21.5 .mu.m, 10% D was 13.1
.mu.m, 50% D was 22.0 .mu.m, and 90%D was 34.4 .mu.m.
[0582] The tap density of the positive electrode active material of
Sample 07 is 2.21 g/cm.sup.3. The tap density was measured with
MULTI TESTER MT-1000 (manufactured by SEISHIN ENTERPRISE Co.,
Ltd.).
[0583] As described above, it is found that the positive electrode
active material of Sample 07 which is one embodiment of the present
invention shows excellent initial charge and discharge capacity,
rate characteristics, and cycle characteristics. In particular, the
initial charge and discharge capacity is high, which is 98% or
higher; thus, it is probable that a side reaction is inhibited. In
addition, even at a high discharge rate of 2 C, an excellent
capacity of 96.1% is shown using 0.2 C as a reference.
EXAMPLE 3
[0584] In this example, a positive electrode active material
including a region containing titanium and magnesium in a
superficial portion was formed by changing the ratio of Li to the
first transition metal of starting materials, and evaluation
results of characteristics are shown.
[Formation of Positive Electrode Active Material]
[0585] In this example, positive electrode active materials of
Samples 11 to 17, Samples 21 to 28, and Samples 31 to 40 in which
cobalt was used as the first transition metal were prepared.
Formation methods and conditions of these samples are as
follows.
<<Samples 11 to 17>>
[0586] First, a source of lithium, a source of cobalt, a source of
magnesium, and a source of fluorine, which are to be starting
materials, were individually weighed. In this example, lithium
carbonate, cobalt oxide, magnesium oxide, and lithium fluoride were
used as the source of lithium, the source of cobalt, the source of
magnesium, and the source of fluorine, respectively.
[0587] At that time, the starting materials of Sample 11 were
weighed so that the ratio of Li to Co was 1.00. The starting
materials of Sample 12 were weighed so that the ratio of Li to Co
was 1.03. The starting materials of Sample 13 were weighed so that
the ratio of Li to Co was 1.05. The starting materials of Sample 14
were weighed so that the ratio of Li to Co was 1.06. The starting
materials of Sample 15 were weighed so that the ratio of Li to Co
was 1.07. The starting materials of Sample 16 were weighed so that
the ratio of Li to Co was 1.08. The starting materials of Sample 17
were weighed so that the ratio of Li to Co was 1.13.
[0588] In addition, the starting materials of each of Samples 11 to
17 were weighed so that, when the number of atoms of cobalt
contained in the starting materials was set to 1, the number of
atoms of magnesium was 0.01 and the number of atoms of fluorine was
0.02.
[0589] Next, the weighed starting materials were separately mixed
with a ball mill for each sample.
[0590] Then, the mixed starting materials were baked. The baking
was performed at 1000.degree. C. for 10 hours under the following
conditions: the temperature rising rate was 200.degree. C./h; and
the flow rate of dry air was 10 L/min.
[0591] Through the above process, particles of a composite oxide
containing lithium, cobalt, fluorine, and magnesium were
synthesized.
[0592] Next, TTIP was added to 2-propanol so that the amount of
TTIP per weight of the positive electrode active material was 0.01
ml/g and then mixing was performed, so that a 2-propanol solution
of tetra-i-propoxy titanium was formed.
[0593] To the 2-propanol solution of TTIP, the particles of a
composite oxide containing lithium, cobalt, fluorine, and magnesium
were added and then mixing was performed.
[0594] The above-described mixed solution was stirred with a
magnetic stirrer for four hours, at 25.degree. C., and at a
humidity of 90% RH. Through the process, water in an atmosphere and
TTIP caused hydrolysis and polycondensation reaction, and a layer
containing titanium was formed on the surface of the lithium cobalt
oxide particle containing magnesium and fluorine.
[0595] The mixed solution which had been subjected to the above
process was filtered to collect the residue. As a filter for
filtration, Kiriyama filter paper (No. 4) was used.
[0596] The collected residue was dried in a vacuum at 70.degree. C.
for one hour.
[0597] The dried particles were heated. The heating was performed
in an oxygen atmosphere under the following conditions: the
temperature was 800.degree. C. (the temperature rising rate was
200.degree. C./h); and the retention time was two hours.
