Polyolefin film, laminate, laminated film, resin current collector, resin current collector for bipolar battery, electrode for bipolar battery, secondary battery, electric vehicle, and electric flying object

Abstract

To provide a polyolefin film having high conductivity and maintaining good conductivity even after stretching, and to provide a resin current collector for a bipolar type battery having uniform conductivity in the thickness direction in a film surface.SOLUTION: The polyolefin film contains conductive particles and satisfies the following (1) and (2): (1) A carbon material and / or metal particles are contained in a total amount of 1.0 mass% or more and 40 mass% or less. (2) The conductive particles contain at least particles having an aspect ratio of 1 or more and less than 5 and particles having an aspect ratio of 5 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyolefin film, a resin current collector, a resin current collector for a bipolar battery, an electrode for a bipolar battery, a secondary battery, an electric vehicle, and an electric flying object. [Background technology]

[0002] In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to protect the environment. The automotive industry is hopeful that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will help reduce carbon dioxide emissions, and efforts are underway to develop secondary batteries for motor drive, which hold the key to their practical application. Secondary batteries include lithium-ion batteries, which can achieve high energy density and high power density, as well as next-generation batteries such as lithium-ion batteries using metallic lithium anodes, all-solid-state batteries, and air-air batteries. In addition to automobiles, development is also underway for next-generation mobility technologies such as drones, flying cars, and flying communication base stations, and lightweight, high-energy-density secondary batteries are in high demand.

[0003] In secondary batteries such as lithium-ion batteries, metal foils (metal current collector foils) have traditionally been used as current collectors, but in recent years, resin film current collectors made of resin films have been proposed as an alternative to metal foils. Resin film current collectors are lighter than metal current collector foils, and are expected to improve the output per unit weight of the battery.

[0004] In recent years, bipolar lithium-ion batteries have been actively developed as compact, high-power lithium-ion batteries. Unlike conventional lithium-ion batteries, in which the current generated in the battery cells is extracted from electrode tabs attached to the edges of the current collector foil and connected between multiple battery cells, bipolar lithium-ion batteries stack multiple power-generating cells with the current collector foils arranged on the outermost layers interposed between them, allowing the current to flow in the thickness direction of the current collector foil. This configuration makes it possible to eliminate the electrode terminals, wiring, and protective exterior body of conventional batteries, thereby significantly reducing the battery size. Furthermore, by shifting the current flowing in the thickness direction of the current collector foil, which previously flowed in the plane direction, the resistance to the flowing current can be reduced, thereby increasing battery output.

[0005] On the other hand, in order to fabricate film current collectors for use in bipolar batteries, they must be made conductive in the thickness direction. Furthermore, there is a demand for thinner film current collectors. This is because thinner film current collectors increase the number of current collectors that can be stacked per unit volume, thereby improving the energy density of lithium-ion batteries. Furthermore, film current collectors must be strong enough for ease of processing and handling.

[0006] For example, Patent Documents 1 to 3 disclose materials for bipolar resin current collectors that contain a combination of polyolefin and elastomer or an imide group-containing resin as the main component and a high concentration of conductive filler, and resin current collectors that contain the resin current collector materials. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2019-179732 [Patent Document 2] International Publication No. 2015 / 005116 [Patent Document 3] International Publication No. 2011 / 092938 Summary of the Invention [Problem to be solved by the invention]

[0008] However, Patent Documents 1 to 3 disclose resin current collector materials that use a combination of polyolefin and elastomer, or that contain an imide group-containing resin as the main component and a high concentration of conductive filler, and resin current collectors that contain the resin current collector materials. However, when sheets formed based on these documents were biaxially stretched, it was found that the conductivity of the film was significantly reduced, the film strength was low, and there was room for improvement in the conductivity after pulling or bending.

