U.S. patent application number 11/400465 was filed with the patent office on 2007-10-11 for multilayer separator exhibiting improved strength and stability.
This patent application is currently assigned to Celgard LLC. Invention is credited to Ronald W. Call, Shizuo Ogura, Premanand Ramadass, Lie Shi, Xiangyun Wei, Zhengming Zhang.
Application Number | 20070238017 11/400465 |
Document ID | / |
Family ID | 38575695 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070238017 |
Kind Code |
A1 |
Call; Ronald W. ; et
al. |
October 11, 2007 |
Multilayer separator exhibiting improved strength and stability
Abstract
A multi-layer microporous battery separator which comprises: a
high molecular weight polypropylene layer having a melt flow index
of .ltoreq.1.2 measured at layer; a polyethylene layer; and a high
molecular weight polypropylene layer having a melt flow index of
.ltoreq.1.2 measured at layer. The resulting microporous battery
separator which is formed by a dry stretch process produces the
microporous battery separator which has a porosity of .ltoreq.37%
while maintaining a gurley from 13-25 seconds and a thickness of
.ltoreq.25 microns.
Inventors: |
Call; Ronald W.; (Rock Hill,
SC) ; Shi; Lie; (Charlotte, NC) ; Zhang;
Zhengming; (Charlotte, NC) ; Ogura; Shizuo;
(Charlotte, NC) ; Wei; Xiangyun; (Charlotte,
NC) ; Ramadass; Premanand; (Charlotte, NC) |
Correspondence
Address: |
HAMMER & HANF, PC
3125 SPRINGBANK LANE
SUITE G
CHARLOTTE
NC
28226
US
|
Assignee: |
Celgard LLC
|
Family ID: |
38575695 |
Appl. No.: |
11/400465 |
Filed: |
April 7, 2006 |
Current U.S.
Class: |
429/145 ;
156/229 |
Current CPC
Class: |
B32B 2305/026 20130101;
Y02E 60/10 20130101; B32B 2038/0048 20130101; B29K 2023/12
20130101; B29C 55/023 20130101; B29K 2023/06 20130101; H01M 50/411
20210101; B32B 37/153 20130101; B32B 38/0032 20130101; B32B
2038/0028 20130101; B32B 2457/10 20130101 |
Class at
Publication: |
429/145 ;
156/229 |
International
Class: |
H01M 2/16 20060101
H01M002/16; B29C 65/00 20060101 B29C065/00 |
Claims
1. A multi-layer microporous battery separator comprising: a high
molecular weight polypropylene layer having a melt flow index (MFI)
of .ltoreq.1.2 measured at layer; a polyethylene layer; a high
molecular weight polypropylene layer having a melt flow index of
.ltoreq.1.2 measured at layer; which forms a microporous battery
separator by a dry stretch process, where said microporous battery
separator has a porosity of .ltoreq.37% while maintaining a Gurley
from 13 to 25 seconds and a thickness of .ltoreq.25 microns.
2. The multi-layer microporous battery separator according to claim
1, where said microporous battery separator exhibits an increase 5%
or more in mixed penetration strength compared to a tri-layer
dry-stretched microporous battery separator of the same
thickness.
3. The multi-layer microporous battery separator according to claim
1, where the net shrinkage of said microporous battery separator is
less than 5% measured for 6 hours at 105.degree. C.
4. The multi-layer microporous battery separator according to claim
1, where the ionic resistance of said microporous battery separator
is less than 2.5 ohms-cm.sup.2.
5. The multi-layer microporous battery separator according to claim
1, where the polyethylene layer is a high density polyethylene.
6. A multi-layer microporous battery separator comprising: a
tri-layer dry-stretched microporous battery separator having an
outer polyolefin layer, an inner polyolefin layer and an outer
polyolefin layer and an overall thickness of .ltoreq.25 microns;
said outer polyolefin layers are a high molecular weight
polypropylene having a melt flow index of .ltoreq.1.2 measured at
layer; said inner polyolefin layer is a polyethylene; and where
said tri-layer dry-stretched microporous battery separator exhibits
an increase 5% or more in mixed penetration strength compared to a
tri-layer dry-stretched microporous battery separator of the same
thickness.
