U.S. patent application number 11/035147 was filed with the patent office on 2005-06-23 for electric double layer capacitor and process for its production.
This patent application is currently assigned to Asahi Glass Company, Limited. Invention is credited to Ikeda, Katsuji, Shinozaki, Yasuo.
Application Number | 20050135044 11/035147 |
Document ID | / |
Family ID | 32040768 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050135044 |
Kind Code |
A1 |
Ikeda, Katsuji ; et
al. |
June 23, 2005 |
Electric double layer capacitor and process for its production
Abstract
An electric double layer capacitor includes, contained in a
casing, an electrolyte and positive and negative electrodes each
containing carbon black, to form an electric double layer at the
interface with the electrolyte, and a separator interposed between
the positive electrode and the negative electrode. At least one
electrode of the positive electrode and the negative electrode has
protruded portions or bent portions formed continuously in the
height direction against the bottom face of the casing. Further, a
space due to the height of the protruded portions or the bent
portions is formed between the at least one electrode and the
separator.
Inventors: |
Ikeda, Katsuji; (Kanagawa,
JP) ; Shinozaki, Yasuo; (Kanagawa, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Asahi Glass Company,
Limited
TOKYO
JP
|
Family ID: |
32040768 |
Appl. No.: |
11/035147 |
Filed: |
January 14, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11035147 |
Jan 14, 2005 |
|
|
|
10681090 |
Oct 9, 2003 |
|
|
|
Current U.S.
Class: |
361/502 ;
438/381 |
Current CPC
Class: |
Y02T 10/70 20130101;
H01G 9/155 20130101; Y02E 60/13 20130101; Y02T 10/7022
20130101 |
Class at
Publication: |
361/502 ;
438/381 |
International
Class: |
H01L 021/20; H01G
009/00; H01L 021/44 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 9, 2002 |
JP |
2002-296583 |
Claims
1. A process for producing an electric double layer capacitor,
comprising steps of: forming a positive electrode and a negative
electrode each containing carbon black to form an electric double
layer at the interface with an electrolyte; forming protruded
portions or bent portions on at least one of the positive electrode
and the negative electrode; interposing a separator between the
positive electrode and the negative electrode to form an element;
containing the element in a casing; impregnating the element with
the electrolyte; and at least one charging operation, wherein a
space due to a height of the protruded portions or the bent
portions is formed between said at least one electrode and the
separator, wherein a plurality of the positive electrode and the
negative electrode are alternately stacked with the separator
between them, or the positive electrode and the negative electrode
each having a long strip shape, are wound with the separator
between them, and contained in the casing having a bottomed
rectangular or cylindrical shape.
2. The process according to claim 1, wherein the protruded portions
or the bent portions on the positive electrode and the negative
electrode are formed continuously in a height direction against the
bottom face of the casing.
3. The process according to claim 1, wherein the protruded portions
or the bent portions include at least one electrode deformed on one
side or both sides, and the protruded portions or the bent portions
are formed in a plurality at every predetermined distance in a
direction perpendicular to the height direction.
4. The process according to claim 1, wherein the electrode includes
a metal current collector and an electrode sheet, and the electrode
sheet contains from 5 to 30 mass % of carbon black.
5. The process according to claim 1, wherein the separator has a
thickness of from 10 to 60 .mu.m, a porosity of from 40 to 85% and
a maximum pore size of at most 1 .mu.m as measured by the test
method prescribed in JIS K3832.
6. The process according to claim 1, wherein the electrolyte
includes a non-aqueous electrolyte containing a quaternary onium
salt as a solute.
7. The process according to claim 1, wherein forming the protruded
portions or the bent portions is carried out on both the positive
electrode and the negative electrode.
8. The process according to claim 1, wherein the electrode is
formed by bonding with an adhesive layer an electrode sheet
containing a carbonaceous material as the main component and having
a thickness of from 80 to 400 .mu.m on at least one side of a metal
current collector.
9. The process according to claim 8, wherein the protruded portions
or the bent portions are formed so that a sum of a thickness of the
electrode and a height of the protruded portions or the bent
portions becomes to be from 1.01 to 1.20 times the thickness of the
electrode, said thickness of the electrode includes a sum of a
thicknesses of the metal current collector, the electrode sheet and
the adhesive layer.
10. The process according to claim 3, wherein the protruded
portions and the bent portions are formed at a distance of at most
20 mm in a direction perpendicular to a height direction of the
casing.
11. The process according to claim 8, wherein the electrode is
formed by bonding an electrode sheet to each side of a metal
current collector with an adhesive layer.
12. The process according to claim 1, wherein the impregnating the
electrolyte and the at least one charging operation causes a
thickness of the electrode to expand from 1.1 to 1.6 times.
13. The process according to claim 8, wherein the impregnating the
electrolyte and the at least one charging operation causes a
thickness of the electrode to expand from 1.1 to 1.6 times.
14. The process according to claim 2, wherein forming the protruded
portions or the bent portions is carried out on both the positive
electrode and the negative electrode.
15. The process according to claim 2, wherein the protruded
portions or the bent portions are formed so that a sum of a
thickness of the electrode and a height of the protruded portions
or the bent portions becomes to be from 1.01 to 1.20 times the
thickness of the electrode, said thickness of the electrode
includes a sum of thicknesses of the metal current collector, the
electrode sheet and the adhesive layer.
16. The process according to claim 2, wherein the protruded
portions and the bent portions are formed at a distance of at most
20 mm in a direction perpendicular to a height direction of the
casing.
Description
[0001] The present invention relates to an electric double layer
capacitor and a process for its production. Particularly, it
relates to an electric double layer capacitor whereby the internal
resistance of the electric double layer capacitor can be made low,
its capacitance density can be made high and its productivity can
be satisfactorily maintained, and a process for producing such an
electric double layer capacitor.
[0002] An electric double layer capacitor is excellent in the power
density and the long term reliability by charge/discharge cycles,
and it is being used as a power source for hybrid electric cars, or
as an emergency power source. In such an application as power
sources, a high voltage at a level of a few hundreds V is
required.
[0003] Usually, the operating voltage of a unit cell of an electric
double layer capacitor is relatively low (at a level of up to 2.6
V). Accordingly, from a few tens to a few hundreds of such unit
cells are used as connected in series to form an electric double
layer module.
[0004] As the structure of such a unit cell, a rectangular cell or
a cylindrical cell is common.
[0005] A perspective view illustrating the structure of a
rectangular cell (partially cut) is shown in FIG. 7.
[0006] As shown in FIG. 7, a rectangular cell 20A is one wherein a
plurality of flat plate positive electrodes 1A and negative
electrodes 1B are alternately stacked with a separator 2 interposed
therebetween to form a rectangular element assembly 3, which is
contained in a rectangular casing 5.
[0007] Further, from positive electrodes 1B and negative electrodes
1B, flat plate lead portions 7A and 7B extend upwardly,
respectively, and they are respectively bundled into lead
connecting portions 8A and 8B as divided into a positive electrode
and a negative electrode. The lead connecting portions 8A and 8B
are connected to a positive electrode terminal 9A and a negative
electrode terminal 9B passing through and fixed to the rectangular
casing 5.
[0008] On the other hand, a perspective view illustrating the
structure of a cylindrical cell (partially cut) is shown in FIG.
8.
[0009] As shown in FIG. 8, a cylindrical cell 20B is constituted in
such a manner that a pair of long strip-shaped positive electrode
11A and negative electrode 11B are wound up with a separator 12
interposed therebetween to form a wound element assembly 13, which
is contained in a cylindrical casing 15.
[0010] Further, leads 17A and 17B are connected to the upper ends
of the positive electrode 11A and negative electrode 11B, and these
leads 17A and 17B are, respectively, connected to a positive
electrode terminal 19A and a negative electrode terminal 19B
passing through and fixed to a sealing insulation plate 16.
[0011] Unit cells 20A and 20B thus constituted, are, respectively,
designed so that, for example, a plurality of them are connected in
series to constitute an electric double layer module.
[0012] A perspective view illustrating an embodiment of such an
electric double layer module structure, is shown in FIG. 9.
[0013] As shown in FIG. 9, the electric double layer module 25 is
constituted by solid module structural members 21 to integrally
secure a plurality of unit cells 20 (cylindrical cells 20B) and
many connecting bus bar members 23 to electrically connect the unit
cells 20 in series.
