U.S. patent application number 11/473328 was filed with the patent office on 2006-12-21 for lithium secondary battery.
Invention is credited to Masato Fujikawa, Shinji Kasamatsu, Hajime Nishino, Mikinari Shimada, Hideharu Takezawa.
Application Number | 20060286438 11/473328 |
Document ID | / |
Family ID | 37532055 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060286438 |
Kind Code |
A1 |
Fujikawa; Masato ; et
al. |
December 21, 2006 |
Lithium secondary battery
Abstract
A lithium secondary battery including: an electrode group, a
non-aqueous electrolyte and a battery case housing the electrode
group and the non-aqueous electrolyte, the electrode group
including a positive electrode, a negative electrode and a
separator layer interposed between the positive electrode and the
negative electrode, wherein an end-of-charge voltage and an
end-of-discharge voltage are set in such a manner that the
electrode group has an energy density of not less than 700 Wh/L,
the separator layer includes a porous heat-resistant layer, and a
short circuit area A produced when an internal short circuit has
occurred between the positive electrode and the negative electrode,
and a reduced area B of the porous heat-resistant layer that is
produced by heat generation satisfy 1.ltoreq.(A+B)/A.gtoreq.10.
Inventors: |
Fujikawa; Masato; (Osaka,
JP) ; Kasamatsu; Shinji; (Osaka, JP) ;
Nishino; Hajime; (Nara, JP) ; Takezawa; Hideharu;
(Nara, JP) ; Shimada; Mikinari; (Kyoto,
JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Family ID: |
37532055 |
Appl. No.: |
11/473328 |
Filed: |
June 23, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11339617 |
Jan 26, 2006 |
|
|
|
11473328 |
Jun 23, 2006 |
|
|
|
Current U.S.
Class: |
429/61 ; 429/120;
429/144; 429/231.95 |
Current CPC
Class: |
H01M 50/446 20210101;
H01M 4/525 20130101; Y02T 10/70 20130101; H01M 10/4235 20130101;
Y02E 60/10 20130101; H01M 50/411 20210101; H01M 50/449 20210101;
H01M 50/46 20210101; H01M 10/052 20130101 |
Class at
Publication: |
429/061 ;
429/144; 429/120; 429/231.95 |
International
Class: |
H01M 2/16 20060101
H01M002/16; H01M 2/18 20060101 H01M002/18; H01M 10/50 20060101
H01M010/50; H01M 4/40 20060101 H01M004/40 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2005 |
JP |
2005-174585 |
Claims
1. A lithium secondary battery comprising: an electrode group, a
non-aqueous electrolyte and a battery case housing said electrode
group and said non-aqueous electrolyte, said electrode group
including a positive electrode, a negative electrode and a
separator layer interposed between said positive electrode and said
negative electrode, wherein an end-of-charge voltage and an
end-of-discharge voltage are set in such a manner that said
electrode group has an energy density of not less than 700 Wh/L,
said separator layer includes a porous resin membrane and a porous
heat-resistant layer, said porous heat-resistant layer being
provided on a surface of said porous resin membrane, said separator
layer has a thickness of not more than 24 .mu.m, and a short
circuit area A produced when an internal short circuit has occurred
between said positive electrode and said negative electrode, and a
reduced area B of said porous heat-resistant layer that is produced
by heat generation satisfy 1.ltoreq.(A+B)/A.ltoreq.10.
2. The lithium secondary battery in accordance with claim 1,
wherein said separator layer has a thickness of not less than 12.5
.mu.m.
3. The lithium secondary battery in accordance with claim 1,
wherein said porous heat-resistant layer has a thickness of not
less than 1 .mu.m and not more than 10 .mu.m.
4. The lithium secondary battery in accordance with claim 1,
wherein said porous heat-resistant layer includes a heat-resistant
resin.
5. The lithium secondary battery in accordance with claim 1,
wherein said porous heat-resistant layer is located at said
positive electrode side of said porous resin membrane.
6. The lithium secondary battery in accordance with claim 1,
wherein said porous heat-resistant layer has an area larger than
that of an active material layer of said negative electrode.
7. The lithium secondary battery in accordance with claim 1,
wherein said negative electrode includes at least one selected from
the group consisting of a lithium metal and a substance comprising
an element capable of being alloyed with lithium.
8. The lithium secondary battery in accordance with claim 1,
wherein said positive electrode includes a lithium-containing
composite oxide including Ni element.
9. The lithium secondary battery in accordance with claim 1,
wherein said end-of-charge voltage is set to a voltage higher than
4.2 V.
10. The lithium secondary battery in accordance with claim 1,
wherein a ratio of a thickness A of said porous resin membrane to a
thickness B of said porous heat-resistant layer, A/B, satisfies
2.4.ltoreq.A/B.ltoreq.8.
11. A lithium secondary battery comprising: an electrode group, a
non-aqueous electrolyte and a battery case housing said electrode
group and said non-aqueous electrolyte, said electrode group
including a positive electrode, a negative electrode and a
separator layer interposed between said positive electrode and said
negative electrode, wherein an end-of-charge voltage and an
end-of-discharge voltage are set in such a manner that said
electrode group has an energy density of not less than 700 Wh/L,
said separator layer includes a porous resin membrane and a porous
heat-resistant layer, and said porous heat-resistant layer
including an insulating filler and a binder, said separator layer
has a thickness of not more than 24 .mu.m, and a short circuit area
A produced when an internal short circuit has occurred between said
positive electrode and said negative electrode, and a reduced area
B of said porous heat-resistant layer that is produced by heat
generation satisfy 1.ltoreq.(A+B)/A.ltoreq.10.
12. The lithium secondary battery in accordance with claim 11,
wherein said separator layer has a thickness of not less than 12.5
.mu.m.
13. The lithium secondary battery in accordance with claim 11, said
porous heat-resistant layer has a thickness of not less than 1
.mu.m and not more than 10 .mu.m.
14. The lithium secondary battery in accordance with claim 11,
wherein said porous heat-resistant layer is provided on a surface
of said positive electrode.
15. The lithium secondary battery in accordance with claim 11,
wherein said negative electrode includes at least one selected from
the group consisting of a lithium metal and a substance comprising
an element capable of being alloyed with lithium.
16. The lithium secondary battery in accordance with claim 11,
wherein said positive electrode includes a lithium-containing
composite oxide containing Ni element.
17. The lithium secondary battery in accordance with claim 11,
wherein said end-of-charge voltage is set to a voltage higher than
4.2 V.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 11/339,617 filed Jan. 26, 2006 which in turn
claims the benefit of Japanese Application No. 2005-174585 filed
Jun. 15, 2005, the disclosures of which Applications are
incorporated by reference herein in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to a lithium secondary battery
including an electrode group with a high energy density, and more
particularly to a lithium secondary battery having excellent short
circuit resistance.
BACKGROUND OF THE INVENTION
[0003] Lithium secondary batteries are attracting attention as
high-capacity power sources, and are considered to be promising
particularly as power sources for portable devices. A commonly used
lithium secondary battery includes an electrode group, a
non-aqueous electrolyte and a battery case housing the electrode
group and the non-aqueous electrolyte. The electrode group includes
a positive electrode, a negative electrode and a porous resin
membrane interposed between the electrodes. The porous resin
membrane serves to provide electronic insulation between the
positive electrode and the negative electrode, and to retain the
non-aqueous electrolyte. For example, the positive electrode
includes lithium cobaltate as its active material, whereas the
negative electrode includes graphite as its active material.
[0004] At present, studies aiming at providing even higher capacity
for lithium secondary batteries are being carried out. However, the
theoretical capacity of lithium cobaltate used as the positive
electrode active material is about 150 mAh/g, and that of graphite
used as the negative electrode active material is about 370 mAh/g.
These theoretical capacities cannot be considered very high. Thus,
it is difficult to stably supply lithium secondary batteries
including an electrode group with an energy density of not less
than 700 Wh/L. Therefore, the uses of active materials with high
theoretical capacities are being investigated.
[0005] For example, there has been proposed a lithium secondary
battery including an electrode group with an energy density
exceeding 700 Wh/L, in which Si is used as the negative electrode
active material (Japanese Laid-Open Patent Publication No. Hei
7-29602). There has also been proposed to set the end-of-charge
voltage of a lithium secondary battery to high so as to increase
the utilization rate of the positive electrode active material,
thereby attaining a high capacity (Japanese Laid-Open Patent
Publication No. 2005-85635). In addition, the energy density of a
lithium secondary battery can be increased when using a positive
electrode active material comprising a lithium-containing composite
oxide containing Ni element.
BRIEF SUMMARY OF THE INVENTION
[0006] As described above, there has been a demand for stable
supply of lithium secondary batteries including an electrode group
with an energy density of not less than 700 Wh/L, recently.
However, the thermal energy released during an internal short
circuit increases with an increase in the energy density of the
electrode group. In particular, the safety significantly decreases
during an internal short circuit resulting from a nail penetration
test.
[0007] The present invention was made in view of the foregoing
problems, and it is an object of the invention to improve the
safety during an internal short circuit and the storage
characteristics of a lithium secondary battery having a high energy
density.
[0008] That is, the present invention relates to a lithium
secondary battery comprising: an electrode group, a non-aqueous
electrolyte and a battery case housing the electrode group and the
non-aqueous electrolyte, the electrode group including a positive
electrode, a negative electrode and a separator layer interposed
between the positive electrode and the negative electrode, wherein
an end-of-charge voltage and an end-of-discharge voltage are set in
such a manner that the electrode group has an energy density of not
less than 700 Wh/L, the separator layer includes a porous
heat-resistant layer, and a short circuit area A produced when an
internal short circuit has occurred between the positive electrode
and the negative electrode, and a reduced area B of the porous
heat-resistant layer that is produced by heat generation satisfy
1.ltoreq.(A+B)/A.ltoreq.10.
[0009] The present invention also relates to a charge/discharge
system including the above lithium secondary battery.
[0010] Here, "short circuit area All refers to the defective area
of the separator layer that has resulted from a short circuit and
is observed immediately after the occurrence of that short circuit
(i.e. before the occurrence of melting or burning of the separator
layer). On the other hand, "reduced area B" refers to the area of
the porous heat-resistant layer that has been melted or burned out
by the heat generated by the short circuit. For example, in the
case of a nail penetration test, the cross-sectional area S of the
nail perpendicular to the length direction thereof corresponds to
the short circuit area A. Further, the defective area of the porous
heat-resistant layer that is observed when 30 seconds or more have
passed since the nail penetration corresponds to A+B.
[0011] It should be noted that "nail penetration test" is a test in
which a nail is put into a completed battery so as to penetrate the
positive electrode, the negative electrode and the separator layer
simultaneously, thereby forcibly forming a short circuit
portion.
[0012] The porous heat-resistant layer may be formed on the surface
of the negative electrode, the positive electrode or a porous resin
membrane.
[0013] The separator layer may include a single or plural layers of
the porous heat-resistant layer.
[0014] The separator layer may include only the porous
heat-resistant layer, or may include the porous heat-resistant
layer and the porous resin membrane.
[0015] When the separator layer includes the porous heat-resistant
layer and the porous resin membrane, it is preferable that the
porous heat-resistant layer has an area (dimension) that is larger
than that of the active material layer of the negative electrode.
It should be noted that the negative electrode includes a
sheet-shaped current collector and a negative electrode active
material layer supported on the current collector. Here, the area
of the active material layer of the negative electrode refers to
the area of the active material layer supported on one side of the
negative electrode, rather than the total area of the active
material layers supported on both sides of the negative
electrode.
[0016] The sheet-shaped current collector of the negative electrode
has the shape of a band having a predetermined width and a
predetermined length. Similarly, the negative electrode active
material layer has the shape of a band having a predetermined width
and a predetermined length. The positive electrode also includes a
sheet-shaped current collector and a positive electrode active
material layer supported on the current collector. The sheet-shaped
current collector of the positive electrode has the shape of a band
having a predetermined width and a predetermined length. Similarly,
the positive electrode active material layer has the shape of a
band having a predetermined width and a predetermined length. The
electrode group is formed by rolling up the positive electrode and
the negative electrode with the separator layer disposed between
the electrodes.
[0017] When the separator layer includes the porous heat-resistant
layer and the porous resin membrane, it is preferable that the
porous heat-resistant layer is formed on a surface of the positive
electrode or the porous resin membrane.
[0018] When the porous heat-resistant layer is formed on the
surface of the porous resin membrane, it is preferable that porous
heat-resistant layer is disposed on the positive electrode side of
the porous resin membrane.
[0019] For example, the porous heat-resistant layer may include an
insulating filler and a binder, or may include a heat-resistant
resin.
[0020] The negative electrode may include, for example, at least
one selected from the group consisting of a lithium metal and a
substance comprising an element capable of being alloyed with
lithium.
[0021] The positive electrode may include, for example, a
lithium-containing composite oxide containing Ni element.
[0022] The present invention is particularly effective when the
end-of-charge voltage of the lithium secondary battery is set to a
voltage higher than 4.2 V.
[0023] In one embodiment of the present invention, the separator
layer has a thickness of not more than 24 .mu.m.
[0024] In another embodiment of the present invention, the
separator layer has a thickness of 10 to 24 .mu.m.
[0025] In still another embodiment of the present invention, the
separator layer has a thickness of 12.5 to 24 .mu.m.
