SINGLE-LAYER LITHIUM ION BATTERY SEPARATORS EXHIBITING HIGH LOW-SHRINKAGE RATES AT HIGH TEMPERATURES

Abstract

An insulating (nonconductive) microporous polymeric battery separator comprised of a single layer of enmeshed microfibers and nanofibers is provided. Such a separator accords the ability to attune the porosity and pore size to any desired level through a single nonwoven fabric. Through a proper selection of materials as well as production processes, the resultant battery separator exhibits isotropic strengths, low shrinkage, high wettability levels, and pore sizes related directly to layer thickness. The overall production method is highly efficient and yields a combination of polymeric nanofibers within a polymeric microfiber matrix and/or onto such a substrate through high shear processing that is cost effective as well. The separator, a battery including such a separator, the method of manufacturing such a separator, and the method of utilizing such a separator within a battery device, are all encompassed within this invention.

Claims

1. A battery separator comprising fibers of length less than 2.54 cm (1 inch), such fibers comprising at least 5% of thermally stable fibers that have no melting point, glass transition temperature or thermal degradation below about 300 C., wherein said fibers are polymeric materials selected from the group consisting essentially of polyacrylonitriles, polyolefins, polyolefin copolymers, polyamides, polyvinyl alcohol, polyethylene terephthalate, polybutylene terephthalate, polysulfone, polyvinyl fluoride, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polymethyl pentene, polyphenylene sulfide, polyacetyl, polyurethane, aromatic polyamide, semi-aromatic polyamide, polypropylene terephthalate, polymethyl methacrylate, polystyrene, synthetic cellulosic polymers, polyaramids, and blends, mixtures and copolymers including said polymeric materials, wherein such battery separator exhibits no observable spectral reflectance of after application of a drop of an electrolyte on the surface thereof, wherein such battery separator has a mean flow pore size less than 2000 nm, wherein said separator allows for lithium ion transport with an electrolyte within a lithium ion battery, and wherein said separator exhibits an apparent density of at least 0.564 g/cm.sup.3.

2. A battery separator of claim 1, said separator further comprised of a single layer of said fibers, and wherein said fiber layer includes enmeshed microfibers and nanofibers, and wherein said separator exhibits a porosity of no greater than 51%.

3. The battery separator of claim 2, wherein said microfibers are entangled with one another; wherein said separator exhibits interstices between each entangled microfiber; and wherein said interstices include nanofibers present therein.

4. The battery separator of claim 2 in which said microfibers comprise fibrillated microfibers.

5. The battery separator of claim 2, wherein said microfibers have an average fiber diameter greater than 1000 nm.

6. The battery separator of claim 2, wherein said microfibers have an average fiber diameter greater than 3000 nm.

7. The battery separator of claim 2, wherein said nanofibers have an average fiber diameter less than 1000 nm.

8. The battery separator of claim 2, wherein said nanofibers have an average fiber diameter less than 700 nm.

9. The battery separator of claim 2 comprising nanofibers with a transverse aspect ratio greater than 1.5:1.

10. The battery separator of claim 1 wherein said separator exhibits a tensile strength greater than 59 kg/cm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 is an SEM microphotograph of a prior art expanded film battery separator.

[0034] FIG. 2 is an SEM microphotograph of a prior art nanofiber nonwoven fabric battery separator.

[0035] FIGS. 3 and 4 are SEM microphotographs of one potentially preferred embodiment of an inventive microfiber/nanofiber nonwoven fabric battery separator structure.

[0036] FIGS. 5, 6, and 7 are SEM microphotographs of another potentially preferred embodiment of an inventive microfiber/nanofiber nonwoven fabric battery separator structure.

[0037] FIG. 8 shows an exploded view of an inventive rechargeable lithium ion battery including an inventive battery separator.

DETAILED DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS

[0038] All the features of this invention and its preferred embodiments will be described in full detail in connection with the following illustrative, but not limiting, drawings and examples.

