U.S. patent application number 10/636115 was filed with the patent office on 2005-02-10 for battery separator and method of making same.
Invention is credited to Call, Ronald W..
Application Number | 20050031943 10/636115 |
Document ID | / |
Family ID | 33552953 |
Filed Date | 2005-02-10 |
United States Patent
Application |
20050031943 |
Kind Code |
A1 |
Call, Ronald W. |
February 10, 2005 |
Battery separator and method of making same
Abstract
A battery separator comprises a multi-layered film, individual
layers of said film having been bonded together by heat and
pressure, having a peel strength of greater than or equal to 40
grams per inch (1.6 g/mm) and a thickness of .ltoreq.25 microns. A
method for making a battery separator comprises the steps of:
extruding and winding up a first precursor film, extruding and
winding up a second precursor film, unwinding the first and second
precursor films, stacking up the first and second precursor films
to form a single stacked precursor, laminating the single stacked
precursor film, winding up the laminated single stacked precursor
film, stacking up a plurality of laminated single stacked precursor
films, and making microporous the stacked plurality of laminated
single stacked precursor films.
Inventors: |
Call, Ronald W.; (Rock Hill,
SC) |
Correspondence
Address: |
ROBERT H. HAMMER III, P.C.
3121 SPRINGBANK LANE
SUITE I
CHARLOTTE
NC
28226
US
|
Family ID: |
33552953 |
Appl. No.: |
10/636115 |
Filed: |
August 7, 2003 |
Current U.S.
Class: |
429/144 ;
156/244.11 |
Current CPC
Class: |
H01M 50/411 20210101;
Y02E 60/10 20130101; H01M 50/449 20210101; B32B 27/32 20130101 |
Class at
Publication: |
429/144 ;
156/244.11 |
International
Class: |
H01M 002/18; B29C
047/00 |
Claims
That which is claimed:
1. A battery separator comprising a multi-layered microporous film,
individual layers of said film having been bonded together by heat
and pressure, having a peel strength of greater than 40 grams per
inch (1.6 g/mm) and a thickness of .ltoreq.25 microns.
2. The battery separator of claim 1 wherein said multi-layered
microporous film being a tri-layered film.
3. The battery separator of claim 2 wherein said tri-layered film
having a polypropylene-polyethylene-polypropylene structure.
4. The battery separator of claim 1 wherein said film having a
thickness of less than or equal to 20 microns.
5. The battery separator of claim 1 wherein said film having a
thickness of less than or equal to 15 microns.
6. A battery separator comprising: a multi-layered microporous
film, individual layers of said film having been bonded together by
heat and pressure, having a peel strength of greater than 40 grams
per inch (1.6 g/mm) wherein at least one layer being substantially
polypropylene, another layer being substantially polyethylene, and
the film having a thickness of less than or equal to 15
microns.
7. A method of making a battery separator comprising the steps of:
extruding and winding up a first precursor film; extruding and
winding up a second precursor film; unwinding the first and second
precursor film; stacking up the first and second precursor films to
form a single stacked precursor; laminating the single stacked
precursor film; winding up the laminated single stacked precursor
film; stacking up a plurality of laminated single stacked precursor
films; and making microporous said plurality of laminated single
stacked precursor films.
8. The method of claim 7 wherein extruding the first or second
precursor further comprises extruding with a slot die, T die, or a
blown film die.
9. The method of claim 7 wherein the single stacked precursor being
a tri-layer precursor.
10. The method of claim 9 wherein the tri-layer precursor being a
polypropylene-polyethylene-polypropylene precursor.
11. The method of claim 7 wherein laminating being at speeds
greater than 100 ft/min (30.5 m/min).
12. The method of claim 11 wherein laminating being at speeds
greater than 125 ft/min (38.1 m/min).
13. The method of claim 12 wherein laminating being at speeds
greater than 150 ft/min (45.7 m/min).
14. The method of claim 13 wherein laminating being at speeds
greater than 200 ft/min (61.0 m/min).
15. The method of claim 7 wherein laminating being conducted
between heated nip rollers.
16. The method of claim 15 wherein the nip roller temperature
ranging from 145.degree. C. to 170.degree. C.
17. The method of claim 16 wherein the nip roller temperature
ranges from 155.degree. C. to 165.degree. C.
18. The method of claim 15 wherein the nip roller pressure ranges
from 100 to 800 pounds per linear inch (pli).
