U.S. patent application number 16/502345 was filed with the patent office on 2019-10-24 for electrode for power storage device, power storage device, and manufacturing method of electrode for power storage device.
The applicant listed for this patent is SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Nobuhiro INOUE, Kazutaka KURIKI, Junpei MOMO, Ryota TAJIMA.
Application Number | 20190326069 16/502345 |
Document ID | / |
Family ID | 50622654 |
Filed Date | 2019-10-24 |
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United States Patent
Application |
20190326069 |
Kind Code |
A1 |
KURIKI; Kazutaka ; et
al. |
October 24, 2019 |
ELECTRODE FOR POWER STORAGE DEVICE, POWER STORAGE DEVICE, AND
MANUFACTURING METHOD OF ELECTRODE FOR POWER STORAGE DEVICE
Abstract
To improve the long-term cycle performance of a lithium-ion
battery or a lithium-ion capacitor by minimizing the decomposition
reaction of an electrolytic solution and the like as a side
reaction of charge and discharge in the repeated charge and
discharge cycles of the lithium-ion battery or the lithium-ion
capacitor. A current collector and an active material layer over
the current collector are included in an electrode for a power
storage device. The active material layer includes a plurality of
active material particles and silicon oxide. The surface of one of
the active material particles has a region that is in contact with
one of the other active material particles. The surface of the
active material particle except the region is partly or entirely
covered with the silicon oxide.
Inventors: |
KURIKI; Kazutaka; (Ebina,
JP) ; TAJIMA; Ryota; (Isehara, JP) ; INOUE;
Nobuhiro; (Atsugi, JP) ; MOMO; Junpei;
(Sagamihara, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMICONDUCTOR ENERGY LABORATORY CO., LTD. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
50622654 |
Appl. No.: |
16/502345 |
Filed: |
July 3, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14069408 |
Nov 1, 2013 |
10388467 |
|
|
16502345 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01G 11/32 20130101;
H01M 4/366 20130101; H01M 4/587 20130101; Y02E 60/13 20130101; H01G
11/42 20130101; H01M 4/1393 20130101 |
International
Class: |
H01G 11/42 20060101
H01G011/42; H01M 4/587 20060101 H01M004/587; H01M 4/1393 20060101
H01M004/1393; H01G 11/32 20060101 H01G011/32 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 7, 2012 |
JP |
2012-245847 |
Claims
1. An electrode for a power storage device, comprising: a current
collector; and an active material layer over the current collector,
the active material layer comprising a plurality of active material
particles and silicon oxide, wherein a surface of one of the
plurality of active material particles has a region that is in
contact with another of the plurality of active material particles,
and wherein the surface of the one of the plurality of active
material particles except the region is at least partly covered
with the silicon oxide.
2. The electrode for a power storage device, according to claim 1,
wherein the plurality of active material particles comprise a
carbon material, and wherein an R value of the carbon material is
less than 0.4.
3. The electrode for a power storage device, according to claim 1,
wherein the plurality of active material particles comprise a
carbon material, and wherein an R value of the carbon material is
less than 1.1.
4. The electrode for a power storage device, according to claim 1,
further comprising a binder, wherein the binder is in contact with
the plurality of active material particles and the silicon
oxide.
5. A power storage device comprising the electrode according to
claim 1.
6. An electrode for a power storage device, comprising: a current
collector; and an active material layer over the current collector,
the active material layer comprising a plurality of active material
particles and metal oxide, wherein a surface of one of the
plurality of active material particles has a region that is in
contact with another of the plurality of active material particles,
and wherein the surface of the one of the plurality of active
material particles except the region is at least partly covered
with the metal oxide.
7. The electrode for a power storage device, according to claim 6,
wherein the plurality of active material particles comprise a
carbon material, and wherein an R value of the carbon material is
less than 0.4.
8. The electrode for a power storage device, according to claim 6,
wherein the plurality of active material particles comprise a
carbon material, and wherein an R value of the carbon material is
less than 1.1.
9. The electrode for a power storage device, according to claim 6,
further comprising a binder, wherein the binder is in contact with
the plurality of active material particles and the metal oxide.
10. A power storage device comprising the electrode according to
claim 6.
Description
TECHNICAL FIELD
[0001] The present invention relates to a negative electrode for a
power storage device and a power storage device.
BACKGROUND ART
[0002] In recent years, a variety of power storage devices, for
example, nonaqueous secondary batteries such as lithium-ion
secondary batteries (LIBs), lithium-ion capacitors (LICs), and air
cells have been actively developed. In particular, demand for
lithium-ion batteries with high output and high energy density has
rapidly grown with the development of the semiconductor industry,
for electronic devices, for example, portable information terminals
such as cell phones, smartphones, and laptop computers, portable
music players, and digital cameras; medical equipment;
next-generation clean energy vehicles such as hybrid electric
vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid
electric vehicles (PHEVs); and the like. The lithium-ion secondary
batteries are essential as rechargeable energy supply sources for
today's information society.
[0003] A negative electrode for power storage devices such as
lithium-ion batteries and the lithium-ion capacitors is a structure
body including at least a current collector (hereinafter referred
to as a negative electrode current collector) and an active
material layer (hereinafter referred to as a negative electrode
active material layer) provided over a surface of the negative
electrode current collector. The negative electrode active material
layer contains an active material (hereinafter referred to as a
negative electrode active material) which can receive and release
lithium ions serving as carrier ions, such as carbon or
silicon.
[0004] At present, a negative electrode of a lithium-ion battery
which contains a graphite-based carbon material is generally formed
by mixing graphite as a negative electrode active material,
acetylene black (AB) as a conductive additive, PVDF, which is a
resin as a binder, to form a slurry, applying the slurry over a
current collector, and drying the slurry, for example.
[0005] Such a negative electrode for a lithium-ion battery and a
lithium-ion capacitor has an extremely low electrode potential and
a high reducing ability. For this reason, an electrolytic solution
containing an organic solvent is subjected to reductive
decomposition. The range of potentials in which the electrolysis of
an electrolytic solution does not occur is referred to as a
potential window. A negative electrode originally needs to have an
electrode potential in the potential window of an electrolytic
solution. However, the negative electrode potentials of a
lithium-ion battery and a lithium-ion capacitor are out of the
potential windows of almost all electrolytic solutions. Actually, a
decomposition product of an electrolytic solution forms a
passivating film (also referred to as solid electrolyte interphase)
on the surface of a negative electrode, and the passivating film
inhibits further reductive decomposition. Consequently, lithium
ions can be inserted into the negative electrode with the use of a
low electrode potential below the potential window of an
electrolytic solution (for example, see Non-Patent Document 1).
REFERENCE
[0006] [Non-Patent Document 1] Zempachi Ogumi, "Lithium Secondary
Battery", Ohmsha, Ltd., the first impression of the first edition
published on Mar. 20, 2008, pp. 116-118
DISCLOSURE OF INVENTION
[0007] A passivating film is a reductive decomposition product
generated by reductive decomposition reaction of an electrolytic
solution or a product of a reaction between a reductive
decomposition product and an electrolytic solution. For example, in
the case where a negative electrode active material is graphite,
which has a layered structure, a passivating film is formed between
layers in an edge surface of the graphite and on a surface (basal
surface) of the graphite. When carrier ions are inserted into the
graphite and thus the volume of the graphite increases, part of the
passivating film is separated from the graphite and part of the
negative electrode active material is exposed.
[0008] Although a generated passivating film kinetically inhibits
the decomposition of an electrolytic solution, the thickness of the
passivating film gradually increases on repeated charge and
discharge. The passivating film having an increased thickness is
susceptible to the volume expansion of a negative electrode active
material, and part of the passivating film is easily separated.
[0009] Another passivating film is formed on a surface of the
negative electrode active material which is exposed by the
separation of the passivating film.
[0010] A passivating film of a conventional negative electrode is
considered as being formed because of battery reaction in charging,
and electric charge used for formation of the passivating film
cannot be discharged. Thus, irreversible capacity resulting from
the electric charge used for forming the passivating film reduces
the initial discharge capacity of a lithium-ion battery. In
addition, separation of the passivating film and formation of other
passivating films on repeated charge and discharge further reduce
the discharge capacity.
[0011] As the electrochemical decomposition of an electrolytic
solution proceeds, the amount of lithium responsible for charge and
discharge is decreased in accordance with the number of electrons
used in the decomposition reaction of the electrolytic solution.
Therefore, as charge and discharge are repeated and other
passivating films are generated, the capacity of a lithium-ion
battery is lost after a while. In addition, the higher the
temperature is, the faster the electrochemical reaction proceeds.
Thus, the capacity of a lithium-ion battery decreases more
significantly as charge and discharge are repeated at high
temperature.
[0012] Not only lithium-ion batteries but also power storage
devices such as lithium-ion capacitors have the above problems.
[0013] In view of the above, an object of one embodiment of the
present invention is to form a stable surface of an active material
of a lithium-ion battery or a lithium-ion capacitor to minimize the
electrochemical decomposition of an electrolytic solution and the
like around an electrode.
[0014] Another object of one embodiment of the present invention is
to improve the long-term cycle performance of a lithium-ion battery
or a lithium-ion capacitor by minimizing the decomposition reaction
of an electrolytic solution and the like as a side reaction of
charge and discharge in the repeated charge and discharge cycles of
the lithium-ion battery or the lithium-ion capacitor.
[0015] One embodiment of the present invention achieves at least
one of the above objects.
[0016] One embodiment of the present invention provides an
electrode for a power storage device that includes a current
collector and an active material layer over the current collector.
The active material layer includes a plurality of active material
particles and either metal oxide or silicon oxide. The surface of
one of the active material particles has a region that is in
contact with one of the other active material particles. The
surface of the active material particle except the region is partly
or entirely covered with the metal oxide or the silicon oxide.
[0017] Another embodiment of the present invention provides a power
storage device including the above electrode for a power storage
device.
[0018] According to one embodiment of the present invention, an
active material of a lithium-ion battery or a lithium-ion capacitor
has a stable surface, which makes it possible to minimize the
electrochemical decomposition of an electrolytic solution and the
like around an electrode.
[0019] Further, according to one embodiment of the present
invention, it is possible to minimize the decomposition reaction of
an electrolytic solution and the like as a side reaction of charge
and discharge in the repeated charge and discharge cycles of a
power storage device such as a lithium-ion battery or a lithium-ion
capacitor, and thus the long-term cycle performance of the
lithium-ion battery or the lithium ion capacitor can be
improved.
BRIEF DESCRIPTION OF DRAWINGS
[0020] In the accompanying drawings:
[0021] FIGS. 1A to 1C2 illustrate an electrode and a negative
electrode active material provided with a metal oxide film;
[0022] FIG. 2 shows a method for forming an active material
provided with a metal oxide film;
[0023] FIGS. 3A and 3B illustrate a negative electrode;
[0024] FIGS. 4A and 4B illustrate a positive electrode;
[0025] FIGS. 5A and 5B illustrate a coin-type lithium-ion
battery;
[0026] FIG. 6 illustrates a laminated lithium-ion battery;
[0027] FIGS. 7A and 7B illustrate a cylindrical lithium-ion
battery;
[0028] FIG. 8 illustrates electronic devices;
[0029] FIGS. 9A to 9C illustrate an electronic device;
[0030] FIGS. 10A and 10B illustrate an electronic device of the
present invention;
[0031] FIG. 11 shows the ratios of the D band to the G band of each
of Electrode A, Comparative Electrode B, and Comparative Electrode
C;
[0032] FIG. 12 shows initial irreversible capacities;
[0033] FIG. 13 shows cycle performance;
[0034] FIGS. 14A and 14B are SEM images;
[0035] FIG. 15 shows cycle performance; and
[0036] FIGS. 16A and 16B are SEM images.
BEST MODE FOR CARRYING OUT THE INVENTION
[0037] Hereinafter, embodiments and examples will be described with
reference to drawings. However, the embodiments and examples can be
implemented in many different modes, and it will be readily
appreciated by those skilled in the art that modes and details
thereof can be changed in various ways without departing from the
spirit and scope of the present invention. Thus, the present
invention should not be interpreted as being limited to the
following descriptions of the embodiments and examples.
Embodiment 1
[0038] In this embodiment, an electrode for a power storage device
of one embodiment of the present invention will be described with
reference to FIGS. 1A to 1C2.
[0039] FIGS. 1A and 1B each illustrate an electrode for a power
storage device of one embodiment of the present invention. FIG. 1A
is a cross-sectional view of an electrode 101. In the schematic
view of FIG. 1A, an active material layer 103 is formed over one
surface of a current collector 102.
[0040] For the current collector 102, a highly conductive material
such as a metal typified by stainless steel, gold, platinum, zinc,
iron, copper, aluminum, or titanium, or an alloy thereof can be
used. Alternatively, an aluminum alloy to which an element which
improves heat resistance, such as silicon, titanium, neodymium,
scandium, or molybdenum, is added can be used. Still alternatively,
a metal element which forms silicide by reacting with silicon can
be used. Examples of the metal element which forms silicide by
reacting with silicon include zirconium, titanium, hafnium,
vanadium, niobium, tantalum, chromium, molybdenum, tungsten,
cobalt, nickel, and the like. The current collector 102 can have a
foil-like shape, a plate-like shape (sheet-like shape), a net-like
shape, a punching-metal shape, an expanded-metal shape, or the like
as appropriate. The current collector 102 preferably has a
thickness of greater than or equal to 10 .mu.m and less than or
equal to 30 .mu.m.