7. The multi-layer microporous battery separator according to claim
6, where said microporous battery separator exhibits a porosity of
.ltoreq.37% while maintaining a Gurley from 13 to 25 seconds.
8. The multi-layer microporous battery separator according to claim
6, where the net shrinkage of said microporous battery separator is
less than 5% measured for 6 hours at 105.degree. C.
9. The multi-layer microporous battery separator according to claim
6, where the ionic resistance of said microporous battery separator
is less than 2.5 ohms-cm.sup.2.
10. A multi-layer microporous battery separator comprising: a
tri-layer dry-stretched microporous battery separator having an
outer polyolefin layer, an inner polyolefin layer and an outer
polyolefin layer; said outer polyolefin layers are a high molecular
weight polypropylene; said inner polyolefin layer is a
polyethylene; and where said tri-layer dry-stretched microporous
battery separator has a thickness of .ltoreq.25 microns, a porosity
of 37% or lower while maintaining a Gurley from 13 to 25 seconds,
and exhibits a 5% increase in mixed penetration strength compared
to a tri-layer dry-stretched microporous battery separator of the
same thickness.
11. The multi-layer microporous battery separator according to
claim 10, where the net shrinkage of said microporous battery
separator is less than 5% measured for 6 hours at 105.degree.
C.
12. The multi-layer microporous battery separator according to
claim 10, where the ionic resistance of said microporous battery
separator is less than 2.5 ohms-cm.sup.2.
13. A process of for the preparation of a multi-layer microporous
battery separator comprising the steps of: providing a
polypropylene having a MFI .ltoreq.1.0 measured at pellet before
processing and a polyethylene; extruding said high molecular weight
polypropylene to form a precursor polypropylene film; extruding
said polyethylene to form a precursor polyethylene film; laminating
said precursor polypropylene films to each side of said precursor
polyethylene film to form a non-porous tri-layer precursor;
annealing said non-porous tri-layer precursor; stretching said
non-porous tri-layer precursor to form a stretched microporous
tri-layer film; allowing a relaxation of said stretched microporous
tri-layer film to form a microporous tri-layer film; and where net
stretch of said microporous tri-layer film is less than 90%.
14. A battery separator made of a microporous polyolefin having an
overall thickness of .ltoreq.25 microns where the net shrinkage of
said separator is less than 5% measured for 6 hours at 105.degree.
C.
15. The battery separator according to claim 14 having a porosity
of 37% or lower.
16. The battery separator according to claim 15 having a Gurley
from 13 to 25 seconds.
17. The battery separator according to claim 14, where said
separator has a porosity of .ltoreq.37% while maintaining a Gurley
from 13 to 25 seconds.
18. A multi-layer battery separator made of a microporous
polyolefin having an outer layer of a high molecular weight
polypropylene layer having a melt flow index of .ltoreq.1.2
measured after processing at said outer layer.
19. The multi-layer battery separator according to claim 18, where
said separator has a porosity of .ltoreq.37% while maintaining a
Gurley from 13 to 25 seconds.
20. The multi-layer microporous battery separator according to
claim 19, where the ionic resistance of said microporous battery
separator is less than 2.5 ohms-cm.sup.2.
Description
FIELD OF THE INVENTION
[0001] The present invention is a battery separator and a method of
making this separator. The invented separator exhibits an increase
in mixed penetration tests and in decreased shrinkage when compared
to other separators made by either a dry stretch process or solvent
extraction process. Surprisingly the separators of the invention
also have a Gurley of 13 to 25 seconds even with a porosity of less
than or equal to 37%.
BACKGROUND OF THE INVENTION
[0002] The use of microporous multi-layered membranes as battery
separators is known. See, for example, U.S. Pat. Nos. 5,480,745;
5,691,047; 5,667,911; 5,691,077; and 5,952,120.
