KR100292978B1 - Monolayer porous membrane and battery separator and cell comprising same - Google Patents

Abstract

It is formed of a first polymer having a melting point of 130 ° C. or higher and a second polymer having a melting point of 120 ° C. or lower, and the walls of the pores of the membrane do not block the permeability of the pores under normal operating temperature conditions, but the membrane has a temperature above the melting point of the second polymer. A monolayer porous membrane having a second polymer in an amount capable of blocking upon reaching.

Description

Monolayer porous membrane and battery separator and cell comprising same

1A and 1B are schematic diagrams of cross-sectional states of a porous membrane having a monolayer structure according to the present invention, respectively, before and after heating at a predetermined temperature.

2 is a schematic representation of a conventional porous membrane cross section with multiple layers, one of which is a melting layer.

3 is a schematic view of a cross section of an apparatus for measuring the ion permeability of a porous membrane having a monolayer structure according to the present invention.

4A and 4B are 5000 magnification scanning electron micrographs of the cross section of the porous membrane having the monolayer structure obtained in Example 1 of the present invention before and after heating at 115 ° C., respectively.

5A and 5B are respectively 5000 magnification scanning electron micrographs of the cross section of the porous membrane having the monolayer structure obtained in Example 2 of the present invention before and after heating at 115 ° C.

6A and 6B are 2000 magnification scanning electron micrographs of the cross section of the porous membrane having the monolayer structure obtained in Example 6 of the present invention before and after heating at 100 ° C., respectively.

The present invention relates to a porous membrane of a single layer structure and a method of manufacturing the same. More particularly, the present invention relates to porous membranes capable of losing permeability above a predetermined temperature. The invention also relates to a cell with a cell separator and a cell separator formed from a porous membrane.

Porous polymer membranes have been used in the field of filtration and separation techniques. Various techniques have been used to impart desired specific properties to such polymer membranes. For example, the membrane can be stretched to improve strength. When the membrane is used for filtration, the membrane can be treated with a surfactant to improve the affinity for the filtrate. Ion exchange has been imparted to membranes by grafting or copolymerizing various monomers having specific functional groups.

For certain applications, porous polymer membranes have been expected that can lose permeability above a predetermined temperature. Such membranes, for example, are highly desirable as separator components in rechargeable lithium secondary batteries. This membrane should have the property of freely passing ions contained in the electrolyte of the battery through the pores of the membrane under normal operating conditions (“ion permeability” or “electrolyte conductivity”). However, this permeability should be sufficiently reduced to interrupt the current when the temperature in the cell exceeds a certain temperature due to malfunction during charging process, short circuit between electrodes or other reasons. If this current interruption is not achieved, the vapor of the solvent used in the electrolyte solution is excessively increased at the pressure in the cell, resulting in a risk of fire or explosion.

In the past, kraft paper or manilama sheet material has been used as separators used in conventional batteries, lithium batteries and capacitors. Recently, nonwoven and porous polyolefin membranes with high mechanical strength have been used for this purpose. However, these membranes do not exhibit the ability to cut electrolyte current by reducing electrolyte conductivity.

In particular, the lithium battery is designed to flow a high-density current, so if a short circuit occurs between the electrodes, the temperature in the battery will increase rapidly. Short-circuit causes overcurrent to promote chemical action at the cathode. It is very dangerous if the temperature in the battery increases rapidly. Without protection, it can reach temperatures above 140 ° C very quickly. This condition may cause the organic solvent used in the electrolyte solution to ignite or cause a battery explosion. To avoid these risks, various countermeasures are currently required. The need for stability of commercial products has increased the need for devices that can protect against such risks.

As an example for solving the above problem, a proposal has been proposed to use a material that can be melted and becomes nonporous above its melting point as a battery separator. Under exothermic conditions, the separator may lose ion permeability, preventing overcurrent flow. Polyolefins have been proposed as materials for this purpose. However, the mechanical strength of polyolefins decreases as the melting point decreases. Thus, materials having sufficient strength suitable as separator materials are polypropylene or high density polyethylene having a melting point of at least 130 ° C. However, these materials have a high risk of fire or explosion due to abnormal heating since the porosity begins to decrease after the battery temperature reaches 130 ° C or higher. Such separators are inadequate to achieve the desired results.

Recently, battery separators having a multi-layer structure have been proposed as a means for blocking battery circuits under heating conditions. This separator comprises a porous polymer membrane having a high melting point as a support layer and a porous polymer membrane having a low melting point as a melting layer. The membranes are stacked such that the low melting porous polymer membrane melts at a temperature above its melting point to form the non-permeable layer while providing the strength of the high melting membrane. For example, a battery separator made of a multilayer porous polyolefin membrane is described in Japanese Patent Laid-Open No. 62-10857, and lamination of a non-crosslinked polyethylene film on a crosslinked polyethylene film is described in Japanese Patent Application Laid-Open No. Hei 3-59947 ( A nonwoven fabric having a wax coating thereon is described in US Pat. No. 4,741,979. In addition, a lithium battery separator having a multilayer structure including a support layer of a porous or microporous polymer and a fused nonwoven fabric thermally bonded to the support layer is described in Japanese Patent Application Laid-Open No. Hei 2-75152 (1990). For reference, a cross-sectional view of a porous membrane having a multilayer structure formed by laminating a fusible porous polymer layer on a porous polymer layer as a supporting layer is shown in FIG.

