Untitled record

Single-layer porous film, battery separator made therefrom, its preparation and battery pack equipped with this battery separator

The present invention relates to a porous film with a single-layer structure and a method for preparing the film. More specifically, the present invention relates to a porous film that can lose its permeability at a temperature equal to or higher than a specified temperature. The present invention also relates to the battery separator produced by the porous film and the battery pack equipped with the battery separator.

Porous polymer membranes have been used in the range of filtration and separation technologies. Various techniques have been used to impart certain desired characteristics to such polymer films. For example, the film can be stretched to improve its strength. When it is used in filtration operations, the film can be treated with a surfactant to improve its affinity for the filtration solution. Various monomers with special functional groups are grafted or copolymerized to give the film an ion exchange function.

In some applications, it is desirable to have a porous polymer film that can lose permeability at a temperature higher than the specified temperature. For example, such a thin film will be highly anticipated as a separate component in a self-rechargeable lithium secondary battery. Under normal operating conditions, the film must have the property of allowing ions contained in the electrolyte of the battery to pass freely through the porosity ("ion permeability" or "electrolyte conductivity") of the film. However, when the temperature in the battery rises above a certain point due to an error during charging or due to a short circuit between the electrodes or other reasons, such permeability must be sufficiently reduced to cut off the current. If such a current cut-off cannot be achieved, the vapor of the solvent used in the electrolyte solution may cause excessive pressure increase in the battery to cause danger, such as fire or explosion.

In the past, separators for conventional batteries, lithium batteries and capacitors used kraft paper or manila hemp sheet materials. Recently, non-woven fabrics and porous polyolefin films with high mechanical strength have been used for this purpose. However, these films do not exhibit the ability to reduce the conductivity of the electrolyte to cut off the current in the battery.

In particular, lithium batteries are designed to allow high-density current to flow, and therefore, when a short circuit occurs between the electrodes, the temperature inside the battery will rise rapidly. This short circuit causes the current to be too high, which in turn accelerates the chemical reaction at the anode electrode. The rapid rise in temperature in the battery is very dangerous. If there is no protection mechanism, it can reach 140°C and higher quickly. This situation may cause the organic solvent used in the electrolyte solution to catch fire or may even cause the battery to explode. To avoid such dangers, a variety of countermeasures are now needed. Since commercial products need to be safe, an organization that can protect them from such dangers has great market demand.

One of the suggestions to solve such problems is to use as a battery separator, a material that melts and, therefore, becomes non-porous when it is above its melting point. Under exothermic conditions, this separator will lose its ion permeability, preventing the flow of excessive current. Some people have suggested polyolefins as materials for such purposes. However, the mechanical strength of polyolefins decreases as the melting point decreases. Therefore, the material having sufficient strength and suitable as the material of the separator is, for example, polypropylene or high-density polyethylene which has a melting point of 130°C or higher. However, with such materials, the porosity only begins to decrease after the battery temperature has reached 130°C or higher. Therefore, the risk of fire or explosion caused by abnormal heating is still high. This type of separator is not suitable for achieving the desired result.

Recently, some people have proposed a multi-layered battery separator as a method to cut off the battery current in the case of heat release. Such a separator includes a porous polymer film with a high melting point as a support layer and a porous polymer film with a low melting point as a meltable layer. These films are laminated so that a porous polymer film with a low melting point can be melted at a temperature above its melting point to form an impermeable layer while providing the strength of the higher melting point film. For example, Japanese Patent Publication (Kokai) SHO 62-10857 (1987) discloses a battery separator made of a multilayer porous polyolefin film, which is disclosed in Japanese Patent Publication by laminating a non-crosslinked polyethylene film on a crosslinked polyethylene film (Kokai) HEI 3-55947 (1991), and US Patent 4,741,979 disclose a battery separator made of non-woven fabric with a layer of wax coating on it. Also, a lithium battery separator has a multilayer structure including a porous or a microporous polymer support layer and a meltable non-woven fabric thermally bonded to this support layer as disclosed in Japanese Patent Publication (Kokai) HEI 2-75152 (1990 )middle. For reference, a cross-sectional schematic diagram of a porous film with a multilayer structure formed on a porous polymer layer as a support layer by laminating a meltable porous polymer layer is shown in FIG. 2.