[0014] Further, another structure of an electric double layer
module 25 may be one as shown in JP-A-2002-353078 wherein
electrodes constituting the positive electrode 11A and the negative
electrode 11B, etc. are specially designed so that the cylindrical
casing 15 of unit cells 20, the module structural members 21 and
the connecting bus bar members 23 are integrated to make the
electric double layer module 25 compact and light in weight.
[0015] Of such a electric double layer capacitor for large
capacitance and large current charging/discharging, it is desired
to further reduce the internal resistance and to increase the
capacitance per unit volume (hereinafter referred to as the
capacitance density).
[0016] Accordingly, it is conceivable to enlarge the surface area
of the electrode and to reduce the thickness of the separator 2 or
12 as far as possible.
[0017] However, in such a case, the following structural problem is
likely to result.
[0018] For example, the separator 2 or 12 is required to have the
porosity set to be high to some extent from the viewpoint of
absorption and retention of the electrolyte. Here, the porosity is
the proportion of the volume occupied by voids (bubbles present in
the object) in the volume of the object.
[0019] Therefore, if it is attempted to reduce the thickness of the
separator 2 or 12, while maintaining the porosity to be high to
some extent, the insulation between the positive electrode 11A and
the negative electrode 11B tends to be inadequate, whereby the
positive electrode 11A and the negative electrode 11B tends to
undergo microscopic short circuiting, thus leading to
self-discharge or a decrease in the production yield of the
capacitor.
[0020] Further, if the thickness of the separator 2 or 12 is made
too thin (e.g. at most 60 .mu.m), it tends to be difficult to
increase the porosity of the separator 2 or 12, whereby the amount
of the electrolyte in the separator 2 or 12 tends to be small,
whereby it tends to be difficult to supply the electrolyte to the
positive electrode 11A and the negative electrode 11B
sufficiently.
[0021] Consequently, no adequate electrolyte will be supplied to
the positive electrode 11A and the negative electrode 11B, whereby
no adequate amount of ions will be present in the vicinity of the
positive electrode 11A and the negative electrode 11B, and at the
time of discharge, the voltage drop is likely to be substantial due
to instantaneous large current discharge.
[0022] Further, as no adequate electrolyte will be supplied to the
positive electrode 11A and the negative electrode 11B, polarization
of ions tends to be inadequate at the positive electrode 11A and
the negative electrode 11B during the charge, whereby the voltage
retention property is likely to be low. Further, no adequate
adsorption of ions required for the external applied voltage for
charging against the positive electrode 11A and the negative
electrode 11B will be carried out, whereby an electrochemical
decomposition reaction or the like other than the adsorption will
take place at the positive electrode 11A and the negative electrode
11B, whereby the internal resistance is likely to increase, or the
capacitance density is likely to decrease.
[0023] Further, if it is attempted to supply the electrolyte
sufficiently to the positive electrode 11A and the negative
electrode 11B to solve such problems, it takes time for injection
of the electrolyte, thus leading to a problem in the productivity
of the electric double layer capacitor.
[0024] As a method for supplying an electrolyte sufficiently to the
electrodes, for example, JP-A-2001-44081 proposes a method wherein
a groove is formed on the surface of the electrodes to maintain the
electrolyte in the vicinity of the electrodes in an amount
corresponding to the amount of the electrolyte expected to be dried
up during the use.
[0025] However, this method is limited to maintain the electrolyte
in an amount corresponding to the amount to be dried up, and, for
example, in a case where the thickness of the separator 2 or 12 is
made thin, no adequate amount of the electrolyte required for the
polarization of ions, can be maintained in the separator 2 or
12.
[0026] The present invention has been made in view of such
conventional problems, and it is an object of the present invention
to provide an electric double layer capacitor whereby the internal
resistance of the electric double layer capacitor can be reduced,
its capacitance density can be made high, and its productivity can
be satisfactorily maintained, and a process for producing such an
electric double layer capacitor.
[0027] Namely, the present invention provides an electric double
layer capacitor having contained in a casing an electrolyte, a
positive electrode and a negative electrode each being an electrode
containing carbon black, to form an electric double layer at the
interface with the electrolyte, and a separator interposed between
the positive electrode and the negative electrode, wherein at least
one electrode of the positive electrode and the negative electrode
has protruded portions or bent portions formed continuously in the
height direction against the bottom face of the casing, and a space
due to the height of the protruded portions or the bent portions is
formed between said at least one electrode and the separator.
[0028] When impregnated with an electrolyte, electrodes usually
expand beyond the thicknesses before impregnation. Consequently,
impregnation of the electrolyte to the electrodes is likely to be
inadequate, and no adequate performance of the electric double
layer capacitor is likely to be performed. Whereas, on the
electrode of the present invention, protruded portions or bent
portions are formed continuously in the height direction against
the bottom face of the casing, whereby a space due to the height of
the protruded portions or the bent portions is formed between the
separator and the electrode, and thus, the impregnation paths for
the electrolyte to the electrode are secured.
[0029] Thus, it is possible to lower the internal resistance of the
electric double layer capacitor and to increase its capacitance
density, and it is possible to maintain its productivity
satisfactorily.
[0030] Further, with respect to the electric double layer capacitor
of the present invention, it is preferred that a plurality of such
positive electrodes and negative electrodes are alternately stacked
with the separator between them, or the positive electrode and the
negative electrode each having a long strip shape, are wound with
the separator between them, and contained in the casing having a
bottomed cylindrical shape.
[0031] Thus, it is possible to construct a rectangular or
cylindrical electric double layer capacitor having a large
capacitance by containing the positive electrode, the negative
electrode and the separator in the casing.
[0032] Further, with respect to the electric double layer capacitor
of the present invention, it is preferred that the protruded
portions or the bent portions are ones having said at least one
electrode deformed on one side or both sides, and they are formed
in a plurality at every predetermined distance in a direction
perpendicular to the height direction. By this construction, the
electrolyte will readily be impregnated to the entire
electrodes.
[0033] Further, with respect to the electric double layer capacitor
of the present invention, it is preferred that the electrode is one
comprising a metal current collector and an electrode sheet
containing a carbonaceous material as the main component and having
a thickness of from 80 to 400 .mu.m, bonded with an adhesive layer
to at least one side of the metal current collector. It is further
preferred that the above-mentioned electrode sheet is bonded to
each side of the metal current collector, and the protruded
portions or the bent portions are formed to protrude or bend
alternately on both sides of the electrode. By this construction,
the electrolyte can sufficiently be supplied to the front and back
sides of the electrode.
[0034] Further, with respect to the electric double layer capacitor
of the present invention, it is preferred that the separator has a
thickness of from 10 to 60 .mu.m, a porosity of from 40 to 85% and
the maximum pore size of at most 1 .mu.m as measured by the test
method prescribed in JIS K3832.
[0035] If the thickness of the separator is less than 10 .mu.m, the
amount of the electrolyte which can be maintained in the separator
tends to be inadequate, whereby the internal resistance is likely
to increase, and because of its thinness, short-circuiting between
electrodes is likely to result. On the other hand, if the thickness
of the separator exceeds 60 .mu.m, such tends to hinder the high
capacitance densification of the cell. Accordingly, the thickness
of the separator is preferably from 10 to 60 .mu.m. More
preferably, the thickness of the separator is from 20 to 50
.mu.m.
[0036] Further, if the porosity of the separator exceeds 85%, the
separator itself tends to be undurable against expansion of the
electrodes, whereby short-circuiting is likely to result between
the electrodes. On the other hand, if the porosity of the separator
is less than 40%, the amount of the electrolyte in the separator
tends to be small, whereby the internal resistance is likely to be
too high. Accordingly, the porosity of the separator is preferably
from 40 to 85%.
[0037] Further, in the electric double layer capacitor of the
present invention, the electrodes will expand by impregnation of
the electrolyte and/or by at least one charging operation which
will be described hereinafter, whereby the separator will be
compressed, and the electrolyte in the electrodes will be squeezed
out, whereupon the internal resistance is likely to further
increase. Accordingly, the porosity of the separator is more
preferably at least 50% and less than 80%.