[0041] The active material layer 103 includes at least an active
material, a film, and a binder. The active material layer 103 may
further include a conductive additive.
[0042] In the case where the electrode 101 is a negative electrode,
graphite, which is a carbon material generally used in the field of
power storage, can be used as a negative electrode active material.
Examples of graphite include low crystalline carbon such as soft
carbon and hard carbon and high crystalline carbon such as natural
graphite, kish graphite, pyrolytic graphite, mesophase pitch based
carbon fiber, meso-carbon microbeads (MCMB), mesophase pitches,
petroleum-based or coal-based coke, and the like. Alternatively,
graphene as a carbon material, which will be specifically described
later, may be used. Carbon black such as acetylene black (AB) can
be used. Alternatively, a carbon material such as a carbon
nanotube, graphene, or fullerene may be used.
[0043] The above carbon materials can each function as an active
material and a conductive additive of a negative electrode. Thus,
the active material layer 103 may include one or more of the above
carbon materials. The carbon material can also function as a
conductive additive of a positive electrode. Note that as the
conductive additive, a carbon material with a large specific
surface area is preferably used. The use of a carbon material with
a large specific surface area as the conductive additive can
increase contact points and the contact area of active
materials.
[0044] For example, graphene has excellent electric characteristics
of high conductivity and excellent physical characteristics such as
sufficient flexibility and high mechanical strength. Thus, the use
of graphene as the conductive additive can increase contact points
and the contact area of active materials.
[0045] Graphene in this specification includes single-layer
graphene and multilayer graphene including two to hundred layers.
Single-layer graphene refers to a one-atom-thick sheet of carbon
molecules having .pi. bonds. Graphene oxide refers to a compound
formed by oxidizing the graphene. When graphene oxide is reduced to
form graphene, oxygen contained in the graphene oxide is not
entirely released and part of the oxygen remains in the graphene.
When the graphene contains oxygen, the proportion of the oxygen,
which is measured by XPS, is higher than or equal to 2 at. % and
lower than or equal to 20 at. % of the whole graphene, preferably
higher than or equal to 3 at. % and lower than or equal to 15 at. %
of the whole graphene.
[0046] In the case where graphene is multilayer graphene including
graphene obtained by reducing graphene oxide, the interlayer
distance between graphenes is greater than or equal to 0.34 nm and
less than or equal to 0.5 nm, preferably greater than or equal to
0.38 nm and less than or equal to 0.42 nm, more preferably greater
than or equal to 0.39 nm and less than or equal to 0.41 nm. In
general graphite, the interlayer distance between single-layer
graphenes is 0.34 nm. Since the interlayer distance between the
graphenes used for the power storage device of one embodiment of
the present invention is longer than that in general graphite,
carrier ions can easily transfer between the graphenes in
multilayer graphene.
[0047] The degree of graphitization of a carbon material might
influence the initial irreversible capacity of a power storage
device. The degree of graphitization is expressed by a
ratio/I.sub.1360/I.sub.1580 (also referred to as an R value), which
is the ratio of the peak intensity I.sub.1360 (what is called a D
band) when a Raman shift of the Raman spectrum observed using Raman
spectroscopy is 1360 cm.sup.-1 to the peak intensity I.sub.1580
(what is called a G band) when a Raman shift is 1580 cm.sup.-1. The
smaller the R value, the higher the degree of graphitization. In
other words, the smaller the R value, the higher the
crystallinity.
[0048] When a carbon material with a low degree of graphitization
is included in a negative electrode, the carbon material itself
causes irreversible capacity of a power storage device. Thus, a
carbon material whose R value is less than 1.1, preferably less
than 0.4 is preferably included in a negative electrode active
material layer. When a carbon material with a high degree of
graphitization is included in a negative electrode active material
layer, the initial irreversible capacity of a power storage device
can be low. It is needless to say that a carbon material whose R
value is 1.1 or more may be included in a negative electrode active
material layer as long as the irreversible capacity of a power
storage device is not influenced by the carbon material
content.
[0049] As the negative electrode active material, other than the
above carbon materials, an alloy-based material which enables
charge-discharge reaction by alloying and dealloying reaction with
carrier ions can be used. In the case where carrier ions are
lithium ions, for example, a material containing at least one of
Mg, Ca, Al, Si, Ge, Sn, Pb, As, Sb, Bi, Ag, Au, Zn, Cd, Hg, In,
etc. can be used as the alloy-based material. Such metals 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.
[0050] In the case where the electrode 101 is a positive electrode,
a material into and from which carrier ions can be inserted and
extracted is used for a positive electrode active material. For
example, a compound such as LiFeO.sub.2, LiCoO.sub.2, LiNiO.sub.2,
LiMn.sub.2O.sub.4, V.sub.2O.sub.5, Cr.sub.2O.sub.5, and MnO.sub.2
can be used.
[0051] Alternatively, a lithium-containing complex phosphate
(LiMPO.sub.4 (general formula) (M is one or more of Fe(II), Mn(II),
Co(II), and Ni(II))) can be used. Typical examples of LiMPO.sub.4
(general formula) which can be used as a material are lithium
compounds such as LiFePO.sub.4, LiNiPO.sub.4, LiCoPO.sub.4,
LiMnPO.sub.4, LiFe.sub.aNi.sub.bPO.sub.4,
LiFe.sub.aCo.sub.bPO.sub.4, LiFe.sub.aMn.sub.bPO.sub.4,
LiNi.sub.aCo.sub.bPO.sub.4, LiNi.sub.aMn.sub.bPO.sub.4
(a+b.ltoreq.1, 0<a<1, and 0<b<1),
LiFe.sub.cNi.sub.dCo.sub.ePO.sub.4,
LiFe.sub.cNi.sub.dMn.sub.ePO.sub.4,
LiNi.sub.cCo.sub.dMn.sub.ePO.sub.4 (c+d+e.ltoreq.1, 0<c<1,
0<d<1, and 0<e<1), and
LiFe.sub.fNi.sub.gCo.sub.hMn.sub.iPO.sub.4 (f+g+h+i.ltoreq.1,
0<f<1, 0<g<1, 0<h<1, and 0<i<1).
[0052] Alternatively, a lithium-containing complex silicate such as
Li.sub.(2-j)MSiO.sub.4 (general formula) (M is one or more of
Fe(II), Mn(II), Co(II), and Ni(II); 0.ltoreq.j.ltoreq.2) may be
used. Typical examples of Li.sub.(2-j)MSiO.sub.4 (general formula)
which can be used as a material are lithium compounds such as
Li.sub.(2-j)FeSiO.sub.4, Li.sub.(2-j)NiSiO.sub.4,
Li.sub.(2-j)CoSiO.sub.4, Li.sub.(2-j)MnSiO.sub.4,
Li.sub.(2-j)Fe.sub.kNi.sub.lSiO.sub.4,
Li.sub.(2-j)Fe.sub.kCo.sub.lSiO.sub.4,
Li.sub.(2-j)Fe.sub.kMn.sub.lSiO.sub.4,
Li.sub.(2-j)Ni.sub.kCo.sub.lSiO.sub.4,
Li.sub.(2-j)Ni.sub.kMn.sub.lSiO.sub.4 (k+l.ltoreq.1, 0<k<1,
and 0<l<1), Li.sub.(2-j)Fe.sub.mNi.sub.nCo.sub.qSiO.sub.4,
Li.sub.(2-j)Fe.sub.mNi.sub.nMn.sub.qSiO.sub.4, Li.sub.(2-j)Ni.sub.m
Co.sub.nMn.sub.qSiO.sub.4 (m+n+q.ltoreq.1, 0<m<1,
0<n<1, and 0<q<1), and
Li.sub.(2-j)Fe.sub.rNi.sub.sCo.sub.tMn.sub.uSiO.sub.4
(r+s+t+u.ltoreq.1, 0<r<1, 0<s<1, 0<t<1, and
0<u<1).
[0053] Examples of carrier ions used for a power storage device are
lithium ions, which are a typical example thereof; alkali-metal
ions other than lithium ions; and alkaline-earth metal ions. In the
case where ions other than lithium ions are used as carrier ions,
the following may be used as the positive electrode active
material: a compound which is obtained by substituting an alkali
metal (e.g., sodium or potassium), an alkaline-earth metal (e.g.,
calcium, strontium, barium, beryllium, or magnesium) for lithium in
any of the above lithium compounds, the lithium-containing complex
phosphates, and the lithium-containing complex silicates and a
composite of the obtained compounds.
[0054] FIG. 1B is an enlarged schematic view of a cross section of
the active material layer 103. FIG. 1B illustrates a plurality of
active material particles 111. There is no particular limitation on
the average diameter of the active material particles 111; active
material particles with general average diameter or diameter
distribution are used. In the case where the active material
particles 111 are negative electrode active material particles used
for a negative electrode, negative electrode active material
particles with an average diameter in the range from 1 .mu.m to 50
.mu.m, for example, can be used. In the case where the active
material particles 111 are positive electrode active material
particles used for a positive electrode and each of the positive
electrode active material particles is a secondary particle, the
average diameter of primary particles composing the secondary
particle can be in the range from 10 nm to 1 .mu.m.
[0055] The plurality of active material particles 111 are in
contact with each other; thus, the surface of one of the active
material particles 111 has a region in contact with one of the
other active material particles 111. The surface of the active
material particle except the region is partly or entirely covered
with a film 112. In is preferable that the film 112 cover the
entire surface except the region where a plurality of the active
material particles 111 are in contact with each other; however, it
may partly covers the surface. Further, the plurality of active
material particles 111 are bound with a binder 113; accordingly,
the film 112 is also in contact with the binder 113. In some cases,
the active material layer 103 includes a space 114 formed by a
plurality of the active material particles 111.
[0056] As the binder 113, a material which can bind the active
material, the conductive additive, and the current collector is
used. For example, any of the following can be used as the binder
113: resin materials such as poly(vinylidene fluoride) (PVDF), a
vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene
fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer
rubber, polytetrafluoroethylene, polypropylene, polyethylene, and
polyimide.
[0057] It is preferable that carrier ions can pass through the film
112. Thus, it is preferable that the film 112 be formed using a
material through which carrier ions can pass, and be thin enough to
allow carrier ions to pass through the film.
[0058] A film containing metal oxide or silicon oxide as a main
component can be used as the film 112. For the film containing
metal oxide as a main component, an oxide film of any one of
niobium, titanium, vanadium, tantalum, tungsten, zirconium,
molybdenum, hafnium, chromium, and aluminum or an oxide film
containing lithium and one or more of these elements can be used.
Alternatively, a film containing silicon oxide as a main component
can be used. Note that "main component" refers to an element
determined by energy dispersive X-ray spectrometry (EDX).
[0059] For example, in the case where graphite is used as an active
material, a film containing silicon oxide as a main component
preferably has a mesh structure where a carbon atom in the graphite
is bonded to a silicon atom through an oxygen atom and the silicon
atom is bonded to another silicon atom through an oxygen atom.
[0060] The thickness of the film 112 is, for example, preferably 1
nm to 10 .mu.m, more preferably 10 nm to 1 .mu.m.
[0061] Further, the product of the electric resistivity and the
thickness of the film 112 at 25.degree. C. is greater than or equal
to 20 .OMEGA.mm, preferably greater than or equal to 200 .OMEGA.mm.
When the product of the electric resistivity and the thickness of
the film 112 at 25.degree. C. is greater than or equal to 20
.OMEGA.mm, the decomposition reaction between the active material
particle 111 and an electrolytic solution can be reduced. Further,
when the product of the electric resistivity and the thickness of
the film 112 at 25.degree. C. is greater than or equal to 200
.OMEGA.mm, the decomposition reaction between the active material
particle 111 and an electrolytic solution can be inhibited. When
the product of the electric resistivity and the thickness of the
film 112 at 25.degree. C. is greater than or equal to 20 .OMEGA.mm,
electrons can be prevented from being supplied into the interface
between the surface of the negative electrode active material and
an electrolytic solution on charge and discharge of a power storage
device, so that the decomposition of the electrolytic solution can
be inhibited. Thus, the irreversible decomposition reaction can be
inhibited.
[0062] In the case of using, as the active material particle 111,
an active material particle whose volume is changed on charge and
discharge, the film 112 is preferably changed in shape accordingly
when the active material particle 111 is changed in shape because
of the change in volume thereof. Therefore, the Young's modulus of
the film 112 is preferably less than or equal to 70 GPa. The film
112 which covers part or the entire of the surface of the active
material particle 111 can be changed following a change in shape
due to the change in volume of the active material particle 111, so
that separation of the film 112 from the active material particle
111 can be suppressed.
[0063] The plurality of active material particles 111 are in
contact with each other and thus the surface of one of the active
material particles 111 has a region in contact with one of the
other active material particles 111, and the surface of the active
material particle except the region is partly or entirely covered
with the film 112, whereby the reductive decomposition of an
electrolytic solution can be inhibited. Accordingly, formation of a
passivating film on the active material particle due to the
reductive decomposition of the electrolytic solution can be
inhibited, resulting in inhibition of a reduction in the initial
capacity of a power storage device.
[0064] The film 112 which covers the surfaces of the active
material particles 111 can be changed in shape accordingly when the
active material particle 111 is changed in shape because of the
change in volume thereof, so that separation of the film 112 from
the active material particle 111 can be prevented. Further, when an
increase in the thickness of a passivating film on repeated charge
and discharge is inhibited, the passivating film is less likely to
be influenced by volume expansion of the active material particle,
so that separation of the passivating film from the active material
particle 111 can be suppressed.
