U.S. patent application number 09/265921 was filed with the patent office on 2001-11-08 for lithium secondary battery.
Invention is credited to KITOH, KENSHIN, KUROKAWA, TERUHISA.
Application Number | 20010038945 09/265921 |
Document ID | / |
Family ID | 26409254 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010038945 |
Kind Code |
A1 |
KITOH, KENSHIN ; et
al. |
November 8, 2001 |
LITHIUM SECONDARY BATTERY
Abstract
A lithium secondary battery includes a battery case, an internal
electrode body contained in the battery case and including a
positive electrode, a negative electrode and a separator made of
porous polymer. The positive electrode and the negative electrode
are wound or laminated so that the positive electrode and negative
electrode are not brought into direct contact with each other via
the separator. The respective resistance value of multiple tabs for
electricity collection to be connected with to the positive
electrode and negative electrode was set to remain within the range
of .+-.20% of the averaged resistance value of the tabs. The
lithium secondary battery maintains a good charge-discharge
characteristic even during high-output cycle operation and in
particular may be preferably used for a drive motor of an electric
vehicle.
Inventors: |
KITOH, KENSHIN;
(NAGOYA-CITY, JP) ; KUROKAWA, TERUHISA; (AMA-GUN,
JP) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Family ID: |
26409254 |
Appl. No.: |
09/265921 |
Filed: |
March 10, 1999 |
Current U.S.
Class: |
429/211 |
Current CPC
Class: |
H01M 6/10 20130101; H01M
10/0585 20130101; Y02E 60/10 20130101; H01M 50/531 20210101; Y02T
10/70 20130101; Y02P 70/50 20151101; H01M 10/052 20130101; H01M
10/0587 20130101; H01M 10/0525 20130101 |
Class at
Publication: |
429/211 |
International
Class: |
H01M 002/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 1998 |
JP |
10-68017 |
Jun 18, 1998 |
JP |
10-171105 |
Claims
What is claimed is:
1. A lithium secondary battery comprising: a battery case; an
internal electrode body contained in the battery case and including
a positive electrode, a negative electrode and a separator made of
porous polymer, the positive electrode and the negative electrode
being wound or laminated so that the positive electrode and
negative electrode are not brought into direct contact with each
other via the separator, and the respective resistance value of
multiple tabs for electricity collection to be connected to the
positive electrode and negative electrode being set to remain
within a range of .+-.20% of the average resistance value of the
tabs.
2. A lithium secondary battery according to claim 1, wherein all
the tabs are concentrated at one place and connected to an electric
current extracting terminal by crimping or welding.
3. A lithium secondary battery according to claim 1, wherein all
the tabs are unified by crimping or welding or eyelet-type
connecting in advance, and are thereafter connected to an electric
current extracting terminal by crimping or welding or screwing.
4. A lithium secondary battery according to claim 1, wherein the
thickness of the tab is within a range from 5 .mu.m to 100
.mu.m.
5. A lithium secondary battery according to claim 1, wherein
battery capacity is not less than 5 Ah.
6. A lithium secondary battery according to claim 1, wherein the
battery is used for an electric vehicle or a hybrid electric
vehicle.
Description
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
[0001] The present invention relates to a lithium secondary battery
which maintains a good charge-discharge characteristic even during
a high output cycle operation and which in particular may be
preferably used for a drive motor of an electric vehicle.
[0002] In recent years, it is eagerly desired to regulate the
emissions of carbon dioxide with the environment protection
movement. In the automobile industry, to replace automobiles using
fossil fuels, such as vehicles driven by gasoline, there is a
movement to promote the introduction of electric vehicles (EV) and
hybrid electric vehicles (HEV) A lithium secondary battery acting
as a motor-drive battery acting as a key for putting EV and HEV
into practical use is required to have not only a huge battery
capacity but also a huge battery output, which greatly affects the
acceleration performance as well as the slope-climbing performance
of the vehicle.
