U.S. patent application number 13/597675 was filed with the patent office on 2013-08-29 for negative electrode of power storage device and power storage device.
This patent application is currently assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. The applicant listed for this patent is Mayumi Mikami, Yumiko SAITO, Rie Yokoi. Invention is credited to Mayumi Mikami, Yumiko SAITO, Rie Yokoi.
Application Number | 20130224581 13/597675 |
Document ID | / |
Family ID | 48188851 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130224581 |
Kind Code |
A1 |
SAITO; Yumiko ; et
al. |
August 29, 2013 |
NEGATIVE ELECTRODE OF POWER STORAGE DEVICE AND POWER STORAGE
DEVICE
Abstract
A mixture of amorphous PAHs and at least one of a carrier ion
storage metal, a Sn compound, a carrier ion storage alloy, a metal
compound, Si, Sb, and SiO.sub.2 is used as the negative electrode
active material. The theoretical capacity of amorphous PAHs greatly
exceeds that of a graphite-based carbon material. Thus, the use of
amorphous PAHs enables the negative electrode active material to
have a higher capacity than in the case of using the graphite-based
carbon material. Further, addition of at least one of the carrier
ion storage metal, the Sn compound, the carrier ion storage alloy,
the metal compound, Si, Sb, and SiO.sub.2 to the amorphous PAHs
enables the negative electrode active material to have a higher
capacity than the case of only using the amorphous PAHs.
Inventors: |
SAITO; Yumiko; (Atsugi,
JP) ; Yokoi; Rie; (Atsugi, JP) ; Mikami;
Mayumi; (Atsugi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SAITO; Yumiko
Yokoi; Rie
Mikami; Mayumi |
Atsugi
Atsugi
Atsugi |
|
JP
JP
JP |
|
|
Assignee: |
SEMICONDUCTOR ENERGY LABORATORY
CO., LTD.
Atsugi-shi
JP
|
Family ID: |
48188851 |
Appl. No.: |
13/597675 |
Filed: |
August 29, 2012 |
Current U.S.
Class: |
429/211 ;
361/502 |
Current CPC
Class: |
H01M 4/13 20130101; H01M
4/0404 20130101; H01M 4/1399 20130101; H01M 4/137 20130101; H01M
4/483 20130101; H01G 11/62 20130101; H01M 4/386 20130101; Y02T
10/70 20130101; H01G 11/48 20130101; H01M 2220/30 20130101; H01M
4/662 20130101; H01M 4/1391 20130101; H01G 11/30 20130101; H01G
11/52 20130101; H01M 4/366 20130101; H01M 4/606 20130101; Y02E
60/10 20130101; H01M 4/131 20130101; H01M 10/0525 20130101; Y02E
60/13 20130101; H01M 4/0471 20130101; H01G 11/32 20130101; H01M
4/387 20130101; H01G 11/50 20130101; H01M 2004/027 20130101 |
Class at
Publication: |
429/211 ;
361/502 |
International
Class: |
H01M 4/38 20060101
H01M004/38; H01G 11/50 20060101 H01G011/50 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2011 |
JP |
2011-189140 |
Claims
1. A negative electrode of a power storage device, the negative
electrode comprising: a negative electrode active material
containing amorphous PAHs and at least one of a carrier ion storage
metal, a Sn compound, a carrier ion storage alloy, a metal
compound, Si, Sb, and SiO.sub.2; and a current collector.
2. The negative electrode of a power storage device, according to
claim 1, wherein the carrier ion storage metal is any one of Sn,
Al, Zn, and Bi.
3. The negative electrode of a power storage device, according to
claim 1, wherein the Sn compound is any one of SnO.sub.2,
Sn.sub.2P.sub.2O.sub.7, SnPBO.sub.6, and SnPO.sub.4Cl.
4. The negative electrode of a power storage device according to
claim 1, wherein the carrier ion storage alloy is any alloy
represented by Sn.sub.2M (M is Fe, Co, Mn, V, or Ti)
5. The negative electrode of a power storage device, according to
claim 1, wherein the metal compound is any one of CoO, NiO,
MnO.sub.2, and FePO.sub.4.
6. The negative electrode of a power storage device, according to
claim 1, wherein a carrier ion of the carrier ion storage metal or
the carrier ion storage alloy is either of a Li ion and a Na
ion.
7. The negative electrode of a power storage device, according to
claim 1, wherein the amorphous PAHs have a spherical shape.
8. The negative electrode of a power storage device, according to
claim 1, wherein any one of the carrier ion storage metal, the
metal compound, Si, Sb and SiO.sub.2 is attached to an outer
surface of the amorphous PAHs.
9. The negative electrode of a power storage device, according to
claim 1, wherein the amorphous PAHs contain at least one of the
carrier ion storage metal, the metal compound, Si, Sb, and
SiO.sub.2 at greater than or equal to 1 wt % and less than or equal
to 50 wt %.
10. A power storage device comprising: a negative electrode; a
positive electrode; and an electrolyte solution containing an
electrolyte, wherein the negative electrode comprises a negative
electrode active material containing amorphous PAHs and at least
one of a carrier ion storage metal, a Sn compound, a carrier ion
storage alloy, a metal compound, Si, Sb, and SiO.sub.2; and a
current collector.
11. An electric device comprising the power storage device
according to claim 10.
12. An electronic device comprising the power storage device
according to claim 10.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a negative electrode of a
power storage device and a power storage device having the negative
electrode.
[0003] 2. Description of the Related Art
[0004] With an increasing concern for the environmental issues,
power storage devices such as secondary batteries and electric
double layer capacitors used for power supply for hybrid vehicles
and the like have been actively developed. As the power storage
devices, lithium (Li)-ion secondary batteries and Li-ion capacitors
which have high energy performance have attracted attention. The
Li-ion secondary battery, which is compact but can have a large
capacity, has already been mounted on a portable information
terminal such as a mobile phone or a laptop personal computer,
thereby contributing to miniaturization of products.
[0005] The power storage device basically has a structure in which
an electrolyte is provided between a positive electrode and a
negative electrode. It is known that each of the positive electrode
and the negative electrode includes a current collector and an
active material provided over the current collector. For example,
in a Li-ion secondary battery, a material capable of storing and
releasing Li ions is used as an active material.