However, battery separators formed of multilayer porous polymer membranes suffer from thickness and uniformity. In small lithium batteries, the thickness of the membrane, which is the battery separator, must be reduced and uniformly produced to provide improved energy density per unit weight or volume. In other words, the state of the art requires the use of a porous membrane formed of a polymer of high melting point (primary transition temperature) to achieve the mechanical strength of the membrane necessary to act as a separator. However, since the electrolyte conductivity needs to be completely blocked when the battery temperature increases to about 130 ° C., it is necessary to use a low melting point material to completely block the membrane pores. It is clear that the need for high mechanical strength and capacity to be substantially nonporous by melting or the like is contrary to the polymer technology of the present invention.

Multilayer separators provide mechanical strength and capacity that is substantially nonporous at a given temperature, but these separators are thicker than the thickness needed to achieve high energy density, resulting in low porosity between laminated layers when stacking different membrane layers. It may form a skin structure. As a result, the permeability of the multilayer membrane can be substantially reduced, lost or blocked by this skin layer.

It is highly desirable to have a monolayer porous membrane that is thin and has high mechanical strength and can be substantially nonporous without losing mechanical strength at low temperatures (eg, 80 ° to 130 ° C.). Such membranes are suitable as separators in cells, providing a safeguard against overheating and resulting disasters.

It is an object of the present invention to provide a monolayer porous polymer membrane which exhibits excellent mechanical strength and high porosity at normal operating temperatures and which can be substantially nonporous at a predetermined elevated temperature, which is suitable as a battery separator for stopping electrolyte conduction in cells. .

Another object of the present invention is to provide a battery separator for a lithium secondary battery which ensures safety from abnormal heat.

The porous membrane having a monolayer structure according to the present invention has a (a) a porous polymer sheet made of a first polymer having micropores therein and having a predetermined melting point, and (b) a melting point lower than a predetermined melting point of the first polymer. And a second polymer. The second polymer is in a state that does not block the micropores inside the micropores, but is present in an amount that blocks the inside of the micropores when the membrane reaches a temperature equal to or higher than the melting point of the second polymer. The porosity of the membrane is substantially lost at the predetermined melting point of the second polymer.

The present invention relates to (i) a first polymer having a predetermined melting point, (b) a second polymer having a predetermined melting point lower than that of the first polymer, and (c) an agent that is not mixed with the first polymer and the second polymer. 3 melt blending the polymer to form a polymer film;

(ii) extracting the third polymer using a solvent capable of dissolving only the third polymer, the method comprising producing a porous membrane having a monolayer structure.

The present invention includes a battery separator formed from the porous membrane of the present invention.

The present invention also includes a battery having at least one positive electrode / cathode pair, a battery between the positive electrode and the negative electrode and having a separator and an electrolyte composition formed of the porous membrane of the present invention described above.

In order to explain the invention in more detail, the following terms have the meanings as set forth below throughout the description of the invention and the appended claims, respectively:

"Film" means a sheet product having a predetermined length and width and a thickness defined by a boundary formed by two major surfaces of the sheet product.

By "single layer structure" is meant a sheet product having a substantially uniform composition throughout the body or thickness of the sheet product with reference to the membrane of the present invention. This uniform composition may be formed from a single polymer or a plurality of polymers, but the single layer structure is not a laminated structure.

By "first polymer" is meant a polymer or copolymer which is inert to other battery components (electrode and electrolyte compositions) having a high melting point of at least about 130 ° C. and which can be considered for use.

"Secondary polymer" means a polymer or copolymer that is inert to battery components (electrode and electrolyte compositions) and that is used to form a film that has a low melting point of 80 ° C to 120 ° C and may be considered for use.

"Melting point" means the primary transition temperature of a polymer.

The cross section of the porous membrane of the present invention has a single layer structure. This membrane has a body of length and width scale necessary to meet the requirements of its end use. For example, the exact scale will depend on the size of the cell, the size of the electrode plate, and the type of use (such as jelly roll-type separators, pocket separators, plates, or leaf separators). The separator has a thickness composed of a polymer mixture and channels or pores exist across the thickness from one major surface to the other. The pores need not be straight through the whole, but are interconnected to lead between the two major surfaces. The polymer mixture of the membrane body consists of a homogeneous mixture of only one or more first polymers or a homogeneous mixture in combination with one or more second polymers.