[0065] Here, a conduction path of electrons of the plurality of
active material particles 111 will be described. As illustrated in
FIG. 1C1, when the active material particles 111 each covered with
the film 112 are in contact with each other, the film inhibits
electron conduction, resulting in an increase in the resistance of
the electrode and a reduction in the substantial capacity of a
power storage device.
[0066] In contrast, when the surface of one of the active material
particles 111 has a region in contact with one of the other active
material particles and the surface except the region is partly or
entirely covered with the film 112 as illustrated in FIG. 1C2,
electron conduction can be prevented from being inhibited by the
film. Accordingly, an increase in the resistance of an electrode
can be suppressed and the capacity of a power storage device can be
increased.
[0067] As described above, in the electrode for a power storage
device of one embodiment of the present invention, the surface of
the active material particle 111 is stabilized by the film 112
which covers part or the entire of the surface of the active
material particle 111, resulting in minimization of the
electrochemical decomposition of an electrolytic solution and the
like around the electrode.
[0068] Further, when the electrode for a power storage device is
used in a power storage device such as a lithium-ion battery or a
lithium-ion capacitor to minimize the decomposition reaction of an
electrolytic solution as a side reaction of charge and discharge in
the repeated charge and discharge cycles of the power storage
device, the long-term cycle performance of the power storage device
such as a lithium-ion battery or a lithium-ion capacitor can be
improved.
[0069] This embodiment can be implemented in combination with any
of the other embodiments and examples as appropriate.
Embodiment 2
[0070] In this embodiment, an example of a manufacturing method of
an electrode for a power storage device will be described with
reference to FIG. 2.
[0071] First, an active material, a binder, and a solvent are mixed
to form a slurry (Step 151). For the active material and the
binder, any of the materials given in Embodiment 1 can be used. For
the solvent, N-methylpyrrolidone (NMP) can be used. In this
embodiment, graphite, PVDF, and NMP are used as the active
material, the binder, and the solvent, respectively. Note that the
slurry may contain a conductive additive.
[0072] Then, the slurry is applied to one of or both the surfaces
of a current collector, and dried (Step 152). In the case where
both the surfaces of the current collector are subjected to the
coating step, the slurry is applied to the surfaces at the same
time or one by one, and dried. After that, rolling with a roller
press machine is performed, whereby active material layers are
formed so that the current collector is sandwiched
therebetween.
[0073] In this embodiment, the film is formed on the active
material by a liquid-phase method such as a dip coating method.
[0074] First, an organometallic compound or an organosilicon
compound, a solvent, and a catalyst are mixed to prepare a
treatment liquid (Step 153).
[0075] Examples of the organometallic compound are an organic
aluminum compound and an organogallium compound. Examples of the
organosilicon compound are ethyl polysilicate, methyl polysilicate,
propyl polysilicate, butyl polysilicate, tetramethoxysilane,
tetraethoxysilane, tetrabutoxysilane, and tetrapropoxysilane.
Further, an oligomer obtained by partial hydrolysis and
condensation of any of the organosilicon compounds may be used. An
organic composite metal compound containing a lithium compound such
as an organic lithium silicate compound or an organic lithium
aluminate compound may be used.
[0076] In the case of using the organosilicon compound, the
concentration of silicon oxide contained in the treatment liquid
is, for example, greater than or equal to 0.1 wt % and less than or
equal to 40 wt %, preferably greater than or equal to 0.8 wt % and
less than or equal to 20 wt %.
[0077] In this embodiment, ethyl silicate as Pentamer is used as
the organosilicon compound.
[0078] As the solvent, ethanol can be used, and as the catalyst,
hydrochloric acid can be used. Further, water may be added as an
additive.
[0079] Next, the active material layer formed over the current
collector is soaked in the treatment liquid either in a vacuum or
in the air (Step 154).
[0080] Then, the active material layer formed over the current
collector is taken out of the treatment liquid, and the solvent in
the treatment liquid permeating the active material layer is
evaporated (Step 155).
[0081] After that, heat treatment is performed on the active
material layer formed over the current collector (Step 156). The
heat treatment is performed, for example, on a hot plate at
70.degree. C. Through the heat treatment, the organometallic
compound or the organosilicon compound attached to the active
material layer reacts with moisture in the air, so that hydrolysis
occurs, and the organometallic compound or the organosilicon
compound after the hydrolysis is condensed in association with the
hydrolysis. Consequently, a film containing metal oxide or silicon
oxide as a main component is formed on the surface of the active
material. Further, when an enclosed space to which water is added
with water vapor is used, time for hydrolysis can be shortened.
[0082] In the active material layer, the plurality of active
material particles are in contact with each other and bound with
the binder. The active material layer in this state is soaked in
the treatment liquid containing the organometallic compound or the
organosilicon compound, whereby the treatment liquid permeates the
whole active material layer while the plurality of active material
particles remain in contact with each other. After that, heat
treatment is performed so that hydrolysis and condensation reaction
of the organometallic compound or the organosilicon compound
occurs, whereby metal oxide films or silicon oxide films can be
formed on the surfaces of the plurality of active material
particles. In this embodiment, silicon oxide films are formed on
the surfaces of the plurality of active material particles.
[0083] For example, in the case where graphite is used as an active
material, a film containing silicon oxide as a main component
preferably has a mesh structure where a carbon atom in the graphite
is bonded to a silicon atom through an oxygen atom and the silicon
atom is bonded to another silicon atom through an oxygen atom.
[0084] For example, in the case where films are formed on the
surfaces of active material particles and then a slurry is formed
to form an active material layer, the films formed on the active
material particles are in contact with each other, so that electron
conduction might be inhibited and thus the resistance of an
electrode might be increased. Consequently, the substantial
capacity of a power storage device might be reduced.
[0085] As described in this embodiment, when an active material
layer is formed over a current collector and then a film is formed
on the active material, the surfaces of the plurality of active
material particles except regions where a plurality of the active
material particles are in contact with each other can be partly or
entirely covered with the film while the plurality of active
material particles remain in contact with each other. Thus, an
increase in the resistance of an electrode due to contact between
films formed on the active material particles can be prevented,
leading to suppression of a reduction in the capacity of a power
storage device.
[0086] The use of a liquid-phase method such as a dip coating
method enables the treatment liquid to permeate the whole active
material layer and enter a space formed by a plurality of the
active material particles. Through hydrolysis and condensation
reaction after the permeation and the entry of the treatment
liquid, the metal oxide film can also be formed in the space formed
by the plurality of the active material particles. Further, the
surfaces of the active material particles can be prevented from
being exposed; therefore, the area of contact between the active
material particles and the electrolytic solution can be reduced.
Consequently, the decomposition of the electrolytic solution can be
inhibited, resulting in prevention of formation of a passivating
film.
[0087] This embodiment can be implemented in combination with any
of the other embodiments and examples as appropriate.
Embodiment 3
[0088] In this embodiment, a power storage device including an
electrode for a power storage device and a manufacturing method of
the power storage device will be described with reference to FIGS.
3A to 7B.
[0089] FIG. 3A is a cross-sectional view of a negative electrode
200 which includes a negative electrode current collector 201 and a
negative electrode active material layer 202 provided over one of
surfaces of the negative electrode current collector 201 or
negative electrode active material layers 202 provided so that the
negative electrode current collector 201 is sandwiched
therebetween. In the drawing, the negative electrode active
material layers 202 are provided so that the negative electrode
current collector 201 is sandwiched therebetween.
[0090] In this embodiment, an example where graphene is used as a
conductive additive added to the negative electrode active material
layer 202 will be described with reference to FIG. 3B.
[0091] FIG. 3B is an enlarged schematic view of a cross section of
the negative electrode active material layer 202 including
graphene. The negative electrode active material layer 202 includes
a plurality of negative electrode active material particles 211, a
film 212, a binder 213, and graphenes 215. The graphenes 215 each
are a thin sheet with a thickness of several micrometers to several
tens of micrometers and thus can cover a plurality of the negative
electrode active material particles 211. The graphenes 215 appear
linear in cross section. One graphene or a plurality of the
graphenes overlap with a plurality of the negative electrode active
material particles 211, or the plurality of the negative electrode
active material particles 211 are at least partly surrounded with
one graphene or a plurality of the graphenes. Note that the
graphene 215 has a bag-like shape, and a plurality of the negative
electrode active materials are at least partly surrounded with the
graphene 215 with the bag-like shape in some cases. The graphene
215 partly has openings where the negative electrode active
material particles 211 are exposed in some cases.
[0092] An example of the negative electrode active material
particle 211 is a material whose volume is expanded by reception of
carrier ions. When such a material is used, the negative electrode
active material layer gets vulnerable and is partly collapsed on
charge and discharge, resulting in lower reliability (e.g.,
inferior cycle characteristics) of a power storage device. However,
the graphene 215 covering the periphery of the negative electrode
active material particles 211 can prevent dispersion of the
negative electrode active material particles 211 and the collapse
of the negative electrode active material layer 202, even when the
volume of the negative electrode active material particles is
increased and decreased on charge and discharge. That is to say,
the graphene 215 has a function of maintaining the bond between the
negative electrode active material particles 211 even when the
volume of the negative electrode active material particles 211 is
increased and decreased on charge and discharge.
[0093] Further, the plurality of negative electrode active material
particles 211 are bound with the binder 213. The negative electrode
active material particles 211 may be bound with the binder 213 in
the state where they are in contact with each other or in the state
where they are bonded to each other with the graphene 215
interposed therebetween. Although FIG. 3B illustrates the case
where the binder 213 is used, the binder 213 does not necessarily
have to be added in the case where the graphenes 215 are included
so many as to sufficiently function as a binder by being bound with
each other.
[0094] That is to say, when a binder is not used in forming the
negative electrode active material layer 202, the proportion of the
negative electrode active material particles in the negative
electrode active material layer 202 with certain weight (certain
volume) can be increased, leading to an increase in charge and
discharge capacity per unit weight (unit volume) of the
electrode.
[0095] The graphene 215 efficiently forms a sufficient electron
conductive path in the negative electrode active material layer
202, so that the conductivity of the negative electrode for a power
storage device can be increased.
[0096] The graphene 215 also functions as a negative electrode
active material capable of receiving and releasing carrier ions,
leading to an increase in the charge and discharge capacity of the
negative electrode for a power storage device.
[0097] Further, as illustrated in FIG. 3B, an exposed surface of
the negative electrode active material particle 211 is partly or
entirely covered with the film 212. The film 212 does not prevent
contact between the negative electrode active material particles
211.
[0098] In the negative electrode illustrated in FIG. 3B, the
plurality of negative electrode active material particles 211 are
in contact with each other and the surfaces of the plurality of
negative electrode active material particles 211 except regions
where a plurality of the negative electrode active material
particles 211 are in contact with each other are partly or entirely
covered with the film 212, whereby the reductive decomposition of
an electrolytic solution can be inhibited. Accordingly, formation
of a passivating film on the negative electrode active material
particles 211 due to the reductive decomposition of the
electrolytic solution can be inhibited, resulting in suppression of
a reduction in the initial capacity of a power storage device.
[0099] Having flexibility, the graphenes 215 and the film 212 can
be changed in shape accordingly when the volume of the negative
electrode active material particles 211 expands because of
reception of carrier ions. Thus, separation of the graphenes 215
and the film 212 from the negative electrode active material
particles 211 can be prevented. Further, when an increase in the
thickness of the film 212 on repeated charge and discharge is
inhibited, the film 212 is less likely to be influenced by volume
expansion of the negative electrode active material particles, so
that separation of the film 212 from the negative electrode active
material particles 211 can be suppressed.
[0100] As described above, in the negative electrode 200, the
surfaces of the negative electrode active material particles 211
are stabilized by the film 212 which covers part or the entire of
the surfaces of the negative electrode active material particles
211, resulting in minimization of the electrochemical decomposition
of an electrolytic solution and the like around the negative
electrode 200.
[0101] Further, when the electrode for a power storage device is
used in a power storage device such as a lithium-ion battery or a
lithium-ion capacitor in order to minimize the decomposition
reaction of an electrolytic solution as a side reaction of charge
and discharge in the repeated charge and discharge cycles of the
power storage device, the long-term cycle performance of the power
storage device such as a lithium-ion battery or a lithium-ion
capacitor can be improved.
[0102] The desired thickness of the negative electrode active
material layer 202 is determined in the range from 20 .mu.m to 150
.mu.m.
[0103] The negative electrode active material layer 202 may be
predoped with lithium in such a manner that a lithium layer is
formed on a surface of the negative electrode active material layer
202 by a sputtering method. Alternatively, lithium foil is provided
on the surface of the negative electrode active material layer 202,
whereby the negative electrode active material layer 202 can be
predoped with lithium.
[0104] Next, a formation method of the negative electrode active
material layer 202 in FIGS. 3A and 3B will be described.
[0105] First, the plurality of negative electrode active material
particles 211 are mixed into a dispersion containing graphene
oxides and then a binder is mixed into the mixture to form a
slurry. By mixing the dispersion containing graphene oxides and the
plurality of negative electrode active material particles 211
first, the graphene oxides can be dispersed uniformly. Since the
binder is added in the state where the graphene oxides are
dispersed uniformly, contact between the negative electrode active
material particles 211 and the graphene oxides can be prevented
from being obstructed by the binder. Note that the binder does not
necessarily have to be added.
[0106] Then, the slurry is applied to one of or both the surfaces
of the negative electrode current collector 201, and dried. After
that, rolling with a roller press machine is performed.
