Shutdown, bilayer battery separator

Abstract: The present invention is directed to a shutdown, bilayer battery separator and a process for making the same. A first microporous membrane with shutdown capability and a second microporous membrane with strength capability are joined together in face-to-face contact. The face of the first membrane being adhered by calendaring, adhesives, or welding, to the face of the second membrane, and the separator having a thickness of less than 3 mils, and a puncture strength, as measured from the second microporous membrane, of greater than 1900 g-mm, and a peel strength of greater than 1 grams/centimeter.

Description: Field of the Invention The present invention is directed to a shutdown, bilayer battery separator. Background of the Invention Shutdown, bilayer battery separators are known. For example, see U.S. Patent Nos. 4,650,730; 4,731,304; 5,240,655; 5,281,491; and Japanese Kokai No. 6-20671. In batteries, the anode and cathode are separated from one another by a separator. Today, "lithium batteries" are very popular because they are able to generate high energy outputs. The lithium battery market can be divided into two groups, the "primary" lithium battery and the "secondary" lithium battery. The primary lithium battery is a disposable battery, while the secondary lithium battery is a rechargeable battery. A problem associated with the secondary lithium battery is its potential for short circuiting. Short circuiting arises when the separator ruptures and allows the anode and the cathode to come into direct electrical communication with one another. This short circuit may manifest itself with a rapid evolution of heat. This rapid evolution of heat can cause the battery to explode. Accordingly, the shutdown battery separator was developed. The shutdown battery separator generally comprises two polymerically dissimilar and juxtaposed microporous membranes. One microporous membrane is chosen for its relatively low melting point and the other for its relative strength. For example, the low melting point membrane may be a polyethylene material and the strength membrane may be a polypropylene material. The polyethylene microporous membrane has a melting point of about 130°C. This is sufficiently low that, in the event of a short circuit in a lithium battery, the heat generated will melt the polyethylene so that it shuts down, or fills in the pores of the separator, and thereby stops or inhibits the likelihood of a short circuit. The polypropylene membrane, which has a substantially higher melting point, approximately 160°C, provides strength to the separator so that it maintains the separator's integrity in the event of a short circuit. In U.S. Patent No. 4,650,730 and 4,731,304, shutdown, bilayer battery separators of the foregoing type are disclosed. In the examples, separator thicknesses of 76-102 µm (3-4 mils) are disclosed. The methods of making these battery separators are disclosed as: 1) two discrete films containing extractable fillers are made, then bonded together, and then the fillers are extracted; 2) a bilayer film, containing extractable fillers, is coextruded and the fillers are subsequently extracted; 3) one film made by an extrusion, annealing and stretching process, and a second film made with extractable fillers are bonded together, and then the fillers are subsequently extracted; 4) a first film is coated with a second material. In U.S. Patent No. 5,240,655, a shutdown, bilayer battery separator, of the above-mentioned type, is made by coextruding the discrete layers, and then stretching and annealing the extruded bilayer film. In U.S. Patent No. 5,281,491, the shutdown, bilayer battery separator, of the above-mentioned type, is made by coextruding the first and second films that contain an extractable material, and the extractable material is subsequently removed. In Japanese Kokai 6-20671, a shutdown, bilayer battery separator, of the above-mentioned type, has a thickness of about 25-51µm (1 to 2 mils) a penetration energy (from Figure 1) of about 1660 g-mm (this value is equivalent to about 1800 g-mm when converted into units comparable with the test method disclosed herein), and a peel strength of 0.1 to 1 gram/centimeter. The separator is made by a process in which two discrete films, made by the process set forth in U.S. Patent Nos. 3,679,538 or 3,801,404, are bonded together by calendaring at a temperature of 134°C. A problem with the foregoing battery separators is that, while they provide the necessary shutdown characteristics, they are deficient in one or more of the following qualities: thinness or penetration energy (or puncture