Methods of forming a monolayer sheet structure with desired properties include first using a porous film made of a first polymer (polyolefin, such as, for example, polypropylene, as described herein). The second polymer is heated and dissolved in a solvent such as toluene or xylene and then maintained at this temperature. The porous first polymer film described below is immersed in a heating solution and the inside of the micropores is filled with a second polymer solution. Subsequently, the porous film filled with the second polymer solution inside the micropores is dried by an air dryer or the like to remove the solvent inside the micropores while leaving the second polymer residue. If the second polymer lacks adhesion to the walls of the micropores in the porous first polymer film and the second polymer tends to separate from the interior walls of the pores, the film is again heated under controlled conditions to partially melt and fine It adheres to the second polymer inside the micropores without blocking the pores, thereby increasing the binding force of the second polymer. The heating should be carried out at a temperature lower than the melting point of the first and second polymers.

In the method described above, the concentration of the second polymer solution should be low so as not to cause the blocking of the micropores by the second polymer. Preferred concentrations depend on the porosity of the porous first polymer film. However, any person can easily determine the desired concentration experimentally or empirically. The concentration of the solution should be mainly 0.05 to 20% by weight. For example, when using a porous film having 55% porosity, the preferred concentration of the second polymer solution is 0.1-15% by weight.

As schematically shown in Figure 1 (a), the first polymer and the second polymer are exposed to form part of the membrane pore wall. The second polymer is present in an amount such that it will not block the micropores as it is. The second polymer may have an additional second polymer present on the wall surface and bonded to a portion of the membrane mixture. Blocking the micropores of the membrane by the second polymer occurs when the membrane is treated at a temperature equal to or higher than the melting point of the second polymer. At this temperature the second polymer making up or contacting the walls of the pores migrates into the pore channel. In this way, the permeability of the porous membrane is lost or substantially reduced.

Under ordinary conditions, ions easily pass through the pores filled with the electrolyte composition. Therefore, the electrolyte conductivity is high and the battery works as desired. However, when the temperature rises to indicate an abnormal condition higher than the normal operating temperature and exceeds the melting point of the second polymer, the second polymer flows down to seal the micropores, so that the passage of ions through the micropores is first (b). It is blocked as shown in the figure.

The second method of forming the film of the present invention comprises (i) (a) a first polymer having a predetermined melting point, (b) a second polymer having a predetermined melting point lower than that of the first polymer and (c) the first polymer and Melt mixing a third polymer that is not mixed with the second polymer to form a polymer film; (ii) extracting only the third polymer using a solvent capable of dissolving only the third polymer.

While not meant to be a limitation of the present invention, in preparing the membrane according to the second preferred manner, the melt mixture comprises (i) molecules of the first polymer, the second polymer and the third polymer mixed together to form a continuous layer. Or (ii) the molecule of the second polymer is bonded at a portion of the surface of the first polymer (at the boundary between the first and third polymer phases) and the first and third polymers each form a continuous phase It is formed into a phase structure, and these continuous phases are mixed with each other to form a network structure.

It is useful that the first polymer and the second polymer as described below can be used in both formation methods and the third polymer is used in combination with the second polymer as described in more detail below.

Polymers having a melting point high enough to withstand cell conditions can be used as the first polymer in forming the porous polymer membrane of the present invention. The first polymer may be a homopolymer or a copolymer or may be a mixture of polymers having high melting point properties. Polymers with high crystallinity and high tensile strength are suitable for use as battery separator membranes. Mechanical strength or modulus of elasticity can be improved by crosslinking the polymer, but should be carried out to a degree that does not harm the object of the present invention. Since the porous polymer membrane preferably has heat resistance and high mechanical strength, it is preferable that the crystallinity and molecular weight of the first polymer forming the porous polymer membrane should be high and / or use physical crosslinking of the first polymer to impart the above characteristics. Do. However, if the molecular weight or degree of crosslinking is excessive, a thin film cannot be formed, or the age or resilience of the desired film cannot be achieved. Such treatment must be controlled and may not be desirable for certain applications. Crosslinking methods usable in the present invention include conventional crosslinking methods by electron beams or irradiation or using silane couplers or peroxides.

Examples of the first polymer that can be used in the present invention and have high tensile strength, good elastic modulus, resilience and high melting point properties of 130 ° C. or higher are polypropylene, high molecular weight, having a melting point of 130 ° C. or higher and a molecular weight of about 30,000 to about 800,000. It is a polyolefin such as (weight average) polypropylene, and may be used alone or in admixture with a low molecular weight polymer. Specific examples include the polypropylene (Mitsui Niseki Polymers Company Limited) having a melting point of 169 ° C., a melt flow index of 0.5 g / 10 min and a density of 0.91 g / cm 3, as well as the trade name “NOBLEN JS” (Mitsui Toatsu Chemical) , Incorporated Products), "CHISSOPOLYPRO" (Chiso Corporation), "IDEMITSU POLYRPO" (Idemitsu PITRO Chemical Company Limited), "MITSUBISHI.POLYPRO" (Mitsubishi PITICAL COMPANY LIMITED) and " Polymers commercially available from TONEN POLYPRO ”(manufactured by Tonen Corporation), and other polymers having the above-described properties. Other polymers can be used as the first polymer, including high molecular weight polyethylene, copolymers of olefins with alpha-beta unsaturated monomers such as ethylene, acrylates, and the like.