[0006] Various approaches have been taken to improve the
characteristics of a power storage device. For example, study of a
negative electrode active material for a power storage device is
one of the approaches to improve the characteristics of a power
storage device. A graphite-based carbon material, which is mainly
used as the negative electrode active material, has the theoretical
capacity of 372 mAh/g and has already been put to practical use
with a capacity close to the theoretical capacity. Thus, an active
material with a higher capacity (charge capacity) is required.
[0007] A material containing a semimetal, a semimetal compound, a
metal, or a metal compound is given as an example of a material
having a higher capacity than a graphite-based carbon material when
it is used as a negative electrode active material for a power
storage device. For example, silicon (Si) is known to have a higher
capacity than a graphite-based carbon material. Patent Document 1
discloses a negative electrode of a Li-ion secondary battery in
which a fiber shaped carbon material, silicon, and a silicon
compound are used in addition to a graphite-based carbon material.
Patent Document 2 discloses a Li-ion secondary battery in which a
graphite-based carbon material and a metal-carbon composite
material are used.
[0008] However, a negative electrode active material with a higher
capacity is required to meet an increasing demand for a compact
power storage device.
REFERENCE
Patent Documents
[0009] [Patent Document 1] Japanese Published Patent Application
No. 2004-182512 [0010] [Patent Document 2] Japanese Published
Patent Application No. 2009-105046
SUMMARY OF THE INVENTION
[0011] An object of one embodiment of the invention is to provide a
negative electrode active material with a higher capacity.
[0012] In order to achieve the object, in one embodiment of the
invention, a mixture of amorphous polycyclic aromatic hydrocarbons
(PAHs) and at least one of a carrier ion storage metal, a carrier
ion storage alloy, a metal compound, Si, Sb, and SiO.sub.2 is used
as a negative electrode active material. Note that in this
specification, "carrier ion storage metal" means a metal which can
store and release carrier ions in a power storage device. Further,
"carrier ion storage alloy" means an alloy which can store and
release carrier ions in a power storage device.
[0013] The theoretical capacity of amorphous PAHs is 1116 mAh/g and
an experimental capacity thereof is 680 mAh/g, both of which
greatly exceed 372 mAh/g that is the theoretical capacity of a
graphite-based carbon material. Therefore, in the case where
amorphous PAHs are used, a negative electrode active material can
have a higher capacity than in the case where a graphite-based
carbon material is used.
[0014] Further, when amorphous PAHs that are materials with a high
capacity and at least one of a carrier ion storage metal, a carrier
ion storage alloy, a metal compound, Si, Sb, and SiO.sub.2 are
mixed, the capacity of the negative electrode active electrode can
be higher than in the case where only the amorphous PAHs are
used.
[0015] One embodiment of the invention is a negative electrode of a
power storage device comprising a negative electrode active
material containing amorphous PAHs and at least one of a carrier
ion storage metal, a carrier ion storage alloy, a metal compound,
Si, Sb, and SiO.sub.2; and a current collector.
[0016] The carrier ion storage metal may be any one of Sn, Al, Zn,
and Bi. The carrier ion storage alloy may be any one of alloys
expressed by a Sn-M alloy (M is Fe, Co, Mn, V, or Ti). Further, as
the metal compound, a Sn compound or a metal compound used as a
positive electrode material in a state where carrier ions are
released (a decarrierionized state) can be used. The Sn compound
may be any one of SnO.sub.2, Sn.sub.2P.sub.2O.sub.7, and
SnPBO.sub.6. Further, the metal compound used as the positive
electrode material in a state where carrier ions are released (a
decarrierionized state) may be any one of SnPO.sub.4ClCoO, NiO,
MnO.sub.2, and FePO.sub.4.
[0017] In the above, the carrier ion may be either of a Li ion and
a Na ion.
[0018] Further, the amorphous PAHs may have a spherical shape.
[0019] Furthermore, at least one of a metal, a metal compound, Si,
Sb, and SiO.sub.2 may be attached to the surfaces of the amorphous
PAHs.
[0020] Furthermore, the amorphous PAHs may contain at least one of
a metal, a metal compound, Si, Sb, and SiO.sub.2 at greater than or
equal to at 1 wt % and less than or equal to 50 wt %.
[0021] Furthermore, one embodiment of the invention is a power
storage device including the negative electrode, a positive
electrode, and an electrolyte solution containing an
electrolyte.
[0022] According to one embodiment of the invention, a negative
electrode active material with a higher capacity can be
provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIGS. 1A to 1C are schematic views of examples of a negative
electrode active material and a negative electrode.
[0024] FIGS. 2A and 2B are a plan view and a cross-sectional view
illustrating one embodiment of a power storage device.
[0025] FIG. 3 is a diagram illustrating application modes of a
power storage device.
[0026] FIGS. 4A and 4B are scanning electron micrograph images each
illustrating an example of a negative electrode active
material.
[0027] FIGS. 5A and 5B are scanning electron micrograph images each
illustrating an example of a negative electrode active
material.
[0028] FIG. 6 shows evaluation results of an example of a negative
electrode.
[0029] FIG. 7 shows evaluation results of an example of a negative
electrode.
[0030] FIG. 8 shows evaluation results of an example of a negative
electrode.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Hereinafter, Embodiments are described in detail using the
drawings. Note that the invention is not limited to the following
description of the embodiments, and it is readily appreciated by
those skilled in the art that modes and details of the invention
can be modified in a variety of ways without departing from the
spirit of the invention disclosed in this specification and the
like. A structure of the different embodiment can be implemented by
combination appropriately. On the description of the invention with
reference to the drawings, a reference numeral indicating the same
part is used in common throughout different drawings, and the
repeated description is omitted.
[0032] Note that the position, the size, the range, or the like of
each structure illustrated in the drawings and the like is not
accurately represented in some cases for easy understanding.
Therefore, the present invention is not necessarily limited to the
position, size, range, or the like disclosed in the drawings and
the like.
Embodiment 1
[0033] In this embodiment, a negative electrode active material for
a power storage device, which is one embodiment of the invention,
and a manufacturing method thereof will be described with reference
to FIGS. 1A to 1C.
Negative Electrode Active Material
[0034] An example of a negative electrode active material 100 will
be described with reference FIG. 1A. The negative electrode active
material 100 includes amorphous PAHs 101 and a fine particle 102
formed of any one of a metal, a metal compound, Si, Sb, and
SiO.sub.2. In addition, the negative electrode active material 100
may include a secondary particle 103 in which a plurality of the
fine particles 102 is aggregated.