Microfiber and Nanofiber Production

[0039] As noted above, the microfiber may be constructed from any polymer (or polymer blend) that accords suitable chemical and heat resistance in conjunction with internal battery cell conditions, as well as the capability to form suitable fiber structures within the ranges indicated. Such fibers may further have the potential to be treated through a fibrillation or like technique to increase the surface area of the fibers themselves for entanglement facilitation during nonwoven fabrication. Such fibers may be made from longstanding fiber manufacturing methods such as melt spinning, wet spinning, solution spinning, melt blowing and others. In addition, such fibers may begin as bicomponent fibers and have their size and/or shape reduced or changed through further processing, such as splittable pie fibers, islands-in-the-sea fibers and others. Such fibers may be cut to an appropriate length for further processing, such lengths may be less than 50 mm, or less than 25 mm, or less than 12 mm even. Such fibers may be also be made long to impart superior processing or higher strength to have a length that is longer than 0.5 mm, longer than 1 mm, or even longer than 2 mm. Such fibers may also be fibrillated into smaller fibers or fibers that advantageously form wet-laid nonwoven fabrics.

[0040] Nanofibers for use in the current invention may be made through several longstanding techniques, such as islands-in-the-sea, centrifugal spinning, electrospinning, film or fiber fibrillation, and the like. Teijin and Hills both market potentially preferred islands-in-the-sea nanofibers (Teijin's is marketed as NanoFront fiber polyethylene terephthalate fibers with a diameter of 500-700 nm). Dienes and FiberRio are both marketing equipment which would provide nanofibers using the centrifugal spinning technique. Xanofi is marketing fibers and equipment to make them using a high shear liquid dispersion technique. Poly-aramids are produced by duPont in nanofiber state that exhibit excellent high temperature resistance, as well as other particularly preferred properties.

[0041] Electrospinning nanofiber production is practiced by duPont, E-Spin Technologies, or on equipment marketed for this purpose by Elmarco. Nanofibers fibrillated from films are disclosed in U.S. Pat. Nos. 6,110,588, 6,432,347 and 6,432,532, which are incorporated herein in their entirety by reference. Nanofibers fibrillated from other fibers may be done so under high shear, abrasive treatment. Nanofibers made from fibrillated cellulose and acrylic fibers are marketed by Engineered Fiber Technologies under the brand name EFTEC. Any such nanofibers may also be further processed through cutting and high shear slurry processing to separate the fibers an enable them for wet laid nonwoven processing. Such high shear processing may or may not occur in the presence of the required microfibers.

[0042] Nanofibers that are made from fibrillation in general have a transverse aspect ratio that is different from those made initially as nanofibers in typical fashion (islands-in-the-sea, for instance). One such transverse aspect ratio is described in full in U.S. Pat. No. 6,110,588, which is incorporated herein by reference. As such, in one preferred embodiment, the nanofibers have a transverse aspect ratio of greater than 1.5:1, preferably greater than 3.0:1, more preferably greater than 5.0:1.

[0043] As such, acrylic, polyester, and polyolefin fibers are particularly preferred for such a purpose, with fibrillated acrylic fibers, potentially most preferred. Again, however, this is provided solely as an indication of a potentially preferred type of polymer for this purpose and is not intended to limit the scope of possible polymeric materials or polymeric blends for such a purpose.

[0044] FIGS. 1 and 2 provide photomicrographs of the typical structures of the Celgard expanded film materials and the duPont nanofiber nonwoven battery separator materials, respectively, and as discussed above. Noticeably, the film structure of the Celgard separator shows similarity in pore sizes, all apparently formed through film extrusion and resultant surface disruptions in a rather uniform format. The duPont separator is made strictly from nanofibers alone as the uniformity in fiber size and diameter is evident. Being a nonwoven structure of such nanofibers themselves, the overall tensile strengths of this separator in both machine and cross directions are very low, although roughly uniform in both directions. Thus, such a material may be handled uniformly, as a result, although overall strength lends itself to other difficulties a manufacturer must face, ultimately, if introducing such a separator into a battery cell. To the contrary, then, the FIG. 1 separator, showing the striations for pore generation in the same direction (and thus extrusion of the film in one direction), provides extremely high machine direction tensile strength; unfortunately, the tensile strength of the same material in the cross direction is very low, leaving, as discussed previously, a very difficult and highly suspect battery separator material to actually utilize in a battery manufacturing setting.