The second polymer forming the melting phase in the porous membrane of the present invention can be variously selected from polymers and copolymers having a melting point lower than that of the first polymer, preferably from 95 ° C to 120 ° C. Examples of the second polymer include low molecular weight polyolefins such as the trade name “MITSUBISHI POLYETHYLENE-LD” (manufactured by Mitsubishi Kasei Corp.), “MIRASON” (manufactured by Mitsui Petrochemical Industries Limited), “SUMIKATHENE” (Sumitomo Chemical Company Limited) ), Low density polyethylene sold as "ULTZEX" (Mitsui Fitro Chemicals Limited), "FLO-THENE" (Sumitomo Seika Chemicals Company Limited), "NISSEKI REXLON" (Nippon Fitro Chemicals Company Limited); Trade name "SUMIKATHENE-L" (Sumitomo Chemical Company Limited),

Linear low-density polyethylene and ethylene / vinyl acetate copolymers, ethylene / butadiene copolymers, ethylene sold as "IDEMISTSU POLYETHYLENE-L" (made by Idemitsu Pyrochemical Co., Ltd.), "LINIREX" (made by Nippon Pyrochemical Co., Ltd.) / Pentadiene copolymers, ethylene / methacrylic acid copolymers, ethylene / acrylate copolymers, ethylene / methacrylate copolymers, ethylene / propylene copolymers, ethylene / propylene / diene terpolymers and ethylene, maleic anhydride and others Polyethylene and its copolymer containing a terpolymer with a monomer are mentioned.

The first polymer and the second polymer described above may be suitably selected and combined to form the porous membrane of the monolayer structure of the present invention. Its selection can be readily made by one skilled in the art based on a second polymer having a melting point at a desired temperature at which the stability and ion permeability of both polymers are lost under normal operating conditions. For example, when the porous membrane of the present invention is used as a separator of a lithium battery, polypropylene having a melting point of 130 ° C. or higher is used as the first polymer and low molecular weight polyethylene having a melting point of about 95 to 120 ° C. is used as the second polymer. This results in a useful cell separator that has the desired mechanical strength and can immediately lose its permeability at the critical temperature of the small intestine, thereby preventing the dangerous destruction of the cell or fixture in which the separator is used.

The ratio of the first polymer and the second polymer forming the porous membrane of the present invention should be selected such that the micropores of the porous membrane are not blocked by the second polymer under normal conditions but exceed the predetermined safe temperature. These preferred ratios vary depending on the thickness of the porosity, the size and shape of the micropores in the porous membrane, the method of making the porous membrane and the method of introducing the second polymer into the micropores, and one skilled in the art can determine the suitable ratio by experiment. have. The weight ratio of the first polymer to the second polymer is generally from 1: 1 to 9: 1 and preferably from 1: 1 to 8: 1. For example, if a porous membrane having a porosity of 50% and a thickness of 25 μm is used, the weight ratio of the first polymer to the second polymer is preferably 1: 1 to 4: 1.

Also, if desired, various kinds of additives such as antioxidants, ultraviolet absorbers, colorants, lubricants and / or antiblocking agents, etc. may be added to the film forming polymer composition. These may be added to the first polymer or the second polymer during the process of forming the sheet product of the present invention without causing adverse effects on the present invention. This amount is about 5% by weight or less, preferably about 1% by weight or less of the resulting composition.

To increase the adhesion of the first polymer to the second polymer, a copolymer of olefin and unsaturated carboxylic acid can be added to the composition that is part of the initial polymer mixture. Examples of such copolymers are the trade names "BONDINE" (Sumitomo Chemical Company Limited), "YUKARON" (Mitsubishi Pyro Chemical Company Limited), "ADMER" (Graft Copolymer of Polyolefin, Mitsui Pyro Chemical Industries Limited) ), "SUMIFARM" (Sumitomo Chemical Co., Ltd.) and "REUBEX" (Asahi Chemical Industries Co., Ltd.). Such copolymers may be added up to 10% by weight, preferably up to 5% by weight for adhesion.

Various methods as described above can be used to prepare the porous membrane having the monolayer structure of the present invention. A second preferred method is described below.