[0035] As the amorphous PAHs 101, PAHs with a hydrogen/carbon
atomic ratio (hereinafter referred to as a H/C ratio) of 0.05 or
more and 0.5 or less are used. As the amorphous PAHs with a H/C
ratio of 0.05 or more and 0.5 or less, for example, a polyacenic
material or a hard carbon-based material can be used.
[0036] The polyacenic material has a higher capacity (about 850
mAh/g) than the graphite-based carbon material. Further, the hard
carbon-based material has a higher capacity (about 400 mAh/g to 700
mAh/g) than the graphite-based carbon material and has discharge
characteristics such that voltage uniformly descends to a discharge
end voltage. Thus, any of these materials is preferably used for
the negative electrode active material because in that case a power
storage device has a high energy density.
[0037] When the amorphous PAHs 101 have a spherical shape,
variation in the contact area with other constituent elements of
the negative electrode can be reduced. Thus, the amorphous PAHs
preferably have a spherical shape because variation in resistance
in the negative electrode active layer can be reduced; further,
because there is little abrasion when it is transferred,
high-density filling is easily achieved, and fluidity in the case
of being mixed with other constituent elements of the negative
electrode can be improved. Specifically, a grain diameter of the
amorphous PAHs 101 is preferably 100 .mu.m or less.
[0038] Note that in this specification and the like, "spherical
shape" does not necessarily mean an accurate spherical shape. For
example, a substantially spherical shape (for example, the smallest
diameter is greater than or equal to 70% and less than 100% of the
longest diameter), a deformed spherical shape, a spherical shape
with a projection on its surface, and an elliptical spherical shape
are included.
[0039] In this embodiment, for the amorphous PAHs 101, a spherical
polyacenic material is used.
[0040] In this embodiment, a metal, a metal compound, Si, Sb, or
SiO.sub.2 is mixed to the amorphous PAHs 101.
[0041] A carrier ion storage metal is used as the metal to be
mixed. Alternatively, a carrier ion storage alloy or a metal
compound is used as the metal compound to be mixed.
[0042] As carrier ions, alkali metal ions such as Li ions or sodium
(Na) ions, alkaline earth metal ions, beryllium (Be) ions,
magnesium (Mg) ions, or the like can be used. The use of Li ions as
the carrier ions enables a power storage device to have a small
memory effect, a high energy density, a high charge/discharge
capacity, and a high output voltage, which is preferable. Further,
Na ions that are abundant in resources are preferably used because
the cost of manufacturing a power storage device can be reduced. In
this embodiment, Li ions are used as the carrier ions.
[0043] As the carrier ion storage metal to be mixed, a metal having
properties of storing carrier ions, for example, Sn, Al, Zn, or Bi
can be used.
[0044] As the carrier ion storage alloy to be mixed, an alloy
having properties of storing carrier ions, for example, an alloy
represented by Sn-M (M is Fe, Co, Mn, V, or Ti) can be used.
[0045] As the metal compound to be mixed, a Sn compound or a metal
compound used as a positive electrode material in a state where
carrier ions are released (a decarrierionized state) can be used.
As the Sn compound, for example, SnO.sub.2, Sn.sub.2P.sub.2O.sub.7,
SnPBO.sub.6, or SnPO.sub.4Cl can be used. Further, as the metal
compound used as a positive electrode material in a state where
carrier ions are released (a decarrierionized state), a stable
substance even in the absence of carrier ions, among substances
that can be used as a positive electrode active material, for
example, CoO, NiO, MnO.sub.2, NiMnO.sub.4, or FePO.sub.4 can be
used.
[0046] The fine particle 102 is a particle composed of a metal, a
metal compound, Si, Sb, or SiO.sub.2. The fine particle 102
preferably has a grain diameter of, for example, 1 .mu.m or less in
order to increase the efficiency of reaction with the carrier
ion.
[0047] The fine particle 102 composed of a metal, a metal compound,
Si, Sb, or SiO.sub.2 may be, but is not necessarily, attached to
the outer surface of the amorphous PAHs 101. Further, the secondary
particle 103 may be generated from the particle 102 composed of a
metal, a metal compound, Si, Sb, or SiO.sub.2. The secondary
particle 103 may be, but is not necessarily, attached to the outer
surface of the amorphous PAHs 101. Note that it is more preferable
that the fine particle 102 composed of a metal, a metal compound,
Si, Sb, or SiO.sub.2 be attached to the PAHs 101 without being
aggregated. This is because the fine particle 102 has a larger
specific surface area than the aggregated secondary particles 103
and easily reacts with carrier ions. When the fine particle 102 is
attached to the amorphous PAHs 101, the secondary particle is not
easily formed.
[0048] It is preferable that the amorphous PAHs 101 contain any one
of a metal, a metal compound, Si, Sb, and SiO.sub.2 at greater than
or equal to 1 wt % and less than or equal to 50 wt %, preferably
greater than or equal to 1 wt % and less than or equal to 30 wt %.
It is not preferable that the amount of a metal, a metal compound,
Si, Sb, or SiO.sub.2 be too small, because the effect of making the
power storage device having a high capacity is decreased. Also, it
is not preferable that the amount of a metal, a metal compound, Si,
Sb, or SiO.sub.2 be too large, because electric conductivity of the
negative electrode active material is too low. In this embodiment,
the amorphous PAHs 101 contain SiO.sub.2 at greater than or equal
to 1 wt % and less than or equal to 30 wt %.
Negative Electrode
[0049] An example of a negative electrode 200 of a power storage
device that is one embodiment of the present invention will be
described with reference to FIGS. 1B and 1C.
[0050] The negative electrode 200 in FIG. 1B includes the negative
electrode active material 100, a conduction auxiliary agent 120,
and a binder (not shown in FIG. 1B) over a negative electrode
current collector 130.
[0051] A conductive material such as copper (Cu), titanium (Ti),
aluminum (Al), or stainless steel, which is processed into a foil
shape, a plate shape, a net shape, or the like can be used for the
negative electrode current collector 130.