strength) or peel strength. In the manufacture of batteries, it is important to,have extremely thin separators, so that: the energy density of the battery maybe increased;and the size of the battery, as well as, the electrical resistance across the separator, may be reduced. Good penetration energy is important in battery manufacture, particularly in the manufacture of "jelly roll" type batteries because the surfaces of the anode and the cathode can be sufficiently rough that they can puncture these extremely thin separators during manufacture. Good peel strength is important in battery manufacture because it prevents delamination of the separator. Accordingly, there is a need to produce an extremely thin, shutdown battery separator that has a sufficient penetration energy and peel strength to withstand the rigors of battery manufacture. Summary of the Invention The present invention is directed to (A) a shutdown, bilayer battery separator comprising: a first microporous membrane having shutdown capability, being made of a material selected from polyethylene, a blend comprising substantially polyethylene and a copolymer of polyethylene and having a face; and a second microporous membrane having strength capability, being made of a material selected from polypropylene, a blend comprising substantially polypropylene and a copolymer of polypropylene and having a face; and said first membrane and said second membrane being joined together in face-to-face contact; and said separator having a thickness less than 76 µm (3 mils); a puncture strength, as measured from said second microporous membrane, of greater than 1900 g-mm; and a peel strength of greater than 1 g/cm; (B) a battery comprising the separator as defined in (A) above; and (C) a process for making a shutdown, bilayer battery separator as defined in (A) above, which process comprises the steps of: 1) forming a first microporous membrane by a) extruding a first polymer selected from polyethylene, a blend comprising substantially polyethylene and a copolymer of polyethylene to form a first sheet, b) annealing said first sheet, and c) stretching said first sheet; 2) forming a second microporous membrane by a) extruding a second polymer selected from polypropylene, a blend comprising substantially polypropylene and a copolymer of polypropylene to form a second sheet, b) annealing said second sheet, and c) stretching said second sheet; and 3) bonding said first membrane in face-to-face contact with said second membrane by a method comprising calendaring at a temperature less than 134 °C, adhering with adhesives , and welding. Description of the Invention The present invention shall be described in further detail below by way of the following detailed description and the examples. Shutdown battery separator, as used herein, refers to the type of separator described in U.S. Patent Nos. 4,640,730; 4,731,304; 5,240,655; 5,281,491; Japanese Kokai No. 6-20671; and U.S. patent application No. 08/341,239, filed November 17, 1994 and entitled "Methods of Making Cross-ply Microporous Membrane Battery Separators, and the Battery Separators Made Thereby". These battery separators are preferably bilayer, i.e., have two different layers, one that provides the shutdown characteristics and the other that provides the strength characteristics. These separators, however, are not so limited and they may have more than two layers. Since the bilayer separator is preferred, it shall be discussed hereinafter. These separators are made with at least two layers, one being a microporous membrane having shutdown capabilities and the other being a microporous membrane having strength capabilities. The shutdown capabilities refer to the principal, but not necessarily the sole, function of the layer which is the ability to close the micropores of the separator in the event of a short circuit. This typically means that the shutdown layer will melt, at some temperature, blind the pores of the separator, and thereby terminate the short circuit by prohibiting ion migration across the separator. Preferably, in lithium batteries, the shutdown capabilities can be provided by a material that will melt at a temperature at least 20°C below the melting temperature of the lowest melting electrode (e.g., the lithium material, the melting point of lithium is about 180°C). In the present invention the first membrane is made from polyethylene or a blend comprising substantially polyethylene, or a copolymer of polyethylene. The strength capabilities refer to the principal, but not necessarily the sole, function of the layer which is the ability to maintain the integrity of the separator in the event of