[0598] The heated particles were cooled and subjected to crushing
treatment. In the crushing treatment, the particles were made to
pass through a sieve. The sieve has an aperture width of 53
.mu.m.
[0599] The particles which were subjected to crushing treatment
were used as the positive electrode active materials of Samples 11
to 17.
<<Samples 21 to 27>>
[0600] The starting materials of Samples 21 to 27 were the same as
those of Samples 11 to 16. At that time, the starting materials of
Sample 21 were weighed so that the ratio of Li to Co was 1.00. The
starting materials of Sample 22 were weighed so that the ratio of
Li to Co was 1.03. The starting materials of Sample 23 were weighed
so that the ratio of Li to Co was 1.05. The starting materials of
Sample 24 were weighed so that the ratio of Li to Co was 1.06. The
starting materials of Sample 25 were weighed so that the ratio of
Li to Co was 1.07. The starting materials of Sample 26 were weighed
so that the ratio of Li to Co was 1.08. The starting materials of
Sample 27 were weighed so that the ratio of Li to Co was 1.13.
[0601] Samples 21 to 27 were formed in manners similar to those of
Samples 11 to 17 except that the concentration of TTIP in the
2-propanol solution was adjusted so that the amount of TTIP per
weight of the positive electrode active material was 0.02 ml/g.
<<Sample 28>>
[0602] The ratio of Li to Co of the starting materials and the
amount of TTIP of Sample 28 were the same as the ratio of Li to Co
of the starting materials and the amount of TTIP of Sample 23. That
is, in Sample 28, the starting materials were weighed so that the
ratio of Li to Co was 1.05, and the amount of TTIP per weight of
the positive electrode active material was 0.02 ml/g.
[0603] Note that in Sample 28, after the starting materials were
mixed, baking was performed at 950.degree. C.
[0604] Sample 28 was formed in a manner similar to that of Sample
23 except for the baking temperature.
[0605] It is probable that Samples 11 to 17 and Samples 21 to 28
are each a positive electrode active material which contains
lithium cobaltate inside and includes a region containing titanium
and magnesium in a superficial portion.
<<Samples 31 to 40>>
[0606] Samples 31 to 40 were formed as comparative examples, each
of which did not include a region containing titanium.
[0607] The starting materials of Sample 31 were weighed so that the
ratio of Li to Co was 1.00. The starting materials of Sample 32
were weighed so that the ratio of Li to Co was 1.01. The starting
materials of Sample 33 were weighed so that the ratio of Li to Co
was 1.02. The starting materials of Sample 34 were weighed so that
the ratio of Li to Co was 1.03. The starting materials of Sample 35
were weighed so that the ratio of Li to Co was 1.035. The starting
materials of Sample 36 were weighed so that the ratio of Li to Co
was 1.04. The starting materials of Sample 37 were weighed so that
the ratio of Li to Co was 1.051. The starting materials of Sample
38 were weighed so that the ratio of Li to Co was 1.061. The
starting materials of Sample 39 were weighed so that the ratio of
Li to Co was 1.081. The starting materials of Sample 40 were
weighed so that the ratio of Li to Co was 1.130.
[0608] In addition, the starting materials of each of Samples 31 to
40 were weighed so that, when the number of atoms of cobalt
contained in the starting materials was set to 1, the number of
atoms of magnesium was 0.01 and the number of atoms of fluorine was
0.02.
[0609] Next, the weighed starting materials were separately mixed
with a ball mill for each sample.
[0610] Then, the mixed starting materials were baked. The baking
was performed at 1000.degree. C. for 10 hours under the following
conditions: the temperature rising rate was 200.degree. C./h; and
the flow rate of dry air was 10 L/min.
[0611] Through the above process, particles of a composite oxide
containing lithium, cobalt, fluorine, and magnesium were
synthesized.
[0612] The synthesized particles were cooled and then heated. The
heating was performed in an oxygen atmosphere under the following
conditions: the temperature was 800.degree. C. (the temperature
rising rate was 200.degree. C./h); and the retention time was two
hours.
[0613] The heated particles were cooled and subjected to crushing
treatment. In the crushing treatment, the particles were made to
pass through a sieve. The sieve has an aperture width of 53
.mu.m.
[0614] The particles which were subjected to crushing treatment
were used as the positive electrode active materials of Samples 31
to 40.