19. The method of claim 18 wherein the nip roller pressure ranges
from 100 to 300 pli.
20. The method of claim 7 wherein a chill roll following the nip
rollers.
21. The method of claim 20 wherein the chill roll temperature
ranges from 20.degree. C. to 45.degree. C.
22. The method of claim 21 wherein the chill roll temperature
ranges from 25.degree. C. to 40.degree. C.
23. The method of claim 20 wherein an air knife being placed
between the nip rollers and the chill roll.
24. The method of claim 20 wherein edge trim knives follow the
chill roll.
25. The method of claim 7 wherein the plurality of laminated single
stacked precursor films being at least six laminated single stacked
precursor films.
26. The method of claim 25 wherein the plurality of laminated
single stacked precursor films being at least twelve laminated
single stacked precursor films.
27. The method of claim 26 wherein the plurality of laminated
single stacked precursor films being at least sixteen laminated
single stacked precursor films.
28. The method of claim 7 wherein making microporous said plurality
of laminated single stacked precursor films being selected from the
group consisting of a dry process and a wet process.
29. A method of making a battery separator comprising the steps of:
extruding a precursor film, laminating together two or more
precursor films to form a multi-layered precursor film, stacking up
at least twelve multi-layered precursor films, and making
microporous the stacked multi-layered precursor films.
30. The method of claim 29 wherein at least sixteen multi-layered
precursor films are stacked up.
31. The method of claim 29 wherein making microporous the stacked
multi-layered precursor films being selected from the group
consisting of a dry process and a wet process.
Description
FIELD OF THE INVENTION
[0001] A microporous laminated membrane useful as a battery
separator, particularly in lithium secondary batteries, and its
method of manufacture are disclosed herein.
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 the 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. There is no mention of
stacking precursors for the step of forming the micropores.
[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. 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. There is no mention of stacking precursors for the step
of forming the micropores.
[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).
[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.
[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).
[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 have such films with
greater interply adhesion (i.e., resistance to peeling individual
layers from one another, measured by peel strength). One route,
mentioned above, is to co-extrude the multi-layered film. From
co-extrusion, an infinite peel strength may be obtained because the
polymers at the interface of the layers are knitted together during
extrusion. However, when individual layers are extruded and
subsequently bonded (or laminated) together, peel strengths have
been limited (as noted above).
[0009] Accordingly, there is a need to improve the peel strength of
multi-layered microporous films made by laminating together
precursors.
SUMMARY OF THE INVENTION
[0010] A battery separator comprises a multi-layered film,
individual layers of said film having been bonded together by heat
and pressure, having a peel strength of greater than or equal to 40
grams per inch (1.6 g/mm) and a thickness of .ltoreq.25 microns. A
method for making a battery separator comprises the steps of:
extruding and winding up a first precursor film, extruding and
winding up a second precursor film, unwinding the first and second
precursor films, stacking up the first and second precursor films
to form a single stacked precursor, laminating the single stacked
precursor film, winding up the laminated single stacked precursor
film, stacking up a plurality of laminated single stacked precursor
films, and making microporous the stacked plurality of laminated
single stacked precursor films.
DETAILED DESCRIPTION OF THE INVENTION
[0011] 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 UHMWPE) 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.