[0026] In one embodiment of the present invention, the porous
heat-resistant layer has a thickness of 1 to 20 .mu.m.
[0027] In another embodiment of the present invention, the porous
heat-resistant layer has a thickness of 1 to 10 .mu.m.
[0028] In one embodiment of the present invention, the porous resin
membrane has a thickness of 8 to 18 .mu.m.
[0029] The present invention particularly relates to a lithium
secondary battery (Battery X) comprising: an electrode group, a
non-aqueous electrolyte, and a battery case housing the electrode
group and the non-aqueous electrolyte, the electrode group
including a positive electrode, a negative electrode, and a
separator layer interposed between the positive electrode and the
negative electrode, wherein an end-of-charge voltage and an
end-of-discharge voltage are set in such a manner that the
electrode group has an energy density of not less than 700 Wh/L,
the separator layer includes a porous resin membrane and a porous
heat-resistant layer, the porous heat-resistant layer is provided
on a surface of the porous resin membrane, the separator layer has
a thickness of not more than 24 .mu.m, and a short circuit area A
produced when an internal short circuit has occurred between the
positive electrode and the negative electrode, and a reduced area B
of the porous heat-resistant layer that is produced by heat
generation satisfy 1.ltoreq.(A+B)/A.ltoreq.10.
[0030] In Battery X, the porous heat resistant layer preferably
includes a heat-resistant resin. The porous heat-resistant layer is
preferably located at a positive electrode side of the porous resin
membrane. The porous heat-resistant layer preferably has an area
larger than the area of an active material layer of the negative
electrode. The ratio of a thickness A of the porous resin membrane
and a thickness B of the porous heat-resistant layer, A/B,
satisfies 2.4.ltoreq.A/B.ltoreq.8, for example.
[0031] The present invention particularly relates to a lithium
secondary battery (Battery Y) comprising: an electrode group, a
non-aqueous electrolyte, and a battery case housing the electrode
group and the non-aqueous electrolyte, the electrode group
including a positive electrode, a negative electrode, and a
separator layer interposed between the positive electrode and the
negative electrode, wherein an end-of-charge voltage and an
end-of-discharge voltage are set in such a manner that the
electrode group has an energy density of not less than 700 Wh/L,
the separator layer includes a porous resin membrane and a porous
heat-resistant layer, the porous heat-resistant layer includes an
insulating filler and a binder, the separator layer has a thickness
of not more than 24 .mu.m, and a short circuit area A produced when
an internal short circuit has occurred between the positive
electrode and the negative electrode, and a reduced area B of the
porous heat-resistant layer that is produced by heat generation
satisfy 1.ltoreq.(A+B)/A.ltoreq.10.
[0032] In Battery Y, the porous heat resistant layer is preferably
provided on a surface of the positive electrode.
[0033] In Batteries X and Y, the negative electrode preferably
includes at least one selected from the group consisting of a
lithium metal and a substance comprising an element capable of
being alloyed with lithium. The positive electrode preferably
includes a lithium-containing composite oxide including Ni element.
The end of charge voltage of the battery is preferably set to
higher than 4.2 V.
[0034] In the case where a short circuit has occurred between the
positive electrode and the negative electrode, conventional
separator layers will be melted by heat generated, since they only
include a porous resin membrane. Consequently, the area of the
short circuit portion increases to the total of the short circuit
area A and the reduced area of the separator layer, further
increasing the amount of heat generated. The amount of heat
generated at the time of occurrence of a short circuit
significantly increases when the energy density of the electrode
group exceeds 700 Wh/L. Particularly, when an internal short
circuit has occurred during storage in a high temperature
environment and the short circuit portion expands, then the heat
generation is greatly accelerated. On the other hand, the separator
layer according to the present invention includes the porous
heat-resistant layer, and the reduced area B of the porous
heat-resistant layer that has resulted from the heat generated
during a short circuit is limited to a small amount. Accordingly,
it is possible to reduce the duration of a short circuit, thereby
suppressing the increase of the amount of heat generated, or the
acceleration of the heat generation. Thus, according to the present
invention, it is possible to provide a lithium secondary battery
that offers a high level of safety in case of an internal short
circuit and excellent storage characteristics, while having a high
energy density.
[0035] While the novel features of the invention are set forth
particularly in the appended claims, the invention, both as to
organization and content, will be better understood and
appreciated, along with other objects and features thereof, from
the following detailed description taken in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWING
[0036] FIG. 1 is a cross-sectional view schematically showing the
vicinity of a short circuit portion of a lithium secondary battery
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The lithium secondary battery according to the present
invention includes an electrode group, a non-aqueous electrolyte
and a battery case housing the electrode group and the non-aqueous
electrolyte. The electrode group includes a positive electrode, a
negative electrode and a separator layer interposed between the
positive electrode and the negative electrode. The electrode group
has an energy density of not less than 700 Wh/L. The separator
layer includes a porous heat-resistant layer. The short circuit
area A produced when an internal short circuit has occurred between
the positive electrode and the negative electrode and the reduced
area B of the porous heat-resistant layer that has resulted from
that internal short circuit satisfy 1.ltoreq.(A+B)/A.ltoreq.10.
[0038] An internal short circuit tends to occur, for example, when
conductive foreign matter is incorporated between the positive
electrode and the negative electrode. Usually, the safety during an
internal short circuit is evaluated by a nail penetration test. In
a nail penetration test, when a nail simultaneously penetrates the
positive electrode, the negative electrode and the separator layer,
a short circuit portion is formed.
[0039] For commonly used conventional lithium secondary batteries,
only a porous resin membrane is used as the separator layer. The
porous resin membrane is composed only of a polyolefin, or contains
a polyolefin as the main component (e.g. contains a polyolefin at
not less than 95 wt %). Accordingly, the porous resin membrane has
low heat resistance, and thus tends to be melted by heat generated.
Consequently, the short circuit portion dramatically expands,
relative to the short circuit area A, and the amount of heat
generated suddenly increases. On the other hand, the separator
layer according to the present invention includes the porous
heat-resistant layer, and the porous heat-resistant layer is
difficult to be melted or burned out by heat generated.
Accordingly, it is possible to prevent the increase of the amount
of heat generated owing to an expanded short circuit portion or the
acceleration of heat generation, resulting in a significantly
improved resistance to an internal short circuit.
[0040] FIG. 1 is a cross-sectional view schematically showing the
vicinity of a short circuit portion produced when an internal short
circuit has occurred between a positive electrode and a negative
electrode. A positive electrode 1 and a negative electrode 2 are
arranged alternately with a separator layer 3 disposed between
them. The separator layer 3 includes a porous heat-resistant layer
(not shown). If conductive foreign matter 4 penetrates the
separator layer 3, then a short circuit portion is formed between
the positive electrode 1 and the negative electrode 2. The lithium
secondary battery of the present invention includes an electrode
group having a high energy density of not less than 700 Wh/L.
Accordingly, when a short circuit portion is formed in a state
where the chemical potential of the positive electrode 1 and the
negative electrode 2 is high (i.e. a state where the battery is
charged), a large amount of heat is generated, leading to melting,
elimination or deformation of the porous heat-resistant layer at
its part in the vicinity of the short circuit portion. As a result,
the defective area of the separator layer 3 (i.e. the area of the
short circuit portion) increases to the total of the short circuit
area A and the reduced area B of the porous heat-resistant layer.
When the separator layer 3 includes a porous resin membrane, the
porous resin membrane will have a greater area reduction than the
porous heat-resistant layer. Thus, the defective area of the
separator layer 3 (i.e. the area of the short circuit portion) can
be in effect regarded as the defective area of the porous
heat-resistant layer.
[0041] When the electrode group has an energy density of not less
than 700 Wh/L, the amount of heat generation increases
acceleratingly if the defective area of the porous heat-resistant
layer expands to the extent that 10<(A+B)/A. Particularly, in
the case of a battery placed under a high temperature environment,
the amount of the current resulting from the short circuit
increases since the internal resistance is reduced at high
temperatures, so that amount of heat generation increases
significantly. Accordingly, the safety of the battery during an
internal short circuit is rapidly reduced. On the other hand, when
(A+B)/A.ltoreq.10, it is possible to suppress the heat generation
to a minimum even if the chemical potential of the positive
electrode and the negative electrode is high. In order to suppress
the heat generation effectively, it is preferable that
(A+B)/A.ltoreq.9 or (A+B)/A.ltoreq.7 be satisfied, for example.
[0042] Additionally, the porous heat-resistant layer includes a
heat-resistant material. Examples of the heat-resistant material
include an inorganic oxide, a ceramic and a heat-resistant resin.
These may be used alone or in combination of two or more of them.
Here, it is preferable that the heat-resistant resin has a heat
deformation temperature of not less than 260.degree. C. Here, "heat
deformation temperature" refers to a deflection temperature under
load determined under a load of 1.82 MPa in accordance with the
testing method ASTM-D648 of American Society for Testing and
Materials. It is also preferable that the heat-resistant resin has
a glass transition temperature (Tg) of not less than 130.degree.
C.
[0043] The separator layer may include a single layer or plural
layers of the above-described porous heat-resistant layer.
[0044] For example, the porous heat-resistant layer is formed on
the surface of the negative electrode, the positive electrode or
the porous resin membrane. However, the porous heat-resistant layer
may be formed on the surfaces of the negative electrode and the
positive electrode, the surfaces of the positive electrode and the
porous resin membrane, the surfaces of the porous resin membrane
and the negative electrode, or the surfaces of the negative
electrode, the positive electrode and the porous resin membrane.
The porous heat-resistant layer may also be interposed between the
positive electrode and the negative electrode in the form of a
sheet that is independent of the negative electrode, the positive
electrode and the porous resin membrane. However, the porous
heat-resistant layer is preferably bonded on or adhered to at least
one surface of the positive electrode, at least one surface of the
negative electrode, or at least one surface of the porous resin
membrane.
[0045] It is preferable that the separator layer includes the
porous heat-resistant layer and the porous resin membrane, instead
of including only the porous heat-resistant layer. Since the porous
resin membrane includes a polyolefin, it has flexibility.
Accordingly, the separator layer including the porous
heat-resistant layer and the porous resin membrane is more durable
than the separator layer including only the porous heat-resistant
layer. Further, the porous resin membrane is also superior in the
capability of retaining the non-aqueous electrolyte.
[0046] From the viewpoint of preventing contact between the
positive electrode and the negative electrode, since the area of
the negative electrode is usually larger than that of the positive
electrode, the porous heat-resistant layer is provided on the
surface of the negative electrode in many cases. However, from the
viewpoint of preventing defect of decrease in battery voltage, it
is preferable that the porous heat-resistant layer is provided on
the surface of the positive electrode, or on the surface of the
porous resin membrane to face the positive electrode.
[0047] In addition, regardless of whether the porous heat-resistant
layer is formed on the surface of the negative electrode, it is
preferable that the area of the porous heat-resistant layer is made
larger than that of the active material layer of the negative
electrode. That is to say, it is preferable that the area of the
porous heat-resistant layer is made larger than that of the active
material layer of the negative electrode also when the porous
heat-resistant layer is formed on the surface of the positive
electrode or the porous resin membrane, from the viewpoint of
improving the reliability of the battery. By making the area of the
porous heat-resistant layer larger than that of the active material
layer of the negative electrode, even if the electrode group is
distorted as a result of repeated charge/discharge or storage,
contact between the positive electrode and the negative electrode
is prevented, thus making it possible to suppress the decrease in
the battery voltage.
[0048] When the separator layer includes the porous heat-resistant
layer and the porous resin membrane, it is preferable that the
porous heat-resistant layer is formed on the surface of the
positive electrode, or on the surface of the porous resin
membrane.
[0049] When conductive foreign matter adheres onto the surface of
the positive electrode in the electrode group, the foreign matter
is dissolved during the subsequent charge/discharge, and thus tends
to be re-deposited on the surface of the negative electrode. In
this case, there is the possibility that the deposit on the
negative electrode may grow and eventually reach the positive
electrode. When such a short circuit occurs, an abnormal voltage
drop occurs in the battery. On the other hand, when the porous
heat-resistant layer is formed on the surface of the positive
electrode, a high potential area on the surface of the positive
electrode is protected by the porous heat-resistant layer, even if
conductive foreign matter enters into the electrode group.
Therefore, the foreign matter becomes difficult to be dissolved,
and therefore is difficult to be deposited on the surface of the
negative electrode. Accordingly, it is possible to prevent an
abnormal voltage drop in the battery.
[0050] When the negative electrode includes an element capable of
being alloyed with lithium, e.g. silicon, volume change of the
negative electrode during charge/discharge becomes large. Thus,
when the porous heat-resistant layer is formed on the surface of
the negative electrode, the porous heat-resistant layer tends to be
damaged. However, even when the negative electrode includes an
element capable of being alloyed with lithium, the porous
heat-resistant layer is prevented from being damaged or fractured
if the porous heat-resistant layer is provided on the surface of
the positive electrode.
[0051] When the porous heat-resistant layer is formed on the
surface of the porous resin membrane, the porous heat-resistant
layer and the porous resin membrane are integrated in one piece, so
that the separator layer has an increased strength, facilitating
the formation of the electrode group and thus improving the
productivity of the battery.