a short circuit, as well as, to facilitate the manufacture of the battery. Preferably, in lithium batteries, the strength capabilities can be provided by a material that will melt at a temperature at about or greater than the melting temperature of the lowest melting electrode (e.g., the lithium material).In the present invention the second membrane is made from polypropylene or a blend comprising substantially polypropylene, or a copolymer of polypropylene. The thickness of these separators is less than 76 µm (3 mils). These separators preferably range in thickness from 23 µm (0.9 mils) to less than 76 µm (3 mils). Measurement details are set forth below. The puncture strength (penetration energy) should be greater than 1900 grams - millimeters, when measured from the strength enabling side of the separator. Preferably, the puncture strength should be greater than 2000 gram - millimeters and, most preferably, the puncture strength should be greater than 2400 grams - millimeters. Measurement details are set forth below. The strength enabling side of a shutdown, bilayer battery separator made of a polyethylene layer and a polypropylene layer is the polypropylene side. The peel strength should be greater than 1 gram/centimeter. Preferably, the peel strength should be greater than or equal to 4 grams/centimeters and, most preferably, the peel strength should be greater than or equal to 8 grams/centimeter. Measurement details are set forth below. The process, by which the inventive separators are made, broadly comprises making a first microporous membrane, making a second microporous membrane, and bonding together the first membrane and the second membrane. Regarding the preferred method for making the first and the second membranes, the process requires the following steps: extruding a polymer to form a sheet; annealing the sheet; and stretching the annealed sheet. The specific methods for making these sheets, particularly polyethylene or polypropylene, are within the skill of the art. The following refernces illustrate possible routes for making the sheets: U.S. Patent Nos: 3,426,754; 3,558,764; 3,679,538; 3,801,404; 3,801,692; 3,843,761; 3,853,601; 4,138,459; 4,539,256; 4,726,989; 4,994,335; and U.S. Patent Application No. 08/341,239, filed November 17, 1994. Further specifics regarding the preferred method for making these sheets may be found in the examples below. The first and second membranes are preferrably oriented, with respect to one another in the same direction. Cross-plying, as set forth in U.S. Application No. 08/341,239 is also contemplated. Regarding the preferred methods for bonding the first membrane to the second membrane, several bonding methods are contemplated. Broadly, the bonding methods include calendaring at a temperature less than 134°C, adhering with adhesives, and welding. Preferably, bonding is accomplished by calendaring, with nips closed, at a temperature of about 130°C (a peel strength of greater than 1 gram/centimeter is thereby obtained). The application of adhesives may include: air atomizing; gravure/screen printing; hydraulic spraying; and ultrasonic spraying. The choice of adhesive and the rate of adhesive application must be chosen so that the separator's porosity is not adversely effected. The welding technique includes thermal welding and ultrasonic welding. The amount of energy for either welding procedure and the pattern of welds should be chosen so that the separator's porosity is not adversely effected. Further information about the foregoing inventions may be obtained from the following examples. The test methods referenced to herein are set forth below. Test Methods Gurley ASTM-D726(B) Gurley is a resistance to air flow measured by the Gurley densometer (e.g. Model 4120). Gurley is the time in seconds required to pass 10 cm 3 of air through 6.5 cm 2 ( one square inch) of product under a pressure of 3.04 kPa (12.2 inches of water). Basis Weight Basis weight is determined by cutting three - 0.092 m 2 (1 square foot) samples across the width of the sample and weighing them on a precision balance with accuracy to 0.0001 grams. The three (3) samples are averaged and compared to tolerance limits. Thickness Method: T411om-83 developed under the auspices of the Technical Association of the Pulp and Paper Industry. Thickness is determined using a precision micrometer with a 12.7mm (1/2 inch) diameter, circular shoe contacting the sample at 483 mbar (seven (7) PSI). Ten (10) individual micrometer readings taken across the width of the sample are averaged. Shrinkage, MD ASTM D-1204 (60 min at 90°C) Three (e) separate lengths of stretched product of approximately ten