U.S. patent application number 09/169563 was filed with the patent office on 2001-12-20 for lithium secondary battery.
Invention is credited to KITOH, KENSHIN, NEMOTO, HIROSHI.
Application Number | 20010053477 09/169563 |
Document ID | / |
Family ID | 17630302 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010053477 |
Kind Code |
A1 |
KITOH, KENSHIN ; et
al. |
December 20, 2001 |
LITHIUM SECONDARY BATTERY
Abstract
A lithium secondary battery includes: a battery case, and an
internal electrode body 1 contained in the battery case and
including a positive electrode 2, a negative electrode 3, and a
separator 4 made of porous polymer. The positive electrode and the
negative electrode are wound and laminated through the separator so
that the positive electrode and the negative electrode are not
brought into direct contact with each other. The battery case is
composed of pure aluminum or aluminum alloy in which one or more
components selected from manganese, magnesium, silicon and copper
is added in aluminum. The lithium secondary battery has high weight
energy density, is superior in safety, and is used for,
particularly, an electric vehicle.
Inventors: |
KITOH, KENSHIN;
(NAGOYA-CITY, JP) ; NEMOTO, HIROSHI; (NAGOYA-CITY,
JP) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Family ID: |
17630302 |
Appl. No.: |
09/169563 |
Filed: |
October 9, 1998 |
Current U.S.
Class: |
429/176 ;
429/224 |
Current CPC
Class: |
H01M 10/052 20130101;
H01M 50/107 20210101; H01M 6/10 20130101; Y02E 60/10 20130101; H01M
50/133 20210101; H01M 50/545 20210101; H01M 10/0585 20130101; H01M
50/119 20210101; Y02P 70/50 20151101; H01M 10/0587 20130101; H01M
50/10 20210101; H01M 50/116 20210101; H01M 4/505 20130101; H01M
50/449 20210101; H01M 2010/4292 20130101; H01M 10/058 20130101;
H01M 4/131 20130101; H01M 2300/0025 20130101; H01M 50/56 20210101;
H01M 50/457 20210101; H01M 50/103 20210101; Y02T 10/70 20130101;
H01M 10/0525 20130101 |
Class at
Publication: |
429/176 ;
429/224 |
International
Class: |
H01M 002/02; H01M
004/50 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 14, 1997 |
JP |
9-280810 |
Claims
What is claimed is:
1. A lithium secondary battery comprising: a battery case, and 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 through the separator so that the positive
electrode and the negative electrode are not brought into direct
contact with each other; wherein said battery case is composed of
pure aluminum or aluminum alloy in which one or more components
selected from manganese, magnesium, silicon and copper is added in
aluminum.
2. A lithium secondary battery according to claim 1, wherein a
relationship of C/(w.multidot.c).ltoreq.0.03 is established where
current capacity is C (Ah), battery weight is w (kg), and specific
heat of the battery is c (W/kg.multidot..degree. C.).
3. A lithium secondary battery according to claim 1, wherein a
relationship of 0.004.ltoreq.t/d.ltoreq.0.04 is established where
said battery case is cylindrical, its outer diameter is d
(mm.phi.), and its wall thickness is t (mm).
4. A lithium secondary battery according to claim 1, wherein
current capacity is 50 Wh or more.
5. A lithium secondary battery according to claim 1, wherein it is
used for an electric vehicle or a hybrid electric vehicle.
6. A lithium secondary battery according to claim 1, wherein
lithium-manganese oxide (LiMn.sub.2O.sub.4) is used as positive
active material.
Description
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
[0001] The present invention relates to a lithium secondary battery
which is superior in safety, and has high weight energy density
(energy stored per unit weight, hereinafter called "energy
density"), and which is suitably used for, particularly, an
electric vehicle.
[0002] In recent years, the lithium secondary battery is being
rapidly and widely used to realize a small power source for
portable electronic equipment. In addition, effort of development
is being also made to realize practical use of the lithium
secondary battery as a motor driving battery for an electric
vehicle which replaces a gasoline-powered vehicle, and as a battery
for storing electric power in the night.