[0052] When the porous heat-resistant layer is formed on the
surface of the porous resin membrane, it is also preferable that
the porous heat-resistant layer is disposed on the positive
electrode side of the porous resin membrane. The polyolefin (e.g.
polyethylene or polypropylene) included in the porous resin
membrane may be oxidized under high voltage and high temperature
environment. When the polyolefin is oxidized on the surface of the
porous resin membrane, there is the possibility that the porous
resin membrane may be denatured, or cause clogging, leading to the
deterioration of the battery characteristics. On the other hand, by
disposing the porous heat-resistant layer on the positive electrode
side of the porous resin membrane, the porous resin membrane is
protected from the positive electrode, making it possible to
prevent the porous resin membrane from being deteriorated by the
positive electrode potential.
[0053] In addition, the voids within the electrode group tend to
decrease with an increase in the energy density of the electrode
group. Further, the porous resin membrane is compressed owing to
the expansion and contraction of the electrodes during
charge/discharge, so that the electrolyte has a greater tendency to
be extruded from the electrode group. Therefore, the ionic
conductivity within the electrodes tends to decrease. The decrease
of the ionic conductivity is more prominent in the positive
electrode, which generally has fewer voids than the negative
electrode. On the other hand, when the porous heat-resistant layer,
which is difficult to be compressed, is disposed on the positive
electrode side, it is possible to secure the electrolyte abundantly
in the vicinity of the positive electrode, thus making it possible
to maintain favorable characteristics.
[0054] In the case of using high-capacity materials for the
positive electrode and the negative electrode for the purpose of
increasing the energy density, an alloy material (e.g. a silicon
alloy or a tin alloy), a silicon oxide (e.g. SiO) or the like can
be suitably used for the negative electrode active material.
However, a high-capacity material undergoes great expansion and
contraction during charge/discharge. Therefore, when the porous
heat-resistant layer is disposed on the negative electrode side,
there is the possibility that the porous heat-resistant layer may
be damaged. In view of the foregoing standpoints, it is preferable
that the porous heat-resistant layer is disposed on the positive
electrode side.
[0055] Although there is no particular limitation with respect to
the porous heat-resistant layer, the porous heat-resistant layer
may include, for example, an insulating filler and a binder, or may
comprise a heat-resistant resin. The porous heat-resistant layer
including an insulating filler and a binder has relatively high
mechanical strength and hence high durability. Here, the porous
heat-resistant layer including an insulating filler and a binder
contains the insulating filler as the main component. For example,
the insulating filler constitutes not less than 80 wt % of the
porous heat-resistant layer. The porous heat-resistant layer
comprising a heat-resistant resin includes the heat-resistant resin
at more than 20 wt %, for example.
[0056] It should be noted that the porous heat-resistant layer
comprising a heat-resistant resin has higher flexibility than the
porous heat-resistant layer containing an insulating filler as the
main component. The reason is that the heat-resistant resin is more
flexible than the insulating filler. Therefore, the porous
heat-resistant layer comprising a heat-resistant resin can more
easily accommodate itself to the expansion and contraction of the
electrode plate that results from charge/discharge, and therefore
can maintain high heat resistance. Accordingly, it provides a high
level of safety against nail penetration.
[0057] The porous heat-resistant layer comprising a heat-resistant
resin may include an insulating filler at less than 80 wt %, for
example. Inclusion of an insulating filler can provide a porous
heat-resistant layer with well-balanced flexibility and durability.
The heat-resistant resin contributes to the flexibility of the
porous heat-resistant layer, whereas the insulating filler, which
has high mechanical strength, contributes to the durability.
Inclusion of an insulating filler in the porous heat-resistant
layer improves the high output characteristics of the battery.
Although the details are unknown, the reason seems to be that the
void structure of the porous heat-resistant layer is optimized by a
synergetic effect produced by flexibility and durability. From the
viewpoint of ensuring good high output characteristics, it is
preferable that the porous heat-resistant layer comprising a
heat-resistant resin includes an insulating filler at 25 wt % to 75
wt %.
[0058] The porous heat-resistant layer can be provided by casting
the source material of the porous heat-resistant layer onto the
surface of at least one of the positive electrode, the negative
electrode and the porous resin membrane. When the porous
heat-resistant layer is in the form of an independent sheet, the
sheet constituted by the porous heat-resistant layer is disposed
between the positive electrode and the negative electrode, between
the positive electrode and the porous resin membrane, or between
the negative electrode and the porous resin membrane.
[0059] Specifically, the porous heat-resistant layer can be formed,
for example, as follows:
[0060] (i) An insulating filler and a binder are mixed with a
liquid component to prepare a paste or slurry, which is then
applied onto the surface of at least one of the positive electrode,
the negative electrode and the porous resin membrane, followed by
removal of the liquid component by drying. The amount of the binder
is preferably, but not limited to, 0.5 to 10 parts by weight per
100 parts by weight of the insulating filler.
[0061] The insulating filler, the binder and the liquid component
are mixed using a double arm kneader, for example. The obtained
paste or slurry is applied onto the surface of the electrodes or
the porous resin membrane with a doctor blade or a die coater, for
example.
[0062] (ii) A resin solution in which a heat-resistant resin is
dissolved in a solvent is applied onto the surface of at least one
of the positive electrode, the negative electrode and the porous
resin membrane, followed by removal of the solvent by drying. The
solvent in which the heat-resistant resin is dissolved is
preferably, but not limited to, a polar solvent such as
N-methyl-2-pyrrolidone (hereinafter, abbreviated as "NMP"). Not
more than 500 parts by weight (preferably 33 parts by weight to 300
parts by weight) of the insulating filler per 100 parts by weight
of the heat-resistant resin may be dispersed in the resin
solution.
[0063] (iii) As in (i) described above, an insulating filler and a
binder are mixed with a liquid component to prepare a paste or
slurry, which is then applied onto a flat substrate, followed by
removal of the liquid component by drying. Then, a sheet
constituted by the porous heat-resistant layer including the
insulating filler and the binder is peeled from the substrate, and
disposed between the electrodes, or between one of the electrodes
and the porous resin membrane. For example, a glass plate or a
plate made of stainless steel (SUS) is used as the substrate.
[0064] (iv) As in (ii) described above, a resin solution in which a
heat-resistant resin is dissolved in a solvent is applied onto a
flat substrate, followed by removal of the solvent by drying. Then,
a sheet constituted by the porous heat-resistant layer including
the heat-resistant resin is peeled from the substrate, and disposed
between the electrodes, or between one of the electrodes and the
porous resin membrane.
[0065] It is also possible to use plural porous heat-resistant
layers selected from (i) to (iv) above in combination. For example,
it is possible to integrate the porous heat-resistant layer
described in (i) or (iii) with the porous heat-resistant layer
described in (ii) or (iv) in one piece.
[0066] While there is no particular limitation with respect to the
insulating filler included in the porous heat-resistant layer, it
is possible to use an inorganic oxide or a ceramic, for example. It
is also possible to use a fibrous heat-resistant resin. Among
these, an inorganic oxide is particularly preferable. As the
inorganic oxide, it is preferable to use alumina, silica, titania,
zirconia, magnesia, yttria or the like, from the viewpoint of the
chemical stability in the environment inside the battery. These
insulating fillers may be used alone or in combination of two or
more of them. The median diameter of the insulating filler is
preferably 0.05 to 10 .mu.m.
[0067] While there is no particular limitation with respect to the
binder included in the porous heat-resistant layer, it is possible
to use, for example, polyvinylidene fluoride (hereinafter,
abbreviated as "PVDF"), acrylic rubber particles, typified by
BM-500B (trade name) manufactured by ZEON Corporation, or
polytetrafluoroethylene (hereinafter, abbreviated as "PTFE"). In
the case of using PTFE or BM-500B, it is preferable to use them in
combination with carboxymethyl cellulose (hereinafter, abbreviated
as "CMC"), polyethylene oxide (hereinafter, abbreviated as "PEO")
or a modified acrylonitrile rubber, typified by BM-720H (trade
name) manufactured by ZEON Corporation as the thickener for the
paste or slurry. These binders may be used alone or in combination
of two or more of them.
[0068] While there is no particular limitation with respect to the
heat-resistant resin constituting the porous heat-resistant layer,
it is possible to use, for example, aramid, polyamide imide and
cellulose. These heat-resistant resins may be used alone or in
combination of two or more of them. The heat-resistant resins may
also be used in combination with other resins.
[0069] The lithium secondary battery according to the present
invention includes an electrode group having an energy density of
not less than 700 Wh/L. Therefore, it is desirable to use materials
having a high theoretical capacity as the active material of each
of the positive electrode and the negative electrode. Further, it
is desirable that the amount of optional components (e.g. a binder
and a conductive agent) included in the electrodes is as small as
possible.
[0070] Usually, the negative electrode includes an active material
and a sheet-shaped current collector (core member) carrying the
active material. For the negative electrode current collector, it
is preferable to use a copper foil, and the thickness of the foil
may be, for example, 5 to 50 .mu.m. As the negative electrode
active material, it is possible to use various substances that have
been conventionally used as the negative electrode active material
of lithium secondary batteries. Specifically, it is possible to
use, for example, a carbon material (e.g. graphite), a substance
comprising an element capable of being alloyed with lithium, and
lithium metal. However, from the viewpoint of increasing the
capacity, it is preferable to use a substance comprising an element
capable of being alloyed with lithium and/or lithium metal.
Examples of the substance comprising an element capable of being
alloyed with lithium include a Si-containing substance (e.g. a
substance composed simply of Si, SiOx (0<x<2)), a substance
composed simply of Sn, SnO and Ti. As the lithium metal, it is
possible to use a lithium alloy such as Li--Al, in addition to a
substance composed simply of lithium. These negative electrode
active materials may be used alone or in combination of two or more
of them. In addition, although it is possible to directly
vapor-deposit the negative electrode active material on the current
collector, it is also possible to carry a material mixture
containing the active material and a small amount of optional
components on the current collector. Examples of the optional
components include a binder (e.g. PVDF and polyacrylic acid) and a
conductive agent (e.g. acetylene black).
[0071] Usually, the positive electrode includes an active material
and a sheet-shaped current collector (core member) carrying the
active material. For the positive electrode current collector, it
is preferable to use an aluminum foil, and the thickness of the
aluminum foil may be, for example, 10 to 30 .mu.m. As the positive
electrode active material, it is possible to use various substances
that have been conventionally used as the positive electrode active
material of lithium secondary batteries. Specifically, it is
possible to use, for example, a lithium-containing transition metal
oxide such as lithium cobaltate, lithium nickelate or lithium
manganate. It is possible to partly replace the transition metal of
the lithium-containing transition metal oxide with another element.
It is also possible to coat the surface of particles of the oxide
with another element. However, from the viewpoint of increasing the
capacity, it is more preferable that the positive electrode active
material includes a lithium-containing composite oxide containing
Ni element (e.g. LiNiO.sub.2 or
LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2). It is possible to partly
replace the Ni element with another element. It is also possible to
use a mixture of a lithium-containing composite oxide containing Ni
element and a material not containing Ni element. These positive
electrode active materials may be used alone or in combination of
two or more of them. It is preferable that the positive electrode
is formed by making a material mixture including a positive
electrode active material and a small amount of a binder (e.g. PVDF
or BM-500B) carried on the current collector. When the binder is
PVDF, it is preferable to select a high molecular weight PVDF,
which can exert adhesion capability even in a small amount. It is
possible to add a small amount of carbon black or the like as a
conductive agent to the positive electrode material mixture. The
total amount of the binder and the conductive agent is preferably 2
to 8 parts by weight per 100 parts by weight of the positive
electrode active material.
[0072] In order to increase the energy density of the electrode
group, it is possible to set the end-of-charge voltage of the
battery to a voltage higher than 4.2 V, which is usually used. For
example, the end-of-charge voltage can be set to 4.4 V, 4.5 V, or
4.6 V. When the end-of-charge voltage is set to a voltage higher
than 4.2 V, the utilization rate of the positive electrode is
increased even for a combination of a positive electrode and a
negative electrode having relatively low theoretical capacities, so
that it is possible to increase the average voltage of the battery.
Accordingly, the energy density of the battery is increased.
[0073] The thickness of the separator layer is, in one embodiment
of the present invention, not more than 24 .mu.m. The thickness of
the separator layer is, for example, 10 to 24 .mu.m, 12.5 to 24
.mu.m or 14 to 20 .mu.m. When the thickness of the separator layer
is too small, the electronic insulation between the positive
electrode and the negative electrode may decrease. On the other
hand, when the thickness of the separator layer is too large, the
design capacity of the battery decreases, high output
characteristics decreases, or the electrode group becomes difficult
to be inserted into the battery case. When the thickness of the
separator layer is 10 to 24 .mu.m or 12.5 to 24 .mu.m, it is
possible to provide a battery of a high capacity design, and to
maintain sufficient electronic insulation between the positive
electrode and the negative electrode.
[0074] The thickness of the porous heat-resistant layer is, in one
embodiment of the present invention, 1 to 20 .mu.m. The thickness
of the porous heat-resistant layer is, for example, 1 to 10 .mu.m
or 2 to 8 .mu.m, regardless of the presence or absence of the
porous resin membrane. When the thickness of the porous
heat-resistant layer is less than 1 .mu.m, it may not be possible
to sufficiently achieve the effect of improving the resistance of
the battery to an internal short circuit. On the other hand, when
the thickness of the porous heat-resistant layer exceeds 20 .mu.m
or 10 .mu.m, the porous heat-resistant layer may become brittle.