[0009] The present invention aims to provide a polyolefin film that has high electrical conductivity and maintains good electrical conductivity even after stretching, and a resin current collector for a bipolar battery that has uniform electrical conductivity in the thickness direction within the film plane. [Means for solving the problem]

[0010] In order to solve the above problems, a preferred embodiment of the present invention has the following configuration. [I] A polyolefin film containing conductive particles, which satisfies the following (1) and (2): (1) The composite material contains carbon material and / or metal particles in a total amount of 1.0 mass % or more and 40 mass % or less. (2) The conductive particles include at least conductive particles having an aspect ratio of 1 or more and less than 5, and conductive particles having an aspect ratio of 5 or more. [II] A polyolefin film according to [I], wherein the conductive particles (C1) contained in the polyolefin film have an aspect ratio of 1 or more but less than 5, and the conductive particles (C2) have an aspect ratio of 5 or more, and the ratio of the area (SC1) occupied by C1 to the area (SC2) occupied by C2 contained in the polyolefin film satisfies formula 1. 5≧SC1 / SC2≧0.1 ···Formula 1 [III] A polyolefin film according to [I] or [II], having an F5 value (the load value when the test piece is elongated by 5% divided by the cross-sectional area of ​​the test piece) of 25 MPa or more in at least one direction within the plane. [IV] The polyolefin film according to [III], wherein the F5 values ​​in both the longitudinal and transverse directions within the plane are 25 MPa or more. [V] Volume resistivity at 23°C 65% RH is 10 0 Ω cm or more 10 8 The polyolefin film according to any one of [I] to [IV], which has a resistivity of less than Ω·cm. [VI] The polyolefin film according to any one of [I] to [V], wherein the conductive particles contained therein have a volume-based average dispersed diameter of 0.1 μm or more and 10 μm or less. [VII] A polyolefin film containing conductive particles, which satisfies the following (3) and (4): (3) The volume resistivity at 23°C and 65% RH is 10 0 Ω cm or more 5.0×10 6 Ω·cm or less. (4) The F5 value (the load value when the test piece is elongated by 5% divided by the cross-sectional area of ​​the test piece) in at least one direction within the plane is 25 MPa or more. [VIII] The polyolefin film according to any one of [I] to [VII], which has a shrinkage rate of 20% or less when kept at 120°C for 15 minutes. [IX] The polyolefin film according to any one of [I] to [VIII], wherein the half-value width of the heat of fusion peak measured by a differential scanning calorimeter (DSC) is 10° C. or more and 40° C. or less. [X] The polyolefin film according to any one of [I] to [IX], which contains at least two or more layers. [XI] The polyolefin film according to any one of [I] to [X], wherein the water contact angle on at least one surface is 75° or more and 100° or less. [XII] A laminate having a layer made of a metal and / or a metal-based compound on at least one surface of the polyolefin film according to any one of [I] to [XI]. [XIII] The laminate according to [XII], wherein the layer made of a metal and / or a metal-based compound contains aluminum element. [XIV] A laminate having a layer made of a metal and / or a metal-based compound on both surfaces of the polyolefin film according to any one of [I] to [XI]. [XV] A laminate according to [XIV], wherein the layer of metal and / or metal-based compound on one surface contains aluminum element, and the layer of metal and / or metal-based compound on the opposite surface contains copper element. [XVI] A laminated film having a layer of a metal and / or a metal-based compound on at least one surface of a polyolefin film containing conductive particles, which satisfies the following (5), and has a penetration resistivity of 1.0 × 10 0 Ωcm or more 1.0×10 8 Laminated film with a resistance of Ωcm or less. (5) The composite material contains carbon material and / or metal particles in a total amount of 1.0 mass % or more and 40 mass % or less. [XVII] A resin current collector comprising the polyolefin film according to any one of [I] to [XI]. [XVIII] A resin current collector for a bipolar battery, comprising the polyolefin film according to any one of [I] to [XI]. [XIX] [XVIII] An electrode for a bipolar battery comprising a negative electrode active material layer on one surface side of the resin current collector for a bipolar battery according to [XIX] [XVIII], and a positive electrode active material layer on the opposite side to the surface layer. A secondary battery comprising the bipolar battery electrode according to [XX][XIX]. [XXI] [XIX] A secondary battery comprising the bipolar battery electrode according to the present invention, wherein the positive electrode active material layer and the negative electrode active material layer are stacked in at least two layers with an electrolyte interposed therebetween. [XXII][XXI] An electric vehicle equipped with the secondary battery described in. [XXIII][XXI] An electric flying object equipped with a secondary battery. [Effects of the Invention]

[0011] The present invention can provide a polyolefin film that has high electrical conductivity and maintains good electrical conductivity even after stretching, and a resin current collector for a bipolar battery that has uniform electrical conductivity in the thickness direction within the film plane. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in detail below. A preferred embodiment of the present invention is a polyolefin film containing conductive particles, which satisfies the following (1) and (2): (1) The composite material contains carbon material and / or metal particles in a total amount of 1.0 mass % or more and 40 mass % or less. (2) The conductive particles include at least conductive particles having an aspect ratio of 1 or more and less than 5, and conductive particles having an aspect ratio of 5 or more.