However, battery separators formed from multilayer porous polymer films have problems with thickness and uniformity. In small lithium batteries, the thickness of the membrane as the battery separator must be reduced and made uniform at the same time to provide an increased energy density per unit weight or volume. In other words, the current state of this art requires the use of a porous film made from a polymer with a high melting point (first-order transition temperature) to achieve the required mechanical strength of the film to maintain its functionality as a separator. However, because the electrolyte conductivity needs to be completely blocked when the battery temperature rises to about 130°C, it is necessary to use a material with a low melting point to completely seal the pores of the film. The need for high mechanical strength and the ability to melt pores to become substantially non-porous according to the current polymer technology is contradictory to the obvious.

Although this multilayer separator provides both strength and ability to be substantially non-porous at a specified temperature, this type of separator has a larger thickness than required to achieve high energy density, and when laminating different layers of the film A skin-like structure with low porosity may be formed between these laminated layers. Its structure is that the permeability of the multilayer film may be greatly reduced, lost or blocked by such a skin layer.

It is highly expected that a single-layer porous film is thin, has high mechanical strength, and can become substantially non-porous at low temperatures (for example, 80-130°C) without loss of mechanical strength. Such a film would be suitable for use as a separator in a battery to provide a safety measure against overheating and the disasters from it.

An object of the present invention is to provide a single-layer porous polymer film suitable for use as a battery separator, which exhibits good mechanical strength and high porosity at normal operating temperatures and can become substantially non-porous at a predetermined high temperature. Stop the electrolyte conductivity in this battery.

Another object of the present invention is to provide a battery separator for a lithium secondary battery to ensure its safety against abnormal heating.

<u style="single">Summary description</u>

A porous film with a single-layer structure according to the present invention includes (a) a porous polymer sheet having micropores in its interior, the polymer sheet being made of a first polymer having a predetermined melting point and (b) a The second polymer has a melting point lower than the prescribed melting point of the first polymer. The second polymer exists inside the micropores in such a state that it does not block the micropores, but in such an amount that it blocks when the film reaches a temperature substantially equal to or higher than the melting point of the second layer of polymer Inside these micropores. At the prescribed melting point of the second polymer, the film substantially loses its porosity.

The present invention includes a method for preparing a porous film with a single-layer structure, which includes the following steps:

(i) Melt mixing (a) a first polymer with a certain prescribed melting point, (b) a second polymer with a certain prescribed melting point lower than the melting point of the first polymer and (c) A third polymer that is not miscible with the first polymer and the second polymer to form a polymer film.

(ii) Extract only the third polymer with one solvent that can only dissolve the third polymer.

The present invention also includes a battery separator formed from the above-mentioned porous film of the present invention.

The present invention further includes a battery having at least a pair of cathode/anode, a separator between the cathode and anode and an electrolyte composition, wherein the separator is formed from the above-mentioned porous film of the present invention.

Fig. 1-(a) and Fig. 1-(b) are schematic diagrams of the cross-sections of a porous film with a single-layer structure before and after heating at a certain temperature according to the present invention. In Figure 1-(b), this micropore is blocked.

Figure 2 is a schematic cross-sectional view of a conventional porous film with multiple layers, one of which is a meltable layer.

Fig. 3 is a schematic cross-sectional view of a device for measuring the ion permeability of a porous membrane with a single-layer structure according to the present invention.

Figure 4-(a) and Figure 4-(b) are scanning electron microscopes of the cross-section of the porous film with a single-layer structure obtained in Example 1 of the present invention before and after heating at 115°C with a magnification of 5000 times Photo.

Figure 5-(a) and Figure 5-(b) are scanning electron microscopes of the cross-section of the porous film with a single-layer structure obtained in Example 2 of the present invention before and after heating at 115°C with a magnification of 5000 times Photo.

Figure 6-(a) and Figure 6-(b) are scanning electron micrographs of the cross-section of the porous film with a single layer structure obtained in Example 6 of the present invention before and after heating at 100°C at 5000 times magnification .

Detailed description

For the purpose of clearly describing the present invention, the following terms throughout this specification and the scope of the appended patent application have the following meanings: "Film" refers to a sheet product with a predetermined length and width and a thickness through which the sheet product It is defined by the boundary generated by the two main surfaces.

"Single-layer structure" means that, with reference to the film of the present invention, a sheet product has a substantially uniform composition throughout the body or thickness of the sheet product. Such a uniform composition can be generated from a single polymer or multiple polymers, but the single-layer structure is not a laminated configuration.

"First polymer" refers to a polymer or copolymer used to produce the subject film which has a high melting point of at least about 130°C and is inactive to the other components (electrodes and electrolyte components) of the battery conceived for use.

"Second polymer" refers to a polymer or copolymer used to form the subject film. It has a low melting point between 80°C and 120°C and is insensitive to other components (electrodes and electrolyte components) of the battery conceived for use. Sexual.

