JP5079544B2 - Method for producing composite microporous membrane and method for producing lithium ion battery - Google Patents

Description

  The present invention relates to a method for producing a composite microporous membrane and a method for producing a lithium ion battery.

  In recent years, lithium ion batteries have been used as power sources for small devices such as mobile phones and notebook personal computers. The separator provided in this battery is required to have excellent ion permeability. Moreover, in order to ensure the safety | security of a battery, it is requested | required of a separator that it can shut down when a battery is overheated to about 130-150 degreeC. Here, “shutdown” means that when the temperature in the battery rises, the communication holes are blocked by the melting of the polymer constituting the microporous membrane, increasing the electrical resistance of the membrane and blocking the flow of lithium ions. It is a phenomenon. In order to satisfy these requirements, a microporous membrane mainly composed of polyethylene (hereinafter simply referred to as “polyethylene microporous membrane”) is mainly employed for the separator. The battery separator desirably has a characteristic that the temperature at which the shutdown occurs (referred to as “shutdown temperature”) is as low as possible.

  Lithium ion batteries are also being applied to electric vehicles and small motorcycles. Lithium ion batteries used for these applications have a large size and a high energy capacity, and therefore higher safety is required. Accordingly, there is a need for a lithium ion battery separator that can ensure safety and maintain various characteristics even at high temperatures assuming, for example, abnormal heat generation of the battery.

  However, the mechanical strength of polyethylene tends to decrease when its crystals melt. Therefore, when a polyethylene microporous membrane is used as a battery separator, if the temperature inside the battery further rises above the shutdown temperature, the membrane shape may not be maintained and the membrane may break (the temperature at this time is referred to as “ Temperature)). As a result, a short circuit may occur between battery electrodes (the temperature at this time is referred to as “short temperature”). Therefore, the problem is how to increase the short-circuit temperature of the separator, maintain the shape even after shutdown, and maintain insulation between the electrodes. In other words, how to raise the membrane breaking temperature of the microporous membrane constituting the separator is a problem. Conventionally, as an attempt to improve the film strength of lithium battery separators at high temperatures, many attempts have been made to laminate a polyethylene microporous film with a microporous film mainly composed of polypropylene. Against this background, recently, there is an increasing demand for laminates of microporous membranes, and various laminates have been proposed.

  For example, Patent Document 1 proposes a specific laminated porous film that is made porous by a stretching method and used for a battery separator. The porous film in this document, that is, the microporous film, is manufactured by a method of stretching and making a laminate obtained by separately extruding and laminating thermoplastic resins having different melting points. Alternatively, this microporous film is made by extruding thermoplastic resins having different melting points separately and making them porous by a stretching method, and laminating them, or by coextrusion of thermoplastic resins having different melting points by a stretching method. It is manufactured by the method to make. Patent Document 2 proposes a method for producing a composite microporous membrane comprising a step of coating a nonporous precursor film with a polymer composition and a step of stretching the coated nonporous precursor by a specific method. .

On the other hand, Patent Documents 3 and 4 propose a method for producing a high-molecular-weight polyolefin microporous film in which a specific polyolefin-impermeable film is microporous by heat treatment. This method selectively melts or dissolves the amorphous portion of the polyolefin in the air-impermeable film in a specific liquid, and then melts or dissolves the polyolefin thus melted or dissolved on each fiber of the fibril that forms the veins remaining as crystals. This is a method for producing a microporous film by performing heat treatment to crystallize and agglomerate microcrystals.
Japanese Patent Laid-Open No. 11-123799 JP 2005-254814 A JP-A-10-306168 Japanese Patent Laid-Open No. 11-302436

  However, the microporous membrane described in Patent Document 1 requires adjustment by longitudinal stretching, and the stretching ratio is low, so that it is difficult to achieve high productivity. In addition, the method described in Patent Document 2 requires a complicated process of coating a nonporous precursor film, and, like the one described in Patent Document 1, it can be stretched only in the longitudinal direction and stretched at a high magnification. Then, the film breaks. Thus, the manufacturing method described in Patent Document 2 is also expected to have a problem that the production efficiency is not sufficient.

  Furthermore, in the methods described in Patent Documents 3 and 4, since a process of heat-treating the air-impermeable film in a liquid is essential, the manufacturing process becomes complicated and it is difficult to achieve high production efficiency. In addition, the air-impermeable film described in these documents has an intrinsic viscosity [η] of 4 dl / g or more, or [η] of 2.5 dl / g or more and Mw / Mn of 10 or less, and plane orientation. It is what is done. And when it replaces with the said air-impermeable film and employs a lower-viscosity air-impermeable film and implements the method of these literature descriptions, when performing heat processing in a liquid, an air-impermeable film melts. It is also a concern that As a result, the problem that the air-impermeable film which can produce a microporous film with the method of these literature description will be limited is anticipated.

  Therefore, the present invention has been made in view of the above circumstances, and it is possible to maintain a high membrane breaking temperature of the microporous membrane, and to produce a composite microporous membrane capable of sufficiently improving production efficiency. It is an object to provide a method and a method for producing a lithium ion battery.