[0003] The structure of lithium secondary battery is roughly
divided into a wound type shown in FIG. 2 and a laminated type
shown in FIG. 3. An internal electrode body 1 of the wound type is
constituted by winding a positive electrode 2 and a negative
electrode 3 through a separator 4, in which the positive electrode
2 with a large area or the like can be contained in a tubular
container. In the case of this wound type, since it is sufficient
that there is at least one lead 5 from each electrode 2, 3, and,
even if it is desired to lower current collection resistance of
each electrode 2, 3, it is sufficient to increase the number of
leads, there is an advantage that the internal structure of battery
does not become complicated to make easy assembly of the
battery.
[0004] On the other hand, an internal electrode body 7 of the
laminated type is constructed by alternately laminating positive
electrodes 8 and negative electrodes 9 in multiple layers through
separators 10, in which area per one positive electrode 8 or the
like is no large, but the electrode area of the entire battery can
be increased by laminating them in multiple layers. The internal
electrode body 7 being produced can be designed into any desired
shape including a rectangular parallelepiped, cylindrical or
tubular shape depending on the shape of each electrode 8, 9 and the
number of laminations. However, since a lead 6 is necessary for
each electrode 8, 9, there is a disadvantage that the internal
structure of battery becomes complicated, and it is inferior to the
wound type in view of assembly workability of battery.
[0005] In both the wound and laminated type structures, the
internal electrode body is housed in a metal battery case so that
each electrode and lead do not contact each other. Conventionally,
stainless steel is most widely used for this battery case, and
sometimes nickel, titanium or the like may be used.
[0006] However, since stainless steel or nickel has higher specific
gravity, there is a disadvantage that, when it is used for the
battery case, the battery itself becomes heavy, so that the energy
density is low. On the other hand, while titanium has an advantage
to have lower specific gravity than stainless steel or nickel, and
to be excellent in corrosion resistance, it is expensive, and its
use is limited to a specific application such as space development,
so that it is difficult to be used as a general purpose battery
component. In addition, in the lithium secondary battery, the
battery case itself is often used as a current path for the
positive or negative, and such material has high electric
resistance, leading to a cause of power loss. In addition, such
metal is not always said to have good workability as the battery
case.
[0007] Under such circumstances, a lithium secondary battery for an
electric vehicle (EV) or hybrid electric vehicle (HEV) is required
to have a cell capacity of at least 50 Wh, to have light weight not
to increase weight of the vehicle itself, and to have high safety.
To meet such requirements, stainless steel with high melting point
and high strength has been conventionally used by particularly
taking safety into consideration. However, as described earlier, it
is difficult to solve the problem for reducing weight of the
battery. In addition, EV and HEV require a high current in
acceleration, and, when the battery case is used as the current
path, magnitude of electric resistance of the battery case cannot
be ignored, and there remains a problem on workability of battery
case the size of which is increased. Also, when nickel or titanium
is used, such problems are also difficult to be solved because of
physical characteristics of these materials.
[0008] Then, to solve such problems, the inventors have studied the
possibility to use aluminum as the battery case which has light
weight, is excellent in electron conductivity, and of good
workability. There is no precedent to use aluminum as a battery
case for a large battery of 50 Wh or more. This may be because the
melting point of aluminum is as relatively low as 660.degree. C., a
temperature significantly lower than those of the above materials,
and, when the battery case is softened or melted due to erroneous
use or the like, electrolyte is feared to be evaporated or burned,
or exploded in the worst case.
[0009] According to Battery Association of Japan, as the "Guideline
for Safety Evaluation on Secondary Lithium Cells" (commonly called
"SBA Guideline"), it regulates that even if entire energy fully
charged is instantaneously discharged by external short-circuit or
internal short-circuiting caused by a nail piercing test or the
like, and then the lithium secondary battery generates heat, the
battery does not burst or fire.
[0010] While such safety is strictly required, the inventors found
that, even when an aluminum battery case is used, the problems on
safety could be solved by accurately measuring temperature rise on
the surface of the battery to calculate specific heat of the
battery, and identifying the relationship between battery capacity
and weight, and that reduction of energy density could be prevented
by optimizing the battery case shape, and thus reached the present
invention.
