U.S. patent application number 17/588956 was filed with the patent office on 2022-05-19 for lithium ion battery separator and lithium ion battery.
This patent application is currently assigned to MITSUBISHI PAPER MILLS LIMITED. The applicant listed for this patent is MITSUBISHI PAPER MILLS LIMITED. Invention is credited to Toshihiro SHIGEMATSU.
Application Number | 20220158299 17/588956 |
Document ID | / |
Family ID | |
Filed Date | 2022-05-19 |
United States Patent
Application |
20220158299 |
Kind Code |
A1 |
SHIGEMATSU; Toshihiro |
May 19, 2022 |
LITHIUM ION BATTERY SEPARATOR AND LITHIUM ION BATTERY
Abstract
It is an object of the present invention to provide a lithium
ion battery separator which can be made thin, is excellent in
tensile strength and cuttability and has low internal resistance, a
low incidence of internal short circuit failure and high cycle
characteristics and a lithium ion battery including the separator.
In a lithium ion battery separator containing heat-resistant fibers
and synthetic resin short fibers, fibrillated heat-resistant fibers
having a modified freeness of not more than 300 ml are contained as
the heat-resistant fibers, and the content of the fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml is not less than 1.0 mass % to less than 5.0 mass % based on
the total of all the fiber components contained in the separator.
Modified freeness: This is a value measured in accordance with JIS
P8121-2:2012 except that an 80-mesh wire net having a wire diameter
of 0.14 mm and an opening of 0.18 mm is used as a screening plate
and the concentration of a sample is 0.1%.
Inventors: |
SHIGEMATSU; Toshihiro;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MITSUBISHI PAPER MILLS LIMITED |
Tokyo |
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JP |
|
|
Assignee: |
MITSUBISHI PAPER MILLS
LIMITED
Tokyo
JP
|
Appl. No.: |
17/588956 |
Filed: |
January 31, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16470674 |
Jun 18, 2019 |
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PCT/JP2017/045327 |
Dec 18, 2017 |
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17588956 |
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International
Class: |
H01M 50/44 20060101
H01M050/44; H01M 10/0525 20060101 H01M010/0525; H01M 50/411
20060101 H01M050/411 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2016 |
JP |
2016-252401 |
Nov 21, 2017 |
JP |
2017-223561 |
Nov 21, 2017 |
JP |
2017-223562 |
Nov 24, 2017 |
JP |
2017-225923 |
Claims
1. A lithium ion battery separator comprising heat-resistant fibers
and synthetic resin short fibers, wherein fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml are contained as the heat-resistant fibers, and the content
of the fibrillated heat-resistant fibers having a modified freeness
of not more than 300 ml is not less than 1.0 mass % to less than
5.0 mass % based on the total of all the fiber components contained
in the separator. wherein modified freeness is a value measured in
accordance with JIS P8121-2:2012 except that an 80-mesh wire net
having a wire diameter of 0.14 mm and an opening of 0.18 mm is used
as a screening plate and the concentration of a sample is 0.1%.
2. The lithium ion battery separator according to claim 1, wherein
the content of the synthetic resin short fibers is 90.0 to 99.0
mass % based on the total of all the fiber components contained in
the separator.
3. The lithium ion battery separator according to claim 1, wherein
the content of the synthetic resin short fibers is more than 95.0
to not more than 99.0 mass % based on the total of all the fiber
components contained in the separator.
4. A lithium ion battery comprising the lithium ion battery
separator of claim 1.
Description
CROSS-REFERENCE APPLICATIONS
[0001] This application is continuation application of U.S.
application Ser. No. 16/470,674 filed Jun. 18, 2019, which is a
national stage application filed under 35 U.S.C. .sctn. 371 of
international application no. PCT/JP2017/045327 filed Dec. 18,
2017, and claims foreign priority to Japanese patent application
no. 2017-225923 filed Nov. 24, 2017, Japanese patent application
no. 2017-223561 filed Nov. 21, 2017, Japanese patent application
no. 2017-223562 filed Nov. 21, 2017, and Japanese patent
application no. 2016-252401 filed Dec. 27, 2016.
TECHNICAL FIELD
[0002] The present invention relates to a lithium ion battery
separator and a lithium ion battery. The term "lithium ion battery
separator" may be simply referred to as "separator". The term
"lithium ion battery" may be simply referred to as "battery".
BACKGROUND ART
[0003] Due to the spread and improved performance of mobile
electronic equipment in recent years, a secondary battery having
high energy density is desired. As this type of battery, a lithium
ion battery containing an organic electrolytic solution has been
attracting attention. Since this lithium ion battery can achieve an
average voltage of about 3.7 V which is about three times as large
as a conventional alkaline secondary battery, it has high energy
density. However, since an aqueous electrolytic solution cannot be
used in this battery unlike the alkaline secondary battery, an
organic electrolytic solution having sufficiently high oxidation
and reduction resistance is used. As the organic electrolytic
solution is combustible, there is the risk of firing and therefore,
close attention is paid to safety at the time of its use. Out of
plural causes of exposure to a firing risk, overcharging is
especially dangerous.
[0004] To prevent overcharging, the current lithium ion battery
performs constant-voltage/constant-current charging and is equipped
with a precision IC (protection circuit). The cost of this
protection circuit is high, which is one of the causes of raising
the cost of the lithium ion battery.
[0005] When overcharging is to be prevented with the protection
circuit, it is naturally possible that the protection circuit does
not operate properly, and it is hard to say that the lithium ion
battery is essentially safe. The current lithium ion battery is
equipped with means such as a safety valve, PTC element or
separator having a heat fuse function in order to destroy the
battery safely when the protection circuit is destroyed at the time
of overcharging and overcharging is performed. However, even when
the lithium ion battery is equipped with the above means, according
to overcharging conditions, safety at the time of overcharging is
not surely ensured and the firing accident of a lithium ion battery
actually occurs even nowadays.
[0006] As the lithium ion battery separator, a porous film made of
a polyolefin such as polyethylene or polypropylene is often used.
The porous film made of a polyolefin has a heat fuse function
(shut-down function) that it melts and closes pores to prevent the
movement of an lithium ion and shut down a current when the inside
temperature of a battery rises up to 130.degree. C. However, when
the temperature further rises in some situation, there is suggested
the possibility that the polyolefin itself melts to cause a short
circuit, resulting in a thermal runaway. Then, a heat-resistant
separator which does not melt and shrink at a temperature close to
200.degree. C. is now under development.
[0007] As the heat-resistant separator, there is disclosed a
separator composed of a polyester nonwoven fabric (refer to Patent
Document 1). However, the separator composed of a nonwoven fabric
has problems that pores in the nonwoven fabric are large, a leakage
current is large, electrolytic solution holding performance is low,
and the internal resistance of the separator is high.
[0008] As a separator having high safety at the time of
overcharging, there is disclosed a lithium ion secondary battery
separator composed of a porous sheet made of a nonwoven fabric or a
porous film incorporating this porous sheet and containing a porous
organic polymer which swells with an electrolytic solution to hold
it (refer to Patent Document 2). However, since the porous sheet
made of a nonwoven fabric of Patent Document 2 has large voids and
a large number of through holes, an internal short circuit or
self-discharge tends to occur. In fact, the separator is caused to
function by forming a porous film including a porous film
containing a porous polymer which swells with the electrolytic
solution to hold it, or polyvinylidene fluoride in examples.
Therefore, in this case, the separator has problems that the film
thickness is large at 24 .mu.m or more, it is difficult to increase
the capacity, and it is necessary to solidify, rinse and dry the
porous polymer for the formation of the porous film, thereby
boosting cost.
[0009] As a separator which is capable of preventing a short
circuit at a high temperature, there are disclosed a lithium ion
secondary battery separator which is composed of a nonwoven fabric
containing heat-resistant pulp fibers having a melting point or
carbonization point of 300.degree. C. or higher and thermoplastic
fibers having a melting point of 200.degree. C. or higher (refer to
Patent Document 3), a separator for electrochemical elements
containing at least one type of organic fibers at least some of
which are fibrillated to a fiber diameter of not more than 1 .mu.m
and at least one type of nonfibrillated organic fibers having a
fineness of not more than 0.5 dtex as a separator which is
excellent in heat resistance, electrolytic solution holding
property, internal short circuit prevention property and
windability and can reduce internal resistance and may have long
service life (refer to Patent Document 4), and a separator for
electrochemical elements which is composed of a nonwoven fabric
containing a fibrillated polymer having a melting point or thermal
decomposition temperature of 250.degree. C. or higher at least part
of which has a fiber diameter of not more than 1 .mu.m and a weight
average fiber length of 0.2 to 2 mm and organic fibers having a
fineness of not more than 3.3 dtex as a separator having excellent
reflow heat resistance, low internal resistance and excellent
high-speed charging and discharging properties (refer to Patent
Document 5).
[0010] Out of these Patent Documents 3 to 5, the separator of
Patent Document 3 has a content of the heat-resistant pulp fibers
of preferably 30 to 80 mass % and the separators of examples have a
basis weight of 20 g/m.sup.2 and a large thickness of 26 .mu.m, the
separator of Patent Document 4 has a content of the fibrillated
organic fibers or liquid crystal polymer fibers of preferably 10 to
70 mass % and the separators of examples have a basis weight of not
less than 16 g/m.sup.2 and a large thickness of not less than 30
.mu.m, and the separator of Patent Document 5 has a content of the
fibrillated polymer of preferably not less than 10 mass % and the
separators of examples have a basis weight of not less than 18
g/m.sup.2 and a large thickness of not less than 55 .mu.m. It is
difficult for all of the above separators to meet the requirement
for high capacity in recent years.
[0011] In the separators disclosed by Patent Documents 3 to 5, the
lower limit of the preferred content of the heat-resistant fibers
such as the heat-resistant pulp fibers, fibrillated organic fibers
and fibrillated polymer is 10 mass %. However, since the
heat-resistant fibers are mostly rigid and have high strength, when
the content thereof is not less than 10 mass %, the separator is
hardly smashed and it is difficult to reduce the thickness of the
separator. When the separator is smashed by applying heat or a
load, the resistance of the separator becomes high, when the
separator is cut to a predetermined width to be used as a
separator, it is difficult to cut the separator in the slitting
step, and it is also difficult to fusion-cut the separator in the
step of laminating an electrode. Further, as bonding force between
heat-resistant fibers and bonding force between a heat-resistant
fiber and another fiber are low, when the content of the
heat-resistant fibers is not less than 10 mass %, the tensile
strength of the separator becomes low, whereby special care must be
paid in the operation of the electrode lamination step.
PRIOR ART DOCUMENTS
Patent Documents
[0012] Patent Document 1: JP-A 2003-123728 [0013] Patent Document
2: JP-A 2007-317675 [0014] Patent Document 3: JP-A 2006-19191
[0015] Patent Document 4: WO01/93350 [0016] Patent Document 5: JP-A
2004-146137
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0017] It is an object of the present invention to provide a
lithium ion battery separator which can be made thin, is excellent
in tensile strength and cuttability and has low internal
resistance, a low incidence of internal short circuit failure and
high cycle characteristics as well as a lithium ion battery
including the separator.
Means for Solving the Problem
[0018] The inventors of the present invention conducted intensive
studies to solve the above problems and made the following
invention.
[0019] (1-1) A lithium ion battery separator comprising
heat-resistant fibers and synthetic resin short fibers, wherein
fibrillated heat-resistant fibers having a modified freeness of not
more than 300 ml are contained as the heat-resistant fibers, and
the content of the fibrillated heat-resistant fibers having a
modified freeness of not more than 300 ml is not less than 1.0 mass
% to less than 5.0 mass % based on the total of all the fiber
components contained in the separator.
[0020] (1-2) The lithium ion battery separator in the above
paragraph (1-1), wherein the content of the synthetic resin short
fibers is 90.0 to 99.0 mass % based on the total of all the fiber
components contained in the separator.
[0021] (1-3) The lithium ion battery separator in the above
paragraph (1-1), wherein the content of the synthetic resin short
fibers is more than 95.0 mass % to not more than 99.0 mass % based
on the total of all the fiber components contained in the
separator.
[0022] (2-1) A lithium ion battery separator comprising
heat-resistant fibers and synthetic resin short fibers, wherein
fibrillated heat-resistant fibers having a modified freeness of not
more than 300 ml and fibrillated heat-resistant fibers having a
modified freeness of more than 300 ml are contained as the
heat-resistant fibers, and the total content of the fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml and the fibrillated heat-resistant fibers having a modified
freeness of more than 300 ml is not less than 2.0 mass % to less
than 9.0 mass % based on the total of all the fiber components
contained in the separator.
[0023] (2-2) The lithium ion battery separator in the above
paragraph (2-1), wherein the content of the fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml is not less than 1.0 mass % to less than 5.0 mass % and the
content of the fibrillated heat-resistant fibers having a modified
freeness of more than 300 ml is not less than 1.0 mass % to less
than 4.0 mass % based on the total of all the fiber components
contained in the separator.
[0024] (2-3) The lithium ion battery separator in the above
paragraph (2-1) or (2-2), wherein the content of the synthetic
resin short fibers is 90.0 to 98.0 mass % based on the total of all
the fiber components contained in the separator.
[0025] (3-1) A lithium ion battery separator comprising
heat-resistant fibers and synthetic resin short fibers, wherein
heat-resistant fibers having a modified freeness of not more than
300 ml and a fibrid made of para-aromatic polyamide having a
modified freeness of not more than 300 ml are contained as the
heat-resistant fibers, and the total content of the heat-resistant
fibers is not less than 1.0 mass % to less than 5.0 mass % based on
the total of all the fiber components contained in the
separator.
[0026] (3-2) The lithium ion battery separator in the above
paragraph (3-1), wherein the content of the synthetic resin short
fibers is 90.0 to 99.0 mass % based on the total of all the fiber
components contained in the separator.
[0027] (3-3) The lithium ion battery separator in the above
paragraph (3-1), wherein the content of the synthetic resin short
fibers is more than 95.0 mass % to not more than 99.0 mass % based
on the total of all the fiber components contained in the
separator.
[0028] (4-1) A lithium ion battery separator comprising
heat-resistant fibers and synthetic resin short fibers, wherein a
fibrid made of para-aromatic polyamide, fibrillated heat-resistant
fibers having a modified freeness of not more than 300 ml and
fibrillated heat-resistant fibers having a modified freeness of
more than 300 ml are contained as the heat-resistant fibers, and
the total content of the heat-resistant fibers is not less than 2.0
mass % to less than 9.0 mass % based on the total of all the fiber
components contained in the separator.
[0029] (4-2) The lithium ion battery separator in the above
paragraph (4-1), wherein the content of the fibrid made of
para-aromatic polyamide is not less than 0.4 mass % to less than
2.5 mass %, the content of the fibrillated heat-resistant fibers
having a modified freeness of more than 300 ml is not less than 1.0
mass % to less than 4.0 mass %, and the content of the fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml is not less than 0.6 mass % to less than 4.6 mass % based on
the total of all the fiber components contained in the
separator.
[0030] (4-3) The lithium ion battery separator in the above
paragraph (4-1) or (4-2), wherein the content of the synthetic
resin short fibers is 90.0 to 98.0 mass % based on the total of all
the fiber components contained in the separator.
[0031] (5) A lithium ion battery including the lithium ion battery
separator in any one of (1-1) to (1-3), (2-1) to (2-3), (3-1) to
(3-3) and (4-1) to (4-3).
[0032] The term "modified freeness" is "a value measured in
accordance with JIS P8121-2:2012 except that an 80-mesh wire net
having a wire diameter of 0.14 mm and an opening of 0.18 mm is used
as a screening plate and the concentration of a sample is
0.1%".
Effect of the Invention
[0033] The lithium ion battery separator of the present invention
can be made thin and is excellent in tensile strength and
cuttability. The lithium ion battery separator of the present
invention can attain effects such as low internal resistance, a low
incidence of internal short circuit failure and high cycle
characteristics.
PRACTICAL EMBODIMENT OF THE INVENTION
[0034] The lithium ion battery in the present invention refers to a
secondary battery in which a lithium ion moves between a negative
electrode and a positive electrode upon discharge and charge.
Examples of the lithium ion battery include a lithium ion secondary
battery and a lithium ion polymer secondary battery. The lithium
ion battery includes a lithium ion secondary battery comprising a
lithium storage material as a negative electrode active material
and a metal lithium secondary battery comprising metal lithium as a
negative electrode active material.