[0003] U.S. Pat. No. 5,480,745 discloses forming a multi-layered
film by co-extruding the multi-layered precursor or by
heat-welding, at 152.degree. C., pre-formed precursor layers. The
multi-layered precursor, formed by either technique, is then made
microporous by annealing and stretching. This membrane, which is
made by a dry stretch process, has a preferable amount of net
stretch is from 100% to 300%.
[0004] U.S. Pat. No. 5,691,047 discloses forming the multi-layered
film by co-extruding the multi-layered precursor or by uniting,
under heat (120-140.degree. C.) and pressure (1-3 kg/cm.sup.2),
three or more precursor layers. The precursor formed under heat and
pressure, at a speed of 0.5 to 8 m/min (1.6-26.2 ft/min), has a
peel strength in the range of 3 to 60 g/15 mm (0.2-4 g/mm). In the
examples, one 34 .mu.m separator has a peel strength of 1 g/mm and
the other, about 0.5 g/mm. The multi-layered precursor, formed by
either technique, is then made microporous by annealing and
stretching. The porosity of these separators is greater than the
present invention while showing a relatively high Gurley.
[0005] U.S. Pat. No. 5,667,911 discloses forming the multi-layered
film by uniting (by heat and pressure or by adhesives) cross-plied
microporous films to form a multi-layered microporous film. The
microporous films are laminated together using heat (110.degree. C.
-140.degree. C.) and pressure (300-450 psi) and at line speeds of
15-50 ft/min (4.6-15.2 m/min). This reference teaches lower Gurley
values, which is a good indication that the porosity of these films
is high.
[0006] U.S. Pat. No. 5,691,077 discloses forming the multi-layered
film by uniting, by heat and pressure (calendering), or by
adhesives, or by pattern welding, microporous films to form a
multi-layered microporous film. Calendering is performed at
125.degree. C. to 130.degree. C. for a residence time of 2 to 10
minutes. Four (4) stacked multi-layered microporous precursors are
calendering between a single nip roll. The porosity of these
separators is greater than the present invention while showing a
relatively high Gurley.
[0007] U.S. Pat. No. 5,952,120 discloses forming the multi-layered
film by extruding nonporous precursors, bonding together nonporous
precursors, annealing the bonded, nonporous precursors, and
stretching the bonded, nonporous precursors to form a multi-layered
microporous film. At least four (4) tri-layer precursors are
simultaneously passed through the steps of bonding, annealing, and
stretching. Bonding was performed between nip rollers at
128.degree. C. (range 125.degree. C.-135.degree. C.) at a line
speed of 30 ft/min (9.1 m/min) to yield a peel strength of 5.7 g/in
(0.2 g/mm) and between nip rollers at 128.degree. C.-130.degree. C.
at a line speed of 40 ft/min (12.2 m/min) to yield a peel strength
of 30 g/in (1.2 g/mm). The net stretch on these separators all tend
to be at least 100% or higher, while the Gurley's are on the high
side.
[0008] While the foregoing processes have produced commercially
viable multi-layered, microporous films suitable for use as battery
separators, there is a desire on the part of both the separator
manufacturers and the battery manufacturers to produce separators
with greater processability. To improve processability a separator
needs be more resistant to failure during the manufacture process.
Two of the big problems that plague the battery manufactures are
leaks and shrinkage of the separator. Shrinkage occurs when the
separator is subjected to a heated environment, which a battery
will go through during use. In the past one way separators had been
tested for leaks was through a puncture strength test. However, it
has been learned that a new test called mixed penetration, is by
far a better indicator of how a separator will do in the
manufacturing process than the puncture strength test. When testing
for shrinkage the separator needs to be exposed to elevated heat
over a time period. The manufactures of the batteries will still
demand that the separators have Gurley numbers in a desirable
range
[0009] Accordingly, there is a need to provide an improved
multi-layered microporous films to be used as separators, which
shows an increase in mixed penetration strength, while still
maintaining low shrinkage values, and still exhibiting Gurley
numbers in a desirable range.