That is, the present invention is as follows.
(1) A raw sheet containing a first thermoplastic resin and a plasticizer, wherein the content of the plasticizer with respect to the total mass of the first thermoplastic resin and the plasticizer is 20 to 80% by mass; A method of producing a composite microporous membrane, comprising: a step of drawing a laminate with an air-impermeable film containing a thermoplastic resin as a main component to obtain a stretched composite film; and a step of extracting a plasticizer from the stretched composite film. The plasticizer is an organic compound capable of melting or dissolving the amorphous part of the second thermoplastic resin contained in the air- impermeable film , and the air- impermeable film has a total mass of On the other hand, the manufacturing method of a composite microporous film containing the said 2nd thermoplastic resin 50-100 mass%.
(2) The film thickness of the air-impermeable film and / or the raw sheet containing the first thermoplastic resin and the plasticizer and the air-impermeable film containing the second thermoplastic resin as a main component are laminated. A method for producing a composite microporous membrane comprising the steps of obtaining a stretched composite film by stretching a laminate in which the mechanical strength is made non-uniform by a plasticizer, and extracting the plasticizer from the stretched composite film, The impervious film is a method for producing a composite microporous membrane, containing 50 to 100% by mass of the second thermoplastic resin with respect to the total mass.
(3) The method for producing a composite microporous membrane according to the above (1) or (2), wherein the air-impermeable film is a polypropylene film.
(4) The method for producing a composite microporous membrane according to any one of (1) to (3), wherein the stretching temperature when stretching the laminate is 90 ° C. or higher.
(5) The method for producing a composite microporous membrane according to any one of the above (1) to (4), wherein the stretched surface magnification when stretching the laminate is 10 times or more.
(6) A method for producing a lithium ion battery, comprising a step of incorporating the composite microporous membrane obtained by the method for producing a composite microporous membrane according to any one of (1) to (5) as a separator.

That is, the present invention is as follows.
(1) A raw sheet containing a first thermoplastic resin and a plasticizer, wherein the content of the plasticizer with respect to the total mass of the first thermoplastic resin and the plasticizer is 20 to 80% by mass; A method for producing a composite microporous membrane, comprising: a step of drawing a laminate of an air-impermeable film containing a thermoplastic resin as a main component to obtain a stretched composite film; and a step of extracting a plasticizer from the stretched composite film.
(2) The film thickness of the air-impermeable film and / or the raw sheet containing the first thermoplastic resin and the plasticizer and the air-impermeable film containing the second thermoplastic resin as a main component are laminated. A method for producing a composite microporous membrane, comprising: a step of obtaining a stretched composite film by stretching a laminate having a mechanical strength made non-uniform by a plasticizer; and a step of extracting a plasticizer from the stretched composite film.
(3) The method for producing a composite microporous membrane according to the above (1) or (2), wherein the air-impermeable film is a polypropylene film.
(4) The method for producing a composite microporous membrane according to any one of (1) to (3), wherein the stretching temperature when stretching the laminate is 90 ° C. or higher.
(5) The method for producing a composite microporous membrane according to any one of the above (1) to (4), wherein the stretched surface magnification when stretching the laminate is 10 times or more.
(6) A method for producing a lithium ion battery, comprising a step of incorporating the composite microporous membrane obtained by the method for producing a composite microporous membrane according to any one of (1) to (5) as a separator.

  According to the present invention, it is possible to provide a method for producing a composite microporous membrane that can maintain a high membrane breaking temperature of the microporous membrane and that is sufficiently excellent in production efficiency.

  Hereinafter, the best mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail with reference to the drawings as necessary.

  The method for producing a composite microporous membrane of the present embodiment comprises a laminate of a raw fabric sheet containing a first thermoplastic resin and a plasticizer and an air-impermeable film containing a second thermoplastic resin as a main component. A step of drawing to obtain a stretched composite film (hereinafter referred to as “stretching step”) and a step of extracting a plasticizer from the stretched composite film (hereinafter referred to as “plasticizer extraction step”). The manufacturing method of the composite microporous membrane of this embodiment may have the process (henceforth a "raw fabric sheet production process") which produces the above-mentioned original fabric sheet before the said extending process. The step of laminating the raw sheet and the air-impermeable film (hereinafter referred to as “lamination step”) may be included. Hereinafter, each process will be described sequentially.

(A) Original Fabric Sheet Manufacturing Step In the present embodiment, the “original fabric sheet” is a sheet containing a first thermoplastic resin and a plasticizer. The raw sheet production process is not particularly limited as long as the original sheet is produced. In the raw fabric sheet production step, it is preferable to obtain a raw fabric sheet by forming a kneaded product obtained by melt-kneading the mixture of the first thermoplastic resin and the plasticizer into a sheet shape.

The first thermoplastic resin is not particularly limited as long as it is a thermoplastic resin that melts and kneads the mixture and cools and then phase-separates with the plasticizer. Such a thermoplastic resin can form the communication hole resulting from a plasticizer in a composite microporous film. Further, from the viewpoint of making the number and size of the communication holes suitable for the separator, the first thermoplastic resin is preferably compatible with the plasticizer when mixed and / or melt-kneaded with the plasticizer. . From these viewpoints, polyolefin is preferable as the first thermoplastic resin. Examples of the polyolefin include polyolefin resins represented by high density polyethylene having a density of 0.93 g / cm ³ or more, low density polyethylene, linear low density polyethylene, polypropylene, polybutene, polymethylpentene, and copolymers thereof. preferable. These may be used alone or in combination of two or more.

  When the first thermoplastic resin is a polyolefin, its viscosity average molecular weight (Mv) is preferably 50,000 to 3,000,000. This Mv is preferably 50,000 or more from the viewpoint of increasing the mechanical strength of the resulting microporous membrane, and preferably 3 million or less from the viewpoint of improving the moldability during production. From the same viewpoint, Mv is more preferably 100,000 to 1,000,000, and further preferably 200,000 to 800,000. Further, in order to control the mechanical strength of the microporous membrane, a mixture of two or more polyolefins having different Mvs may be used as the first thermoplastic resin. Further, when a polyolefin sheet having an Mv of more than 800,000 is contained in the raw sheet, the content of the polyolefin having an Mv of more than 800,000 with respect to the total amount of the polyolefin contained in the raw sheet from the viewpoint of suppressing deterioration of moldability. The proportion is preferably less than 50% by mass.