[0035] The lithium ion battery includes a positive electrode, a
separator and a negative electrode as members. In general, it has a
structure that a positive electrode, a separator and a negative
electrode are laminated in the mentioned order. An electrolytic
solution is absorbed into the positive electrode, the negative
electrode and the separator. The types of the laminate structure
include a cylindrical type in which all the members are laminated
together and then rolled, a roll flat type in which the cylindrical
type is squashed to form two flat surfaces and curved both end
parts, a ninety-nine-fold type in which leaf electrodes are
inserted into a separator which has been folded 99 times, and a
leaf laminate type in which a leaf separator and leaf electrodes
are laminated together.
[0036] A lithium storage material is used as the negative electrode
active material of the lithium ion battery. Examples of the lithium
storage material include carbon-based materials, silicon-based
materials and composite oxides of a transition metal and lithium.
The carbon-based materials are preferably used as they are
excellent in balance between the amount of lithium able to be
stored per mass and the difficulty of deterioration caused by the
absorption and release of lithium. The carbon-based materials
include graphite's such as natural graphite and artificial
graphite; amorphous carbons such as hard carbon, soft carbon and
mesoporous carbon; and nanocarbon materials such as carbon nanotube
and graphene. The silicon-based materials are preferably used as
the amount of lithium able to be stored per mass is large. The
silicon-based materials include silicon, silicon monoxide (SiO) and
silicon dioxide (SiO.sub.2). Lithium titanate which is a kind of
composite oxide of a transition metal and lithium is preferably
used as deterioration caused by the absorption and release of
lithium hardly occurs.
[0037] As the negative electrode active material used together with
the lithium ion battery separator in the present invention, lithium
titanateis particularly preferably used. Examples of lithium
titanate include spinel type lithium titanate
(Li.sub.4+xTi.sub.5O.sub.12 ("x" changes between 0 and 3 according
to a charge or discharge state)) and ramsdellite type lithium
titanate (Li.sub.2+xTi.sub.3O.sub.7 ("x" changes between 0 and 2
according to a charge or discharge state)). When lithium titanate
is used as the negative electrode active material, a single lithium
titanate may be used or a mixture of two or more lithium titanates
may be used.
[0038] An example of the negative electrode of the lithium ion
battery is an electrode produced by applying a negative electrode
material containing the above negative electrode active material to
a metal foil. The negative electrode material may be mixed with a
binder such as polyvinylidene fluoride or styrene-butadiene
copolymer; conductive agent such as carbon black or nanocarbon
material; dispersant; and thickener as required. Examples of the
metal used in the metal foil include copper and aluminum.
[0039] Examples of the positive electrode active material of the
lithium ion battery include a composite oxide of a transition metal
and lithium, a composite salt having an olivine structure of a
transition metal and lithium, and sulfur. An example of the
composite oxide of a transition metal and lithium is a composite
oxide of lithium and at least one transition metal selected from
cobalt, nickel and manganese. A typical metal such as aluminum or
magnesium and a transition metal such as titanium or chromium may
be further combined with the composite oxide. An example of the
composite salt having an olivine structure of a transition metal
and lithium is a composite salt having an olivine structure of
lithium and at least one transition metal selected from iron and
manganese.
[0040] An example of the positive electrode of the lithium ion
battery is an electrode produced by applying a positive electrode
material containing the above positive electrode active material to
a metal foil. The positive electrode material may be mixed with a
binder such as polyvinylidene fluoride or acrylic acid ester
copolymer; conductive agent such as carbon black or nanocarbon
material; dispersant; and thickener as required. An example of the
metal used in the metal foil is aluminum.
[0041] Examples of the electrolytic solution of the lithium ion
battery include a solution prepared by dissolving a lithium salt in
a polar solvent and a solution prepared by dissolving a lithium
salt in an ion liquid. Examples of the polar solvent used in the
electrolytic solution of the lithium secondary battery include
carbonates such as propylene carbonate (PC), ethylene carbonate
(EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC); and
fatty acid esters such as ethyl acetate, propyl acetate and ethyl
propionate. Examples of the lithium salt used in the electrolytic
solution of the lithium secondary battery include lithium
hexafluorophosphate (LiPF.sub.6) and lithium tetrafluoroborate
(LiBF.sub.4). As a solid electrolyte, a solution prepared by
dissolving a lithium salt in polyethylene glycol or derivative
thereof, polymethacrylic acid derivative, polysiloxane or
derivative thereof, or gel-like polymer such as polyvinylidene
fluoride is used.
[0042] The lithium ion battery separator of the present invention
is characterized in that it contains heat-resistant fibers and
synthetic resin short fibers and fibrillated heat-resistant fibers
having a modified freeness of not more than 300 ml are contained as
the heat-resistant fibers and an essential component.
[0043] In the present invention, the following brief expressions
may be used for three components below.
Fibrid made of para-aromatic polyamide: fibrid Fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml: fibrillated heat-resistant fibers (I) Fibrillated
heat-resistant fibers having a modified freeness of more than 300
ml: fibrillated heat-resistant fibers (II)
[0044] The separator (1) of the present invention is characterized
in that it contains heat-resistant fibers and synthetic resin
fibers, the fibrillated heat-resistant fibers (I) are contained as
the heat-resistant fibers, and the content of the fibrillated
heat-resistant fibers (I) is not less than 1.0 mass % to less than
5.0 mass % based on the total of all the fiber components contained
in the separator.
[0045] The content of the fibrillated heat-resistant fibers (I) is
not less than 1.0 mass % to less than 5.0 mass % based on the total
of all the fiber components contained in the separator (1). The
content is preferably not less than 2.0 mass %, more preferably not
less than 3.0 mass %. It is preferably less than 4.5 mass %. When
the content of the fibrillated heat-resistant fibers (I) is not
less than 5.0 mass %, it is difficult to smash the separator to
reduce the thickness of the separator. When the thickness is
reduced by applying heat or a pressure, internal resistance becomes
worse and high-rate discharge characteristics deteriorate.
Meanwhile, when the content of the fibrillated heat-resistant
fibers (I) is less than 1.0 mass %, though the tensile strength of
the separator becomes high, a leakage current or the incidence of
internal short circuit failure may become worse.
[0046] The content of the synthetic resin short fibers is
preferably not less than 90.0 mass %, more preferably not less than
92.0 mass %, much more preferably not less than 94.0 mass %,
particularly preferably more than 95.0 mass % based on the total of
all the fiber components contained in the separator (1). The
content is preferably not more than 99.0 mass %, more preferably
not more than 98.0 mass %, much more preferably not more than 96.0
mass %. When the content of the synthetic resin short fibers is
more than 99.0 mass %, though the tensile strength of the separator
becomes high, a leakage current or the incidence of internal short
circuit failure may become worse. When the content of the synthetic
resin short fibers is less than 90.0 mass % and the basis weight of
the separator is reduced, mechanical strength becomes low, whereby
the separator may be broken at the time of forming a battery.
[0047] The most preferred separator (1) contains fibrillated
heat-resistant fibers and synthetic resin short fibers as the fiber
components. In this case, the content of the synthetic resin short
fibers is more than 95.0 mass % to not more than 99.0 mass % based
on the total of all the fiber components contained in the separator
(1). The content is preferably not more than 98.0 mass %, more
preferably not more than 97.0 mass %. It is preferably more than
95.5 mass %.
[0048] The separator (2) of the present invention is characterized
in that it contains heat-resistant fibers and synthetic resin short
fibers, the fibrillated heat-resistant fibers (I) and the
fibrillated heat-resistant fibers (II) are contained as the
heat-resistant fibers, and the total content of the fibrillated
heat-resistant fibers (I) and the fibrillated heat-resistant fibers
(II) is not less than 2.0 mass % to less than 9.0 mass % based on
the total of all the fiber components contained in the
separator.
[0049] The total content of the fibrillated heat-resistant fibers
(I) and the fibrillated heat-resistant fibers (II) is not less than
2.0 mass % to less than 9.0 mass %, preferably not less than 3.0
mass % to less than 8.0 mass %, more preferably not less than 3.5
mass % to less than 6.0 mass %, particularly preferably not less
than 3.5 mass % to less than 5.0 mass % based on the total of all
the fiber components contained in the separator (2). When the total
content of the fibrillated heat-resistant fibers (I) and (II) is
not less than 9.0 mass %, it is difficult to reduce the thickness
of the separator. Further, tensile strength and internal resistance
deteriorate. When the total content of the fibrillated
heat-resistant fibers (I) and (II) is less than 2.0 mass %, though
the tensile strength of the separator becomes high, the effect of
improving a leakage current becomes small.
[0050] The content of the fibrillated heat-resistant fibers (I) is
preferably not less than 1.0 mass % to less than 5.0 mass %, and
the content of the fibrillated heat-resistant fibers (II) is
preferably not less than 1.0 mass % to less than 4.0 mass % based
on the total of all the fiber components contained in the separator
(2).
[0051] The content of the fibrillated heat-resistant fibers (I) is
preferably not less than 1.0 mass % to less than 5.0 mass %, more
preferably not less than 2.0 mass % to less than 5.0 mass %, much
more preferably not less than 2.5 mass % to less than 4.0 mass %,
particularly preferably not less than 2.5 mass % to less than 3.5
mass % based on the total of all the fiber components contained in
the separator (2). Within this range, the number of thick stem
fibers before the fibrillation of the heat-resistant fibers is
small and the fibrillated fibers are fine, whereby there is no
hindrance to the control of the thickness of the separator and the
cuttability of the separator, and the pore diameter becomes small.
Therefore, the retainability of the electrolytic solution becomes
high, thereby producing the effect of improving the resistance of
the separator. When the content is not less than 5.0 mass %,
tensile strength may lower and the resistance of the separator may
become worse. When the content is less than 1.0 mass %, the effect
obtained by blending the fibrillated heat-resistant fibers may be
rarely observed.
[0052] The content of the fibrillated heat-resistant fibers (II) is
preferably not less than 1.0 mass % to less than 4.0 mass %, more
preferably 1.0 to 3.0 mass %, much more preferably 1.0 to 2.0 mass
%, particularly preferably 1.0 to 1.5 mass % based on the total of
all the fiber components contained in the separator (2). When the
fibrillated heat-resistant fibers (II) are used in this range,
though the fibrillated heat-resistant fibers (I) are contained,
there is no hindrance to the control of the thickness of the
separator and the cuttability of the separator, and the resistance
of the separator rarely becomes worse. Since the fibrillated
heat-resistant fibers (II) include relatively thick stem fibers
remaining besides the fibrillated fine fibers, the compression
resistance of the separator improves and the short-circuit
resistance of the separator becomes high.
[0053] The content of the synthetic resin short fibers is
preferably not less than 90.0 mass %, more preferably not less than
92 mass %, much more preferably not less than 94.0 mass %,
particularly preferably more than 95.0 mass % based on the total of
all the fiber components contained in the separator (2). The
content is preferably not more than 98.0 mass %, more preferably
not more than 97.0 mass %, much more preferably not more than 96.0
mass %. When the content of the synthetic resin short fibers is
more than 98.0 mass %, though the tensile strength of the separator
becomes high, the effect of improving a leakage current may become
small in the case of a separator having a low basis weight. When
the content of the synthetic resin short fibers is less than 90.0
mass % and the basis weight of the separator is reduced, mechanical
strength becomes low, whereby the separator may be broken in the
step of assembling a battery.
[0054] The most preferred separator (2) contains the fibrillated
heat-resistant fibers (I) and (II) and the synthetic resin short
fibers as fiber components. In this case, the content of the
synthetic resin short fibers is preferably more than 91.0 mass % to
not more than 98.0 mass % based on the total of all the fiber
components contained in the separator of the present invention. The
content is more preferably not more than 97.0 mass %, much more
preferably not more than 96.0 mass %. It is more preferably more
than 95.0 mass %.
[0055] The separator (3) of the present invention is characterized
in that it contains heat-resistant fibers and synthetic resin short
fibers, the fibrillated heat-resistant fibers (I) and the fibrid
are contained as the heat-resistant fibers, and the total content
of the fibrillated heat-resistant fibers (I) and the fibrid is not
less than 1.0 mass % to less than 5.0 mass % based on the total of
all the fiber components contained in the separator.
[0056] The content of the heat-resistant fibers having a modified
freeness of not more than 300 ml, that is, the total content of the
two different types of heat-resistant fibers, i.e., the fibrid
having a modified freeness of not more than 300 ml and the
fibrillated heat-resistant fibers (I) having a modified freeness of
not more than 300 ml is not less than 1.0 mass % to less than 5.0
mass % based on the total of all the fiber components contained in
the separator (3). The total content is preferably not less than
1.6 mass %, more preferably not less than 2.1 mass %, particularly
preferably not less than 2.5 mass %. When the total content of the
heat-resistant fibers is not less than 5.0 mass %, it is difficult
to reduce the thickness of the separator. The internal resistance
may become high. When the total content of the heat-resistant
fibers is less than 1.0 mass %, though the tensile strength of the
separator becomes high, the effect of improving the incidence of
internal short circuit failure may become small in the case of a
separator having a low basis weight.
[0057] In the heat-resistant fibers having a modified freeness of
not more than 300 ml contained in the separator (3), the content of
the fibrillated heat-resistant fibers (I) is preferably more than
50 mass %, more preferably not less than 60 mass %, much more
preferably not less than 70 mass %, particularly preferably not
less than 80 mass % based on the total of the fibrid and the
fibrillated heat-resistant fibers (I). The content of the
fibrillated heat-resistant fibers (I) is preferably not more than
92 mass %, more preferably not more than 88 mass %, much more
preferably not more than 84 mass %.
[0058] In the separator (3), when the content of the fibrillated
heat-resistant fibers (I) is more than 50 mass % and the fibrid and
the fibrillated heat-resistant fibers (I) are used in combination,
the separator can be made thin, there is no hindrance to the
cuttability of the separator, the pore diameter becomes small, and
the retainability of the electrolytic solution improves, whereby
the resistance of the separator rarely becomes worse. The fibrid
has a leaf-like fibrous form and greatly shrinks when moisture
existent in the crystal structure is dried off and removed, thereby
strengthening a network formed by the fibrillated heat-resistant
fibers (I) and the synthetic resin short fibers. Therefore, even
when the basis weight is low, the strength characteristics of the
separator can be retained and the incidence of internal short
circuit failure can be prevented from becoming worse.
[0059] When the content of the fibrillated heat-resistant fibers
(I) is not more than 50 mass % in the separator (3) and the
separator is made thin, the resistance may rise. When the content
of the fibrillated heat-resistant fibers (I) is not more than 50
mass % and the basis weight of the separator is reduced, the
strength characteristics of the separator may be hardly retained,
and it may be difficult to prevent the incidence of internal short
circuit failure from becoming worse. When the content of the
fibrillated heat-resistant fibers (I) is more than 92 mass %, the
tensile strength may lower or the resistance of the separator may
become worse.
[0060] The content of the synthetic resin short fibers is
preferably not less than 90.0 mass %, more preferably not less than
92.0 mass %, much more preferably not less than 94.0 mass %,
particularly preferably more than 95.0 mass % based on the total of
all the fiber components contained in the separator (3). The
content is preferably not more than 99.0 mass %, more preferably
not more than 98.0 mass %, much more preferably not more than 97.0
mass %. When the content of the synthetic resin short fibers is
more than 99.0 mass %, though the tensile strength of the separator
becomes high, the effect of improving the incidence of internal
short circuit failure may become small. When the content of the
synthetic resin short fibers is less than 90.0 mass % and the basis
weight of the separator is reduced, mechanical strength becomes
low, whereby the separator may be broken at the time of assembling
a battery.
[0061] The most preferred separator (3) is a separator containing
the fibrillated heat-resistant fibers (I), the fibrid and the
synthetic resin short fibers as fiber components. In this case, the
content of the synthetic resin short fibers is preferably 96.0 to
99.0 mass % based on the total of all the fiber components
contained in the separator (3). The content is more preferably not
more than 98.0 mass %, much more preferably not more than 97.0 mass
%. It is particularly preferably not less than 96.5 mass %.
[0062] The separator (4) of the present invention is characterized
in that it contains heat-resistant fibers and synthetic resin short
fibers, the fibrillated heat-resistant fibers (I), the fibrillated
heat-resistant fibers (II) and the fibrid are contained as the
heat-resistant fibers, and the total content of the fibrillated
heat-resistant fibers (I) and (II) and the fibrid is not less than
2.0 mass % to less than 9.0 mass % based on the total of all the
fiber components contained in the separator.