When the thickness of the porous heat-resistant layer is too large,
high output characteristics may decrease, and it may be difficult
to provide an electrode group having an energy density of not less
than 700 Wh/L. When the thickness of the porous heat-resistant
layer is 1 to 20 .mu.m or 1 to 10 .mu.m, it is possible to provide
a battery having a high energy density and well-balanced
characteristics.
[0075] The thickness of the porous resin membrane is preferably 8
to 18 .mu.m. When the thickness of the porous resin membrane is
less than 8 .mu.m, it may be difficult to maintain the electronic
insulation between the positive electrode and the negative
electrode if the thickness of the porous heat-resistant layer is
small. On the other hand, when the thickness of the porous resin
membrane exceeds 18 .mu.m, it may be difficult to provide an
electrode group having an energy density of not less than 700 Wh/L.
When the thickness of the porous resin membrane is 8 to 18 .mu.m,
it is possible to provide a battery having a high energy density
and well-balanced characteristics.
[0076] The porous heat-resistant layer including the insulating
oxide filler and the binder is preferably formed on or adhered to
at least one surface of the positive electrode, and further
preferably formed on or adhered to both surfaces of the positive
electrode. The porous heat-resistant layer comprising
heat-resistant resin is preferably formed on or adhered to at least
one surface of the porous resin membrane, and further preferably
formed on or adhered to only one surface of the porous resin
membrane, since the porous heat-resistant layer is comparatively
brittle. When the porous heat-resistant layer comprising the
heat-resistant resin is formed only on one surface of the porous
resin membrane, the ratio of a thickness A of the porous resin
membrane and a thickness B of the porous heat-resistant layer, A/B,
satisfies, for example, 2.4.ltoreq.A/B.ltoreq.8 or
2.8.ltoreq.A/B.ltoreq.6, in view of the balance between flexibility
of the separator layer and heat resistance of the battery.
[0077] Next, the design standard of the lithium secondary battery
and the method for calculating the energy density will be described
in detail.
[0078] The theoretical capacity of the positive electrode can be
determined as follows:
[0079] First, a cell is assembled using a part of the fabricated
positive electrode (a positive electrode piece). The weight of the
active material included in the positive electrode piece can be
determined by calculation, for example. After the weight of the
positive electrode active material is determined, a Li foil in an
amount excessive to the theoretical capacity of the positive
electrode piece is placed facing the positive electrode piece, and
the whole is immersed in a good amount of an electrolyte, thereby
obtaining a cell. This cell is subjected to charge/discharge in a
voltage range defined by an end-of-discharge voltage and an
end-of-charge voltage that are respectively 0.1 V higher than the
end-of-discharge voltage and the end-of-charge voltage defining the
working voltage range of the battery. For example, when the working
voltage range of the desired lithium secondary battery is from 3.0
to 4.2 V (i.e. end-of-discharge voltage: 3.0 V, an end-of-charge
voltage: 4.2 V), the cell is subjected to charge/discharge in a
voltage range from 3.1 to 4.3 V (i.e. end-of-discharge voltage: 3.1
V, end-of-charge voltage: 4.3 V). The theoretical capacity per unit
weight (mAh/g) of the positive electrode active material can be
determined from the discharge capacity obtained at the second
cycle. The theoretical capacity of the positive electrode is the
product of the weight of the active material included in the
positive electrode and the theoretical capacity per unit weight of
the positive electrode active material.
[0080] The theoretical capacity of the negative electrode can be
determined as follows:
[0081] First, a cell is assembled using a part of the fabricated
negative electrode (a negative electrode piece). The weight of the
active material included in the negative electrode piece can be
determined by calculation, for example. After the weight of the
negative electrode active material is determined, a Li foil in an
amount excessive to the theoretical capacity of the negative
electrode piece is placed facing the negative electrode piece, and
the whole is immersed in a good amount of an electrolyte, thereby
obtaining a cell. This cell is subjected to charge/discharge in a
voltage range from 0 to 1.5 V (i.e. end-of-discharge voltage: 0 V,
end-of-charge voltage: 1.5 V), with respect to the electric
potential of Li metal. The theoretical capacity per unit weight
(mAh/g) of the negative electrode active material can be determined
from the charge capacity obtained at the second cycle. The
theoretical capacity of the negative electrode is the product of
the weight of the active material included in the negative
electrode and the theoretical capacity per unit weight of the
negative electrode active material.
[0082] By determining the theoretical capacity from the charge
capacity obtained at the second cycle, the amount of Li captured
into the negative electrode active material that corresponds to the
irreversible capacity can be reflected in the theoretical capacity.
Here, "charge" means a reaction in which lithium is desorbed from
the negative electrode active material. When lithium metal is used
for the negative electrode, the theoretical capacity of the
negative electrode can be directly determined from the weight of
the Li metal used. However, from the viewpoint of optimizing the
battery characteristics, it is preferable that the design capacity
is set to 40% of the weight of the Li metal used. When lithium
metal is used for the negative electrode, no current collector will
be used for the negative electrode, so that it becomes difficult to
maintain the shape of the negative electrode if the battery is
designed such that the whole amount of lithium metal is reacted. In
addition, it is possible to use an active material having an
irreversible capacity in combination with lithium metal, and to
compensate for the irreversible capacity of that active material by
the lithium metal.
[0083] Next, using the positive electrode and the negative
electrode whose theoretical capacities have been clearly
determined, a battery is designed by the following procedure.
[0084] First, from the viewpoint of optimizing the battery life,
the design capacity of the positive electrode is set to 0.97 times
the theoretical capacity of the positive electrode. The design
capacity of the negative electrode is set to 105% of the design
capacity of the positive electrode in the working voltage range of
the desired lithium secondary battery. Next, from the viewpoint of
improving the process yield, the cross-sectional area of the
electrode group is set to (0.95).sup.2 times the cross-sectional
area of the internal space of the battery case housing that
electrode group.
[0085] Here, in the case of a cylindrical battery, the energy
density (Wh/L) of the electrode group can be determined using the
formula:
1000.times..alpha..times..beta./(.gamma..times..pi..times.(0.95.epsilon./-
2).sup.2). However, The theoretical capacity (0.97.times.
theoretical capacity of the positive electrode) of the battery is
taken as .alpha. (Ah), the intermediate voltage (the voltage at
which one half of the amount of the initial battery capacity has
been discharged) during discharge is taken as .beta. (V), the width
of the negative electrode active material layer is taken as .gamma.
(cm), and the inner diameter of the battery case is taken as
.epsilon. (cm). It should be noted that the theoretical capacity of
the battery means the same as the design capacity of the positive
electrode.
[0086] In the case of a square battery, the energy density (Wh/L)
of the electrode group can be determined using the formula:
1000.times..alpha..times..beta./(.gamma..times.0.95.sup.2.times..epsilon.-
). However, the theoretical capacity (0.97.times. theoretical
capacity of the positive electrode) of the battery is taken as
.alpha. (Ah), the intermediate voltage (the voltage at which one
half of the amount of the initial battery capacity has been
discharged) during discharge is taken as .beta. (V), the width of
the negative electrode active material layer is taken as .gamma.
(cm), and the area of the inner bottom surface of the battery case
is taken as .epsilon. (cm.sup.2)
[0087] As the non-aqueous electrolyte and the battery case, it is
possible to use those that have conventionally been used for
lithium secondary batteries. It is also possible to apply publicly
known techniques that have been conventionally used for lithium
secondary batteries to the lithium secondary battery of the present
invention.
[0088] As the non-aqueous electrolyte, it is preferable to use a
non-aqueous solvent in which a lithium salt is dissolved as the
solute.
[0089] As the lithium salt, it is possible to use, for example,
lithium hexafluorophosphate (LiPF.sub.6), lithium perchlorate
(LiClO.sub.4), lithium tetrafluoroborate (LiBF.sub.4),
LiAlCl.sub.4, LiSbF.sub.6, LiSCN, LiCl, LiCF.sub.3SO.sub.3,
LiCF.sub.3CO.sub.2, Li(CF.sub.3SO.sub.2).sub.2, LiAsF.sub.6,
LiN(CF.sub.3SO.sub.2).sub.2, LiB.sub.10Cl.sub.10, lithium lower
aliphatic carboxylate, LiCl, LiBr, LiI, lithium tetrachloroborate,
lithium tetraphenylborate and a lithium imide salt. These may be
used alone or in combination of two or more of them. While there is
no particular limitation with respect to the amount of the lithium
salt dissolved in the non-aqueous solvent, the concentration of the
lithium salt is preferably 0.2 to 2 mol/L, more preferably 0.5 to
1.5 mol/L.
[0090] As the non-aqueous solvent, it is possible to use, for
example, cyclic carbonates such as ethylene carbonate (EC),
propylene carbonate (PC) and butylene carbonate (BC), non-cyclic
carbonates such as dimethyl carbonate (DMC), diethyl carbonate
(DEC), ethylmethyl carbonate (EMC) and dipropyl carbonate (DPC),
aliphatic carboxylic acid esters such as methyl formate, methyl
acetate, methyl propionate and ethyl propionate, lactones such as
y-butyrolactone and y-valerolactone, non-cyclic ethers such as
1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE) and
ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran
and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane,
formamide, acetamide, dimethylformamide, dioxolane, acetonitrile,
propionitrile, nitromethane, ethyl monoglyme, phosphoric acid
triester, trimethoxymethane, a dioxolane derivative, sulfolane,
methylsulfolane, 1,3-dimethyl-2-imidazolidinone,
3-methyl-2-oxazolidinone, a propylene carbonate derivative, a
tetrahydrofuran derivative, ethyl ether, 1,3-propanesultone,
anisole, dimethyl sulfoxide and N-methyl-2-pyrrolidone. Although
these may be used alone, it is preferable to mix two or more of
them. Among these, it is preferable to use a mixed solvent of a
cyclic carbonate and a non-cyclic carbonate, or a mixed solvent of
a cyclic carbonate, a non-cyclic carbonate and an aliphatic
carboxylic acid ester.
[0091] A variety of additives may be added for the purpose of
improving the charge/discharge characteristics of the battery. As
the additive, it is preferable to use, for example, vinylene
carbonate (VC), vinyl ethylene carbonate (VEC), cyclohexylbenzene
(CHB) and fluorobenzene. These additives form a good coating on the
positive electrode and/or negative electrode, thus improving the
stability at the time of overcharge.
[0092] The battery case should be electrochemically stable in the
working voltage range of the lithium secondary battery. For
example, it is preferable to use a battery case made of iron, and
the battery case may also be plated with nickel or tin. After the
electrode group is inserted into the battery case, the non-aqueous
electrolyte is injected into the battery case. Then, the opening of
the battery case is sealed by clamping, with a lid placed at the
opening, thereby a lithium secondary battery is completed.
[0093] Hereinafter, the present invention will be described
specifically by way of examples.
EXAMPLES
Battery A1
[0094] A lithium secondary battery was designed such that its
working voltage range was from 2.5 V to 4.2 V (end-of-discharge
voltage: 2.5 V, end-of-charge voltage: 4.2 V).
(i) Production of Positive Electrode
[0095] A positive electrode material mixture paste was prepared by
stirring, with a double arm kneader, 5 kg of a lithium nickelate
powder (median diameter: 20 .mu.m) serving as a positive electrode
active material, 1 kg of an N-methyl-2-pyrrolidone (NMP) solution
(#1320 (trade name) manufactured by KUREHA CORPORATION) containing
12 wt % of polyvinylidene fluoride (PVDF) serving as a binder, 90 g
of acetylene black serving as a conductive agent and a suitable
amount of NMP serving as a dispersion medium. The positive
electrode material mixture paste was applied onto both sides of a
band-shaped positive electrode current collector comprising an
aluminum foil with a thickness of 15 .mu.m. The applied positive
electrode material mixture paste was dried, and rolled with rollers
to form a positive electrode active material layer. The obtained
electrode plate was cut into a width (57 mm) that could be inserted
into a cylindrical battery case (diameter: 18 mm, height: 65 mm,
inner diameter: 17.85 mm), thereby obtaining a positive
electrode.
(ii) Production of Negative Electrode
[0096] A negative electrode material mixture paste was prepared by
stirring, with a double arm kneader, 3 kg of a powder composed
simply of silicon (Si) (median diameter: 10 .mu.m) serving as a
negative electrode active material, 750 g of an aqueous dispersion
(BM-400B (trade name) manufactured by ZEON Corporation) containing
40 wt % of modified styrene butadiene rubber particles serving as a
binder, 600 g of acetylene black serving as a conductive agent, 300
g of carboxymethyl cellulose (CMC) serving as a thickener and a
suitable amount of water serving as a dispersion medium. The
negative electrode material mixture paste was applied onto both
sides of a band-shaped negative electrode current collector
comprising a copper foil with a thickness of 10 .mu.m. The applied
negative electrode material mixture paste was dried, and rolled
with rollers to form a negative electrode active material layer.
The obtained electrode plate was cut into a width (58.5 mm) that
could be inserted into the battery case, thereby obtaining a
negative electrode. It should be noted that the width of the
negative electrode and that of the negative electrode active
material layer were the same.
(iii) Formation of Porous Heat-Resistant Layer.