  In particular, when the finally obtained composite microporous membrane is used as a battery separator, the raw sheet contains high-density polyethylene as the polyolefin from the viewpoint of obtaining good processability and determining an appropriate shutdown temperature. Is preferred. From the same viewpoint, the content ratio of the high-density polyethylene in the raw sheet is preferably 20 to 100% by mass and more preferably 40 to 100% by mass with respect to the total amount of polyolefin contained in the original sheet. Preferably, it is more preferable in it being 60-100 mass%. Further, the viscosity average molecular weight of the high density polyethylene is preferably 1,000,000 or less, more preferably 800,000 or less, and further preferably 500,000 or less from the same viewpoint.

  The plasticizer is preferably an organic compound that is compatible with the resin at a temperature equal to or higher than the melting point of the resin when mixed with the first thermoplastic resin. Thereby, the function as a plasticizer can be exhibited more effectively. Moreover, the plasticizer can contribute to the opening of the air-impermeable film described later. From this point of view, the plasticizer is preferably an organic compound that can melt or dissolve the amorphous part of the thermoplastic resin used in the air-impermeable film. Examples of such plasticizers include hydrocarbons typified by liquid paraffin and paraffin wax, phthalic acid esters typified by di-2-ethylhexyl phthalate (DOP), diisodecyl phthalate, and diheptyl phthalate. Among these, liquid paraffin is preferable. A plasticizer is used individually by 1 type or in combination of 2 or more types.

  A suitable combination of the first thermoplastic resin and the plasticizer is a combination of high-density polyethylene and liquid paraffin from the viewpoint of further balancing the various effects of the first thermoplastic resin and the various effects of the plasticizer. The combination is preferably a combination of high-density polyethylene and DOP.

  In the raw sheet, the content ratio of the plasticizer with respect to the total mass of the first thermoplastic resin and the plasticizer is not particularly limited as long as it is a ratio that enables non-uniformity in the air-impermeable film described later. However, the content of the plasticizer is preferably 20 to 80% by mass, more preferably 30 to 75% by mass, and further preferably 40 to 70% by mass. The content ratio of 80% by mass or less is preferable from the viewpoint of realizing good air permeability as a lithium ion battery separator, and thus good ion permeability of the finally obtained composite microporous membrane. Moreover, it is preferable to make this content rate into 20 mass% or more from a viewpoint of promoting the nonuniformity of the mechanical strength of an air-impermeable film, and the viewpoint of improving the stretchability of the said air-impermeable film. In this case, the production efficiency of the composite microporous membrane can be improved. This is due to the fact that the plasticizer is contained more than a certain ratio, and thus acts more effectively on the second thermoplastic resin contained in the air-impermeable film.

  Further, the total content of the first thermoplastic resin and the plasticizer with respect to the total mass of the raw sheet is not particularly limited as long as the object of the present embodiment can be achieved, and is 30 to 100% by mass. And preferred.

  In order to prevent resin deterioration when the first thermoplastic resin is blended and formed into a raw sheet, an antioxidant may be added to the raw sheet. The timing of this addition may be when mixing the first thermoplastic resin and the plasticizer, or may be when melt kneading them. The content ratio of the antioxidant in the raw sheet is preferably 0.2% by mass or more from the viewpoint of further preventing resin deterioration with respect to the total amount of the first thermoplastic resin, from the viewpoint of economy. It is preferable that it is 3 mass% or less. From the same viewpoint, the content of the antioxidant is more preferably 0.4 to 3% by mass, and still more preferably 0.5 to 2% by mass.

  As said antioxidant, the phenolic antioxidant which is a primary antioxidant from a viewpoint of exhibiting the function more effectively is preferable. Examples of the antioxidant include 2,6-di-t-butyl-4-methylphenol, pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate. ], Octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate. These are used singly or in combination of two or more. Moreover, you may contain a secondary antioxidant in a raw fabric sheet together with a primary antioxidant. Examples of secondary antioxidants include tris (2,4-di-t-butylphenyl) phosphite and tetrakis (2,4-di-t-butylphenyl) -4,4-biphenylene-diphosphonite. And phosphorus antioxidants typified by (2) and sulfur antioxidants typified by dilauryl-thio-dipropionate. These are used singly or in combination of two or more.

  In the raw sheet production process, it is represented by polymers other than the above polyolefins, organic materials other than the above components, silica and alumina, as long as the film formability is not impaired and the achievement of the object of the present embodiment is not hindered. An inorganic material may be added to the mixture and / or kneaded product.

  The method for obtaining a raw sheet by forming into a sheet is not particularly limited, and a conventionally known method may be adopted. For example, a method may be mentioned in which a first thermoplastic resin and a plasticizer are supplied to a known extruder, mixed and melt-kneaded, and the gel-like kneaded product is formed into a sheet while cooling. Examples of the cooling method at this time include a method in which the kneaded product is brought into direct contact with a cooling medium such as cold air or cooling water, and a method in which the kneaded product is brought into contact with a roll or a press machine cooled with a refrigerant. In the former case, the gel-like kneaded material may be formed into a sheet shape by spreading into a film shape by its own weight or the like. In the latter case, the kneaded material may be stretched by a roll or a press to be formed into a sheet shape. Among these, a method of bringing the kneaded product into contact with a roll cooled by a refrigerant and a press is preferable in terms of excellent control of the thickness of the raw sheet.

  The thickness of the obtained raw sheet is adjusted in consideration of the first thermoplastic resin and the magnification in the stretching process described later. The thickness of the original fabric sheet is preferably 100 μm or more from the viewpoint of preventing film breakage in the stretching process described later, and is preferably less than 3000 μm from the viewpoint of suppressing excessive film thickness unevenness after the stretching process.