[0107] After that, the graphene oxides are electrochemically
reduced with electric energy or thermally reduced by heat treatment
to form the graphenes 215, and the negative electrode active
material layer 202 including the plurality of negative electrode
active material particles 211 and the graphenes 215 is formed over
the negative electrode current collector 201. Particularly in the
case where electrochemical reduction treatment is performed, the
proportion of .pi. bonds of graphene formed by the electrochemical
reduction treatment is higher than that of graphene formed by heat
treatment; therefore, the graphenes 215 having high conductivity
can be formed.
[0108] After that, the negative electrode active material layer 202
formed over the negative electrode current collector 201 is soaked
in a treatment liquid containing an organometallic compound or an
organosilicon compound, whereby the treatment liquid permeates the
negative electrode active material layer 202. The details of the
treatment liquid are described in Embodiment 2.
[0109] Then, the negative electrode active material layer 202 is
taken out of the treatment liquid, and the solvent in the treatment
liquid permeating the negative electrode active material layer 202
is evaporated. After that, heat treatment is performed on the
negative electrode active material layer 202 formed over the
negative electrode current collector 201. Through the heat
treatment, the organometallic compound or the organosilicon
compound attached to the negative electrode active material layer
202 reacts with moisture in the air, so that hydrolysis occurs, and
the organometallic compound or the organosilicon compound after the
hydrolysis is condensed in association with the hydrolysis.
Consequently, the film 212 containing metal oxide or silicon oxide
as a main component is formed on the surfaces of the negative
electrode active material particles 211.
[0110] Through the above steps, the negative electrode 200 in which
the negative electrode active material layer 202 including the
plurality of negative electrode active material particles 211, the
film 212, the binder 213, and the graphenes 215 is formed over the
negative electrode current collector 201 can be formed. Although
the case where the negative electrode active material layer 202
includes the graphenes 215 is described in this embodiment, the
negative electrode active material layer 202 does not necessarily
include the graphenes 215.
[0111] Next, a positive electrode and a formation method thereof
will be described with reference to FIGS. 4A and 4B.
[0112] FIG. 4A is a cross-sectional view of a positive electrode
250 which includes a positive electrode current collector 251 and a
positive electrode active material layer 252 provided over one of
surfaces of the positive electrode current collector 251 or
positive electrode active material layers 252 provided so that the
positive electrode current collector 251 is sandwiched
therebetween. In the drawing, the positive electrode active
material layers 252 are provided so that the positive electrode
current collector 251 is sandwiched therebetween.
[0113] The positive electrode active material layer 252 does not
necessarily have to be formed on and in direct contact with the
positive electrode current collector 251. Any of the following
functional layers may be formed using a conductive material such as
a metal between the positive electrode current collector 251 and
the positive electrode active material layer 252: an adhesion layer
for increasing the adhesion between the positive electrode current
collector 251 and the positive electrode active material layer 252;
a planarization layer for reducing the roughness of the surface of
the positive electrode current collector 251; a heat dissipation
layer; a stress relaxation layer for relaxing the stress on the
positive electrode current collector 251 or the positive electrode
active material layer 252; and the like.
[0114] In this embodiment, an example where graphenes are used as a
conductive additive added to the positive electrode active material
layer 252 will be described with reference to FIG. 4B.
[0115] FIG. 4B is an enlarged schematic view of a cross section of
the positive electrode active material layer 252 including
graphene. The positive electrode active material layer 252 includes
a plurality of positive electrode active material particles 261, a
film 262, a binder 263, and graphenes 265. The graphenes 265 each
are a thin sheet with a thickness of several micrometers to several
tens of micrometers and thus can cover a plurality of the positive
electrode active material particles 261. The graphenes 265 appear
linear in cross section. A plurality of the positive electrode
active material particles are at least partly surrounded with one
graphene or a plurality of the graphenes or sandwiched between a
plurality of the graphenes. Note that the graphene has a bag-like
shape, and a plurality of the positive electrode active material
particles are surrounded with the graphene with the bag-like shape
in some cases. In addition, part of the positive electrode active
material particles is not covered with the graphenes 265 and
exposed in some cases.
[0116] The positive electrode active material layer 252 includes
positive electrode active material particles 261 which are capable
of receiving and releasing carrier ions, and graphenes 265 which
cover a plurality of the positive electrode active material
particles 261 and at least partly surround the plurality of the
positive electrode active material particles 261. The different
graphenes 265 cover the surfaces of the plurality of the positive
electrode active material particles 261. The positive electrode
active material particles 261 may partly be exposed.
[0117] The size of the positive electrode active material particle
261 is preferably greater than or equal to 20 nm and less than or
equal to 100 nm. Note that the size of the positive electrode
active material particle 261 is preferably smaller because
electrons transfer in the positive electrode active material
particles 261.
[0118] Sufficient characteristics can be obtained even when the
surface of the positive electrode active material particle 261 is
not covered with a graphite layer; however, it is preferable to use
both the graphene and the positive electrode active material
particle covered with a graphite layer because current flows.
[0119] Further, the plurality of positive electrode active material
particles 261 are bound with the binder 263. The positive electrode
active material particles 261 may be bound with the binder 263 in
the state where they are in contact with each other or in the state
where they are bonded to each other with the graphene 265
interposed therebetween. Although FIG. 4B illustrates the case
where the binder 263 is used, the binder 263 does not necessarily
have to be added in the case where the graphenes 265 are included
so many as to sufficiently function as a binder by being bound with
each other.
[0120] That is to say, when a binder is not used in forming the
positive electrode active material layer 252, the proportion of the
positive electrode active material particles in the positive
electrode active material layer 252 with certain weight (certain
volume) can be increased, leading to an increase in charge and
discharge capacity per unit weight (unit volume) of the
electrode.
[0121] The graphene 265 efficiently forms a sufficient electron
conductive path in the positive electrode active material layer
252, so that the conductivity of the positive electrode for a power
storage device can be increased.
[0122] As illustrated in FIG. 4B, an exposed surface of the
positive electrode active material particle 261 is partly or
entirely covered with the film 262. The film 262 does not prevent
contact between the positive electrode active material particles
261.
[0123] In the positive electrode illustrated in FIG. 4B, the
plurality of positive electrode active material particles 261 are
in contact with each other and the surfaces of the plurality of
positive electrode active material particles 261 except regions
where a plurality of the positive electrode active material
particles 261 are in contact with each other are partly or entirely
covered with the film 262, whereby the oxidative decomposition of
an electrolytic solution can be inhibited. Accordingly, formation
of a passivating film on the positive electrode active material
particles 261 due to the oxidative decomposition of the
electrolytic solution can be inhibited, resulting in suppression of
a reduction in the initial capacity of a power storage device.
[0124] Having flexibility, the graphenes 265 and the film 262 can
be changed in shape accordingly following expansion of the volume
of the positive electrode active material particles 261 due to
reception of carrier ions. Thus, separation of the graphenes 265
and the film 262 from the positive electrode active material
particles 261 can be prevented.
[0125] The desired thickness of the positive electrode active
material layer 252 is determined in the range from 20 .mu.m to 100
.mu.m. It is preferable to adjust the thickness of the positive
electrode active material layer 252 as appropriate so that a crack
and flaking are not caused.
[0126] Note that the positive electrode active material layer 252
may include acetylene black particles having a volume 0.1 times to
10 times as large as that of the graphene, carbon particles having
a one-dimensional expansion such as carbon nanofibers, or other
known conductive additives.
[0127] Depending on a material of positive electrode active
material particles, the volume is expanded because of reception of
ions serving as carriers. When such a material is used, a positive
electrode active material layer gets vulnerable and is partly
collapsed on charge and discharge, resulting in lower reliability
of a power storage device. However, graphene covering the periphery
of positive electrode active material particles allows prevention
of dispersion of the positive electrode active material particles
and the collapse of a positive electrode active material layer,
even when the volume of the positive electrode active material
particles is increased and decreased on charge and discharge. That
is to say, the graphene has a function of maintaining the bond
between the positive electrode active material particles even when
the volume of the positive electrode active material particles is
increased and decreased on charge and discharge.
[0128] The graphene 265 is in contact with a plurality of the
positive electrode active material particles and serves also as a
conductive additive. Further, the graphene 265 has a function of
holding the positive electrode active material particles capable of
receiving and releasing carrier ions. Thus, a binder does not have
to be mixed into the positive electrode active material layer.
Accordingly, the proportion of the positive electrode active
material particles in the positive electrode active material layer
can be increased, which allows an increase in charge and discharge
capacity of a power storage device.
[0129] Next, a method for forming the positive electrode active
material layer 252 will be described.
[0130] First, a slurry containing positive electrode active
material particles and graphene oxide is formed. Then, the slurry
is applied to the positive electrode current collector 251. After
that, heating is performed in a reducing atmosphere for reduction
treatment so that the positive electrode active material particles
are baked and part of oxygen is released from graphene oxide to
form graphene. Note that oxygen in the graphene oxide might not be
entirely released and partly remains in the graphene. Through the
above steps, the positive electrode active material layer 252 can
be provided over the positive electrode current collector 251.
Consequently, the positive electrode active material layers 252
have higher conductivity.
[0131] Graphene oxide contains oxygen and thus is negatively
charged in a polar liquid. As a result of being negatively charged,
graphene oxide is dispersed in the polar liquid. Accordingly, the
positive electrode active material particles contained in the
slurry are not easily aggregated, so that the size of the positive
electrode active material particle can be prevented from
increasing. Thus, the transfer of electrons in the positive
electrode active material particles is facilitated, resulting in an
increase in conductivity of the positive electrode active material
layer.
[0132] Subsequently, the positive electrode active material layer
252 formed over the positive electrode current collector 251 is
soaked in a treatment liquid containing an organometallic compound,
whereby the treatment liquid permeates the positive electrode
active material layer 252. The details of the treatment liquid are
described in Embodiment 2.
[0133] Then, the positive electrode active material layer 252 is
taken out of the treatment liquid, and the solvent in the treatment
liquid permeating the positive electrode active material layer 252
is evaporated. After that, heat treatment is performed on the
positive electrode active material layer 252 formed over the
positive electrode current collector 251. Through the heat
treatment, the organometallic compound attached to the positive
electrode active material layer 252 reacts with moisture in the
air, so that hydrolysis occurs, and the organometallic compound
after the hydrolysis is condensed in association with the
hydrolysis. Consequently, the film 262 containing metal oxide as a
main component is formed on the surfaces of the positive electrode
active material particles 261.
[0134] Through the above steps, the positive electrode 250 in which
the positive electrode active material layer 252 including the
plurality of positive electrode active material particles 261, the
film 262, the binder 263, and the graphenes 265 is formed over the
positive electrode current collector 251 can be formed. Although
the case where the positive electrode active material layer 252
includes the graphenes 265 is described in this embodiment, the
positive electrode active material layer 252 does not necessarily
include the graphenes 265.
[0135] Next, a power storage device and a manufacturing method
thereof will be described. Here, the structure and a manufacturing
method of a lithium-ion battery, which is one mode of the power
storage device, will be described with reference to FIGS. 5A to 7B.
Here, a cross-sectional structure of the lithium-ion battery will
be described below.
(Coin-Type Secondary Battery)
[0136] FIG. 5A is an external view of a coin-type (single-layer
flat type) secondary battery. FIG. 5B is a cross-sectional view of
the coin-type secondary battery.
[0137] In a coin-type lithium-ion 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. A separator 310 and an
electrolytic solution (not illustrated) are provided between the
positive electrode active material layer 306 and the negative
electrode active material layer 309.
[0138] As at least one of the positive electrode 304 and the
negative electrode 307, the electrode for a power storage device of
one embodiment of the present invention can be used.
[0139] Next, as the separator 310, a porous insulator such as
cellulose, polypropylene (PP), polyethylene (PE), polybutene,
nylon, polyester, polysulfone, polyacrylonitrile, polyvinylidene
fluoride, or tetrafluoroethylene can be used. Alternatively,
nonwoven fabric of a glass fiber or the like, or a diaphragm in
which a glass fiber and a polymer fiber are mixed may be used.
[0140] As a solvent for the electrolytic solution, an aprotic
organic solvent is preferably used. For example, one of ethylene
carbonate (EC), propylene carbonate (PC), butylene carbonate,
chloroethylene carbonate, vinylene carbonate,
.gamma.-butyrolactone, .gamma.-valerolactone, dimethyl carbonate
(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC),
methyl formate, methyl acetate, methyl butyrate, 1,3-dioxane,
1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl
ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran,
sulfolane, and sultone can be used, or two or more of these
solvents can be used in an appropriate combination in an
appropriate ratio.
[0141] When a gelled high-molecular material is used as the solvent
for the electrolytic solution, safety against liquid leakage and
the like is improved. Further, a secondary battery can be thinner
and more lightweight. Typical examples of the gelled high-molecular
material include a silicone gel, an acrylic gel, an acrylonitrile
gel, polyethylene oxide, polypropylene oxide, a fluorine-based
polymer, and the like.
[0142] Alternatively, the use of one or more of ionic liquids (room
temperature ionic liquids) which has non-flammability and
non-volatility as the solvent for the electrolytic solution can
prevent the secondary battery from exploding or catching fire even
when the secondary battery internally shorts out or the internal
temperature increases because of overcharging or the like.
[0143] As an electrolyte dissolved in the above 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.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.
[0144] As an electrolyte of the electrolytic solution, a material
which contains carrier ions is used. Typical examples of the
electrolyte of the electrolytic solution include lithium salts such
as LiClO.sub.4, LiAsF.sub.6, LiBF.sub.4, LiPF.sub.6, and
Li(C.sub.2F.sub.5SO.sub.2).sub.2N.