centimeters are measured across the width of the sample in the machine direction (MD). The sample is exposed to air at 90°C for one hour, the lengths are re-measured, the percentage shrinkage of the original length is calculated for each sample, and the results averaged. Tensile Strength ASTM D-882 Puncture Strength 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/s 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 film. The penetration energy (puncture strength) was defined as the product of the resistance force and the displacement at the maximum point. Peel strength Peel Strength is measured using a tension and compression tester to determine the force in grams required to separate two one-inch wide sections of bonded membrane. The peel rate is 6 inches/minute. Three measurements are taken across the web and averaged. Melt Index ASTM D 1238, (P); PP: 230°C, 2.16 Kg; PE: 190°C, 2.16 Kg EXAMPLE 1 A polyethylene sheet product is formed by extruding a high density polyethylene with a melt index of 0.6 g/10 min and a density of 0.96 g/cm 3 . Extrusion conditions are summarized in Table 1. A polypropylene sheet product is formed by extruding an isotactic polypropylene homopolymer having a melt index of 1.5 g/10 min and a density of 0.90 g/cm 3 . Extrusion conditions are summarized in Table 1. Extrusion conditions Polyethylene Polypropylene Die Temperature, °C 195 212 Melt Temperature, °C 220 228 Air ring height, cm (inches) 1.9 (0.75) 5.7 (2.25) Take up speed, fpm 80 80 Draw down ratio 90 to 1 82 to 1 Blow up ratio 1 1 Air plenum pressure, Pa (inches of water) 870 (3.5) 620 (2.5) Weighted gauge, µm (mil) 14.7 (0.58) 16.0 (0.63) Birefringence 0.031 0.016 The sheet products are annealed in an oven under slight tension at a constant temperature for 10 minutes. The annealing temperature for polypropylene is 140 °C and for polyethylene, 115 °C. The annealed material is then subjected to a cold stretch at room temperature, followed by a hot stretch at elevated temperatures, and then by heat setting with relaxation of the film. Stretching conditions are summarized in Table 2. The stretched products are microporous films with the properties summarized in Table 3. Stretching Conditions Polyethylene (PE) Polypropylene (PP) Cold Stretch, % 30 50 Hot Stretch, % 200 150 Hot Stretch Temperature, °C 115 140 Relaxation, % 50 50 Relaxation Temperature, °C 120 140 The two ply stretched film is slit along the edges and deplied. Using four unwind stands, two plies of polyethylene and two plies of polypropylene are mounted in the sequence of polyethylene on the top, the two polypropylene plies, and a polyethylene ply on the bottom. The four plies are passed through the oven under slight tension at 130 °C with nips closed. Two polyethylene-polypropylene bilayers are produced after separating the polypropylene plies. Adhesion between the polyethylene and polypropylene is about 8 grams/cm (the peel strength, at these values, does not appear to be significantly dependent upon peel rate of 0.65 to 15.2 cm/min (0.25 to 6 inch/min)). The properties of the bilayer separator are summarized in Table 3. Stretched Microporous Film and Bilayer Properties PE PP Bilayer Gurley, s·cm 2 (sec-in 2 ) 64.5 (10) 103 (16) 161 (25) Thickness, µm (mil) 13.0 (0.51) 12.7 (0.50) 26.9 (1.06) Basis Weight, mg/cm 2 0.77 0.73 Porosity, % 36 38 Machine direction shrinkage, % 1 2 3 Machine direction stress, kbar (kpsi) 1.65 (24) 1.72 (25) 1.72 (25) Machine direction elongation, % 62 48 43 Transverse direction stress, kbar (kpsi) 124 (1.8) 138 (2.0) 124 (1.8) Transverse direction elongation, % 860 690 800 Electrical resistance, mΩ·cm 2 (mOhms-in 2 ) 32 (5) 26 (4) 39 (6) Puncture strength, grams-mm 2400 EXAMPLE 2 A polyethylene sheet product is formed by extruding a high density polyethylene with a melt index of 0.3 g/10 min and a density of 0.96 g/cm 3 . Extrusion conditions are summarized in Table 4. A polypropylene sheet product is formed by extruding an isotactic polypropylene homopolymer having a melt index of 1.5 g/10 min and a density of 0.90 g/cm 3 . Extrusion conditions are summarized in Table 4. Extrusion conditions PE PP Die Temperature, °C 180 210 Melt Temperature, °C 204 224 Air ring height, cm (inches) 4.45 (1.75) 5.08 (2.0) Take up speed, fpm 60 50 Draw down ratio 115 to 1 120 to 1 Blow up ratio 1 1 Air plenum pressure, Pa (inches of water) 299 (1.2) 299 (1.2) Weighted gauge, µm (mil) 15.5 (0.61) 14.5 (0.57) Birefringence 0.035 0.016 The sheet products are annealed in an oven under slight tension at a constant temperature for 10 minutes. The annealing temperature for polypropylene is 140 °C and for polyethylene, 115 °C. The annealing step