SUMMARY OF THE INVENTION
[0011] That is, according to the present invention, there is
provided a lithium secondary battery comprising:
[0012] a battery case, and
[0013] 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 through the separator
so that the positive electrode and the negative electrode are not
brought into direct contact with each other;
[0014] wherein the battery case is composed of pure aluminum or
aluminum alloy in which one or more components selected from
manganese, magnesium, silicon and copper is added in aluminum.
[0015] In addition, in the lithium secondary battery of the present
invention, it is preferable in view of assuring safety of the
battery that a relationship of C(w.multidot.c).ltoreq.0.03 is
established where current capacity is C (Ah), battery weight is w
(kg), and specific heat of the battery is c (W/kg.multidot..degree.
C.). It is also preferable in view of attaining both high energy
density and safety that a relationship of
0.004.ltoreq.t/d.ltoreq.0.04 is established where the battery case
is cylindrical, its outer diameter is d (mm.phi.), and its wall
thickness is t (mm). Moreover, such conditions are preferably
applied to a lithium secondary battery with battery capacity of 50
Wh or more. The lithium secondary battery satisfying such
conditions is suitably used as a battery for an electric vehicle or
a hybrid electric vehicle. With this regard, the lithium secondary
battery of the present invention preferably uses lithium-manganese
oxide (LiMn.sub.2O.sub.4) as positive active material.
[0016] As described above, the lithium secondary battery of the
present invention reduces weight of the battery case while assuring
high safety, so that it has a high energy density.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a sectional view showing a structure at the end of
a lithium secondary battery produced according to an
embodiment.
[0018] FIG. 2 is a perspective view showing a structure of a
wound-type internal electrode body.
[0019] FIG. 3 is a perspective view showing a structure of a
laminated-type internal electrode body.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] Now, embodiments of the present invention will be described,
but the present invention is not limited to these embodiments.
[0021] In the lithium secondary battery of the present invention,
an internal electrode body is composed by winding or laminating
positive and negative electrodes through separator films of porous
polymer such that the positive electrodes do not directly contact
the negative electrodes. Specifically, it includes structures shown
in FIGS. 2 and 3, that is, internal electrode bodies 1 and 7.
[0022] The positive electrode used is an aluminum foil applied with
mixture of positive active material and carbon powder to improve
conductivity. As the positive active material, there can be used,
for example, lithium-cobalt oxide (LiCoO.sub.2), lithium-nickel
oxide (LiNiO.sub.2), or lithium-manganese oxide
(LiMn.sub.2O.sub.4). The present invention preferably uses
LiMn.sub.2O.sub.4. In addition, as the carbon powder, there can be
used, for example, acetylene black, graphite powder or the like. It
is preferable to use high a purity material for the aluminum foil
constituting the positive electrode to prevent the battery
performance from lowering due to corrosion by an electrochemical
reaction of the battery.
[0023] On the other hand, for the negative electrode it is
preferable to use a copper foil coated with an amorphous carbon
material such as soft carbon or hard carbon, or carbon powder such
as natural graphite as negative active material. Here, similarly to
the aluminum foil used for the positive electrode, it is preferable
to use a high purity material for the copper foil used as the
negative electrode to withstand the corrosion due to an
electrochemical reaction.
[0024] When the above-mentioned carbon material is used for the
negative electrode, it is known that a part of the lithium ions
adsorbed to the carbon material at the initial charging reaction of
the battery becomes the so-called dead lithium which is kept
adsorbed to the carbon material and does not contribute to the
subsequent charging and discharging reactions, so that the capacity
of the battery is lowered. Thus, it is preferable to select a
material in which the amount of the dead lithium is small as the
carbon material for the negative active material.
[0025] As a material of the separator film, it is preferable to use
a three-layer structural material 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 in 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 are collapsed to suppress the movement of lithium ions,
that is, the battery reaction. When the polyethylene film is
sandwiched between the polypropylene films having a softening
temperature higher than the polyethylene film, it is possible to
prevent the contact between the positive and negative electrodes
even after softening of polyethylene.