"Melting point" refers to the primary transition temperature of a polymer.

The porous film of the present invention has a single-layer structure in cross section. This film has stretched length and width dimensions to provide its end-use needs. For example, the correct size will depend on the size of the battery, the electrode plate and the type used (for example as a separator in jelly-roll, as a bag separator, as a plate or leaf separator). The separator has a thickness composed of a polymer substance with channels or pores extending from one main surface through this thickness to the other surface. These pores need not be straight but can be interconnected to provide communication between the two main surfaces. The polymer material of the film body is composed of at least one first polymer alone or a homogeneous mixture combined with at least one second polymer.

One method of producing a single-layer sheet structure with the desired properties involves initially using a porous film, such as, for example, such a first polymer (as described herein such as, for example, a polyolefin (e.g., polypropylene)) as The porous film. Heat and dissolve a second polymer in a solvent such as toluene or xylene and maintain the temperature. Immerse the above-mentioned porous first polymer film in the heated solution to fill the micropores The inside of the porous polymer film is filled with the second polymer solution. Then the inside of the micropores of the porous polymer film is filled with the second polymer solution and dried in an air dryer or the like to remove the micropores. The solvent in the pores leaves the second polymer residue at the same time. If the adhesion of the second polymer to the walls of the micropores in the porous first polymer film is insufficient and the second polymer has a gap from the pores The tendency of the inner wall to separate, the film can be reheated under controlled conditions to partially melt and adhere the second polymer inside the micropores without blocking the micropores and to improve the adhesion of the second polymer Focus on. This heating operation must be performed below the melting point of the first and second polymers.

In the above-mentioned method, the concentration of the second polymer solution must be low so as not to cause the micropores to be blocked by the second polymer. The preferred concentration varies with the porosity of the porous first polymer film and the like. However, anyone can easily determine the required concentration by experiment or experience. The concentration should usually be from 0.05 to 20% by weight of the solution. For example, in the case of using a porous film with a porosity of 55%, the required concentration of the second polymer solution is normally from 0.1 to 15% by weight.

As shown schematically in Fig. 1-(a), the first polymer and the second polymer are both exposed and part of the wall forming the pores of the film. The second polymer is in such a state and in a quantity that it does not block the micropores. The second polymer is present on the wall surface and may have other second polymers attached to it, which is part of the film material. When the film is subjected to a temperature equal to or higher than the melting point of the second polymer, it occurs that the micropores of the film are blocked by the second polymer. The second polymer constituting the walls of the micropores or in contact with the walls of the micropores flows into the pore channels at this temperature. In this way, the permeability of the porous film is lost or substantially disappeared.

Under normal conditions, ions can easily move through these pores which are filled with electrolyte components. Thus, the electrolyte conductivity is high and the battery functions as required. However, when the temperature rises above the normal operating temperature to indicate abnormal conditions and therefore exceed the melting point of the second polymer, the second polymer becomes a fluid to close these micropores, and thus block the movement of ions through these micropores as shown in Figure 1-(b).

The second method of forming the subject film includes the following steps: (i) melt-blending (a) a first polymer with a specified melting point, (b) a second polymer with a specified melting point lower than The melting point of the first polymer and (c) a third polymer that is immiscible with the first polymer and the second polymer to form a polymer film; (ii) a polymer film that can only dissolve the first polymer The solvent extraction of the three polymer is only the third polymer.

Although it is not intended as a limitation on the present invention, it is believed that according to the second preferred method of the present invention, a thin film is produced. The melt blend produces a sheet-like structure in which (i) the first polymer, the first polymer The molecules of the second polymer and the third polymer are entangled with each other to form a continuous layer. (ii) The molecules of the second polymer are attached to a part of the surface of the first polymer (between the first and second Between the three polymer phases) and the first and third polymers form a continuous phase, and these continuous phases are entangled with each other to form a network structure.

The first polymer and the second polymer described below can be used in either of these two production methods and the third polymer is used in connection with this second method, and will be described in more detail below.

Any polymer with a high enough melting point to endure the use conditions of a battery can be used as the first polymer to form the porous polymer film of the present invention. The first polymer may be a simple polymer or copolymer or may be a mixture of polymers with high melting point characteristics. Polymers with high crystallinity and high tensile strength are preferable for battery separator films. The mechanical strength and modulus can be improved by cross-linking the polymer, provided that it is not performed to the extent that it adversely affects the purpose of the present invention. Since the porous polymer film has high heat resistance and high mechanical strength, it is preferable that the crystallinity and molecular weight of the first polymer forming the porous polymer film should be high and/or the physical cross-linking of the first polymer is Used to impart such properties. However, if the molecular weight and the degree of cross-linking are too high, a thin film may not be formed, or the required softness or elasticity of the film may not be achieved. Such processing must be controlled and may not be welcome for certain purposes. Cross-linking methods that can be used in the present invention include these conventional methods by electron beam or radiation or with a silane coupling agent or a peroxide or the like.