[0615] Table 4 shows the formation conditions of Samples 11 to 17,
Samples 21 to 28, and Samples 31 to 40.
TABLE-US-00004 TABLE 4 Baking Li/Co TTIP temperature Sample 11 1.00
0.01 ml/g 1000.degree. C. Sample 12 1.03 Sample 13 1.05 Sample 14
1.06 Sample 15 1.07 Sample 16 1.08 Sample 17 1.13 Sample 21 1.00
0.02 ml/g 1000.degree. C. Sample 22 1.03 Sample 23 1.05 Sample 24
1.06 Sample 25 1.07 Sample 26 1.08 Sample 27 1.13 Sample 28 1.05
0.02 ml/g 950.degree. C. Sample 31 1.00 -- 1000.degree. C. Sample
32 1.01 Sample 33 1.02 Sample 34 1.03 Sample 35 1.035 Sample 36
1.04 Sample 37 1.051 Sample 38 1.061 Sample 39 1.081 Sample 40
1.130
[XPS]
[0616] The positive electrode active materials of Samples 11 to 17,
Samples 21 to 28, and Samples 31 to 40 were subjected to an XPS
analysis. Table 5 shows results of the XPS analysis of Samples 11
to 17, Table 6 shows results of the XPS analysis of Samples 21 to
28, and Table 7 shows results of the XPS analysis of Samples 31 to
40. Tables 5 to 7 show a relative value of the concentration of
each element under the condition where the concentration of cobalt
is 1.
TABLE-US-00005 TABLE 5 Relative value under condition where
concentration of Co is 1 Li/Co Li Co O C F Mg Ti Si Ca Na S Sample
11 1.00 0.82 1.00 2.87 0.25 0.37 0.57 0.15 0.00 0.04 0.27 0.01
Sample 12 1.03 0.69 1.00 2.84 0.93 0.63 0.63 0.14 0.00 0.05 0.04
0.00 Sample 13 1.05 0.96 1.00 3.22 0.66 0.47 0.61 0.14 0.00 0.03
0.09 0.03 Sample 14 1.06 1.04 1.00 3.13 0.34 0.77 0.77 0.15 0.00
0.05 0.34 0.04 Sample 15 1.07 0.84 1.00 3.42 0.58 0.60 0.23 0.18
0.00 0.08 0.27 0.00 Sample 16 1.08 1.08 1.00 3.10 0.66 0.59 0.04
0.16 0.00 0.03 0.16 0.02 Sample 17 1.13 1.29 1.00 3.70 0.83 0.87
0.00 0.19 0.00 0.04 0.20 0.02
TABLE-US-00006 TABLE 6 Relative value under condition where
concentration of Co is 1 Li/Co Li Co O C F Mg Ti Si Ca Na S Sample
21 1.00 1.08 1.00 3.23 0.47 0.45 0.49 0.20 0.00 0.06 0.35 0.01
Sample 22 1.03 0.73 1.00 3.05 1.06 0.70 0.59 0.17 0.00 0.03 0.09
0.00 Sample 23 1.05 1.19 1.00 3.40 0.73 0.48 0.58 0.23 0.00 0.05
0.10 0.04 Sample 24 1.06 1.10 1.00 3.44 0.33 0.92 0.91 0.25 0.00
0.05 0.30 0.04 Sample 25 1.07 1.05 1.00 4.26 0.74 0.20 0.25 0.29
0.00 0.07 0.30 0.02 Sample 26 1.08 1.12 1.00 3.32 0.74 0.52 0.14
0.21 0.00 0.05 0.15 0.00 Sample 27 1.13 1.49 1.00 3.80 0.60 0.41
0.00 0.12 0.00 0.00 0.16 0.02 Sample 28 1.05 0.99 1.00 3.11 0.72
0.86 0.56 0.18 0.00 0.05 0.12 0.02
TABLE-US-00007 TABLE 7 Relative value under condition where
concentration of Co is 1 Li/Co Li Co O C F Mg Ca Na Sample 31 1.00
0.51 1.00 2.45 0.69 0.08 0.27 0.03 0.08 Sample 32 1.01 0.67 1.00
2.65 0.77 0.08 0.28 0.03 0.08 Sample 33 1.02 0.53 1.00 2.51 0.66
0.09 0.27 0.02 0.06 Sample 34 1.03 0.79 1.00 2.93 0.92 0.09 0.35
0.04 0.14 Sample 35 1.04 0.65 1.00 2.33 0.48 0.11 0.32 0.03 0.11
Sample 36 1.04 0.69 1.00 2.73 0.56 0.11 0.38 0.05 0.16 Sample 37
1.05 0.67 1.00 3.04 0.64 0.09 0.35 0.04 0.21 Sample 38 1.06 0.83
1.00 2.65 1.03 0.29 0.10 0.05 0.11 Sample 39 1.08 0.80 1.00 2.79
1.04 0.26 0.03 0.08 0.10 Sample 40 1.13 0.77 1.00 2.72 0.22 0.99
0.00 0.01 0.26
[0617] FIGS. 46A and 46B are graphs in which the relative value of
magnesium and the relative value of titanium are extracted from the
analysis results of Tables 5 to 7. FIG. 46A is a graph showing the
ratio of Li to Co and the relative value of magnesium. FIG. 46B is
a graph showing the ratio of Li to Co and the relative value of
titanium.