[0052] An electron-conductive material which does not cause
chemical change in the power storage device is used for the
conduction auxiliary agent 120. For example, graphite; a carbon
particle; a carbon fiber; a metal material such as Cu, nickel (Ni),
Al, or silver (Ag); or powder, fiver, and the like of mixtures
thereof can be used. In the negative electrode 200 in FIG. 1B,
acetylene black that is one of carbon particles is used.
[0053] The binder exists between the negative electrode active
material 100, the conduction auxiliary agent 120, and the current
collector 130, and these substances are bonded one another by the
binder. As the binder, polysaccharides such as starch,
carboxymethyl cellulose, hydroxypropyl cellulose, regenerated
cellulose, and diacetyl cellulose; vinyl polymers such as polyvinyl
chloride, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl
pyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride,
ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM
rubber, styrene-butadiene rubber, butadiene rubber, and fluorine
rubber; polyether such as polyethylene oxide; and the like can be
given. In this embodiment, PVdF is used.
[0054] Note that the negative electrode active material layer may
be predoped with carrier ions. When Li ions are used as the carrier
ions, a Li layer is formed on a surface of the negative electrode
active material layer by a sputtering method. Alternatively, Li
foil is provided on the surface of the negative electrode active
material layer, whereby the negative electrode active material
layer can be predoped with Li.
[0055] Further, as shown in FIG. 1C, graphene or multilayer
graphene 121 may be used instead of the conduction auxiliary agent
120 and the binder. The use of graphene or multilayer graphene 121
can suppress adverse effects (pulverization of the negative
electrode active material 100 and separation of the negative
electrode active material layer) of expansion and contraction of
the negative electrode active material 100 due to storing and
releasing of carrier ions. Further, graphene or the multilayer
graphene 121 stores carrier ions and functions as a negative
electrode active material and thus a negative electrode with a
higher capacity can be obtained.
Method for Manufacturing Negative Electrode Active Material
[0056] An example of a method for manufacturing a negative
electrode active material will be described below.
[0057] First, materials of amorphous PAHs are prepared. When a
polyacenic material is used as the amorphous PAHs 101, for example,
a phenol resin can be used for raw materials of the polyacenic
material. When the polyacenic material is used, a baking
temperature at the time of baking that is to be described later can
be low; thus, productivity is improved.
[0058] When a hard carbon-based material is used as the amorphous
PAHs 101, for example, a furfuryl alcohol resin, a saccharide such
as saccharose or cellulose can be used for raw materials of the
hard carbon-based material.
[0059] Note that cleaning is preferably performed to remove an
organic impurity attached to the amorphous PAHs 101. For example,
ultrasonic cleaning in an organic solvent may be conducted.
[0060] Next, a metal, a metal compound, Si, Sb, or SiO.sub.2 is
mixed to the amorphous PAHs 101. As for a metal, a metal compound,
Si, Sb, or SiO.sub.2, the description of FIG. 1A can be referred
to.
[0061] In this embodiment, a polyacenic material is used as the
amorphous PAHs 101, and a spherical phenol resin is used for the
raw materials of the polyacenic material. A fine particle of
SiO.sub.2 is mixed to the spherical phenol resin.
[0062] There is no limitation on a method for mixing, and for
example, a dry-mixing method can be performed. In the case where a
spherical phenol resin is used, a method in which the shape of the
resin can be maintained is preferably employed; for example, mixing
with the use of a rotatable roller is preferably employed.
[0063] Next, a mixture of the material of the amorphous PAHs 101
and at least one of a metal, a metal compound, Si, Sb, and
SiO.sub.2 is baked. The baking is preferably performed under an
inert atmosphere, for example, a nitrogen atmosphere. The
temperature and time at the baking may be set under sufficient
conditions for carbonization of the amorphous PAHs 101. In the case
of using a phenol resin, for example, the baking temperature can be
set to greater than or equal to 600.degree. C. and less than or
equal to 800.degree. C. There is no limitation on the method of
baking; for example, baking using a muffle furnace can be
performed.
[0064] By the above method, the negative electrode active material
100 that is one embodiment of the invention can be
manufactured.
Method for Manufacturing Negative Electrode
[0065] An example of a method for manufacturing the negative
electrode 200 including the negative electrode active material 100
will be described below.
[0066] First, the negative electrode active material 100, the
conduction auxiliary agent 120, and the binder are mixed using a
solvent, so that slurry is formed. There is no particular
limitation on the solvent, for example, an organic solvent such as
N-methyl-2-pyrrolidone (NMP) can be used.
[0067] Note that graphene or the multilayer graphene 121 may be
used instead of the conduction auxiliary agent 120 and the binder.
Note that in this specification, graphene refers to a
one-atom-thick sheet of carbon molecules having sp.sup.2 bonds.
Further, multilayer graphene refers to a stack of 2 to 100 sheets
of graphene, and may contain less than or equal to 30 at. % of an
element other than carbon, such as oxygen or hydrogen.
Alternatively, the multilayer graphene may contain less than or
equal to 15 at. % of an element other than carbon and hydrogen.
Note that an alkali metal such as Li, Na, or potassium (K) may be
added to graphene or multilayer graphene.
[0068] Next, the slurry is applied onto the negative electrode
current collector 130. An anchor coat may be applied before the
slurry is applied to the negative electrode current collector 130
so as to improve adhesion between the negative electrode current
collector 130 and the negative electrode active material 100.
Further, the slurry containing the negative electrode active
material 100 may be applied to one surface of the negative
electrode current collector 130 as shown in FIGS. 1B and 1C or both
surfaces thereof.
[0069] Next, after the negative electrode current collector 130 and
the slurry are dried to form the negative electrode 200 into a
desired shape, the negative electrode 200 is further dried.
[0070] Through the above steps, the negative electrode 200 that is
one embodiment of the invention can be manufactured.
Embodiment 2
[0071] In this embodiment, an example of a power storage device
that is one embodiment of the present invention will be described
with reference to FIGS. 2A and 2B.
[0072] The power storage device that is one embodiment of the
present invention includes at least a positive electrode, a
negative electrode, a separator, and an electrolyte solution. The
negative electrode is the one described in Embodiment 1.