[0063] In the present invention, the total content of the fibrid
and the fibrillated heat-resistant fibers (I) and (II) is
preferably not less than 2.0 mass % to less than 9.0 mass %, more
preferably not less than 3.0 mass % to less than 9.0 mass %, much
more preferably not less than 3.5 mass % to less than 7.0 mass %,
particularly preferably not less than 3.5 mass % to less than 5.0
mass % based on the total of all the fiber components contained in
the separator (4). When the total content of the fibrid and the two
different types of fibrillated heat-resistant fibers (I) and (II)
is not less than 9.0 mass %, it is difficult to reduce the
thickness of the separator. Tensile strength may lower, and
internal resistance may become worse. When the total content of the
fibrid and the two different types of fibrillated heat-resistant
fibers (I) and (II) is less than 2.0 mass %, though the tensile
strength of the separator becomes high, the effect of improving the
incidence of internal short circuit failure may become small.
[0064] The content of the fibrid is preferably not less than 0.4
mass % to less than 2.5 mass %, more preferably not less than 0.4
mass % to less than 2.0 mass %, much more preferably not less than
0.4 mass % to less than 1.5 mass %, particularly preferably not
less than 0.4 mass % to less than 1.0 mass % based on the total of
all the fiber components contained in the separator (4). When the
fibrid is used in this range, though the fibrillated heat-resistant
fibers (I) and (II) are contained, there is no hindrance to the
control of the thickness of the separator and the cuttability of
the separator, and the resistance of the separator rarely becomes
worse. Further, the fibrid has a thin leaf-like fibrous form and
greatly shrinks when moisture existent in the crystal structure is
dried off and removed, thereby strengthening a network formed by
the fibrillated heat-resistant fibers (I) and (II). Therefore, even
when the basis weight is low, the strength characteristics of the
separator can be retained and internal short circuit failure can be
prevented, thereby improving the short-circuit resistance of the
separator.
[0065] The content of the fibrillated heat-resistant fibers (II) is
preferably not less than 1.0 mass % to less than 4.0 mass %, more
preferably 1.0 to 3.0 mass %, much more preferably 1.0 to 2.0 mass
%, particularly preferably 1.0 to 1.5 mass % based on the total of
all the fiber components contained in the separator (4). When the
fibrillated heat-resistant fibers (II) are used in this range,
though the fibrid and the fibrillated heat-resistant fibers (I) are
contained, there is no hindrance to the control of the thickness of
the separator and the cuttability of the separator, and the
resistance of the separator rarely becomes worse. Since the
fibrillated heat-resistant fibers (II) include relatively thick
stem fibers remaining besides the fibrillated fine fibers, the
compression resistance of the separator improves and the
short-circuit resistance of the separator become high.
[0066] The content of the fibrillated heat-resistant fibers (I) is
preferably not less than 0.6 mass % to less than 4.6 mass %, more
preferably not less than 1.0 mass % to less than 4.6 mass %, much
more preferably not less than 1.5 mass % to less than 4.0 mass %,
particularly preferably 2.0 to 3.0 mass % based on the total of all
the fiber components contained in the separator (4). When the
fibrillated heat-resistant fibers (I) are used in this range, the
number of thick stem fibers before the fibrillation of the
heat-resistant fibers is small and the fibrillated fibers are fine,
whereby there is no hindrance to the control of the thickness of
the separator and the cuttability of the separator, and the pore
diameter becomes small. Therefore, the retainability of the
electrolytic solution becomes high, thereby producing the effect of
improving the resistance of the separator. When the content of the
fibrillated heat-resistant fibers (I) is not less than 4.6 mass %,
tensile strength may lower, and the resistance of the separator may
become worse. When the content of the fibrillated heat-resistant
fibers (I) is less than 0.6 mass %, the effect obtained by blending
the fibrillated heat-resistant fibers (I) may not be observed.
[0067] Even when the total content of the fibrid and the
fibrillated heat-resistant fibers (I) and (II) is not less than 2.0
mass % to less than 9.0 mass % based on the total of all the fiber
components contained in the separator (4), in the case of a total
content of the fibrid and the fibrillated heat-resistant fibers (I)
of not less than 5.0 mass %, internal resistance may become worse.
In addition, when the content of the fibrid is not less than 2.5
mass % and the basis weight of the separator is low, internal
resistance may become worse.
[0068] The content of the synthetic resin short fibers is
preferably not less than 90.0 mass %, more preferably not less than
92.0 mass %, much more preferably not less than 93.0 mass %,
particularly preferably more than 95.0 mass % based on the total of
all the fiber components contained in the separator (4). The
content is preferably not more than 98.0 mass %, more preferably
not more than 97.5 mass %, much more preferably not more than 97.0
mass %, particularly preferably not more than 96.5 mass %. When the
content of the synthetic resin short fibers is more than 98.0 mass
%, though the tensile strength of the separator becomes high, the
effect of improving the incidence of internal short circuit failure
may become small. When the content of the synthetic resin short
fibers is less than 90.0 mass % and the basis weight of the
separator is reduced, mechanical strength becomes low, whereby the
separator may be broken at the time of laminating an electrode or
assembling a battery.
[0069] In the present invention, as the fibrillated heat-resistant
fibers, fibers obtained by fibrillating heat-resistant fibers made
of a heat-resistant resin such as wholly aromatic polyamide
(aramid), wholly aromatic polyester, polyimide, polyamide imide,
polyether ether ketone, polyphenylene sulfide, polybenzimidazole,
poly-p-phenylene benzobisthiazole, poly-p-phenylene benzobisoxazole
or polytetrafluoroethylene are used. Out of these, wholly aromatic
polyamide is preferred as it has high affinity for an electrolytic
solution and is easily fibrillated.
[0070] The fibrillated heat-resistant fibers can be obtained by
processing heat-resistant fibers with a refiner, beater, mill,
grinding machine, rotary homogenizer which applies shear force with
a high-speed rotary blade, double cylinder type high-speed
homogenizer which produces shear force between an cylindrical inner
blade rotating at a high speed and a fixed outer blade, ultrasonic
crusher which refines with ultrasonic shock, or high-pressure
homogenizer which lets a fiber suspension pass through a
small-diameter orifice by applying a pressure difference to
accelerate the speed of the suspension and collides the fiber
suspension to rapidly decelerate it so as to apply shear force and
cutting force to the fibers.
[0071] In the present invention, the term "fibrid" refers to a thin
leaf-like or flaky piece having fine fibrils and fine
heat-resistant fiber in which a water molecule or moisture is
existent in the crystal structure in an amorphous state without
forming a strong fiber crystal structure. The fibrid is obtained by
collecting a product obtained by introducing a fiber forming
high-molecular weight polymer solution into an aqueous coagulation
bath without drying it and fibrillating it by beating as required.
It is, for example, a fibrid which is produced by mixing a polymer
solution with a precipitating agent for it in the presence of shear
force or an amorphous hydrous product having molecular orientation
formed from a high-molecular weight polymer solution showing
optical anisotropy and may be beaten as required.
[0072] The fibrid can be obtained by beating with a refiner,
beater, mill, grinding machine, rotary homogenizer which applies
shear force with a high-speed rotary blade, double cylinder type
high-speed homogenizer which produces shear force between an
cylindrical inner blade rotating at a high speed and a fixed outer
blade, ultrasonic crusher which refines with ultrasonic shock, or
high-pressure homogenizer which lets a fiber suspension pass
through a small-diameter orifice by applying a pressure difference
to accelerate the speed of the suspension and collides the fiber
suspension to rapidly decelerate it so as to apply shear force and
cutting force to the fibers.
[0073] In the present invention, when moisture existent in the
crystal structure is removed by heating or pressure reduction, the
fibrid greatly shrinks to strengthen the fiber network, thereby
improving the strength characteristics of the separator.
[0074] The modified freeness of the fibrillated heat-resistant
fibers (I) in the present invention is not more than 300 ml,
preferably 0 to 200 ml, more preferably 0 to 100 ml. When the
modified freeness of all the fibrillated heat-resistant fibers is
more than 300 ml, a large number of thick stem fibers are existent,
thereby making it difficult to control the thickness and
deteriorating the cuttability of the separator. Since ion
permeability is hindered and the retainability of the electrolytic
solution is deteriorated by the existence of the thick stem fibers,
the resistance of the separator becomes high. Further, since there
is a great need to increase the load or temperature of a heat
calender for the control of thickness, the fusion area of a binder
fiber is widened and ion permeability is hindered, whereby the
resistance of the separator may become high. When the modified
freeness of all the fibrillated heat-resistant fibers is less than
0 ml, the fibrillation of the fibrillated heat-resistant fibers
proceeds too much, whereby the number of fine fibers to be bonded
by a predetermined amount of a binder fiber is increased and
therefore, tensile strength may lower. When the fibrillation of the
fibrillated heat-resistant fibers proceeds, the modified freeness
keeps falling. When fibrillation further proceeds after the
modified freeness reaches 0 ml, the fibers pass through the mesh,
whereby the modified freeness begins to rise reversely. In the
present invention, the state that the modified freeness begins to
rise reversely is referred to as "modified freeness of less than 0
ml".
[0075] The modified freeness of the fibrillated heat-resistant
fibers (II) in the present invention is more than 300 ml,
preferably more than 300 ml to less than 700 ml, more preferably
more than 300 ml to less than 600 ml, much more preferably more
than 300 ml to less than 450 ml. When the modified freeness of the
fibrillated heat-resistant fibers (II) is not less than 700 ml,
fibrillation does not proceed much, whereby it is difficult to
control the thickness of the separator and the cuttability of the
separator may deteriorate due to the existence of a large number of
thick stem fibers. Since ion permeability is hindered and the
retainability of the electrolytic solution is deteriorated by the
existence of the thick stem fibers, the internal resistance of the
separator may become high.
[0076] In the present invention, the modified freeness of the
fibrid is preferably 0 to 300 ml, more preferably 0 to 200 ml, much
more preferably 0 to 100 ml. When the modified freeness of the
fibrid is more than 300 ml, the fiber width of the fibrid becomes
large and the internal resistance of the separator may become high.
Further, the texture (density unevenness) of the separator becomes
worse and the mechanical strength of the separator becomes low,
whereby the separator may be broken at the time of assembling a
battery. Further, the cuttability of the separator may
deteriorate.
[0077] Preferably, the fibrillated heat-resistant fibers (I) have a
mass-weighted average fiber length of 0.02 to 1.00 mm. The
length-weighted average fiber length is preferably 0.02 to 0.50 mm.
When the average fiber length is smaller than the preferred range,
the fibrillated heat-resistant fibers may fall off from the
separator. When the average fiber length is larger than the
preferred range, the disintegration of the fibers deteriorates and
a dispersion failure tends to occur.
[0078] Preferably, the fibrillated heat-resistant fibers (II) have
a mass-weighted average fiber length of 1.00 to 1.50 mm. The
length-weighted average fiber length is preferably 0.50 to 1.00 mm.
When the average fiber length is smaller than the preferred range,
the effect of improving short-circuit resistance becomes small.
When the average fiber length is larger than the preferred range,
it may be difficult to reduce the thickness of the separator and
cuttability may deteriorate.
[0079] Preferably, the fibrid has a mass-weighted average fiber
length of 0.30 to 1.00 mm. The length-weighted average fiber length
of the fibrid is preferably 0.10 to 0.50 mm. When the average fiber
length is smaller than the preferred range, the fibrid may fall off
from the separator. When the average fiber length is larger than
the preferred range, the textile of the separator may become worse,
the internal resistance of the separator may become high, and the
incidence of internal short circuit failure may become worse.
[0080] When the fibrid has the above mass-weighted average fiber
length and the above length-weighted average fiber length, if the
content of the fibrid in the separator is low, a fine network
structure is formed by the fibers between fibrids and between the
fibrid and the synthetic resin short fibers, whereby a thin
separator having high tensile strength is easily obtained without
impairing cuttability.
[0081] When the mass-weighted average fiber lengths and the
length-weighted average fiber lengths of the fibrillated
heat-resistant fibers (I) and (II) and the fibrid fall within the
above preferred ranges, if the contents of the fibrid and the
fibrillated heat-resistant fibers in the separator are low, a fine
network structure is formed by the fibers between the
heat-resistant fibers and between the heat-resistant fibers and the
synthetic resin short fibers, whereby a thin separator having high
tensile strength is easily obtained without impairing cuttability.
Further, a separator having low internal resistance, a low
incidence of internal short circuit failure and high rate discharge
characteristics is easily obtained.
[0082] In the present invention, the mass-weighted average fiber
length and the length-weighted average fiber length are
mass-weighted average fiber length (L(w)) and length-weighted
average fiber length (L(I)) measured in a projection fiber length
(Proj) mode with KajaaniFiberLab V3.5 (manufactured by Metso
Automation), respectively.
[0083] The average fiber width of the fibrillated heat-resistant
fibers (I) is preferably 0.5 to 20.0 .mu.m, more preferably 3.0 to
16.0 .mu.m, much more preferably 5.0 to 15.0 .mu.m. When the
average fiber width is larger than 20.0 .mu.m, it may be difficult
to reduce the thickness of the separator or cuttability may
deteriorate. When the average fiber width is smaller than 0.5
.mu.m, the fibrillated heat-resistant fibers (I) may fall off from
the separator.
[0084] The average fiber width of the fibrillated heat-resistant
fibers (II) is preferably 5.0 to 40.0 .mu.m, more preferably 5.0 to
35.0 .mu.m, much more preferably 5.0 to 30.0 .mu.m. When the
average fiber width is larger than 40.0 .mu.m, it may be difficult
to reduce the thickness of the separator or cuttability may
deteriorate. When the average fiber width is smaller than 5.0
.mu.m, short-circuit resistance may not improve.
[0085] The average fiber width of the fibrid is preferably 3.0 to
40.0 .mu.m, more preferably 5.0 to 35.0 .mu.m, much more preferably
10.0 to 30.0 .mu.m. When the average fiber width is larger than
40.0 .mu.m, the internal resistance of the separator tends to
become high, it may be difficult to reduce the thickness, or
cuttability may deteriorate. When the average fiber width is
smaller than 3.0 .mu.m, the fibrid beating time becomes long,
thereby greatly reducing productivity.
[0086] In the present invention, the average fiber width of the
fibrid is a fiber width measured by using KajaaniFiberLab V.3.5
(manufactured by Metso Automation).
[0087] In Examples of the present invention, the modified freeness,
mass-weighted average fiber length, length-weighted average fiber
length and average fiber width of the heat-resistant fibers are the
measurement values (A) of raw materials before the manufacture of
the separator. In the present invention, the measurement values (B)
of the heat-resistant fibers taken out from the separator were
obtained to check the differences between the measurement values
(A) and the measurement values (B). A separator containing
polyethylene terephthalate-based synthetic resin short fibers and
the fibrillated heat-resistant fibers (I) was first put in an
alkali-resistant plastic vessel. Then, a highly concentrated alkali
solution (for example, a 48 mass % potassium hydroxide aqueous
solution) was added to the vessel, and a stirrer was inserted into
the vessel. Thereafter, the solution was stirred for one day while
the vessel was immersed in a 40 to 60.degree. C. hot bath to
dissolve the PET-based synthetic resin short fibers completely.
Then, the fibrillated heat-resistant fibers (I) in the vessel were
filtered out with a filter, neutralized, rinsed fully and dried to
obtain the fibrillated heat-resistant fibers (I) taken out from the
separator. The modified freeness and others of the fibrillated
heat-resistant fibers (I) taken out from the separator were
measured as the measurement values (B). When the measurement values
(A) before the manufacture of the separator were compared with the
measurement values (B), there were not so much differences between
the measurement values (A) and (B) and it could be confirmed that
"measurement values (A)/measurement values (B)".
TABLE-US-00001 <measurement values (A)> Modified freeness 50
ml Mass-weighted average fiber length 0.58 mm Length-weighted
average fiber length 0.33 mm Average fiber width 15.5 .mu.m
TABLE-US-00002 <measurement values (B)> Modified freeness 49
ml Mass-weighted average fiber length 0.55 mm Length-weighted
average fiber length 0.31 mm Average fiber width 16.1 .mu.m
[0088] In the present invention, the synthetic resin short fibers
are short fibers (staples) which are not fibrillated and are made
of a synthetic resin such as polyolefin, polyester, polyvinyl
acetate, ethylene-vinyl acetate copolymer, polyamide, acryl,
polyvinyl chloride, polyvinylidene chloride, polyvinyl ether,
polyvinyl ketone, polyether, polyvinyl alcohol, diene,
polyurethane, phenol, melamine, furan, urea, aniline, unsaturated
polyester, fluorine, silicon or derivative thereof. The tensile
strength and thrust strength of the separator can be enhanced by
blending the synthetic resin short fibers.