[0618] From the analysis results of Samples 31 to 40 in FIG. 46A,
it is found that, in the case where a coating layer containing
titanium is not included, the concentration of magnesium is high in
samples where the ratio of Li to Co is greater than or equal to
1.00 and less than or equal to 1.05. This is probably because
magnesium contained in the starting materials is segregated in a
range where the element concentration can be detected by XPS by
heating. In contrast, in samples where the ratio of Li to Co is
greater than or equal to 1.06, the concentration of magnesium is
low; thus, it is probable that the segregation of magnesium does
not easily occur when the amount of lithium is too large.
[0619] From the analysis results of Samples 11 to 16 and Samples 21
to 26 in FIG. 46A, it is found that the concentration of magnesium
in a range where the element concentration can be detected by XPS
is higher in the case where a region containing titanium is
included in a superficial portion than in the case where the region
containing titanium is not included.
[0620] Moreover, in the case where the ratio of Li to Co is 1.06,
in samples where the region containing titanium is not included,
the concentration of magnesium in a range where the element
concentration can be detected by XPS is low; in contrast, in
samples where the region containing titanium is included, the
concentration of magnesium in a range where the element
concentration can be detected by XPS is high. That is, when the
region containing titanium is formed in the superficial portion,
magnesium is sufficiently segregated even in the case where the
ratio of Li to Co is high.
[0621] Note that even if the region containing titanium is
included, the concentration of magnesium is lower in the case where
the ratio of Li to Co is 1.07 than in the case where the ratio of
Li to Co is 1.06. Furthermore, it is probable that, in the case
where the ratio of Li to Co is greater than or equal to 1.08, the
segregation of magnesium does not easily occur even if the region
containing titanium is included.
[Evaluation of Cycle Characteristics]
<<Energy Density Retention Rate>>
[0622] Next, cycle characteristics were evaluated in a manner
similar to that of Example 1 using positive electrode active
materials of Samples 11 to 14, Sample 16, Samples 21 to 24, and
Sample 26.
[0623] The shape of the secondary battery, the materials and the
mixture ratios of the positive electrode active material, the
conductive additive, and the binder in the positive electrode, the
counter electrode, the electrolyte solution, the exterior body, the
conditions of the cycle characteristics test, and the like are the
same as those in Example 1.
[0624] FIG. 47A is a graph showing the energy density retention
rates and the number of charge and discharge cycles at the time of
charging at 4.6 V of secondary batteries using the positive
electrode active materials of Samples 11 to 14 and Sample 16 which
were formed so that the amount of TTIP per weight of the positive
electrode active material was 0.01 ml/g. FIG. 47B is a graph
showing the energy density retention rates and the number of charge
and discharge cycles at the time of charging at 4.6 V of secondary
batteries using the positive electrode active materials of Samples
21 to 24 and Sample 26 which were formed so that the amount of TTIP
per weight of the positive electrode active material was 0.02
ml/g.