[0073] The electrolyte is a nonaqueous solution containing an
electrolyte salt or a solution containing an electrolyte salt. Any
electrolyte salt can be used as the electrolyte salt as long as it
contains carrier ions such as alkali metal ions, alkaline earth
metal ions, Be ions, or Mg ions. Examples of the alkali metal ions
include Li ions, Na ions, and K ions. Examples of the alkaline
earth metal ions include calcium (Ca) ions, strontium (Sr) ions,
and barium (Ba) ions. In this embodiment, the electrolyte salt is
an electrolyte salt containing Li ions (hereinafter, referred to as
a Li-containing electrolyte salt).
[0074] With the above structure, the power storage device can be a
secondary battery or a capacitor. Further, an electric double layer
capacitor can be obtained by using only a solvent for an
electrolyte solution without using the electrolyte salt.
[0075] Here, the power storage device will be described with
reference to the drawing.
[0076] FIG. 2A shows a structural example of a power storage device
351. FIG. 2B is a cross-sectional view along dashed dotted line X-Y
in FIG. 2A.
[0077] The power storage device 351 shown in FIG. 2A includes a
power storage cell 355 in an exterior member 353. The power storage
device 351 further includes terminal portions 357 and 359 which are
connected to the power storage cell 355. For the exterior member
353, a laminate film, a polymer film, a metal film, a metal case, a
plastic case, or the like can be used.
[0078] As shown in FIG. 2B, the power storage cell 355 includes a
negative electrode 363, a positive electrode 365, a separator 367
between the negative electrode 363 and the positive electrode 365,
and an electrolyte 369 with which the exterior member 353 is
filled.
[0079] The negative electrode 363 is the one described in
Embodiment 1. The negative electrode current collector 371 is
connected to the terminal portion 359. A positive electrode current
collector 375 is connected to the terminal portion 357.
[0080] Further, the terminal portions 357 and 359 each partly
extend outside the exterior member 353.
[0081] The positive electrode layer 365 is formed to include a
positive electrode current collector 375 and a positive electrode
active material layer 377. The positive electrode active material
layer 377 is formed on one or both surfaces of the positive
electrode current collector 375. Further, the positive electrode
365 may include a binder, a conduction auxiliary agent, and the
like besides the positive electrode current collector 375 and the
positive electrode active material layer 377.
[0082] Although a sealed thin power storage device is described as
the power storage device 351 in this embodiment, the external shape
of the power storage device 351 is not limited thereto. A power
storage device having any of a variety of shapes, such as a button
power storage device, a cylindrical power storage device, or a
rectangular power storage device can be used as the power storage
device 351. Further, although the structure where the positive
electrode, the negative electrode, and the separator are stacked is
described in this embodiment, a structure where the positive
electrode, the negative electrode, and the separator are rolled may
be employed.
[0083] For the positive electrode current collector 375, a
conductive material such as Al or stainless steel which is
processed into a foil shape, a plate shape, a net shape, or the
like can be used. Alternatively, a conductive layer provided by
deposition separately on a substrate and then separated from the
substrate can be used as the positive electrode current collector
375.
[0084] The positive electrode active material layer 377 can be
formed using LiFeO.sub.2, LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.4,
LiFePO.sub.4, LiCoPO.sub.4, LiNiPO.sub.4, LiMn.sub.2PO.sub.4,
V.sub.2O.sub.5, MnO.sub.2, or another Li compound as a material.
Note that when carrier ions are alkali metal ions other than Li
ions, alkaline earth metal ions, Be ions, or Mg ions, the positive
electrode active material layer 377 can be formed using, instead of
Li in the above Li compounds, an alkali metal (e.g., Na or K), an
alkaline earth metal (e.g., Ca, Sr, or Ba), Be, or Mg. For example,
when carrier ions are Na ions, NaNi.sub.0.5Mn.sub.0.5O.sub.2 can be
used.
[0085] The positive electrode active material layer 377 is formed
over the positive electrode current collector 375 by a coating
method or a physical vapor deposition method (e.g., a sputtering
method), whereby the positive electrode 365 can be formed. In the
case where a coating method is employed, the positive electrode 365
is formed in such a manner that a paste in which a conduction
auxiliary agent (e.g., acetylene black), a binder (e.g., PVDF), or
the like is mixed with any of the above materials for the positive
electrode active material layer 377 is applied to the positive
electrode current collector 375 and dried. In this case, the
positive electrode 365 is preferably molded by applying pressure as
needed.
[0086] The positive electrode active material layer 377 may be
formed using a paste of a mixture of the positive electrode active
material and graphene or multilayer graphene instead of a
conductive auxiliary agent and a binder.
[0087] The use of graphene or multilayer graphene instead of a
conductive auxiliary agent and a binder leads to a reduction in
amount of the conductive auxiliary agent and the binder in the
positive electrode 365. In other words, the weight of the positive
electrode 365 can be reduced; accordingly, the charge/discharge
capacity of the power storage device per unit weight of the
negative electrode can be increased.
[0088] Note that strictly speaking, "positive electrode active
material" or "negative electrode active material" refers only to a
material that relates to storing and releasing of ions functioning
as carriers. In this specification, however, in the case of using a
coating method to form an active material layer, for the sake of
convenience, the active material layer collectively refers to the
materials of the active material layer, that is, a substance that
is actually an "active material", a conductive auxiliary agent, a
binder, and the like.
[0089] The electrolyte 369 is a nonaqueous solution containing an
electrolyte salt or a solution containing an electrolyte salt. In
particular, in a Li-ion secondary battery, a Li-containing
electrolyte salt in which Li ions as carrier ions can transfer and
stably exist is used. Examples of the Li-containing electrolyte
salt includes LiClO.sub.4, LiAsF.sub.6, LiBF.sub.4, LiPF.sub.6, and
Li(C.sub.2F.sub.5SO.sub.2).sub.2N. Note that when carrier ions are
alkali metal ions other than Li ions or alkaline earth metal ions,
alkali metal salt (e.g., Na salt or K salt), alkaline earth metal
salt (e.g., Ca salt, Sr salt or Ba salt), Be salt, Mg salt, or the
like can be used for the solute of the electrolyte 369. For
example, when Na ions are used as the carrier ions, NaPF.sub.6,
NaClO.sub.4, or the like can be used as the solute (electrolyte
salt).
[0090] The electrolyte 369 is preferably a nonaqueous solution
containing an electrolyte salt. That is, as a solvent of the
electrolyte 369, an aprotic organic solvent is preferably used.