  In addition, the manufacturing method of the composite microporous film | membrane of this embodiment does not need to be provided with the original fabric sheet preparation process, For example, you may obtain the original fabric sheet which has the above structures.

(B) Lamination process In a lamination process, the said original fabric sheet and an air-impermeable film are laminated | stacked, and a sheet-like laminated body is obtained. In the present embodiment, the “impermeable film” is a film having an air permeability measured by a Gurley type air permeability meter described later of 10,000 seconds / 100 mL or more. The air-impermeable film according to this embodiment is not particularly limited as long as it contains the second thermoplastic resin as a main component. The second thermoplastic resin is preferably a polyolefin from the viewpoint of affinity with a plasticizer. Specific examples thereof include high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, polybutene, polymethylpentene, and copolymers thereof. Among these, polypropylene is preferable from the viewpoint of easily forming a microporous film suitable for a lithium battery separator. These may be used alone or in combination of two or more.

The air-impermeable film may be a commercially available film as long as the air permeability satisfies the above-described conditions and contains the second thermoplastic resin as a main component, and may be produced by a known manufacturing method. . In the present embodiment, the “main component” is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 90% by mass or more as a ratio of the specific component in the matrix component. Means that it may be 100% by mass.
Further, the content ratio of the second thermoplastic resin is preferably 50 to 100% by mass with respect to the total mass of the air-impermeable film from the viewpoint of efficiently achieving the object of the present embodiment, and 70 to 100%. It is more preferable that it is mass%, and it is still more preferable that it is 90-100 mass%.

  Moreover, the combination of a raw fabric sheet and an air-impermeable film is preferably a combination of an original fabric sheet having a shutdown temperature of 150 ° C. or lower and an air-impermeable film having a short-circuit temperature (fracture resistance temperature) exceeding 150 ° C. Thereby, the safety of the battery can be obtained more reliably. When polyethylene is used as the first thermoplastic resin contained in the raw sheet, the second thermoplastic resin contained in the air-impermeable film is made of polypropylene from the viewpoint of keeping the film shape well after shutdown and reducing the cost. Is preferred.

  The combination of the plasticizer contained in the raw sheet and the second thermoplastic resin contained in the air-impermeable film is a combination of liquid paraffin and polypropylene, DOP from the viewpoint of more effectively achieving the object of the present embodiment. A combination of polypropylene and polypropylene is preferred.

  In the present embodiment, since the raw sheet and the impermeable film are stretched in a laminated state, the viscosity of the impermeable film is not particularly limited. However, when a composite microporous membrane is used as a battery separator, if the surface layer of the separator in contact with the electrode in the battery has a low viscosity, the separator tends to stick to the electrode when the temperature rises in the battery, and the membrane shape is changed. Easy to maintain. From this viewpoint, the intrinsic viscosity [η] of the air-impermeable film is preferably 5 dL / g or less. From the same viewpoint, the intrinsic viscosity [η] is more preferably 4 dL / g or less, and further preferably 2.5 dL / g or less. Further, from the viewpoint of preventing film breakage during stretching and improving the strength of the obtained film, the intrinsic viscosity [η] is preferably 0.5 dL / g or more.

  The method for laminating (superimposing) the raw sheet and the air-impermeable film is not particularly limited. For example, a method of laminating the raw sheet and the air-impermeable film against each other before the stretching process described below, and laminating during the stretching process, that is, laminating while stretching the raw sheet and the air-impermeable film A method of laminating an impermeable film when the kneaded product is brought into contact with a roll cooled by a refrigerant or a press, and while rolling the raw sheet and the impermeable film using a hot rolling roll The method of laminating is mentioned. At least one layer of the air-impermeable film is laminated on one side or both sides of the raw sheet. However, from the viewpoint of curling prevention of the obtained laminate, it is preferable to laminate an air-impermeable film on both sides of the raw sheet.

  The film thickness of the impermeable film is preferably 1 to 50%, more preferably 2 to 30% of the thickness of the raw sheet. A film thickness of 1% or more of the thickness of the original sheet is preferable from the viewpoint of securing the film strength at high temperature, and a film thickness of 50% or less of the thickness of the original sheet from the viewpoint of preventing film breakage during stretching. Is preferred.

  By this lamination step, the plasticizer contained in the raw sheet acts on the second thermoplastic resin that is the main component of the air-impermeable film. For example, the plasticizer dissolves a part of the second thermoplastic resin, thereby making the film thickness of the air-impermeable film nonuniform between the dissolved part and the undissolved part. Alternatively, the plasticizer may swell part of the second thermoplastic resin or react with part of the part to change the quality of the impermeable film between the part that has acted and the part that has not acted. Uneven mechanical strength. At this time, the communication holes may be partially formed in the air-impermeable film.

  In addition, the manufacturing method of the composite microporous film of this embodiment does not need to be equipped with the lamination process, For example, you may obtain simply the lamination sheet which has the above structures.

(C) Stretching step In the stretching step, the above laminate is stretched in the in-plane direction to obtain a stretched composite film. Examples of the stretching method include uniaxial stretching using a roll stretching machine, sequential biaxial stretching using a combination of a roll stretching machine and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter. Among these, sequential biaxial stretching and simultaneous biaxial stretching are preferable from the viewpoint that the strength and permeability of the membrane can be adjusted with good balance and productivity is high.

  The “stretched surface ratio” when the laminate is stretched means the ratio of the area of the stretched composite film main surface obtained after stretching to the area of the laminate main surface before stretching. From the viewpoint of improving productivity and the strength of the obtained stretched composite film, this stretched surface ratio is preferably 10 times or more, and in order to prevent an increase in heat shrinkage stress due to excessive stretching, this stretched surface ratio is preferably 200 times or less. . From the same viewpoint, the stretch plane magnification is more preferably 15 to 100 times, and still more preferably 20 to 60 times.