[0625] As shown in FIG. 47A, in the case where TTIP is 0.01 ml/g,
Samples 11 to 14, that is, the positive electrode active materials
in which the ratio of Li to Co is greater than or equal to 1.00 and
less than or equal to 1.06 have excellent cycle characteristics. In
particular, Samples 11 and 12, that is, the positive electrode
active materials in which the ratio of Li to Co is greater than or
equal to 1.00 and less than or equal to 1.03 have extremely
excellent cycle characteristics. In contrast, in Sample 16 in which
the ratio of Li to Co is 1.08, the energy density retention rate
decreases at a relatively early stage.
[0626] As shown in FIG. 47B, in the case where TTIP is 0.02 ml/g,
Samples 21 to 24, that is, the positive electrode active materials
in which the ratio of Li to Co is greater than or equal to 1.00 and
less than or equal to 1.06 have excellent cycle characteristics. In
particular, Samples 23 and 24, that is, the positive electrode
active materials in which the ratio of Li to Co is greater than or
equal to 1.05 and less than or equal to 1.06 have extremely
excellent cycle characteristics.
[0627] FIG. 48 is a graph showing comparison between Sample 11
having the most excellent cycle characteristics in Samples 11 to 15
and Sample 23 having the most excellent cycle characteristics in
Samples 21 to 25.
[0628] As shown in FIG. 48, both Sample 11 and Sample 23 have
excellent cycle characteristics; however, Sample 23 in which TTIP
is 0.02 ml/g has more excellent cycle characteristics.
<<Discharge Capacity Retention Rate>>
[0629] Next, FIG. 49 shows evaluation results of a discharge
capacity retention rate, which is one of the cycle characteristics
of each of Samples 21 to 26 and Sample 28.
[0630] The shape of the secondary battery, the material and the
mixture ratio of the positive electrode active material, the
conductive additive, and the binder in the positive electrode, the
counter electrode, the electrolyte solution, the exterior body, the
conditions of the cycle characteristics test, and the like of
Samples 21 to 26 are the same as those in Example 1.
[0631] The secondary battery using Sample 28 was formed in a manner
similar to those of the secondary batteries using Samples 21 to 26
except that PVDF was used as a binder and the positive electrode
active material (LCO), AB, and PVDF were mixed such that the weight
ratio of LCO to AB and PVDF was 95:3:2, and evaluated.
[0632] As shown in FIG. 49, Samples 21 to 24 and Sample 28 have
excellent cycle characteristics. In particular, Sample 28 has
extremely excellent cycle characteristics. In Sample 28, the
discharge capacity retention rate after 50 cycles was higher than
or equal to 85%.
[0633] In contrast, in Sample 25 and Sample 26 in which the ratios
of Li to Co are 1.07 and 1.08, respectively, the discharge capacity
retention rates decrease at a relatively early stage.
[0634] From the above results, it is found that in the case where
TTIP per weight of the positive electrode active material is 0.02
ml/g, the ratio of Li to Co preferably has a range of greater than
or equal to 1.00 and less than 1.07. Moreover, it is found that a
sample in which the ratio of Li to Co has a range of greater than
or equal to 1.05 and less than or equal to 1.06 has extremely
excellent cycle characteristics.
[0635] FIGS. 50A to 50C show charge and discharge curves of the
secondary batteries using Sample 28, which has extremely excellent
cycle characteristics in FIG. 49, Sample 24, and Sample 25, in
which the degradation occurs at a relatively early stage.
[0636] FIG. 50A, FIG. 50B, and FIG. 50C show charge and discharge
curves of the secondary batteries using Sample 28, Sample 24, and
Sample 25, respectively. Each of the figures shows overlap of
results of 50 cycles of charge and discharge. As indicated by an
arrow in each figure, the charge and discharge capacity decreases
from the first cycle to the fiftieth cycle.
[0637] As shown in FIGS. 50A and 50B, Sample 28 and Sample 24,
which are positive electrode active materials of one embodiment of
the present invention, have high charge and discharge capacity and
excellent charge and discharge characteristics. In addition, it is
found that a decrease in charge and discharge capacity of each of
Sample 28 and Sample 24 in FIGS. 50A and 50B is significantly
suppressed as compared with Sample 25 in FIG. 50C.
EXAMPLE 4
[0638] In this example, SEM observation results and SEM-EDX
analysis results of the positive electrode active material of
Sample 24 formed in Example 2 are described.