[0089] The synthetic resin short fibers may be fibers (single
fibers) made of a single resin or composite fibers made of two or
more resins. The synthetic resin short fibers contained in the
separator of the present invention may be of one type or two or
more different types. Examples of the composite fibers include
core-sheath, eccentric, side-by-side, sea-island, orange and
multiple bimetal type composite fibers.
[0090] The fineness of the synthetic resin short fibers is
preferably 0.01 to 0.6 dtex, more preferably 0.02 to 0.3 dtex. When
the fineness of the synthetic resin short fibers is more than 0.6
dtex, the number of fibers in the thickness direction becomes
small, whereby the pore size distribution of the separator becomes
wide with the result that the incidence of internal short circuit
failure becomes high. Further, it is difficult to reduce the
thickness, and strength characteristics tend to deteriorate. When
the fineness of the synthetic resin short fibers is less than 0.01
dtex, the fibers become very expensive, the stable production of
the fibers may be difficult, and dewaterability may deteriorate at
the time of producing a separator by a wet papermaking method.
[0091] The fiber length of the synthetic resin short fibers is
preferably 1 to 10 mm, more preferably 1 to 5 mm. When the fiber
length is larger than 10 mm, the texture may become worse. When the
fiber length is smaller than 1 mm, the mechanical strength of the
separator lowers, whereby the separator may be broken at the time
of forming a battery.
[0092] The separator of the present invention may contain fibers
other than the heat-resistant fibers and the synthetic resin short
fibers. Examples of the fibers include cellulose fibers, pulp
products and fibrillated products of cellulose fibers, pulp
products of synthetic resins and inorganic fibers. The inorganic
fibers include glass, alumina, silica, ceramic and Rockwool fibers.
The cellulose fibers may be either natural cellulose fibers or
regenerated cellulose fibers.
[0093] The thickness of the separator of the present invention is
preferably not smaller than 10 .mu.m, more preferably not smaller
than 11 .mu.m, much more preferably not smaller than 12 .mu.m.
Further, it is preferably not larger than 20 .mu.m, more preferably
not larger than 18 .mu.m, much more preferably not larger than 15
.mu.m. Even when the thickness of the separator is set to the above
range, internal resistance can be reduced and tensile strength
required for the electrode laminating step can be retained in the
separator of the present invention, whereby work efficiency in each
step including the papermaking property of the separator is not
impaired. When the thickness of the separator is larger than 20
.mu.m, the internal resistance of the separator may become too
high. Further, a high capacity battery may not be obtained. When
the thickness of the separator is smaller than 10 .mu.m, the
strength of the separator becomes too low, whereby the separator
may be broken at the time of handling, laminating an electrode or
assembling a battery.
[0094] The density of the lithium ion battery separator of the
present invention is preferably not lower than 0.40 g/cm.sup.3,
more preferably not lower than 0.45 g/cm.sup.3, and preferably not
higher than 0.75 g/cm.sup.3, more preferably not higher than 0.70
g/cm.sup.3. When the density is lower than 0.40 g/cm.sup.3, the
strength of the separator becomes too low, whereby the separator
may be broken at the time of handling or laminating an electrode.
When the density is higher than 0.75 g/cm.sup.3, the internal
resistance of the separator may become too high.
[0095] The lithium ion battery separator of the present invention
is preferably a wet nonwoven fabric produced by a wet paper-making
method. The wet paper-making method is to produce the wet nonwoven
fabric by dispersing fibers in water to prepare a homogeneous
paper-making slurry and filtering the paper-making slurry with a
paper machine. Examples of the paper machine include a cylinder
paper machine, fourdrinier paper machine, inclination type paper
machine, inclination short foundrinier paper machine (?) and
composite machines thereof. In the process for producing the wet
nonwoven fabric, a water-jet interlacing treatment may be carried
out as required. As the treatment of the wet nonwoven fabric, a
heat treatment, calendering or heat calendering may be carried
out.
EXAMPLES
[0096] The following examples are provided for the purpose of
further illustrating the present invention but are in no way to be
taken as limiting. Percentages (%) and parts in Examples are all
based on mass unless otherwise noted.
[0097] The following physical properties of the lithium ion battery
separators of Examples and Comparative Examples were measured and
evaluated.
<Basis Weight of Separator>
[0098] The basis weight of the separator was measured in accordance
with JIS P8124.
<Thicknesses of Separator>
[0099] The thickness of the separator was measured under a load of
5 N by using an external micrometer specified in JIS B7502.
<Evaluation of Tensile Strength>
[0100] A sample piece measuring 250 mm in a flow direction x 50 mm
in a width direction was cut out from each separator in such a
manner that its long side was arranged in the flow direction to
carry out a tensile test at a tensile speed of 200 mm/min by using
a desk-top material testing machine (STA-1150 of ORIENTEC Co.,
Ltd.) in accordance with JIS P8113. The maximum value of tensile
stress is taken as "tensile strength". In general, higher tensile
strength is more preferred. When tensile strength is low, tension
applied to the separator must be precisely controlled at the time
of manufacturing a battery, thereby causing a problem that a
large-scaled apparatus is required for this control. When tensile
strength is evaluated as .DELTA., the separator may be usable for
practical application by precisely controlling tension applied to
the separator.
.circleincircle.: tensile strength of not less than 800 N/m
.largecircle.: tensile strength of not less than 600 N/m to less
than 800 N/m .DELTA.: tensile strength of not less than 400 N/m to
less than 600 N/m X: tensile strength of less than 400 N/m
<Cuttability of Separator>
[0101] Each separator having a width of 120 mm was melt-cut with a
heat sealing apparatus to check the melt-down surface visually and
evaluate it based on the following criteria.
.smallcircle.: no fluff is seen on melt-down surface .DELTA.: fluff
is slightly seen on melt-down surface X: fluff is striking on
melt-down surface
<Manufacture of Evaluation Battery>
[0102] A slurry was prepared by mixing together 95 mass % of
lithium titanate having a spinel structure represented by
Li.sub.4Ti.sub.5O.sub.12 which had an average particle diameter of
0.7 .mu.m and a Li absorption potential of 1.55 V as a negative
electrode active material, 2.5 mass % of acetylene black as a
conductive material and 2.5 mass % of polyvinylidene fluoride and
dispersing the resulting mixture in N-methyl-2-pyrrolidone and
coated to both sides of a 15 .mu.m-thick aluminum foil which was
then rolled and vacuum dried at 150.degree. C. for 2 hours to
manufacture a 100 .mu.m-thick negative electrode for lithium ion
secondary batteries.
[0103] A slurry was prepared by mixing together 90 mass % of
lithium cobalt oxide (LiCoO.sub.2) as a positive electrode active
material, 3 mass % of acetylene black, 3 mass % of graphite and 4
mass % of polyvinylidene fluoride and dispersing the resulting
mixture in N-methyl-2-pyrrolidone. This slurry was coated to both
sides of a collector made of an aluminum foil having a thickness of
15 .mu.m which was then rolled and vacuum dried at 150.degree. C.
for 2 hours to manufacture a 100 .mu.m-thick positive electrode for
lithium ion secondary batteries.
[0104] Terminals were connected to the collector of the positive
electrode and the negative electrode, the positive electrode, the
separator, the negative electrode and the separator were laminated
together in the mentioned order, and the obtained laminate was
wound in such a manner that the terminals of the positive electrode
and the negative electrode were arranged perpendicular to the
longitudinal direction of the separator. Subsequently, this wound
laminate was thermally pressed at 90.degree. C. to manufacture flat
electrodes measuring 70 mm.times.100 mm and having a thickness of
3.0 mm. Then, a laminate film pack (bag-like exterior material)
having a thickness of 0.1 mm which was composed of an aluminum foil
having a thickness of 40 .mu.m and a polyethylene film on both
sides was prepared, and the obtained electrodes were stored in this
bag-like exterior material in such a manner that the terminals of
the negative electrode and the positive electrode projected outward
from the opening of the exterior material and vacuum dried at
80.degree. C. for 24 hours. After the above electrodes were stored
and a solution prepared by dissolving 1.5 mol/L lithium
tetrafluoroborate as an electrolyte in a mixed solvent of ethylene
carbonate (EC) and .gamma.-butyrolactone (BL) (volume ratio of
25:75) as an electrolytic solution was injected into the bag-like
exterior material, the opening of the bag-like exterior material
was completely sealed up by heat sealing to manufacture a lithium
ion secondary battery.
<Evaluation of Resistance>
[0105] After the manufactured separator was immersed in an
electrolytic solution (1M-LiPF.sub. /EC+diethyl carbonate
(DEC)+dimethyl carbonate (DMC) (volume ratio of 1:1:1)), it was
sandwiched between two substantially cylindrical copper electrodes
to measure the resistance component of AC impedance at 200 kHz by
using an LCR meter (LCR-821 of Instec).
<Incidence of Internal Short Circuit Failure>
[0106] After the electrodes were manufactured by winding the
manufactured separator between electrodes made of aluminum foil,
conduction between the electrodes was checked with a tester without
impregnating the electrodes with the electrolytic solution to
confirm the existence or nonexistence of a short circuit. The
incidence of internal short circuit failure (%) was calculated from
the number of short-circuited electrodes based on the total number
of all electrodes by checking 200 electrodes.
<Cycle Characteristics>
[0107] A charging/discharging cycle test was carried out on each
lithium ion secondary battery at a 1C rate in a 45.degree. C.
environment to measure discharge capacity at a 1,000-th cycle so as
to calculate cycle characteristics as the discharge capacity
retention ratio (%) to the discharge capacity at a 5-th cycle.
Examples 1
Example 1-1
[0108] 55.5 parts by mass of oriented crystallized polyethylene
terephthalate (PET)-based synthetic resin short fibers having a
fineness of 0.06 dtex and a fiber length of 3 mm, 40.0 parts by
mass of undrawn PET-based synthetic resin short fibers for
single-component binders having a fineness of 0.2 dtex and a fiber
length of 3 mm (softening point of 120.degree. C., melting point of
230.degree. C.) and 4.5 parts by mass of fibrillated heat-resistant
fibers obtained by fibrillating a pulp-like product of wholly
aromatic polyamide fibers (average fiber length of 1.7 mm, average
fiber diameter of 10 .mu.m) to a modified freeness of up to 50 ml
with a high-pressure homogenizer were dispersed in water with a
pulper to prepare a homogeneous paper-making slurry having a
concentration of 0.5 mass %, and a wet paper web was obtained by
using an inclination type paper machine and dried with a cylinder
drier having a surface temperature of 135.degree. C. to obtain a
sheet. The obtained sheet was calendered with a heat calender
having a steel roll plated with chromium and having a surface
temperature of 195.degree. C. as one of the rolls and a resin roll
having a hardness of Shore D92 as the other roll and a linear
pressure of 100 kN/m to produce a separator having a basis weight
of 10 g/m.sup.2 and a thickness of 15 .mu.m.
Example 1-2
[0109] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 1-1 except that 59.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 1.0
part by mass of fibrillated heat-resistant fibers having a modified
freeness of 50 ml were used.
Example 1-3
[0110] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 11 .mu.m was manufactured in the same manner as in
Example 1-1 except that 55.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 4.9
parts by mass of fibrillated heat-resistant fibers having a
modified freeness of 50 ml were used.
Example 1-4
[0111] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 1-1 except that 55.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 4.5
parts by mass of fibrillated heat-resistant fibers obtained by
fibrillating cut fibers (average fiber length of 3.0 mm, average
fiber diameter of 20 .mu.m) of polyarylate fibers (wholly aromatic
polyester fibers) to a modified freeness of up to 250 ml with a
high-pressure homogenizer were used.
Comparative Example 1-1
[0112] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 1-1 except that 60.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm and 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm were
used.
Comparative Example 1-2
[0113] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 11 .mu.m was manufactured in the same manner as in
Example 1-1 except that 54.9 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 5.1
parts by mass of fibrillated heat-resistant fibers having a
modified freeness of 50 ml were used and hot calendering was
carried out at a steel roll surface temperature of 195.degree. C.
and a linear pressure of 120 kN/m to make the separator uniform in
thickness.
Comparative Example 1-3
[0114] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 11 .mu.m was manufactured in the same manner as in
Example 1-1 except that 50.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 10.0
parts by mass of fibrillated heat-resistant fibers having a
modified freeness of 50 ml were used and hot calendering was
carried out at a steel roll surface temperature of 195.degree. C.
and a linear pressure of 175 kN/m to make the separator uniform in
thickness.
Comparative Example 1-4
[0115] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 11 .mu.m was manufactured in the same manner as in
Example 1-3 except that the fibrillated heat-resistant fibers were
changed to wholly aromatic polyamide fibers which had been
fibrillated to a modified freeness of up to 320 ml by using a
grinding apparatus.
Example 1-5
[0116] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 1-1 except that 54.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 4.5
parts by mass of fibrillated heat-resistant fibers having a
modified freeness of 50 ml were used.
Comparative Example 1-5
[0117] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 11 .mu.m was manufactured in the same manner as in
Example 1-1 except that the paper-making slurry of Comparative
Example 1-1 was used.
Example 1-6
[0118] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 1-1 except that 50.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 4.5 parts
by mass of fibrillated heat-resistant fibers and 5.5 pars by mass
of beaten solvent spun cellulose fibers having a modified freeness
of 90 ml obtained by refining solvent spun cellulose fibers having
an average fiber diameter of 10 .mu.m and a fiber length of 4 mm
with a refiner were used.
Example 1-7
[0119] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 1-1 except that 49.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 4.5 parts
by mass of fibrillated heat-resistant fibers having a modified
freeness of 50 ml and 6.5 pars by mass of beaten solvent spun
cellulose fibers having a modified freeness of 90 ml were used.
Example 1-8
[0120] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 1-1 except that 55.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 4.0
parts by mass of fibrillated heat-resistant fibers having a
modified freeness of 50 ml were used.
Example 1-9
[0121] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 1-1 except that 56.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm and 3.0
parts by mass of fibrillated heat-resistant fibers having a
modified freeness of 50 ml were used.
Example 1-10
[0122] A separator having a basis weight of 12 g/m.sup.2 and a
thickness of 18 .mu.m was manufactured in the same manner as in
Example 1-1 except that the paper-making slurry of Example 1-8 was
used.
Comparative Example 1-6
[0123] A separator having a basis weight of 12 g/m.sup.2 and a
thickness of 18 .mu.m was manufactured in the same manner as in
Example 1-1 except that the paper-making slurry of Comparative
Example 1-3 was used and hot calendering was carried out at a steel
roll surface temperature of 195.degree. C. and a linear pressure of
120 kN/m to make the separator uniform in thickness.
TABLE-US-00003 TABLE 1 Ex. Ex. Ex. Ex. Ex. 1-1 1-2 1-3 1-4 1-5
Fiber Oriented crystallized PET-based synthetic pbm 55.5 59.0 55.1
55.5 54.5 blending resin short fibers 0.06 dtex 3 mm Undrawn
PET-based synthetic resin short pbm 40.0 40.0 40.0 40.0 41.0 fibers
0.2 dtex 3 mm Fibrillated heat-resistant fibers (I) pbm 4.5 1.0 4.9
4.5 4.5 Beaten solvent spun cellulose fibers pbm 0.0 0.0 0.0 0.0
0.0 Fibrillated Modified freeness ml 50 50 50 250 50 heat-
Mass-weighted average fiber length mm 0.58 0.58 0.58 0.82 0.58
resistant Length-weighted average fiber length mm 0.33 0.33 0.33
0.45 0.33 fibers (I) Fiber width .mu.m 15.5 15.5 15.5 19.5 15.5
Linear pressure of heat calender kN/m 100 100 100 100 100
Evaluation Weight of separator g/m.sup.2 10 10 8 10 7 Thickness of
separator .mu.m 15 15 11 15 10 Tensile strength of separator --
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.circleincircle. Cuttability of separator -- .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. Resistance
component (impedance) .OMEGA. 0.55 0.55 050 0.54 0.45 Incidence of
internal short circuit failure % 0.0 0.0 0.0 0.0 0.0 Cycle
characteristics % 90 88 88 90 85 Ex. Ex. Ex. Ex. Ex. 1-6 1-7 1-8
1-9 1-10 Fiber Oriented crystallized PET-based synthetic pbm 50.0
49.0 55.0 56.0 55.0 blending resin short fibers 0.06 dtex 3 mm
Undrawn PET-based synthetic resin short pbm 40.0 40.0 41.0 41.0
41.0 fibers 0.2 dtex 3 mm Fibrillated heat-resistant fibers (I) pbm
4.5 4.5 4.0 3.0 4.0 Beaten solvent spun cellulose fibers pbm 5.5
6.5 0.0 0.0 0.0 Fibrillated Modified freeness ml 50 50 50 50 50
heat- Mass-weighted average fiber length mm 0.58 0.58 0.58 0.58
0.58 resistant Length-weighted average fiber length mm 0.33 0.33
0.33 0.33 0.33 fibers (I) Fiber width .mu.m 15.5 15.5 15.5 15.5
15.5 Linear pressure of hot calender kN/m 100 100 100 100 100
Evaluation Weight of separator g/m.sup.2 10 10 7 7 12 Thickness of
separator .mu.m 15 15 10 10 18 Tensile strength of separator --
.circleincircle. .largecircle. .circleincircle. .circleincircle.