[0097] A heat-resistant layer slurry was prepared by stirring, with
a double arm kneader, 970 g of an alumina powder (median diameter:
0.3 .mu.m), 375 g of an NMP solution (BM-720H (trade name)
manufactured by ZEON Corporation) containing 8 wt % of a modified
polyacrylonitrile rubber serving as a binder and a suitable amount
of NMP serving as a dispersion medium. The heat-resistant layer
slurry was applied onto the surface of the positive electrode so as
to cover the positive electrode active material layer. The applied
heat-resistant layer slurry was dried for 10 hours at 120.degree.
C. under vacuum and reduced pressure to form a porous
heat-resistant layer. The thickness of the porous heat-resistant
layer was 5 .mu.m.
(iv) Assembly of Battery
[0098] The positive electrode and the negative electrode having the
porous heat-resistant layer were rolled up with a 14 .mu.m thick
porous resin membrane made of polyethylene (Hipore (trade name)
manufactured by Asahi Kasei Corporation) interposed between them,
thus forming an electrode group. Accordingly, the separator layer
was constituted by the porous heat-resistant layer and the porous
resin membrane, and the total thickness of the porous
heat-resistant layer and the porous resin membrane was 19
.mu.m.
[0099] After the electrode group was inserted into a cylindrical
nickel plated battery case made of iron (diameter: 18 mm, height:
65 mm, inner diameter: 17.85 mm), 5.0 g of a non-aqueous
electrolyte was injected into the battery case, and the opening of
the battery case was sealed with a lid, thereby completing a
lithium secondary battery. A non-aqueous electrolyte in which
LiPF.sub.6 was dissolved in a mixed solvent of ethylene carbonate
(EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) at a
concentration of 1 mol/L was used as the non-aqueous electrolyte.
The volume ratio of EC:DMC:EMC in the mixed solvent was 1:1:1. 3 wt
% of vinylene carbonate (VC) was added to the non-aqueous
electrolyte. The theoretical capacity of the battery was 3606 mAh,
and the energy density of the electrode group was 928 Wh/L.
Battery A2
[0100] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that a SiO powder (median diameter: 8
.mu.m) was used in place of the powder composed simply of silicon,
and that the dimensions of the positive electrode and the negative
electrode were changed as necessary based on the above-described
design standard (i.e. with the working voltage range of the lithium
secondary battery being set to 2.5 V to 4.2 V, and the volume of
the electrode group being set to be the same as that in Battery
A1). The theoretical capacity of the battery was 3203 mAh, and the
energy density of the electrode group was 824 Wh/L.
Battery A3
[0101] A negative electrode material mixture paste was prepared by
stirring, with a double arm kneader, 4 kg of a powder composed
simply of tin (Sn) (median diameter: 10 .mu.m) serving as a
negative electrode active material, 250 g of an aqueous dispersion
(BM-400B (trade name) manufactured by ZEON Corporation) containing
40 wt % of modified styrene butadiene rubber particles serving as a
binder, 200 g of acetylene black serving as a conductive agent, 100
g of carboxymethyl cellulose (CMC) serving as a thickener and a
suitable amount of water serving as a dispersion medium. A lithium
secondary battery was fabricated in the same manner as Battery A1,
except that this negative electrode material mixture paste was
used, and that the dimensions of the positive electrode and the
negative electrode were changed as necessary based on the
above-described design standard. The theoretical capacity of the
battery was 3395 mAh, and the energy density of the electrode group
was 873 Wh/L.
Battery A4
[0102] A lithium secondary battery was fabricated in the same
manner as Battery Al, except that a 140 .mu.m thick lithium metal
foil was used for the negative electrode, and that the dimensions
of the positive electrode and the negative electrode were changed
as necessary based on the above-described design standard. The
theoretical capacity of the battery was 3242 mAh, and the energy
density of the electrode group was 932 Wh/L.
Battery A5
[0103] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that a 5 .mu.m thick vapor-deposited
film of lithium metal was formed on the surface of the negative
electrode, and that the dimensions of the positive electrode and
the negative electrode were changed as necessary based on the
above-described design standard. The theoretical capacity of the
battery was 3529 mAh, and the energy density of the electrode group
was 908 Wh/L.
Battery A6
[0104] A lithium secondary battery was fabricated in the same
manner as Battery A2, except that a 5 .mu.m thick vapor-deposited
film of lithium metal was formed on the surface of the negative
electrode, and that the dimensions of the positive electrode and
the negative electrode were changed as necessary based on the
above-described design standard. The theoretical capacity of the
battery was 3135 mAh, and the energy density of the electrode group
was 807 Wh/L.
Batteries A7 to A10
[0105] Lithium secondary batteries A7, A8, A9 and A10 were
fabricated in the same manner as Batteries A5, A6, A4 and A1,
respectively, except that the porous heat-resistant layer was not
formed on the surface of the positive electrode, but was formed on
the surface of the negative electrode so as to cover the negative
electrode active material layer.
Batteries A11 to A13
[0106] Lithium secondary batteries A11, A12 and A13 were fabricated
in the same manner as Battery A1, except that the alumina in the
porous heat-resistant layer was changed to magnesia, silica and
zirconia having substantially the same particle size distribution
as the alumina in Batteries A11, A12 and A13, respectively.
Battery A14
[0107] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the separator
layer was changed to 22 .mu.m by changing the thicknesses of the
porous heat-resistant layer and the porous resin membrane to 2
.mu.m and 20 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3510 mAh, and the energy
density of the electrode group was 903 Wh/L.
Battery A15
[0108] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the separator
layer was changed to 20 .mu.m by changing the thicknesses of the
porous heat-resistant layer and the porous resin membrane to 2
.mu.m and 18 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3587 mAh, and the energy
density of the electrode group was 923 Wh/L.
Battery A16
[0109] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the separator
layer was changed to 16 .mu.m by changing the thicknesses of the
porous heat-resistant layer and the porous resin membrane to 2
.mu.m and 14 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3702 mAh, and the energy
density of the electrode group was 952 Wh/L.
Battery B1
[0110] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the separator
layer was changed to 10 .mu.m by changing the thicknesses of the
porous heat-resistant layer and the porous resin membrane to 2
.mu.m and 8 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3913 mAh and the energy
density of the electrode group was 1007 Wh/L.
Battery B2
[0111] A lithium secondary battery was fabricated in the same
manner Battery B1, except that the thickness of the separator layer
was changed to 8 .mu.m by changing the thicknesses of the porous
heat-resistant layer and the porous resin membrane to 2 .mu.m and 6
.mu.m, respectively, and that the dimensions of the positive
electrode and the negative electrode were changed as necessary
based on the above-described design standard. The theoretical
capacity of the battery was 3970 mAh, and the energy density of the
electrode group was 1021 Wh/L.
Battery A17
[0112] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the separator
layer was changed to 24 .mu.m by changing the thickness of the
porous heat-resistant layer to 10 .mu.m, and that the dimensions of
the positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3453 mAh, and the energy
density of the electrode group was 888 Wh/L.
Battery A18
[0113] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the separator
layer was changed to 15 .mu.m by changing the thickness of the
porous heat-resistant layer to 1 .mu.m, and that the dimensions of
the positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3740 mAh, and the energy
density of the electrode group was 962 Wh/L.
Battery B3
[0114] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the separator
layer was changed to 14.5 .mu.m by changing the thickness of the
porous heat-resistant layer to 0.5 .mu.m, and that the dimensions
of the positive electrode and the negative electrode were changed
as necessary based on the above-described design standard. The
theoretical capacity of the battery was 3759 mAh, and the energy
density of the electrode group was 967 Wh/L.
Battery A19
[0115] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the porous
heat-resistant layer was changed to 1 .mu.m, the thickness of the
separator layer was changed to 12.5 .mu.m, and that the dimensions
of the positive electrode and the negative electrode were changed
as necessary based on the above-described design standard. The
theoretical capacity of the battery was 3836 mAh, and the energy
density of the electrode group was 987 Wh/L.
Battery B4
[0116] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the porous
heat-resistant layer was changed to 10 .mu.m, the thickness of the
separator layer was changed to 26 .mu.m, and that the dimensions of
the positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3357 mAh, and the energy
density of the electrode group was 864 Wh/L.
Battery B5
[0117] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the porous
heat-resistant layer was changed to 12 .mu.m, the thickness of the
separator layer was changed to 26 .mu.m, and that the dimensions of
the positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3357 mAh, and the energy
density of the electrode group was 864 Wh/L.
Battery A20
[0118] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the porous heat-resistant layer
was not formed on the surface of the positive electrode, but was
formed on the surface (only one side) of the porous resin membrane,
thus disposing the porous heat-resistant layer on the positive
electrode side.
Battery A21
[0119] A lithium secondary battery was fabricated in the same
manner as Battery A20, except that the porous heat-resistant layer
was formed on the surface of the porous resin membrane by the
following procedure.
[0120] First, 65 g of dry anhydrous calcium chloride was added to 1
kg of NMP, and the mixture was heated to 80.degree. C. in a
reaction vessel to completely dissolve the anhydrous calcium
chloride. After the obtained NMP solution of calcium chloride was
cooled down to ordinary temperature, 32 g of paraphenylenediamine
was added to the solution, and completely dissolved. Thereafter,
the reaction vessel was placed in a constant temperature bath at
20.degree. C., and 58 g of dichloroterephthalate was added dropwise
to the NMP solution over one hour. Then, the NMP solution was stood
still for one hour in the constant temperature bath at 20.degree.
C. to allow a polymerization reaction to proceed, thus synthesizing
polyparaphenylene terephthalamide (hereinafter, abbreviated as
"PPTA").
[0121] After completion of the reaction, the NMP solution
(polymerized solution) was moved from the constant temperature bath
into a vacuum vessel, and degassed under stirring for 30 minutes
under reduced pressure. Further, the obtained polymerized solution
was diluted with an NMP solution of calcium chloride to prepare an
NMP solution of aramid resin with a PPTA concentration of 1.4 wt
%.
[0122] The obtained NMP solution of aramid resin was applied onto
one side of the porous resin membrane with a doctor blade, and
dried with hot air having 80.degree. C. (air velocity: 0.5 m/sec).
Thereafter, the aramid resin film was sufficiently washed with pure
water to remove the calcium chloride, while forming micro pores in
the film, and the film was dried. Thus, a porous heat-resistant
layer with a thickness of 5 .mu.m was formed on one side of the
porous resin membrane. In addition, the aramid resin was removed
from the NMP solution, and its heat deformation temperature
(deflection temperature under load) measured in accordance with the
ASTM was 321.degree. C.
Battery A22
[0123] A lithium secondary battery was fabricated in the same
manner as Battery A20, except that the porous heat-resistant layer
was formed on the surface of the porous resin membrane by the
following procedure.
[0124] First, 21 g of trimellitic acid anhydride monochloride and
20 g of diamine (diaminodiphenyl ether) were added to 1 kg of NMP,
and the whole was mixed at room temperature to prepare an NMP
solution of polyamic acid (polyamic acid concentration: 3.9 wt %).
The obtained NMP solution of polyamic acid was applied onto one
side of the porous resin membrane with a doctor blade. Thereafter,
the coated film was dried with hot air having 80.degree. C. (air
velocity: 0.5 m/sec), while causing cyclodehydration of the
polyamic acid to form polyamide imide. In addition, the heat
deformation temperature (deflection temperature under load) of the
polyamide imide measured in accordance with the ASTM was
280.degree. C.
Battery A23 to A25
[0125] Lithium secondary batteries A23, A24, and A25 were
fabricated in the same manner as Batteries A20, A21, and A22
respectively, except that the porous heat-resistant layer carried
on the porous resin membrane was disposed on the negative electrode
side.
Battery B6
[0126] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 12.5 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 12
.mu.m and 0.5 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3836 mAh, and the energy
density of the electrode group was 987 Wh/L.
Battery A26
[0127] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 13 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 12
.mu.m and 1 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3817 mAh, and the energy
density of the electrode group was 982 Wh/L.
Battery A27
[0128] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 14 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 12
.mu.m and 2 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3778 mAh, and the energy
density of the electrode group was 972 Wh/L.
Battery A28
[0129] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 17 .mu.m by changing the thickness of the
porous resin membrane to 12 .mu.m, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3683 mAh, and the energy
density of the electrode group was 948 Wh/L.
[0130] Battery A29 A lithium secondary battery was fabricated in
the same manner as Battery A21, except that the thickness of the
separator layer was changed to 22 .mu.m by changing the thicknesses
of the porous resin membrane and the porous heat-resistant layer to
12 .mu.m and 10 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3510 mAh, and the energy
density of the electrode group was 903 Wh/L.
Battery A30
[0131] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 24 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 14
.mu.m and 10 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3453 mAh, and the energy
density of the electrode group was 888 Wh/L.
Battery A31
[0132] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 12.5 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 11.5
.mu.m and 1 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3836 mAh, and the energy
density of the electrode group was 987 Wh/L.
Battery A32
[0133] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 13.5 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 12
.mu.m and 1.5 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3817 mAh, and the energy
density of the electrode group was 982 Wh/L.
Battery B7
[0134] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 26 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 16
.mu.m and 10 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3357 mAh, and the energy
density of the electrode group was 864 Wh/L.
Battery B8
[0135] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 24 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 12
.mu.m and 12 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3453 mAh, and the energy
density of the electrode group was 888 Wh/L.
Battery B9
[0136] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the thickness of the separator
layer was changed to 12 .mu.m by changing the thicknesses of the
porous resin membrane and the porous heat-resistant layer to 11
.mu.m and 1 .mu.m, respectively, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3836 mAh, and the energy
density of the electrode group was 984 Wh/L.