Examples of the aprotic organic solvent include ethylene carbonate,
propylene carbonate, dimethyl carbonate, diethyl carbonate,
.gamma.-butyrolactone, acetonitrile, dimethoxyethane, and
tetrahydrofuran, and one or more of these materials can be used.
Alternatively, as the aprotic organic solvent, one ionic liquid or
a plurality of ionic liquids may be used. Owing to non-flammability
and non-volatility of an ionic liquid, it is possible to suppress
explosion, inflammation, and the like of the power storage device
351 at the time when the internal temperature of the power storage
device 351 rises, resulting in improvement in safety.
[0091] When a gelled high-molecular material containing an
electrolyte salt is used as the electrolyte 369, safety against
liquid leakage and the like is improved and the power storage
device 351 can be thinner and more lightweight. Examples of the
gelled high-molecular material include a silicon gel, an acrylic
gel, an acrylonitrile gel, polyethylene oxide, polypropylene oxide,
and a fluorine-based polymer.
[0092] As the electrolyte 369, a solid electrolyte such as
Li.sub.3PO.sub.4 can be used.
[0093] As the separator 367, an insulating porous material is used.
For example, paper; nonwoven fabric; a glass fiber; a synthetic
fiber containing nylon (polyamide), vinylon (polyvinyl alcohol
based fiber), polyester, acrylic, polyolefin, or polyurethane; or
ceramics may be used. Note that a material which does not dissolve
in the electrolyte 369 should be selected.
[0094] In the case where the power storage device that is one
embodiment of the present invention is a Li-ion capacitor, instead
of the positive electrode active material layer 377, a material
capable of reversibly storing and releasing one of or both Li ions
and anions may be used. Examples of the material include active
carbon, graphite, a conductive polymer, and a polyacenic
material.
[0095] With the use of the negative electrode active material, a
power storage device with a high capacity can be obtained.
[0096] Note that this embodiment can be implemented in appropriate
combination with any of the structures of the other embodiments and
example.
Embodiment 3
[0097] The power storage device that is one embodiment of the
present invention can be used for power supplies of a variety of
electric and electronic devices which are operated with power.
[0098] Specific examples of electric and electronic devices each
utilizing the power storage device that is one embodiment of the
present invention are as follows: display devices, lighting
devices, desktop personal computers and laptop personal computers,
image reproduction devices which reproduce still images and moving
images stored in recording media such as digital versatile discs
(DVDs), mobile phones, portable game machines, portable information
terminals, tablet terminals, e-book readers, video cameras, digital
still cameras, high-frequency heating appliances such as microwave
ovens, electric rice cookers, electric washing machines,
air-conditioning systems such as air conditioners, electric
refrigerators, electric freezers, electric refrigerator-freezers,
freezers for preserving DNA, and medical electrical and electronic
equipment such as dialyzers. In addition, moving objects driven by
an electric motor using electric power from a power storage device
are also included in the category of electric and electronic
devices. As examples of the moving objects, electric vehicles,
hybrid vehicles which include both an internal-combustion engine
and a motor, motorized bicycles including motor-assisted bicycles,
and the like can be given.
[0099] In the electric and electronic devices, the power storage
device that is one embodiment of the present invention can be used
as a power storage device for supplying enough electric power for
almost the whole power consumption (such a power storage device is
referred to as a main power supply). Alternatively, in the electric
and electronic devices, the power storage device that is one
embodiment of the present invention can be used as a power storage
device which can supply electric power to the electric and
electronic devices when the supply of power from the main power
supply or a commercial power supply is stopped (such a power
storage device is referred to as an uninterruptible power supply).
Further alternatively, in the electric and electronic device, the
power storage device that is one embodiment of the present
invention can be used as a power storage device for supplying
electric power to the electric and electronic devices at the same
time as the electric power supply from the main power supply or a
commercial power supply (such a power storage device is referred to
as an auxiliary power supply).
[0100] FIG. 3 illustrates specific structures of the electric and
electronic devices. In FIG. 3, a display device 1000 is an example
of an electronic device including a power storage device 1004 that
is one embodiment of the present invention. Specifically, the
display device 1000 corresponds to a display device for TV
broadcast reception and includes a housing 1001, a display portion
1002, speaker portions 1003, the power storage device 1004, and the
like. The power storage device 1004 that is one embodiment of the
present invention is provided in the housing 1001. The display
device 1000 can receive electric power from a commercial power
supply. Alternatively, the display device 1000 can use electric
power stored in the power storage device 1004. Thus, the display
device 1000 can be operated with use of the power storage device
1004 that is one embodiment of the present invention as an
uninterruptible power supply even when electric power cannot be
supplied from the commercial power supply due to power failure or
the like.
[0101] A semiconductor display device such as a liquid crystal
display device, a light-emitting device provided with a
light-emitting element such as an organic EL element in each pixel,
an electrophoresis display device, a digital micromirror device
(DMD), a plasma display panel (PDP), a field emission display
(FED), and the like can be used for the display portion 1002.
[0102] Note that the display device includes, in its category, all
of information display devices for personal computers,
advertisement displays, and the like other than TV broadcast
reception.
[0103] In FIG. 3, an installation lighting device 1100 is an
example of an electric device including a power storage device 1103
that is one embodiment of the present invention. Specifically, the
lighting device 1100 includes a housing 1101, a light source 1102,
a power storage device 1103, and the like. FIG. 3 illustrates the
case where the power storage device 1103 is provided in a ceiling
1104 on which the housing 1101 and the light source 1102 are
installed; alternatively, the power storage device 1103 may be
provided in the housing 1101. The lighting device 1100 can receive
electric power from a commercial power supply. Alternatively, the
lighting device 1100 can use electric power stored in the power
storage device 1103. Thus, the lighting device 1103 can be operated
with the use of the power storage device 1103 that is one
embodiment of the present invention as an uninterruptible power
supply even when electric power cannot be supplied from the
commercial power supply due to power failure or the like.
[0104] Note that although the installation lighting device 1100
provided in the ceiling 1104 is illustrated in FIG. 3 as an
example, the power storage device that is one embodiment of the
present invention can be used in an installation lighting device
provided in, for example, a wall 1105, a floor 1106, a window 1107,
or the like other than the ceiling 1104. Alternatively, the power
storage device can be used in a tabletop lighting device and the
like.