[0013] (Polyolefin film) Examples of polyolefins in the present invention include homopolymers of α-olefins having about 2 to 8 carbon atoms, such as ethylene, propylene, 1-butene, 3-methyl-1-butene, 1-pentene, 4-methyl-1-pentene, and 1-hexene; copolymers of these α-olefins with other α-olefins having about 2 to 12 carbon atoms, such as ethylene, propylene, 1-butene, 3-methyl-1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, and 1-decene; copolymers of these α-olefins with vinyl compounds, such as vinyl acetate, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, styrene, and vinyl chloride; and graft polymers in which the vinyl compounds are grafted onto the homopolymers or copolymers of the above α-olefins. Among these, polypropylene resins or polyethylene resins are preferred, and polypropylene resins are more preferred, from the viewpoints of mechanical strength and film-forming properties.

[0014] The polypropylene resin may be a homopolymer of propylene (homopolypropylene resin) or a copolymer of propylene and an olefin having 2 to 20 carbon atoms (random polypropylene resin). One or more types of olefins may be copolymerized with propylene. Specific examples of random polypropylene resins include propylene-ethylene copolymers, propylene-1-butene copolymers, and propylene-ethylene-1-butene copolymers. Among these, homopolypropylene resins are preferred from the viewpoints of mechanical properties and moldability.

[0015] The method for producing polyolefin is not particularly limited, and known polymerization methods, such as radical polymerization and ionic polymerization using a Ziegler catalyst, a metallocene catalyst, a Phillips catalyst, or the like, can be employed.

[0016] The polyolefin film of the present invention is preferably biaxially oriented from the viewpoint of improving the mechanical strength and thermal dimensional stability of the film. Biaxial orientation as used herein refers to a film that exhibits a biaxially oriented pattern in wide-angle X-ray diffraction. Biaxially oriented polyolefin films can generally be obtained by stretching an unstretched thermoplastic resin film in the axial direction of the film-forming machine (hereinafter sometimes referred to as the longitudinal direction) and the width direction, followed by heat treatment to complete the oriented crystallization.

[0017] The polyolefin film of the present invention has a volume resistivity of 1.0×10 at 23° C. and 65% RH, as determined by the volume resistivity measurement described below. 0 Ωcm or more 1.0×10 8 It is preferably less than Ωcm.

[0018] The volume resistivity is a value that reflects the conductivity in the thickness direction of the film and is a value that is independent of the film thickness. The volume resistivity of the polyolefin film of the present invention at 23°C and 65% RH is 1.0 × 10 8By making the volume resistivity of the polyolefin film of the present invention less than 1.0×10 Ωcm, the film has excellent conductivity, and when used as a resin current collector for a bipolar battery, the current value flowing in the battery can be increased, and sufficient battery performance can be achieved. 8 By making the volume resistivity less than Ωcm, a conductive path is sufficiently formed in the thickness direction, and a significant decrease in conductivity can be suppressed even when the polyolefin film of the present invention is subjected to deformation such as pulling or bending when processed into a battery component. 6 Ωcm or less, more preferably 1.0×10 4 Ωcm or less, and most preferably 5.0×10 2 It is less than Ωcm.

[0019] The volume resistivity of the polyolefin film of the present invention at 23°C and 65% RH is 1.0 × 10 0 By setting the resistivity to Ωcm or more, when a cast film is produced by an electrostatic application method in the casting step of film production described below, an unstretched film can be obtained without wrinkles or uneven thickness due to uneven application of electric current. This can prevent surface defects from occurring when a layer (M layer) made of a metal and / or metal-based compound described below is provided on the polyolefin film after film production, due to wrinkles or uneven thickness of the polyolefin film after film production, and can prevent a decrease in current value or an increase in variation when the polyolefin film is incorporated as a resin current collector in an electrode for a bipolar battery.