[0003] In general, the internal electrode body of a lithium
secondary battery has a positive electrode, a negative electrode
and a separator made of porous polymer film, the positive electrode
and the negative electrode being wound or laminated so that the
positive electrode and negative electrode are not brought into
direct contact with each other via the separator. For example, as
shown in FIG. 1, an internal electrode body 1 of a winding type is
formed by winding a positive plate 2 and a negative plate 3 having
a separator 4 in between, and tab 5 is provided for each of the
positive and negative electrode plates 2, 3 (hereafter referred to
as "electrodes 2, 3") respectively. The end opposite to the end
connected to the electrodes 2, 3 of each tab 5, is attached to an
external terminal 11 or an electric current extracting terminal 13
such as an internal terminal member 12 being electrically connected
to the external terminal 11. That is, the tab 5 serves as a lead
line which is electrically connected to the electric current
extracting terminal 13 while conducting electricity collection from
the electrodes 2, 3.
[0004] Here, a plan view of the electrodes 2, 3 when the internal
electrode body 1 is spread out is shown in FIG. 2. The electrodes
2, 3 are formed with an electrode active material 16 coated
respectively onto metal foils 15 made of aluminum, etc., for the
positive electrode 2 and made of copper, etc., for the negative
electrode 3. Since a tab 5 is provided on a side of such metal foil
15, a tab having a thin band shape is preferably used. The tabs are
disposed at approximately a uniform distance so that each tab 5
conducts electricity collection in a constant area in electrode 2,
3. Incidentally, in general, the material qualities of the tabs 5
are the same as the material qualities of the metal foil 15 to
which the tabs 5 are attached.
[0005] With respect to a lithium secondary battery for EV or HEV,
there are cases in which a current equal to or more than 100 A
flows per battery. In the case where such a huge current flows,
there is a need for the internal resistance of all the batteries to
be made as low as possible in order to reduce the output loss of
the batteries.
[0006] Therefore, it goes without saying that it is preferred to
make the resistance of the internal electrode body low, but now,
paying attention to the connection path from the formerly-described
internal electrode body 1 to electric current extracting terminal
13, it is preferable that the resistance of the members of metal
foil 15, tab 5 and electric current extracting terminal 13 be low.
However, concerning the metal foil 15 and electric current
extracting terminal 13, there is a certain limit to making the
resistance value lower due to the fact that the material is limited
as well as due to limitations from the point of view of the shape
of the battery and energy density.
[0007] On the other hand, the tab 5 has an allowable range in which
to set a resistance value from the point of view of the feasibility
of setting its shape freely since the shape of tab 5 is to be
housed in the space between the battery case housing the internal
electrode body 1 therein and the internal electrode body 1. In
addition, concerning the tabs 5 and the metal foil 15, the
connection resistance of these does not vary much, since they are
unified by welding, except for with extremely faulty welding.
Concerning the connection between the tabs 5 and the electric
current extracting terminal 13, however, there is room left to
reduce the contact resistance, since various methods may be
considered. For example, for tabs with the shape of a thin band, a
method of bundling by piling up in one direction is preferably
easiest in terms of a forming process for the battery, and also is
preferred since the structure inside the battery will not become
complicated. In this case, however, there will be a need to aim to
reduce the contact resistance on the contact surface of each tab,
since contact between tabs will occur more times.
[0008] Concerning the tabs 5 and the method used to connect the tab
5 to the electric current extracting terminal 13, however, the
resistance of these members themselves and the connection
resistance, which did not occupy a larger percentage from the view
point of internal resistance of the entire battery, were not
considered to be important, and how the dispersion (distribution)
of this resistance may affect the output characteristic or
charge-discharge cycle operation characteristic has not been
explained.
SUMMARY OF THE INVENTION
[0009] The present invention was achieved by considering the
problems of the prior art mentioned above. That is, according to
the present invention, there is provided a lithium secondary
battery, comprising a battery case, an internal electrode body
contained in the battery case and including a positive electrode, a
negative electrode and a separator made of porous polymer, the
positive electrode and the negative electrode being wound or
laminated so that the positive electrode and negative electrode are
not brought into direct contact with each other via the separator,
and at least plural tabs, having been connected to the positive
electrode and negative electrode for electricity collection so that
the respective resistance value of a tab remains within the range
of .+-.20% of the average resistance value of the tabs.
[0010] In a lithium secondary battery of the present invention, it
is preferred that all the tabs are concentrated at one place and
are connected to the electric current extracting terminal by
crimping or welding. In addition, it is also preferred that all the
tabs are connected to the electric current extracting terminal by
crimping, or welding, or screwing after they have been unified by
crimping, welding, or eyelet-type connecting in advance.