[0145] Note that when carrier ions are alkali metal ions other than
lithium ions, or alkaline-earth metal ions, instead of lithium in
the above lithium salts, an alkali metal (e.g., sodium or
potassium), an alkaline-earth metal (e.g., calcium, strontium,
barium, beryllium, or magnesium) may be used for the
electrolyte.
[0146] Instead of the electrolytic 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 macromolecular material such as a
polyethylene oxide (PEO)-based macromolecular material may
alternatively be used. When the solid electrolyte is used, a
separator is not necessary. Further, the battery can be entirely
solidified; therefore, there is no possibility of liquid leakage
and thus the safety of the battery is dramatically increased.
[0147] For the positive electrode can 301 and the negative
electrode can 302, a metal having corrosion resistance to an
electrolytic solution, such as nickel, aluminum, or titanium, an
alloy of such metals, or an alloy of such a metal and another metal
(stainless steel or the like) can be used. Alternatively, it is
preferable to cover the positive electrode can 301 and the negative
electrode can 302 with nickel, aluminum, or the like in order to
prevent corrosion due to the electrolytic 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.
[0148] The negative electrode 307, the positive electrode 304, and
the separator 310 are immersed in the electrolytic solution. Then,
as illustrated in FIG. 5B, the positive electrode 304, the
separator 310, the negative electrode 307, and the negative
electrode can 302 are stacked in this order with the positive
electrode can 301 positioned at the bottom, and the positive
electrode can 301 and the negative electrode can 302 are subjected
to pressure bonding with the gasket 303 interposed therebetween. In
such a manner, the coin-type lithium-ion battery 300 can be
fabricated.
(Laminated Secondary Battery)
[0149] Next, an example of a laminated secondary battery will be
described with reference to FIG. 6.
[0150] A laminated lithium-ion battery 400 illustrated in FIG. 6
can be obtained in such a manner that a positive electrode 403
including a positive electrode current collector 401 and a positive
electrode active material layer 402, a separator 407, and a
negative electrode 406 including a negative electrode current
collector 404 and a negative electrode active material layer 405
are stacked and sealed in an exterior body 409 and then an
electrolytic solution 408 is injected into the exterior body 409.
Although the laminated lithium-ion battery 400 in FIG. 6 has a
structure where one sheet-like positive electrode 403 and one
sheet-like negative electrode 406 are stacked, it is preferable to
roll the stack or to stack a plurality of the stacks and then
laminate them in order to increase battery capacity. Particularly
in the case of the laminated lithium-ion battery, the battery has
flexibility and thus is suitable for applications which require
flexibility.
[0151] In the laminated lithium-ion battery 400 illustrated in FIG.
6, the positive electrode current collector 401 and the negative
electrode current collector 404 also function as terminals for
electrical contact with an external portion. For this reason, each
of the positive electrode current collector 401 and the negative
electrode current collector 404 is provided so as to be partly
exposed on the outside of the exterior body 409.
[0152] As the exterior body 409 in the laminated lithium-ion
battery 400, for example, a laminate film having a three-layer
structure where a highly flexible metal thin film of aluminum,
stainless steel, copper, nickel, or the like is provided over a
film formed of a material such as polyethylene, polypropylene,
polycarbonate, ionomer, or polyamide, and an insulating synthetic
resin film of a polyamide resin, a polyester resin, or the like is
provided as the outer surface of the exterior body over the metal
thin film can be used. With such a three-layer structure,
permeation of an electrolytic solution and a gas can be blocked and
an insulating property and resistance to the electrolytic solution
can be obtained.
[0153] The positive electrode 403 and the negative electrode 406
and the laminated lithium-ion battery 400 are formed in a manner
similar to those of the positive electrode and the negative
electrode in the above coin-type lithium-ion battery and the
coin-type lithium-ion battery
(Cylindrical Secondary Battery)
[0154] Next, an example of a cylindrical secondary battery will be
described with reference to FIGS. 7A and 7B. As illustrated in FIG.
7A, a cylindrical lithium-ion battery 500 includes a positive
electrode cap (battery cap) 501 on the top surface and a battery
can (outer can) 502 on the side surface and bottom surface. The
positive electrode cap 501 and the battery can 502 are insulated
from each other by a gasket (insulating gasket) 510.
[0155] FIG. 7B is a diagram schematically illustrating a cross
section of the cylindrical lithium-ion battery. Inside the battery
can 502 having a hollow cylindrical shape, a battery element in
which a strip-like positive electrode 504 and a strip-like negative
electrode 506 are wound with a stripe-like separator 505 interposed
therebetween is provided. Although not illustrated, the battery
element is wound around a center pin. One end of the battery can
502 is close and the other end thereof is open. For the battery can
502, a metal having corrosion resistance to an electrolytic
solution, such as nickel, aluminum, or titanium, an alloy of such a
metal, or an alloy of such a metal and another metal (stainless
steel or the like) can be used. Further, it is preferable to cover
the metal or the like with nickel, aluminum, or the like in order
to prevent corrosion by the electrolytic solution. Inside the
battery can 502, the battery element in which the positive
electrode, the negative electrode, and the separator are wound is
interposed between a pair of insulating plates 508 and 509 which
face each other. Further, an electrolytic solution (not
illustrated) is injected inside the battery can 502 provided with
the battery element. As the electrolytic solution, an electrolytic
solution which is similar to those in the above coin-type secondary
battery and the above laminated secondary battery can be used.
[0156] Although the positive electrode 504 and the negative
electrode 506 can be formed in a manner similar to that of the
positive electrode and the negative electrode of the coin-type
lithium-ion battery described above, the difference lies in that,
since the positive electrode and the negative electrode of the
cylindrical lithium-ion battery are wound, active materials are
formed on both sides of the current collectors. A positive
electrode terminal (positive electrode current collecting lead) 503
is connected to the positive electrode 504, and a negative
electrode terminal (negative electrode current collecting lead) 507
is connected to the negative electrode 506. Both the positive
electrode terminal 503 and the negative electrode terminal 507 can
be formed using a metal material such as aluminum. The positive
electrode terminal 503 and the negative electrode terminal 507 are
resistance-welded to a safety valve mechanism 512 and the bottom of
the battery can 502, respectively. The safety valve mechanism 512
is electrically connected to the positive electrode cap 501 through
a positive temperature coefficient (PTC) element 511. The safety
valve mechanism 512 cuts off electrical connection between the
positive electrode cap 501 and the positive electrode 504 when the
internal pressure of the battery exceeds a predetermined threshold
value. Further, the PTC element 511, which serves as a thermally
sensitive resistive element whose resistance increases as
temperature rises, limits the amount of current by increasing the
resistance, in order to prevent abnormal heat generation. Note that
barium titanate (BaTiO.sub.3)-based semiconductor ceramic or the
like can be used for the PTC element.
[0157] Note that in this embodiment, the coin-type lithium-ion
battery, the laminated lithium-ion battery, and the cylindrical
lithium-ion battery are given as examples of the lithium-ion
battery; however, any of lithium-ion batteries with a variety of
shapes, such as a sealed lithium-ion battery and a square-type
lithium-ion battery, can be used. Further, a structure in which a
plurality of positive electrodes, a plurality of negative
electrodes, and a plurality of separators are stacked or wound may
be employed.
[0158] As the negative electrodes of the lithium-ion battery 300,
the lithium-ion battery 400, and the lithium-ion battery 500, which
are described in this embodiment, the negative electrode for a
power storage device of one embodiment of the present invention are
used. Thus, the lithium-ion batteries 300, 400, and 500 can have
favorable long-term cycle performance.
[0159] This embodiment can be implemented in combination with any
of the other embodiments and examples as appropriate.
Embodiment 4
[0160] In this embodiment, a lithium-ion capacitor will be
described as a power storage device.
[0161] A lithium-ion capacitor is a hybrid capacitor including a
combination of a positive electrode of an electric double layer
capacitor (EDLC) and a negative electrode of a lithium-ion
secondary battery formed using a carbon material and is also an
asymmetric capacitor where power storage principles of the positive
electrode and the negative electrode are different from each other.
The positive electrode enables charge and discharge by adsorption
and desorption of charge carrying ions across electrical double
layers as in the "electric double layer capacitor", whereas the
negative electrode enables charge and discharge by the redox
reaction as in the "lithium-ion battery". A negative electrode in
which lithium is received in a negative electrode active material
such as a carbon material is used, whereby energy density is much
higher than that of a conventional electric double layer capacitor
whose negative electrode is formed using porous activated
carbon.
[0162] In a lithium-ion capacitor, instead of the positive
electrode active material layer in the lithium-ion battery
described in Embodiment 3, a material capable of reversibly having
at least one of lithium ions and anions is used. Examples of such a
material include active carbon, a conductive polymer, and a
polyacenic semiconductor (PAS).
[0163] The lithium-ion capacitor has high charge and discharge
efficiency which allows rapid charge and discharge and has a long
life even when it is repeatedly used.
[0164] As the negative electrode of such a lithium-ion capacitor,
the electrode for a power storage device which is described in the
above embodiment is used. Thus, the decomposition reaction of an
electrolytic solution and the like as a side reaction of charge and
discharge can be minimized and therefore, a power storage device
having long-term cycle performance can be manufactured.
[0165] This embodiment can be implemented in combination with any
of the other embodiments and examples as appropriate.
Embodiment 5
[0166] The power storage device of one embodiment of the present
invention can be used for power supplies of a variety of electric
appliances which can be operated with electric power.
[0167] Specific examples of electrical appliances each utilizing
the power storage device of one embodiment of the present invention
are as follows: display devices of televisions, monitors, and the
like, lighting devices, desktop personal computers and laptop
personal computers, word processors, image reproduction devices
which reproduce still images and moving images stored in recording
media such as digital versatile discs (DVDs), portable CD players,
portable radios, tape recorders, headphone stereos, stereos, table
clocks, wall clocks, cordless phone handsets, transceivers,
portable wireless devices, mobile phones, car phones, portable game
machines, calculators, portable information terminals, electronic
notebooks, e-book readers, electronic translators, audio input
devices, video cameras, toys, digital still cameras, electric
shavers, high-frequency heating appliances such as microwave ovens,
electric rice cookers, electric washing machines, electric vacuum
cleaners, water heaters, electric fans, hair dryers,
air-conditioning systems such as air conditioners, humidifiers, and
dehumidifiers, dishwashers, dish dryers, clothes dryers, futon
dryers, electric refrigerators, electric freezers, electric
refrigerator-freezers, freezers for preserving DNA, flashlights,
electrical tools such as chain saws, smoke detectors, and medical
equipment such as dialyzers. Further, industrial equipment such as
guide lights, traffic lights, belt conveyors, elevators,
escalators, industrial robots, power storage systems, and power
storage devices for leveling the amount of power supply and smart
grid can be given. In addition, moving objects driven by electric
motors using electric power from the lithium secondary batteries
are also included in the category of electrical appliances.
Examples of the moving objects are electric vehicles (EV), hybrid
electric vehicles (HEV) which include both an internal-combustion
engine and a motor, plug-in hybrid electric vehicles (PHEV),
tracked vehicles in which caterpillar tracks are substituted for
wheels of these vehicles, motorized bicycles including
motor-assisted bicycles, motorcycles, electric wheelchairs, golf
carts, boats, ships, submarines, helicopters, aircrafts, rockets,
artificial satellites, space probes, planetary probes, and
spacecrafts.
[0168] In the electrical devices, the power storage device of one
embodiment of the present invention can be used as a main power
supply for supplying enough electric power for almost the whole
power consumption. Alternatively, in the electrical devices, the
power storage device of one embodiment of the present invention can
be used as an uninterruptible power supply which can supply
electric power to the electrical devices when the supply of
electric power from the main power supply or a commercial power
supply is stopped. Still alternatively, in the electrical devices,
the power storage device of one embodiment of the present invention
can be used as an auxiliary power supply for supplying electric
power to the electrical devices at the same time as the power
supply from the main power supply or a commercial power supply.
[0169] FIG. 8 illustrates specific structures of the electrical
devices. In FIG. 8, a display device 600 is an example of an
electrical device including a power storage device 604 of one
embodiment of the present invention. Specifically, the display
device 600 corresponds to a display device for TV broadcast
reception and includes a housing 601, a display portion 602,
speaker portions 603, and the power storage device 604. The power
storage device 604 of one embodiment of the present invention is
provided in the housing 601. The display device 600 can receive
electric power from a commercial power supply. Alternatively, the
display device 600 can use electric power stored in the power
storage device 604. Thus, the display device 600 can be operated
with the use of the power storage device 604 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.
[0170] A semiconductor display device such as a liquid crystal
display device, a light-emitting device in which a light-emitting
element such as an organic EL element is provided in each pixel, an
electrophoresis display device, a digital micromirror device (DMD),
a plasma display panel (PDP), or a field emission display (FED) can
be used for the display portion 602.
[0171] Note that the display device includes, in its category, all
of information display devices for personal computers,
advertisement displays, and the like besides TV broadcast
reception.
[0172] In FIG. 8, an installation lighting device 610 is an example
of an electrical device including a power storage device 613 of one
embodiment of the present invention. Specifically, the lighting
device 610 includes a housing 611, a light source 612, and a power
storage device 613. Although FIG. 8 illustrates the case where the
power storage device 613 is provided in a ceiling 614 on which the
housing 611 and the light source 612 are installed, the power
storage device 613 may be provided in the housing 611. The lighting
device 610 can receive electric power from a commercial power
supply. Alternatively, the lighting device 610 can use electric
power stored in the power storage device 613. Thus, the lighting
device 610 can be operated with the use of the power storage device
613 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.