[0038] Further, if the maximum pore size as prescribed in JIS
K3832, of the separator, exceeds 1 .mu.m, the electrodes are likely
to penetrate through the separator to cause short-circuiting, or
metal impurities contained in the electrodes are likely to
precipitate to cause microscopic short-circuiting. Accordingly, the
maximum pore size is preferably at most 1 .mu.m. More preferably,
the average pore size as prescribed in JIS K3832, of the separator,
is from 0.1 to 0.3 .mu.m.
[0039] Further, the present invention provides a process for
producing an electric double layer capacitor, which comprises a
step of forming a positive electrode and a negative electrode each
being an electrode containing carbon black, to form an electric
double layer at the interface with an electrolyte, a step of
forming protruded portions or bent portions on at least one
electrode of the positive electrode and the negative electrode, a
step of interposing a separator between the positive electrode and
the negative electrode to form an element, a step of containing the
element in a casing, a step of impregnating the element with an
electrolyte, and at least one charging operation, in this order,
wherein a space due to the height of the protruded portions or the
bent portions is formed between said at least one electrode and the
separator.
[0040] By this construction, a space due to the height of the
protruded portions or the bent portions is formed between said at
least one electrode and the separator, and impregnation paths of
the electrolyte to the electrode can be secured, whereby it is
possible to prepare an electric double layer capacitor having a low
internal resistance and high capacitance.
[0041] Further, with respect to the process for producing an
electric double layer capacitor of the present invention, it is
preferred that the electrode is formed by bonding with an adhesive
layer an electrode sheet containing a carbonaceous material as the
main component and having a thickness of from 80 to 400 .mu.m, on
at least one side of a metal current collector, the protruded
portions or the bent portions are formed so that the sum of the
thickness of the electrode and the height of the protruded portions
or the bent portions becomes to be from 1.01 to 1.20 times the
thickness of the electrode comprising the sum of the thicknesses of
the metal current collector, the electrode sheet and the adhesive
layer, and the protruded portions and the bent portions are formed
at a distance of at most 20 mm in a direction perpendicular to the
height direction of the casing.
[0042] In order to secure a large capacitance of the electrodes,
the electrodes are preferably ones containing a carbonaceous
material as the main component. Further, in order to sufficiently
secure the electrical conductivity, it is necessary to incorporate
carbon black in the electrode sheet.
[0043] By this construction, ions in an amount required for
polarization can be secured in the electrodes, and the
electrolyte-retention property can be improved.
[0044] Further, if the thickness of the electrode sheet is less
than 80 .mu.m, the space between the separator and the electrode
due to the protruded portions or the bent portions tends to be too
wide relative to the thickness of the electrode. Accordingly, a
vacant distance between the electrode will be substantial, thus
hindering reduction of the resistance of the electric double layer
capacitor or leading to a decrease of the capacitance density. On
the other hand, if the thickness of the electrode sheet exceeds 400
.mu.m, the space formed between it and the separator is likely to
be reduced by the self expansion. Accordingly, impregnation paths
of the electrolyte to the electrodes are likely to be closed, and
no adequate supply of the electrolyte to the electrodes will be
possible, such being undesirable. Thus, the thickness of the
electrode sheet is preferably from 80 to 400 .mu.m.
[0045] Further, if the sum of the thickness of this electrode and
the height of the protruded portions or the bent portions is less
than 1.01 times to the thickness of the electrode, the space
between the separator and the electrode tends to be inadequate,
whereby the impregnation of the electrolyte to the electrode tends
to be low. On the other hand, if it exceeds 1.20 times, the
distance between the electrodes tends to be so much that the
internal resistance tends to be high, or the capacitance density is
likely to decrease. Accordingly, the sum of the thickness of the
electrode and the height of the protruded portions or the bent
portions is preferably from 1.01 times to 1.20 times to the
thickness of the electrode.
[0046] Further, if the distance between the protruded portions or
between the bent portions is larger than 20 mm, the space between
the separator and the electrode tends to be closed down, and no
adequate amount of the electrolyte tends to be supplied to the
electrodes. Accordingly, it is preferred that the protruded
portions or the bent portions are formed at a distance of at most
20 mm. More preferably, this distance is at most 15 mm.
[0047] Further, if the distance between the protruded portions or
between the bent portions is less than the thickness of the
electrode, processing to provide the distance between the protruded
portions or between the bent portions itself tends to be difficult,
and such being not practical. Accordingly, it is preferred that the
protruded portions or the bent portions are provided at a distance
wider than the thickness of the electrode.
[0048] Further, with respect to the process for producing an
electric double layer capacitor of the present invention, it is
preferred that the thickness of the electrode expands by from 1.1
times to 1.6 times by the step of impregnating the electrolyte and
the charging operation.
[0049] If the expansion of the thickness of the electrode is less
than 1.1 times, an excess space tends to form between the electrode
and the separator even after impregnation of the electrolyte, etc.,
which is likely to hinder reduction of the resistance. On the other
hand, if the thickness of this electrode expands beyond 1.6 times,
the space between the separator and the electrode tends to be
closed down during the preparation of the electric double layer
capacitor, whereby no adequate amount of the electrolyte tends to
be supplied to the electrode. Further, the separator tends to be
not durable against the expansion of the electrode, whereby
short-circuiting is likely to take place between the electrodes.
Accordingly, it is preferred that the thickness of the electrode
expands by from 1.1 to 1.6 times by the step of impregnating the
electrolyte and at least one charging operation. More preferably,
this expansion is from 1.15 to 1.5 times.
[0050] In the accompanying drawings:
[0051] FIG. 1 is a cross-sectional view of an electrode as an
embodiment of the present invention.
[0052] FIG. 2 is a cross-sectional view of a processed electrode as
an embodiment of the present invention.
[0053] FIG. 3 is a cross-sectional view of a processed electrode as
another embodiment of the present invention.
[0054] FIG. 4 is a cross-sectional view of a wound element assembly
as an embodiment of the present invention.
[0055] FIG. 5 is a cross-sectional view of a wound element assembly
as another embodiment of the present invention.
[0056] FIG. 6 is a perspective view of a cylindrical cell as an
embodiment of the present invention.
[0057] FIG. 7 is a perspective view illustrating the structure of a
rectangular cell partially cut.
[0058] FIG. 8 is a perspective view showing the structure of a
cylindrical cell partially cut.
[0059] FIG. 9 is a perspective view illustrating an embodiment of
the structure of a high pressure power source module.
[0060] Now, the preferred embodiments of the present invention will
be described with reference to the drawings. Various members
constituting the electric double layer capacitors according to the
embodiments of the present invention will be described. In the
following description, various members will be described with
reference to those to be used for a cylindrical cell.
[0061] Firstly, the electrode will be described. A cross-section of
the electrode is shown in FIG. 1.
[0062] In FIG. 1, an electrode 31 has a structure in which
electrode sheets 37A and 37B are bonded to both sides of a metal
current collector foil 33 of a band or strip shape, with an
adhesive layer 35. The electrode 31 may be prepared by bonding an
electrode sheet 37A only on one side of the metal current collector
foil 33 with an adhesive layer 35. However, it is preferred to
prepare it by bonding electrode sheets 37A and 37B on both sides of
the metal current collector foil 33, to facilitate high capacitance
of the cell.
[0063] This metal current collector foil 33 is not particularly
limited so long as it is excellent in electrochemical corrosion
resistance on the positive electrode side, and a foil, net or the
like of aluminum, stainless steel or the like may be used. However,
one composed mainly of aluminum is preferred, since it is light in
weight and has low resistance, and even when it is rolled into a
thin foil, it has sufficient strength and is electrochemically
stable.
[0064] Further, the thickness of the metal current collector foil
33 is made as thin as possible within a range where the strength
allows, and it is usually preferably within a range of from 20 to
100 .mu.m. Further, for the purpose of reducing the bonding
resistance or improvement of the bonding strength with the
electrode sheets 37A and 37B, chemical, electrochemical or
mechanical surface etching treatment may be applied.
[0065] Further, the metal current collector foil 33 has an end
strip portion not shown, where the electrode sheets 37A and 37B are
not bonded, and this end strip portion is to take an electrical
connection with the exterior. Accordingly, in order to increase the
cell capacitance, the end strip portion should better be as narrow
as possible, and is preferably from about 2 to 6 mm.
[0066] The electrode sheets 37A and 37B contain a carbonaceous
material as the main component and contain carbon black. And, the
electric double layer capacitor is based on a principle such that
an electric charge is accumulated in the electric double layer
formed at the interface between the electrode and the
electrolyte.