Examples of the first polymer with these properties that can be used in the present invention are polyolefins having high tensile strength, good modulus, elasticity, and a high melting point of at least 130°C, such as those having a melting point of 130°C and higher and Polypropylene with a molecular weight of about 30,000 to about 800,000 (weight average) can be used alone or in combination with lower molecular weight polymers. Specific examples include a polypropylene that has a melting point of 169°C, a melt flow index of 0.5 g/10 minutes, and a density of 0.91 g/cm3 (product of Mitsui Nisseki Polymers Co., Ltd.), and this type of polymer is under the brand name "NOBLEN" JS" (product of Mitsui Toatsu Chemicals, Inc.), "CHISSOPOLYPRO" (product of Chisso Corporation), "IDEMITSU POLYPRO" (product of Idemitsu Petrochemical Co., Ltd.), "MITSUBISHI POLYPRO" (Mitsubishi Petrochemical Co., Ltd.) . Products), and "TONEN POLYPRO" (Tonen Corporations products) sellers and other polymers with the above properties. Other polymers that can be used as the first polymer include high molecular weight polyethylenes, copolymers of an olefin and an α, β-unsaturated monomer such as ethylene, acrylates and the like.

The second polymer which forms a meltable phase in the porous film of the present invention can be widely selected from polymers and copolymers having a melting point lower than the melting point of the first polymer and preferably from 95°C to 120°C. Examples of this second polymer are low-molecular-weight polyolefins, such as polyethylenes and their copolymers, including low-density polyethylenes such as "MITSUBISHI POLYETHYLENE-LD" (product of Mitsubishi Kasei Corporation), "MIRASON" (Product of Mitsui Petrochemical Industries, Ltd.), "SUMIKATHENE" (product of Sumitomo Chemical Co., Ltd.), "ULTZEX" (product of Mitsui Petrochemical Industries, Ltd.), "FLO-THENE" (Sumitomo Seika Chemicals Co. ., Ltd. product), "MISSEKI REXLON" (Nippon Petrochemical Co., Ltd. product) seller; linear low-density polyethylene such as "SUMIKATHENE-L" (Sumitomo Chemical Co., Ltd. product), "IDEMITSU POLYETHYLENE-L" (product of Idemitsu Petrochemical Co., Ltd.), "LINIREX" (product of Nippon Petrochemical Co., Ltd.) and ethylene/vinyl acetate copolymers, ethylene/methacrylic acid copolymers, ethylene /Acrylate copolymers, ethylene/methacrylate copolymers, ethylene/propylene copolymers, ethylene/propylene/diene terpolymers and ethylene, maleic anhydride and other monomers Meta copolymers.

The first polymer and the second polymer described above can be appropriately selected and combined to form a porous film with the single-layer structure of the present invention. Those who are proficient in this technique, based on the stability of the two polymers under normal operating conditions and based on the fact that the second polymer has a melting point and loses ion permeability at the required temperature, are what they want, and they can make a simple choice. For example, when the porous film of the present invention is used for a lithium battery as a separator, polypropylene with a melting point of 130°C or higher is used as the first polymer and a low molecular weight polymer with a melting point of about 95-120°C is used. Ethylene as the second polymer will produce a useful battery separation device. The battery itself has a better mechanical strength and its permeability can be quickly lost at the specified critical temperature to prevent the battery and the device used in it from being catastrophic. Sexual destruction.

The ratio of the first polymer and the second polymer to form the porous film of the present invention must be such that the micropores of the porous film will not be blocked by the second polymer under normal conditions, but when the predetermined safe temperature is exceeded Can be blocked by this second polymer. But such a popular ratio can be based on the thickness of the porous film, the size and shape of the micropores in the porous film, the method of preparing the porous film, and the method of introducing the second polymer into the micropores. Change, anyone who is proficient in this skill can determine the best ratio through experimentation. The weight ratio of the first polymer to the second polymer should usually be from 1:1 to 9:1 and preferably from 1:1 to 8:10. For example, when using a porosity with a porosity of 50% and a thickness of 25 microns When filming, the weight ratio of the first polymer to the second polymer should be 1:1 to 4:1.