[0026] The internal electrode body produced using such material is
housed in the battery case. The present invention uses a battery
case composed of pure aluminum or aluminum alloy in which one or
more components selected from manganese, magnesium, silicon and
copper is added in aluminum. Here, pure aluminum does not refer to
aluminum with 100% purity, but may contain impurities which are
unavoidably mixed during an ordinary refining or manufacturing
process, and, more specifically, the purity is preferably 99% or
more. In addition, as for the aluminum alloy, it does not mean that
impurities unavoidably mixed during an ordinary manufacturing
process are not similarly excluded from aluminum which is the main
component. Specific examples of aluminum alloy include alloy No.
3203 (aluminum-manganese alloy) prescribed in JIS.
[0027] Here, the aluminum battery case means that a main portion of
the battery case, that is, a container into which the internal
wound body is inserted for storage is made of aluminum, and a
sealing member for sealing an opening of the battery case is not
necessarily made of aluminum. For example, when the internal
electrode body is a cylindrical wound body shown in FIG. 2, it is
sufficient that at least a cylindrical container opened at both
ends, or a bottomed cylindrical container opened at only one end is
made of aluminum. When the internal electrode body is a rectangular
parallelepiped laminated body shown in FIG. 3, as long as at least
a tubular container with a rectangular section or a rectangular
parallelepiped box-like container opened only at one side is made
of aluminum, it is preferably used for the present invention.
[0028] The reason why the opening or the like through which the
internal electrode body is inserted is excluded from the battery
case lies in that the sealing member for sealing the opening of the
battery case is sometimes preferably constituted by an insulating
material such as heat resistance resin or ceramics for the purpose
of installing an external terminal for taking out electric energy
from the internal electrode body or isolating an electric path of
the positive and negative electrodes within the battery. Of course,
the above example of battery case does not exclude a battery case
which can be entirely composed of aluminum for the outer shell of
battery by disposing insulating materials at appropriate locations
to assure electric paths for the positive and negative electrodes,
and using an aluminum part for the sealing member.
[0029] Then, the battery is produced by the internal electrode
body, the battery case and other necessary members such as
electrode terminals. In this case, as for the structure for the
battery being produced, it may be possible to adopt a structure
which is a structure of a known small battery enlarged as it is. In
addition, the inventors have proposed in Japanese Patent
Application No. 9-202963 a structure of lithium secondary battery
in which various pressure releasing mechanisms are disposed at
appropriate locations, and such structure may be preferably
employed. Moreover, the battery thus produced preferably has at
least one pressure release valve which releases the battery
internal pressure to the ambient air pressure when the battery
internal pressure rises and reaches a predetermined pressure due to
erroneous use of the battery or the like, thereby preventing
explosion due to rise of internal pressure of the battery.
[0030] According to the present invention, where current capacity
of the battery produced by using the aluminum battery case is C
(Ah), the battery weight is w (kg), and the specific heat of the
battery is c (W/kg.multidot..degree. C.), the battery is preferably
designed such that a relationship of C/(w.multidot.c).ltoreq.0.03
is established. Here, the specific heat c of battery is defined to
be power (W) necessary for raising temperature of a battery of 1 kg
by 1.degree. C. Therefore, even if the same battery case is used in
producing the battery, the battery has different specific heat if
components other than the battery case differ, while, even if the
volume of battery is the same, the battery has different specific
heat depending on the material and wall thickness of the battery
case, the size of internal electrode body or the like.
[0031] However, when the construction conditions are established to
assure safety even if all energy which the battery can store is
used to raise temperature of the battery, that is, the battery is
arranged such that the relationship of C/(w.multidot.c).ltoreq.0.03
is established, it is possible to obtain a battery in which
temperature rise due to generated heat does not cause softening or
melting of the battery case, and which can clear the safety
criteria of the SBA Guideline even if the energy charged in the
battery is instantaneously discharged as external short-circuiting
is caused or as internal short-circuiting is caused by the nail
piercing test.