Incidentally, the thickness of a tab is preferably 5 .mu.m or more
and 100 .mu.m or less, and the battery structure using such tabs is
preferably adopted to a lithium secondary battery with a battery
capacity of 5 Ah or more, and especially to a lithium secondary
battery for an EV or a hybrid electric vehicle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a perspective view showing the structure of a
wound-type internal electrode body.
[0012] FIG. 2 is a plan view showing the spread state of each
positive electrode and negative electrode in a wound-type internal
electrode body.
[0013] FIG. 3 is a perspective view showing one embodiment of the
structure of a lamination-type internal electrode body.
[0014] FIG. 4 is a cross-sectional view showing one embodiment of a
method used to connect tabs to an electric current extracting
terminal according to the lithium secondary battery of the present
invention.
[0015] FIG. 5 is an explanatory drawing showing another embodiment
of a method used to connect tabs to an electric current extracting
terminal according to the lithium secondary battery of the present
invention.
[0016] FIG. 6 is an explanatory drawing showing a method of
measuring resistance of tabs according to the lithium secondary
battery of the present invention.
[0017] FIG. 7 is an explanatory drawing showing an embodiment of a
method used to connect tabs to an electric current extracting
terminal according to the lithium secondary battery of Comparative
Example 2.
[0018] FIG. 8 is an explanatory drawing showing the dispersion in
resistance of tabs according to Examples and Comparative
Examples.
[0019] FIG. 9 is an explanatory drawing showing the flow of current
from tabs to an electric current extracting terminal of a bolt and
a nut.
[0020] FIG. 10 is an explanatory drawing showing the flow of
current from tabs to an electric current extracting terminal of a
rivet.
[0021] FIG. 11 is an explanatory drawing showing the process up to
tab connection to an electric current extracting terminal in
forming a lithium secondary battery of the present invention.
[0022] FIG. 12 is an explanatory drawing showing the test results
of cycle operations on an Example and Examples for comparison.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0023] The lithium secondary battery of this invention yields an
excellent effect in that it is possible to discharge a huge
electric current constantly, enabling uniform electricity
collection from the positive electrode and negative electrode
together with a uniform battery reaction within the positive
electrode and negative electrode, since the resistance from the
positive and negative electrodes to the electric current extracting
terminal is uniformalized. As a result of this, it also yields
another excellent effect in that local deterioration of battery
materials may be suppressed and thus excellent endurance may be
provided during cycle operations.
[0024] While the embodiments of the present invention are described
below, it goes without saying that the present invention is not
limited to the following embodiments.
[0025] The internal electrode body of the lithium secondary battery
of the present invention (hereinafter referred to as a "battery")
is constituted by a positive electrode, a negative electrode and a
separator made of porous polymer film, the positive electrode and
the negative electrode being wound or laminated so that the
positive electrode and negative electrode are not brought into
direct contact with each other via the separator. In particular, as
shown in FIG. 1 before, an internal electrode body 1 of a winding
type is formed by winding a positive electrode 2 and a negative
electrode 3 having a separator 4 in between, and tabs 5 are
provided for electrodes 2, 3.
[0026] On the other hand, as shown in FIG. 3, the lamination-type
internal electrode body 7 laminates the positive plate 8 and the
negative plate 9 alternately via the separator 10 with tabs 6 being
connected to the positive and negative electrodes plates 8 and 9
(hereinafter referred to as "electrodes" 8, 9) respectively. Such
an internal electrode body 1, 7 is basically configured to have a
plurality of element batteries being connected in parallel, an
element battery consisting of positive electrodes 2, 8 and negative
electrodes 3, 9 facing each other.
[0027] The positive electrodes 2, 8 and the negative electrodes 3,
9 are formed in the shape of a thin plate with an electrode active
material being coated respectively onto metal foil as base
materials. Here, aluminum foil is used as the base material for the
positive electrodes 2, 8, and copper foil as the base material for
the negative electrodes 3, 9 respectively.
[0028] For a battery with any of the above-described structures,
lithium transition metal compound oxides such as lithium cobalt
oxide(LiCoO.sub.2), lithium nickel oxide(LiNiO.sub.2), or lithium
manganese oxide (LiMn.sub.2O.sub.4), etc., are generally used as
positive active materials. In addition, in order to improve the
conductivity of these positive active materials, a carbon powder
such as acetylene black, graphite powder, etc., is frequently mixed
with the electrode active material. On the other hand, as the
negative active materials, an amorphous carbon material such as
soft carbon or hard carbon, or carbon powder such as natural
graphite, etc., is used.