[0020] The electrostatic application method involves charging a polyolefin resin layer, adhering it to a metal casting drum, and cooling it to obtain an unstretched sheet. However, if the volume resistivity of the polyolefin film is low, the charge generated by the electrification may flow toward the casting drum, reducing adhesion to the casting drum and resulting in uneven application of the charge.

[0021] The polyolefin film of the present invention preferably has a film thickness determined by the method described below of 1 μm or more and less than 500 μm.

[0022] A film thickness of 1 μm or more can suppress the occurrence of tearing during film formation, improving film formability. Furthermore, the film has increased tensile strength, preventing a significant decrease in conductivity even when the film is deformed during processing into a battery component. The film thickness is more preferably 2 μm or more, even more preferably 3 μm or more, and most preferably 4 μm or more. Furthermore, a film thickness of less than 500 μm can minimize an increase in battery size when incorporated into a resin current collector for a bipolar battery. The film thickness is more preferably 300 μm or less, particularly preferably 150 μm or less, and most preferably 80 μm or less.

[0023] Furthermore, when the thickness of the polyolefin film of the present invention is 80 μm or less, it is more preferable that the polyolefin film of the present invention is a uniaxially oriented or biaxially oriented polyolefin film, and particularly a biaxially oriented polyolefin film, from the viewpoint of increasing the strength of the film and preventing a significant decrease in conductivity even when the film is deformed during processing as a battery component.

[0024] The conductive particles of the present invention preferably contain one or more particles selected from metal particles, metal oxide particles, conductive resin particles, and carbon materials, more preferably the particles contain carbon materials, and even more preferably the particles are carbon materials.

[0025] Examples of metal particles used as conductive particles include particles containing one selected from the group consisting of gold, silver, copper, tin, nickel, indium, aluminum, and iron as a main component, as well as metal particles made of stainless steel, nickel-indium alloy, etc. Alternatively, the surface of a core made of any resin may be plated with the above metal. Examples of metal oxide particles used as conductive particles include tin oxide, zinc oxide, and zinc oxide. Also, inorganic particles such as aluminum oxide, calcium carbonate, mica, talc, and glass may have a coating layer formed on their surfaces, the coating layer being made of a metal or a metal oxide, both of which are listed as examples of the metal particles and metal oxides. Alternatively, the surface of a core made of any resin may be plated with the metal oxide.

[0026] Examples of conductive resin particles used as conductive particles include resins having a conjugated double bond structure in the main chain skeleton, such as polyacetylene, poly-p-phenylene, polyfluorene, and poly-p-phenylene vinylene; thiophene-based resins such as polyethylenedioxythiophene (PEDOT) / polystyrene sulfonate (PSS), polythiophene, and polythienylene vinylene; polyaniline; and polypyrrole.

[0027] The carbon material is a material that has a graphite structure (sp 2 bond) at 1580 cm -1 The diamond structure (sp 3 bond) at 1350 cm -1 This indicates a material for which a peak is observed around

[0028] The polyolefin film of the present invention preferably contains a carbon material and / or metal particles in a total amount of 1.0% by mass or more and 40% by mass or less.

[0029] By including these particles in an amount of 1.0% by mass or more and 40% by mass or less, the volume resistivity of the film can be controlled within a preferred range, and good conductivity can be exhibited in the thickness direction of the polyolefin film.

[0030] By containing these particles in a polyolefin film in an amount of 1.0% by mass or more, the volume resistivity can be reduced and the conductivity of the film can be increased. Furthermore, the conductivity in the thickness direction can be made uniform. The concentration of these particles is more preferably 1.5% by mass or more, and even more preferably 2.0% by mass or more.

[0031] By containing these particles in a polyolefin film at 40% by mass or less, it is possible to prevent discharge disturbances during the film-forming process of the polyolefin film of the present invention and to prevent numerous breaks or tiny air spaces (hereinafter sometimes referred to as voids) originating from the conductive particles during stretching, which can lead to breakage of the film during stretching. Since the generation of numerous voids can be prevented, uniform conductivity can be achieved in the thickness direction. Furthermore, by containing these particles in a polyolefin film at 40% by mass or less, it is possible to prevent a significant decrease in the mechanical strength of the film. This prevents a significant decrease in conductivity even when the film is deformed during processing as a battery component. Furthermore, when the polyolefin film of the present invention is incorporated as a current collector in a secondary battery, it is possible to prevent the polyolefin film from breaking due to thermal deformation accompanying battery charge / discharge or minute deformation due to external stress, which can result in a deterioration in battery performance. The amount of these particles contained in the polyolefin film is more preferably 30% by mass or less, even more preferably 25% by mass or less, particularly preferably 15% by mass or less, and most preferably 10% by mass or less.