In addition, if necessary or desired, various additives such as antioxidants, ultraviolet absorbers, colorants, lubricants and/or release agents can be added to the polymer composition forming the subject film. These additives can be added to the first polymer or the second polymer in a range that does not deleteriously affect the present invention during the process of producing the subject sheet product. Such an amount is normally up to 5% by weight and preferably up to about 1% by weight of the resulting composition.

To increase the adhesion of the first polymer to the second polymer, a copolymer of an olefin and an unsaturated carboxylic acid can be added to the composition as a part of the starting polymer mixture. Examples of such copolymers are "BONDINE" (product of Sumitomo Chemical Co., Ltd.), "YUKARON" (product of Mitsubishi Petrochemical Co., Ltd.), and "ADMER" (a kind of polymer sold under the trade name). Olefin graft copolymers, products of Mitsui Petrochemical Industries, Ltd.), "SUMIFARM" (products of Sumitomo Chemical Co., Ltd.) and "REUBEX" (products of Asahi Chemical Industrial Co., Ltd.). Such copolymers can be added for adhesion purposes in amounts up to 10% by weight, preferably up to 5% by weight.

Various methods such as those described above can be used to prepare the porous film with a single layer structure of the present invention. The second way to describe this is as follows.

Melt-blending a first polymer and a second polymer which constitutes the porous film and a third polymer which is immiscible with the first polymer and the second polymer (as described in detail below) and This melt blending produces a film with a polymer network structure (hereinafter referred to as "IPN structure") penetrating each other, in which the molecules of the second polymer are attached to the surface of the first polymer A part of the first polymer and the third polymer form a continuous phase, and these continuous phases are entangled with each other to form a network structure. The film can be produced by known techniques such as casting, extrusion and the like. After the film formation operation, the third polymer is extracted with a flux that can only dissolve the third polymer to obtain a porous sheet with a single-layer structure of the present invention. The schematic diagram of the IPN structure is shown in Figure 1.3 on page 3 of Takashi Inoue and Shoji Ichihara, "POLYMER ALLOY" (edited by Society of Polymer Science, Japan, published by Kyoritsu Shuppan).

The third polymer can be selected from polymers, which are immiscible with the first polymer and the second polymer that form the porous film of the present invention, can form an IPN structure with the first polymer, and can use a special Type of solvent extraction "a special type of solvent" refers to such a type of solvent at a specified processing temperature which is lower than the melting point of the first polymer and the second polymer, preferably lower than the lowest melting point At least 30°C and most preferably from the surrounding temperature to 60°C, it can dissolve only the third polymer but not the first polymer and the second polymer. Anyone skilled in this art can decide, by experiment or by experience, that this special type of solvent can be used as a solvent for the third polymer and its combination can be used for one of the first polymer and the second polymer Special combination. An example of this third polymer that can be used in the present invention is a styrene/hydrogenated butadiene/styrene band polymer (referred to herein as "SEBS") having a number average molecular weight from about 10,000 to 300,000 including "KRATON" (Product of Shell Chemical Co.); styrene/hydrogenated isoprene/styrene block copolymer (herein referred to as "SEPS") has a number average molecular weight from about 10,000 to 300,000 and styrene/hydrogenated isoprene The olefin block copolymer has a number average molecular weight from about 5,000 to 200,000.

When, for example, this SEBS or this SEPS is blended with polyolefins such as polypropylene and polyethylene melts, it generates an IPN structure. After the film is formed from the melt blend, the SEBS or the SEPS can be extracted with a special type of solvent to prepare a porous film with through-pores. Can be used for this styrene/hydrogenated butadiene/styrene block polymer, this styrene/hydrogenated isoprene/styrene block polymer and this styrene/hydrogenated isoprene block copolymer Solvents are solvents with solubility parameters ranging from 7.5 to 9.3 including, for example, toluene, xylene, cyclohexane, methyl alkyl hexane, methyl ethyl ketone, carbon tetrachloride and chloroform.

Although the above description is a representative example of the method of making the film from the first polymer, the second polymer, and the third polymer, other conventional film-forming techniques such as expansion, casting, and extrusion can also be used. Method through a T-shaped or similar. In addition, in order to increase the mechanical strength of the obtained porous film as a final product, the film can be stretched at a temperature higher than the secondary transition point of the first polymer but lower than its melting point and, if required, this is done. The stretched film can be annealed at a temperature higher than the stretching temperature to relax the residual tension in the film. The stretch ratio that can be used in the present invention is typically 1.1 to 10 times, preferably 1.1 to 5 times. Stretching can be performed before or after (preferably after) the third polymer is extracted from the sheet produced therefrom.