[0639] Sample 24 was formed so that Li/Co was 1.06 and TTIP per
weight of the positive electrode active material was 0.02 ml/g.
FIG. 51A shows a SEM image of Sample 24. FIGS. 51B and 51C each
show an enlarged image of a part in FIG. 51A.
[0640] As shown in FIGS. 51A to 51C, there are a large number of
projected regions in a superficial portion of the positive
electrode active material.
[0641] Next, FIGS. 52A-1, 52A-2, 52B-1, 52B-2, 52C-1, and 52C-2
show analysis results of the positive electrode active material of
Sample 24 using SEM-EDX. FIG. 52A-1 shows a SEM image of a
superficial portion of the positive electrode active material, FIG.
52A-2 shows a mapping image of titanium, FIG. 52B-1 shows a mapping
image of magnesium, FIG. 52B-2 shows a mapping image of oxygen,
FIG. 52C-1 shows a mapping image of aluminum, and FIG. 52C-2 shows
a mapping image of cobalt. Note that in EDX element mapping images
in FIGS. 52A-2, 52B-1, 52B-2, 52C-1, and 52C-2, a region where the
number of elements is less than or equal to a lower limit of the
detection is indicated in black, and as the number of elements is
increased, the black region becomes white.
[0642] The same regions in FIGS. 52A-1, 52A-2, and 52B-1 are
surrounded by dotted lines. When the regions surrounded by dotted
lines are compared with each other, it is found that in the
projected regions in the superficial portion of the positive
electrode active material, titanium and magnesium are
distributed.
[0643] Thus, it is found that Sample 24 is a positive electrode
active material including the projected fourth region 104
containing titanium and magnesium over the third region 103.
[0644] As shown in Example 2, Sample 24 is one of the samples which
show extremely excellent cycle characteristics. Thus, it is found
that, even if the fourth region is provided in the superficial
portion or when the fourth region is provided, the positive
electrode active material with excellent cycle characteristics can
be obtained.
[0645] From the above results of Examples 1 to 3, it is found that
when the region containing titanium is formed in the superficial
portion, the positive electrode active material with excellent
cycle characteristics can be obtained. In addition, it is found
that, when the ratio of Li to Co is increased to increase the
particle diameter of the positive electrode active material, the
cycle characteristics might be degraded; however, the region
containing titanium is formed in the superficial portion, whereby
the range of the ratio of Li to Co in which excellent cycle
characteristics is obtained can be widened. Furthermore, it is
found that even when the fourth region containing titanium and
magnesium is provided in the superficial portion of the positive
electrode active material, excellent cycle characteristics is
obtained.
EXAMPLE 5
[0646] In this example, an example of a method for producing a
positive electrode active material coated with graphene oxide is
shown, and observation results of the positive electrode active
material produced by the method with an electron microscope are
described.
[0647] As shown in a process flow chart in FIG. 53, a process for
forming a coating film on a positive electrode active material
includes the steps of: weighing graphene oxide (S11); mixing and
stirring the graphene oxide and pure water (S12); controlling pH
(S13); adding an active material (S14); completing a suspension
(S15); spraying the suspension using a spray dry apparatus (S16);
and collecting particles in a container (S17).
[0648] Note that in (S12), pure water is used as a dispersion
medium; however, a dispersion medium is not particularly limited
and ethanol or the like may be used. In addition, in (S14), the
active material is a positive electrode active material.
[0649] FIG. 54 is a schematic view of a spray dry apparatus 280.
The spray dry apparatus 280 includes a chamber 281 and a nozzle
282. A suspension 284 is supplied to the nozzle 282 through a tube
283. The suspension 284 is supplied from the nozzle 282 to the
chamber 281 in the form of mist and dried in the chamber 281. The
nozzle 282 may be heated with a heater 285. Here, a region of the
chamber 281 which is close to the nozzle 282, for example, a region
surrounded by dashed-two dotted line in FIG. 54, is also heated
with the heater 285.
[0650] In the case of using a suspension containing a positive
electrode active material and graphene oxide as the suspension 284,
powder of the positive electrode active material coated with the
graphene oxide is collected in a collection container 286 through
the chamber 281.
[0651] The air in the chamber 281 may be suctioned by an aspirator
or the like through a path indicated by an arrow 288.