[0067] Accordingly, in order to increase the capacitance of the
electric double layer capacitor, the specific surface area of the
carbonaceous material is preferably as large as possible, and it is
preferred to use a carbonaceous material having a specific surface
area of from 100 to 2,500 m.sup.2/g as the main component. The
carbonaceous material may, for example, be activated carbon, carbon
black, polyacene or carbon aerogel.
[0068] Further, as carbon black, one having high electrical
conductivity and large oil absorptivity, such as Ketjenblack, is
used particularly preferably. The content of carbon black in the
electrode sheets 37A and 37B is preferably made to be from 5 to 30
mass % in the total amount of the carbonaceous material and the
binder. If the content of carbon black is less than 5 mass %, it
tends to be difficult to maintain the electrolyte sufficiently, and
it tends to be difficult to supply ions in an amount necessary for
polarization. On the other hand, if the content of carbon black
exceeds 30 mass %, the electrodes are likely to expand too much by
impregnation of the electrolyte, such being undesirable.
[0069] The adhesive layer 35 is one to be used to bond the
electrode sheets 37A and 37B to the metal current collector foil
33. Accordingly, it is required to have a sufficient adhesive
property and high electrical conductivity. Further, the adhesive
layer 35 is required to have heat resistance sufficiently durable
against drying at the time of removing moisture from the electrode
31, and at the same time, it is required to be stable against the
electrolyte to be used in the present invention and to be
electrochemically stable within the voltage range in which the
electric double layer capacitor is used.
[0070] Accordingly, an electrically conductive adhesive is
preferably used in which graphite or carbon black is dispersed as
an electroconductive material, using a polyamideimide resin, a
polyvinylidene fluoride, a polytetrafluoroethylene, a polyimide
resin or the like as the binder. Further, the electrode 31 of the
present invention may be prepared also by coating an electrode
material containing a carbonaceous material as the main component,
on a metal current collector foil 33. However, from the viewpoint
of efficient processability, the above-mentioned method of bonding
the electrode sheets 37A and 37B to the metal current collector 33
with the adhesive layer 35, is more preferred.
[0071] Now, the electrolyte will be described.
[0072] The electrolyte to be used for the electric double layer
capacitor includes an aqueous electrolyte and a non-aqueous
electrolyte. Here, the withstand voltage of a unit cell in a case
where the aqueous electrolyte is used, is about 0.8 V, and the
withstand voltage in a case where the non-aqueous electrolyte is
used, is about 2.6 V. The electrostatic energy of an electric
double layer capacitor is proportional to the square of the
withstand voltage. Accordingly, when a non-aqueous electrolyte is
used, the electrostatic energy can be made larger by at least 10
times than when an aqueous electrolyte is used. Accordingly, in the
present invention, a non-aqueous electrolyte is preferred from the
viewpoint of the energy density.
[0073] The solute contained in the non-aqueous electrolyte is
preferably a quaternary onium salt from the viewpoint of the
electrical conductivity, the solubility in a solvent and the
electrochemical stability.
[0074] Particularly preferred is either one or a mixture of two or
more of salts comprising a quaternary onium cation represented by
R.sup.1R.sup.2R.sub.3R.sup.4N.sup.+ or
R.sup.1R.sup.2R.sup.3R.sup.4P.sup.- + (wherein each of R.sup.1,
R.sup.2, R.sup.3 and R.sup.4 which are independent of one another,
is a C.sub.1-6 alkyl group), or as a cyclic quaternary nitrogen
compound, an imidazolium cation represented by
R.sup.1R.sup.2C.sub.3H.sub.3N.sub.2.sup.+ (wherein each of R.sup.1
and R.sup.2 which are independent of each other, is a C.sub.1-6
alkyl group) or a morpholinium cation represented by
R.sup.1R.sup.2C.sub.4H.sub.8ON.su- p.+ (wherein each of R.sup.1 and
R.sup.2 which are independent of each other, is a C.sub.1-6 alkyl
group), and an anion such as BF.sub.6.sup.-, PF.sub.6.sup.-,
CF.sub.3SO.sub.3--, AsF.sub.6.sup.-,
N(SO.sub.2CF.sub.3).sub.2.sup.- or ClO.sub.4.sup.-. More preferred
is an ammonium salt wherein at least one of R.sup.1, R.sup.2,
R.sup.3 and R.sup.4 is different, or an imidazolium salt wherein
R.sup.1 and R.sup.2 are different.
[0075] Further, it is possible to use a salt containing no solvent,
i.e. a molten salt, in a case where the temperature range in which
the electric double layer capacitor is used, is limited to some
extent, or in a case where ion conductivity is shown in such a
practical temperature range. For example, an imidazolium salt such
as (C.sub.2H.sub.5)
(CH.sub.3)C.sub.2H.sub.3N.sub.2N(SO.sub.2CF.sub.3).sub.2 is in a
molten state at room temperature and exhibits an ion conductivity.
Accordingly, even when this is used as the electrolyte, such a
construction will function as an electric double layer capacitor of
the present invention.
[0076] Further, in a case where an organic solvent is used to
dissolve the solute, it is preferred to use a cyclic carbonate such
as ethylene carbonate, propylene carbonate or butylene carbonate, a
nitrile such as acetonitrile, a chain carbonate such as dimethyl
carbonate, ethylmethyl carbonate or diethyl carbonate, sulfolane or
a sulfolane derivative. Particularly preferred is at least one
member selected from the group consisting of propylene carbonate,
ethylene carbonate, dimethyl carbonate, diethyl carbonate,
methylethyl carbonate, acetonitrile, sulfolane and
methylsulfolane.
[0077] Now, the separator will be described.
[0078] The separator 12 electrically insulates a positive electrode
51A and a negative electrode 51B which are described hereinafter,
while it facilitates movement of ions in the electrolyte and in the
positive electrode 51A and the negative electrode 51B, which takes
place along charging or discharging.
[0079] Accordingly, it is preferred to employ e.g. a polyethylene
porous film, a polypropylene porous film, a polyethylene non-woven
fabric, a polypropylene non-woven fabric, a polyester non-woven
fabric, a cellulose paper, a craft paper, a rayon fiber/sisal fiber
blend sheet, a Manila hemp sheet, a polyester fiber sheet or a
glass fiber sheet, having ion permeability.
[0080] Further, the positive electrode 51A and the negative
electrode 51B will usually expand in the process of being
impregnated with the electrolyte, and at least one charging
operation. Accordingly, the separator 12 is preferably one having a
strength durable against the pressure due to this expansion and
being capable of maintaining the electrolyte and having
stretchability so that it will not break even in a stretched
state.
[0081] In the process of being impregnated with the electrolyte and
in the case when the positive electrode 51A and the negative
electrode 51B are expanded to a thickness of at least 1.2 times by
at least one charging operation, particularly preferred is a porous
film of ultrahigh molecular weight polyethylene having a high
porosity of from 70 to 85% and excellent in elongation at berakage,
a porous film of ultrahigh molecular weight polyethylene having
inorganic particles packed, a polyester non-woven fabric made of a
polyethylene terephthalate fiber or polybutylene terephthalate, or
a sheet having inorganic particles packed therein. Also, when the
positive electrode 51A and the negative electrode 51B are expanded
to a thickness of at least 1.1 times and less than 1.2 times, it is
preferable to use a cellulose paper as a separator, which is
excellent in denseness and has a maximum pore size of at most 1
.mu.m defined in accordance with JIS K3832, even if it is thin
film-like. Particularly, it is preferable to use a cellulose paper
separator prepared from solvent-spinned rayon.
[0082] Now, the processed electrode will be described. A
cross-sectional view of the processed electrode is shown in FIG. 2.
Here, the same elements as in FIG. 1 are identified by the same
symbols, and their description will be omitted.
[0083] In FIG. 2, a processed electrode 41A is modified so that,
against the electrode 31, it has a long strip shape strip portion
43 similar to the conventional one and protruded portions each
having a convex portion 45a and a concave portion 45b.
[0084] In the after-mentioned wound type element assembly 53, the
convex portion 45a is one formed to provide a prescribed space D
between the separator 12 and the strip portion 43, and the metal
current collector foil 33, the electrode sheets 37A and 37B, etc.
constituting the processed electrode 41A are integrally raised by
the space D.