[0032] In addition, it is preferable in the present invention that
the relationship of 0.004.ltoreq.t/d.ltoreq.0.04 is established
where the battery case is cylindrical, the outer diameter of the
battery case is d (mm.phi.), and the wall thickness is t (mm). For
example, in the case where the battery case has a thin wall
thickness t when the outer diameter d of the battery case is
constant, that is, the value of t/d is small, the energy density of
the battery increases since the weight of battery case is reduced
as the battery capacity is increased, and there arises a problem in
safety since the strength of battery case is lowered. On the other
hand, in the case where the battery case has a thick wall thickness
t, that is the value of t/d is large, it is desirable from the
viewpoint of safety since the strength of battery case is
heightened, but there arises a problem that the energy density is
reduced as a whole since the weight of battery case is increased,
and the battery capacity is also reduced,
[0033] Thus, when the battery is arranged to have a ratio of the
outer dimension of battery case to the wall thickness in a specific
range, it becomes possible to assure safety while maintaining the
energy density of battery at a proper high value.
[0034] In the case where the battery case is a rectangular
parallelepiped, the above relationship can be analogously applied
by assuming that the outer diameter of a circle having the same
area as the section perpendicular in the longitudinal direction is
the outer diameter d of the battery case.
[0035] As described earlier, the present invention is attained as
the result of study mainly on the possibility of use of an aluminum
battery case for a battery with large capacity which has not been
produced, and the technical features of the present invention are
suitably employed in a lithium secondary battery having battery
capacity of 50 Wh or more. However, it is needless to say that
there is no problem to employ the structure of battery with large
capacity for which the safety criteria are strict as above for a
battery with smaller capacity.
[0036] Thus, the battery with large capacity per unit cell produced
by using a battery case composed of aluminum has excellent
advantages that the battery has lighter weight, and that it has
higher energy density. When compared with a case where a plurality
of batteries with small capacity are connected to obtain a battery
with equivalent capacity, contact resistance due to battery
connection can be reduced as the number of series/parallel
connections is reduced in the battery, and mounting space for the
battery can be saved. Therefore, the lithium secondary battery of
the present invention is suitable in applications such as the power
supply for an electric or hybrid electric vehicle or as power
supply for various mobile equipment.
[0037] Now, examples of lithium secondary battery according to the
present invention are described, but it is needless to say that the
present invention is not limited to these examples.
[0038] First, description is given of members commonly used for the
examples and the battery structure. The positive electrode was
formed of an aluminum foil coated with a mixture in which carbon
powder (acetylene black) for improving the conductivity was added
to lithium-manganese oxide (LiMn.sub.2O.sub.4) as a positive active
material. The negative electrode was formed of a copper foil coated
with graphite powder. As a separator for separating the positive
electrode from the negative electrode, a microporous separator made
of polypropylene was used. The electrolyte was prepared by
dissolving an LiPF.sub.6 electrolyte in a mixed solution of
ethylene carbonate (EC) and diethyl carbonate (DEC). The battery
was a cylindrical type which was formed by inserting a cylindrical
internal electrode body, in which the positive and negative
electrodes were wound through the separator, into a cylindrical
battery case, both ends of the case being sealed with a structure
shown in FIG. 1.
[0039] Here, in FIG. 1, a lead 32 for electricity collection
connected to either one of the positive or negative electrode (not
shown) was connected to a metal rivet 33 as an internal terminal
mounted on a disk 34 for sealing a battery case 39. Then, the disk
34 was provided with a pressure release valve 35 which was burst
when the internal pressure of the battery reached a predetermined
pressure, and crimped onto the battery case 39 through ethylene
propylene rubber 38 so that an external terminal 37 was
electrically connected to the disk 34 through a metal ring 36, and
that the disk 34, the metal ring 36 and the external terminal 37
were electrically insulated from the battery case. Thus, there was
formed a battery of cylindrical type with both terminals in which
the external terminal for either one of the positive or negative
electrode was disposed on one end of the battery case 39.