[0045] The inventive materials, shown in photomicrograph form in FIGS. 3 and 4, are of totally different structure from these two prior art products. One potentially preferred embodiment of the initial combination of microfiber and nanofibers is the EFTEC. A-010-4 fibrillated polyacrylonitrile fibers, which have high populations of nanofibers as well as residual microfibers. The resultant nanofibers present within such a combination are a result of the fibrillation of the initial microfibers. Nonwoven sheets made of these materials are shown in FIGS. 3 and 4. By way of example, these fibers can be used as a base material, to which can be added further microfibers or further nanofibers as a way of controlling the pore size and other properties of the nonwoven fabric, or such a material may be utilized as the nonwoven fabric battery separator itself. Examples of such sheets with additional microfibers added are shown in FIGS. 5, 6 and 7. Typical properties of the acrylic Micro/Nanofibers are shown below.

TABLE-US-00001 TABLE 1 Acrylic Micro/Nanofiber Properties Density, g/cm.sup.3 1.17 Tensile Strength, MPa 450 Modulus, GPa 6.0 Elongation, % 15 Typical Fiber Length, mm 4.5-6.5 Canadian Standard Freeness, ml 10-700 BET Surface Area, m.sup.2/g. 50 Moisture Regain, % <2.0 Surface Charge Anionic

[0046] Such fibers are actually present, as discussed above, in a pulp-like formulation, thereby facilitating introduction within a wetlaid nonwoven fabric production scheme.

Nonwoven Production Method

[0047] Material combinations were then measured out to provide differing concentrations of both components prior to introduction together into a wet-laid manufacturing process. Handsheets were made according to TAPPI Test Method T-205, which is incorporated here by reference (basically, as described above, mixing together in a very high aqueous solvent concentration formulation and under high shear conditions as are typically used in wet laid manufacturing and described as refining of fibers, ultimately laying the wet structure on a flat surface to allow for solvent evaporation). Several different combinations were produced to form final nonwoven fabric structures. The method was adjusted only to accommodate different basis weights by adjusting the initial amount of material incorporated into each sheet. Materials and ratios are shown in Table 2.

[0048] FIGS. 5, 6, and 7 correlate in structure to Example 3 below, as well. The similarity in structure (larger microfibers and smaller nanofibers) are clarified, and the presence of fewer amounts of nanofibers in these structures is evident from these photomicrographs, as well.

[0049] The fabric was measured for thickness and then cut into suitable sizes and shapes for introduction within lithium ion rechargeable battery cells. Prior to any such introduction, however, samples of the battery separator fabrics were analyzed and tested for various properties in relation to their capability as suitable battery separators. Furthermore, comparative examples of battery separator nanofiber membranes according to U.S. Pat. No. 7,112,389, which is hereby incorporated by reference, as well as battery separator films from Celgard, are reported from the tests in the patent and from Celgard product literature.

EXAMPLES

[0050] Examples 36-51 were made according to TAPPI Test Method T-205 using Engineered Fiber Technologies EFTEC. A-010-04 fibrillated acrylic fiber (combination of microfiber and nanofiber)(listed as Base Fiber) and Fiber Visions T426 fiber, which is 2 denier per filament, cut to 5 mm length, a bicomponent fiber made from polypropylene and polyethylene, and has a diameter of approximately 17 microns (listed as Added Fiber). The sheets were calendered between two hard steel rolls at 2200 pounds/linear inch at room temperature (25 C.). The amount of each fiber, conditioned basis weight, caliper (or thickness), apparent density and porosity of the examples are shown in Table 4. Conditioned Basis Weight, Caliper, Apparent Density, and Tensile were tested according to TAPPI T220, which is hereby incorporated by reference.

TABLE-US-00002 TABLE 2 Separator Properties Con- % % ditioned Apparent Base Added Basis Wt Caliper Density Porosity Example Fiber Fiber g/m.sup.2 mm g/cm.sup.3 % 36 100 0 39.9 0.065 0.614 56.2% 37 90 10 40.2 0.067 0.600 55.6% 38 80 20 39.8 0.068 0.585 55.0% 39 70 30 39.9 0.07 0.570 54.4% 40 100 0 29.98 0.051 0.588 58.0% 41 90 10 29.89 0.053 0.564 58.2% 42 80 20 28.91 0.054 0.535 58.8% 43 70 30 30.9 0.074 0.418 66.6% 44 100 0 23.58 0.044 0.536 61.7% 45 90 10 24.8 0.046 0.539 60.1% 46 80 20 24.75 0.047 0.527 59.5% 47 70 30 24.15 0.053 0.456 63.5% 48 100 0 14.8 0.03 0.493 64.8% 49 90 10 16.6 0.036 0.461 65.8% 50 80 20 16.4 0.033 0.497 61.8% 51 70 30 16.5 0.037 0.446 64.3%