SUMMARY OF THE INVENTION
[0010] The invention is a multi-layer microporous battery
separator, having a high molecular weight polypropylene layer,
indicated by a melt flow index of .ltoreq.1.2 measured at the
layer, a polyethylene layer, and a high molecular weight
polypropylene layer, which has a melt flow index of .ltoreq.1.2
measured at layer. This resulting microporous battery separator is
formed by a dry stretch process. The microporous battery separator
has a porosity of .ltoreq.37% while maintaining a gurley from 13-25
seconds for a separator with a thickness of .ltoreq.25 microns.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing will become more readily apparent by referring
to the following detailed description and the appended drawings in
which:
[0012] FIG. 1 is a side view of a multilayer separator in a mixed
penetration test.
[0013] FIG. 2 is side view showing the electrodes and the separator
after pressure is applied.
[0014] FIG. 3 is a graph showing a slope of the ionic resistance of
a separator.
[0015] FIG. 4 is a schematic view of a four probe AC impedence
technique for measuring the ionic resistance of separator
membranes.
[0016] FIG. 5 is a graph of the percentage increase in mixed
penetration strength of the multilayer membranes made by the
current process of multilayer dry stretch membranes of the same
thickness.
DETAILED DESCRIPTION OF THE INVENTION
[0017] A battery separator refers to a microporous film or membrane
for use in electrochemical cells or capacitors. Electrochemical
cells include primary (non-rechargeable) and secondary
(rechargeable) batteries, such as batteries based on lithium
chemistry. These films are commonly made of polyolefins, for
example, polyethylene, polypropylene, polybutylene,
polymethylpentene, mixtures thereof and copolymers thereof.
Polypropylene (including isotactic and atactic) and polyethylene
(including LDPE, LLDPE, HDPE, and UHM WPE) and blends thereof and
their copolymers are the preferred polyolefins that are used to
make commercially available films for these applications. These
films may be made by the CELGARD.RTM. process (also known as the
dry process, i.e., extrude-anneal-stretch) or by a solvent
extraction process (also known as the wet process or phase
inversion process or TIPS, thermally induced phase separation,
process) or by a particle stretch process. Some of these films,
those made by the dry process, are often multi-layered films.
Multi-layered films are preferred because they have shutdown
capability (i.e., can stop the flow of ions in the event of short
circuiting). A common multi-layered film is the tri-layered film. A
popular tri-layered film has a polypropylene (PP)/polyethylene
(PE)/polypropylene (PP) structure, another structure is
PE/PP/PE.
[0018] The present invention is to a multi-layer microporous
battery separator which has three layers. The first layer is a high
molecular weight polypropylene layer having a melt flow index of
less than or equal to (.ltoreq.) 1.2 measured at layer, a second
polyethylene layer, and a third high molecular weight polypropylene
layer, which has a melt flow index of .ltoreq.1.2 measured at the
layer. This microporous battery separator is formed by a dry
stretch process. The process of the invention produces the
microporous battery separator which has a porosity of less than or
equal to .ltoreq.37% while maintaining a Gurley from 13-25 seconds
and a thickness of less than or equal to .ltoreq.25 microns.
[0019] This multi-layer microporous battery separator exhibits an
increase of 5% or more in mixed penetration strength compared to a
tri-layer dry-stretched microporous battery separator of the same
thickness. The net shrinkage of this microporous battery separator
is less than 5% after an exposure of 6 hours at 105.degree. C. The
ionic resistance of this microporous battery separator is less than
2.5 ohms-cm.sup.2. The polyethylene layer of this separator is a
high density polyethylene.
[0020] In another embodiment of the invention the multi-layer
microporous battery separator comprises a tri-layer dry-stretched
microporous battery separator, which has an outer polyolefin layer,
an inner polyolefin layer and an outer polyolefin layer. The
overall thickness of the separator is .ltoreq.25 microns. The outer
polyolefin layers are a high molecular weight polypropylene. The
inner polyolefin layer is a polyethylene. This tri-layer
dry-stretched microporous battery separator, exhibits an increase
in mixed penetration strength of 5% or more compared to a tri-layer
dry-stretched microporous battery separator of the same thickness.