[0029] As a separator 4, 10, it is preferable to use a three-layer
structural one in which a polyethylene film having lithium ion
permeability and including micropores is sandwiched between porous
polypropylene films having lithium ion permeability. This serves
also as a safety mechanism with which, when the temperature of the
internal electrode body is raised, the polyethylene film is
softened at about 130.degree. C. so that the micropores collapse to
suppress the movement of lithium ions, that is, battery reaction.
With this polyethylene film being sandwiched between the
polypropylene films having a softening temperature higher than the
polyethylene film, it is possible to prevent contact/welding
between the electrodes (2, 3), and (8, 9).
[0030] Next, the method of connection between the tabs and the
internal electrode body (positive electrodes and negative
electrodes) as well as the electric current extracting terminal
will be explained using the case of a wound-type internal electrode
body 1 as an example. As described before, FIG. 2 is a plan view of
the electrodes 2, 3 when a wound-type internal electrode body 1
shown in FIG. 1 is spread out, wherein in the case of the battery
capacity being constant, the length L toward its winding direction
can be shortened if the width D of the electrodes 2, 3 is
lengthened.
[0031] However, in the case where the width D of the electrodes 2,
3 is long, there arises an inconvenience in that the internal
resistance becomes large since the distance between the tabs 5 and
the electrode active material 16 in the vicinity of a side facing
the side where the tabs 5 are attached becomes long. Therefore,
normally the width of the electrodes 2, 3 is preferably set within
the range from 10 cm to 40 cm, and when the width of the electrodes
2, 3 is within such a range, the number of tabs 5 to be placed
along the length L toward the winding direction of electrode 2, 3
is preferably around 6 to 10 per 1 m.
[0032] The tabs 5 (tab 5A and tab 5B in FIG. 11) are preferably
arranged to make an approximately straight line toward the
direction of the diameter of the internal electrode body 1 when the
electrodes 2, 3 are wound, so that arrangement of the tabs 5 to the
electrode 2, 3 is not configured to have a complicated structure as
shown in FIG. 11 to be described below, but is easily attachable to
the electric current extracting terminal 13.
[0033] As a method for connecting the electrodes 2, 3 to the tabs
5, resistance welding or ultrasonic welding may be used. There are
no cases where the contact resistance between the tabs 5 and the
electrode 2, 3 will become large except in the case of extremely
faulty welding. However, it goes without saying that it is
preferred to make the contact resistance between the tabs 5 and the
electrodes 2, 3 to be approximately constant.
[0034] Thus, the tabs 5 disposed approximately at an almost equal
distance respectively serve to transfer electrons having a
relationship to the battery reaction in an approximately equal
electrode area of the electrodes 2, 3. However, if there is any
dispersion in resistance from the electric current extracting
terminal 13 to the respective tabs 5, there will arise dispersion
in extraction of electricity as well. That is, there is a
possibility that electricity will be concentrated in low resistance
tabs 5, and in this case, not only will the battery reaction be
uneven, but also a problem will arise in that rapid deterioration
of material will take place at the portions where the battery
reaction will be most active (at the portions where the low
resistance tabs 5 are connected).
[0035] In order to avoid such a problem, according to the present
invention, the plural tabs 5, having been connected to each
electrode 2, 3 for electricity collection, are designed so that the
respective resistance values of tabs 5 at least remain within the
range of .+-.20% of the average resistance value of the tabs 5. Due
to this, it is preferred that all the tabs 5 are concentrated at
one place and are connected to the electric current extracting
terminal 13 by crimping or welding.
[0036] In particular, as shown in FIG. 4, it is possible to use a
rivet 22 as an electric current extracting terminal 13 to be
attached to the battery cap 23, and to collect the tabs 5 to
connect to the rivet 22 by crimping. In this case, based on the
pressure used for the crimping, the resistance value of the tabs 5
can be within the above-described range. Incidentally, the tabs 5
may be connected to the rivet 22 by welding instead of crimping, or
the tabs 5 may be pressed into contact with the rivet 22 and the
connection part may be unified further by welding.