[0173] Note that although the installation lighting device 610
provided in the ceiling 614 is illustrated in FIG. 8 as an example,
the power storage device of one embodiment of the present invention
can be used in an installation lighting device provided in, for
example, a wall 615, a floor 616, a window 617, or the like other
than the ceiling 614. Alternatively, the power storage device can
be used in a tabletop lighting device or the like.
[0174] As the light source 612, an artificial light source which
emits light artificially by using electric power can be used.
Specifically, an incandescent lamp, a discharge lamp such as a
fluorescent lamp, and light-emitting elements such as an LED and an
organic EL element are given as examples of the artificial light
source.
[0175] In FIG. 8, an air conditioner including an indoor unit 620
and an outdoor unit 624 is an example of an electrical device
including a power storage device 623 of one embodiment of the
invention. Specifically, the indoor unit 620 includes a housing
621, an air outlet 622, and a power storage device 623. Although
FIG. 8 illustrates the case where the power storage device 623 is
provided in the indoor unit 620, the power storage device 623 may
be provided in the outdoor unit 624. Alternatively, the secondary
batteries 623 may be provided in both the indoor unit 620 and the
outdoor unit 624. The air conditioner can receive electric power
from a commercial power supply. Alternatively, the air conditioner
can use electric power stored in the power storage device 623.
Particularly in the case where the secondary batteries 623 are
provided in both the indoor unit 620 and the outdoor unit 624, the
air conditioner can be operated with the use of the power storage
device 623 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.
[0176] Note that although the split-type air conditioner including
the indoor unit and the outdoor unit is illustrated in FIG. 8 as an
example, the power storage device 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.
[0177] In FIG. 8, an electric refrigerator-freezer 630 is an
example of an electrical device including a power storage device
634 of one embodiment of the present invention. Specifically, the
electric refrigerator-freezer 630 includes a housing 631, a door
for a refrigerator 632, a door for a freezer 633, and the power
storage device 634. The power storage device 634 is provided in the
housing 631 in FIG. 8. The electric refrigerator-freezer 630 can
receive electric power from a commercial power supply.
Alternatively, the electric refrigerator-freezer 630 can use
electric power stored in the power storage device 634. Thus, the
electric refrigerator-freezer 630 can be operated with the use of
the power storage device 634 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.
[0178] Note that among the electrical devices described above, a
high-frequency heating apparatus such as a microwave oven and an
electrical device such as an electric rice cooker require high
power in a short time. The tripping of a breaker of a commercial
power supply in use of an electrical device can be prevented by
using the power storage device of one embodiment of the present
invention as an auxiliary power supply for supplying electric power
which cannot be supplied enough by a commercial power supply.
[0179] In addition, in a time period when electrical devices are
not used, particularly when the proportion of the amount of
electric power which is actually used to the total amount of
electric power which can be supplied from a commercial power supply
source (such a proportion referred to as a usage rate of electric
power) is low, electric power can be stored in the power storage
device, whereby the usage rate of electric power can be reduced in
a time period when the electrical devices are used. For example, in
the case of the electric refrigerator-freezer 630, electric power
can be stored in the power storage device 634 in night time when
the temperature is low and the door for a refrigerator 632 and the
door for a freezer 633 are not often opened or closed. On the other
hand, in daytime when the temperature is high and the door for a
refrigerator 632 and the door for a freezer 633 are frequently
opened and closed, the power storage device 634 is used as an
auxiliary power supply; thus, the usage rate of electric power in
daytime can be reduced.
[0180] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 6
[0181] Next, a portable information terminal which is an example of
electrical devices will be described with reference to FIGS. 9A to
9C.
[0182] FIGS. 9A and 9B illustrate a tablet terminal 650 which can
be folded. FIG. 9A illustrates the tablet terminal 650 in the state
of being unfolded. The tablet terminal includes a housing 651, a
display portion 652a, a display portion 652b, a display-mode
switching button 653, a power button 654, a power-saving-mode
switching button 655, and an operation button 656.
[0183] A touch panel area 657a can be provided in part of the
display portion 652a, in which area, data can be input by touching
displayed operation keys 658. Note that half of the display portion
652a has only a display function and the other half has a touch
panel function. However, the structure of the display portion 652a
is not limited to this, and all the area of the display portion
652a may have a touch panel function. For example, a keyboard can
be displayed on the whole display portion 652a to be used as a
touch panel, and the display portion 652b can be used as a display
screen.
[0184] A touch panel area 657b can be provided in part of the
display portion 652b like in the display portion 652a. When a
keyboard display switching button 659 displayed on the touch panel
is touched with a finger, a stylus, or the like, a keyboard can be
displayed on the display portion 652b.
[0185] The touch panel area 657a and the touch panel area 657b can
be controlled by touch input at the same time.
[0186] The display-mode switching button 653 allows switching
between a landscape mode and a portrait mode, color display and
black-and-white display, and the like. The power-saving-mode
switching button 655 allows optimizing the display luminance in
accordance with the amount of external light in use which is
detected by an optical sensor incorporated in the tablet terminal.
In addition to the optical sensor, other detecting devices such as
sensors for determining inclination, such as a gyroscope or an
acceleration sensor, may be incorporated in the tablet
terminal.
[0187] Although the display area of the display portion 652a is the
same as that of the display portion 652b in FIG. 9A, one embodiment
of the present invention is not particularly limited thereto. The
display area of the display portion 652a may be different from that
of the display portion 652b, and further, the display quality of
the display portion 652a may be different from that of the display
portion 652b. For example, one of the display portions 652a and
652b may display higher definition images than the other.
[0188] FIG. 9B illustrates the tablet terminal 650 in the state of
being closed. The tablet terminal 650 includes the housing 651, a
solar cell 660, a charge and discharge control circuit 670, a
battery 671, and a DC-DC converter 672. FIG. 9B illustrates an
example where the charge and discharge control circuit 670 includes
the battery 671 and the DC-DC converter 672. The power storage
device of one embodiment of the present invention, which is
described in the above embodiment, is used as the battery 671.
[0189] Since the tablet terminal 650 can be folded, the housing 651
can be closed when the tablet terminal is not in use. Thus, the
display portions 652a and 652b can be protected, which permits the
tablet terminal 650 to have high durability and improved
reliability for long-term use.
[0190] The tablet terminal illustrated in FIGS. 9A and 9B can also
have a function of displaying various kinds of data (e.g., a still
image, a moving image, and a text image), a function of displaying
a calendar, a date, the time, or the like on the display portion, a
touch-input function of operating or editing data displayed on the
display portion by touch input, a function of controlling
processing by various kinds of software (programs), and the
like.
[0191] The solar cell 660, which is attached on a surface of the
tablet terminal, can supply electric power to a touch panel, a
display portion, an image signal processor, and the like. Note that
the solar cell 660 can be provided on one or both surfaces of the
housing 651 and thus the battery 671 can be charged efficiently.
The use of the power storage device of one embodiment of the
present invention as the battery 671 has advantages such as a
reduction in size.
[0192] The structure and operation of the charge and discharge
control circuit 670 illustrated in FIG. 9B will be described with
reference to a block diagram of FIG. 9C. FIG. 9C illustrates the
solar cell 660, the battery 671, the DC-DC converter 672, a
converter 673, switches SW1 to SW3, and a display portion 652. The
battery 671, the DC-DC converter 672, the converter 673, and the
switches SW1 to SW3 correspond to the charge and discharge control
circuit 670 in FIG. 9B.
[0193] First, an example of operation in the case where electric
power is generated by the solar cell 660 using external light will
be described. The voltage of electric power generated by the solar
cell is raised or lowered by the DC-DC converter 672 so that the
electric power has a voltage for charging the battery 671. When the
display portion 652 is operated with the electric power from the
solar cell 660, the switch SW1 is turned on and the voltage of the
electric power is raised or lowered by the converter 673 to a
voltage needed for operating the display portion 652. In addition,
when display on the display portion 652 is not performed, the
switch SW1 is turned off and the switch SW2 is turned on so that
the battery 671 may be charged.
[0194] Although the solar cell 660 is described as an example of a
power generation means, there is no particular limitation on the
power generation means, and the battery 671 may be charged with any
of the other means such as a piezoelectric element or a
thermoelectric conversion element (Peltier element). For example,
the battery 671 may be charged with a non-contact power
transmission module capable of performing charging by transmitting
and receiving electric power wirelessly (without contact), or any
of the other charge means used in combination.
[0195] It is needless to say that one embodiment of the present
invention is not limited to the electrical device illustrated in
FIGS. 9A to 9C as long as the power storage device described in the
above embodiment is included.
Embodiment 7
[0196] Further, an example of the moving object which is an example
of the electrical devices will be described with reference to FIGS.
10A and 10B.
[0197] The power storage device described in the above embodiment
can be used as a control battery. The control battery can be
externally charged by electric power supply using a plug-in
technique or contactless power feeding. Note that in the case where
the moving object is an electric railway vehicle, the electric
railway vehicle can be charged by electric power supply from an
overhead cable or a conductor rail.
[0198] FIGS. 10A and 10B illustrate an example of an electric
vehicle. An electric vehicle 680 is equipped with a battery 681.
The output of the electric power of the battery 681 is adjusted by
a control circuit 682 and the electric power is supplied to a
driving device 683. The control circuit 682 is controlled by a
processing unit 684 including a ROM, a RAM, a CPU, or the like
which is not illustrated.
[0199] The driving device 683 includes a DC motor or an AC motor
either alone or in combination with an internal-combustion engine.
The processing unit 684 outputs a control signal to the control
circuit 682 based on input data such as data on operation (e.g.,
acceleration, deceleration, or stop) of a driver or data during
driving (e.g., data on an upgrade or a downgrade, or data on a load
on a driving wheel) of the electric vehicle 680. The control
circuit 682 adjusts the electric energy supplied from the battery
681 in accordance with the control signal of the processing unit
684 to control the output of the driving device 683. In the case
where the AC motor is mounted, although not illustrated, an
inverter which converts direct current into alternate current is
also incorporated.
[0200] The battery 681 can be charged by external electric power
supply using a plug-in technique. For example, the battery 681 is
charged through a power plug from a commercial power supply. The
battery 681 can be charged by converting the supplied power into DC
constant voltage having a predetermined voltage level through a
converter such as an AC-DC converter. The use of the power storage
device of one embodiment of the present invention as the battery
681 can be conducive to an increase in battery capacity, leading to
an improvement in convenience. When the battery 681 itself can be
more compact and more lightweight as a result of improved
characteristics of the battery 681, the vehicle can be lightweight,
leading to an increase in fuel efficiency.
[0201] Note that it is needless to say that one embodiment of the
present invention is not limited to the electrical device described
above as long as the power storage device of one embodiment of the
present invention is included.
[0202] This embodiment can be implemented in combination with any
of the other embodiments and the example as appropriate.
Example 1
[0203] The present invention will be described in detail below with
examples. Note that the present invention is not limited to the
examples below.
(Formation of Negative Electrode)
[0204] First, a method for forming an electrode used in this
example will be described.
[0205] First, graphite produced by JFE Chemical Corporation was
used as an active material and PVDF was used as a binder to form a
slurry in which the weight ratio of the graphite to the PVDF is
90:10. As a solvent of the slurry, NMP was used.
[0206] Copper foil was used as a current collector. The slurry
containing the graphite was applied to the current collector, dried
at 70.degree. C., and then dried at 170.degree. C. in a vacuum
atmosphere for 10 hours. In the above manner, an active material
layer containing graphite was formed.
[0207] Then, the active material layer was soaked in a treatment
liquid used to form a film on the active material for ten minutes.
The treatment liquid contains 2 wt % of an organosilicon compound
containing ethyl polysilicate as a main component, 97.8 wt % of
ethanol, 0.2 wt % of water, and 4.times.10.sup.-4 wt % of
hydrochloric acid. In the case of this compounding ratio, the
proportion of silicon oxide in the treatment liquid is 0.8 wt % of
the weight of the treatment liquid.
[0208] After that, heat treatment was performed on the active
material layer on a hot plate at 70.degree. C. for an hour, whereby
ethyl silicate contained in the treatment liquid reacted with
moisture in the air, so that hydrolysis occurred, and the ethyl
silicate after the hydrolysis was condensed by dehydration reaction
following the hydrolysis reaction. In the above manner, the active
material layer was covered with silicon oxide.
[0209] Through the above steps, Electrode A was formed.
[0210] Next, a method for forming Comparative Electrode B will be
described. Comparative Electrode B was formed by forming a film on
graphite in advance by a sol-gel method and then forming an active
material layer.
[0211] In the case of Comparative Electrode B, graphite used as an
active material was covered with silicon oxide by a sol-gel method.
As the graphite, graphite produced by JFE Chemical Corporation was
used.
[0212] First, silicon ethoxide, ethyl acetoacetate, and toluene
were mixed and stirred to form a Si(OEt).sub.4 toluene solution. At
this time, the amount of the silicon ethoxide was determined so
that the weight ratio of silicon oxide formed later to graphite was
1 wt % (weight percent). The compounding ratio of this solution was
as follows: the Si(OEt).sub.4 was 3.14.times.10.sup.-4 mol; the
ethyl acetoacetate, 6.28.times.10.sup.-4 mol; and the toluene, 2
ml.