The first polymer and the second polymer constituting the porous membrane are melt mixed with a third polymer (as described above sufficiently) that does not mix with the first polymer and the second polymer, and the molten mixture is mixed with the molecules of the second polymer. It is formed into a film having an internally penetrating polymer network structure (hereinafter referred to as "IPN structure") that adheres to the surface of the polymer and such first and third polymers form a continuous phase and the continuous phases mix with each other to form a network structure. . This film may be formed by a known method such as casting, extrusion, or the like. Following film formation, the third polymer is extracted using a solvent capable of dissolving only the third polymer to form a porous sheet having a monolayer structure of the present invention. A schematic view of the IPN structure is described in detail by POIMER ALLOY (edit: Society of Polymer Science, Japan, published by Kyoritsu Shuppan), page 3, Figure 1.3 by Inoshi Takashi and Shoji Ichihara.

The third polymer is a polymer that can form an IPN structure with the first polymer without being mixed with the first polymer and the second polymer forming the porous membrane of the present invention and can be removed by extraction with a specific type of solvent. Can be selected from. The term “particular type of solvent” means that at a predetermined processing temperature below the melting point of the first and second polymers, preferably at least 30 ° C. or less, and most preferably from ambient temperature to 60 ° C. below the lowest temperature of the melting point. By means a solvent of the type capable of dissolving only the third polymer without dissolving the first polymer and the second polymer. Those skilled in the art can empirically and empirically determine the specific solvent types that can be used as solvents for the third polymer and combinations thereof that can be used as solvents for the particular combination of the first and second polymers. Examples of third polymers that may be used in the present invention are styrene / hydrogenated butadiene / styrene block polymers (hereinafter referred to as “SEBS”) having a number average molecular weight of about 10,000 to 300,000 and comprising “KRATON G” (manufactured by Shell Chemical Company). : Styrene / hydrogenated isoprene / styrene block copolymer having a number average molecular weight of about 10,000 to 300,000 (hereinafter referred to as “SEPS”) and a styrene / hydrogenated isoprene block copolymer having a number average molecular weight of about 5,000 to 200,000.

For example, when SEBS or SEPS is melt mixed with polyolefins such as polypropylene and polyethylene, an IPN structure is formed. After forming the film from the molten mixture, a porous membrane having direct pores can be prepared by extracting only SEBS or SEPS using a specific type of solvent. Solvents that can be used for styrene / hydrogenated butadiene / styrene block polymers, styrene / hydrogenated isoprene / styrene block polymers and styrene / hydrogenated isoprene block polymers are solvents having solubility variables 7.5 to 9.3, such as toluene, xylene, cyclohexane, methylcyclo Hexyn, methylethylketone, carbon tetrachloride and chloroform.

Although a typical example of a method for producing a film composed of the first polymer, the second polymer, and the third polymer is described above, conventional film forming methods such as expansion through T-die, casting, and extrusion can be applied. . Also in order to increase the mechanical strength of the porous membrane obtained as the final product, the film is stretched at a temperature higher than the secondary transition temperature of the first polymer but below its melting point, and if necessary the film is stretched at a temperature higher than the stretching temperature. Annealing may relax the residual stresses inside the film. The draw ratios that can be applied in the present invention are typically 1.1 to 10 times, preferably 1.1 to 5 times. Stretching can be carried out before or after (preferably) extruding the third polymer from the formed sheet.

The porosity of the porous membrane of the present invention is typically 80% or less. If the porosity is greater than 80%, the mechanical strength of the porous membrane is drastically reduced so that the porous membrane cannot be actually used. In addition, if the porosity is greater 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, thus losing the balance of the composition of the first polymer and the second polymer. Most porous membranes are made of a second polymer. Therefore, the mechanical strength of the porous membrane is not suitable for various purposes.

When the porous membrane of the present invention is used as a battery separator, the porosity is typically 30% to 80%, preferably 40% to 70%. If the porosity is less than 30%, the retention of the electrolyte solution in the separator can be reduced to dryness. If the flanks and porosities are greater than 80%, the mechanical strength of the porous membrane is too low for practical use, and short-circuits are likely to occur due to intrusion and compression of dendrites formed on the anode.

The porosity of the porous membrane of the present invention can be controlled by the amount of the third polymer forming pores after extracting the third polymer from the film made of the first polymer, the second polymer and the third polymer. The amount of the third polymer used to form the initial sheet is preferably 0.5 to 9 times the total weight of the first polymer and the second polymer. If this amount is less than 0.5 times the total amount of the first polymer and the second polymer, the desired porosity cannot be obtained. On the other hand, if the amount of the third polymer is 9 weight times or more based on the total amount of the first polymer and the second polymer, the mechanical strength of the resulting porous membrane is too low for practical use.

Also to improve porosity, films formed of the first polymer, the second polymer and the third polymer can be annealed at a temperature below the melting point of the second polymer. As a result of the annealing, the stress of each polymer is relaxed during film formation to reduce the shrinkage in the vertical direction of the porous membrane after extruding the third polymer from the sheet.

The average pore size of the porous membrane is typically from 0.05 μm to 10 μm, depending on the application. If a porous membrane is used as the battery separator, the average pore size is preferably 0.1 μm to 5 μm. If the average pore size is less than 0.05 µm, the electrical insulation is increased and the battery function is lowered. On the other hand, when the average pore size is 10 µm or more, the possibility of short circuit due to the formation of dendrites increases rapidly.