[0085] On the other hand, the concave portion 45b is one formed as
a result of such that at the time of forming the convex portion
45a, the constituting members of the processed electrode 41a were
integrally raised, and the cross-sectional shape of this concave
45b is a shape along the periphery of the prescribed semiellipse
(the long diameter corresponds to the width B, and a half of its
short diameter corresponds to the depth D).
[0086] And, the deformation of such convex 45a and concave 45b
(hereinafter, both may be altogether referred to as a
convexoconcave portion 45) continuously extends vertically from the
front side to the rear side of the paper plane in the FIG. of the
processed electrode 41A.
[0087] Accordingly, the convexoconcave portion 45 of the processed
electrode 41A can be formed, for example, by pressing the electrode
31 against the side surface of an elliptic cylindrical rod having a
prescribed elliptic cross-section.
[0088] Further, the convexoconcave portions 45 alternately have
convex portions 45a/concave portions 45b directed to
upward/downward in the FIG., and the space between the adjacent
convex portion 45a and concave portion 45b has a distance A. Here,
the distance A represents the distance between the adjacent two
protruded portions and corresponds to the distance from the most
raised portion of the convex 45a to the most recessed portion of
the adjacent concave portion 45b. By this construction, a space D
from the separator 12 is formed on the front and rear sides of the
processed electrode 41A, whereby the electrolyte becomes to be
readily impregnated to the processed electrode 41A, such being
preferred since the electrolyte can sufficiently be supplied to the
front and rear sides of the electrode 31 especially in a case where
the electrode sheets 37A and 37B are bonded on both sides of the
metal current collector foil 33 as shown in FIG. 2.
[0089] Further, the cross-sectional shape of the concave portion
45b is not limited to the above-described case where it is a shape
along the periphery of the prescribed semiellipse, and it may be a
shape along an angular side. Further, so long as it is continuous
in the height direction against the bottom face of the casing, it
is not limited to the above-mentioned shape perpendicular to the
paper face and may, for example, be an oblique line shape.
[0090] A cross-sectional view of another embodiment of this
processed electrode is shown in FIG. 3. Here, the same elements as
in FIG. 2 are identified by the same symbols, and their description
will be omitted.
[0091] In FIG. 3, the cross-sectional shape of the concave 47b of
the processed electrode 41B has a shape along two sides other than
the bottom of the prescribed isosceles triangle (the bottom being
width B, and the height being D). Further, the processed electrode
41B is constituted solely by a convexoconcave portion 47 without
being provided with a portion corresponding to the strip portion 43
of the processed electrode 41A.
[0092] Accordingly, in other words, the processed electrode 41B can
be said to be one having the electrode 31 (simply) deformed to have
prescribed bent portions. Accordingly, the processed electrode 41B
can be formed, for example, by pressing the electrode 31 to an
angular hard plate having the prescribed isosceles triangle.
[0093] By this construction, the processed electrode 41B is also
capable of forming a space D on average between it and the
separator 12 on its front and rear sides.
[0094] Further, in the following, the processed electrode 41A and
the processed electrode 41B are commonly referred to as a processed
electrode 41.
[0095] Now, the wound element assembly will be described.
Cross-sectional views of the wound element assemblies are shown in
FIGS. 4 and 5. The same elements as in FIGS. 2, 3 and 8 are
identified by the same symbols, and their descriptions will be
omitted.
[0096] Here, the cross-sectional view of the wound element assembly
shown in FIG. 4 corresponds to the processed electrode 41A shown in
FIG. 2, and the cross-sectional view of the wound element assembly
shown in FIG. 5 corresponds to the processed electrode 41B shown in
FIG. 3.
[0097] In each of FIGS. 4 and 5, the wound element assembly 53 is
constituted by a positive electrode 51A and negative electrode 51B
using the processed electrode 41, as is different from the wound
element assembly 13 using conventional long strip shaped positive
electrode 11A and negative electrode 11B. The processed electrode
41 may be used for either one of the positive electrode 51A and the
negative electrode 51B. However, it is preferred to use it for each
of the positive electrode 51A and the negative electrode 51B,
whereby the electrolyte can better be maintained.
[0098] And, the wound element assembly 53 is formed by sandwiching
a separator 12 by the positive electrode 51A and the negative
electrode 51B and winding them up by a winding core not shown.
[0099] At that time, the processed electrodes 41 constituting the
positive electrode 51A and the negative electrode 51B are arranged
and wound up so that "from the front to the rear direction in the
FIG." being a direction in which the deformation of the
convexoconcave portions 45 or 47 extends, corresponds to the
winding axis direction of the wound element assembly 53 (the height
direction against the bottom of the casing), so that
"upward/downward in the FIG." being the direction of the convex
45a/the concave 45b or the convex 47a/the concave 47b, corresponds
to the winding diameter direction (the direction parallel with the
bottom of the casing), and so that "a left/right direction in the
FIG." being the direction of the distance A of the convexoconcave
portions 45 or 47, corresponds to the rolling direction of the
winding (the direction perpendicular to the height direction
against the bottom of the casing).
[0100] Accordingly, between the separator 12 and the positive
electrode 51A or the negative electrode 51B, a space D is formed
continuously over the entirety in the axis direction of the winding
of the wound element assembly 53. By this construction,
impregnation paths of the electrolyte can be secured over the
entire axis direction of the winding.
[0101] Now, the cylindrical cell will be described. A perspective
view of the cylindrical cell is shown in FIG. 6. Here, the same
elements as in FIGS. 4 and 8 are identified by the same symbols,
and their description will be omitted.
[0102] In FIG. 6, a cylindrical cell 60 is constituted by having
the wound element assembly 53 contained therein, although not
shown.
[0103] This wound element assembly 53 is contained so that the
height direction against the bottom of the cylindrical casing 15
corresponds to the axis direction in the same manner as in the
conventional structure.
[0104] Further, to this wound element assembly 53, leads 17A and
17B are connected to perform electrical connection to the exterior,
respectively, to the respective end strip portions of the metal
current collector foils 33 of the positive electrode 51A and the
negative electrode 51B.
[0105] Here, the method for connecting the end strip portions and
the leads 17A and 17B may be electroconductive bonding by
mechanical pressing or by a conductive adhesive, but is preferably
a welding method which is mechanically and electrically highly
reliable. For such a welding method, an ultrasonic welding, a laser
welding by YAG or an electron beam welding method is suitably
employed.
[0106] For the leads 17A and 17B, the material is not particularly
limited so long as it has high electrical conductivity and
electrochemical corrosion resistance, but aluminum or an aluminum
alloy is, for example, preferred. Further, its shape is not
particularly limited, but it is necessary not to hinder the
impregnation of the electrolyte at the end face of the wound
element assembly 53.
[0107] Further, to the leads 17A and 17B, the terminal 19A of a
positive electrode and the terminal 19B of a negative electrode are
connected, respectively. Such terminal 19A of the positive
electrode and the terminal 19B of a negative electrode extend
through and fixed to a sealing insulation plate 66 having an
electrolyte injection hole 61, and they are air-tightly attached to
the sealing insulation plate 66 with an insulating resin.
[0108] In such a construction, it is conceivable to make the
thickness of the separator 12 as thin as possible as mentioned
above in order to accomplish high densification and lower
resistance of the electric double layer capacitor, but in such a
case, supply of the electrolyte to the processed electrode 41 was
likely to be inadequate.
[0109] In order to supply the electrolyte sufficiently, it is known
to be effective to impregnate a sufficient amount of the
electrolyte in pores in the carbonaceous material of the processed
electrode 41 in addition to permitting the electrolyte present in
the separator 12.
[0110] However, with the processed electrode 41, its thickness
usually expands beyond the initial thickness upon impregnation of
the electrolyte. This is likely to result by increasing the packing
amount of the electrode sheet 37A or 37B or by adding carbon black
into the processed electrode 41, in order to increase the
capacitance of the electric double layer capacitor with the
processed electrode 41 as an embodiment of the present invention.
Further, such expansion is likely to take place particularly when
carbon black having a large electrolyte-retaining amount per unit
volume, such as Ketjenblack, is used.
[0111] In such a case, by the expansion of the processed electrode
41 itself, the electrolyte tends to hardly penetrate into the
interior of the wound element assembly 53, and it tends to be
difficult to supply the electrolyte to the processed electrode 41
itself.