[0213] Next, the Si(OEt).sub.4 toluene solution to which graphite
was added was stirred in a dry room. Then, the solution was held at
70.degree. C. in a humid environment for 3 hours so that the
Si(OEt).sub.4 in the Si(OEt).sub.4 toluene solution to which the
graphite was added was hydrolyzed and condensed. In other words,
the Si(OEt).sub.4 in the solution gradually reacted with moisture
in the air, so that hydrolysis reaction gradually occurred, and the
Si(OEt).sub.4 after the hydrolysis was condensed by dehydration
reaction following the hydrolysis reaction. In such a manner,
gelled silicon was attached to the surfaces of graphite particles
to form a net-like structure of a C--O--Si bond.
[0214] Then, baking was performed at 500.degree. C. in a nitrogen
atmosphere for three hours, whereby graphite covered with silicon
oxide was formed. Further, the graphite covered with silicon oxide
and PVDF were mixed to form a slurry, and the slurry was applied to
a current collector and dried, so that Comparative Electrode B was
formed. In this case, the weight ratio of the graphite to the PVDF
was 90:10.
[0215] Comparative Electrode C was formed using an active material
not covered with silicon oxide. In the case of Comparative
Electrode C, graphite produced by JFE Chemical Corporation was used
as an active material and PVDF was used as a binder to form a
slurry in which the ratio of the graphite to the PVDF is 90:10. As
a solvent of the slurry, NMP was used.
[0216] Copper foil was used as a current collector. The slurry
containing the graphite was applied to the current collector, dried
at 70.degree. C., and then dried at 170.degree. C. in a vacuum
atmosphere for 10 hours. In the above manner, an active material
layer containing graphite was formed.
[0217] Through the above steps, Comparative Electrode C was
formed.
(Evaluation of Degree of Graphitization)
[0218] Next, Raman spectra obtained by Raman spectroscopy
measurement on graphites in Electrode A, Comparative Electrode B,
and Comparative Electrode C will be described. Three-point
measurement was performed on each of Electrode A,
[0219] Comparative Electrode B, and Comparative Electrode C using a
PL microscope LabRAM manufactured by HORIBA, Ltd.
[0220] FIG. 11 shows the peak intensity ratios of D band to G band
of each of graphites in Electrode A, Comparative Electrode B, and
Comparative Electrode C. "D band" is a peak around 1360 cm.sup.-1
in Raman spectrum, and "G band" is a peak around 1580 cm.sup.-1 in
Raman spectrum. As shown in FIG. 11, the peak intensity ratio of D
band to G band (D band/G band) of graphite in Electrode A and the
peak intensity ratio of D band to G band of graphite in Comparative
Electrode C are each approximately 0.3. On the other hand, the peak
intensity ratio of D band to G band of graphite in Comparative
Electrode B is approximately 0.6.
[0221] According to the results in FIG. 11, the degree of
graphitization of graphite in Electrode A was comparable to that of
graphite in Comparative Electrode C, and the degree of
graphitization of graphite in Comparative Electrode B was lower
than that of graphite in Electrode A. The results in FIG. 11
suggest that the degree of graphitization of graphite in
Comparative Electrode B was decreased in the process where graphite
was covered with silicon oxide. Thus, Electrode A probably has less
factors of a decrease in the degree of graphitization in the
process where graphite is covered with silicon oxide than
Comparative Electrode B.
[0222] In the method for forming Comparative Electrode B in which
graphite particles are covered with silicon oxide in advance by a
sol-gel method, a graphite particle was damaged by hydrolysis
reaction, leading to lower crystallinity. In contrast, in the
method for forming Electrode A in which graphite particles are
covered with silicon oxide after formation of a coated electrode,
it is presumable that the number of contact points between the
graphite particles and silicon oxide was reduced in hydrolysis
reaction (because there are a portion coated with a binder and a
contact point between graphite particles) and thus damage to the
graphite particles was inhibited, resulting in suppression of a
reduction in the crystallinity of the graphite particles.
(Evaluation of Initial Irreversible Capacity of Half Cell)
[0223] Next, half cells including Electrode A, Comparative
Electrode B, and Comparative Electrode C were fabricated.
Measurement results of the initial irreversible capacities of the
half cells are as follows.
[0224] The performance was evaluated using coin cells. Electrode A,
Comparative Electrode B, or Comparative Electrode C was used as a
positive electrode; a lithium metal was used as a negative
electrode; polypropylene (PP) was used as a separator; and an
electrolytic solution formed in such a manner that lithium
hexafluorophosphate (LiPF.sub.6) was dissolved at a concentration
of 1 mol/L in a solution in which ethylene carbonate (EC) and
diethyl carbonate (DEC) were mixed at a volume ratio of 1:1 was
used. Two half cells including Electrodes A, two half cells
including Comparative Electrodes B, and two half cells including
Comparative Electrodes C were fabricated. Discharging was performed
in such a manner that capacity of 200 mAh/g was discharged at a
constant current and a rate of 0.2 C (it takes five hours for
discharging). Charging was performed at a rate of 0.2 C (it takes
five hours for charging) until the voltage reached a termination
voltage of 1 V. The environmental temperature was set to 25.degree.
C. Under such conditions, the measurements were performed. FIG. 12
shows the difference between discharge capacity and charge capacity
as irreversible capacity.
[0225] The results in FIG. 12 indicate that the irreversible
capacity of Electrode A was lower than that of Comparative
Electrode B. This is probably because since the degree of
graphitization of Electrode A is higher than that of Comparative
Electrode B, the amount of Li inserted into low crystalline carbon,
which requires a high potential for release of Li, was smaller. The
results also indicate that the irreversible capacity of Electrode A
was lower than that of Comparative Electrode C. This is probably
because the decomposition of the electrolytic solution was
inhibited by silicon oxide covering the surfaces of graphite.
[0226] Thus, in Electrode A, the graphite active material was able
to be covered with silicon oxide while the degree of graphitization
of the graphite active material was maintained, so that the
irreversible capacity was able to be reduced.
(Evaluation of Cycle Performance)
[0227] A full cell including Electrode A as a negative electrode,
an electrolytic solution, and a positive electrode was fabricated
and charged and discharged once, whereby Lithium-ion Secondary
Battery A was fabricated. Then, the cycle performance of the
secondary battery was measured. In a similar manner, full cells
including Comparative Electrode B/Comparative Electrode C as a
negative electrode, an electrolytic solution, and a positive
electrode were fabricated and charged and discharged once, whereby
Lithium-ion Secondary Battery B and Lithium-ion Secondary Battery C
were fabricated.
[0228] The performance was measured using coin cells. An electrode
including LiFePO.sub.4 as an active material was used as a positive
electrode; polypropylene (PP) was used as a separator; and an
electrolytic solution formed in such a manner that lithium
hexafluorophosphate (LiPF.sub.6) was dissolved at a concentration
of 1 mol/L in a solution in which ethylene carbonate (EC) and
diethyl carbonate (DEC) were mixed at a volume ratio of 1:1 was
used. Charge and discharge in the first cycle were performed at a
rate of 0.2 C (it takes five hours for charge), and charge and
discharge in the second and the subsequent cycles were performed at
a rate of 1 C (it takes an hour for charge). In every 200 cycles,
charge and discharge were performed at a rate of 0.2 C (it takes 5
hours for charge) to measure discharge capacity. Constant current
charging and discharging were performed at voltages ranging from 2
V to 4 V and an environmental temperature of 60.degree. C. Under
such conditions, the measurements were performed.
[0229] FIG. 13 shows the measurement results of cycle performance.
The horizontal axis represents the number of cycles (times) and the
vertical axis represents discharge capacity (mAh/g) of the
secondary batteries. In FIG. 13, a curve 701 shows the cycle
performance of Lithium-ion Secondary Battery A; a curve 702 shows
the cycle performance of Lithium-ion Secondary Battery B; and a
curve 703 shows the cycle performance of Lithium-ion Secondary
Battery C.
[0230] As shown by the curve 703 in the measurement results, in the
case of the lithium-ion secondary battery including a negative
electrode active material layer where graphite particles are not
covered with a silicon oxide film, the discharge capacity was
decreased as the number of cycles increased. That is to say,
degradation was significant.
[0231] In contrast, as shown by the curves 701 and 702, in the case
of the lithium-ion secondary batteries in each of which graphite
was provided with a silicon oxide film, although the discharge
capacity had a tendency to be reduced, the reduction was not
significant compared with that in the case of the lithium-ion
secondary battery where graphite was not provided with a silicon
oxide film, which indicates that degradation was sufficiently
inhibited.
[0232] Further, comparison between the curve 701 and the curve 702
shows that the cycle performance of Lithium-ion Secondary battery A
was better than that of Lithium-ion Secondary Battery B.
[0233] The results shown in FIG. 13 suggest that the
electrochemical deposition of the electrolytic solution and the
like in Electrode A was able to be minimized because the surface of
graphite was partly or entirely covered with silicon oxide and thus
was stabilized and that in Lithium-ion Secondary Battery A
including Electrode A, generation of other passivating films on
repeated charge and discharge was inhibited, leading to an
improvement in cycle performance.
(Observation of Electrode A with Electron Microscope)
[0234] FIGS. 14A and 14B show images of Electrode A observed with a
scanning electron microscope (SEM). FIGS. 14A and 14B are SEM
images of graphite partly provided with a silicon oxide film. FIG.
14B is the enlarged SEM image of a part of the SEM image in FIG.
14A. Observation was performed at a low acceleration voltage of 0.1
kV by a retarding method using a top detector. As shown in FIGS.
14A and 14B, a formation region of silicon oxide appears dark gray,
and the surface of graphite appears light gray or white.
[0235] FIG. 14A shows the state where graphite particles each
having a diameter of approximately 10 .mu.m were provided over a
current collector to serve as a mixture electrode.
[0236] In FIG. 14B, a graphite particle 751 is shown by light gray,
and a dark gray region in the surface of the graphite particle is a
portion where a silicon oxide film 752 was formed. Thus, a
difference in contrast in the SEM image makes it possible to
distinguish the formation region of the silicon oxide film from
other regions. The silicon oxide film not completely covering the
surface of the graphite particle but partly covering the surface
was observed.
[0237] As described above, the result of observation with SEM shows
how the silicon oxide films were formed on the surfaces of the
graphite particles.
(Evaluation)
[0238] According to the above results, the use of the electrode of
one embodiment of the present invention in the lithium-ion battery
enabled a reduction in the initial irreversible capacity of the
lithium-ion battery. Further, the decomposition reaction of the
electrolytic solution as a side reaction of charge and discharge
was minimized on repeated charge and discharge of the lithium-ion
battery, resulting in an improvement in the cycle performance of
the lithium-ion battery.
Example 2
[0239] In this example, evaluation results of the cycle performance
of a lithium-ion secondary battery including the electrode of one
embodiment of the present invention will be described.
[0240] First, a method for forming an electrode used as a negative
electrode will be described.
[0241] First, graphite produced by JFE Chemical Corporation was
used as an active material and PVDF was used as a binder to form a
slurry in which the weight ratio of the graphite to the PVDF is
90:10. As a solvent of the slurry, NMP was used.
[0242] Copper foil was used as a current collector. The slurry
containing the graphite was applied to the current collector, dried
at 70.degree. C., and then dried at 170.degree. C. in a vacuum
atmosphere for 10 hours. In the above manner, an active material
layer containing graphite was formed.
[0243] Next, punching was performed on the current collector
provided with the active material layer to obtain round shapes, so
that Comparative Electrode D1 and Comparative Electrode D2 were
formed.
[0244] In addition, Comparative Electrodes E1 to E4 and Comparative
Electrodes F1 to F4, which are different from Comparative
Electrodes D1 and D2, were formed.
[0245] First, silicon ethoxide, ethyl acetoacetate, and toluene
were mixed and stirred to form a Si(OEt).sub.4 toluene solution.
The compounding ratio of this solution was as follows: the
Si(OEt).sub.4 was 3.14.times.10.sup.-4 mol; the ethyl acetoacetate,
6.28.times.10.sup.-4 mol; and the toluene, 2 ml. Note that two
different amounts of the silicon ethoxide were determined so that
the weight ratios of silicon oxide formed later to graphite were 1
wt % and 3 wt %.
[0246] Next, the Si(OEt).sub.4 toluene solution to which graphite
was added was stirred in a dry room. Then, the solution was held at
70.degree. C. in a humid environment for 3 hours so that the
Si(OEt).sub.4 in the Si(OEt).sub.4 toluene solution to which the
graphite was added was hydrolyzed and condensed. In other words,
the Si(OEt).sub.4 in the solution reacted with moisture in the air,
so that hydrolysis reaction gradually occurred, and the
Si(OEt).sub.4 after the hydrolysis was condensed by dehydration
reaction following the hydrolysis reaction. In such a manner,
gelled silicon was attached to the surfaces of graphite particles
to form a net-like structure of a C--O--Si bond.
[0247] Then, baking was performed at 500.degree. C. in a nitrogen
atmosphere for three hours, whereby three kinds of graphites
covered with silicon oxide were formed.
[0248] The graphite covered with 1 wt % of silicon oxide and PVDF
were mixed to form a slurry, and the slurry was applied to a
current collector and dried, so that an active material layer was
formed. In this case, the weight ratio of the graphite to the PVDF
was 90:10. As a solvent of the slurry, NMP was used. Next, punching
was performed on the current collector provided with the active
material layer to obtain round shapes, so that Comparative
Electrodes E1 to E4 were formed.