The porosity of the porous film of the present invention is typically not higher than 80%. When the porosity is higher than 80%, the mechanical strength of the porous film is drastically reduced and therefore, the porous film cannot be used for practical purposes. In addition, if the porosity is higher than 80%, a larger amount of the second polymer must be used to block the micropores of the porous polymer layer by melting the second polymer, and thus completely lose the first polymer and The composition of the second polymer is balanced and, as a result, the porous film will be mainly made of this second polymer. Therefore, the mechanical strength of this porous film cannot be ensured for various uses.

When the porous film of the present invention is used as a battery separator, the porosity is typically 30% to 80%, preferably 40% to 70%. When the porosity is less than 30%, it is very likely to reduce the maintenance of an electrolyte solution in the separator membrane and cause it to have a dry state. On the other hand, when the porosity is higher than 80%, the mechanical strength of the porous film is too low for practical purposes, and it is also prone to short-circuit due to piercing and squeezing of the dendrites generated on the cathode.

The porosity of the porous film of the present invention can be adjusted by the amount of the third polymer, and the film made from the first polymer, the second polymer and the third polymer will be produced after the third polymer is extracted therefrom Pores. The amount of the third polymer used to form the starting sheet is preferably from 0.5 to 9 times the weight of the total weight of the first and second polymers. When this amount is less than 0.5 of the total weight of the first and second polymers, the required porosity cannot be obtained. On the other hand, when the weight of the third polymer is greater than 9 times the total weight of the first and second polymers, the mechanical strength of the resulting porous film is usually too low for practical purposes.

In addition, in order to increase the porosity, the film formed from the first polymer, the second polymer and the third polymer can be annealed at a temperature lower than the melting point of the second polymer. As a result of this annealing, the tension of each polymer in the formation of the film is relaxed to reduce the vertical shrinkage of the porous film after the third polymer is extracted from the sheet.

The average pore size of this porous film varies depending on its use and is typically 0.05 to 10 microns. When the porous film is used as a battery separator, the average pore size is preferably 0.1 μm to 5 μm. When the average pore size is less than 0.05 microns, the increase in resistance causes the battery to have low functionality. On the other hand, when the average pore size is greater than 10 microns, the possibility of short-circuiting due to the formation of dendrites increases rapidly.

The pore size of the porous film is determined by the fineness of the third polymer dispersed in the mixture of the first polymer and the second polymer during melt-blending, that is, it can be used to make each polymer How small the structural elements are, decide. This is because the thin films formed from the first, second and third polymers extract the third polymer and generate pores. Therefore, the pore size of the porous film can be adjusted by the temperature and shear stress during melt-blending and the melt viscosity of the first polymer, the second polymer and the third polymer. For example, when the first polymer, the second polymer and the third polymer are blended at a lower temperature by a kneader, the melt viscosity of each polymer is blended in an increased state, and the porous film is The pore size becomes smaller. On the other hand, when the melt blending is performed at an elevated temperature, the pore size becomes larger. When the shear stress is increased during melt-blending, the pore size becomes smaller, and when the shear stress is reduced, the pore size becomes larger. In addition, when a higher molecular weight third polymer is used as the third polymer to increase the melt viscosity, a porous film with a smaller pore size can be obtained. On the other hand, when a lower molecular weight polymer is used as the third polymer to reduce the melt viscosity, a porous film with a larger pore size can be obtained. As described above, the pore size of the porous film of the present invention can be easily changed depending on the selection of the above-mentioned factors.

The thickness of the porous film of the present invention can vary depending on its use and is typically 15 micrometers to 200 micrometers. The preferred thickness is 20 microns to 120 microns. When the porous film is used as a battery separator, the thickness of the porous film is typically 15 micrometers to 200 micrometers, preferably 20 micrometers to 120 micrometers. When the thickness is less than 15 microns, the mechanical strength of the separator is significantly reduced so that it is easy to cause a short circuit due to dendritic compression or puncture. On the other hand, when the thickness is greater than 200 microns, the occupied volume of the battery separator in the battery increases and this reduces its ability to adapt to market requirements such as miniaturization and high energy density.

Various types of batteries can be prepared by using the battery separator of the present invention. The battery separator of the present invention is particularly useful when the temperature in the battery has risen abnormally above the specified temperature, and the current flow path of the battery must be substantially cut off to prevent the danger of fire or explosion.