[0112] Further, the thickness of the processed electrode 41 usually
expands also by at least one charging operation, although such may
depend also on the type of the carbonaceous material to be used for
the electrode sheet 37A or 37B. This is caused by acceleration of
the adsorption of the electrolyte in the pores of the carbonaceous
material of the processed electrode 41 as the application of the
voltage to the processed electrode 41 becomes a driving force.
Further, this expansion is likely to take place especially when a
readily graphitizable alkali-activated active carbon or the like is
used as the carbonaceous material.
[0113] Also in such a case, the electrolyte tends to be hardly
supplied into the interior of the wound element assembly 53, and
impregnation into the processed electrode 41 tends to be
inadequate, whereby no adequate performance of the electric double
layer capacitor is likely to be obtainable.
[0114] However, with the electric double layer capacitor of the
present invention, a prescribed space D is formed between the
separator 12 and the positive electrode 51A or the negative
electrode 51B, whereby impregnation paths of the electrolyte into
the positive electrode 51A or the negative electrode 51B are
secured. Accordingly, even if expansion of the positive electrode
51A and the negative electrode 51B takes place during the injection
of the electrolyte or the subsequent charging operation, the
electrolyte can be supplied to the entire wound element assembly
53.
[0115] Accordingly, at the time of discharging, the electrolyte is
sufficiently present in the vicinity of the positive electrode 51A
and the negative electrode 51B, whereby even if an instantaneous
large current discharge happens, the voltage drop can be minimized.
Further, even at the time of charging, the electrolyte is
sufficiently supplied to the positive electrode 51A and the
negative electrode 51B, whereby polarization of ions can
sufficiently be carried out, and the voltage-maintaining property
can be improved.
[0116] Further, it does not take time for injection of the
electrolyte, whereby the productivity of the electric double layer
capacitor will be improved.
[0117] Further, with respect to the thicknesses, the expansion
degrees, etc. of the positive electrode 51A and the negative
electrode 51B, the distance A, the depth D, etc. of the
convexoconcave portions 45 and 47 of the processed electrode 41,
the thickness, the porosity, the maximum pore diameter (the
definition will be described hereinafter), etc., it is advisable to
decide their degrees taking into the following points into
consideration.
[0118] Firstly, the thicknesses of the positive electrode 51A and
the negative electrode 51B will be considered.
[0119] If the thicknesses of the positive electrode 51A and the
negative electrode 51B are too thin, excess spaces are likely to be
present between the separator 12 and the positive electrode 51A or
the negative electrode 51B even after the positive electrode 51A
and the negative electrode 51B have expanded. Accordingly, the
distance between the positive electrode 51A and the negative
electrode 51B is likely to increase, thus hindering the reduction
of the resistance or leading to a decrease of the capacitance
density.
[0120] On the other hand, if the thicknesses of the positive
electrode 51A and the negative electrode 51B are too thick, the
space D formed between the separator 12 and the positive electrode
51A or the negative electrode 51B is likely to be closed down when
the expansion thereof takes place. Accordingly, during the
injection of the electrolyte or the like, impregnation paths of the
electrolyte to the positive electrode 51A and the negative
electrode 51B are likely to be closed, whereby no adequate supply
of the electrolyte is likely to be carried out.
[0121] Accordingly, it is advisable to determine the thicknesses of
the positive electrode 51A and the negative electrode 51B so that a
proper space will be formed between them and the separator 12 when
the positive electrode 51A and the negative electrode 51B have
expanded. It is particularly preferred that the thicknesses of the
electrode sheets 47A and 47B are set to be from 80 to 400
.mu.m.
[0122] Now, the degree of expansion of the positive electrode 51A
and the negative electrode 51B will be considered.
[0123] If the degree of expansion of the positive electrode 51A and
the negative electrode 51B is small, an excessive space is likely
to be formed between the separator 12 and the positive electrode
51A or the negative electrode 51B depending upon the relation to
the thicknesses of the positive electrode 51A and the negative
electrode 51B or the depth D of the convexoconcave portion 45 or 47
of the processed electrode 41.
[0124] On the other hand, if the degree of expansion of positive
electrode 51A and the negative electrode 51B is large, the space D
formed between the separator 12 and the positive electrode 51A or
the negative electrode 51B is likely to be closed down. Further,
the separator 12 may not be durable against the expansion of the
positive electrode 51A and the negative electrode 51B, whereby the
positive electrode 51A and the negative electrode 51B are likely to
break through the separator 12 to cause short-circuiting.
Accordingly, it is preferred to adjust so that the thicknesses of
the positive electrode 51A and the negative electrode 51B will
expand from 1.1 to 1.6 times by the step of impregnating the
electrolyte and at least one charging operation.
[0125] Now, the distance A and the width B of the convexoconcave
portions 45 or 47 of the processed electrode 41 will be
considered.
[0126] If the distance A between the convexoconcave portions 45 or
47 is too large, the space D is likely to be closed down when the
processed electrode 41 is wound up as a wound element assembly 54,
whereby it tends to be difficult to supply an adequate amount of
the electrolyte to the positive electrode 51A and the negative
electrode 51B.
[0127] Further, it is not practical to make the distance A between
the convexoconcave portions 45 or 47 to be a length shorter than
the thickness of the positive electrode 51A and the negative
electrode 51B, since such processing itself tends to be difficult.
Accordingly, it is advisable to determine the distance A of the
convexoconcave portions 45 or 47 within a range where processing is
easy at the time of the preparation of the wound element assembly
53 and so that the space D will not be closed down. It is
particularly preferred to set the distance A to be at least the
thicknesses of the positive electrode 51A and the negative
electrode 51B and at most 20 mm.
[0128] Further, the width B of the convexoconcave portions 45 or 47
is preferably at least equal to and at most two times of the
thicknesses of the positive electrode 51A and the negative
electrode 51B, but there is no problem in effects, if it exceeds
this range. Further, with respect to the convexoconcave portions
45, they may be constituted solely by convexes 45a without being
provided with concaves 45b.
[0129] Now, the depth D of the convexoconcave portions 45 or 47
will be considered.
[0130] The depth D of the convexoconcave portions 45 or 47 can be
calculated by the ratio of the apparent thickness to the initial
thickness of the electrode before expansion of the electrode 31 by
impregnation of the electrolyte or by charging operation. Here, the
ratio of the apparent thickness to the initial thickness of the
electrode means the ratio of the stacked thickness of the processed
electrode 41 after forming the convexoconcave portions 45 or 47, to
the stacked thickness before forming the convexoconcave portions 45
or 47 on the electrode 31.
[0131] If this ratio of the apparent thickness to the initial
thickness of the electrode is too small (i.e. the ratio is too
close to 1), the space D formed between the separator 12 and the
positive electrode 51A or the negative electrode 51B will be
inadequate, whereby no effect for improving the impregnation
property of the electrolyte to the positive electrode 51A and the
negative electrode 51B is likely to be obtained. On the other hand,
if this ratio is too large, although a sufficient amount of the
electrolyte will be supplied to the positive electrode 51A and the
negative electrode 51B, the space D from the separator 12 tends to
be large even after impregnation of the electrolyte or charging
operation, whereby the internal resistance is likely to be high,
and the capacitance density is likely to be low. Accordingly, it is
preferred to set this ratio to be from 1.01 to 1.20 times.
[0132] Now, the thickness or the porosity of the separator 12 will
be considered.
[0133] As mentioned above, if the thickness of the separator 12 is
too thin, the amount of the electrolyte which can be maintained in
the separator 12, tends to be inadequate, whereby the internal
resistance is likely to increase. Further, if the separator 12 is
too thin, short-circuiting is likely to result between the positive
electrode 51A and the negative electrode 51B.
[0134] Further, if the thickness of the separator 12 is too thick,
such will hinder to increase the capacitance of the positive
electrode 51A and the negative electrode 51B, whereby it tends to
be difficult to improve the capacitance density of the electric
double layer capacitor.
[0135] On the other hand, if the porosity of the separator 12 is
too high, the separator 12 tends to be hardly durable against
expansion of the positive electrode 51A and the negative electrode
51B, whereby short-circuiting is likely to result between the
positive electrode 51A and the negative electrode 51B.