This multi-layer microporous battery separator has a porosity of
.ltoreq.37% while maintaining a gurley from 13-25 seconds.
Surprisingly, the net shrinkage of this microporous battery
separator is less than 5% measured for 6 hours at 105.degree. C.
This multi-layer microporous battery separator has an ionic
resistance of less than 2.5 ohm-cm.sup.2.
[0021] Another embodiment of the invention is a multi-layer
microporous battery separator which comprises a tri-layer
dry-stretched microporous battery separator. This separator has an
outer polyolefin layer an inner polyolefin layer and an outer
polyolefin layer. The outer polyolefin layers are a high molecular
weight polypropylene. The inner polyolefin layer is a polyethylene.
This tri-layer dry-stretched microporous battery separator has a
thickness of .ltoreq.25 microns, a porosity of 37% or lower and a
Gurley from 13-25 seconds. This separator exhibits a 5% increase in
mixed penetration strength compared to a tri-layer dry-stretched
microporous battery separator of the same thickness. This
multi-layer microporous battery separator surprisingly exhibits a
net shrinkage of less than 5% measured for 6 hours at 105.degree.
C. The multi-layer microporous battery separator also has an ionic
resistance of the microporous battery separator is less than 2.5
ohm-cm.sup.2.
[0022] The invented separator can be prepared by the following
process of the preparation of a multi-layer microporous battery
separator. A polypropylene having a MFI .ltoreq.1.0 measured at
pellet before processing and a polyethylene is provided. The
polypropylene, which is a high molecular weight polypropylene, is
extruded to form a precursor polypropylene film. Then a
polyethylene is provided and is extruded to form a precursor
polyethylene film. The precursor polypropylene films are then
laminated to each side of the precursor polyethylene film to form a
non-porous tri-layer precursor. This non-porous tri-layer precursor
is then annealed. After annealing, the non-porous tri-layer
precursor is then stretched to form a stretched microporous
tri-layer film. The stretched microporous tri-layer film is then
allowed to relaxed to form a microporous tri-layer film. The net
stretch in this process is less than 90%. Net stretch is determined
by the percentage of stretch given to the film minus the amount
relax. Stretch may be done either hot or cold or as mixture of hot
and cold. The relaxation may also be performed either hot or cold
or as a mixture of both hot and cold.
[0023] In another embodiment of the invention a battery separator
made of a microporous polyolefin is provided, which has an overall
thickness of .ltoreq.25 microns, where the net shrinkage of the
separator is less than 5% measured for 6 hours at 105.degree. C.
This battery separator has a porosity of 37% or lower. Yet
surprisingly this battery separator has a Gurley from 13-25
seconds. Traditionally in order to obtain a Gurley level in the
13-25 second range a separator had to have a porosity of more than
37% and in most cases the porosity was at least 40% or more. It has
been seen that even small changes in porosity tend to have a big
impact of the Gurley for a separator. A battery separator made of a
microporous polyolefin having an overall thickness of .ltoreq.25
microns, where the net shrinkage of the separator is less than 5%
measured for 6 hours at 105.degree. C., where the separator has a
porosity of .ltoreq.37% while maintaining a gurley from 13-25
seconds is surprising for a separator made by a wet process as well
as a separator made by a dry process.
[0024] In another embodiment of the invention a multi-layer battery
separator is made of a microporous polyolefin having an outer layer
of a high molecular weight polypropylene layer, which has a melt
flow index of .ltoreq.1.2 measured after processing at the outer
layer. The measurement at the layer is important because many
polypropylenes that may be referred to as high density will see a
significant fall off in the melt flow performance after processing.
In the past when melt flow index was used it always referred to the
melt flow prior to processing.