[0652] An example of conditions for forming the coating film is
shown below.
[0653] First, a suspension was formed by dispersing graphene oxide
into a solvent.
[0654] Although in pure water, graphene oxide is highly
dispersible, pure water might react with an active material added
later, so that Li might be dissolved or an active material might be
damaged to change the surface structure. Thus, the graphene oxide
was dispersed into a solution such that the ratio between ethanol
and pure water was 4:6.
[0655] Stirring to disperse the graphene oxide into a solution was
performed under the following conditions: a stirrer and an
ultrasonic wave generator were used; a rotation rate was 750 rpm;
and irradiation time with ultrasonic waves was 2 minutes.
[0656] Then, a LiOH aqueous solution was dropped to adjust pH to be
pH7 (25.degree. C.).
[0657] The positive electrode active materials (in this example,
lithium cobalt oxide particles produced by NIPPON CHEMICAL
INDUSTRIAL CO., LTD. (product name: C-20F)) were added, and
stirring was performed using a stirrer and an ultrasonic wave
generator under the following conditions: a rotation rate was 750
rpm; and irradiation time with ultrasonic waves was 1 minute.
Through the above process, the suspension was prepared. The above
lithium cobalt oxide particles produced by NIPPON CHEMICAL
INDUSTRIAL CO., LTD. (product name: C-20F) contain at least
fluorine, magnesium, calcium, sodium, silicon, sulfur, and
phosphorus, and each have a diameter of approximately 20 .mu.m.
[0658] Next, the suspension was sprayed uniformly with a spray
nozzle (having a nozzle diameter of 20 .mu.m) of the spray dry
apparatus to obtain powder. The inlet temperature was 160.degree.
C. and the outlet temperature was 40.degree. C. as the hot-air
temperature of the spray dry apparatus, and the N.sub.2 gas flow
rate was 10 L/min.
[0659] FIG. 55 shows a cross-sectional TEM image of the obtained
powder. In addition, FIG. 56 shows a SEM image of the obtained
powder. When a positive electrode active material which was the
same as the sprayed positive electrode active material (C-20F,
produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.) was mixed with
the graphene oxide with a planetary centrifugal mixer as a
comparative example, coating was insufficient. FIG. 57 shows a SEM
image of the comparative example
[0660] It is found that, as compared with FIG. 57, the coating film
is uniformly formed on the surface of the powder in FIG. 56.
[0661] FIGS. 58A and 58B illustrate a cross-sectional structure
example of an active material layer 200 which is coated with
graphene oxide with a spray dry apparatus and includes a graphene
compound as a conductive additive.
[0662] FIG. 58A is a longitudinal sectional view of the active
material layer 200. The active material layer 200 includes positive
electrode active material particles 100 coated with graphene oxide,
a graphene compound 201 as a conductive additive, and a binder (not
illustrated). Here, graphene or multilayer graphene may be used as
the graphene compound 201, for example. The graphene compound 201
preferably has a sheet-like shape. The graphene compound 201 may
have a sheet-like shape formed of a plurality of sheets of
multilayer graphene and/or a plurality of sheets of graphene that
partly overlap with each other.
[0663] In the longitudinal section of the active material layer
200, as illustrated in FIG. 58B, the positive electrode active
material 100 coated with a coating film 105 formed of graphene
oxide is in contact with the graphene compound 201. A plurality of
graphene compounds 201 are formed in such a way as to be partly in
contact with the positive electrode active material 100 coated with
the coating film 105 and adhere to the coating film 105 of the
adjacent positive electrode active material 100, so that the
graphene compounds 201 are in contact with each of the positive
electrode active material 100.
[0664] The graphene compound 201 and the coating film 105 are
formed using carbon-based materials; thus, an excellent conductive
path can be formed.
[0665] The coating film 105 is effective in protecting the crystal
structure of the positive electrode active material 100 so as not
to be in contact with the electrolyte solution and in forming the
excellent conductive path.