[0051] The higher the porosity, the higher the peak power output within the subject battery. With such high results, theoretically, at least, the number of batteries necessary to accord the necessary power levels to run certain devices (such as hybrid automobiles, for instance) would be reduced through an increase in the available power from individual batteries. Such a benefit would be compounded with an effective air resistance barrier as well. The porosity of the inventive separator may also be controlled by the ratio of nanofiber to microfibers, the types of nanofibers, and also by post processing such as calendaring, as can be seen below.

Battery Separator Base Analysis and Testing

[0052] The test protocols were as follows:

[0053] Porosity was calculated according to the method in U.S. Pat. No. 7,112,389, which is hereby incorporated by reference. Results are reported in %, which related to the portion of the bulk of the separator that is filled with air or non-solid materials, such as electrolyte when in a battery.

[0054] Gurley Air Resistance was tested according to TAPPI Test Method T460, which is hereby incorporated by reference. The instrument used for this test is a Gurley Densometer Model 4110. To run the test, a sample is inserted and fixed within the densometer. The cylinder gradient is raised to the 100 cc (100 ml) line and then allowed to drop under its own weight. The time (in seconds) it takes for 100 cc of air to pass through the sample is recorded. Results are reported in seconds/100 cc, which is the time required for 100 cubic centimeters of air to pass through the separator.

[0055] Mean Flow Pore Size was tested according to ASTM E-1294 Standard Test Method for Pore Size Characteristics of Membrane Filters Using Automated Liquid Porosimeter which uses an automated bubble point method from ASTM F 316 using a capillary flow porosimeter. Tests were performed by Porous Materials, Inc., Ithaca, N.Y.

The air permeability of a separator is a measurement of the time required for a fixed volume of air to flow through a standard area under light pressure. The procedure is described in ASTM D-726-58.

TABLE-US-00003 TABLE 3 Tensile properties and Mean Flow Pore Size Mean Flow MD Tensile CD Tensile Pore Size Example kg/cm.sup.2 kg/cm.sup.2 microns 36 94 94 0.13 37 85 85 0.13 38 67 67 0.15 39 59 59 0.20 40 88 88 0.15 41 69 69 0.18 42 51 51 0.25 43 29 29 0.62 44 74 74 0.19 45 65 65 0.23 46 56 56 0.27 47 40 40 0.69 48 52 52 49 57 57 50 42 42 51 34 34

[0056] The inventive example thus shows a very small pore size mean, indicating a capability to permit a large number of recharge cycles for the subject battery. In addition, the ability to control the pore size is indicated by the change in pore size with the proportional change in the ratio of nanofiber and microfiber materials. This is a key advantage that is not present in any previous art, such that with this technology the pore size can be dialed in by the battery manufacturer depending on the requirements of the end user. Thus, a separator can be designed for a power tool or automotive application to have different characteristics from a rechargeable watch battery, cell phone or laptop computer.

[0057] The tensile properties in the examples given are isotropic, that is, the same in all directions, with no distinction between machine and cross directions. Comparative examples show tensile properties that vary considerably between machine direction (MD) and cross direction (CD) tensile strength. In general, nanofiber-based battery separators are quite weak. Thus, one advantage of the current invention is the tensile strength, which allows faster processing in battery manufacture, tighter winding of the batteries, and more durability in battery use. Such MD tensile strength is preferably greater than 25 kg/cm.sup.2, more preferably greater than 50 kg/cm.sup.2, and most preferably greater than 100 kg/cm.sup.2. The requirements on the CD tensile strength are lower, preferably being greater than 10 kg/cm.sup.2, more preferably being greater than 25 kg/cm.sup.2, and most preferably greater than 50 kg/cm.sup.2.

[0058] As noted above, calendering and an increased population of nanofibers relative to microfibers will reduce the overall pore size mean, even further, thus indicating, again, the ability to target certain measurements on demand for the inventive technology. Sheet production of the initial separator was then undertaken on a paper making machine (to show manufacturing may be simplified in such a manner) with such a calendering, etc., step undertaken as well.