.circleincircle. Cuttability of separator -- .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. Resistance
component (impedance) .OMEGA. 0.50 0.49 0.45 0.45 0.56 Incidence of
internal short circuit failure % 0.0 0.0 0.0 0.0 0.0 Cycle
characteristics % 90 90 83 80 90 Ex.: Example pbm: part by mass
Ex.: Example
TABLE-US-00004 TABLE 2 C. Ex. C. Ex. C. Ex. C. Ex. C. Ex. C. Ex.
1-1 1-2 1-3 1-4 1-5 1-6 Fiber Oriented crystallized PET-based
synthetic pbm 60.00 54.9 50.00 55.1 60.0 50.0 blending resin short
fibers 0.06 dtex 3 mm Undrawn PET-based synthetic resin short pbm
40.0 40.0 40.0 40.0 40.0 40.0 fibers 0.2 dtex 3 mm Fibrillated
heat-resistant fibers (I) pbm 0.0 5.1 10.0 4.9 0.0 10.0 Beaten
solvent spun cellulose fibers pbm 0.0 0.0 0.0 0.0 0.0 0.0
Fibrillated Modified freeness ml -- 50 50 320 -- 50 heat-
Mass-weighted average fiber length mm -- 0.58 0.58 320 -- 0.50
resistant Length-weighted average fiber length mm -- 0.33 0.33 0.59
-- 0.33 fibers (I) Fiber width .mu.m -- 15.5 15.5 25.2 -- 15.5
Linear pressure of heat calender kN/m 100 120 175 100 100 120
Evaluation Weight of separator g/m.sup.2 10 8 8 8 8 12 Thickness of
separator .mu.m 15 11 11 11 11 18 Tensile strength of separator --
.circleincircle. .largecircle. .DELTA. .circleincircle.
.circleincircle. .DELTA. Cuttability of separator -- .largecircle.
.DELTA. X .DELTA. .largecircle. X Resistance component (impedance)
.OMEGA. 0.54 0.58 0.65 0.55 0.47 0.58 Incidence of internal short
circuit failure % 0.5 0.0 0.0 0.5 1.0 0.0 Cycle characteristics %
35 30 75 85 70 80 C. Ex.: Comparative Example pbm: part by mass C.
Ex.: Comparative Example
[0124] The separators manufactured in Examples 1-1 to 1-10
contained fibrillated heat-resistant fibers (fibrillated
heat-resistant fibers (I)) and synthetic resin short fibers, and
the content of the fibrillated heat-resistant fibers (I) was not
less than 1.0 mass to less than 5.0 mass % based on the total of
all the fiber components contained in the separator. The separators
of Examples 1-1 to 1-10 were excellent in tensile strength and
cuttability. Further, they had a low impedance as a resistance
component and a low incidence of internal short circuit failure.
Especially the separators of Examples 1-1 to 1-9 were excellent in
cycle characteristics though they had a small thickness of not more
than 15 .mu.m.
[0125] The separators of Examples 1-6 and 1-7 contained fibers
other than the fibrillated heat-resistant fibers (I) and the
synthetic resin short fibers. From comparison between Example 1-6
and Example 1-7, the separator of Example 1-6 having a content of
the synthetic resin short fibers of not less than 90.0 mass % was
superior in strength characteristics.
[0126] Since the separators of Comparative Examples 1-1 and 1-5 did
not contain the fibrillated heat-resistant fibers (I), when the
basis weight of the separator was reduced, internal short circuit
failure was apt to occur.
[0127] The separators of Comparative Examples 1-2 and 1-3 contained
the fibrillated heat-resistant fibers (I) and the synthetic resin
short fibers, and the content of the fibrillated heat-resistant
fibers (I) was not less than 5.0 mass % based on the total of all
the fiber components contained in the separator. Therefore, tensile
strength lowered and fluff remained on the melt-down surface in the
evaluation of cuttability. Further, it was found that the linear
pressure had to be raised to control the thickness of the separator
to a predetermined value by hot calendering, thereby making it
difficult to reduce the thickness of the separator. As a result,
the impedance indicative of the resistance of the separator became
high. In Comparative Example 1-3, as tensile strength lowered, the
separator shrank in width or was broken as it was difficult to
control tension in the step of laminating an electrode.
[0128] Since the modified freeness of the fibrillated
heat-resistant fibers of the separator of Comparative Example 1-4
exceeded 300 ml, cuttability was low and fluff was slightly seen on
the melt-down surface. Since the degree of fibrillation was low,
the incidence of internal short circuit failure was apt to become
worse when the basis weight of the separator was low.
[0129] Although the separators of Examples 1-10 and Comparative
Example 1-6 had the same thickness of 18 .mu.m, the content of the
fibrillated heat-resistant fibers (I) was 4.0 mass % in the
separator of Example 1-10 and 10.0 mass % in the separator of
Comparative Example 1-6. In the case of a thick separator, even
when the content of the fibrillated heat-resistant fibers (I) was
not less than 5.0 mass %, the resistance component was not high.
Although the separators of Example 1-8 and Comparative Example 1-3
had a thickness of 10 to 11 .mu.m, the content of the fibrillated
heat-resistant fibers (I) was 4.0 mass % in the separator of
Example 1-8 and 10.0 mass % in the separator of Comparative Example
1-3. In the case of a thin separator, as the content of the
fibrillated heat-resistant fibers (I) was less than 5.0 mass %,
internal resistance was low.
Examples 2
Example 2-1
[0130] 55.5 parts by mass of oriented crystallized polyethylene
terephthalate (PET)-based synthetic resin short fibers having a
fineness of 0.06 dtex and a fiber length of 3 mm, 40.0 parts by
mass of undrawn PET-based synthetic resin short fibers for
single-component binders having a fineness of 0.2 dtex and a fiber
length of 3 mm (softening point of 120.degree. C., melting point of
230.degree. C.), 1.0 part by mass of fibrillated heat-resistant
fibers obtained by fibrillating a pulp-like product of wholly
aromatic polyamide fibers (average fiber length of 1.7 mm, average
fiber diameter of 10 .mu.m) to a modified freeness of up to 350 ml
with a high-pressure homogenizer and 3.5 parts by mass of
fibrillated heat-resistant fibers obtained by fibrillating a
pulp-like product of wholly aromatic polyamide fibers to a modified
freeness of up to 50 ml with a high-pressure homogenizer were
dispersed in water with a pulper to prepare a homogeneous
papermaking slurry having a concentration of 0.5 mass %, and a wet
paper web was obtained by using an inclination type paper machine
and dried with a cylinder drier having a surface temperature of
135.degree. C. to obtain a sheet. The obtained sheet was calendered
with a hot calender having a steel roll plated with chromium and
having a surface temperature of 195.degree. C. as one of the rolls
and a resin roll having a hardness of Shore D92 as the other roll
and a linear pressure of 100 kN/m to manufacture a separator having
a basis weight of 10 g/m.sup.2 and a thickness of 15 .mu.m.
Example 2-2
[0131] A separator having a basis weight of 10 g/m- and a thickness
of 15 .mu.m was manufactured in the same manner as in Example 2-1
except that 55.5 parts by mass of oriented crystallized PET-based
synthetic resin short fibers having a fineness of 0.06 dtex and a
fiber length of 3 mm, 40.0 parts by mass of undrawn PET-based
synthetic resin short fibers for binders having a fineness of 0.2
dtex and a fiber length of 3 mm, 3.5 parts by mass of fibrillated
wholly aromatic polyamide fibers having a modified freeness of 350
ml and 1.0 part by mass of fibrillated wholly aromatic polyamide
fibers having a modified freeness of 50 ml were used.
Example 2-3
[0132] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 2-1 except that 58.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 1.0 part by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml and 1.0 part by mass of fibrillated wholly
aromatic polyamide fibers having a modified freeness of 50 ml were
used.
Example 2-4
[0133] A separator having a basis weight of 9 g/m.sup.2 and a
thickness of 13 .mu.m was manufactured in the same manner as in
Example 2-1 except that 54.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 2.5 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml and 3.5 parts by mass of fibrillated wholly
aromatic polyamide fibers having a modified freeness of 50 ml were
used.
Example 2-5
[0134] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that 50.2 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 3.9 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml and 4.9 parts by mass of fibrillated wholly
aromatic polyamide fibers having a modified freeness of 50 ml were
used and hot calendering was carried out at a steel roll surface
temperature of 195.degree. C. and a linear pressure of 115 kN/m to
make the separator uniform in thickness.
Example 2-6
[0135] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that 53.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 2.5 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml and 3.5 parts by mass of fibrillated polyarylate
fibers obtained by fibrillating cut fibers (average fiber length of
3.0 mm, average fiber diameter of 20 .mu.m) of polyarylate fibers
(wholly aromatic polyester fibers) to a modified freeness of up to
250 ml with a high-pressure homogenizer were used.
Example 2-7
[0136] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that 51.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 2.5 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml and 5.0 parts by mass of fibrillated wholly
aromatic polyamide fibers having a modified freeness of 50 ml were
used and hot calendering was carried out at a steel roll surface
temperature of 195.degree. C. and a linear pressure of 120 kN/m to
make the separator uniform in thickness.
Example 2-8
[0137] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that 51.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 4.0 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml and 3.5 parts by mass of fibrillated
heat-resistant fibers having a modified freeness of 50 ml were used
and hot calendering was carried out at a steel roll surface
temperature of 195.degree. C. and a linear pressure of 125 kN/m to
make the separator uniform in thickness.
Example 2-9
[0138] A separator having a basis weight of 9 g/m.sup.2 and a
thickness of 13 .mu.m was manufactured in the same manner as in
Example 2-1 except that 50.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 3.5 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml, 4.5 parts by mass of fibrillated wholly
aromatic polyamide fibers having a modified freeness of 50 ml and
2.0 parts by mass of beaten solvent spun cellulose fibers having a
modified freeness of 90 ml obtained by refining solvent spun
cellulose fibers having an average fiber diameter of 10 .mu.m and a
fiber length of 4 mm with a refiner were used.
Example 2-10
[0139] A separator having a basis weight of 9 g/m.sup.2 and a
thickness of 13 .mu.m was manufactured in the same manner as in
Example 2-1 except that 49.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 3.5 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml, 4.5 parts by mass of fibrillated wholly
aromatic polyamide fibers having a modified freeness of 50 ml and
3.0 parts by mass of beaten solvent spun cellulose fibers having a
modified freeness of 90 ml obtained by refining solvent spun
cellulose fibers having an average fiber diameter of 10 .mu.m and a
fiber length of 4 mm with a refiner were used.
Comparative Example 2-1
[0140] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 2-1 except that 60.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm and 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm were used.
Comparative Example 2-2
[0141] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that hot calendering was carried out at a steel
roll surface temperature of 195.degree. C. and a linear pressure of
90 kN/m by using the papermaking slurry of Comparative Example 2-1
to make the separator uniform in thickness.
Comparative Example 2-3
[0142] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that 50.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm and 8.9 parts by
mass of fibrillated wholly aromatic polyamide fibers having a
modified freeness of 350 ml were used and hot calendering was
carried out at a steel roll surface temperature of 195.degree. C.
and a linear pressure of 175 kN/m to make the separator uniform in
thickness.
Comparative Example 2-4
[0143] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that 50.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm and 8.9 parts by
mass of fibrillated heat-resistant fibers having a modified
freeness of 50 ml were used and hot calendering was carried out at
a steel roll surface temperature of 195.degree. C. and a linear
pressure of 160 kN/m to make the separator uniform in
thickness.
Comparative Example 2-5
[0144] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 2-1 except that 50.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.8 parts by mass of undrawn
PET-based synthetic resin short fibers for binders having a
fineness of 0.2 dtex and a fiber length of 3 mm, 4.1 parts by mass
of fibrillated wholly aromatic polyamide fibers having a modified
freeness of 350 ml and 5.1 parts by mass of fibrillated
heat-resistant fibers having a modified freeness of 50 ml were used
and hot calendering was carried out at a steel roll surface
temperature of 195.degree. C. and a linear pressure of 140 kN/m to
make the separator uniform in thickness.
TABLE-US-00005 TABLE 3 Ex. Ex. Ex. Ex. Ex. 2-1 2-2 2-3 2-4 2-5
Fiber Oriented crystallized PET-based synthetic pbm 55.5 55.5 58.0
54.0 50.2 blending resin short fibers 0.06 dtex 3 mm Undrawn
PET-based synthetic resin short pbm 40.0 40.0 40.0 40.0 41.0 fibers
0.2 dtex 3 mm Fibrillated heat-resistant fibers (II) pbm 1.0 3.5
1.0 2.5 3.9 having a modified freeness of more than 300 ml
Fibrillated heat-resistant fibers (I) pbm 3.5 1.0 1.0 3.5 4.9
having a modified freeness of not more than 300 ml Beaten solvent
spun cellulose fibers pbm 0.0 0.0 0.0 0.0 0.0 Fibrillated Modified
freeness ml 350 350 350 350 350 heat- Mass-weighted average fiber
length mm 1.30 1.30 1.30 1.30 1.30 resistant Length-weighted
average fiber length mm 0.59 0.59 0.59 0.59 0.59 fibers (II) Fiber
width .mu.m 25.2 25.2 25.2 25.2 25.2 Fibrillated Modified freeness
ml 50 50 50 50 50 heat- Mass-weighted average fiber length mm 0.58
0.58 0.58 0.58 0.58 resistant Length-weighted average fiber length
mm 0.33 0.33 0.33 0.33 0.33 fibers (I) Fiber width .mu.m 15.5 15.5
15.5 15.5 15.5 Linear pressure of hot calender kN/m 100 100 100 100
100 Evaluation Weight of separator g/m.sup.2 10 10 10 9 7 Thickness
of separator .mu.m 15 15 15 13 10 Tensile strength of separator --
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.largecircle. Cuttability of separator -- .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. Resistance
component (impedance) .OMEGA. 0.55 0.57 0.54 0.52 0.47 Incidence of
internal short circuit failure % 0.0 0.0 0.0 0.0 0.0 Cycle
characteristics % 90 88 88 90 85 Ex. Ex. Ex. Ex. Ex. 2-6 2-7 2-8
2-9 2-10 Fiber Oriented crystallized PET-based synthetic pbm 53.0
51.5 51.5 50.0 49.0 blending resin short fibers 0.06 dtex 3 mm
Undrawn PET-based synthetic resin short pbm 41.0 41.0 41.0 40.0
40.0 fibers 0.2 dtex 3 mm Fibrillated heat-resistant fibers (II)
pbm 2.5 2.5 4.0 3.5 3.5 having a modified freeness of more than 300
ml Fibrillated heat-resistant fibers (I) pbm 3.5 5.0 3.5 4.5 4.5
having a modified freeness of not more than 300 ml Beaten solvent
spun cellulose fibers pbm 0.0 0.0 0.0 2.0 3.0 Fibrillated Modified
freeness ml 350 350 350 350 350 heat- Mass-weighted average fiber
length mm 1.30 1.30 1.30 1.30 1.30 resistant Length-weighted
average fiber length mm 0.59 0.59 0.59 0.59 0.59 fibers (II) Fiber
width .mu.m 25.2 25.2 25.2 25.2 25.2 Fibrillated Modified freeness
ml 250 50 50 50 50 heat- Mass-weighted average fiber length mm 0.82
0.58 0.58 0.58 0.58 resistant Length-weighted average fiber length
mm 0.45 0.33 0.33 0.33 0.33 fibers (I) Fiber width .mu.m 19.5 15.5
15.5 15.5 15.5 Linear pressure of hot calender kN/m 100 120 125 100
100 Evaluation Weight of separator g/m.sup.2 7 7 7 9 9 Thickness of
separator .mu.m 10 10 10 13 13 Tensile strength of separator --
.largecircle. .largecircle. .largecircle. .largecircle. .DELTA.