[0136] Further, if the porosity of the separator 12 is too low, the
amount of the electrolyte to be impregnated in the separator 12
tends to be small, whereby the internal resistance is likely to be
high. Further, the separator 12 is likely to be pressed by
expansion of the positive electrode 51A and the negative electrode
51B, and the electrolyte in the separator 12 will be squeezed out,
whereby the internal resistance tends to further increase.
[0137] Accordingly, it is advisable to set the thickness and the
porosity of the separator 12 so as to prevent the increase of the
internal resistance and to prevent short-circuiting between the
positive electrode 51A and the negative electrode 51B. It is
particularly preferred to set the thickness of the separator 12 to
be from 10 to 60 .mu.m and to set the porosity to be from 40 to
85%.
[0138] Further, the maximum pore size of the separator 12 will be
considered. Here, the maximum pore size means the maximum pore size
by the test method stipulated in JIS K3832.
[0139] If the maximum pore size of the separator 12 is too large,
the positive electrode 51A and the negative electrode 51B are
likely to pass through the separator 12 to cause short-circuiting,
or metal impurities contained in the positive electrode 51A and the
negative electrode 51B are likely to precipitate to cause
microscopic short-circuiting. Accordingly, it is preferred to set
the maximum pore size of the separator 12 to be at most 1
.mu.m.
[0140] Now, specific examples of the electric double layer
capacitor of the present invention will be described.
EXAMPLE 1
[0141] A process for preparing an electrode 31 will be described in
detail. To prepare the electrode 31, a mixture comprising 80 mass %
of steam-activated active carbon powder having a specific surface
area of 1,800 m.sup.2/g made of a phenol resin as the starting
material, as a carbonaceous material, 10 mass % of carbon black as
an electroconductive material and 10 mass % of
polytetrafluoroethylene as a binder, was prepared. And, to this
mixture, propylene glycol was added as a kneading adjuvant, and
this mixture was kneaded and then continuously rolled to obtain a
long sheet having a thickness of 200 .mu.m.
[0142] Then, this long sheet was continuously dried at 300.degree.
C. to remove the kneading adjuvant, followed by further rolling and
slitting to obtain electrode sheets 37A and 37B having a thickness
of 140 .mu.m and a width of 100 mm.
[0143] Such electrode sheets 37A and 37B were continuously bonded
to both sides of a metal current collector foil 33 by means of an
electrically conductive adhesive as an adhesive layer 35. As the
metal current collector foil 33, aluminum having a thickness of 40
.mu.m was used, and the electrode sheets 37A and 37B were bonded to
the portions having a width of 100 mm, of this metal current
collector foil 33.
[0144] And, these electrode sheets 37A and 37B, the metal current
collector foil 33, etc., were altogether roll-pressed and
integrated to obtain a long strip shaped electrode 31 having a
thickness of 320 .mu.m.
[0145] Then, a process for preparing the processed electrode 41
will be described in detail.
[0146] To prepare the processed electrode 41, the side face of an
oval columnar rod was pressed against the long strip shaped
electrode 31, whereby groove-shaped recesses as convexoconcave
portions 45 having a width B of concaves 45b being 0.8 mm and a
depth D being 0.2 mm, were formed alternately on the front and rear
sides so that the distance A became about 15 mm. By this
construction, a processed electrode 41A as generally shown in FIG.
2, will be formed.
[0147] Then, a process for preparing a wound element assembly 53
will be described in detail.
[0148] To prepare the wound element assembly 53, a separator 12 was
sandwiched between a positive electrode 51A and a negative
electrode 51B using the processed electrode 41, and the position in
the width direction was adjusted so that the separator 12 protrude
from the width of the positive electrode 51A and the negative
electrode 51B by 4 mm on the left and right hand sides,
respectively. And the one having the position so adjusted was wound
up on a winding core having a diameter of 8 mm to prepare a wound
element assembly 53 having a diameter of 40 mm and a length of 108
mm.
[0149] Here, as the separator 12, one obtained by slitting a porous
film made of an ultrahigh molecular weight polyethylene and having
a thickness of 40 .mu.m, a porosity of 80% and a maximum pore size
of 0.8 .mu.m by the test method stipulated in JIS K3832, with a
width of 108 mm, was used.
[0150] Now, a process for preparing the cylindrical cell 60 will be
described.
[0151] To prepare the cylindrical cell 60, the wound element
assembly 53 was contained in an aluminum cylindrical casing 15
having a height of 120 mm, a diameter of 41 mm and a wall thickness
of 0.4 mm. And, a terminal 19A of the positive electrode and a
terminal 19B of the negative electrode were air-tightly attached
with an insulating resin to a sealing insulation plate 66 made of
aluminum and having an electrolyte injection hole 61. And, to such
a terminal 19A of the positive electrode and a terminal 19B of the
negative electrode, leads 17A and 17B were, respectively, welded by
supersonic welding, and then, the sealing insulation plate 66 was
fixed in the cylindrical casing 15, followed by laser welding to
seal the cylindrical casing 15.
[0152] Then, in a state where the electrolyte injection hole 61 of
the sealing insulation plate 66 was opened, vacuum drying treatment
was carried out for 72 hours in an atmosphere of 90.degree. C. The
interior of the element was vacuumed to 30 Pa, and an electrolyte
having 1.5 mol/kg of (C.sub.2H.sub.5).sub.3(CH.sub.3)NBF.sub.4
dissolved in propylene carbonate, was injected from the electrolyte
injection hole 61 under atmospheric pressure. And after expiration
of 30 minutes, an excess electrolyte was removed, and a safety
valve was put on the electrolyte injection hole 61 for air-tight
sealing.
[0153] Thus, a cylindrical cell 60 was prepared as the electric
double layer capacitor.
EXAMPLE 2
[0154] With the electrode 31 in Example 1, an angular hard plate
having a prescribed isosceles triangle was pressed against this
electrode 31 to bend it to form convexoconcave portions 47
alternately on the front and rear sides and to make the distance A
to be 10 mm. By this construction, a processed electrode 41B as
generally shown in FIG. 3, will be formed. Otherwise, preparation
was carried out in the same manner as in Example 1.
EXAMPLE 3
[0155] Preparation was carried out in the same manner as in Example
1 except that in the electrode 31 in Example 1, an alkali-activated
active carbon powder having a specific surface area of 800
m.sup.2/g and made of petroleum pitch as the starting material, was
used as the carbonaceous material, and a non-woven fabric made of a
polyethylene terephthalate and having a thickness of 50 .mu.m, a
porosity of 60% and a maximum pore diameter of 0.9 .mu.m by the
test method stipulated in JIS K3832, was used as the separator
12.
EXAMPLE 4
[0156] Preparation was carried out in the same manner as in Example
2 except that in the electrode 31 in Example 2, an alkali-activated
active carbon powder having a specific surface area of 800
m.sup.2/g and made of petroleum pitch as the starting material, was
used as the carbonaceous material, and a non-woven fabric made of a
polyethylene terephthalate and having a thickness of 50 .mu.m, a
porosity of 60% and a maximum pore diameter of 0.9 .mu.m by the
test method stipulated in JIS K3832, was used as the separator
12.
EXAMPLE 5
[0157] Preparation was carried out in the same manner as in Example
1 except that in Example 1, a cellulose paper prepared from a
solvent spinning rayon (Rayocel, tradename) and having a thickness
of 50 .mu.m, a porosity of 55% and a maximum pore size of 0.7 .mu.m
by the test method stipulated in JIS K3832, was used as the
separator 12.
EXAMPLE 6
[0158] Preparation was carried out in the same manner as in Example
1 except that in the processed electrode 41 in Example 1, the
convexoconcave portions 45 were formed so that the distance A would
be 30 mm.
EXAMPLE 7
[0159] Preparation was carried out in the same manner as in Example
1 except that a non-woven fabric made of a polyethylene
terephthalate and having a thickness of 60 .mu.m, a porosity of 80%
and a maximum pore size of 2.5 .mu.m by the test method stipulated
in JIS K3832, was used as the separator 12.
COMPARATIVE EXAMPLE 1
[0160] Preparation was carried out in the same manner as in Example
1 except that no convexoconcave portions 45 were formed in the
electrode 31 of Example 1.
COMPARATIVE EXAMPLE 2
[0161] Preparation was carried out in the same manner as in Example
1 except that in the electrode 31 in Example 1, a graphite powder
was used instead of the carbon black, as the conductive
material.