[0025] This invention is further illustrated with reference to the
examples set forth below. In the following example, Gurley is
measured by the ASTM D-726(B) method. As used herein, Gurley is the
resistance to air flow measured by the Gurley Densometer (e.g.
Model 4120). The Gurley values set forth herein are expressed as
the time in seconds required to pass 10 cc of air through one
square inch of product under a pressure 12.2 inches of water.
[0026] The tensile strength along MD and TD is measured with the
ASTM D-638 method. The tear resistance is measured by ASTM
D-1004.
[0027] The thickness of the battery separator is measured by the
T411 om-83 method developed under the auspices of the Technical
Association of the Pulp and Paper Industry. Thickness is determined
using a precision micrometer with a 1/2 inch diameter, circular
shoe contacting the sample at seven (7) psi. Ten (10) individual
micrometer readings taken across the width of the sample are
averaged.
[0028] The porosity of a microporous film is measured by the method
of ASTM D-2873.
[0029] Puncture strength is measured as follows: ten measurements
are made across the width of the stretched product and averaged. A
Mitech Stevens LFRA Texture Analyzer is used. The needle is 1.65 mm
in diameter with 0.5 mm radius. The rate of descent is 2 mm/sec and
the amount of deflection is 6 mm. The film is held tight in the
clamping device with a central hole of 11.3 mm. The displacement
(in mm) of the film that was pierced by the needle was recorded
against the resistance force (in gram force) developed by the
tested film. The maximum resistance force is the puncture
strength.
[0030] Mixed penetration is the force required to create a short
through a separator due to mixed penetration. In this test one
starts with a base of a metal plate 10, FIG. 1, on top of this
plate is placed a sheet of cathode material 15, on top of cathode
is placed a multilayer separator 20 and on top of the multilayer
separator 20 is placed a sheet of anode material 25. A ball tip of
3 mm, 30 is then provided attached to a force gauge 35. The ball
tip 30 is connected to the metal plate 10 by a resistance meter 40.
Pressure 45, FIG. 2, is applied to the ball tip 30, which is
recorded on the force gauge 35, FIG. 1. Once force is applied,
there builds up an anode mix 50, FIG. 2 and a cathode mix 55 on
either side of the separator 20. When the resistance falls
dramatically it indicates a short through the separator due to
mixed penetration.
[0031] Mixed penetration measures the strength of the separator and
resistance towards mixed penetration. This has been found to more
accurately simulate the behavior of a real cell. It is a better
indicator than puncture strength of how a separator will behave in
a cell. This test is used to indicate the tendency of separators to
allow short-circuits during battery assembly.
[0032] Melt Index is measured according to ASTM DS 1238; PE:
190.degree. C./2.16 Kg; PP: 230.degree. C./2.16 Kg. It is measured
as g/10 minutes.
[0033] The shrinkage is measured at 105 .degree. C. for 6 hours.
Both width and length of a separator membrane are measured before
and after the said heat treatment. The net shrinkage is calculated
by the following formula: Net Shrinkage
percent=100*((L0-L1)/L0+(W0-W1)/W0) Where L0 is the length before
heat treatment, L1 is the length after heat treatment, W0 is the
width before heat treatment, and W1 is the width after heat
treatment.
[0034] The measurement ionic resistance of separator soaked with a
certain electrolyte is very important to the art of battery
manufacture, because of the influence the separator has on
electrical performance. Ionic resistance is a more comprehensive
measure of permeability than the Gurley number, in that the
measurement is carried out in the actual electrolyte solution for
real battery application. The ionic resistance of the porous
membrane is essentially the ionic resistance of the electrolyte
that is embedded in the pores of the separator. Typically, a
microporous separator, immersed in an electrolyte has an electrical
resistance about 6-7 times that of a comparable volume of
electrolyte, which it displaces. It is a function of the membrane's
porosity, tortuosity, the resistance of the electrolyte, the
thickness of the membrane, and the extent to which the electrolyte
wets the pores of the membrane.