increases the crystallinity of the sheet products. The annealed material is then subjected to a cold stretch at room temperature, followed by a hot stretch at elevated temperatures, and then by heat setting with relaxation of the film. Stretching conditions are summarized in Table 5. The stretched products are microporous films with the properties summarized in Table 6. Stretching Conditions Polyethylene Polypropylene Cold Stretch, % 45 25 Hot Stretch, % 150 140 Hot Stretch Temperature, °C 115 140 Relaxation, % 50 50 Relaxation Temperature, °C 115 140 The two ply stretched film is slit along the edges and deplied. Using four unwind stands, two plies of polyethylene and two plies of polypropylene are mounted in the sequence of polyethylene on the top, the two polypropylene plies, and a polyethylene ply on the bottom. The four plies are passed through the oven under slight tension at 130 °C with nips closed. Two polyethylene-polypropylene bilayers are produced after separating the polypropylene plies. Adhesion between the polyethylene and polypropylene is 8 grams/cm (the peel strength, at these values, does not appear to be significantly dependent on peel rate of 0.65 to 15.2 cm/min (0.25 to 6 inch/min)). The properties of the bilayer are summarized in Table 6. Stretched Microporous Film and Bilayer Properties Polyethylene Polypropylene Bilayer Gurley, s·cm 2 (sec-in 2 ) 51.6 (8) 64.5 (10) 103.2 (16) Thickness, µm (mil) 12.7 (0.50) 12.7 (0.50) 26.2 (1.03) Basis Weight, mg/cm 2 0.75 0.74 Porosity, % 40 39 Puncture strength, grams-mm 2060

Claims: A shutdown, bilayer battery separator comprising: a first microporous membrane having shutdown capability, being made of a material selected from polyethylene, a blend comprising substantially polyethylene and a copolymer of polyethylene and having a face; and a second microporous membrane having strength capability, being made of a material selected from polypropylene, a blend comprising substantially polypropylene and a copolymer of polypropylene and having a face; and said first membrane and said second membrane being joined together in face-to-face contact; and said separator having a thickness less than 76 µm (3 mils); a puncture strength, as measured from said second microporous membrane, of greater than 1900 g-mm; and a peel strength of greater than 1 g/cm. The separator according to claim 1 wherein said puncture strength is greater than 2000 g-mm. The separator according to claim 1 wherein said puncture strength is greater than or equal to 2400 g-mm. The separator according to claim 1 wherein said peel strength is greater than or equal to 4 g/cm. The separator according to claim 1 wherein said peel strength is greater than or equal to 8 g/cm. The separator according to claim 1 wherein said thickness ranges from 23 µm to less than 76 µm (0.9 mils to less than 3 mils). The separator according to claim 1 wherein said first membrane's melting temperature is at least 20 °C less than said second membrane's melting temperature. The separator according to claim 1, wherein the first microporous membrane is a polyethylene membrane; the second microporous membrane is a polypropylene membrane; said separator has a puncture strength, as measured from the polypropylene membrane, greater than or equal to 2400 g-mm; and a peel strength greater than or equal to 8 g/cm. A battery comprising the separator set forth in claim 1 or 8. A process for making a shutdown bilayer battery separator as defined in claims 1 to 8, which process comprises the steps of: 1) forming a first microporous membrane by a) extruding a first polymer selected from polyethylene, a blend comprising substantially polyethylene and a copolymer of polyethylene to form a first sheet, b) annealing said first sheet, and c) stretching said first sheet; 2) forming a second microporous membrane by a) extruding a second polymer selected from polypropylene, a blend comprising substantially polypropylene and a copolymer of polypropylene to form a second sheet, b) annealing said second sheet, and c) stretching said second sheet; and 3) bonding said first membrane in face-to-face contact with said second membrane by a method comprising calendaring at a temperature less than 134 °C, adhering with adhesives , and welding. The process according to claim 10 wherein bonding is by calendaring at a temperature of 130 °C. The process according to claim 11 wherein the calendaring nip rolls are closed.

Inventors: GEIGER MARGARET W, YU WEI-CHING, GEIGER, MARGARET W., YU, WEI-CHING
Applicants: HOECHST CELANESE CORP, HOECHST CELANESE CORPORATION
Application year: 1995
Main classification: H01M2
IPC codes: B32B5, B29C55, C08J9, H01M2