Cuttability of separator -- .largecircle. .DELTA. .DELTA.
.largecircle. .largecircle. Resistance component (impedance)
.OMEGA. 0.45 0.49 0.50 0.50 0.49 Incidence of internal short
circuit failure % 0.0 0.0 0.0 0.0 0.0 Cycle characteristics % 85 85
85 90 90 Ex.: Example pbm: part by mass Ex.: Example
TABLE-US-00006 TABLE 4 C. Ex. C. Ex. C. Ex. C. Ex. C. Ex. 2-1 2-2
2-3 2-4 2-5 Fiber Oriented crystallized PET-based synthetic pbm
60.0 60.0 50.1 50.1 50.0 blending resin short fibers 0.06 dtex 3 mm
Undrawn PET-based synthetic resin short pbm 40.0 40.0 41.0 41.0
40.8 fibers 0.2 dtex 3 mm Fibrillated heat-resistant fibers (II)
pbm 0.0 0.0 8.9 0.0 4.1 having a modified freeness of more than 300
ml Fibrillated heat-resistant fibers (I) pbm 0.0 0.0 0.0 8.9 5.1
having a modified freeness of not more than 300 ml Beaten solvent
spun cellulose fibers pbm 0.0 0.0 0.0 0.0 0.0 Fibrillated Modified
freeness ml -- -- 350 -- 350 heat- Mass-weighted average fiber
length mm -- -- 1.30 -- 1.30 resistant Length-weighted average
fiber length mm -- -- 0.59 -- 0.59 fibers (II) Fiber width .mu.m --
-- 25.2 -- 25.2 Fibrillated Modified freeness ml -- -- -- 50 50
heat- Mass-weighted average fiber length mm -- -- -- 0.58 0.58
resistant Length-weighted average fiber length mm -- -- -- 0.33
0.33 fibers (I) Fiber width .mu.m -- -- -- 15.5 15.5 Linear
pressure of hot calender kN/m 100 90 175 160 140 Evaluation Weight
of separator g/m.sup.2 10 7 7 7 7 Thickness of separator .mu.m 15
10 10 10 10 Tensile strength of separator -- .circleincircle.
.largecircle. .DELTA. .DELTA. .DELTA. Cuttability of separator --
.largecircle. .largecircle. X X X Resistance component (impedance)
.OMEGA. 0.54 0.39 0.73 0.60 0.58 Incidence of internal short
circuit failure % 0.5 1.5 0.0 0.0 0.0 Cycle characteristics % 85 70
65 70 75 C. Ex.: Comparative Example pbm: part by mass C. Ex.:
Comparative Example
[0145] The separators manufactured in Examples 2-1 to 2-10
contained fibrillated heat-resistant fibers and synthetic resin
short fibers, and the total content of the fibrillated
heat-resistant fibers having a modified freeness of more than 300
ml (fibrillated heat-resistant fibers (II)) and the fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml (fibrillated heat-resistant fibers (I)) was not less than
2.0 mass % to less than 9.0 mass % based on the total of all the
fiber components contained in the separator. The separators of
Examples 2-1 to 2-10 were excellent in tensile strength and
cuttability. They had a low impedance as a resistance component and
a low incidence of internal short circuit failure and were
excellent in cycle characteristics though they had a small
thickness of not more than 15 .mu.m.
[0146] Out of the separators manufactured in Examples 2-1 to 2-10,
the separators of Examples 2-1 to 2-6 and Examples 2-9 and 2-10
which had a total content of the fibrillated heat-resistant fibers
(I) and the fibrillated heat-resistant fibers (II) of not less than
2.0 mass % to less than 9.0 mass %, a content of the fibrillated
heat-resistant fibers (II) of not less than 1.0 mass % to less than
4.0 mass % and a content of the fibrillated heat-resistant fibers
(I) of not less than 1.0 mass % to less than 5.0 mass % were
excellent in curability.
[0147] Out of the separators manufactured in Examples 2-1 to 2-10,
the separator of Example 2-7 contained the fibrillated
heat-resistant fibers (I), the fibrillated heat-resistant fibers
(II) and the synthetic resin short fibers and had a total content
of the fibrillated heat-resistant fibers (I) and the fibrillated
heat-resistant fibers (II) of not less than 2.0 mass % to less than
9.0 mass % and a content of the fibrillated heat-resistant fibers
(I) of not less than 5.0 mass % based on the total of all the fiber
components contained in the separator. Therefore, in the evaluation
of cuttability, fluff slightly remained on the edge surface, and it
was found that the linear pressure had to be raised at the time of
controlling the thickness of the separator to a predetermined value
by hot calendering, thereby making it difficult to reduce the
thickness of the separator. As a result, the impedance indicative
of the resistance component of the separator was the same as that
of a separator having a basis weight of 9 g/m.sup.2 though the
basis weight of the separator was 7 g/m.
[0148] Out of the separators manufactured in Examples 2-1 to 2-10,
the separator of Example 2-8 contained the fibrillated
heat-resistant fibers (I), the fibrillated heat-resistant fibers
(II) and the synthetic resin short fibers and had a total content
of the fibrillated heat-resistant fibers (I) and the fibrillated
heat-resistant fibers (II) of not less than 2.0 mass % to less than
9.0 mass % but a content of the fibrillated heat-resistant fibers
(II) of not less than 4.0 mass % based on the total of all the
fiber components contained in the separator. From comparison
between Example 2-5 and Example 2-8, when the fibrillated
heat-resistant fibers (II) were contained in an amount of not less
than 4.0 mass %, a large number of thick stem fibers remained,
whereby a large amount of fluff remained on the edge surface in the
evaluation of cuttability. Further, it was found that the linear
pressure had to be raised at the time of controlling the thickness
of the separator to a predetermined value by hot calendering,
thereby making it difficult to reduce the thickness of the
separator. As a result, the impedance indicative of the resistance
component of the separator was the same as that of a separator
having a basis weight of 9 g/m.sup.2 though the basis weight of the
separator was 7 g/m.sup.2.
[0149] Out of the separators manufactured in Examples 2-1 to 2-10,
the separators of Examples 2-9 and 2-10 contained fibers other than
the fibrillated heat-resistant fibers (I) and (II) and the
synthetic resin short fibers. From comparison between Example 2-9
and Example 2-10, the separator of Example 2-9 having a content of
the synthetic resin short fibers of not less than 90.0 mass % was
superior in strength characteristics to the separator of Example
2-10 having a content of the synthetic resin short fibers of less
than 90.0 mass %.
[0150] Since the separators of Comparative Examples 2-1 and 2-2 did
not contain fibrillated heat-resistant fibers, when the basis
weight of the separator was reduced, internal short circuit failure
was apt to occur. Although the separators of Examples 2-1 to 2-3
had the same basis weight and the same thickness as the separator
of Comparative Example 2-1, the cycle characteristics of
Comparative Example 2-1 deteriorated. Although the separators of
Examples 2-5 to 2-8 had the same basis weight and the same
thickness as the separator of Comparative Example 2-2, the cycle
characteristics of Comparative Example 2-2 deteriorated as
well.
[0151] The separators of Comparative Examples 2-3 contained only
the fibrillated heat-resistant fibers (II), the separator of
Comparative Example 2-4 contained only the fibrillated
heat-resistant fibers (I) as the heat-resistant fibers, and both of
the separators had a content of the fibrillated heat-resistant
fibers of 8.9 mass %. Although the content of the fibrillated
heat-resistant fibers was not less than 2.0 mass % to less than 9.0
mass %, when the separator contained only one of the two different
types of fibrillated heat-resistant fibers, tensile strength
lowered and fluff remained on the edge surface in the evaluation of
cuttability. Further, it was found that the linear pressure had to
be raised at the time of controlling the thickness of the separator
to a predetermined value by hot calendering, thereby making it
difficult to reduce the thickness of the separator. As a result,
the impedance indicative of the resistance component and cycle
characteristics of the separator deteriorated.
[0152] The separator of Comparative Example 2-5 had a total content
of the fibrillated heat-resistant fibers (II) and the fibrillated
heat-resistant fibers (I) of not less than 9.0 mass %, a content of
the fibrillated heat-resistant fibers (II) of not less than 4.0
mass % and a content of the fibrillated heat-resistant fibers (I)
of not less than 5.0 mass % based on the total of all the fiber
components contained in the separator. In this case, tensile
strength lowered and a large amount of fluff remained on the edge
surface in the evaluation of cuttability. Further, it was found
that the linear pressure had to be raised at the time of
controlling the thickness of the separator to a predetermined value
by hot calendering, thereby making it difficult to reduce the
thickness of the separator. As a result, the impedance indicative
of the resistance component and cycle characteristics of the
separator deteriorated.
Examples 3
Example 3-1
[0153] 55.5 parts by mass of oriented crystallized polyethylene
terephthalate (PET)-based synthetic resin short fibers having a
fineness of 0.06 dtex and a fiber length of 3 mm, 40.0 parts by
mass of undrawn PET-based synthetic resin short fibers for
single-component binders having a fineness of 0.2 dtex and a fiber
length of 3 mm (softening point of 120.degree. C., melting point of
230.degree. C.), 1.0 part by mass of a fibrid made of para-aromatic
polyamide having a modified freeness of 88 ml obtained by
defibrating and beating with a high-speed homogenizer and 3.5 parts
by mass of fibrillated heat-resistant fibers obtained by
fibrillating a pulp-like product of para-aromatic polyamide fibers
to a modified freeness of up to 50 ml with a high-pressure
homogenizer as heat-resistant fibers were dispersed in water with a
pulper to prepare a homogeneous papermaking slurry having a
concentration of 0.1 mass %, and a wet paper web was obtained by
using an inclination type paper machine and dried with a cylinder
drier having a surface temperature of 135.degree. C. to obtain a
sheet. The obtained sheet was thermally calendered with a hot
calender having a steel roll plated with chromium and having a
surface temperature of 195.degree. C. as one of the rolls and a
resin roll having a hardness of Shore D92 as the other roll and a
linear pressure of 100 kN/m to produce a separator having a basis
weight of 10 g/m.sup.2 and a thickness of 15 .mu.m.
Example 3-2
[0154] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 3-1 except that 55.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.0 parts
by mass of the fibrid used in Example 3-1 and 2.5 parts by mass of
the fibrillated heat-resistant fibers used in Example 3-1 were
used.
Example 3-3
[0155] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 3-1 except that 55.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.5 parts
by mass of the fibrid used in Example 3-1 and 2.0 parts by mass of
the fibrillated heat-resistant fibers used in Example 3-1 were
used.
Example 3-4
[0156] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 3-1 except that 55.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 3.5 parts
by mass of the fibrid used in Example 3-1 and 1.0 part by mass of
the fibrillated heat-resistant fibers used in Example 3-1 were
used.
Example 3-5
[0157] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 3-1 except that 59.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 0.4 part
by mass of the fibrid used in Example 3-1 and 0.6 part by mass of
the fibrillated heat-resistant fibers used in Example 3-1 were
used.
Example 3-6
[0158] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 3-1 except that 56.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 1.0 part
by mass of the fibrid used in Example 3-1 and 2.5 parts by mass of
the fibrillated heat-resistant fibers used in Example 3-1 were
used.
Example 3-7
[0159] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 3-1 except that 53.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 42.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 1.0 part
by mass of the fibrid used in Example 3-1 and 3.9 parts by mass of
the fibrillated heat-resistant fibers used in Example 3-1 were
used.
Example 3-8
[0160] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 3-1 except that 48.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 42.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.0 parts
by mass of the fibrid used in Example 3-1, 2.5 parts by mass of the
fibrillated heat-resistant fibers used in Example 3-1 and 5.5 parts
by mass of beaten solvent spun cellulose fibers having a modified
freeness of 90 ml obtained by refining solvent spun cellulose
fibers having an average fiber diameter of 10 .mu.m and a fiber
length of 4 mm with a refiner were used.
Example 3-9
[0161] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 3-1 except that 47.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 42.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.0 parts
by mass of the fibrid used in Example 3-1, 2.5 parts by mass of the
fibrillated heat-resistant fibers used in Example 3-1 and 6.5 parts
by mass of the beaten solvent spun cellulose fibers used in Example
3-8 were used.
Comparative Example 3-1
[0162] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 3-1 except that 60.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm and 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.) were
used.
Comparative Example 3-2
[0163] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 3-1 except that 60.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm and 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.) were used
and hot calendering was carried out at a steel roll surface
temperature of 195.degree. C. and a linear pressure of 90 kN/m to
make the separator uniform in thickness.
Comparative Example 3-3
[0164] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 3-1 except that 53.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 42.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.5 parts
by mass of the fibrid used in Example 3-1 and 2.5 parts by mass of
the fibrillated heat-resistant fibers used in Example 3-1 were
used.
Comparative Example 3-4
[0165] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 3-1 except that 55.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.), 2.0
parts by mass of a fibrid made of para-aromatic polyamide having a
modified freeness of 310 ml and 2.5 parts by mass of fibrillated
heat-resistant fibers having a modified freeness of 320 ml were
used and hot calendering was carried out at a steel roll surface
temperature of 195.degree. C. and a linear pressure of 120 kN/m to
make the separator uniform in thickness.
TABLE-US-00007 TABLE 5 Ex. Ex. Ex. Ex. Ex. 3-1 3-2 3-3 3-4 3-5
Fiber Oriented crystallized PET-based synthetic pbm 55.5 55.5 55.5
55.5 59.0 blending resin short fibers 0.06 dtex 3 mm Undrawn
PET-based synthetic resin short pbm 40.0 40.0 40.0 40.0 40.0 fibers
0.2 dtex 3 mm fibrid made of para-aromatic polyamide pbm 1.0 2.0
2.5 3.5 0.4 Fibrillated heat-resistant fibers made of pbm 3.5 2.5
2.0 1.0 0.6 para-aromatic polyamide beaten solvent spun cellulose
fibers pbm 0.0 0.0 0.0 0.0 0.0 Content of heat-resistant fibers
having a mass % 4.5 4.5 4.5 4.5 1.0 modified freeness of not more
than 300 ml Fibrid Modified freeness ml 88 88 88 88 88
Mass-weighted average fiber length mm 0.52 0.52 0.52 0.52 0.52
Length-weighted average fiber length mm 0.32 0.32 0.32 0.32 0.32
Fiber width .mu.m 24.1 24.1 24.1 24.1 24.1 Fibrillated Modified
freeness ml 50 50 50 50 50 heat- Mass-weighted average fiber length
mm 0.58 0.58 0.58 0.58 0.58 resistant Length-weighted average fiber
length mm 0.33 0.33 0.33 0.33 0.33 fibers Fiber width .mu.m 15.5
15.5 15.5 15.5 15.5 Linear pressure of hot calender kN/m 100 100
100 100 100 Evaluation Weight of separator g/m.sup.2 10 10 10 10 10
Thickness of separator .mu.m 15 15 15 15 15 Tensile strength of
separator -- .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .circleincircle. Cuttability of separator --
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. Resistance component (impedance) .OMEGA. 0.55 0.57
0.59 0.60 0.55 Incidence of internal short circuit failure % 0.0
0.0 0.0 0.0 0.0 Cycle characteristics % 90 88 83 80 88 Ex.: Example
pbm: part by mass Ex.: Example
TABLE-US-00008 TABLE 6 Ex. Ex. Ex. Ex. 3-6 3-7 3-8 3-9 Fiber
Oriented crystallized PET-based synthetic pbm 56.5 53.1 48.0 47.0
blending resin short fibers 0.06 dtex 3 mm Undrawn PET-based
synthetic resin short pbm 40.0 42.0 42.0 42.0 fibers 0.2 dtex 3 mm
fibrid made of para-aromatic polyamide pbm 1.0 1.0 2.0 2.0
Fibrillated heat-resistant fibers made of pbm 2.5 3.9 2.5 2.5
para-aromatic polyamide beaten solvent spun cellulose fibers pbm
0.0 0.0 5.5 6.5 Content of heat-resistant fibers having a mass %
3.5 4.9 4.5 4.5 modified freeness of not more than 300 ml Fibrid
Modified freeness ml 88 88 88 88 Mass-weighted average fiber length
mm 0.52 0.52 0.52 0.52 Length-weighted average fiber length mm 0.32
0.32 0.32 0.32 Fiber width .mu.m 24.1 24.1 24.1 24.1 Fibrillated
Modified freeness ml 50 50 50 50 heat- Mass-weighted average fiber
length mm 0.58 0.58 0.58 0.58 resistant Length-weighted average
fiber length mm 0.33 0.33 0.33 0.33 fibers Fiber width .mu.m 15.5
15.5 15.5 15.5 Linear pressure of hot calender kN/m 100 100 100 100
Evaluation Weight of separator g/m.sup.2 8 7 8 8 Thickness of
separator .mu.m 12 10 12 12 Tensile strength of separator --
.largecircle. .largecircle. .largecircle. .DELTA. Cuttability of
separator -- .largecircle. .largecircle. .largecircle.