The pore size of the porous membrane is determined by how finely the third polymer can be dissolved in the mixture of the first polymer and the second polymer during melt mixing, i.e., how small the structural elements of each polymer can be made. This is by pore formation after the third polymer is extruded from the film formed of the first, second and third polymers. Therefore, the pore size of the porous membrane can be controlled by the temperature and shear stress during melt mixing and the melt viscosity of the first, second and third polymers. For example, when the first polymer, the second polymer and the third polymer are melt mixed with the kneader at a low temperature at which the melt viscosity of each polymer is increased, the pore size of the porous membrane becomes smaller. On the other hand, when melt mixing is performed at an elevated temperature, the pore size becomes larger. If the shear stress is increased during melt mixing, the pore size is smaller, while if the shear stress is reduced, the pore size is larger. In addition, when a high molecular weight third polymer is used as the third polymer to increase the melt viscosity, a porous membrane having a smaller pore size can be obtained. On the other hand, if a low molecular weight polymer is used as the third polymer to reduce the melt viscosity, a porous membrane having a larger pore size can be obtained. As mentioned above, the pore size of the porous membrane of the present invention may vary with the above selected factors.

The thickness of the porous membrane of the present invention may vary depending on its use and is typically 15 μm to 200 μm. Preferred thicknesses are between 20 μm and 120 μm. If a porous membrane is used as the battery separator, the thickness of the porous membrane is 15 µm to 200 µm, preferably 20 µm to 120 µm. If the thickness is smaller than 15 mu m, the mechanical strength of the separator is significantly reduced, which is likely to cause a short circuit due to crimped or penetrated dendritic. On the other hand, if the thickness exceeds 200 mu m, the volume occupied by the battery separator in the battery is increased, thereby reducing the ability to meet market requirements such as miniaturization and high energy density.

Various types of batteries can be produced using the battery separator of the present invention. The battery separator of the present invention is particularly suitable for use in batteries which must substantially block the flow of current when the temperature in the battery rises abnormally above a predetermined temperature to prevent the risk of fire or explosion.

In connection with the present, the utility of the battery separator of the present invention will be most effectively shown in lithium batteries using organic electrolyte solutions. Combinations of positive electrode materials, negative electrode materials and electrolyte solutions used in lithium batteries are known to those skilled in the art and the battery separator of the present invention can be used as a combination of these constituent elements. Preferred organic electrolyte solutions that can be used in lithium batteries are solutions prepared by dissolving lithium perchlorate in a mixture of propylene carbonate and 1,2-dimethoxyethane. Preferred combinations of other organic solvents and electrolytes include organic solvents such as propylene carbonate, dimethoxyethane, dioxolane, tetrahydrofuran, 1,2-dimethoxypropane and lithium trifluoromethanesulfonate (CF ₃ SO ₃ Li) , LiA _s F ₆ , LiBF ₄ and LiClO ₄ can be selected from other organic solvents that can be used to make batteries with conventional lithium anodes and lithium salts dissolved in these solvents. Examples of useful negative materials include lithium metals, lithium salts contained in solid carriers (such as carbon, etc.), lithium / aluminum alloys, lithium / silicon alloys, lithium / boron alloys, and metals of group IA and IIA of the periodic table. Examples of metals that can be used as current collectors and support members include nickel, stainless steel, aluminum and titanium. A wide range of positive electrode active materials are known and also knives such as MnO ₂ , TiS, FeS ₂ , FeS, MoS ₂ , CuO, V ₆ O ₁₃ , Bi ₂ O _{3 in} various carriers such as polyfluorocarbons and polyolefins. Cogenide. The battery separator of the present invention can be used for batteries other than lithium batteries.

The following examples are illustrative only and do not limit the invention.

In the examples below, the thickness of the porous membrane is measured with a dial gauge, and the average pore size is measured from scanning electron micrographs of the surface and cross section of the porous membrane. The porosity of the porous membrane was measured by cutting the porous membrane into 20 mm × 20 mm sizes, immersing the sample in n-butyl alcohol, measuring the weight of the deposited sample, and weighing the completely dried sample according to the following equation. Calculate

Porosity = (pore volume / volume of porous membrane) × 100

At this time, the pore volume = (weight of the porous membrane containing n-butyl alcohol-weight of the completely dried porous membrane) / 0.81

The blocking of the micropores of the test porous membrane by the second polymer component is determined by comparing the ion permeability of the porous membrane before and after heating at a predetermined temperature. More specifically, after the porous membrane is heated at a predetermined temperature for a predetermined period of time, the porous membrane is fixed between the two chambers of the apparatus as shown in FIG. Into one chamber of the device, an integer of 50 cc is introduced and in another chamber 50 cc of 1% by weight aqueous sodium chloride solution with sodium lauryl sulfate is introduced in an amount that reduces the surface tension of the solution to below the critical surface tension of the porous membrane. do. Due to the concentration difference between these two solutions, ions migrate to the purified water through the porous membrane. The movement of these ions to the constant is measured with a conductivity meter over time. The ion permeability of the porous membrane before and after heating at a predetermined temperature is compared to determine whether ion transport is blocked after heating. In addition, before and after heating at a predetermined temperature, the cross section of the tested porous membrane was measured by scanning electron microscopy to determine whether the pores were blocked by the melting of the second polymer.