Paper Machine Production

[0059] Two materials were then made on a rotoformer paper machine. The first, Example 52, was made from 75% EFTec A-010-4 and 25% 0.5 denier/filament polyethylene terephthalate (PET) fiber with cut length 6 mm. The second, Example 53, was made from 37.5% EFTec A-010-4, 37.5% EFTec L-010-4 and 25% PET fiber with cut length 6 mm. The fiber materials were dispersed using high shear mixing and mixed at high dilution in water, then fed into the rotoformer head box and made to sheets of weight 20 grams/m.sup.2 and dried in a hot air oven. The resultant rolls were calendered at 325 F. at 2200 pounds/linear inch, resulting in thicknesses of .about.40 microns for the first sheet and 30 microns for the second sheet. Shrinkage was measured at 90 C., 130 C., and 160 C. by measuring a 12 length in each of machine and cross direction, placing in an oven stabilized at the measuring temperature for 1 hour, and measuring the length again. The shrinkage is the change in length expressed as a percentage of the original length. Properties of the sheets are shown below in Table 4.

TABLE-US-00004 TABLE 4 Membrane Properties Unit of Example Example Basic Membrane Property Measure 52 53 Thickness m 40 30 Gurley (JIS) seconds 20 110 Porosity % 60% 55% Mean Flow Pore Size m 0.5 0.5 TD Shrinkage @ 90 C/1 % 0 0 Hour MD Shrinkage @ 90 C/1 % 0 0 Hour TD Shrinkage @ 130 C/1 % 0 0 Hour MD Shrinkage @ 130 C/1 % 2 1 Hour TD Shrinkage @ 160 C/1 % 1 0 Hour MD Shrinkage @ 160 C/1 % 4 2 Hour TD Shrinkage @ 190 C/1 % 5 0 Hour MD Shrinkage @ 190 C/1 % 7 2 Hour TD Strength Kg/cm.sup.2 70 100 MD Strength Kg/cm.sup.2 190 170 Elongation % 4% 4%

[0060] As can be seen, the materials with both acrylic (EFTec A-010-4) and lyocell (EFTec L-010-4) materials show very good properties at high temperature. For example, many current stretched film separators may be made from polyethylene, which melts at 135 C. and shows significant shrinkage at over 110 C., or from polypropylene, which melts at 160 C. and shows significant shrinkage over 130 C. One problem that is known in the industry, especially for large format cells that might be used in electric vehicles, is that shrinkage upon exposure to high temperature can expose the electrodes to touching each other on the edges if the separator shrinks, causing a short and potentially a catastrophic thermal runaway leading to an explosion. Separators with high temperature stability thus are safer in these environments, allowing larger format cells to be used with higher energy per cell. Preferred separator performance might be to have less than 10% shrinkage at 130 C., 160 C. or 190 C. in both directions, or preferably less than 6% shrinkage or most preferably less than 3% shrinkage. In addition, the separator might be made with a component that has high temperature stability such as a lyocell, rayon, para-aramid, meta-aramid, or other fiber, that when formed into a sheet with other materials imparts a low shrinkage result, as is shown in Example 53.

[0061] Additional examples were made and tested for different calendering conditions. The paper was constructed on a Rotoformer at the Hefty Foundation facility, and consisted of 27% EFTec A-010-04 acrylic nanofiber, 53% EFTec L-010-04 lyocell nanofiber, and 20% 0.5 denier/filament polyester fiber with 5 mm cut length. The materials were mixed for 40 minutes in a 1000 gallon hydropulper, and then fed into the machine at approximately 0.25% fiber content, and a sheet made that was 15 grams/m.sup.2 in areal density. This paper was calendered under different conditions, which are listed below and shown as Examples 56-60 in the Table 5 below.

Legend for Examples 56-60:

[0062] 56: Calendered using the conditions above, except the rolls were not heated.

[0063] 57: Sheet was fed through the calender with a second sheet of Example 56, plying the sheets together.

[0064] 58: Sheet from 56 was fed through the calender with a roll of copy paper, then peeled from the copy paper.

[0065] 59: Sheet from 56 was calendered with a second pass under the same conditions.

[0066] 60: The plies of 57 were peeled apart, resulting in two separate sheets.