[0032] The carbon material preferably contains, based on 100% by mass of the carbon material, a total of 80% by mass or more of one or more selected from furnace black, acetylene black, carbon nanotubes represented by single-walled carbon nanotubes, multi-walled carbon nanotubes, and thin-walled carbon nanotubes, carbon fiber, graphene, ketjen black, fibrous carbon, fullerene, and graphite. From the viewpoint of imparting conductivity with the addition of a small amount, the carbon material more preferably contains, based on 100% by mass of the carbon material, a total of 80% by mass or more and 100% by mass or less of one or more selected from ketjen black, carbon nanotubes, and acetylene black.

[0033] In the polyolefin film of the present invention, the conductive particles preferably contain at least two types of conductive particles: conductive particles C1 having an aspect ratio of 1 or more but less than 5, and conductive particles C2 having an aspect ratio of 5 or more. A method for determining the aspect ratio of the conductive particles contained in the polyolefin film will be described later. The aspect ratio can be determined using the same measurement method regardless of whether the conductive particles are metal particles, metal oxide particles, conductive resin particles, or carbon materials.

[0034] By containing at least two types of conductive particles, namely, conductive particles C1 having an aspect ratio of 1 or more but less than 5, and conductive particles C2 having an aspect ratio of 5 or more, the volume resistivity of the film can be kept within a preferred range with a smaller amount than when either conductive particle is contained alone. Furthermore, sufficient conductive paths are formed in the thickness direction, and even when the polyolefin film of the present invention is subjected to deformation such as pulling or bending during processing into a battery component, a significant decrease in conductivity can be suppressed. Furthermore, since a smaller content of conductive particles is required, the mechanical strength of the film can be increased.

[0035] The reason why a significant decrease in conductivity can be prevented even when the film is subjected to deformation such as pulling or bending can be explained as follows. Conductive particles with an aspect ratio of 5 or more have high conductivity in the long axis direction of the conductive particles and have the function of transmitting electrons over long distances. However, when the conductive particles are subjected to deformation such as pulling or bending, the spacing between the conductive particles increases, making the connection points more likely to be broken. Conductive particles with an aspect ratio of 1 to 5 but less than 5 are smaller in size and more spherical in shape than conductive particles with an aspect ratio of 5 or more. When the polyolefin film contains at least two types of conductive particles with different aspect ratios, the conductive particles with an aspect ratio of 1 to 5 can penetrate between the broken conductive paths of the conductive particles with an aspect ratio of 5 or more when the film is subjected to deformation such as pulling or bending, thereby preventing the breakage of the conductive paths.

[0036] Furthermore, when the polyolefin film contains at least two types of conductive particles, namely, conductive particles C1 having an aspect ratio of 1 or more but less than 5 and conductive particles C2 having an aspect ratio of 5 or more, the polyolefin film of the present invention can be prevented from decreasing in conductivity when heated. This is because the mobility of resin molecular chains in the polyolefin film of the present invention increases under high temperature conditions, and even when the dispersion state of the contained conductive particles changes, conductive particles having an aspect ratio of 1 or more but less than 5 can enter between the disconnected conductive paths of the conductive particles having an aspect ratio of 5 or more, thereby preventing disconnection of the conductive paths. Therefore, an energy storage element using the polyolefin film of the present invention as a resin current collector for a bipolar battery can be prevented from decreasing in current value even when the temperature rises during operation.

[0037] From the same viewpoint as above, the polyolefin film of the present invention preferably contains at least two kinds of carbon materials: carbon material C1 having an aspect ratio of 1 or more and less than 5, and carbon material C2 having an aspect ratio of 5 or more.

[0038] Here, the carbon material C1 having an aspect ratio of 1 or more and less than 5 is preferably one or more selected from furnace black, acetylene black, ketjen black, and fullerene, and from the viewpoint of excellent conductivity, the carbon material C1 having an aspect ratio of 1 or more and less than 5 is more preferably acetylene black and / or ketjen black.