.largecircle. Resistance component (impedance) .OMEGA. 0.48 0.43
0.48 0.47 Incidence of internal short circuit failure % 0.0 0.0 0.0
0.0 Cycle characteristics % 88 85 88 88 Ex.: Example pbm: part by
mass Ex.: Example
TABLE-US-00009 TABLE 7 C. Ex. C. Ex. C. Ex. C. Ex. 3-1 3-2 3-3 3-4
Fiber Oriented crystallized PET-based synthetic pbm 60.0 60.0 53.0
55.5 blending resin short fibers 0.06 dtex 3 mm Undrawn PET-based
synthetic resin short pbm 40.0 40.0 42.0 40.0 fibers 0.2 dtex 3 mm
fibrid made of para-aromatic polyamide pbm 0.0 0.0 2.5 2.0
Fibrillated heat-resistant fibers made of pbm 0.0 0.0 2.5 2.5
para-aromatic polyamide beaten solvent spun cellulose fibers pbm 0
0 0 0 Content of heat-resistant fibers having a mass % 0.0 0.0 5.0
0.0 modified freeness of not more than 300 ml Fibrid Modified
freeness ml -- -- 88 310 Mass-weighted average fiber length mm --
-- 0.52 0.78 Length-weighted average fiber length mm -- -- 0.32
0.56 Fiber width .mu.m -- -- 24.1 31.2 Fibrillated Modified
freeness ml -- -- 50 320 heat- Mass-weighted average fiber length
mm -- -- 0.58 1.30 resistant Length-weighted average fiber length
mm -- -- 0.33 0.59 fibers Fiber width .mu.m -- -- 15.5 25.2 Linear
pressure of hot calender kN/m 100 80 100 100 Evaluation Weight of
separator g/m.sup.2 10 8 7 8 Thickness of separator .mu.m 15 12 10
12 Tensile strength of separator -- .circleincircle.
.circleincircle. .DELTA. .DELTA. Cuttability of separator --
.largecircle. .largecircle. .largecircle. .DELTA. Resistance
component (impedance) .OMEGA. 0.54 0.43 0.50 0.50 Incidence of
internal short circuit failure % 0.5 1.0 0.0 0.5 Cycle
characteristics % 85 70 75 70 C. Ex.: Comparative Example pbm: part
by mass C. Ex.: Comparative Example
[0166] The separators manufactured in Examples 3-1 to 3-9 were
lithium ion battery separators containing heat-resistant fibers and
synthetic resin short fibers, which contained a fibrid made of
para-aromatic polyamide and fibrillated heat-resistant fibers
(fibrillated heat-resistant fibers (I)) having a modified freeness
of not more than 300 ml as the heat-resistant fibers and had a
total content of the heat-resistant fibers of not less than 1.0
mass % to less than 5.0 mass based on the total of all the fiber
components contained in the separator. The separators of Examples
3-1 to 3-9 had high tensile strength and excellent cuttability.
They had a low impedance as the resistance component and a low
incidence of internal short circuit failure. They were excellent in
cycle characteristics though they had a small thickness of not more
than 15 .mu.m.
[0167] The separators of Examples 3-6 and 3-7 had content ratios of
the fibrillated heat-resistant fibers (I) of 71 mass % and 80 mass
% based on the total of all the heat-resistant fibers having a
modified freeness of not more than 300 ml, respectively. The number
of fiber networks increased, the fibrid firmly bonded these fiber
networks, the impedance as the resistance component was low though
the basis weights of the separators were 8 g/m.sup.2 and 7
g/m.sup.2, respectively, the incidence of internal short circuit
failure was satisfactory, and strength characteristics could be
retained.
[0168] It was found from comparison between Examples 3-1 to 3-2 and
Examples 3-3 to 3-4 that, when the basis weight of the separator
was 10 g/m.sup.2, Examples 3-1 and 3-2 having higher content ratios
of the fibrillated heat-resistant fibers (I) based on the total of
all the heat-resistant fibers having a modified freeness of not
more than 300 ml had a lower impedance as the resistance component
and more excellent cycle characteristics.
[0169] The separators of Examples 3-8 and 3-9 contained fibers
other than the fibrillated heat-resistant fibers having a modified
freeness of not more than 300 ml and the synthetic resin short
fibers. From comparison between Example 3-8 and Example 3-9, the
separator of Example 3-8 having a content of the synthetic resin
short fibers of not less than 90.0 mass % was superior in strength
characteristics.
[0170] Since the separators of Comparative Examples 3-1 and 3-2 did
not contain heat-resistant fibers having a modified freeness of not
more than 300 ml, when the basis weight of the separator was
reduced, internal short circuit failure was apt to occur.
[0171] The separator of Comparative Example 3-3 which had a basis
weight of 7 g/m.sup.2 and contained not less than 5 mass % of
heat-resistant fibers having a modified freeness of not more than
300 ml had a higher impedance as the resistance component and lower
strength and cycle characteristics than those of Example 3-7.
[0172] Since the modified freeness's of the fibrid and the
fibrillated heat-resistant fibers of the separator of Comparative
Example 3-4 exceeded 300 ml, the fiber width of the fibrid and the
fiber diameter of the stem part of each of the fibrillated
heat-resistant fibers were large, whereby the dispersibility of the
fibers and entanglement between the fibers became worse, thereby
deteriorating the texture of the separator. As a result, the
strength characteristics, cuttability, impedance as the resistance
component and incidence of internal short circuit failure of the
separator became worse. The cycle characteristics of the separator
also became worse.
Examples 4
Example 4-1
[0173] 55.5 parts by mass of oriented crystallized polyethylene
terephthalate (PET)-based synthetic resin short fibers having a
fineness of 0.06 dtex and a fiber length of 3 mm, 40.0 parts by
mass of undrawn PET-based synthetic resin short fibers for
single-component binders having a fineness of 0.2 dtex and a fiber
length of 3 mm (softening point of 120.degree. C., melting point of
230.degree. C.), 1.0 part by mass of a fibrid made of para-aromatic
polyamide having a modified freeness of 88 ml obtained by
defibrating and beating with a high-speed homogenizer, 1.0 part by
mass of fibrillated heat-resistant fibers (fibrillated
heat-resistant fibers (II)) obtained by fibrillating a pulp-like
product (average fiber length of 1.7 mm, average fiber diameter of
10 .mu.m) of para-aromatic polyamide fibers to a modified freeness
of up to 350 ml with a high-pressure homogenizer and 2.5 parts by
mass of fibrillated heat-resistant fibers (fibrillated
heat-resistant fibers (I)) obtained by fibrillating a pulp-like
product (average fiber length of 1.7 mm, average fiber diameter of
10 .mu.m) of para-aromatic polyamide fibers to a modified freeness
of up to 50 ml with a high-pressure homogenizer as heat-resistant
fibers were dispersed in water with a pulper to prepare a
homogeneous papermaking slurry having a concentration of 0.1 mass
%, and a wet paper web was obtained by using an inclination type
paper machine and dried with a cylinder drier having a surface
temperature of 135.degree. C. to obtain a sheet. The obtained sheet
was calendered with a hot calender having a steel roll plated with
chromium and having a surface temperature of 195.degree. C. as one
of the rolls and a resin roll having a hardness of Shore D92 as the
other roll and a linear pressure of 100 kN/m to produce a separator
having a basis weight of 10 g/m.sup.2 and a thickness of 15
.mu.m.
Example 4-2
[0174] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 4-1 except that 55.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 0.4 part
by mass of the fibrid used in Example 4-1, 3.5 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 0.6
part by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-3
[0175] A separator having a basis weight of 10 g/m.sup.2 and a
thickness of 15 .mu.m was manufactured in the same manner as in
Example 4-1 except that 58.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 0.4 part
by mass of the fibrid used in Example 4-1, 1.0 part by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 0.6
part by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-4
[0176] A separator having a basis weight of 9 g/m.sup.2 and a
thickness of 13 .mu.m was manufactured in the same manner as in
Example 4-1 except that 55.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 1.0 part
by mass of the fibrid used in Example 4-1, 1.5 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 2.5
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-5
[0177] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.6 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.0 parts
by mass of the fibrid used in Example 4-1, 3.9 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 2.5
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-6
[0178] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 4-1 except that 51.6 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 0.4 part
by mass of the fibrid used in Example 4-1, 2.5 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 4.5
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-7
[0179] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 54.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 0.8 part
by mass of the fibrid used in Example 4-1, 1.2 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 2.5
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-8
[0180] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 51.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.5 parts
by mass of the fibrid used in Example 4-1, 2.5 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 2.5
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-9
[0181] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.0 parts
by mass of the fibrid used in Example 4-1, 4.0 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 2.5
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used and hot calendering was carried out at a
steel roll surface temperature of 195.degree. C. and a linear
pressure of 110 kN/m to make the separator uniform in
thickness.
Example 4-10
[0182] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 51.5 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 0.4 part
by mass of the fibrid used in Example 4-1, 2.5 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 4.6
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used.
Example 4-11
[0183] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.4 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.5 parts
by mass of the fibrid used in Example 4-1, 1.5 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 4.6
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used and hot calendering was carried out at a
steel roll surface temperature of 195.degree. C. and a linear
pressure of 110 kN/m to make the separator uniform in
thickness.
Example 4-12
[0184] A separator having a basis weight of 9 g/m.sup.2 and a
thickness of 13 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.0 parts
by mass of the fibrid used in Example 4-1, 2.0 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1, 3.0
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 and 3.0 parts by mass of beaten solvent spun cellulose
fibers having a modified freeness of 90 ml obtained by refining
solvent spun cellulose fibers having an average fiber diameter of
10 .mu.m and a fiber length of 4 mm with a refiner were used.
Example 4-13
[0185] A separator having a basis weight of 9 g/m.sup.2 and a
thickness of 13 .mu.m was manufactured in the same manner as in
Example 4-1 except that 49.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.0 parts
by mass of the fibrid used in Example 4-1, 2.0 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1, 3.0
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 and 4.0 parts by mass of the beaten solvent spun
cellulose fibers used in Example 4-12 were used.
Comparative Example 4-1
[0186] A separator having a basis weight of 8 g/m.sup.2 and a
thickness of 12 .mu.m was manufactured in the same manner as in
Example 4-1 except that 60.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm and 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.) were
used.
Comparative Example 4-2
[0187] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 60.0 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm and 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.) were
used.
Comparative Example 4-3
[0188] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.9 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 40.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.4 parts
by mass of the fibrid used in Example 4-1, 3.9 parts by mass of the
fibrillated heat-resistant fibers (II) used in Example 4-1 and 2.8
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used and hot calendering was carried out at a
steel roll surface temperature of 195.degree. C. and a linear
pressure of 120 kN/m to make the separator uniform in
thickness.
Comparative Example 4-4
[0189] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.) and 8.9
parts by mass of the fibrid used in Example 4-1 were used and hot
calendering was carried out at a steel roll surface temperature of
195.degree. C. and a linear pressure of 110 kN/m to make the
separator uniform in thickness.
Comparative Example 4-5
[0190] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.) and 8.9
parts by mass of the fibrillated heat-resistant fibers (II) used in
Example 4-1 were used and hot calendering was carried out at a
steel roll surface temperature of 195.degree. C. and a linear
pressure of 120 kN/m to make the separator uniform in
thickness.
Comparative Example 4-6
[0191] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.) and 8.9
parts by mass of the fibrillated heat-resistant fibers (I) used in
Example 4-1 were used and hot calendering was carried out at a
steel roll surface temperature of 195.degree. C. and a linear
pressure of 120 kN/m to make the separator uniform in
thickness.
Comparative Example 4-7
[0192] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.), 3.9
parts by mass of the fibrid used in Example 4-1 and 5.0 parts by
mass of the fibrillated heat-resistant fibers (I) used in Example
4-1 were used and hot calendering was carried out at a steel roll
surface temperature of 195.degree. C. and a linear pressure of 120
kN/m to make the separator uniform in thickness.
Comparative Example 4-8
[0193] A separator having a basis weight of 7 g/m.sup.2 and a
thickness of 10 .mu.m was manufactured in the same manner as in
Example 4-1 except that 50.1 parts by mass of oriented crystallized
PET-based synthetic resin short fibers having a fineness of 0.06
dtex and a fiber length of 3 mm, 41.0 parts by mass of undrawn
PET-based synthetic resin short fibers for single-component binders
having a fineness of 0.2 dtex and a fiber length of 3 mm (softening
point of 120.degree. C., melting point of 230.degree. C.), 3.9
parts by mass of the fibrillated heat-resistant fibers (II) used in
Example 4-1 and 5.0 parts by mass of the fibrillated heat-resistant
fibers (I) used in Example 4-1 were used and hot calendering was
carried out at a steel roll surface temperature of 195.degree. C.
and a linear pressure of 120 kN/m to make the separator uniform in
thickness.
TABLE-US-00010 TABLE 8 Ex. Ex. Ex. Ex. Ex. Ex. 4-1 4-2 4-3 4-4 4-5
4-6 Fiber Oriented crystallized PET-based synthetic pbm 55.5 55.5
58.0 55.0 50.6 51.6 blending resin short fibers 0.06 dtex 3 mm
Undrawn PET-based synthetic resin short pbm 40.0 40.0 40.0 40.0
41.0 41.0 fibers 0.2 dtex 3 mm Fibrid made of para-aromatic
polyamide pbm 1.0 0.4 0.4 1.0 2.0 0.4 Fibrillated heat-resistant
fibers (II) pbm 1.0 3.5 1.0 1.5 3.9 2.5 having a modified freeness
of more than 300 ml Fibrillated heat-resistant fibers (I) pbm 2.5
0.6 0.6 2.5 2.5 4.5 having a modified freeness of not more than 300
ml Heat resistant fibers pbm 4.5 4.5 2.0 5.0 0.4 7.4 Beaten solvent
span cellulose fibers pbm 0.0 0.0 0.0 0.0 0.0 0.0 Fibrid Modified
freeness ml 88 88 88 88 88 88 Mass-weighted average fiber length mm
0.52 0.52 0.52 0.52 0.52 0.52 Length-weighted average fiber length
mm 0.32 0.32 0.32 0.32 0.32 0.32 Fiber width .mu.m 24.1 24.1 24.1
24.1 24.1 24.1 Fibrillated Modified freeness ml 350 350 350 350 350
350 heat-resistant Mass-weighted average fiber length mm 1.30 1.30
1.30 1.30 1.30 1.30 fibers (II) Length-weighted average fiber
length mm 0.59 0.59 0.59 0.59 0.59 0.59 having a Fiber width .mu.m
25.2 25.2 25.2 25.2 25.2 25.2 modified freeness of more than 300 ml
Fibrillated Modified freeness ml 50 50 50 50 50 50 heat-resistant
Mass-weighted average fiber length mm 0.58 0.58 0.58 0.58 0.58 0.58
fibers (I) Length-weighted average fiber length mm 0.33 0.33 0.33
0.33 0.33 0.33 having a Fiber width .mu.m 15.5 15.5 15.5 15.5 15.5
15.5 modified freeness of not more than 300 ml Linear pressure of
hot calender kN/m 100 100 100 100 100 100 Evaluation Weight of
separator g/m.sup.2 10 10 10 9 8 8 Thickness of separator .mu.m 15
15 15 13 12 12 Tensile strength of separator -- .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .largecircle.