[0035] The separator resistance is characterized by cutting small
pieces of separators from the finished material and then placing
them between two blocking electrodes. The separators are completely
saturated with the battery electrolyte with 1.0M LiPF.sub.6 salt in
EC/EMC solvent of 3:7 ratio by volume. The resistance, R (.OMEGA.)
of the separator is measured by 4-probe AC impedance technique. In
order to reduce the measurement error on the electrode/separator
interface, multiple measurements are needed by adding more
separator layers.
[0036] Based on the multiple layer measurements, the ionic
resistance, Rs (.OMEGA.) of the separator saturated with
electrolyte is then calculated by the formula, R s = .rho. s
.times. l A ( 1 ) ##EQU1## where .rho..sub.s is the ionic
resistivity of the separator in .OMEGA.-cm, A is the electrode area
in cm.sup.2 and l is the thickness of the separator membrane in cm.
The ratio .rho..sub.s/A is the slope calculated for the variation
of separator resistance with multiple separator layers which is
given by, slope = .rho. s A = .DELTA. .times. .times. R .DELTA.
.times. .times. .delta. ( 2 ) ##EQU2## where .DELTA.R and
.DELTA..delta. are defined in the FIG. 3. Calculation of slope in
FIG. 3 is used to estimate the ionic resistance of separator
membrane using multiple layer measurement approach.
[0037] Ionic resistance of separator membranes is measured by using
a four probe AC impedance technique. FIG. 4 shows the schematic 60
of the cell used to measure the resistance. The lead coming out of
the top 65 and bottom 70 probes of the cell has two wires each 75,
80 one for sensing current and other for voltage. Electrolyte used
for all resistance measurement is 1.0 M LiPF6 salt in EC:EMC
solvent of a 3:7 ratio by volume. Place a sample of separator on
the bottom electrode 85. The separator should completely cover the
bottom electrode and the separator should be completely wet with
electrolyte. Then slide the second electrode 90 on top of the
bottom electrode 85 and measure the impedance value. The impedance
value is measured with an impedance meter 95 from Potentiostat.
Start adding more separator layers and measure cumulative
resistance in order to reduce the measurement error. It is possible
to test the resistance of just the electrolyte by adding a Teflon
spacer 100 which has a hollow center 105 which can be placed over
the bottom probe 70. Then electrolyte is added to fill the hollow
center 105 then the top probe 65 is placed over the spacer 100.
EXAMPLES
[0038] The description above will be clear when one looks at the
examples in Table A. Sample A & B are for a competitive
trilayer separator made by a dry stretch process. Sample A is a 20
micron separator sample B is for a 25 micron separator. Examples
C300 and C500 are for the invented separator made by the invented
process. C300 is a 20 micron separator and C500 is for a 25 micron
separator. In the table: IR stands for ionic resistance, P.S. is
puncture strength, MP is mixed penetration and TD is traverse
direction compared to the machine direction. TABLE-US-00001 TABLE A
Description A C300 B C500 Thickness, Microns 20 21 25 24.5 Gurley
15 19 21 18 IR, ohm-cm.sup.2 1.7 2.1 2.0 2.3 P.S. Grams 337 367 412
424 MP % deviation -10 -2 0 5 from 2300 TD Tensile strength 165 180
168 174 kgf/cm.sup.2 Porosity % 43 35 42 37 Net Shrinkage @
105.degree., 6 hr % 6.4 3.0 6.4 2.7
[0039] In the mixed penetration test, the invented material is
compared against a trilayer separator made by the Celgard.RTM.
process which does not use a high molecular weight polypropylene
and is not made in accordance with the process of the invention. In
FIG. 5 the improvement in mixed penetration strength can be seen.
Also see table A, where the a standard 20 micron separator shows a
10% reduction in mixed penetration strength compared to a standard
25 micron trilayer separator. The 20 micron invented separator made
by the invented process shows only a 2% reduction in mixed
penetration strength. The standard 25 micron separator shows no
change in mixed penetration strength, where the 25 micron separator
made by the invent process shows an increase in mixed penetration
strength of 5%.
* * * * *