[0040] (Test for Selecting Battery Case Material)
[0041] Then, batteries in the battery size of outer diameter 50
mm.phi. and length 245 mm and having the above-mentioned structure
were formed by using battery cases with outer diameter 50 mm and
wall thickness 1 mm composed of various materials listed in Table
1, and energy density of each battery was measured. Here, aluminum
alloy was aluminum added with manganese, while SUS-304 was used as
stainless steel. In addition, the disk 34 for sealing the end of
battery case 39 was made of the same material as the battery case
39, and area of the electrode was made equal so that capacity of
all batteries became 100 Wh.
1 TABLE 1 Energy density Battery case material (Wh/kg) Example 1
Aluminum 116 Example 2 Aluminum alloy 115 Comparative example 1
Stainless steel 94 Comparative example 2 Nickel 89 Comparative
example 3 Titanium 107
[0042] The energy densities of the produced batteries are also
listed in Table 1. It is significant that a battery case material
with higher density tends to provide lower resultant energy
density. That is, in the case of comparative example 2 where nickel
with the highest density was used as the battery case material, the
energy density was the lowest of 89 Wh/kg, and the energy density
became higher as the density of battery case material decreased in
the order of stainless steel (comparative example 1), titanium
(comparative example 3), and aluminum (examples 1 and 2). Examples
1 and 2 using aluminum according to the present invention provided
energy density of about 115 Wh/kg. Since the energy density was 94
Wh/kg for comparative example 1 using stainless steel which had
been generally used as the battery case material, the
characteristic of energy density was improved by about 20% by using
aluminum or aluminum alloy for the battery case. Examples 1 and 2
were believed to have similar energy density because there was no
significant difference in density between aluminum and aluminum
alloy. In addition, in this test, since the battery case was not
used as a current path, and distance was very short between the
lead connected to the disk for sealing the battery case and the
external terminal, impact on the energy density due to difference
of conductivity of the battery case materials (disk for sealing the
battery case) used can be ignored.
[0043] (Test for Identifying Battery Case Shape)
[0044] Effectiveness in using aluminum for the battery case was
demonstrated from the result of test for selecting battery case
material described above. Then, batteries were produced with
various wall thickness t by using aluminum for the battery case,
fixing the outer diameter d of the battery case to 50 mm, and
length of the battery to 245 mm, and varying the wall thickness t
(mm) in view of improvement of energy density and securing of
safety, and measured for energy density and bulging (deformation)
of the battery case after completing 100 charging/discharging
cycles with discharging rate of 0.2C and depth of discharge
(D.O.D.) 100%. Table 2 lists values of t/d and results of the
produced battery cases.
2 TABLE 2 Energy density Bulging after 100 t/d (Wh/kg) cycles (mm)
Comparative example 4 0.002 141 >0.5 Example 3 0.004 137 0.2
Example 4 0.01 130 0.1 Example 5 0.02 117 <0.1 Example 6 0.04
101 0.0 Comparative example 5 0.06 82 0.0 Comparative example 6 0.1
57 0.0
[0045] Although the outer diameter of battery case is fixed, since
the inner diameter of battery case is reduced as the wall thickness
of battery case is thickened, the size of internal electrode body
which can be housed in the battery case is reduced, that is, the
area of electrodes is made small, so that the absolute value of
battery capacity is decreased. In addition, as the wall thickness
of battery case is thickened, ratio of the battery case to the
weight of entire battery is increased. This increases the value of
t/d as listed in Table 2. That is, as the wall thickness of battery
case is thickened, the energy density significantly tends to
decrease.
[0046] Here, since comparative example 4 has as small t/d as 0.002,
it had a light battery case, and very high energy density of about
140 Wh/kg. However, it has large bulging of outer diameter of
battery case after the charging/discharging test of 100 cycles, and
is found to have a problem in safety. On the other hand,
comparative example 5 had as large t/d as 0.06, so that no
deformation of battery case was observed after the
charging/discharging test of 100 cycles, but it could not provide
desired energy density of 100 Wh/kg or more due to increase of
weight of the battery case and decrease of volume of the internal
electrode body which could be housed in the battery case.
[0047] It is revealed from Table 2 that
0.004.ltoreq.t/d.ltoreq.0.04 is preferable as the condition for
assuring safety as well as output density of 100 Wh/kg, as shown in
examples 3 through 6. In addition, the most preferable
characteristic can be attained with bulging suppressed to as low as
0.1 mm or less while maintaining high energy density by making
0.01.ltoreq.t/d.ltoreq.0.02.