  In addition, the stretching temperature when stretching the laminate is the material of the raw sheet such as the first thermoplastic resin, the material of the air-impermeable film such as the second thermoplastic resin, the material sheet and the air-impermeable film. The film thickness can be selected in consideration of the film thickness. However, it is preferable that the stretching temperature is in a range from a temperature that is 50 ° C. lower than the melting point of the first thermoplastic resin and the air-impermeable film that is the lowest melting point to a temperature that is 5 ° C. higher than the melting point. As a result, it is possible to more effectively suppress membrane breakage due to stretching in the stretching step and to increase the strength of the resulting film. In addition, from the viewpoint of more effectively preventing film breakage of the raw sheet during stretching in the stretching step, it is preferable that the melting point of the air-impermeable film is higher than the melting point of the first thermoplastic resin. In the present embodiment, the “melting point” is the peak top temperature of the maximum endothermic peak in the endothermic curve obtained by DSC measurement.

  When adopting polyethylene as the first thermoplastic resin and adopting a film containing polypropylene as the second thermoplastic resin as the air-impermeable film, from the viewpoint of preventing film breakage during stretching in the stretching step, the stretching temperature Is preferably 90 ° C. or higher. On the other hand, the stretching temperature is preferably 135 ° C. or less from the viewpoint of increasing the strength of the obtained composite microporous membrane.

  By this stretching step, it is desirable that fine communication holes are formed in the air-impermeable film around a part of the second thermoplastic resin to which the plasticizer has acted in the laminating step. In addition, a communication hole may be formed in a part of the original sheet.

(D) Plasticizer extraction process In a plasticizer extraction process, a plasticizer is extracted from a stretched composite film. The extraction solvent used in this case is a poor solvent for the first and second thermoplastic resins constituting the stretched composite film, a good solvent for the plasticizer, and a boiling point that constitutes the film. A thing lower than melting | fusing point of the 1st and 2nd thermoplastic resin is desirable. Examples of such extraction solvents include hydrocarbons typified by n-hexane and cyclohexane, chlorinated solvents typified by methylene chloride and 1,1,1-trichloroethane, and hydrofluoroethers and hydrofluorocarbons. Non-chlorinated solvents, alcohols represented by ethanol and isopropanol, ethers represented by diethyl ether and tetrahydrofuran, and ketones represented by acetone and methyl ethyl ketone. These extraction solvents may be used alone or in combination of two or more. Among these, methylene chloride and methyl ethyl ketone are preferable from the viewpoint of versatility of the solvent and solubility of the plasticizer.

Examples of the plasticizer extraction method include a method of extracting the plasticizer by immersing the stretched composite film obtained in the stretching step in an extraction solvent, and a method of extracting the plasticizer by showering the extraction solvent on the stretched composite film. . By this plasticizer extraction step, fine communication holes are formed in the portion of the raw sheet where the plasticizer is extracted.
A composite microporous membrane is obtained by removing the extraction solvent from the film after the plasticizer extraction step by drying or the like.

  Moreover, you may extend | stretch the composite microporous film after a plasticizer extraction process as needed for adjustment of a film thickness or air permeability. Examples of stretching in this case include uniaxial stretching, simultaneous biaxial stretching, and sequential biaxial stretching, and simultaneous biaxial stretching and sequential biaxial stretching are preferable. The stretching temperature at this time is such that when the first thermoplastic resin and the impermeable film have the lowest melting point is polyethylene (when the first thermoplastic resin is polyethylene, or the impermeable film Is a polyethylene film), it is preferably 100 to 135 ° C. from the viewpoint of more effectively suppressing membrane breakage due to stretching in the stretching step. In addition, the stretched surface magnification is preferably more than 1 time and 3 times or less from the viewpoint of improving the productivity and the strength of the obtained composite microporous membrane and preventing an increase in heat shrinkage stress due to excessive stretching.

  Further, a relaxation operation may be performed after the stretching. The “relaxation operation” is a reduction operation of the composite microporous membrane in the MD direction and / or the TD direction. From the viewpoint of obtaining a good heat shrinkage rate, the dimension in the MD direction or TD direction of the composite microporous membrane after the relaxation operation is preferably 1.0 times or less, more preferably 0. 95 times or less, more preferably 0.9 times or less. Further, from the viewpoint of preventing the generation of wrinkles, the dimension after the relaxation operation is preferably 0.6 times or more than the dimension before the relaxation operation. The temperature during the relaxation operation is preferably 100 ° C. or higher, more preferably 110 ° C. or higher, from the viewpoint of obtaining a good heat shrinkage rate. Further, from the viewpoint of obtaining a porosity and permeability suitable as a battery separator, 140 ° C. or lower is preferable, and 135 ° C. or lower is more preferable.

According to the manufacturing method of the composite microporous membrane of this embodiment described above, the plasticizer contained in the original fabric sheet is brought into contact with the impermeable film by laminating the impermeable film and the original fabric sheet. Thereby, it is thought that the amorphous part of an impermeable film melt | dissolves or swells. Thereafter, the laminate is preferably stretched by 10 times or more at a stretched surface ratio, and a plasticizer is extracted to produce a highly permeable microporous membrane with high production efficiency.
When a conventional wet method, that is, to produce a microporous film obtained by coextruding a resin composition, a special die capable of coextruding different materials is required. However, in the above-described embodiment, there is an advantage that a composite microporous film can be obtained simply by superimposing an air-impermeable film. Moreover, when making it microporous by spraying a plasticizer directly on an air-impermeable film etc., it is difficult to stretch | stretch at high magnification with an air-impermeable film alone. However, in the above-described embodiment, since the air-impermeable film is stretched together with the raw sheet as the support, it becomes possible to easily stretch at a high magnification. Can be easily microporous. Moreover, by laminating the air-impermeable film and the original fabric sheet, a higher film breaking temperature can be realized than when the original fabric sheet is used alone.