.largecircle. Cuttability of separator -- .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. Resistance component (impedance) .OMEGA. 0.55 0.57
0.54 0.53 0.52 0.50 Incidence of internal short circuit failure %
0.0 0.0 0.0 0.0 0.0 0.0 Cycle characteristics % 90 88 90 90 83 85
Ex.: Example pbm: part by mass Ex.: Example
TABLE-US-00011 TABLE 9 Ex. Ex. Ex. Ex. Ex. Ex. Ex. 4-7 4-8 4-9 4-10
4-11 4-12 4-13 Fiber Oriented crystallized PET-based synthetic pbm
54.5 51.5 50.5 51.5 50.4 50.0 49.0 blending resin short fibers 0.06
dtex 3 mm Undrawn PET-based synthetic resin short pbm 41.0 41.0
41.0 41.0 41.0 40.0 40.0 fibers 0.2 dtex 3 mm Fibrid made of
para-aromatic polyamide pbm 0.8 2.5 2.0 0.4 2.5 2.0 2.0 Fibrillated
heat-resistant fibers (II) pbm 1.2 2.5 4.0 2.5 1.5 2.0 2.0 having a
modified freeness of more than 300 ml Fibrillated heat-resistant
fibers (I) pbm 2.5 2.5 2.5 4.6 4.6 3.0 3.0 having a modified
freeness of not more than 300 ml Heat resistant fibers pbm 4.5 7.5
8.5 7.5 8.6 7.0 7.0 Beaten solvent span cellulose fibers pbm 0.0
0.0 0.0 0.0 0.0 3.0 4.0 Fibrid Modified freeness ml 88 88 88 88 88
88 88 Mass-weighted average fiber length mm 0.52 0.52 0.52 0.52
0.52 0.52 0.52 Length-weighted average fiber length mm 0.32 0.32
0.32 0.32 0.32 0.32 0.32 Fiber width .mu.m 24.1 24.1 24.1 24.1 24.1
24.1 24.1 Fibrillated Modified freeness ml 350 350 350 350 350 350
350 heat-resistant Mass-weighted average fiber length mm 1.30 1.30
1.30 1.30 1.30 1.30 1.30 fibers (II) Length-weighted average fiber
length mm 0.59 0.59 0.59 0.59 0.59 0.59 0.59 having a Fiber width
.mu.m 25.2 25.2 25.2 25.2 25.2 25.2 25.2 modified freeness of more
than 300 ml Fibrillated Modified freeness ml 50 50 50 50 50 50 50
heat-resistant Mass-weighted average fiber length mm 0.58 0.58 0.58
0.58 0.58 0.58 0.58 fibers (I) Length-weighted average fiber length
mm 0.33 0.33 0.33 0.33 0.33 0.33 0.33 having a Fiber width .mu.m
15.5 15.5 15.5 15.5 15.5 15.5 15.5 modified freeness of not more
than 300 ml Linear pressure of hot calender 100 100 110 100 110 100
100 Evaluation Weight of separator g/m.sup.2 7 7 7 7 7 9 9
Thickness of separator .mu.m 10 10 10 10 10 13 13 Tensile strength
of separator -- .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .DELTA. Cuttability of
separator -- .largecircle. .largecircle. .DELTA. .largecircle.
.largecircle. .largecircle. .largecircle. Resistance component
(impedance) .OMEGA. 0.45 0.50 0.52 0.50 0.52 0.50 0.49 Incidence of
internal short circuit failure % 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Cycle
characteristics % 87 82 82 82 80 88 88 Ex.: Example pbm: part by
mass Ex.: Example
TABLE-US-00012 TABLE 10 C. EX. C.EX. C. EX. C. EX. C. EX. C. EX. C.
EX. C. EX. 4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8 Fiber Oriented
crystallized PET-based synthetic pbm 60.0 60.0 50.9 50.1 50.1 50.1
50.1 50.1 blending resin short fibers 0.06 dtex 3 mm Undrawn
PET-based synthetic resin short pbm 40.0 40.0 40.0 41.0 41.0 41.0
41.0 41.0 fibers 0.2 dtex 3 mm Fibrid made of para-aromatic
polyamide pbm 0.0 0.0 2.4 8.9 0.0 0.0 3.9 0.0 Fibrillated
heat-resistant fibers (II) pbm 0.0 0.0 3.9 0.0 8.9 0.0 0.0 3.9
having a modified freeness of more than 300 ml Fibrillated
heat-resistant fibers (I) pbm 0.0 0.0 2.8 0.0 0.0 8.9 5.0 5.0
having a modified freeness of not more than 300 ml Heat resistant
fibers pbm 0.0 0.0 9.1 8.9 8.9 8.9 8.9 8.9 Beaten solvent span
cellulose fibers pbm 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Fibrid
Modified freeness ml -- -- 88 88 -- -- 88 -- Mass-weighted average
fiber length mm -- -- 0.52 0.52 -- -- 0.52 -- Length-weighted
average fiber length mm -- -- 0.32 0.32 -- -- 0.32 -- Fiber width
.mu.m -- -- 24.1 24.1 -- -- 24.1 -- Fibrillated Modified freeness
ml -- -- 350 -- 350 -- -- 350 heat-resistant Mass-weighted average
fiber length mm -- -- 1.30 -- 1.30 -- -- 1.30 fibers (II)
Length-weighted average fiber length mm -- -- 0.59 -- 0.59 -- --
0.59 having a Fiber width .mu.m -- -- 25.2 -- 25.2 -- -- 25.2
modified freeness of more than 300 ml Fibrillated Modified freeness
ml -- -- 50 -- -- 50 50 50 heat-resistant Mass-weighted average
fiber length mm -- -- 0.58 -- -- 0.58 0.58 0.58 fibers (I)
Length-weighted average fiber length mm -- -- 0.33 -- -- 0.33 0.33
0.33 having a Fiber width .mu.m -- -- 15.5 -- -- 15.5 15.5 15.5
modified freeness of not more than 300 ml Linear pressure of hot
calender kN/m 100 100 120 110 120 120 120 120 Evaluation Weight of
separator g/m.sup.2 8 7 7 7 7 7 7 7 Thickness of separator .mu.m 12
10 10 10 10 10 10 10 Tensile strength of separator --
.circleincircle. .largecircle. .DELTA. .largecircle. .DELTA.
.DELTA. .DELTA. .DELTA. Cuttability of separator -- .largecircle.
.largecircle. .DELTA. .largecircle. X X X .DELTA. Resistance
component (impedance) .OMEGA. 0.43 0.39 0.65 0.75 0.73 0.60 0.70
0.55 Incidence of internal short circuit failure % 1.0 1.5 0.0 0.0
0.0 0.0 0.0 0.0 Cycle characteristics % 70 70 68 60 65 70 65 78 C.
Ex.: Comparative Example pbm: part by mass C. Ex.: Comparative
Example
[0194] The separators manufactured in Examples 4-1 to 4-13
contained a fibrid made of para-aromatic polyamide, fibrillated
heat-resistant fibers having a modified freeness of more than 300
ml (fibrillated heat-resistant fibers (II)), fibrillated
heat-resistant fibers having a modified freeness of not more than
300 ml (fibrillated heat-resistant fibers (I)) and synthetic resin
short fibers and had a total content of the three different types
of heat-resistant fibers of not less than 2.0 mass % to less than
9.0 mass % based on the total of all the fiber components contained
in the separator. Although the separators of Examples 4-1 to 4-13
had a small thickness of not more than 15 .mu.m, they were
excellent in strength characteristics and cuttability. They had a
low impedance as the resistance component, a low incidence of
internal short circuit failure and excellent cycle characteristics.
The separators of Examples 4-1 to 4-7 and the separators of
Examples 4-12 to 4-13 in which the total content of the
heat-resistant fibers was not less than 2.0 mass % to less than 9.0
mass %, the content of the fibrid made of para-aromatic polyamide
was not less than 0.4 mass % to less than 2.5 mass %, the content
of the fibrillated heat-resistant fibers (II) was not less than 1.0
mass % to less than 4.0 mass % and the content of the fibrillated
heat-resistant fibers (I) was not less than 0.6 mass % to less than
4.6 mass % had excellent cuttability, a low impedance as the
resistance component, a low incidence of internal short circuit
failure and excellent cycle characteristics.
[0195] Out of the separators manufactured in Examples 4-1 to 4-13,
the separator of Example 4-7 had a total content of the three
different types of heat-resistant fibers of not less than 2.0 mass
% to less than 9.0 mass %, a content of the fibrid of not less than
0.4 mass % to less than 2.5 mass %, a content of the fibrillated
heat-resistant fibers (II) of not less than 1.0 mass % to less than
4.0 mass % and a content of the fibrillated heat-resistant fibers
(I) of not less than 0.6 mass % to less than 4.6 mass %, all of
which are preferred contents. Although the separator had a basis
weight of 7 g/m.sup.2 and a thickness of 10 .mu.m, they had no
internal short circuit failure, a low impedance as the resistance
component and excellent cycle characteristics.
[0196] Out of the separators manufactured in Examples 4-1 to 4-13,
the separator of Example 4-8 had a total content of the three
different types of heat-resistant fibers of not less than 2.0 mass
% to less than 9.0 mass %, a content of the fibrillated
heat-resistant fibers (II) of not less than 1.0 mass % to less than
4.0 mass % and a content of the fibrillated heat-resistant fibers
(I) of not less than 0.6 mass % to less than 4.6 mass % but a
content of the fibrid of not less than 2.5 mass %. Although the
separator had a basis weight of 7 g/m.sup.2, the impedance as the
resistance component of the separator was the same as that of a
separator having a basis weight of 8 g/m.sup.2 and the cycle
characteristics were slightly lower than those of the separator of
Example 4-7.
[0197] Out of the separators manufactured in Examples 4-1 to 4-13,
the separator of Example 4-9 had a total content of the three
different types of heat-resistant fibers of not less than 2.0 mass
% to less than 9.0 mass, a content of the fibrid of not less than
0.4 mass % to less than 2.5 mass % and a content of the fibrillated
heat-resistant fibers (I) of not less than 0.6 mass % to less than
4.6 mass % but a content of the fibrillated heat-resistant fibers
(II) of not less than 4.0 mass %. Therefore, as a large number of
thick para-aromatic polyamide stem fibers remained, fluff slightly
remained on the edge surface. Further, it was found that a high
linear pressure had to be applied at the time of controlling the
thickness of the separator to a predetermined value by hot
calendering, thereby making it difficult to reduce the thickness of
the separator. The impedance as the resistance component of the
separator was the same as that of a separator having a basis weight
of 8 g/m.sup.2 and the cycle characteristics were slightly lower
than those of the separator of Example 4-7.
[0198] Out of the separators manufactured in Examples 4-1 to 4-13,
the separator of Example 4-10 had a total content of the three
different types of heat-resistant fibers of not less than 2.0 mass
% to less than 9.0 mass, a content of the fibrid of not less than
0.4 mass % to less than 2.5 mass % and a content of the fibrillated
heat-resistant fibers (II) of not less than 1.0 mass % to less than
4.0 mass % but a content of the fibrillated heat-resistant fibers
(I) of not less than 4.6 mass %. Therefore, the impedance as the
resistance component of the separator was the same as that of a
separator having a basis weight of 8 g/m.sup.2 and the cycle
characteristics were slightly lower than those of the separator of
Example 4-7.
[0199] Out of the separators manufactured in Examples 4-1 to 4-13,
the separator of Example 4-11 had a total content of the three
different types of heat-resistant fibers of not less than 2.0 mass
% to less than 9.0 mass and a content of the fibrillated
heat-resistant fibers (II) of not less than 1.0 mass % to less than
4.0 mass % but a content of the fibrid of not less than 2.5 mass %
and a content of the fibrillated heat-resistant fibers (I) of not
less than 4.6 mass %. Therefore, the impedance as the resistance
component of the separator was the same as that of a separator
having a basis weight of 8 g/m.sup.2 and the cycle characteristics
were slightly lower than those of the separator of Example 4-7.
[0200] The separators of Examples 4-12 and 4-13 contained fibers
other than the fibrid, the fibrillated heat-resistant fibers (I),
the fibrillated heat-resistant fibers (II) and the synthetic resin
short fibers. From comparison between Example 4-12 and Example
4-13, the separator of Example 4-12 having a content of the
synthetic resin short fibers of not less than 90.0 mass % was
superior in strength characteristics.
[0201] Since the separators of Comparative Examples 4-1 and 4-2 did
not contain the three different types of heat-resistant fibers,
when the basis weight of the separator was reduced, the incidence
of internal short circuit failure became worse.
[0202] In Comparative Example 4-3, the content of the fibrid was
not less than 0.4 mass % to less than 2.5 mass %, the content of
the fibrillated heat-resistant fibers (II) was not less than 1.0
mass % to less than 4.0 mass % and the content of the fibrillated
heat-resistant fibers (I) was not less than 0.6 mass % to less than
4.6 mass % but the total content of the three different types of
heat-resistant fibers was more than 9.0 mass %. Since the number of
fine fibril fibers greatly increased, bonding between fibers became
loose, thereby deteriorating strength characteristics. In the
evaluation of cuttability, fluff slightly remained on the edge
surface. Further, a high linear pressure had to be applied at the
time of controlling the thickness of the separator to a
predetermined value by hot calendering. As a result, the film
formation of the PET fibers for binders proceeded, thereby raising
the impedance as the resistance component and deteriorating the
cycle characteristics.
[0203] In Comparative Example 4-4, only the fibrid was contained in
an amount of 8.9 mass %. Although the content of the heat-resistant
fibers was not less than 2.0 mass % to less than 9.0 mass, when
only the fibrid was contained, fluff slightly remained on the edge
surface in the evaluation of cuttability. Since the fibrous form of
the fibrid was thin leaf-like, the impedance as the resistance
component greatly rose and the cycle characteristics
deteriorated.
[0204] In Comparative Example 4-5, only the fibrillated
heat-resistant fibers (II) were contained in an amount of 8.9 mass
%. Although the content of the heat-resistant fibers was not less
than 2.0 mass % to less than 9.0 mass %, when only the fibrillated
heat-resistant fibers (II) were contained, a large number of thick
para-aromatic polyamide stem fibers remained, cutting was difficult
and fluff remained on the edge surface. Further, a high linear
pressure had to be applied at the time of controlling the thickness
of the separator to a predetermined value by hot calendering. As a
result, the film formation of the PET fibers for binders proceeded,
thereby raising the impedance as the resistance component and
deteriorating the cycle characteristics. Since there are a large
number of thick aromatic polyamide stem fibers, a fiber network
became weak, thereby deteriorating strength characteristics.
[0205] In Comparative Example 4-6, only the fibrillated
heat-resistant fibers (I) were contained in an amount of 8.9 mass
%. Although the content of the heat-resistant fibers was not less
than 2.0 mass % to less than 9.0 mass %, when only the fibrillated
heat-resistant fibers (I) were contained, the number of fine
fibrillated fibers greatly increased, whereby cutting was difficult
and fluff remained on the edge surface. Further, a high linear
pressure had to be applied at the time of controlling the thickness
of the separator to a predetermined value by hot calendering. As a
result, the film formation of the PET fibers for binders proceeded,
thereby raising the impedance as the resistance component and
deteriorating the cycle characteristics. Since the number of fine
fibril fibers increased too much, bonding between fibers became
loose, thereby deteriorating strength characteristics.
[0206] In Comparative Example 4-7, 3.9 mass % of the fibrid and 5.0
mass % of the fibrillated heat-resistant fibers (I) were contained.
Although the total content of the heat-resistant fibers was not
less than 2.0 mass % to less than 9.0 mass %, as the number of fine
fibrillated fibers greatly increased, cutting was difficult and
fluff remained on the edge surface. Further, a high linear pressure
had to be applied at the time of controlling the thickness of the
separator to a predetermined value by hot calendering. As a result,
the film formation of the PET fibers for binders proceeded and the
fibrous form of the fibrid was thin leaf-like, thereby greatly
raising the impedance as the resistance component and deteriorating
the cycle characteristics. Further, since the number of fine fibril
fibers increased too much, bonding between fibers became loose,
thereby deteriorating strength characteristics.
[0207] In Comparative Example 4-8, 3.9 mass % of the fibrillated
heat-resistant fibers (II) and 5.0 mass % of the fibrillated
heat-resistant fibers (I) were contained. Although the total
content of the heat-resistant fibers was not less than 2.0 mass %
to less than 9.0 mass %, since a large number of thick
para-aromatic polyamide stem fibers remained and the number of fine
fibrillated fibers increased, cutting was difficult and fluff
slightly remained on the edge surface. Further, a high linear
pressure had to be applied at the time of controlling the thickness
of the separator to a predetermined value by hot calendering. As a
result, the film formation of the PET fibers for binders proceeded,
thereby raising the impedance as the resistance component and
deteriorating the cycle characteristics. Further, as the number of
fine fibril fibers increased too much, bonding between fibers
became loose, thereby deteriorating strength characteristics.
INDUSTRIAL FEASIBILITY
[0208] The lithium ion battery separator of the present invention
can be advantageously used in lithium ion secondary batteries,
lithium ion polymer secondary batteries and lithium ion
batteries.
* * * * *