Battery A33
[0137] The heat-resistant layer slurry was applied on to a smooth
SUS plate, and the coated film was dried for 10 hours at
120.degree. C. under vacuum and reduced pressure to form a porous
heat-resistant layer with a thickness of 10 .mu.m. This layer was
removed from the SUS plate to give an independent sheet constituted
by the porous heat-resistant layer. A lithium secondary battery was
fabricated in the same manner as Battery A1, except that the
obtained sheet was disposed on the positive electrode side of the
porous resin membrane, instead of forming the porous heat-resistant
layer on the surface of the positive electrode, that the thickness
of the separator layer was changed to 20 .mu.m by changing the
thickness of the porous resin membrane to 10 .mu.m, and that the
dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. In addition, the dimension of the porous heat-resistant
layer was made larger than that of the negative electrode. The
theoretical capacity of the battery was 3587 mAh, and the energy
density of the electrode group was 923 Wh/L.
Battery A34
[0138] The NMP solution of aramid resin with a PPTA concentration
of 1.4 wt % was applied onto a smooth SUS plate, and the coated
film was dried with hot air having 80.degree. C. (air velocity: 0.5
m/sec). Thereafter, the aramid resin film was sufficiently washed
with pure water, and dried again to form a 10 .mu.m thick porous
heat-resistant layer comprising the aramid resin. This layer was
removed from the SUS plate to give an independent sheet constituted
by the porous heat-resistant layer. A lithium secondary battery was
fabricated in the same manner as Battery A33, except that the thus
obtained sheet was used.
Battery A35
[0139] The NMP solution of polyamic acid was applied onto a smooth
SUS plate, and the coated film was dried with hot air having
80.degree. C. (air velocity: 0.5 m/sec), while causing
cyclodehydration of the polyamic acid to produce polyamide imide,
thus forming a 10 .mu.m thick porous heat-resistant layer
comprising the polyamide imide. This layer was removed from the SUS
plate to give an independent sheet constituted by the porous
heat-resistant layer. A lithium secondary battery was fabricated in
the same manner as Battery A33, except that the thus obtained sheet
was used.
Battery B10
[0140] A lithium secondary battery was fabricated in the same
manner as Battery A33, except that the porous resin membrane was
not used, the thickness of the sheet constituted by the porous
heat-resistant layer was changed to 14 .mu.m, and that the
dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. The theoretical capacity of the battery was 3768 mAh, and
the energy density of the electrode group was 972 Wh/L.
Battery B11
[0141] A lithium secondary battery was fabricated in the same
manner as Battery A33, except that the porous resin membrane was
not used, the thickness of the sheet constituted by the porous
heat-resistant layer was changed to 20 .mu.m, and that the
dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. The theoretical capacity of the battery was 3587 mAh, and
the energy density of the electrode group was 923 Wh/L.
Battery B12
[0142] A lithium secondary battery was fabricated in the same
manner as Battery A33, except that the porous resin membrane was
not used, the thickness of the sheet constituted by the porous
heat-resistant layer was changed to 22 .mu.m, and that the
dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. The theoretical capacity of the battery was 3510 mAh, and
the energy density of the electrode group was 903 Wh/L.
Battery B13
[0143] A lithium secondary battery was fabricated in the same
manner as Battery A34, except that the porous resin membrane was
not used, that the thickness of the sheet constituted by the porous
heat-resistant layer was changed to 14 .mu.m, and that the
dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard.
Battery B14
[0144] A lithium secondary battery was fabricated in the same
manner as Battery A35, except that the porous resin membrane was
not used, that the thickness of the sheet constituted by the porous
heat-resistant layer was changed to 14 .mu.m, and that the
dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard.
Battery B15
[0145] A lithium secondary battery was fabricated in the same
manner as Battery B10, except that a 16 .mu.m thick nonwoven fabric
made of cellulose was used in place of the sheet constituted by the
porous heat-resistant layer, and that the dimensions of the
positive electrode and the negative electrode were changed as
necessary based on the above-described design standard. The
theoretical capacity of the battery was 3702 mAh, and the energy
density of the electrode group was 952 Wh/L.
Battery A36
[0146] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the positive electrode was formed
by the following procedure, and that the dimensions of the positive
electrode and the negative electrode were changed as necessary
based on the above-described design standard. The theoretical
capacity of the battery was 3164 mAh, and the energy density of the
electrode group was 838 Wh/L.
[0147] A positive electrode material mixture paste was prepared by
stirring, with a double arm kneader, 3 kg of a lithium cobaltate
powder (median diameter: 15 .mu.m) serving as a positive electrode
active material, 1 kg of an N-methyl-2-pyrrolidone (NMP) solution
(#1320 (trade name) manufactured by KUREHA CORPORATION) containing
12 wt % of polyvinylidene fluoride (PVDF) serving as a binder, 90 g
of acetylene black serving as a conductive agent and a suitable
amount of NMP serving as a dispersion medium. The positive
electrode material mixture paste was applied onto both sides of a
band-shaped positive electrode current collector comprising an
aluminum foil with a thickness of 15 .mu.m. The applied positive
electrode material mixture paste was dried, and rolled with rollers
to form a positive electrode active material layer. The obtained
electrode plate was cut into a width (57 mm) that could be inserted
into a cylindrical battery case (diameter: 18 mm, height: 65 mm,
inner diameter: 17.85 mm), thereby obtaining a positive
electrode.
Battery A37
[0148] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the positive electrode was formed
by the following procedure, and that the dimensions of the positive
electrode and the negative electrode were changed as necessary
based on the above-described design standard. The theoretical
capacity of the battery was 3129 mAh, and the energy density of the
electrode group was 817 Wh/L.
[0149] A positive electrode material mixture paste was prepared by
stirring, with a double arm kneader, 3 kg of a lithium nickel
manganese cobalt oxide (LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2)
powder (median diameter: 15 .mu.m) serving as a positive electrode
active material, 1 kg of an N-methyl-2-pyrrolidone (NMP) solution
(#1320 (trade name) manufactured by KUREHA CORPORATION) containing
12 wt % of polyvinylidene fluoride (PVDF) serving as a binder, 90 g
of acetylene black serving as a conductive agent and a suitable
amount of NMP serving as a dispersion medium. The positive
electrode material mixture paste was applied onto both sides of a
band-shaped positive electrode current collector comprising an
aluminum foil with a thickness of 15 .mu.m. The applied positive
electrode material mixture paste was dried, and rolled with rollers
to form a positive electrode active material layer. The obtained
electrode plate was cut into a width (57 mm) that could be inserted
into a cylindrical battery case (diameter: 18 mm, height: 65 mm,
inner diameter: 17.85 mm), thereby obtaining a positive
electrode.
Battery A38
[0150] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the positive electrode was formed
by the following procedure, and that the dimensions of the positive
electrode and the negative electrode were changed as necessary
based on the above-described design standard. The theoretical
capacity of the battery was 3537 mAh, and the energy density of the
electrode group was 923 Wh/L.
[0151] A positive electrode material mixture paste was prepared by
stirring, with a double arm kneader, 1.5 kg of a lithium cobaltate
powder (median diameter: 15 .mu.m) serving as a first positive
electrode active material, 1.5 kg of a lithium nickel manganese
cobalt oxide (LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2) powder
(median diameter: 15 .mu.m) serving as a second positive electrode
active material, 1 kg of an N-methyl-2-pyrrolidone (NMP) solution
(#1320 (trade name) manufactured by KUREHA CORPORATION) containing
12 wt % of polyvinylidene fluoride (PVDF) serving as a binder, 90 g
of acetylene black serving as a conductive agent and a suitable
amount of NMP serving as a dispersion medium. The positive
electrode material mixture paste was applied onto both sides of a
band-shaped positive electrode current collector comprising an
aluminum foil with a thickness of 15 .mu.m. The applied positive
electrode material mixture paste was dried, and rolled with rollers
to form a positive electrode active material layer. The obtained
electrode plate was cut into a width (57 mm) that could be inserted
into a cylindrical battery case (diameter: 18 mm, height: 65 mm,
inner diameter: 17.85 mm), thereby obtaining a positive
electrode.
Battery A39
[0152] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the negative electrode was formed
by the following procedure, and that the dimensions of the positive
electrode and the negative electrode were changed as necessary
based on the above-described design standard. The theoretical
capacity of the battery was 2633 mAh, and the energy density of the
electrode group was 717 Wh/L.
[0153] A negative electrode material mixture paste was prepared by
stirring, with a double arm kneader, 3 kg of an artificial graphite
powder (median diameter: 20 .mu.m) serving as a negative electrode
active material, 75 g of an aqueous dispersion (BM-400B (trade
name) manufactured by ZEON Corporation) containing 40 wt % of
modified styrene butadiene rubber particles serving as a binder, 30
g of carboxymethyl cellulose (CMC) serving as a thickener and a
suitable amount of water serving as a dispersion medium. The
negative electrode material mixture paste was applied onto both
sides of a band-shaped negative electrode current collector
comprising a copper foil with a thickness of 10 .mu.m. The applied
negative electrode material mixture paste was dried, and rolled
with rollers to form a negative electrode active material layer.
The obtained electrode plate was cut into a width (58.5 mm) that
could be inserted into the battery case, thereby obtaining a
negative electrode.
Battery A40
[0154] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the battery design was changed
such that the working voltage range of the lithium secondary
battery was from 2.5 V to 4.4 V (end-of-discharge voltage: 2.5 V,
end-of-charge voltage: 4.4 V), that the positive electrode was
produced in the same manner as Battery A36, that the negative
electrode was produced in the same manner as Battery A39, and that
the dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. The theoretical capacity of the battery was 2514 mAh, and
the energy density of the electrode group was 742 Wh/L.
Battery A41
[0155] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the battery design was changed
such that the working voltage range of the lithium secondary
battery was from 2.5 V to 4.4 V (end-of-discharge voltage: 2.5 V,
end-of-charge voltage: 4.4 V), that the positive electrode was
produced in the same manner as Battery A38, that the negative
electrode was produced in the same manner as Battery A39, and that
the dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. The theoretical capacity of the battery was 2601 mAh, and
the energy density of the electrode group was 728 Wh/L.
Battery A42
[0156] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the battery design was changed
such that the working voltage range of the lithium secondary
battery was from 2.5 V to 4.4 V (end-of-discharge voltage: 2.5 V,
end-of-charge voltage: 4.4 V), that the positive electrode was
produced in the same manner as Battery A36, that the negative
electrode was produced in the same manner as Battery A39, and that
the dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. The theoretical capacity of the battery was 2514 mAh, and
the energy density of the electrode group was 742 Wh/L.
Battery A43
[0157] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that the battery design was changed
such that the working voltage range of the lithium secondary
battery was from 2.5 V to 4.4 V (end-of-discharge voltage: 2.5 V,
end-of-charge voltage: 4.4 V), that the positive electrode was
produced in the same manner as Battery A38, that the negative
electrode was produced in the same manner as Battery A39, and that
the dimensions of the positive electrode and the negative electrode
were changed as necessary based on the above-described design
standard. The theoretical capacity of the battery was 2601 mAh, and
the energy density of the electrode group was 728 Wh/L.
Battery B16
[0158] A lithium secondary battery was fabricated in the same
manner as Battery A1, except that the thickness of the porous resin
membrane was changed to 20 .mu.m, that the porous heat-resistant
layer was not provided, and that the dimensions of the positive
electrode and the negative electrode were changed as necessary
based on the above-described design standard. The theoretical
capacity of the battery was 3587 mAh, and the energy density of the
electrode group was 923 Wh/L.
Battery B17
[0159] A lithium secondary battery was fabricated in the same
manner as Battery A40, except that the battery design was changed
such that the working voltage range of the lithium secondary
battery was from 2.5 V to 4.2 V (end-of-discharge voltage: 2.5 V,
end-of-charge voltage: 4. 2 V). The theoretical capacity of the
battery was 2314 mAh, and the energy density of the electrode group
was 648 Wh/L.
Battery A44
[0160] A lithium secondary battery was fabricated in the same
manner as Battery A21, except that 200 parts by weight of an
alumina powder was added to the NMP solution of aramid resin of
Battery A21, per 100 parts by weight of the aramid resin. As the
alumina powder, the one used for the porous heat-resistant layer of
Battery Al was used.
Battery A45
[0161] A lithium secondary battery of Battery A49 was fabricated in
the same manner as Battery A44, except that the porous
heat-resistant layer carried on the porous resin membrane was
disposed on the negative electrode side.
[0162] 50 pieces of batteries each were fabricated for all the
examples, and they were evaluated as follows. Tables 1-1, 1-2, 2-1,
2-2, 3-1, 3-2 show the constitution and the evaluation results for
the batteries.
(Insulation Failure Test)
[0163] For each of the electrode groups to which the non-aqueous
electrolyte had not been added, the direct current resistance was
measured by application of a voltage of 25 V. The batteries
including the electrode groups with a measured value of not more
than 1 M.OMEGA. were regarded as batteries having an internal short
circuit, and the rates of occurrence of such batteries are shown as
"insulation failure rate" in Tables 3-1 and 3-2.