[0037] In addition, as shown in FIG. 5, using metal plates 26 such
as copper plates, aluminum plates and the like, all the tabs 5 are
unified by crimping in advance, and thereafter fitted onto the bolt
24 which is an electric current extracting terminal 13 and there
are screwed tightly by a nut 25, a connection method that is used
preferably. Incidentally, for unification of the tabs 5, in
addition to crimping, a method such as welding or eyelet-type
connecting may be employed, and for attaching the unified tabs 5 to
the electric current extracting terminal 13, methods such as
welding as well as crimping may be employed.
[0038] Now, as described before, since metal foils are used as the
base material for the electrodes 2, 3, tabs 5 in a thin band shape
are preferably used as well, the thickness of which are preferred
to be equal to or more than 5 .mu.m and equal to or less than 100
.mu.m. Here, in the present invention, aluminum foils are
preferably used as the ones for the positive tabs and copper or
nickel foils are preferably used as the ones for the negative
tabs.
[0039] Incidentally, since connection points between the tabs 5 and
the electrodes 2, 3 are numerous, while the attachment point of the
tabs 5 to the electric current extracting terminal 13 is only one,
it is not preferred to use the shortest ones in length respectively
for each tab 5, giving rise to differences in resistance value for
the tabs 5. Due to this, it is preferred to adjust the length of
the tabs 5 for use to that of the tab 5 requiring the longest
length, or when tabs 5 with different lengths are used, to equalize
resistance values by adjusting their thickness and width.
[0040] Up until now, description has been centered on a wound-type
internal electrode body 1. It goes without saying that a similar
description is applicable to electrodes 8, 9 forming a
lamination-type internal electrode body 7, where plural tabs are
placed per plate of the electrodes 8, 9 and connectable to the
electric current extracting terminal.
[0041] By adopting the above-described battery structure,
especially in a large capacity battery requiring placement of
numerous tabs having a battery capacity of 5 Ah or more, for
example, for batteries for EV or HEV, a good charge-discharge
characteristic becomes obtainable since electricity is extracted
from the internal electrode bodies in the batteries equally through
each tab.
[0042] The present invention is described below by way of examples.
It goes without saying that the present invention is not limited by
the following examples.
EXAMPLES
Measurement of Resistance of Tab
[0043] When forming the battery, at first, in order to examine
resistance dispersion due to differences in methods of tab
connection to the electric current extracting terminal, 30 sheets
of positive tabs made of bundled aluminum foil were crimped to a
positive rivet made of aluminum, using a connection method that
connects the tabs 5 with a rivet 22 which is an electric current
extracting terminal by crimping as shown in FIG. 4, while 30 sheets
of negative tabs made of bundled copper foil were crimped to a
negative rivet made of copper. Thus, a positive rivet and a
negative rivet to which tabs were crimped constitute a pair. Those
with a pressure for crimping of 1 ton/cm.sup.2 are regarded as
Example 1 and those with a pressure for crimping of 2 ton/cm.sup.2
are regarded as Example 2.
[0044] Next, as shown in FIG. 6, the resistance value for each tab
5 was measured by measuring the voltage when a current of 1 A
flowed at each tab 5 and the rivet 22. The obtained resistance
values were calculated to obtain an average value, and a resistance
value distribution was obtained by standardizing the resistance
values for each tab 5 with the average value as 100%.
[0045] On the other hand, as Comparative Example 1, the tabs 5 were
crimped to the rivet 22 by a similar method as adopted in the
above-described Examples 1 and 2, setting pressure of 500
kg/cm.sup.2 for crimping, and the resistance value of each tab 5
was measured. In addition, as Comparative Example for comparison 2,
as shown in FIG. 7, the resistance value of each tab 5 was measured
by a similar method as adopted in the above-described Examples 1
and 2, using an electric current extracting terminal consisting of
a bolt 24 and a nut 25 without unifying the 30 sheets of the tabs 5
in advance, as shown in FIG. 5, but having a structure in which
each tab 5 is individually screwed tightly.
[0046] The dispersion in resistance values of the tabs 5 in the
above-described Examples 1, 2 and Comparative Examples 1, 2 is
shown in FIG. 8. Examples 1, 2 show that dispersion in resistance
values of the tabs 5 is within .+-.20% of the average value, while
Comparative Examples show that the dispersion in resistance values
is larger.