[0067] Two things can be seen from the examples below. First, the lamination of two sheets gives more than twice the Gurley air resistance of a single sheet, while lowering the total porosity. Second, calendaring a second time had the effect of increasing the porosity and lowering the Gurley. Last, the two sheets that were fed through with another sheet had the effect of increasing the Gurley and increasing the porosity at the same time. Tensile strength was decreased in all cases with additional calendering.

TABLE-US-00005 TABLE 5 Calendered Sheet Results Conditioned Apparent MD TD Gurley Air Basis Wt Caliper Density Porosity Tensile Tensile Resistance Example g/m.sup.2 mm g/cm.sup.3 % kg/cm.sup.2 kg/cm.sup.2 seconds 56 14.7 0.031 0.474 59.6% 155 69 38 57 30.0 0.060 0.500 57.4% 136 53 105 58 15.2 0.037 0.412 64.9% 102 44 48 59 15.1 0.036 0.419 64.2% 99 40 34 60 15.0 0.036 0.415 64.6% 94 43 40

Wettability Testing

[0068] A square of Example 39 was taken along with a square of Celgard 2320, and a drop of 1 M LiPF6 in EC:DMC:DEC mixture (1:1:1 by volume) electrolyte was placed on the surface. After 5 seconds, the electrolyte had been completely absorbed into Example 39, with no spectral reflectance (as from the shiny surface of a liquid drop) observable. The electrolyte drop on the Celgard 2320 remained for several minutes. This is highly desirable for a lithium ion battery separator to increase the processing rate of dispersing the electrolyte, as well as to ensure uniform dispersion of the electrolyte. Non-uniform dispersion of the electrolyte is known to promote dendrite formation on repeated charge and discharge, which become defects in the cells and can lead to short circuits.

[0069] As such, it may be desirable to have a separator upon which the spectral reflectance of a drop of electrolyte deposited on the material disappears is less than 5 minutes, or less than 2 minutes, or more preferably less than 1 minute. In addition, it may be desirable to make an energy storage device from two electrodes, a separator and an electrolyte, such that the spectral reflectance of a drop of said electrolyte deposited on the separator disappears in less than 5 minutes, or less than 2 minutes, or more preferably less than 1 minute.

[0070] Other tests were undertaken involving Differential Scanning calorimetry and Thermogravimetric Analysis for Wettability measurements as well. Example 53 was tested for thermogravimetric analysis from room temperature to 1000 C. The sample showed 1.39% mass loss, ending near 100 C., which is consistent with water loss from the cellulose nanofibers and microfibers. The material showed no further degradation until approximately 300 C., when oxidation set in and a sharp decrease of approximately 60% mass between 335 and 400 C. The Example 53 was also tested for differential scanning calorimetry from room temperature to 300 C. There was a broad exotherm centered around 100 C., consistent with a release of water, and a sharper exotherm at 266 C. which onset at 250 C., consistent with the melting point of PET.

[0071] Example 52 was tested for thermogravimetric analysis from room temperature to 1000 C. The sample showed very little mass loss below 300 C., with an onset of mass loss at 335 C., and an approximately 40% mass loss up to 400 C. The Example 52 was also tested for differential scanning calorimetry from room temperature to 300 C. There was almost no signature shown between room temperature and a sharp exotherm at 266 C., onset at 250 C., consistent with the melting point of PET. In short, the curve showed no signature other than the melting of the PET microfibers.

Aramid Samples

[0072] Additional samples were made on a Rotoformer machine, similar to Examples 52 and 53.

[0073] In Example 61, four types of fibers were combined at low dilution, approximately 60 lbs in 7000 gallons of water, under very high shear conditions. The fibers were: [0074] EFTec A-010-04 20 lbs [0075] EFTec L-010-04 20 lbs [0076] Teijin 1094 wet pulp 10 lbs [0077] 0.3 dpf PET 5 mm 10 lbs
Sheets were made at 18 grams/meter.sup.2, and calendered at 2200 pounds/inch and 250 F. The properties of the sheets are shown below in Table NN.

[0078] In Example 62 and 63, three types of fibers were combined similar to Example 61. The fibers were: [0079] EFTec L-010-04 20 lbs [0080] Teijin 1094 aramid wet pulp 20 lbs [0081] 0.3 dpf PET 5 mm 10 lbs
Sheets were made at 18 and 15 grams/meter.sup.2, Examples 62 and 63 respectively. The properties of the sheets are shown below in Table 6.