  The composite microporous membrane according to the present embodiment is suitably used as a battery (including a secondary battery) separator, more preferably a lithium battery separator. In this case, the thickness of the battery separator is preferably 5 μm or more from the viewpoint of preventing battery insulation failure, and is preferably 100 μm or less from the viewpoint of securing battery capacity. From the same viewpoint, this film thickness is more preferably 7 to 50 μm.

  In addition, the air permeability of the battery separator (in terms of film thickness 20 μm) is preferably 50 seconds / 100 mL or more from the viewpoint of improving mechanical strength, and from the viewpoint of improving the cycle characteristics and rate characteristics of the battery, It is preferable that it is 900 seconds / 100 mL or less. From the same viewpoint, the air permeability is more preferably 100 seconds / 100 mL to 800 seconds / 100 mL. Here, the air permeability of the battery separator was measured with a Gurley air permeability meter in accordance with JIS P-8117. This was multiplied by 20 (μm) / the thickness of the battery separator (μm) to obtain the air permeability (converted to a thickness of 20 μm).

  The puncture strength (converted to a film thickness of 20 μm) of the battery separator is preferably 3N or more, more preferably 3.5N or more. By setting the puncture strength to 3N or more, the film breakage of the separator due to the dropped active material or the like can be suppressed. Further, from the viewpoint of improving the workability of the separator, the puncture strength is preferably less than 20N. Here, the puncture strength of the battery separator was measured using a compression tester (for example, a handy compression tester manufactured by Kato Tech Co., Ltd. (trade name “KES-G5”)) with a radius of curvature of 0.5 mm at the tip of the needle. A piercing test was performed under the condition of a speed of 2 mm / sec. The maximum piercing load (N) at this time was defined as the piercing strength, and this was multiplied by 20 (μm) / the thickness of the battery separator (μm) to obtain the piercing strength (converted to a film thickness of 20 μm).

The porosity of the battery separator is preferably more than 20% and less than 80%, more preferably more than 30% and less than 60%. This porosity is preferably more than 20% from the viewpoint of improving the permeability of the battery separator, and is preferably less than 80% from the viewpoint of increasing the mechanical strength. Here, the porosity of the battery separator is calculated from the volume and mass according to the following equation.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of polymer composition) / volume (cm ³ ) × 100

  The average pore size of the battery separator is preferably 0.01 to 0.5 μm, more preferably 0.02 to 0.1 μm. The average pore diameter is preferably 0.01 μm or more from the viewpoint of improving ion permeability, and is preferably 1 μm or less from the viewpoint of improving the film strength and heat resistance. Here, the average pore diameter of the battery separator is an average pore diameter of the separator surface derived by image analysis from three electronic images with a magnification of 30,000 times obtained from a scanning electron microscope.

  The shutdown temperature of the battery separator is preferably less than 150 ° C., more preferably less than 140 ° C., from the viewpoint of ensuring safety when the battery is heated.

  The short-circuit temperature of the battery separator is preferably 190 ° C. or higher, more preferably 195 ° C. or higher, from the viewpoint of ensuring safety when the battery is heated and heat resistance.

  The method for producing a lithium ion battery according to the present embodiment includes a step of incorporating the composite microporous membrane according to the present embodiment as a battery separator. A conventionally well-known process can be employ | adopted about processes other than the said integration process at the time of manufacturing a lithium ion battery.

  Although the best mode for carrying out this embodiment has been described above, this embodiment is not limited to the above embodiment. The present embodiment can be variously modified without departing from the gist thereof. For example, the use of the composite microporous membrane according to the above-described embodiment is not limited to a battery separator, and for example, it can be used as a microfiltration membrane or a capacitor separator.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, these do not restrict | limit the scope of the present invention. In addition, the measuring method of the various characteristics in an Example is as follows.

(1) Film thickness Measured with a dial gauge (manufactured by Ozaki Seisakusho, trade name “PEACOCK No. 25”). A sample having a dimension of 10 mm in the MD direction × 10 mm in the TD direction was cut out from the microporous film, and the local film thickness was measured at nine locations (3 points × 3 points) in a lattice shape. The arithmetic average value of the nine local film thicknesses obtained was taken as the film thickness of the microporous film.
(2) Air permeability (film thickness 20μm conversion)
The air permeability of the microporous membrane was measured with a Gurley type air permeability meter in accordance with JIS P-8117. By multiplying this by 20 (μm) / thickness (μm) of the microporous membrane, the air permeability of the microporous membrane (converted to a thickness of 20 μm, unit: second / 100 mL) was calculated.
(3) Puncture strength (converted to a film thickness of 20 μm)
Using a handy compression tester manufactured by Kato Tech Co., Ltd. under the trade name “KES-G5”, a puncture test was performed on the microporous membrane under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. The piercing load (N) was measured. By multiplying this by 20 (μm) / film thickness (μm) of the microporous film, the puncture strength of the microporous film (converted to a film thickness of 20 μm, unit: N) was calculated.