Example 1

50 parts of polypropylene (Mitsui Niseki Polymers Company Limited) having a melting point of 169 ° C, a melt flow index of 0.5 g / 10 minutes and a density of 0.91 g / cm 3, a melting point of 109 ° C. and a melt flow index of 7.0 g Styrene / hydrogenated isoprene / styrene block polymer with 50 parts of low density polyethylene (from Nippon Pyro Chemical Company Limited) with a density of 0.917 g / cm3 and a melt flow index of 4 g / 10 min and density of 0.93 g / cm3 100 parts are thoroughly melt mixed with a kneader at a temperature of 200 ° C. to 220 ° C., and then the melt mixture is extruded through a T-die to form a film. The film thus obtained is compressed at 180 ° C. and 200 kg / cm 2 for 1 minute to form a 150 μm thick film. The film is immersed in cyclohexane to completely extract the styrene / hydrogenated isoprene / styrene block polymer and wash the surface with fresh cyclohexane to obtain a monolayer porous membrane composed of polypropylene and polyethylene. From the weight change before and after extraction, the membrane shows a weight loss equal to the content of styrene / hydrogenated isoprene / styrene block polymer. In addition, IR analysis of the formed porous membrane shows that there is no styrene component. Thus, it can be seen that a single layer porous membrane composed of polypropylene and polyethylene is obtained. The properties of the porous membrane thus obtained are shown in Table 1 below, and the ion permeability of the porous membrane is shown in Table 2. The results shown in Table 2 show that the porous membrane is heated at 115 ° C. and loses its permeability. Scanning electron micrographs of the cross section of the porous membrane before and after heating at 115 ° C. are shown in FIGS. 4 (a) and 4 (b), respectively. It can be seen from these photographs that the micropores in the porous membrane are blocked.

[Examples 2 to 4]

Except for changing the amount of polypropylene, polyethylene and block polymer, the same procedure as in Example 1 was repeated to obtain the porous membrane shown in Table 1. After extraction, the membrane contains styrene / hydrogenated isoprene / styrene block polymers and their properties are shown in Table 1 and ion permeability is shown in Table 2.

Scanning electron micrographs of the cross sections of the porous membrane of Example 2 taken before and after heating at 115 ° C. are shown in FIGS. 5 (a) and 5 (b), respectively.

Example 5

The same procedure as in Example 1 is repeated except that 70 parts of polypropylene and 30 parts of polyethylene having a melt flow index of 100 g / 10 min and a density of 0.914 g / cm 3 instead of polypropylene (from Toso Corporation) are used instead of polypropylene.

As a result, a porous membrane having a monolayer structure free of styrene / hydrogenated isoprene / styrene block polymer and composed of polypropylene and polyethylene is obtained. The characteristics of the porous membrane thus obtained are shown in Table 1 below and the ion permeability of the porous membrane is shown in Table 2 below. The results shown in Table 2 show that the porous membranes lose their ion permeability after being treated at 105 ° C and 115 ° C.

Example 6

80 parts of polypropylene (Mitsui Niseki Polymers Company Limited) having a melting point of 130 ° C or higher, 20 parts of low density polyethylene ("FLO-THENE G801", product of Sumitomo Seika Chemical Company Limited) having a melting point of 104 ° C, and styrene / hydrogenated isoprene 100 parts of styrene block polymer ("SEPTON 2006", Kuraray Co., Ltd.) are thoroughly melt mixed with a kneader at a temperature of 220 ° C., and then the melt mixture is extruded through a T-die to form a film. The film thus obtained is soaked in cyclohexane for 1 hour to completely extract the styrene / hydrogenated isoprene / styrene block polymer and wash the surface with fresh cyclohexane to obtain a porous membrane. From the weight change before and after extraction, the membrane shows a weight loss equal to the content of styrene / hydrogenated isoprene / styrene block polymer. In addition, IR analysis of the formed porous membrane shows that there is no styrene component. Thus, it can be seen that a single layer porous membrane composed of polypropylene and polyethylene is obtained.

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

When the porous membrane was immersed in polypropylene carbonate, a poor solvent for polyolefins, and then the treated porous membrane was heated at 100 ° C., the cross-section of the porous membrane was observed by scanning electron microscopy, and the internal state of the micropores changed before and after heating. It can also be seen that the micropores are blocked. Scanning electron micrographs of the cross section of the porous membrane before and after heating are shown in FIGS. 6 (a) and 6 (b), respectively. Ion permeability of the porous membrane is shown in Table 2 below. The results shown in Table 2 show that after heating at 115 ° C., the porous membrane substantially lost its ion permeability.