[0047] This result can be described as follows. That is, in the
case of Comparative Example 2 where the bolt 24 and the nut 25 were
used, as shown in FIG. 9, the tabs 5 are provided with a hole to
insert them into the screw thread 27 of the bolt 24, the diameter
of the hole being larger than the diameter of the screw thread 27
of the bolt 24, and further the area where the side surface of the
hole of the tabs 5 contacts the screw thread 27 being fairly small
due to the thinness of the tabs 5. Therefore, the current of each
tab 5 flows toward the bolt 24 through an adjacent tab 5, as shown
by the arrow 50, or to the bolt 24 through an adjacent tab 5 and
the nut 25, making it difficult for it to flow in the direction
shown by the arrow 60.
[0048] On the other hand, as in the case of Examples 1, 2 and
Comparative Example 1, the case of using the rivet 22 is similar.
In this case shown in FIG. 10, the current of each tab 5 flows
toward the direction shown by the arrow 50, namely to the rivet 22
through an adjacent tab 5 since the area where the side surface of
a tab 5 contacts the rivet 22 is fairly small, thus making it
difficult for the current to flow in the direction shown by the
arrow 60.
[0049] That is, in any case of the above-described Examples 1, 2,
and Comparative Examples 1, 2, the current is to flow through the
contact surface between the tabs 5. Due to this, among the bundled
tabs 5, ones located in the center will yield larger resistance
values since there are more contact surfaces with the bolt 24 or
the rivet 22.
[0050] Therefore, it is necessary to reduce the resistance of the
contact surfaces occurring at each tab 5 so that dispersion in
resistance values of the respective tabs 5 is reduced. Here it is
known that for tabs made of aluminum foil, an insulating film of
alumina is apt to be formed on its surface, and as the number of
bundled thin-band-shaped tabs 5 increases, and the insulating film
will have more effect on the resistance distribution of the tabs 5.
Under the circumstances, it is necessary to destroy the insulating
film, and thereby ensure contact at the metal material portion
originating in the tab so as to reduce dispersion in resistance
values of the tabs 5.
[0051] This is applicable to tabs made of copper foil.
Nevertheless, since the oxide film to be generated on the surface
of copper foil is a semiconductor, the contact surfaces are
affected to a smaller extent than in the case where the aluminum
foil is used. However, concerning the conductivity of electrons,
since metal is naturally superior to a semiconductor, with regard
to copper foil as well, it is preferable to ensure the contact
between metals by destroying the oxide film on the surfaces.
[0052] Therefore, a method capable of ensuring metal-to-metal
contact by crimping, etc., is preferable for connecting the tabs 5
to the electric current extracting terminal. In Examples 1, 2, it
is thought that the dispersion in resistance of the tabs 5 was
suppressed as a result of contact having been ensured between the
metal materials with the oxide films on the surfaces of the tabs 5
having been destroyed by conducting crimping at an appropriate
pressure. Nevertheless, it is thought that even if a crimping is
used, as shown in the result of Comparative Example 1, in the case
where the pressure of the crimping is low and inappropriate,
dispersion in resistance of the connection of tabs 5 will increase,
making the effect of crimping unobtainable. In addition, with
Comparative Example 2, it is thought that with the clamp pressure
for screwing it will be difficult to reach the necessary pressure
to enable the oxide film on the tabs 5 to be destroyed, and as a
result it is presumed that the dispersion of resistance became
larger.
[0053] Incidentally, in order to limit dispersion in the resistance
values of the tabs 5 within an average value of .+-.20%, in the
case where aluminum foil is used as the tabs 5, the pressure of
crimping had to be within the range from 1 ton/cm.sup.2 to 50
ton/cm.sup.2, while in the case where copper foil is used, the
pressure of the crimping had to be within the range from 500
kg/cm.sup.2 to 100 ton/cm.sup.2.
[0054] Thus, one of the reasons contributing to differences in the
required range of pressure between aluminum foil and copper foil to
limit dispersion in resistance values within a given range is
thought to be due to the fact that an oxide film is more easily
formed in an aluminum foil than in a copper foil, as well as due to
the above-described difference in the electric characteristic of
the oxide film. Incidentally, the upper limit of pressure for
crimping for each tab 5 is the pressure when damage such as cuts on
the tab 5 take place at the end portions of the rivet 22.