(4) Surface observation The surface reflection electron image of the microporous film (photographing magnification 30,000 times) is accelerated using a scanning electron microscope (SEM, manufactured by Hitachi High-Technologies Corporation, trade name “S-5500”). The voltage was obtained at 1.5 kV.
(5) Shut-down temperature, short-circuit temperature 10 μm thick electrode A (length 100 mm × width 25 mm), 10 μm thick electrode B (length 100 mm × width 15 mm), separator immersed in a predetermined electrolyte for 30 minutes or more ( MD direction length 75mm x TD direction length 25mm), Aramika film (registered trademark, 75mm x 75mm) with a rectangular window 5 of 10mm x 10mm in the center, slide glass (length 75mm x width 25mm), glass plate (Length 25 mm × width 20 mm) was prepared. Subsequently, as shown in FIG. 1, the glass slide 3, the electrode A, the separator (measurement object) 1, the Aramika film 4, the electrode B, and the glass plate 2 are stacked in this order and fixed with a commercially available double clip. To complete the cell. Next, the thermocouple 6 was connected to the said cell, and it left still in oven. Thereafter, the cell was heated at a rate of 5 ° C./min, and the impedance change between the electrodes A and B at this time was measured with an LCR meter under conditions of AC 1V and 1 kHz. In this measurement, the temperature at which the impedance reached 1000Ω was taken as the shutdown temperature, and the temperature at which the impedance dropped below 1000Ω was taken as the short-circuit temperature. The composition ratio of the predetermined electrolytic solution is as follows.
Composition ratio (volume ratio) of solvent: propylene carbonate / ethylene carbonate / δ-butyllactone = 1/1/2
Solute composition ratio: LiBF ₄ was dissolved in the above solvent to a concentration of 1 mol / liter.
(6) Melting point 6 to 7 mg of the microporous membrane was put into an aluminum pan, and heated from room temperature to 600 ° C. at a rate of temperature increase of 10 ° C./min under a nitrogen stream. The endothermic curve at this time was measured using a product name “DSC60” which is a DSC manufactured by Shimadzu Corporation. The peak top temperature of the maximum endothermic peak of the obtained endothermic curve was taken as the melting point.

Examples 1 to 3
The weight average molecular weight (Mv) is 250,000, the intrinsic viscosity [η] is 2.8 dL / g, the melting point is 136 parts by mass of high-density polyethylene (HDPE), the viscosity average molecular weight (Mv) is 700,000, the intrinsic viscosity [ η] is 5.6 dL / g, melting point is 138 ° C HDPE 19 parts by mass, Mv is 400,000, intrinsic viscosity [η] is 3.3 dL / g, melting point is 163 ° C 2 parts by mass of polypropylene (PP) 40 mass% of 1 thermoplastic resin and 60 mass% of liquid paraffin (manufactured by Matsumura Oil Co., Ltd., trade name “P350P”) were mixed to obtain a mixture. Next, the mixture was melt-kneaded and then cooled with cold air to prepare a 1000 μm-thick original fabric sheet (original fabric sheet manufacturing step). Separately, a cast polypropylene (CPP) film having an Mv of 160,000, an intrinsic viscosity [η] of 1.6 dL / g, a melting point of 162 ° C., a film thickness of 30 μm, and an air permeability of 10,000 seconds / 100 mL is impervious to the film. Prepared as. Next, an air-impermeable film was laminated on one side of the original fabric sheet to obtain a sheet-like laminate (lamination step). Furthermore, simultaneous biaxial stretching was performed on the laminate in the in-plane direction to try to make it microporous (stretching step). The stretching speed at this time was constant, and the strain speed was 20% / second with respect to the laminate length. The stretched composite film thus obtained was immersed in methylene chloride to extract and remove liquid paraffin (plasticizer extraction step). Thereafter, drying was performed to remove methylene chloride, and composite microporous membranes of Examples 1 to 3 were obtained. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane. Moreover, the observation result by SEM of the surface derived from the raw sheet of the composite microporous membrane produced in Example 3 and the surface derived from the air-impermeable film is shown in FIGS.

[Examples 4 and 5]
In the same manner as in Examples 2 and 3, except that the first thermoplastic resin was changed from 40% by mass to 30% by mass and the liquid paraffin was changed from 60% by mass to 70% by mass. A porous membrane was produced. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

Example 6
The same procedure as in Example 2 was performed except that the first thermoplastic resin was changed from 40% by mass to 20% by mass, the liquid paraffin was changed from 60% by mass to 80% by mass, and the stretching temperature was changed from 110 ° C. to 100 ° C. The composite microporous membrane of Example 6 was produced. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

[Examples 7 and 8]
In the same manner as in Examples 2 and 3, except that the first thermoplastic resin was changed from 40% by mass to 20% by mass and liquid paraffin was changed from 60% by mass to 80% by mass. A porous membrane was produced. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

Example 9
A composite microporous membrane of Example 9 was produced in the same manner as in Example 3 except that the CPP film was laminated on both sides of the original sheet instead of one side. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

[Comparative Examples 1 and 2]
A CPP film having an Mv of 160,000, an intrinsic viscosity [η] of 1.6 dL / g, a melting point of 162 ° C., a film thickness of 30 μm, and an air permeability of 10,000 seconds / 100 mL was obtained by liquid paraffin (trade name “P350P, manufactured by Matsumura Oil Co., Ltd.). )) And then biaxially stretching in the in-plane direction to try to make it microporous. Stretching was attempted under various conditions. In particular, in Comparative Example 1, the film was not microporous under the same stretching conditions as in Example 1, and the membrane was broken. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

[Comparative Example 3]
A microporous film of Comparative Example 3 was produced in the same manner as in Example 3 except that the original sheet was stretched as a single layer without using a CPP film. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