Example 7

80 parts of polypropylene ("NOBLENJS", manufactured by Mitsui Toatsu Chemicals Incorporated) with a melt flow index of 1.7 g / 10 min and a melting point of 170 ° C, low density polyethylene ("MIRASON ACE 30N", melting point 100 ° C) 20 parts of Local Chemical Limited) and 100 parts of the same styrene / hydrogenated isoprene / styrene block polymer as in Example 1 were thoroughly melt mixed with a 220 ° C. kneader and then the melt mixture was extruded through a T-die to form a film. The film is then stretched twice in the machine direction, annealed at 100 ° C. and quenched. The film thus obtained was immersed in cyclohexane for 1 hour to completely extract the styrene / isoprene / styrene block polymer and wash its surface with fresh cyclohexane to obtain a monolayer porous membrane composed of polypropylene and polyethylene. The properties of the porous membrane thus obtained are shown in Table 1 below. When the porous membrane was immersed in propylene carbonate and the treated membrane was heated at 100 ° C., when the cross section of the porous membrane was observed by scanning electron microscopy, the internal state of the micropores was changed before and after heating and the micropores were blocked by one heating. It can be seen. Ion permeability of the porous membrane is shown in Table 2 below. The results shown in Table 2 show that after heating at 115 ° C., the porous membrane substantially loses its ion permeability.

Heating time: 4 to 10 minutes in 1 minute practice in Examples 1,2,3,5,6 and 7

Claims (14)

A monolayer porous membrane having a predetermined length and width and a thickness between two major surfaces and having micropores within said thickness that communicate with each major surface, wherein the porous sheet comprises a first polymer and a first polymer having a predetermined melting point A substantially uniform mixture of second polymers having a melting point of at least 10 ° C. below the predetermined melting point of; The pores of the porous sheet have walls composed of a first polymer and a second polymer; And wherein the second polymer is present in mutual communication on the wall in an amount that can block pores and substantially reduce the permeability of the membrane when the membrane is treated at a temperature at least equal to the melting point of the second polymer. plate. The separator of claim 1, wherein the first polymer has a melting point of 130 ° C. or higher and the second polymer has a melting point of 80 ° C. to 120 ° C. 7. The separator of claim 2 wherein the first polymer is polypropylene having a weight average molecular weight of 30,000 to 800,000. The separator of claim 2, wherein the second polymer is polyethylene having a melting point of 95 ° C. to 120 ° C. 4. The method of claim 1 wherein the second polymer has a melting point of 80 ° C. to 120 ° C. and is a low density polyethylene, linear low density polyethylene, ethylene / vinylacetate copolymer, ethylene / butadiene copolymer, ethylene / acrylate or alkylacrylate copolymer. Separators selected from ethylene / acrylic or alkacrylic acid copolymers, ethylene / propylene copolymers or mixtures thereof The separator according to claim 2, wherein the porosity is 30 to 80% by volume of the membrane and the average pore diameter is 0.05 μm to 10 μm. And a porous membrane having a predetermined length and width and a thickness between two major surfaces and having micropores within said thickness that communicate with each major surface, wherein the porous membrane has a first polymer having a predetermined melting point (primary transition temperature) And the pores of the porous membrane have walls partially covered with a second polymer having a melting point of at least 10 ° C. lower than the predetermined melting point of the first polymer and the second polymer has pores between the first and second major surfaces. And a cell separator present on the wall in an amount capable of blocking the pores when the membrane is treated at a temperature at least equal to the melting point of the second polymer and freely permeable therethrough. The separator according to claim 7, wherein the first polymer has a melting point of 130 ° C or higher and the second polymer has a melting point of 80 ° C to 120 ° C. The separator of claim 8, wherein the first polymer is polypropylene having a weight average molecular weight of 30,000 to 800,000. The separator of claim 8 wherein the second polymer is polyethylene having a melting point of 95 ° C. to 120 ° C. 10. The method of claim 7, wherein the second polymer has a melting point of 80 ° C. to 120 ° C. and is a low density polyethylene, linear low density polyethylene, ethylene / vinylacetate copolymer, ethylene / butadiene copolymer, ethylene / acrylate or alkylacrylate copolymer. , Separators selected from ethylene / acrylic or alkacrylic acid copolymers, ethylene / propylene copolymers or mixtures thereof. The separator according to claim 8, wherein the porosity is 30 to 80% by volume of the membrane, and the average pore diameter is 0.05 µm to 10 µm. A cell having at least one negative electrode-anode pair, a separator membrane between the negative electrode and the positive electrode, and an electrolyte composition, wherein the separator membrane is the separator according to any one of claims 1 to 12. The battery according to claim 13, wherein the negative electrode is lithium metal, a lithium salt in a solid carrier or a lithium alloy.