TABLE-US-00006 TABLE 6 Aramid-containing Sheet Properties Unit of Example Example Example Basic Membrane Property Measure 61 62 63 Basis Weight Grams/meter.sup.2 18 18 15 Thickness m 43 45 43 Gurley (JIS) seconds 76 28 19 Porosity % 51% 36% 43% TD Shrinkage @ 90 C. % 0 0 0 MD Shrinkage @ 90 C. % 0 0 0 TD Shrinkage @ 160 C. % 2 1 0 MD Shrinkage @ 160 C. % 3 1 0 TD Shrinkage @ 200 C. % 3 1 1 MD Shrinkage @ 200 C. % 4 1 1 TD Shrinkage @ 240 C. % 2 2 MD Shrinkage @ 240 C. % 3 2 TD Strength Kgf/cm.sup.2 59 39 24 MD Strength Kgf/cm.sup.2 121 70 62 Elongation % 1.6% 1.6% 1.8%

[0082] As can be seen, the use of a high temperature fiber such as an aramid pulp like Teijin 1094 aramid pulp provides very low shrinkage even at high temperatures. Since batteries can be subject to thermal degradation, and this degradation can result in thermal runaway, having a component in the battery that will retain its structural shape and integrity and possibly prevent or slow down thermal runaway could be very desirable. Thus it is desirable to have a material that would shrink less than 5% at 200 C., or even less than 3%. It also may be desirable to have a material that would shrink less than 5% at 240 C., or even less than 3%. To achieve this, it may be desirable to have a component of the separator that has no melting point, glass transition temperature or thermal degradation below about 300 C. In order to make the battery separator itself have thermal stability, it may be desirable to have this thermally stable fiber component at more than 5% of the total ingredients of the separator, or even more than 10%, or even more than 20%.

Battery Formation and Actual Battery Testing Results

[0083] FIG. 8 shows the typical battery 10 structure with the outside housing 12 which includes al of the other components and being securely sealed to prevent environmental contamination into the cell as well as any leakage of electrolyte from the cell. An anode 14 is thus supplied in tandem with a cathode 16, with at least one battery separator 18 between the two. An electrolyte 20 is added to the cell prior to sealing to provide the necessary ion generation. The separator 18 thus aids in preventing contact of the anode 14 and cathode 16, as well as to allow for selected ion migration from the electrolyte 20 therethrough. The general format of a battery cell follows this structural description, albeit with differing structures sizes and configurations for each internal component, depending on the size and structure of the battery cell itself. In this situation, button battery of substantially circular solid components were produced for proper testing of separator effectiveness within such a cell.

Further Battery Products and Tests

[0084] Additional pouch cell batteries were built as follows: Standard cell phone battery electrodes have a coat weight that is approximately 2.5 mAh/cm.sup.2. Electrodes were produced for test procedures exhibiting a coat weight of 4 mAh/cm.sup.2 (NCA) to demonstrate that the capability limits of the separator were exceeded versus standard practices as it pertained to rate capability. One cell (hand built) of each separator type was built with Celgard 2325 (Example 54, below) and Example 53 (Example 55, below). The electrodes were coated, calendered, dried, welded with tabs, put into laminate packaging, and filled with a 1M Li salt in a standard battery solvent electrolyte, and sealed. The cells were tested for discharge capacity at C/10, C/4, C/2 and C rates with several discharges at each rate, and the results are shown in Table 7 below as a percentage of the first discharge at C/10 capacity after formation. The specific discharge capacity at C/10 for the Example 54 cell was 141 mAh/g and for Example 55 cell was 145 mAh/g.

TABLE-US-00007 TABLE 7 Pouch Battery Measurements Rate Example 54 Example 55 C/10 100.3% 101.3% C/4 95.5% 98.3% C/2 69.5% 88.7% C 36.4% 57.1%

[0085] As can be seen from these examples, and consistent with the testing of the batteries of Examples 32-35, the battery made using the inventive separator had higher discharge capacity at higher rates, with a small advantage at C/4, but larger and significant advantages at rates of C/2 and C.

[0086] It should be understood that various modifications within the scope of this invention can be made by one of ordinary skill in the art without departing from the spirit thereof. It is therefore wished that this invention be defined by the scope of the appended claims as broadly as the prior art will permit, and in view of the specification if need be.