[0039] Furthermore, the carbon material C2 having an aspect ratio of 5 or more is preferably one or more selected from single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon fibers, and graphene, and more preferably one selected from single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon fibers.

[0040] In the polyolefin film of the present invention, it is preferable that the ratio of the area (SC1) occupied by the conductive particles C1 to the area (SC2) occupied by the conductive particles C2 contained in the polyolefin film, determined by the following method, satisfies formula 1. 5.0≧SC1 / SC2≧0.1 ··· Formula 1

[0041] <Method for measuring the area occupied by each particle> Cross-sectional images were taken at a magnification of 10,000 times using a scanning electron microscope (JEOL, JSM-6700), and the following analysis was performed using image analysis software (ImageJ, manufactured by the National Institutes of Health, USA).

[0042] (Image analysis conditions) (i) Pretreatment After capturing the cross-sectional image using the software, execute the "8-bit" command under "Type" in the "Image" menu to perform 8-bit grayscale processing. Next, use the "Straight" menu to draw a straight line of a known distance, and set the scale of the captured image using the "Set Scales" command under the "Analyze" menu. After that, use the "Subtract" command under "Math" in the "Process" menu to subtract "Value" by 30. Use the "Enhance Contrast" command from the "Process" menu to set "Saturated pixels" to 5%, check "Normalize", and press the "OK" button. Use the "Gaussian Blur" command under "Filters" in the "Process" menu to perform blurring with a "Sigma (Radius)" of 1.

[0043] (ii) Binarization From the "Image" menu, select "Adjust" and then "Threshold," check only "Dark background" and "Don't reset range," then press "Auto" and then "Apply."

[0044] (iii) Noise reduction From the "Process" menu, select "Noize" and then the "Remove Outliers" command, set "Saturated pixels" to 1 pixel, "Threshold" to 50, and "Which Outliers" to Bright, then press the "OK" button.

[0045] (iv) Particle analysis conditions In the "Analyze" menu, select the "Set Measurements" command, check "Area" and "Feret's diameter," and click the "OK" button. Next, in the "Analyze" menu, select the "Analyze Particles" command, set each item as follows, and click the "OK" button to display the analysis results. Size: 0-Infinity Circularity: 0.00-1.00 Show:Nothing Check "Display Results", "Clear Results", "Exclude on edges" and "include holes".

[0046] From the results obtained, the volume-based average dispersed particle diameter, aspect ratio, and area of ​​each particle are calculated. The aspect ratio is calculated by dividing the "Feret" of each particle in the Results by the "MinFeret." The area is the "Area" value of each particle.

[0047] (Volume-based average dispersed particle diameter) Using the "Feret" of each particle detected above, the volume-based average dispersed diameter is calculated according to the following formula 2. The volume-based average dispersed diameters are calculated for five different fields of view, and their average value is taken as the volume-based average dispersed diameter R (μm) of the particles in the sample. R(μm)=(Σ(Feret) 2 × number) / (Σ(Feret) × number) Equation 2

[0048] (aspect ratio and area of ​​each particle) A count histogram is created by plotting the aspect ratios obtained for the five different fields of view on the horizontal axis in 0.5 intervals in the range of 1 to 10, and in 10 intervals in the range of 10 or more. If a maximum value exists in the region of the number histogram where the aspect ratio is 1 to less than 5, the minimum value in the range showing that maximum value (for example, if the maximum value is in the range of 2 to 2.5, 2 is used as the maximum value) is taken as the aspect ratio of the conductive particles (conductive particles C1) existing in the region where the aspect ratio is 1 to less than 5. If multiple maximum peaks exist in the region where the aspect ratio is 1 to less than 5, the weighted average of those vertical axis values ​​is taken as the aspect ratio of the conductive particles (conductive particles C1) existing in the region where the aspect ratio is 1 to less than 5.

[0049] Similarly, if a maximum value exists in a region where the aspect ratio is 5 or more, that maximum value is taken as the aspect ratio of the conductive particle (conductive particle C2) existing in the region where the aspect ratio is 5 or more. If multiple maximum peaks exist in the region where the aspect ratio is 5 or more, the weighted average of those vertical axis values ​​is taken as the aspect ratio of the conductive particle (conductive particle C2) existing in the region where the aspect ratio is 5 or more.