[0162] In the foregoing Examples 1 to 7 and Comparative Examples 1
and 2, the following measurements were respectively carried
out.
[0163] Measurement 1
[0164] Prior to injection of the electrolyte, the cross-section of
the wound element assembly 53 was observed, and the thickness of
the stack at the portion having the positive electrode 51A/the
separator 12/the negative electrode 51B/the separator 12 stacked in
this order, was measured.
[0165] Measurement 2
[0166] In addition to the measurement 1, the ratio of the apparent
thickness to the initial thickness of the electrode was
calculated.
[0167] Measurement 3
[0168] Then, the amount of the electrolyte at the time when the
cylindrical cell 60 was obtained, was measured.
[0169] Measurement 4
[0170] Further, the cylindrical cell 60 prepared separately from
the one used in Measurement 3, was disassembled, and the initial
expansion rate was measured.
[0171] Here, the initial expansion rate is a ratio of the thickness
of the positive electrode 51A and the negative electrode 51B after
injecting the electrolyte to the thickness of the positive
electrode 51A and the negative electrode 51B before injection of
the electrolyte.
[0172] Measurement 5
[0173] Then, using a cylindrical cell 60 prepared separately from
the one used in Measurement 3 or Measurement 4, constant voltage
charging was carried out at a voltage of 2.6 V for 30 minuets,
whereupon discharging was carried out at a constant current of 30 A
to a voltage of 1.0 V.
[0174] At that time, the capacitance of the entire cylindrical cell
60 was obtained from the inclination of the discharge curve from a
voltage of 2.6 V to a voltage of 1.0 V.
[0175] Measurement 6
[0176] Further, in addition to Measurement 5, the internal
resistance was calculated from the voltage drop at the initial
stage of discharge.
[0177] Measurement 7
[0178] Further, in addition to Measurement 6, the retention voltage
after being left for 72 hours in an open-circuit state after
constant voltage charging at a voltage of 2.6 V for 24 hours, was
measured.
[0179] Measurement 8
[0180] Then, the cylindrical cell 60 after completion of
Measurement 7 was disassembled, and the thicknesses of the positive
electrode 51A and the negative electrode 51B were measured, and the
expansion rate was measured.
[0181] This expansion rate will be referred to as the expansion
rate after charging/discharging against the initial expansion rate
by Measurement 4.
[0182] The results of the foregoing Measurements 1 to 8, are shown
in Table 1.
1 TABLE 1 Measurement Measure- Measure- Measure- 2 Ratio ment 3
Measure- Measure- ment 8 ment 1 of the apparent Amount of ment 4
Measure- Measure- ment 7 Expansion Stacked thickness to the the
Initial ment 5 ment 6 Retention rate after thickness initial
thickness electrolyte expansion Capacitance Resistance voltage
charging and (.mu.m) of the electrode (g) rate (F) (m.OMEGA.) (V)
discharging Ex. 1 735 1.021 111 1.13 1650 2.21 2.444 1.15 Ex. 2 760
1.056 115 1.13 1550 2.41 2.451 1.15 Ex. 3 780 1.054 109 1.11 2210
3.22 2.391 1.48 Ex. 4 790 1.068 112 1.11 2180 3.31 2.401 1.51 Ex. 5
780 1.055 117 1.13 1570 2.33 2.460 1.16 Ex. 6 725 1.007 100 1.11
1619 3.53 2.055 1.12 Ex. 7 736 1.022 113 1.13 1650 2.18 0.507 1.15
Comp. 720 1.000 79 1.11 1412 5.42 0.221 1.11 Ex. 1 Comp. 737 1.024
75 1.01 890 20.8 0.011 1.05 Ex. 2
[0183] In Table 1, from the results of measurements of Example 1
and Comparative Example 1, it is evident that if the ratio of the
apparent thickness to the initial thickness of the electrode is
1.000 (the same as the conventional one having no convexoconcave
portions 45 in the processed electrode 41), the resistance value is
increased. Further, it is evident that also in the retention
voltage, a decrease in the voltage retention properties is brought
about. It is considered that in Comparative Example 1, since no
space D was formed between the separator 12 and the positive
electrode 51A or the negative electrode 51B, the impregnation
property of the electrolyte to the positive electrode 51A and the
negative electrode 51B became poor, whereby an increase of the
resistance or a decrease of the voltage-retention properties was
brought about.
[0184] Further, from the results of measurements in Example 1 and
Example 2, it is evident that if the distance A of the
convexoconcave portions 45 or 47 in the processed electrode 41 is
within a range of from 10 to 15 mm, the characteristics of the
electric double layer capacitor are excellent. Further, it is
evident that even if the shape of the cross-section of the
convexoconcave portion 45 or 47 is a semielliptic shape as in the
processed electrode 41A or a rectangular shape as in the processed
electrode 41B, the characteristics are excellent.
[0185] On the other hand, from the results of measurement in
Example 6, it is evident that if the distance A of the
convexoconcave portions 45 is 30 mm, the resistance value tends to
increase as compared with Example 1. It is considered that in
Example 6, the distance A of the convexoconcave portions 45 was so
wide that at the time of preparation of the wound element assembly
53, the space D between the separator 12 and the positive electrode
51A or the negative electrode 51B was closed down, whereby it was
impossible to supply an adequate amount of the electrolyte to the
positive electrode 51A and the negative electrode 51B, and the
increase of the resistance, etc., were brought about.
[0186] Further, it is evident that in Example 4 wherein the shape,
the distance A, etc. of the convexoconcave portions 47 in Example 2
and the shape of the separator 12, etc. in Example 3 are combined,
no substantial difference in the characteristics is observed as
compared with Examples 1 to 3.
[0187] On the other hand, from the results of measurement in
Example 7, it is evident that if the maximum pore size of the
separator 12 is large, the voltage-retention properties tend to
decrease as compared with Example 1. It is considered that in
Example 7, the maximum pore size of the separator was so large that
the positive electrode 51A and the negative electrode 51B tended to
pass though the separator 12 to cause short-circuiting, or metal
impurities contained in the positive electrode 51A and the negative
electrode 51B precipitated to cause microscopic short-circuiting,
whereby the retention voltage decreased.
[0188] Further, from the results of measurements in Example 1 and
Comparative Example 2, it is evident that when graphite powder is
employed as the electroconductive material for the electrode sheets
37A and 37B, the resistance value tends to increase as compared
with Example 1. Further, it is evident that the capacitance and the
retention voltage are also poor as compared with Example 1.
[0189] Further, as compared with Example 1, in Comparative Example
2, the amount of the electrolyte is small. It is considered that in
Comparative Example 2, since the material for the electroconductive
material was changed, whereby the electroconductivity of the
electrode sheets 37A and 37B decreased, whereby it was impossible
to retain the electrolyte in an amount required for polarization in
the positive electrode 51A and the negative electrode 51B, whereby
the increase of the resistance or the decrease of the voltage
retention property was brought about.
[0190] Especially from the results of measurements in Examples 1 to
5, it is evident that when the specific surface area of the
carbonaceous material for the electrode sheets 37A and 37B is from
800 to 1,800 m.sup.2/g, the thickness of the separator 12 is from
40 to 50 .mu.m, the porosity is from 60 to 80%, and the maximum
pore size is from 0.8 to 0.9 .mu.m, the characteristics of the
electric double layer capacitor are excellent. Accordingly, the
electric double layer capacitors of Examples 1 to 5 are
particularly suitable as electric double layer capacitors for
application primarily for a large capacitance and large current,
particularly for a discharge capacitance of from 10 to 20,000 F or
a discharge current of from 1 to 1,000 A.
[0191] In the foregoing Examples, Examples of electric double layer
capacitors employing cylindrical cells 60, are shown. However,
these examples are likewise applicable to electric double layer
capacitors employing rectangular cells to obtain similar
effects.
[0192] As described in the foregoing, according to the present
invention, protruded portions or bent portions are formed on the
electrode, so that a space is formed between the separator and the
electrode, whereby the internal resistance of the electric double
layer capacitor can be reduced, its capacitance density can be
increased, and its productivity can be maintained excellently.
[0193] The entire disclosure of Japanese Patent Application No.
2002-296583 filed on Oct. 9, 2002 including specification, claims,
drawings and summary is incorporated herein by reference in its
entirety.
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