[0105] As the light source 1102, an artificial light source which
emits light artificially by using power can be used. Specifically,
discharge lamps such as an incandescent lamp and a fluorescent
lamp, and a light-emitting element such as an LED and an organic EL
element are given as examples of the artificial light source.
[0106] In FIG. 3, an air conditioner including an indoor unit 1200
and an outdoor unit 1204 is an example of an electric device
including a power storage device 1203 that is one embodiment of the
present invention. Specifically, the indoor unit 1200 includes a
housing 1201, a ventilation duct 1202, the power storage device
1203, and the like. FIG. 3 illustrates the case where the power
storage device 1203 is provided in the indoor unit 1200;
alternatively, the power storage device 1203 may be provided in the
outdoor unit 1204. Further alternatively, the power storage devices
1203 may be provided in both the indoor unit 1200 and the outdoor
unit 1204. The air conditioner can receive power from the
commercial power supply. Alternatively, the air conditioner can use
power stored in the power storage device 1203. Particularly in the
case where the power storage devices 1203 are provided in both the
indoor unit 1200 and the outdoor unit 1204, the air conditioner can
be operated with the use of the power storage device 1203 that is
one embodiment of the present invention as an uninterruptible power
supply even when power cannot be supplied from a commercial power
supply due to power failure or the like.
[0107] Note that although the separated air conditioner including
the indoor unit and the outdoor unit is illustrated in FIG. 3 as an
example, the power storage device that is 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.
[0108] In FIG. 3, an electric refrigerator-freezer 1300 is an
example of an electric device including a power storage device 1304
that is one embodiment of the present invention. Specifically, the
electric refrigerator-freezer 1300 includes a housing 1301, a door
for a refrigerator 1302, a door for a freezer 1303, and the power
storage device 1304. The power storage device 1304 is provided in
the housing 1301 in FIG. 3. The electric refrigerator-freezer 1300
can receive power from a commercial power supply. Alternatively,
the electric refrigerator-freezer 1300 can use power stored in the
power storage device 1304. Thus, the electric refrigerator-freezer
1300 can be operated with the use of the power storage device 1304
that is one embodiment of the present invention as an
uninterruptible power supply even when power cannot be supplied
from a commercial power supply due to power failure or the
like.
[0109] Note that among the electric devices described above, a
high-frequency heating apparatus such as a microwave and an
electric device such as an electric rice cooker require high
electric power in a short time. The tripping of a breaker of a
commercial power supply in use of electric devices can be prevented
by using the power storage device that is 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.
[0110] In addition, in a time period when electric and electronic
devices are not used, particularly when the proportion of the
amount of power which is actually used to the total amount of power
which can be supplied from a commercial power supply source (such a
proportion referred to as a usage rate of power) is low, power can
be stored in the power storage device, whereby the usage rate of
power can be reduced in a time period when the electric and
electronic devices are used. For example, in the case of the
electric refrigerator-freezer 1300, power can be stored in the
power storage device 1304 in night time when the temperature is low
and the door for a refrigerator 1302 and the door for a freezer
1303 are not often opened or closed. On the other hand, in daytime
when the temperature is high and the door for a refrigerator 1302
and the door for a freezer 1303 are frequently opened and closed,
the power storage device 1304 is used as an auxiliary power supply;
thus, the usage rate of power in daytime can be reduced.
[0111] In FIG. 3, a tablet terminal 1400 is an example of an
example of an electronic device including a power storage device
1403 that is one embodiment of the present invention. Specifically,
the tablet terminal 1400 includes a housing 1401, a housing 1402, a
power storage device 1403, and the like. The housings 1401 and 1402
each have a display portion having a touch panel function. By
touching the display portion with a finger or the like, contents
displayed on the display portion can be controlled. Further, the
tablet terminal 1400 can be folded with the housings 1401 and 1402
and the display portion placed inward; thus, the tablet terminal
1400 can be compact and the display portion can be protected. With
the use of the power storage device 1403 that is one embodiment of
the invention, the tablet terminal 1400 can be compact and used as
a mobile application for a long period.
[0112] Note that this embodiment can be implemented in appropriate
combination with any of the structures of the other embodiments and
example.
Example 1
[0113] In this example, a negative electrode active material for a
power storage device, which is one embodiment of the present
invention, was actually manufactured, and the results of evaluating
the characteristics are described with reference to FIGS. 4A and
4B, FIGS. 5A and 5B, FIG. 6, FIG. 7, and FIG. 8.
Manufacture of Negative Electrode Active Material
[0114] In this example, a spherical phenol resin ("Maririn" HF-008;
Gun Ei Chemical Industry Co., Ltd.) was used as a raw material of
amorphous PAHs. An average grain diameter measured with a particle
size analyzer was 9.6 .mu.m.
[0115] Then, SiO.sub.2 was mixed to the raw material of the
amorphous PAHs. Note that SiO.sub.2 nanopowder (manufactured by
Sigma-Aldrich Corporation) with a grain diameter of 10 nm to 20 nm
was used as SiO.sub.2.
[0116] First, ultrasonic cleaning was conducted in acetone so as to
remove organic impurities attached to the spherical phenol
resin.
[0117] Next, SiO.sub.2 nanopowder was added to the cleaned
spherical phenol resin, and dry-mixing was performed using a
rotatable roller. As shown in Table 1, the additive amounts of
SiO.sub.2 nanopowder to 5 g of the spherical phenol resin were 0 wt
% (0 g, (reference example), 1 wt % (0.05 g), 10 wt % (0.50 g), 20
wt % (1.00 g), and 30 wt % (1.50 g).
TABLE-US-00001 TABLE 1 Baking Material Yield 0 wt % addition (Ref.)
Spherical phenol resin (5 g) 51 .quadrature. 1 w % addition
Spherical phenol resin (5 g) + SiO.sub.2 (0.05 g) 52 .quadrature.
20 wt % addition Spherical phenol resin (5 g) + SiO.sub.2 (1.00 g)
57 .quadrature. 30 wt % addition Spherical phenol resin (5 g) +
SiO.sub.2 (1.50 g) 59 .quadrature.
[0118] Next, the mixture of the spherical phenol resin and
SiO.sub.2 nanopowder were baked so as to be a negative electrode
active material. The baking was performed at 700.degree. C. using a
muffle furnace under a nitrogen atmosphere (N.sub.2, 5 L/min) for
10 hours. Table 1 shows the weight yield after the baking.