(Voltage Failure Test)
[0164] The batteries having no internal short circuit were
subjected to break-in charge/discharge twice, and subsequently
subjected to charge with a current value of 400 mA until their
voltages reached to 4.1 V. Thereafter, the batteries were stored
for seven days under an environment with 45.degree. C. The rates of
occurrence of batteries whose open circuit voltage after storage
decreased by 50 mV or more as compared with that before storage are
shown as "voltage failure rate" in Tables 3-1 and 3-2.
(High Output Characteristics)
[0165] The discharge capacity during high output discharge was
determined by performing charge/discharge under an environment with
20.degree. C. under the following conditions.
Constant Current Charge
[0166] hourly rate: 0.7 C, end-of-charge voltage: design end
voltage (4.2 V for Batteries A1 to A39, A44 and A45 and Batteries
B1 to B17, 4.4 V for Batteries A40 to A43)
Constant Voltage Charge
[0167] charge voltage value: design end voltage, end-of-charge
current: 100 mA
Constant Current Discharge
[0168] hourly rate: 0.2 C, end-of-discharge voltage: design end
voltage (2.5 V)
Constant Current Charge
[0169] hourly rate: 1 C, end-of-charge voltage: design end
voltage
Constant Voltage Charge
[0170] charge voltage value: design end voltage, end-of-charge
current: 100 mA
Constant Current Discharge
[0171] hourly rate: 2 C, end-of-discharge voltage: design end
voltage
[0172] Here, the hourly rate X (C) indicates that the theoretical
capacity of the battery is discharged over 1/X hour, and a larger
value of X indicates a larger current value. The ratios of the
capacity during 2 C discharge to the capacity during 0.2 C
discharge are shown as "high output characteristics" in Tables 3-1
and 3-2.
(Storage Characteristics)
[0173] Each battery was subjected to charge/discharge under an
environment with 20.degree. C. under the following conditions, and
then stored for 20 days under an environment with 60.degree. C.
Thereafter, each battery was subjected to charge/discharge under
the following conditions. The ratios of the capacity during 1 C
discharge after storage to the capacity during 1 C discharge before
storage are shown as "storage characteristics" in Tables 3-1 and
3-2.
Constant Current Charge
[0174] hourly rate: 0.7 C, end-of-charge voltage: design end
voltage+0.1 V
Constant Voltage Charge
[0175] charge voltage value: design end voltage+0.1 V,
end-of-charge current: 100 mA
Constant Current Discharge
[0176] hourly rate: 1 C, end-of-discharge voltage: design end
voltage
Constant Current Charge
[0177] hourly rate: 0.7 C, end-of-charge voltage: design end
voltage+0.1 V
Constant Voltage Charge
[0178] charge voltage value: design end voltage+0.1 V,
end-of-charge current: 100 mA
High Temperature Storage
[0179] 6.degree. C., 20 days
Constant Current Discharge
[0180] hourly rate: 0.2 C, end-of-discharge voltage: design end
voltage
Constant Current Charge
[0181] hourly rate: 0.7 C, end-of-charge voltage: design end
voltage+0.1V
Constant Voltage Charge
[0182] charge voltage value: design end voltage+0.1 V,
end-of-charge current: 100 mA
Constant Current Discharge
[0183] hourly rate: 1 C, end-of-discharge voltage: design end
voltage
(Nail Penetration Test)
[0184] Each battery was subjected to charge at 0.7 C until the
voltage reached a voltage that was 0.1 V higher than the design end
voltage. Thereafter, an iron nail (having a diameter of 2.5 mm of a
cross section perpendicular to its length direction) was allowed to
penetrate into each of the charged batteries from its side at a
speed of 5 mm/sec under an environment with 20.degree. C., and the
heat generation state of the batteries was observed. The
temperature of each battery was measured with a thermocouple
attached on the side of the battery. The temperature that was
reached after 90 seconds was shown in Tables 3-1 and 3-2.
[0185] Further, each battery was disassembled after its temperature
had dropped, and the short circuit area A (i.e. the cross-sectional
area perpendicular to the length direction of the nail) and the
reduced area B of the porous heat-resistant layer or the separator
layer that resulted from an internal short circuit were measured.
The maximum values of (A+B)/A were shown in Tables 2-1 and 2-2.
TABLE-US-00001 TABLE 1-1 Positive Negative Porous Porous Heat
Separator Electrode Electrode Resin Resistant Layer Layer Active
Active Membrane Thickness Thickness Material Material (.mu.m)
Filler (.mu.m) (.mu.m) Battery A1 LiNiO.sub.2 Si 14 Alumina 5 19
Battery A2 LiNiO.sub.2 SiO 14 Alumina 5 19 Battery A3 LiNiO.sub.2
Sn 14 Alumina 5 19 Battery A4 LiNiO.sub.2 Li 14 Alumina 5 19
Battery A5 LiNiO.sub.2 Si + Li 14 Alumina 5 19 Battery A6
LiNiO.sub.2 SiO + Li 14 Alumina 5 19 Battery A7 LiNiO.sub.2 Si + Li
14 Alumina 5 19 Battery A8 LiNiO.sub.2 SiO + Li 14 Alumina 5 19
Battery A9 LiNiO.sub.2 Li 14 Alumina 5 19 Battery A10 LiNiO.sub.2
Si 14 Alumina 5 19 Battery A11 LiNiO.sub.2 Si 14 Magnesia 5 19
Battery A12 LiNiO.sub.2 Si 14 Silica 5 19 Battery A13 LiNiO.sub.2
Si 14 Zirconia 5 19 Battery A14 LiNiO.sub.2 Si 20 Alumina 2 22
Battery A15 LiNiO.sub.2 Si 18 Alumina 2 20 Battery A16 LiNiO.sub.2
Si 14 Alumina 2 16 Battery B1 LiNiO.sub.2 Si 8 Alumina 2 10 Battery
B2 LiNiO.sub.2 Si 6 Alumina 2 8 Battery A17 LiNiO.sub.2 Si 14
Alumina 10 24 Battery A18 LiNiO.sub.2 Si 14 Alumina 1 15 Battery B3
LiNiO.sub.2 Si 14 Alumina 0.5 14.5 Battery A19 LiNiO.sub.2 Si 11.5
Alumina 1 12.5 Battery B4 LiNiO.sub.2 Si 16 Alumina 10 26 Battery
B5 LiNiO.sub.2 Si 14 Alumina 12 26 Battery A20 LiNiO.sub.2 Si 14
Alumina 5 19 Battery A21 LiNiO.sub.2 Si 14 Aramid 5 19 Battery A22
LiNiO.sub.2 Si 14 Polyamide 5 19 imide Battery A23 LiNiO.sub.2 Si
14 Alumina 5 19 Battery A24 LiNiO.sub.2 Si 14 Aramid 5 19 Battery
A25 LiNiO.sub.2 Si 14 Polyamide 5 19 imide
[0186] TABLE-US-00002 TABLE 1-2 Positive Negative Porous Porous
Heat Separator Electrode Electrode Resin Resistant Layer Layer
Active Active Membrane Thickness Thickness Material Material
(.mu.m) Filler (.mu.m) (.mu.m) Battery B6 LiNiO.sub.2 Si 12 Aramid
0.5 12.5 Battery A26 LiNiO.sub.2 Si 12 Aramid 1 13 Battery A27
LiNiO.sub.2 Si 12 Aramid 2 14 Battery A28 LiNiO.sub.2 Si 12 Aramid
5 17 Battery A29 LiNiO.sub.2 Si 12 Aramid 10 22 Battery A30
LiNiO.sub.2 Si 14 Aramid 10 24 Battery A31 LiNiO.sub.2 Si 11.5
Aramid 1 12.5 Battery A32 LiNiO.sub.2 Si 12 Aramid 1.5 13.5 Battery
B7 LiNiO.sub.2 Si 16 Aramid 10 26 Battery B8 LiNiO.sub.2 Si 12
Aramid 12 24 Battery B9 LiNiO.sub.2 Si 11 Aramid 1 12 Battery A33
LiNiO.sub.2 Si 10 Alumina 10 20 Battery A34 LiNiO.sub.2 Si 10
Aramid 10 20 Battery A35 LiNiO.sub.2 Si 10 Polyamide 10 20 imide
Battery B10 LiNiO.sub.2 Si -- Alumina 14 14 Battery B11 LiNiO.sub.2
Si -- Alumina 20 20 Battery B12 LiNiO.sub.2 Si -- Alumina 22 22
Battery B13 LiNiO.sub.2 Si -- Aramid 14 14 Battery B14 LiNiO.sub.2
Si -- Polyamide 14 14 imide Battery B15 LiNiO.sub.2 Si -- Cellulose
16 16 Battery A36 LiCoO2 Si 14 Alumina 5 19 Battery A37
LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2 Si 14 Alumina 5 19 Battery
A38 LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2 + LiCoO.sub.2 Si 14
Alumina 5 19 Battery A39 LiNiO.sub.2 Graphite 14 Alumina 5 19
Battery A40 LiCoO.sub.2 Graphite 14 Alumina 5 19 Battery A41
LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2 + LiCoO.sub.2 Graphite 14
Alumina 5 19 Battery A42 LiCoO.sub.2 Graphite 14 Aramid 5 19
Battery A43 LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2 + LiCoO.sub.2
Graphite 14 Aramid 5 19 Battery A44 LiNiO.sub.2 Si 14 Alumina +
Aramid 5 19 Battery A45 LiNiO.sub.2 Si 14 Alumina + Aramid 5 19
Battery B16 LiNiO.sub.2 Si 20 -- -- 20 Battery B17 LiCoO.sub.2
Graphite 14 Alumina 5 19
[0187] TABLE-US-00003 TABLE 2-1 Short Position of End Energy
Circuit Decreased Porous Heat Voltage Density Area Area Resistant
Layer (V) (Wh/L) A(mm.sup.2) B(mm.sup.2) (A + B)/A Battery A1
Positive Electrode 4.2 928 5.0 26 5.2 Battery A2 Positive Electrode
4.2 824 5.0 24 4.8 Battery A3 Positive Electrode 4.2 873 5.0 27 5.4
Battery A4 Positive Electrode 4.2 932 5.0 28 5.6 Battery A5
Positive Electrode 4.2 908 5.0 26 5.2 Battery A6 Positive Electrode
4.2 807 5.0 24 4.8 Battery A7 Negative Electrode 4.2 908 5.0 45 9.0
Battery A8 Negative Electrode 4.2 807 5.0 42 8.4 Battery A9
Negative Electrode 4.2 932 5.0 40 8.0 Battery A10 Negative
Electrode 4.2 928 5.0 42 8.4 Battery A11 Positive Electrode 4.2 928
5.0 27 5.4 Battery A12 Positive Electrode 4.2 928 5.0 29 5.8
Battery A13 Positive Electrode 4.2 928 5.0 25 5.0 Battery A14
Positive Electrode 4.2 903 5.0 34 6.8 Battery A15 Positive
Electrode 4.2 923 5.0 35 7.0 Battery A16 Positive Electrode 4.2 952
5.0 37 7.4 Battery B1 Positive Electrode 4.2 1007 5.0 38 7.6
Battery B2 Positive Electrode 4.2 1021 5.0 40 8.0 Battery A17
Positive Electrode 4.2 888 5.0 21 4.2 Battery A18 Positive
Electrode 4.2 962 5.0 33 6.6 Battery B3 Positive Electrode 4.2 967
5.0 47 9.4 Battery A19 Positive Electrode 4.2 987 5.0 38 7.6
Battery B4 Positive Electrode 4.2 864 5.0 20 4.0 Battery B5
Positive Electrode 4.2 864 5.0 19 3.8 Battery A20 Porous Resin 4.2
928 5.0 28 5.6 Membrane/Positive Electrode Side Battery A21 Porous
Resin 4.2 928 5.0 26 5.2 Membrane/Positive Electrode Side Battery
A22 Porous Resin 4.2 928 5.0 29 5.8 Membrane/Positive Electrode
Side Battery A23 Porous Resin 4.2 928 5.0 31 6.2 Membrane/Negative
Electrode Side Battery A24 Porous Resin 4.2 928 5.0 33 6.6
Membrane/Negative Electrode Side Battery A25 Porous Resin 4.2 928
5.0 29 5.8 Membrane/Negative Electrode Side
[0188] TABLE-US-00004 TABLE 2-2 Position of End Energy Short
Circuit Decreased Porous Heat Voltage Density Area Area Resistant
Layer (V) (Wh/L) A(mm.sup.2) B(mm.sup.2) (A + B)/A Battery B6
Porous Resin 4.2 987 5.0 48 9.6 Membrane/ Positive Electrode Side
Battery Porous Resin 4.2 982 5.0 43 8.6 A26 Membrane/ Positive
Electrode Side Battery Porous Resin 4.2 972 5.0 37 7.4 A27
Membrane/ Positive Electrode Side Battery Porous Resin 4.2 948 5.0
30 6.0 A28 Membrane/ Positive Electrode Side Battery Porous Resin
4.2 903 5.0 26 5.2 A29 Membrane/ Positive Electrode Side Battery
Porous Resin 4.2 888 5.0 24 4.8 A30 Membrane/ Negative Electrode
Side Battery Porous Resin 4.2 987 5.0 43 8.6 A31 Membrane/ Positive
Electrode Side Battery Porous Resin 4.2 982 5.0 38 7.6 A32
Membrane/ Positive Electrode Side Battery B7 Porous Resin 4.2 864
5.0 23 4.6 Membrane/ Positive Electrode Side Battery B8 Porous
Resin 4.2 888 5.0 22 4.4 Membrane/ Positive Electrode Side Battery
B9 Porous Resin 4.2 984 5.0 45 9.0 Membrane/ Positive Electrode
Side Battery Independent 4.2 923 5.0 14 2.8 A33 Battery Independent
4.2 923 5.0 16 3.2 A34 Battery Independent 4.2 923 5.0 10 2.0 A35
Battery Independent 4.2 972 5.0 12 2.4 B10 Battery Independent 4.2
923 5.0 10 2.0 B11 Battery Independent 4.2 903 5.0 8 1.6 B12
Battery Independent 4.2 972 5.0 11 2.2 B13 Battery Independent 4.2
972 5.0 9 1.8 B14 Battery Independent 4.2 952 5.0 38 7.6 B15
Battery Positive Electrode 4.2 838 5.0 24 4.8 A36 Battery Positive
Electrode 4.2 817 5.0 24 4.8 A37 Battery Positive Electrode 4.2 923
5.0 30 6.0 A38 Battery Positive Electrode 4.2 717 5.0 28 5.6 A39
Battery Positive Electrode 4.4 742 5.0 29 5.8 A40 Battery Positive
Electrode 4.4 728 5.0 31 6.2 A41 Battery Positive Electrode 4.4 742
5.0 30 6.0 A42 Battery Positive Electrode 4.4 728 5.0 28 5.6 A43
Battery Porous Resin 4.2 928 5.0 27 5.4 A44 Membrane/ Positive
Electrode Side Battery Porous Resin 4.2 928 5.0 32 6.4 A45
Membrane/ Negative Electrode Side Battery -- 4.2 923 5.0 197 39.4
B16 Battery Positive Electrode 4.2 648 5.0 22 4.4 B17
[0189] TABLE-US-00005 TABLE 3-1 Battery Temperature after 90 sec.