In this respect, the usefulness of the battery separator of the present invention will be most effectively shown in a lithium battery which uses an organic electrolyte solution. The combination of cathode material, anode material and electrolyte solution used in lithium batteries is well known to those skilled in the art, and the battery separator of the present invention can be used for any combination of such constituent elements. A preferred organic electrolyte solution that can be used in this lithium battery is a solution prepared by dissolving lithium perchlorate in a mixture of propylene carbonate and 1,2-dimethoxyethane. The preferable combination of other organic solvents and electrolytes can be selected from organic solvents including propylene carbonate, dimethoxyethane, dioxolane, tetrahydrofuran, 1,2-dimethoxypropane and other organic solvents, which can be used for production A battery has a conventional lithium anode and lithium salt which are dissolved in such solvents including lithium trifluoromethanesulfonate (CF₃SO₃Li), LiAsF₆, LiBF₄And LiClO₄. Examples of useful anode materials include lithium metal, lithium salt contained in a solid support (such as carbon or the like), lithium/aluminum alloy, lithium/silicon alloy, lithium/boron alloy, and metals of groups IA and IIA of the periodic table. Examples of metals that can be used as current collectors and supporting elements include nickel, stainless steel, aluminum, and titanium. A wide range of cathode effective materials are known and include chalcogenides such as MnO₂, TiS, FeS₂, FeS, MoS₂, CuO, V₆O₁₃, Bi₂O₃In a variety of carriers such as polyfluorocarbons and polyolefins. The battery separator of the present invention can also be used for batteries other than lithium batteries.

The following examples are for illustrative purposes and are not intended to limit the present invention.

In the second example, the thickness of the porous film is measured by a gauge, and the average pore size is measured from scanning electron micrographs of the surface and cross-section of the porous film. The porosity of the porous film is obtained by cutting a porous film into a size of 20 mm x 20 mm, immersing the sample in n-butanol, and measuring the weight of the impregnated sample and the absolute dry sample The weight and the weight are calculated according to the following formula.

Porosity = (pore volume/porous film volume) x 100

Wherein pore volume = (weight of porous film containing n-butanol-weight of absolutely dry porous film)/0.81

The blocking of the micropores of the tested porous film due to the second polymer component was judged by comparing the ion permeability of the porous film before and after heating at a specified temperature. More specifically, after the porous film is heated at a predetermined temperature for a specific period of time, the porous film is installed between the two chambers of the device as shown in FIG. 3. In one chamber of this device, 50 cubic centimeters of pure water was introduced, and 50 cubic centimeters of 1% by weight sodium chloride aqueous solution and sodium lauryl sulfate were introduced in the other chamber in one amount to reduce the surface tension of the solution to below The critical surface tension of this porous film. Due to the difference in concentration between the two solutions, ions move to pure water through the porous membrane. Use a conductivity meter to measure the migration of ions into pure water over time. Compare the ion permeability of the porous film before and after heating to a predetermined temperature to determine whether ion migration is blocked after heating. In addition, the cross-section of the tested porous film before and after heating at a specified temperature was observed by a scanning electron microscope to see if the pores were blocked by the melting of the second polymer.

<u style="single">example 1</u>

Melting point is 169°C, melt flow index is 0.5 g/10 minutes and density is 0.91 g/cm ^ 3 polypropylene (product of Mitsui Nisseki Polymers Co., Ltd.) 50 parts, melting point is 109°C, melt flow index is 7.0 g/10 Minutes with a density of 0.917 g/cm3 of polyethylene (product of Nippon Petrochemicals Co., Ltd) 50 parts and a melt flow index of 4g/10 minutes with a density of 0.93 g/cm3 of styrene/hydrogenated isoprene/ Styrene block polymer ("SEPTON 2007", Kuraray Co., Ltd.'s product) 100 parts is thoroughly melt blended with a kneader at a temperature of 200°C to 220°C, and then the melt blend is extruded through a T-shape to produce a film. The obtained film is then subjected to a pressure of 200 kg/cm² at a temperature of 180°C for one minute to produce a 150 micron-thick film. The film was immersed in cyclohexanone to completely extract the styrene/hydrogenated isoprene/styrene block polymer, and then washed with fresh cyclohexanone to obtain a mixture of polypropylene and polyethylene It is a porous film with a single layer structure. Since the weight change before and after extraction, the weight reduction of the film is equal to the content of the styrene/hydrogenated isoprene/styrene block polymer. Infrared analysis of the formed porous film showed that there was no such styrene component. Thus, it was confirmed that a porous film with a single-layer structure made of polypropylene and polyethylene was obtained. The properties of the porous film thus obtained are shown in Table 1 and the ion permeability of the porous film is shown in Table 2. From the results shown in Table 2, it was found that heating the porous film lost its permeability at 115°C. The scanning electron micrographs of the cross-section of the porous film before and after heating at 115°C are shown in Figure 4-(a) and Figure 4-(b), respectively. It can be seen from these photos that the micropores in this porous film are blocked (Figure 4-(b)).