[0055] (Forming of Battery)
[0056] A lithium secondary battery was formed by the method
described below. At first, a paste was formed with a
LiMn.sub.2O.sub.4 powder body as a positive active material, to
which acetylene black was added to provide conductivity to it, and
further a binder and a solvent were mixed therein. With this paste
being coated on both sides of 20 .mu.m-thick aluminum foil, a
positive electrode was formed having an electrode plane shape with
a length towards the winding direction of 3600 mm.times.a width of
200 mm. On the other hand, a paste was formed with a highly
graphitized carbon powder as a positive active material, and
further a binder and a solvent are mixed therein, which is then
coated on both sides of 10 .mu.m-thick copper foil, and thereby a
negative electrode was formed having an electrode plane shape with
a length towards the winding direction of 4000 mm.times.a width of
200 mm.
[0057] Next, as shown in FIG. 11, the thus-formed positive
electrode 2 and negative electrode 3 were wound with insulation
being provided employing 210 mm-wide separators 4 made of
polyprophylene. At the same time 30 sheets each of positive tabs 5
A made of aluminum and negative tabs SB made of copper which were
used in the above-described Example 1, etc., were attached to the
electrodes 2, 3 by ultrasonic-welding so that they were arranged to
make an approximately straight line along the direction of the
diameter of the internal electrode body 1, and so that each
electrode 2, 3 was placed in an approximate distance in between
when they were spread out, and further so that one of the
electrodes was formed at one end of the internal electrode body
1.
[0058] The thus formed internal electrode body 1 was fitted into
the aluminum-made battery case 17, crimping each positive tab and
negative tab 5A, 5B to the positive electrode and negative
electrode rivets 22A, 22B respectively, which are the electric
current extracting terminals, under a pressure of 1 ton/cm.sup.2,
using the same method as in the above-described Example 1,
attaching a battery cap onto the negative rivet 22B to seal the
negative side of the battery case 17. Thereafter, from the open
side of the positive terminal of the battery case 17, the
electrolyte, a mixed solvent of EC (ethylene carbonate) and DEC
(diethyle carbonate) where electrolyte LiPF.sub.6 was dissolved to
yield 1 mol % density was injected into the battery case 17.
Thereafter, a battery cap was attached onto the positive rivet 22A
to tightly seal the battery case 17. Incidentally, the battery case
17 may be sealed from the positive side.
[0059] Thus, the battery formed using the method of connecting the
tabs to the electric current extracting terminal in the
above-described Example 1 is deemed to be the battery for the
1.
[0060] Subsequently, respective batteries were formed, using the
above-described Example 2 and Comparative Examples 1, 2 as the
method for connecting the tabs to the electric current extracting
terminal in the same way as in the battery for Example 1, except
for the method used to connect the tabs to the electric current
extracting terminal. The thus-formed batteries are deemed to be the
battery for Example 2, the battery for Comparative Example 1, and
the battery for Comparative Example 2 respectively.
[0061] The battery capacity of the formed batteries was 25 Ah, and
their charge-discharge characteristic was assessed by a cycle
operation test. Here, charging was conducted at a constant current
of 25 A and a constant voltage of 4.1V, and discharging was
conducted with constant current at a discharge rate of 1 C (25 A)
until discharge was finalized at 2.5V, whereupon
charging/discharging was repeated. The discharge capacity for each
time was standardized using the discharge capacity at the first
time as 100%.
[0062] FIG. 12 shows how the discharge capacity changed during the
cycle operation test. For the batteries for Examples 1, 2 where the
dispersion in resistance of the tabs was small, the capacity drop
was small, and there were no major differences between the two. On
the other hand, for the batteries for Comparative Example 2, where
the dispersion in resistance of the tabs was large, the capacity
drop was remarkable. Such a result is thought to be caused by
unevenness that occurred in the battery reaction in the internal
electrode body due to dispersion in resistance of the tabs, and
deterioration which occurred partially in the positive electrodes
and negative electrodes.
[0063] As described above, the lithium secondary battery of this
invention yields an excellent effect in that it is possible to
discharge a huge electric current constantly, enabling uniform
electricity collection from the positive electrodes and negative
electrodes together with a uniform battery reaction within the
positive electrodes and negative electrodes, since any
inconsistency in the resistance of multiple tabs is limited within
a certain range. As a result of this, it also yields another
excellent effect in that local deterioration of battery materials
may be suppressed and thus excellent endurance may be provided
during cycle operations.
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