[Comparative Example 4]
An HDPE sheet having an Mv of 250,000, [η] of 2.8 dL / g, and a melting point of 136 ° C. was melt-kneaded and cooled to prepare an unstretched PE sheet. Separately, PP having Mv of 400,000, [η] of 3.3 dL / g, and melting point of 163 ° C. was melt-kneaded and cooled to prepare an unstretched PP sheet. After heat-treating the unstretched PE sheet at 95 ° C and the unstretched PP sheet at 120 ° C, a three-layer laminated sheet with PP as the outer layer and PE as the inner layer is produced by thermocompression bonding, and stretched in the MD direction at room temperature and then at 120 ° C did. In this way, a three-layer microporous film of Comparative Example 4 made of PP / PE / PP was produced by the stretch opening method. Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

[Comparative Example 5]
A composite microporous membrane of Comparative Example 5 was produced in the same manner as in Example 3 except that the first thermoplastic resin was changed from 40% by mass to 90% by mass and the liquid paraffin was changed from 60% by mass to 10% by mass. . Table 1 shows the film configuration of the microporous film, the content ratio of the plasticizer, the stretching temperature, and the stretching ratio. Table 2 shows measurement results of various characteristics of the obtained microporous membrane.

  From the surface observation result of the composite microporous membrane of Example 3 shown in FIGS. 2 and 3, both surfaces (the surface derived from the original sheet and the surface derived from the air-impermeable film) are both microporous by the manufacturing method of the present embodiment. It became clear. As a result of calculating the average pore diameter on each surface by image analysis from the photograph of this electronic image, the average pore diameter on the surface derived from the raw sheet was 0.05 μm, and the average pore diameter on the surface derived from the air-impermeable film was 0.02 μm. .

  Further, from the comparison between Examples 1 and 3 and Comparative Examples 1 and 2, the CPP film alone, which is an air-impermeable film, cannot be stretched at a high magnification and it is difficult to achieve high productivity. It was found that a microporous membrane can be produced at a high draw ratio by using as a support. Furthermore, it became clear from the comparison between Examples 1 and 3 that the microporous membrane exhibits sufficient high permeability and high strength by stretching at a high magnification.

  From the comparison between Examples 1 to 9 (particularly Example 3) and Comparative Example 3, it is possible to obtain a composite microporous membrane having a high short-circuit temperature by laminating and stretching an impermeable film on the raw sheet. all right. In Examples 2 to 9, since the film can be stretched at the same magnification as in Comparative Example 3, the composite microporous film that is comparable to the strength and permeability of the film obtained by the conventional wet method is as good as the conventional wet method. It became clear that it was possible to produce.

  From the comparison between Example 9 and Comparative Example 4, it is clear that the composite microporous membrane produced by the production method of this embodiment can produce a composite microporous membrane at a higher draw ratio than the conventional so-called stretch opening method. became. From this, it has confirmed that the manufacturing method of this embodiment was excellent in productivity rather than the extending | stretching aperture method.

  From the result of Comparative Example 5, it was found that when the plasticizer contained in the raw sheet was small, the amount thereof was insufficient, so that it was difficult to make the air-impermeable film microporous.

  Although not shown in the table, an attempt was made to produce a composite microporous membrane in the same manner as in Example 3 except that the stretching temperature was changed from 120 ° C to 85 ° C. A porous membrane could not be obtained. A composite microporous membrane was prepared in the same manner as in Example 3 except that the stretched surface magnification was changed from 49 times to 9 times (MD magnification: 3 times, TD magnification: 3 times). In this case, although it became clear that a composite microporous membrane could be produced, it was difficult to achieve high production efficiency.

It is sectional drawing which shows roughly the cell used when measuring shutdown temperature and short circuit temperature. It is a photograph which shows the scanning electron microscope electron image (magnification 30,000 times) of the surface derived from the original fabric sheet in the composite microporous film of an Example. It is a photograph which shows the scanning electron microscope microscope electron image (magnification 30,000 times) of the surface derived from the air-impermeable film in the composite microporous film of an Example.

Explanation of symbols

A and B electrodes 1 Measurement object 2 Glass plate 3 Slide glass 4 Aramika film 5 Rectangular window 6 Thermocouple

Claims (6)

A raw sheet containing a first thermoplastic resin and a plasticizer, wherein the content of the plasticizer relative to the total mass of the first thermoplastic resin and the plasticizer is 20 to 80% by mass; and a second Stretching a laminate with an impermeable film containing a thermoplastic resin as a main component to obtain a stretched composite film;
Extracting the plasticizer from the stretched composite film;
A method for producing a composite microporous membrane comprising:
The plasticizer is an organic compound capable of melting or dissolving the amorphous part of the second thermoplastic resin contained in the air-impermeable film,
The said air-impermeable film is a manufacturing method of a composite microporous film containing 50-100 mass% of said 2nd thermoplastic resins with respect to the total mass. A film thickness and / or a mechanical property of the air-impermeable film is obtained by laminating an original sheet containing the first thermoplastic resin and a plasticizer and an air-impermeable film containing the second thermoplastic resin as a main component. A step of stretching a laminate in which the strength is made non-uniform by the plasticizer to obtain a stretched composite film;
Extracting the plasticizer from the stretched composite film;
A method for producing a composite microporous membrane comprising:
The said air-impermeable film is a manufacturing method of a composite microporous film containing 50-100 mass% of said 2nd thermoplastic resins with respect to the total mass.   The manufacturing method of the composite microporous film of Claim 1 or 2 whose said air-impermeable film is a polypropylene film.   The manufacturing method of the composite microporous film as described in any one of Claims 1-3 whose extending | stretching temperature at the time of extending | stretching the said laminated body is 90 degreeC or more.   The manufacturing method of the composite microporous film as described in any one of Claims 1-4 whose draw surface magnification at the time of extending | stretching the said laminated body is 10 times or more.   The manufacturing method of a lithium ion battery which has the process of incorporating the composite microporous film obtained by the manufacturing method of the composite microporous film as described in any one of Claims 1-5 as a separator.