<u style="single">Examples 2 to 4</u>

The same procedure of Example 1 was repeated except that the amounts of polypropylene, polyethylene and the block polymer were changed to obtain the porous film and are described in Table 1. These films, after extraction, contain styrene/hydrogenated isoprene/styrene block polymers and their properties are shown in Table 1 and their ion permeability are shown in Table 2.

The scanning electron micrographs of the cross-section of the porous film of Example 2, taken before and after heating at 115°C, are shown in Figure 5-(a) and Figure 5-(b), respectively.

<u style="single">Example 5</u>

The difference in the same procedure of Weight Example 1 is that instead of 50 parts of the polypropylene, 70 parts of the polypropylene is used, and instead of the polyethylene, a polyethylene with a melt flow index of 100 g/10 min and a density of 0.914 g/cm ^ 3 is used (Tosoh Corporation The product) 30 copies.

As a result, a single-layer porous film made of polypropylene and polyethylene does not contain the styrene/hydrogenated isoprene/styrene block polymer. The properties of the porous film thus obtained are shown in Table 1 and the ion permeability of the porous film is shown in Table 2. It can be seen from the results shown in Table 2 that this porous film substantially lost its permeability after being heated to 105°C and 115°C.

<u style="single">Example 6</u>

80 parts of polypropylene (product of Mitsui Nisseki) with a melting point of no less than 130°C, 20 parts of low-density polyethylene ("FLO-THENE G801", product of Sumitomo Seika Chemicals Co., Ltd.) with a melting point of 104°C and styrene /Hydrogenated isoprene/styrene block polymer ("SEPTON 2006", a product of Kuraray Co., Ltd.) 100 parts in a kneader at 220 °C thoroughly melt-blended and then extruded The melt blend is passed through a T-mouth shape to form a film. The film thus obtained was immersed in cyclohexane for one hour to completely extract the styrene/hydrogenated isoprene/styrene block polymer, and then the surface was washed with fresh cyclohexane to obtain a porous film. Since the weight change before and after extraction, the weight reduction of the film is equal to the content of the styrene/hydrogenated isoprene/styrene block polymer. Infrared analysis showed that there was no such styrene component. Thus, it was confirmed that a porous film with a single-layer structure made of polypropylene and polyethylene was obtained.

The properties of the porous film thus obtained are shown in Table 1.

Immerse the porous film in propylene carbonate, which is a poor solvent for polyolefin, and heat the treated porous film at 100°C. Observe the cross-section of the porous film under a scanning electron microscope to show the micropores before and after heating. The internal conditions change and these micropores are blocked after heating. The scanning electron micrographs of the cross-section of the porous film before and after heating are shown in Figure 6-(a) and Figure 6-(b), respectively. The ion permeability of this porous film is shown in Table 2. From the results shown in Table 2, it can be seen that the porous film substantially lost its ion permeability after heating at 115°C.

<u style="single">Example 7</u>

Polypropylene ("NOBLENJS", a product of Mitsui Toatsu Chemicals, Inc.) with a melt flow index of 1.7 g/10 minutes and a melting point of 170°C ("NOBLENJS", a product of Mitsui Toatsu Chemicals, Inc.) 80 parts; Products of Industries, Ltd.) 20 parts and the same styrene/hydrogenated isoprene/styrene block polymer as in Example 1 were thoroughly melt-blended and extruded in a kneader at 220°C The melt blend is pressed through a T-mouth shape to form a film. Then, the film was stretched twice in this machine direction, annealed and quenched at 100°C. The film thus obtained was immersed in cyclohexane for one hour to completely extract the styrene/hydrogenated isoprene/styrene block copolymer, and after that, the surface was washed with fresh cyclohexane to obtain a polymer Single-layer porous film made of propylene and polyethylene. The properties of the porous film thus obtained are shown in Table 1. Immerse the porous film in propylene carbonate and heat the treated film at 100°C. Observe the cross-section of the porous film under the scanning electron microscope to show the changes in the internal conditions of the micropores before and after heating. The pores are blocked. The ion permeability of this porous film is shown in Table 2. From the results shown in Table 2, it can be seen that the porous film substantially lost its permeability after being heated at 115°C.

<tables><img file="TW222011B_D0001.tif" /></tables>

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