REFERENCE NUMERALS
[0666] 11a: positive electrode, 11b: negative electrode, 12a: lead,
12b: lead, 14: separator, 15a: bonding portion, 15b: bonding
portion, 17: fixing member, 50: secondary battery, 51: exterior
body, 61: folded portion, 62: seal portion, 63: seal portion, 71:
crest line, 72: trough line, 73: space, 100: positive electrode
active material, 101: first region, 101p: crystal plane, 102:
second region, 102p: crystal plane, 103: third region, 103p:
crystal plane, 104: fourth region, 105: coating film, 106: crack
portion, 110: particle, 111: region, 112: layer containing
titanium, 114: cobalt oxide layer, 120: particle, 121: region, 122:
layer containing titanium, 124: cobalt oxide layer, 125: layer
containing lithium titanate, 200: active material layer, 201:
graphene compound, 214: separator, 280: spray dry apparatus, 281:
chamber, 282: nozzle, 283: tube, 284: suspension, 285: heater, 286:
collection container, 288: arrow, 300: coin-type 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, 500:
laminated 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, 510:
positive electrode lead electrode, 511: negative electrode lead
electrode, 600: cylindrical 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, 611: PTC element, 612: safety valve mechanism,
613: conductive plate, 614: conductive plate, 615: module, 616:
wiring, 617: temperature control device, 900: circuit board, 910:
label, 911: terminal, 912: circuit, 913: secondary battery, 914:
antenna, 915: antenna, 916: layer, 917: layer, 918: antenna, 920:
display device, 921: sensor, 922: terminal, 930: housing, 930a:
housing, 930b: housing, 931: negative electrode, 932: positive
electrode, 933: separator, 950: wound body, 951: terminal, 952:
terminal, 980: laminated secondary battery, 981: film, 982: film,
993: wound body, 994: negative electrode, 995: positive electrode,
996: separator, 997: lead electrode, 998: lead electrode, 7100:
portable display device, 7101: housing, 7102: display portion,
7103: operation button, 7104: secondary battery, 7200: portable
information terminal, 7201: housing, 7202: display portion, 7203:
band, 7204: buckle, 7205: operation button, 7206: input output
terminal, 7207: icon, 7300: display device, 7304: display portion,
7400: mobile phone, 7401: housing, 7402: display portion, 7403:
operation button, 7404: external connection port, 7405: speaker,
7406: microphone, 7407: secondary battery, 7408: lead electrode,
7409: current collector, 7500: vaporizer, 7501: atomizer, 7502:
cartridge, 7504: secondary battery, 8000: display device, 8001:
housing, 8002: display portion, 8003: speaker portion, 8004:
secondary battery, 8021: ground-based charging apparatus, 8022:
cable, 8024: secondary battery, 8100: lighting device, 8101:
housing, 8102: light source, 8103: secondary battery, 8104:
ceiling, 8105: wall, 8106: floor, 8107: window, 8200: indoor unit,
8201: housing, 8202: air outlet, 8203: secondary battery, 8204:
outdoor unit, 8300: electric refrigerator-freezer, 8301: housing,
8302: refrigerator door, 8303: freezer door, 8304: secondary
battery, 8400: automobile, 8401: headlight, 8406: electric motor,
8500: automobile, 8600: motor scooter, 8601: side mirror, 8602:
secondary battery, 8603: indicator, 8604: storage unit under seat,
9600: tablet terminal, 9625: power saving mode changing switch,
9626: display mode changing switch, 9627: power switch, 9628:
operation switch, 9629: fastener, 9630: housing, 9630a: housing,
9630b: housing, 9631: display portion, 9631a: display portion,
9631b: display portion, 9633: solar cell, 9634: charge and
discharge control circuit, 9635: power storage unit, 9636: DCDC
converter, 9637: converter, 9640: movable portion.
[0667] This application is based on Japanese Patent Application
Serial No. 2016-133997 filed with Japan Patent Office on Jul. 6,
2016, Japanese Patent Application Serial No. 2016-133143 filed with
Japan Patent Office on Jul. 5, 2016, Japanese Patent Application
Serial No. 2017-002831 filed with Japan Patent Office on Jan. 11,
2017, Japanese Patent Application Serial No. 2017-030693 filed with
Japan Patent Office on Feb. 22, 2017, Japanese Patent Application
Serial No. 2017-084321 filed with Japan Patent Office on Apr. 21,
2017, and Japanese Patent Application Serial No. 2017-119272 filed
with Japan Patent Office on Jun. 19, 2017 the entire contents of
which are hereby incorporated by reference.
* * * * *