Battery separators with reduced splitting propensity

Details Battery separators with reduced splitting propensity Battery separators with reduced splitting propensity

1999-08-30

2003-08-05

Morris, Terrel (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C428S304400, C428S315500, C428S315900, C429S254000

Reexamination Certificate

active

06602593

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to battery separators, in particular to a battery separator with reduced splitting propensity and method of making same.
BACKGROUND OF THE INVENTION
Microporous film battery separators are used in various batteries, particularly rechargeable batteries, such as lithium batteries. Such battery separators allow electrolytes to cross through the battery separator while preventing any contact between electrodes of opposite polarity. Typically, the microporous film comprises one or more layers of microporous membranes.
In lithium batteries, particularly secondary lithium batteries, overheating problems can occur and cause thermal runaway in the battery. Thus, shutdown separators, including mono-layer shutdown separators and multi-layer separators, were developed to prevent thermal runaway. See e.g., U.S. Pat. No. 4,650,730 and U.S. Pat. No. 4,731,304. A shutdown battery separator has a microporous membrane that closes its pores at a temperature substantially lower than the temperature that could cause thermal runaway in the lithium battery. Multi-layer shutdown separators are known in the art and have been disclosed in, e.g., U.S. Pat. Nos. 5,565,281 and 5,691,077, Japanese Patent Application Nos. JP7-304110A and JP8-250097A, and UK Patent Publication No. GB 2,298,817. Typically, multi-layer shutdown separators comprise one or more shutdown layers and at least one strength layer. The shutdown layer or layers are capable of melting and filling the pores at a temperature (shutdown temperature) below the melting point of the strength layers. As a result, when the micropores are eliminated in the shutdown layers at the shutdown temperature, the strength layers retain substantially their dimensional stability and thus maintain the integrity of the separator in the event of short circuit and prevent the ion flow between the electrodes.
A microporous shutdown separator should be thin in order to minimize the space it occupies in the battery and also to reduce electrolytic resistance. Nevertheless the shutdown separator must also have sufficient strength to resist puncture. One problem often encountered with thin battery separators known in the art is that they are prone to splitting, i.e., tearing as a result of puncture. This creates difficulties in handling the separators especially in the battery separator manufacturing processes. Tom separators are not only defective in preventing direct contact of electrodes and thus are ineffective in preventing thermal runaway. Thus, there is great need in the art to develop split resistant battery separators.
SUMMARY OF THE INVENTION
The present invention provides a split resistant microporous membrane for use in preparing a battery separator. The microporous membrane is made by a process which includes the steps of preparing a film precursor by a blown film extrusion process at a blow-up ratio of at least 1.5, annealing the film precursor, and stretching the resultant annealed film precursor to form the microporous membrane.
Generally, in a blown film extrusion process, the polymer film precursor exhibits a crystalline row structure in which lamellae are arranged in rows with their long axis perpendicular to the take-off direction (machine direction or MD). Such a crystalline structure is important to the formation of micropores in the subsequent annealing and stretching steps. Although it has been generally believed that expansion of a blown film in the transverse direction would disrupt such a crystalline structure and interfere with the micropore formation in the subsequent stretching step, it has now been found that even when the extruded film precursor is greatly oriented in the transverse direction (TD) as a result of the increase in blow-up ratio, the resultant film precursor is still suitable for subsequent annealing and stretching operations and micropores can be formed.
It has been discovered that when the blow-up ratio used in the extrusion process is at least 1.5, the microporous film exhibits improved split resistance. As the blow-up ratio increases, a blown film is increasingly oriented in the transverse direction (TD), i.e., the direction perpendicular to the machine direction (MD). As a result, the tensile strength in the transverse direction of the blown film is increased. In accordance with the present invention, a microporous film precursor for use in a battery separator is typically substantially oriented in the machine direction. Thus, as the blow-up ratio increases, the difference between the tensile strength in the transverse direction and the machine direction is reduced even in absence of an extra stretching step in the transverse direction. As a result, the resultant microporous film is less prone to split.
The microporous film of this invention exhibits significantly improved split resistance while still possessing other mechanical properties desired for battery separators.
Typically, the microporous film of this invention is made of a polyolefin, preferably polyethylene, polypropylene, or copolymers, terpolymers and derivatives thereof. The blow-up ratio is at least about 1.5, preferably at least about 2.0, and more preferably at least about 2.5. The TD tensile strength to MD tensile strength ratio of the microporous film is from about 0.1 to about 1.0, preferably from about 0.12 to about 1.0, more preferably from about 0.5 to about 1.0. Typically, the film has a Gurley value of from about 5 seconds to about 100 seconds, preferably from about 10 seconds to about 60 seconds, as measured by the ASTM-D726(B) method, and a shutdown temperature of from about 80° C. to about 160° C., preferably from about 90° C. to about 130° C., more preferably from about 100° C. to about 120° C. The tear strength in the transverse direction of the film is at least about 40 kgf/cm
2
, preferably at least about 50 kgf/cm
2
, more preferably at least about 60 kgf/cm
2
, even more preferably at least about 70 kgf/cm
2
, and most preferably at least about 80 kgf/cm
2
, as measured by the method of ASTM D-1004.
In a preferred embodiment of the invention, a multi-layer shutdown battery separator is provided having one or more microporous shutdown layers each being sandwiched between two microporous strength layers. The microporous membrane of this invention is used as at least one of the microporous layers in the multi-layer battery separator.
In the multi-layer shutdown separator of this invention, different layers of the separator can be prepared separately and subsequently laminated together to form the multi-layer separator. Optionally, cross-plied separators can be provided wherein the microporous membranes are laminated such that the axis of one ply is angularly biased (preferably orthogonally) relative to the axis of another ply. Generally, cross-plied microporous membrane separators exhibit increased strength and puncture-resistance properties.
Alternatively, the multiple layers of the shutdown battery separator can be made by a co-extrusion process, in which all layers are extruded together and are subsequently annealed and stretched to form a multilayer separator.
The battery separator of the present invention exhibits substantially increased split-resistance characteristics, thus making the separator much easier to handle in both the separator production process and during the process of making a lithium battery using the separator. However, no extra components are added in the polymer resins for making the battery separator. In addition, the orientation in the transverse direction is achieved as a film precursor is extruded in the blown film extrusion process. No separate steps are required. Thus, the present invention provides battery separators with substantially improved mechanical properties without requiring additional materials and complex steps.
The foregoing and other advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunc

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