[0249] In addition, the graphite covered with 3 wt % of silicon
oxide and PVDF were mixed to form a slurry, and the slurry was
applied to a current collector and dried, so that an active
material layer was formed. In this case, the weight ratio of the
graphite to the PVDF was 90:10. As a solvent of the slurry, NMP was
used. Next, punching was performed on the current collector
provided with the active material layer to obtain round shapes, so
that Comparative Electrodes F1 to F4 were formed.
[0250] Comparative Electrodes E4 and F4 were formed to be observed
with an electron microscope.
[0251] In addition, Comparative Electrodes H1 to H3, which are
different from the above comparative electrodes, were formed.
[0252] First, silicon ethoxide, ethyl acetoacetate, and toluene
were mixed and stirred to form a Si(OEt).sub.4 toluene solution. At
this time, the amount of the silicon ethoxide was determined so
that the weight ratio of silicon oxide formed later to graphite was
3 wt % (weight percent). The compounding ratio of this solution was
as follows: the Si(OEt).sub.4 was 3.14.times.10.sup.-4 mol; the
ethyl acetoacetate, 6.28.times.10.sup.-4 mol; and the toluene, 2
ml.
[0253] Next, the Si(OEt).sub.4 toluene solution to which graphite
was added was stirred in a dry room. Then, the solution was held at
70.degree. C. in a humid environment for 3 hours so that the
Si(OEt).sub.4 in the Si(OEt).sub.4 toluene solution to which the
graphite was added was hydrolyzed and condensed. In other words,
the Si(OEt).sub.4 in the solution gradually reacted with moisture
in the air, so that hydrolysis reaction gradually occurred, and the
Si(OEt).sub.4 after the hydrolysis was condensed by dehydration
reaction following the hydrolysis reaction. In such a manner,
gelled silicon was attached to the surfaces of graphite particles
to form a net-like structure of a C--O--Si bond.
[0254] Then, baking was performed at 500.degree. C. in a nitrogen
atmosphere for three hours, whereby graphite covered with silicon
oxide was formed.
[0255] The graphite covered with 3 wt % of silicon oxide, PVDF, and
acetylene black (AB) were mixed to form a slurry, and the slurry
was applied to a current collector and dried, so that an active
material layer was formed. In this case, the weight ratio of the
graphite to the PVDF and the AB was 88:10:2. As a solvent of the
slurry, NMP was used. Next, punching was performed on the current
collector provided with the active material layer to obtain round
shapes, so that Comparative Electrodes H1 to H3 were formed.
[0256] Then, the electrodes of this example were formed.
[0257] First, graphite produced by JFE Chemical Corporation was
used as an active material and PVDF was used as a binder to form a
slurry in which the weight ratio of the graphite to the PVDF is
90:10. As a solvent of the slurry, NMP was used.
[0258] Copper foil was used as a current collector. The slurry
containing the graphite was applied to the current collector, dried
at 70.degree. C., and then dried at 170.degree. C. in a vacuum
atmosphere for 10 hours. In the above manner, an active material
layer containing graphite was formed.
[0259] Then, the active material layer was soaked in a treatment
liquid used to cover graphite with silicon oxide for ten minutes.
The treatment liquid contains 2 wt % of an organosilicon compound
containing ethyl polysilicate as a main component, 97.8 wt % of
ethanol, 0.2 wt % of water, and 4.times.10.sup.-4 wt % of
hydrochloric acid. In the case of this compounding ratio, the
proportion of silicon oxide in the treatment liquid is 0.8 wt % of
the weight of the treatment liquid.
[0260] After that, heat treatment was performed on the active
material layer on a hot plate at 70.degree. C. for an hour, whereby
ethyl silicate contained in the treatment liquid reacts with
moisture in the air, so that hydrolysis occurred, and the ethyl
silicate after the hydrolysis was condensed by dehydration reaction
following the hydrolysis reaction. In the above manner, the surface
of graphite in the active material layer was covered with silicon
oxide.
[0261] Next, punching was performed on the current collector
provided with the active material layer to obtain round shapes, so
that Electrode I1 and Electrode I2 were formed.
[0262] Next, a full cell including Comparative Electrode D1 formed
in the above steps as a negative electrode, an electrolytic
solution, and a positive electrode was fabricated and charged and
discharged once, whereby Lithium-ion Secondary Battery D1 was
fabricated. Then, the cycle performance of the secondary battery
was measured. In a similar manner, with the use of Comparative
Electrode D2, Comparative Electrodes E1 to E3, Comparative
Electrodes F1 to F3, Comparative Electrodes H1 to H3, and
Electrodes I1 and I2, Lithium-ion Secondary Battery D2, Lithium-ion
Secondary Batteries E1 to E3, Lithium-ion Secondary Batteries F1 to
F3, Lithium-ion Secondary Batteries H1 to H3, and Lithium-ion
Secondary Batteries I1 and I2 were fabricated. Then, the cycle
performance of each of the lithium-ion secondary batteries was
measured.
[0263] The performance was measured using coin cells. An electrode
including LiFePO.sub.4 as an active material was used as a positive
electrode; polypropylene (PP) was used as a separator; and an
electrolytic solution formed in such a manner that lithium
hexafluorophosphate (LiPF.sub.6) was dissolved at a concentration
of 1 mol/L in a solution in which ethylene carbonate (EC) and
diethyl carbonate (DEC) were mixed at a volume ratio of 1:1 was
used. Charging and discharging in the first cycle were performed at
a rate of 0.2 C (it takes five hours for charging), and charge and
discharge in the second and the subsequent cycles were performed at
a rate of 1 C (it takes an hour for charging). In every 200 cycles,
charge and discharge were performed at a rate of 0.2 C (it takes 5
hours for charging) to measure discharge capacity. Constant current
charging and discharging were performed at voltages ranging from 2
V to 4 V and an environmental temperature of 60.degree. C. Under
such conditions, the measurements were performed.
[0264] FIG. 15 shows the measurement results of cycle performance.
The horizontal axis represents the number of cycles (times) and the
vertical axis represents discharge capacity (mAh/g) of the
secondary batteries.
[0265] As shown in FIG. 15, the discharge capacities of Lithium-ion
Secondary
[0266] Batteries F1 to F3 each including an electrode which
includes graphite covered with 3 wt % of silicon oxide and
Lithium-ion Secondary Batteries D1 and D2 each including an
electrode which includes graphite not covered with silicon oxide
were significantly reduced as the number of cycles increased.
[0267] In contrast, the discharge capacities of Lithium-ion
Secondary Batteries E1 to E3 each including an electrode which
includes graphite covered with 1 wt % of silicon oxide, Lithium-ion
Secondary Batteries H1 to H3 each including an electrode which
includes graphite covered with 3 wt % of silicon oxide and AB, and
Lithium-ion Secondary Batteries I1 and I2 each including an
electrode which was obtained by forming an active material layer
and then covering graphite with silicon oxide had tendencies to be
reduced; however, the reduction was not significant, which
indicates that degradation was sufficiently inhibited.
[0268] The cycle performances of Lithium-ion Secondary Batteries I1
and I2 were better than that of Lithium-ion Secondary Batteries E1
to E3 and Lithium-ion Secondary Batteries H1 to H3.
[0269] Table 1 shows average charge and discharge efficiencies and
discharge capacity maintenance factors which are obtained from the
results of FIG. 15. The average charge and discharge efficiencies
shown in Table 1 are the average values of charge and discharge
efficiencies of 500 cycles of the lithium-ion secondary batteries.
The discharge capacity maintenance factor is the value obtained by
dividing the discharge capacity of each lithium-ion secondary
battery in the second cycle by the discharge capacity thereof after
500 cycles. The average charge and discharge efficiency and the
discharge capacity maintenance factor of Lithium-ion Secondary
Battery H3 were not able to be calculated because the measurement
was stopped before completion of 500 cycles.
TABLE-US-00001 TABLE 1 Average Charge and Discharge Capacity
Lithium-ion Secondary Discharge Efficiency Maintenance Factor
Battery [%] [%] D1 99.52 17.89 D2 99.44 11.68 E1 99.77 58.88 E2
99.72 39.51 E3 99.75 50.50 F1 99.45 10.21 F2 99.35 9.90 F3 99.40
10.38 H1 99.77 41.59 H2 99.68 35.04 H3 -- -- I1 99.80 62.67 I2
99.75 63.72
[0270] As shown in Table 1, the discharge capacity maintenance
factors of Lithium-ion Secondary Batteries D1 and D2 and
Lithium-ion Secondary Batteries F1 to F3 were each less than 20%.
Further, the discharge capacity maintenance factors of Lithium-ion
Secondary Batteries I1 and I2 were higher than those of Lithium-ion
Secondary Batteries E1 to E3, F1 to F3, and H1 and H2.
(Observation of Comparative Electrode E4 and Comparative Electrode
F4 with Electron Microscope)
[0271] FIGS. 16A and 16B show SEM images of Comparative Electrode
E4 and Comparative Electrode F4. FIG. 16A is the SEM image of
Comparative Electrode E4 and FIG. 16B is the SEM image of
Comparative Electrode F4. Note that the SEM image in FIG. 16A and
the SEM image in FIG. 16B were observed under the same
magnification.
[0272] In Comparative Electrode E4 in FIG. 16A, a slight amount of
silicon oxide film 752 formed on the surface of the graphite
particle 751 was observed.
[0273] In Comparative Electrode F4 in FIG. 16B, the silicon oxide
film 752 having an area larger than that of Comparative Electrode
E4 in FIG. 16A was observed.
[0274] However, according to the results of the cycle performance
in FIG. 15, the cycle performances of Comparative Electrodes F1 to
F3 formed in a manner similar to that of Comparative Electrode F4
was worse than those of Comparative Electrodes E1 to E3 formed in a
manner similar to that of Comparative Electrode E4. The degradation
in cycle characteristics is presumably due to contact between the
graphite particles that is obstructed by the silicon oxide as in
the model illustrated in FIG. 1C1, resulting in a reduction of
graphite effectually serving as a negative electrode active
material.
[0275] On the other hand, Electrode A in FIGS. 14A and 14B had a
larger area where the silicon oxide covers the graphite than
Comparative Electrodes E4 and F4. The cycle performances of
Lithium-ion Secondary Batteries I1 and I2 including Electrodes I1
and I2 formed by a formation method similar to that of Electrode A
were better than those of Lithium-ion Secondary Batteries E1 to E3
including Comparative Electrodes E1 to E3 and Lithium-ion Secondary
Batteries F1 to F3 including Comparative Electrodes F1 to F3. This
is presumably because the conductive path of electrons was able to
be maintained by covering the coated electrode with the silicon
oxide even in the case where the contact interface between the
graphite and the electrolytic solution was covered with the film as
in the model illustrated in FIG. 1C2.
[0276] The results in FIG. 15 and FIGS. 16A and 16B show that in
the case where the electrode was formed after the graphite was
covered with the silicon oxide by a sol-gel method, when the
graphite particles were covered with a certain amount or more of
silicon oxide, the cycle performance of the lithium-ion secondary
battery was degraded. In contrast, in the case where the coated
electrode was covered with the silicon oxide, even when the
graphite particles were covered with a certain amount or more of
silicon oxide, degradation of the cycle performance of the
lithium-ion secondary battery was inhibited.
EXPLANATION OF REFERENCE
[0277] 101: electrode, 102: current collector, 103: active material
layer, 111: active material, 112: film, 113: binder, 114: space,
200: negative electrode, 201: negative electrode current collector,
202: negative electrode active material layer, 211: negative
electrode active material, 212: film, 213: binder, 215: graphene,
250: positive electrode, 251: positive electrode current collector,
252: positive electrode active material layer, 261: positive
electrode active material, 262: film, 263: binder, 265: graphene,
300: lithium-ion 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, 400: lithium-ion battery, 401:
positive electrode current collector, 402: positive electrode
active material layer, 403: positive electrode, 404: negative
electrode current collector, 405: negative electrode active
material layer, 406: negative electrode, 407: separator, 408:
electrolytic solution, 409: exterior body, 500: lithium-ion
battery, 501: positive electrode cap, 502: battery can, 503:
positive electrode terminal, 504: positive electrode, 505:
separator, 506: negative electrode, 507: negative electrode
terminal, 508: insulating plate, 509: insulating plate, 511: PTC
element, 512: safety valve mechanism, 600: display device, 601:
housing, 602: display portion, 603: speaker portion, 604: power
storage device, 610: lighting device, 611: housing, 612: light
source, 613: power storage device, 614: ceiling, 615: wall, 616:
floor, 617: window, 620: indoor unit, 621: housing, 622: air
outlet, 623: power storage device, 624: outdoor unit, 630: electric
refrigerator-freezer, 631: housing, 632: door for refrigerator,
633: door for freezer, 634: power storage device, 650: tablet
terminal, 651: housing, 652: display portion, 652a: display
portion, 652b: display portion, 653: display-mode switching button,
654: power button, 655: power-saving-mode switching button, 656:
operation button, 657a: touch panel area, 657b: touch panel area,
658: operation key, 659: keyboard display switching button, 660:
solar cell, 670: charge and discharge control circuit, 671:
battery, 672: DC-DC converter, 673: converter, 680: electric
vehicle, 681: battery, 682: control circuit, 683: driving device,
684: processing unit, 701: curve, 702: curve, 703: curve, 751:
graphite particle, and 752: silicon oxide film
[0278] This application is based on Japanese Patent Application
serial no. 2012-245847 filed with Japan Patent Office on Nov. 7,
2012, the entire contents of which are hereby incorporated by
reference.
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