[0048] (Test for Measuring Specific Heat of Battery)
[0049] Then, specific heat was measured on example 5 which had the
value of t/d of 0.02 or the wall thickness of 1 mm, which was
believed to be preferable from the viewpoint of the energy density
and safety in the above-mentioned test for identifying shape of
battery case. The specific heat was measured by attaching a T-type
thermocouple at the longitudinal center of side of battery,
discharging the battery at a current of 27 A to 2.5 V in a
25.degree. C. constant temperature bath after constant current
charging at 10 A and constant voltage charging at 4.1 V (6 hours in
total), and measuring temperature rise of the battery. As a result,
temperature rise was 6.degree. C. Assuming that all heat generation
from the battery when it is discharged is caused by internal
resistance of the battery, since the internal resistance of battery
was 4 m.OMEGA., total power consumption in discharge
(resistance.times.(current).sup.2.times.di- scharging time) was
8923 W. Therefore, for battery weight of 0.86 kg and temperature
rise of 6.degree. C., the specific heat of battery was calculated
as 1729 W/kg.multidot..degree. C.
[0050] When all energy (100 Wh) of this battery was assumed to be
instantaneously discharged from the full charged state due to
external short-circuiting caused by erroneous use or internal
short-circuiting, since 100 Wh corresponded to 360000 W
(100.times.3600 seconds), when this value is divided by the weight
and specific heat of the battery, the temperature rise of the
battery was calculated as 242.degree. C., and it was found that the
highest temperature reached was lower than the melting point of
660.degree. C. of aluminum. Then, when the external
short-circuiting test was conducted in a state where the battery
was actually fully charged, the pressure release valve was actuated
but there was caused no burst or firing, so that safety of the
battery was confirmed to be assured.
[0051] [Internal and External Short-circuiting Tests]
[0052] Batteries having various C/(w.multidot.c) values as shown in
Table 3 were produced using an aluminum battery case by noticing
the parameter of C/(w.multidot.c) consisting of the battery
capacity C (Ah), the battery weight w (kg), and the specific heat c
of battery (W/kg.multidot..degree. C.) calculated with the above
method based on the result of the test for measuring specific heat
of battery, and subjected to the nail piercing test (internal
short-circuiting test) according to the SBA Guideline. Table 3 also
lists the test results.
3 TABLE 3 C/(w .multidot. c) Situation after test Evaluation
Example 7 0.015 Pressure release valve actuated; .largecircle. good
no burst nor firing Example 8 0.018 Pressure release valve
actuated; .largecircle. good no burst nor firing Example 9 0.03
Pressure release valve actuated; .largecircle. good no burst nor
firing Comparative 0.035 Pressure release valve actuated; X no good
example 7 burst and firing occurred
[0053] As listed in Table 3, in the case of examples 7-9 with
C/(w.multidot.c) value of 0.03 or less, although the pressure
release valve was activated, no significant change of shape was
observed due to softening or melting of the battery case. However,
in the case of comparative example 7 with C/(w.multidot.c) value of
0.035, the battery case was significantly deformed and partially
cracked, and traces which were believed to indicate partial melting
were observed. In addition, as for examples 7-9 and comparative
example 7, when similar batteries were again produced, and
subjected to the external short-circuiting test by short-circuiting
the external terminal, there were provided the same results as the
internal short-circuiting test shown in Table 3. From this, it was
confirmed that the safety criteria prescribed in the SBA Guideline
could be passed by making the C/(w.multidot.c) value 0.03 or
less.
[0054] As described, according to the lithium secondary battery of
the present invention, since it uses for the battery case aluminum
which has lightweight and is excellent in conductivity, it has a
very excellent advantage that the battery has light weight, and is
significantly improved for the energy density than the prior art.
Moreover, it is possible to provide a battery with excellent safety
which can pass the criteria of SBA Guideline because the specific
heat design of battery for the battery capacity and determination
of shape of battery case are properly conducted.
* * * * *