[0119] FIGS. 4A and 4B and FIGS. 5A and 5B each show scanning
electron micrographs of the negative electrode active materials
manufactured in the above manner. FIG. 4A shows the negative
electrode active material in which 0 wt % of SiO.sub.2 nanopowder
(reference example) was added to the spherical phenol resin; FIG.
4B shows the negative electrode active material in which 1 wt % of
SiO.sub.2 nanopowder was added to the spherical phenol resin; FIG.
5A shows the negative electrode active material in which 20 wt % of
SiO.sub.2 nanopowder was added to the spherical phenol resin; and
FIG. 5B shows the negative electrode active material in which 30 wt
% of SiO.sub.2 nanopowder was added to the spherical phenol resin.
The state where SiO.sub.2 nanopowder was attached to the outer
surface of the spherical phenol resin was observed in each of FIG.
4B, and FIGS. 5A and 5B.
Manufacture of Negative Electrode
[0120] Negative electrodes were manufactured using the
above-described negative electrode active materials. For the
material of the negative electrode, in addition to the negative
electrode active material, acetylene black was used as a conduction
auxiliary agent, PVdF was used as a binder, and Cu foil was used as
a current collector. The combination ratio of negative electrode
active material to acetylene black and PVdF was set to 82:8:10 (wt
%).
[0121] First, the negative electrode active material and PVdF were
mixed using a homogenizer with NMP used as a solvent; then
acetylene black was added thereto to be mixed, whereby slurry was
formed by adjusting the viscosity using the NMP. After an anchor
coat is applied to the Cu foil current collector to a thickness of
about 1 .mu.m to 2 .mu.m, the slurry was applied to the Cu foil
current collector.
[0122] Next, the Cu foil current collector and the slurry were
dried at 70.degree. C. for 15 minutes using a circulation drier.
This was punched into a round hole with a diameter of 16.15 mm and
baked at 170.degree. C. for 10 hours using a vacuum furnace,
whereby the negative electrode was obtained.
[0123] Table 2 shows the thicknesses and the densities of thus
manufactured negative electrodes. The negative electrodes each
having a similar thickness and density were manufactured under
conditions as shown in Table 2.
TABLE-US-00002 TABLE 2 Electrode Thickness Density 0 wt % addition
(Ref.) 44.6 .quadrature.m 0.88 g/cm.sup.3 1 wt % addition 41.8
.quadrature.m 0.92 g/cm.sup.3 10 wt % addition 41.4 .quadrature.m
0.90 g/cm.sup.3 20 wt % addition 43.1 .mu.m 0.93 g/cm.sup.3 30 wt %
addition 43.8 .quadrature.m 0.94 g/cm.sup.3
Evaluation of Negative Electrode
[0124] The charge/discharge capacity and efficiency of thus
manufactured negative electrodes were measured and charge/discharge
characteristics were evaluated.
[0125] In order to evaluate the charge/discharge characteristics, a
cell was formed using thus manufactured negative electrode as a
working electrode and using Li metal with a diameter of 15 mm as
the opposite electrode. A glass fiber filter was used as a
separator, and an electrolyte in which 1 mol/L lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a mixed solution
of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (the
volume ratio was 3:7) was used.
[0126] A constant-current constant-voltage (CCCV) charging with a
current value of 0.2 C (1.3 mA), the lower limit voltage of 1 mV,
and an end current of 10 .mu.A was employed for the charging. A
constant current (CC) discharging with a current value of 0.2 C
(1.3 mA) and the upper limit voltage of 2 V was employed for the
discharging.
[0127] FIG. 6, FIG. 7, FIG. 8, and Table 3 each show the results of
evaluating the charge/discharge characteristics. FIG. 6 shows the
charge/discharge characteristics of the negative electrode using
the negative electrode active material in which only the spherical
phenol resin was used (0 wt % of SiO.sub.2 addition, reference
example); FIG. 7 shows the charge/discharge characteristics of the
negative electrode using the negative electrode active material in
which 1 wt % of SiO.sub.2 nanopowder was added to the spherical
phenol resin; and FIG. 8 shows the charge/discharge characteristics
of the negative electrode using the negative electrode active
material in which 30 wt % of SiO.sub.2 nanopowder was added to the
spherical phenol resin. The measurement was performed using two
samples in each case. In each of the graphs, the vertical axis
represents voltage and the horizontal axis represents capacity.
TABLE-US-00003 TABLE 3 Li Charge Capacity Li Discharge Capacity
Effi- (mAh/g) (mAh/g) ciency 0 wt % addition 1011.9 412.7 40.8
.quadrature. (Ref.) 1076.7 433.2 40.2 .quadrature. 1 wt % addition
1116.6 460.5 41.2 .quadrature. 1106.3 467.6 42.3 .quadrature. 30 wt
% addition 1532.9 604.5 .quadrature..quadrature..4 .quadrature.
1730.0 633.2 .quadrature..quadrature..6 .quadrature.
[0128] As shown in FIG. 6, in the case of using only the spherical
phenol resin (0 wt % of SiO.sub.2 addition, reference example), the
maximum charge capacity was 1076.7 mAh/g; the maximum discharge
capacity was 433.2 mAh/g; and the maximum efficiency was 40.8%
[0129] Further, as shown in FIG. 7, in the case of adding 1 wt % of
SiO.sub.2 nanopowder to the spherical phenol resin, the maximum
charge capacity was 1116.6 mAh/g; the maximum discharge capacity
was 467.6 mAh/g; and the maximum efficiency was 42.3%.
[0130] Furthermore, in the case of adding 30 wt % of SiO.sub.2
nanopowder to the spherical phenol resin in FIG. 8, the maximum
charge capacity was 1730.0 mAh/g; the maximum discharge capacity
was 633.2 mAh/g; and the maximum efficiency was 39.4%.
[0131] The results revealed that the charge capacity and discharge
capacity were improved by the addition of SiO.sub.2 nanopowder to
the spherical phenol resin. Moreover, it was revealed that the
charge capacity and discharge capacity were much more improved when
30 wt % of SiO.sub.2 nanopowder was added than when 1 wt % of
SiO.sub.2 nanopowder was added.
[0132] This application is based on Japanese Patent Application
serial no. 2011-189140 filed with Japan Patent Office on Aug. 31,
2011, the entire contents of which are hereby incorporated by
reference.
* * * * *