of Nail High Output Storage Insulation Voltage Penetration
Characteristics Characteristics Failure Failure (.degree. C.) (%)
(%) Rate(%) Rate(%) Battery A1 92 90.2 92.1 0 2 Battery A2 88 91.3
90.5 0 2 Battery A3 84 90.5 91.3 0 2 Battery A4 86 92.2 89.7 0 4
Battery A5 84 91.8 88.6 0 0 Battery A6 88 90.3 90 0 2 Battery A7
112 90.5 84.6 0 16 Battery A8 118 91.2 84.7 0 18 Battery A9 115
89.8 83.6 0 20 Battery A10 121 90.1 85.1 0 8 Battery A11 90 90.1
90.5 0 2 Battery A12 92 90.7 91.8 0 0 Battery A13 93 90.5 90.9 0 2
Battery A14 87 82.2 90.5 0 0 Battery A15 90 87.2 91.2 0 2 Battery
A16 92 90.5 91.4 2 0 Battery B1 99 92.3 92.2 8 10 Battery B2 101
95.2 92 18 20 Battery A17 78 79.7 87.2 0 0 Battery A18 95 92.8 88.5
0 2 Battery B3 131 93.3 85.2 0 6 Battery A19 96 91.5 91.9 4 2
Battery B4 77 74.3 87.5 0 0 Battery B5 73 74 87.1 0 0 Battery A20
98 90.8 90.1 0 0 Battery A21 89 91.5 89.8 0 2 Battery A22 90 90.7
90 0 4 Battery A23 99 91.2 80.5 0 0 Battery A24 87 90.1 82.2 0 6
Battery A25 88 90.3 83.4 0 6
[0190] TABLE-US-00006 TABLE 3-2 Battery Temperature after 90 sec.
of Nail High Output Storage Insulation Voltage Penetration
Characteristics Characteristics Failure Failure (.degree. C.) (%)
(%) Rate(%) Rate(%) Battery B6 128 91.5 90.2 0 4 Battery A26 100
90.1 91.3 0 6 Battery A27 90 88.6 90.2 0 6 Battery A28 82 86.5 89.6
0 4 Battery A29 78 80.3 88.5 0 4 Battery A30 77 79.2 88.4 0 4
Battery A31 104 91.6 90.6 2 4 Battery A32 95 89.4 90.5 0 6 Battery
B7 78 74.1 88.5 0 4 Battery B8 78 71.2 88.2 0 4 Battery B9 100 92.1
90.8 8 12 Battery A33 80 88.6 90.1 0 12 Battery A34 82 88.5 87.5 0
10 Battery A35 79 88.8 86.9 0 14 Battery B10 95 92.1 90.5 6 12
Battery B11 82 87.4 92.3 6 8 Battery B12 74 79.2 91.4 4 10 Battery
B13 89 91.9 90.6 4 8 Battery B14 91 92.4 91.2 4 10 Battery B15 98
90.8 90.6 8 2 Battery A36 86 89.2 90.1 0 2 Battery A37 91 90.5 89.8
0 0 Battery A38 90 90.5 90.9 0 0 Battery A39 95 90.2 90.5 0 0
Battery A40 95 92.1 85.5 0 0 Battery A41 98 91.1 94.9 0 2 Battery
A42 98 89.9 87.1 0 0 Battery A43 93 90.8 92.3 0 0 Battery A44 91
92.8 89.6 0 0 Battery A45 90 92.6 82.7 0 2 Battery B16 145 91.3
83.5 0 0 Battery B17 84 94.5 90.5 0 2
[0191] Both lithium cobaltate (the positive electrode active
material) and graphite (the negative electrode active material)
that were used in Battery B17 have a low theoretical capacity.
Accordingly, although batteries with well-balanced characteristics
and high reliability were obtained, it was not possible to achieve
the desired high energy density (700 Wh/L) when the end-of-charge
voltage was 4.2 V. In order to achieve an electrode group having an
energy density of not less than 700 Wh/L, it is necessary to use a
negative electrode active material having a higher energy density
than graphite, as in Batteries A1 to A38, A44 and A45, or to use a
positive electrode active material having a higher energy density
than lithium cobaltate, as in Battery A39, or to set the
end-of-charge voltage to a voltage higher than 4.2 V, as in
Batteries A40 to A43.
[0192] In the case of Battery B16, in which the separator layer did
not include the porous heat-resistant layer, the separator layer
had a large reduced area when an internal short circuit was caused
in the nail penetration test, and a large amount of heat therefore
was generated in the battery. It seems that the heat generated as a
result of an internal short circuit caused the porous resin
membrane having a low melting point to melt, thus expanding the
short circuit portion, increasing the amount of the short circuit
current and thus promoting the heat generation. On the other hand,
in the examples of the present invention, in which the separator
layer included the porous heat-resistant layer, it was possible to
suppress the reduced area B of the porous heat-resistant layer to a
small extent when an internal short circuit was caused in the nail
penetration test. Thus, it was possible to suppress the heat
generation in the batteries. As can be suggested from Batteries A1
to A6, the effect of suppressing heat generation could be achieved
regardless of the type of the negative electrode active
material.
[0193] In the cases of Batteries A7 to A10, in which the porous
heat-resistant layer was provided on the surface of the negative
electrode, a large amount of heat was generated in the batteries in
the nail penetration test, and the voltage failure rate was
somewhat higher. Additionally, the storage characteristics were
lower. In the cases of Batteries A1 to A6, in which the porous
heat-resistant layer was provided on the surface of the positive
electrode, and Batteries A20 to A21, in which the porous
heat-resistant layer was formed on the porous resin membrane, more
favorable results were obtained. A negative electrode active
material having a high energy density tends to undergo a
significant volume change, or to experience a change of state.
Therefore, it seems that, when the porous heat-resistant layer is
formed on the surface of the negative electrode, the porous
heat-resistant layer, which is structurally fragile, is partly
damaged. In the cases of Batteries A7 to A9, in which the negative
electrode included lithium metal, the voltage failure rate was
particularly high. The reason seems to be that the electric
potential of the negative electrode was lowered by the presence of
the lithium metal, and the conductive foreign matter dissolved in
the positive electrode therefore was more easily deposited at the
negative electrode. On the other hand, in the cases of Batteries A4
to A6, in which the porous heat-resistant layer was provided on the
surface of the positive electrode, the voltage failure rate was
low, because the conductive foreign matter was difficult to be
dissolved in the positive electrode.
[0194] The porous heat-resistant layers of Batteries A1 to A20,
A23, A33, A36 to A41 comprised an insulating filler and a binder,
and therefore had a relatively high mechanical strength and high
durability. In contrast, the porous heat-resistant layer of
Batteries A21, A24, A26 to A32, A34, A42 and A43 comprised aramid
resin, and thus were relatively inferior in the mechanical
strength. This was also the same for the porous heat-resistant
layers of Batteries A22, A25, A35 and B14 that comprised
polyamidoimide resin. Accordingly, the batteries including the
porous heat-resistant layer comprising a heat-resistant resin had a
relatively high voltage failure rate. However, the porous
heat-resistant layers comprising a heat-resistant resin had high
flexibility, so that their electrode groups were easy to form,
leading to an improved productivity.
[0195] Further, as can be seen from a comparison of Batteries A20
to A25, in which the porous heat-resistant layer was provided on
the porous resin membrane, the use of the heat-resistant resin
improved the heat resistance of the batteries and thus provided a
higher safety against nail penetration than that provided by the
use of the insulating filler. This seems to be due to the fact that
the heat-resistant resin, which has higher flexibility than the
insulating filler, can more easily accommodate itself to the
expansion and contraction of the electrode plates during
charge/discharge. In particular, in the cases of Batteries A44 and
A45, in which the insulating filler and the heat-resistant resin
were used in combination, there were observed improved high output
characteristics, in addition to the merit of the insulating filler
(a lower voltage failure rate) and that of the heat-resistant resin
(an improved safety against nail penetration). It seems that the
high output characteristics were improved because the void
structure within the porous heat-resistant layer was improved by
some action.
[0196] As described above, the porous heat-resistant layer has
somewhat low mechanical strength. Therefore, when the porous
heat-resistant layer was provided on the surface of the positive
electrode or the porous resin membrane, as in Batteries A23 to A29,
the voltage failure rate was lower than when the independent sheet
constituted by the porous heat-resistant layer was used as in
Batteries A33 to A35.
[0197] When the porous heat-resistant layer was provided on the
surface of the porous resin membrane, the storage characteristics
tended to decrease if the porous heat-resistant layer was disposed
on the negative electrode side, as in Batteries A23 to A25. Better
storage characteristics were achieved when the porous
heat-resistant layer was disposed on the positive electrode side,
as in Batteries A20 to A22 and A26 to A29. It seems that disposing
the porous heat-resistant layer on the positive electrode side
enables suppression of oxidation of the porous resin membrane
comprising a polyolefin, thus preventing a reduction in electric
characteristics, even when the battery was stored in a state of
high temperature and high voltage.
[0198] Favorable results were obtained when the separator layer had
a thickness in the range of 12.5 to 24 .mu.m. In the case of
Batteries B1, B2, and B9, in which the separator layer had a
thickness of less than 12.5 .mu.m, the insulation failure rate was
high. In the cases of Batteries B4, B5, and B7, in which the
separator layer had a thickness exceeding 24 .mu.m, the energy
density of the electrode group was decreased even when using the
same negative electrode active material. In addition, the high
output characteristics tended to decrease.
[0199] Regardless of the presence or absence of the porous resin
membrane, favorable results were observed when the porous
heat-resistant layer had a thickness in the range of 1 to 10 .mu.m.
When the thickness of the porous heat-resistant layer exceeded 10
.mu.m in Batteries B5 and B8, high output characteristics declined.
When the thickness of the porous case when the thickness of the
porous heat-resistant layer comprising the insulating filler and
the binder exceeds 10 .mu.m. This is probably because the porosity
of the porous heat-resistant layer comprising the heat-resistant
resin is comparatively low. Therefore, the thickness of the porous
heat-resistant layer comprising the heat-resistant resin has to be
controlled with a particular precision. When the thickness of the
porous heat-resistant layer was less than 1 .mu.m, (A+B)/A value
became not less than 9, and the heat generation during the nail
penetration test became relatively significant.
[0200] Favorable results were obtained when the porous resin
membrane had a thickness in the range of 8 to 18 .mu.m.
[0201] The lithium secondary battery of the present invention has a
high energy density and a high level of safety, so that it is
highly applicable to the power sources for portable devices such as
personal digital assistants (PDAs) and mobile electronic devices.
However, the lithium secondary battery of the present invention can
also be used for, for example, compact home electrical energy
storage devices, and the power sources for motorcycles, electric
cars and hybrid electric cars, and there is no particular
limitation with respect to its use. While there is no particular
limitation with respect to the shape of the lithium ion secondary
battery of the present invention, a cylindrical shape and a square
shape are preferable, for example. The lithium secondary battery of
the present invention has high output characteristics, so that it
is highly applicable to the power sources for PDAs, electric
operated tools, personal computers (PCs), electric operated toys or
electric operated robots, and large scale back-up power sources,
uninterruptible power supplies (UPS), load leveling power source
system utilizing natural energy or regenerative energy utilization
system.
[0202] Although the present invention has been described in terms
of the presently preferred embodiments, it is to be understood that
such disclosure is not to be interpreted as limiting. Various
alterations and modifications will no doubt become apparent to
those skilled in the art to which the present invention pertains,
after having read the above disclosure. Accordingly, it is intended
that the appended claims be interpreted as covering all alterations
and modifications as fall within the true spirit and scope of the
invention.
* * * * *