Another separator is a 5-layered film with a PP/PE/PP/PE/PP or a
PE/PP/PE/PP/PE structure. Such separators have a thickness less
than 3 mils (75 microns, .mu.). Preferably, the thickness ranges
from 0.5 to 1.5 mils (12 to 38.mu.) (thickness is the average of 30
measurements across the width of the film, using a precision
micrometer with a 0.25-inch diameter circular shoe contacting the
sample at eight (8) psi). Most preferably, the thickness ranges
from 0.5 to 1.0 mils (12 to 25.mu.). Adhesion (interply adhesion,
measured by peel strength--using a Chatillon TCD-20 Peel force
Tester, Digital Gram Gauge Model DFG-2, and GF6 cam type Grips,
sample--1 inch (2.54 cm).times.6-8 inch (15.24-20.32 cm), peel back
1 inch (2.54 cm) of outside layers from the middle layer with
transparent tape and place one outside layer and middle layer in
grips) is greater than 40 grams/inch (1.6 g/mm), preferably greater
than 50 g/in (2.0 g/mm), and most preferably greater than 60 g/in
(2.4 g/mm). Other film properties are: Gurley <30 seconds
(Gurley--ASTM-D726(B)--a resistance to air flow measured by the
Gurley Densometer (e.g. Model 4120), the time (sec) required to
pass 10 cc of air through one square inch of product under a
pressure of 12.2 inches of water, 10 samples are averaged). Basis
weight ranging from 0.5-2.0 mg/cm.sup.2 (basis weight is the
average of 3--one foot square samples from across the width of the
sample weighted on a precision balance with an accuracy of 0.0001
grams). Shrinkage (%) is less than or equal to 5.0% (shrinkage is
the average of three 10 cm samples from across the width of the
film, they are measured, exposed to 90.degree. C. air for 60
minutes and re-measured, the average is reported. Puncture strength
.gtoreq.360 grams (puncture strength is the average of ten
measurements made from across the width of the sample. A Mitech
Stevens LFRA Texture Analyzer is used. The needle is 1.65 mm in
diameter with a 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 maximum
resistance force is the puncture strength.) The pore size is about
0.04.times.0.09.mu.. The calculated porosity is less than 60%,
preferably about 40%. The calculated density is 100--(apparent
density/resin density) and for multi-layered films, calculated
porosity is 100--.SIGMA. (apparent density/resin
density).sub.i.
[0012] In the manufacture of these films, the process generally
comprises: extruding nonporous precursors; bonding together the
nonporous precursors; and making microporous the bonded nonporous
precursors. For example, in a wet process, a mixture of matrix
components and extractable components are extruded to form a
nonporous precursor film. Precursor films are stacked for bonding,
the stacking being in the configuration of the desired end product.
The stacked precursor films are then bonded. Thereafter, the bonded
stacked precursor films are made microporous by subjecting that
film to an extraction bath where solvents would be used to remove
the extractable components from matrix components. In the dry
process, on the other hand, the matrix components are extruded to
form a nonporous precursor film. Precursor films are stacked for
bonding, the stacking being in the configuration of the desired end
product. The stacked precursor films are then bonded. Thereafter,
the bonded stacked precursor films are made microporous by
subjecting that film to an annealing and then stretching steps
where stretching induces pore formation at the interface of
crystalline and amphorous regions in the matrix components. The
invention will be further described with reference to the dry
process.
[0013] Extruding the precursor film is conventional. For example,
see U.S. Pat. Nos. 5,480,945; 5,691,047; 5,667,911; 5,691,077;
5,952,120; and 6,602,593. Matrix components are polyolefins. The
polyolefins are preferably any polyolefin suitable for blown film
or slot die film productions. Most preferred are polyethylene and
polypropylenes suitable for blown film or slot die film production.
Nonporous precursor films are extruded and wound up. For example,
in a blown film process, a tubular parison is extruded, collapsed,
and the wound up and in a slot die or T die process, the flat
parison is extruded and wound up. Each of these nonporous precursor
films will become a layer of the multi-layered microporous
membrane.
[0014] Laminating (e.g., bonding with heat and pressure via nip
rollers) of two or more of the nonporous precursor films is
performed next. The nonporous precursor films are unwound and
stacked in a conventional manner before bonding in a laminator. The
unwinding and stacking may be performed as illustrated in U.S. Pat.
Nos. 5,691,077 and 5,952,120, except only one set of stacked
nonporous precursor films (i.e., a set being a stack of precursor
films laid up in the configuration of the desired final microporous
membrane) is run through the heated nip rolls of the precursor at a
time. A preferred configuration is a tri-layer precursor with a
PP/PE/PP lay-up pattern. It is preferred that the higher melting
point material (e.g., PP in a PP/PE/PP) precursor be wider than the
lower melting point material (e.g., PE in a PP/PE/PP) so to prevent
sticking on the heated nip rolls. Line speeds through the heated
nip rolls are greater than 50 feet per minute (15.2 m/min) and
typically range from 50-200 fpm (15.2-61 m/min). Preferably, the
line speeds are greater than 100 fpm (30.5 m/min), more preferably
125 fpm (38.1 m/min), and most preferably, 150 fpm (45.7 m/min).
The heated nip roll temperature ranges from 100-175.degree. C.,
preferably 145 to 170.degree. C., and most preferably
155-165.degree. C. Nip roll pressure ranges from 100 to 800 pounds
per linear inch (pli) (17.7-141.7 kg.per linear cm), preferably 100
to 300 pli (17.7-53.1 kg per linear cm).
[0015] After the now bonded stacked nonporous precursor, which is
heated for bonding, is wound up. Prior to wind up, however, it is
desirable to cool the film. This cooling is preferably accomplished
by the use of a chill roll. The chill roll temperatures may range
from 20-45.degree. C., preferably 25-40.degree. C. It is most
preferred that this film be below the glass transition temperature
(Tg) of the outer most layer prior to contact with the chill roll,
this prevents the film from sticking to the chill roll. To assist
cooling and uniformity of cooling across the width of the film, an
air knife may be employed between the heat nip rollers and the
chill roll. Finally, the bonded, nonporous stacked precursor may
curl along the lateral edges of the film. If so, trim knives may be
used to remove the curl prior to winding. Two sets of stacked
nonporous precursor films may be simultaneously wound onto a single
roll.
[0016] Thereafter, the bonded, stacked precursor film is ready to
made microporous. A plurality of the bonded stacked precursor films
are stacked. At least four (4) bonded stacked precursor films are
stacked for further processing, preferably at least six (6), most
preferably at least twelve (12), and still more preferably at least
sixteen (16) may be stacked for further processing. The plurality
of bonded stacked precursor films are then simultaneously annealed
and then stretched in a conventional manner. For example, see: U.S.
Pat. Nos. 5,480,945; 5,691,047; 5,667,911; 5,691,077; 5,952,120;
and 6,602,593 for typical annealing and stretching conditions.
[0017] The foregoing invention will be further illustrated by way
of the following examples:
[0018] In the following examples, the films were made by identical
processes except Examples 1 and 3 were bonded together by the
inventive process and Comparative Examples 2 and 4 were prepared
according to the process set out in U.S. Pat. No. 5,952,120.
Lamination parameters fro the inventive process are as set forth
above, reference preferred ranges. Example 1 and Comparative
Example 2 have a nominal thickness of 25.mu., and Example 3 and
Comparative Example 4 have a nominal thickness of 20.mu..
1 EX 1 CEX 2 EX 3 CEX 4 Gurley 25.0 22.9 18.8 18.5 Thickness 26.5
25.0 20.7 20.2 Basis Weight 1.5 1.4 1.1 1.1 Shrinkage % 2.5 2.2 1.7
1.6 Adhesion 63.1 37.8 62.2 39.6 Porosity % 38.7 39.8 42.2 45.5
Puncture 471 476 423 446 Strength MD Strength 1521 1996 1977 1997
(Kg/cm.sup.2) MD % 46 46 43 41 Elongation TD strength 157 139 157
145 (Kg/cm.sup.2) TD % 151 555 931 788 Elongation Electrical 8.3
7.6 7.4 7.7 Resistance (ER)
[0019] Tensile properties (TD & MD strength and TD & MD %
Elongation) were measured using an INSTRON MODEL 4201 (with Series
IX Automated Materials Testing Software for Windows), crosshead
speed 508.00 mm/min, samples 5-1/2 inch (1.27 cm).times.6-8 inches
(15.24-20.32 cm), clamp pressure--90 psi (6.33 Kg.sub.f/cm.sup.2).
Electrical Resistance (ER) is reported as MacMullen Number
(N.sub.mac=r.sub.separator/.rho..sub.electro- lytet.sub.separator,
r.sub.separator=R(measured resistance of separator)A.sub.probe
(area of probe, cm.sup.2), .rho..sub.electrolyte=el- ectrolyte
resistivity (ohm-cm), t.sub.separator=separator thickness (cm))
using an EG&G Princeton Applied Research of Oak Ridge, Tenn.,
273A Potentiostat with 5210 Lock-in Amplifier and the PowerSuite
software. The test cell has a 1 square inch (6.45 square cm)
electrode faces that contact the wetted separator. Separators are
wetted with a 1 molar LiPF.sub.6 electrolyte in a 3:7 weight ratio
ethyl carbonate (EC) to ethyl methyl carbonate (EMC). Measurements
are taken at AC amplitude of 5 mV and a frequency range of 22,000
to 24,000 Hz. The report results are the average of four membranes,
4 membranes are stacked and measured, them remove one membrane and
measure 3 membranes and so forth, the differences are averaged and
reported.
[0020] The present invention may be embodied in other forms without
departing from the spirit and the essential attributes thereof,
and, accordingly, reference should be made to the appended claims,
rather than to the foregoing specification, as indicated the scope
of the invention.
* * * * *