[0050] For each of the five different fields of view, the aspect ratios of the conductive particles (conductive particles C1) present in the region with an aspect ratio of 1 or more but less than 5, and the conductive particles (conductive particles C2) present in the region with an aspect ratio of 5 or more are calculated, and the average value of the five fields of view in each region is taken as the aspect ratio of the conductive particles C1 and C2 in the polyolefin film.

[0051] The sum (SC1) of the areas of conductive particles (conductive particles C1) present in the region with an aspect ratio of 1 or more and less than 5 confirmed in the number histogram is determined. Next, the sum (SC2) of the areas of conductive particles (conductive particles C2) present in the region with an aspect ratio of 5 or more is determined to calculate SC1 / SC2. Note that the area ratio can be determined using the same measurement method regardless of whether the conductive particles are metal particles, metal oxide particles, conductive resin particles, or carbon materials.

[0052] By satisfying SC1 / SC2 ≧0.1, conductive particles C1 having an aspect ratio of 1 or more but less than 5 can penetrate between conductive particles C2 having an aspect ratio of 5 or more, thereby further suppressing the disconnection of conductive paths when subjected to deformation such as pulling or bending. In particular, this can suppress the deterioration of electrical properties after subjecting a polyolefin film to a tensile process such as metal vapor deposition. Furthermore, by satisfying SC1 / SC2 ≧5.0, conductivity can be obtained without increasing the particle content, resulting in a film with excellent mechanical strength and a film that is less likely to stretch even when tension is applied, thereby suppressing the disconnection of conductive paths when subjected to deformation such as pulling or bending. Furthermore, SC1 / SC2 ≧0.2 is more preferable. SC1 / SC2 is more preferably ≧4.5, and even more preferably ≧4.0.

[0053] In the polyolefin film of the present invention, the conductive particles contained are carbon material, and the volume-based average dispersion diameter of the contained carbon material, determined by the method described below, is preferably 0.1 μm or more and 10 μm or less. A volume-based average dispersion diameter of 0.1 μm or more indicates that the contained carbon material is adequately aggregated, thereby fulfilling its role as a relay point for electron conduction through the film thickness. This is because when the carbon material is excessively dispersed and exists alone, the molecular structure of the carbon material restricts electron movement to linear or planar directions, which may result in poor electronic conductivity through the thickness direction. On the other hand, when multiple carbon materials are aggregated without adequate regularity, the carbon material spreads three-dimensionally, expanding the range of electron movement. This allows electrons to be efficiently transmitted through the carbon material aggregates present within the polyolefin film, thereby improving electronic conductivity through the thickness direction. The volume-based average dispersion diameter of the carbon material contained in the polyolefin film is more preferably 0.2 μm or more.

[0054] Furthermore, by having a volume-based average dispersion diameter of 10 μm or less, film tearing originating from the carbon material can be suppressed when the film is stretched or when tensile tension is applied. The volume-based average dispersion diameter of the carbon material tends to increase depending on the carbon content concentration, and when the carbon material is contained in an amount exceeding 40 mass%, it becomes very difficult to achieve a volume-based average dispersion diameter of 10 μm or less. A more preferable range for the volume-based average dispersion diameter is 5 μm or less.

[0055] The polyolefin film of the present invention can achieve both electrical conductivity and mechanical strength by controlling the half-width of the melting peak of the polyolefin film measured by a differential scanning calorimeter (DSC) to 10°C or more and 40°C or less. The half-width of the melting peak of the polyolefin film can be controlled, for example, by including at least two polyolefin resins with different crystallinity. This resin structure allows the conductive particles to be unevenly distributed in the resin with the lower crystallinity among the at least two polyolefin resins with different crystallinity, thereby enabling the conductive particles to be uniformly dispersed while maintaining the conductive paths. From the viewpoint of conductive path formation, the half-width of the melting peak is more preferably 15°C or more. Furthermore, from the viewpoint of mechanical strength, the half-width of the melting peak is more preferably 30°C or less. The at least two polyolefin resins with different crystallinity are preferably selected from polypropylene resins from the viewpoint of mechanical strength, and among polypropylene resins, a homopolypropylene resin and a random polypropylene resin are more preferably included.

[0056] When multiple melting peaks are observed, it is preferable that the half-width of the peak with the largest heat of fusion is 10°C or more and 40°C or less.

[0057] The polyolefin film of the present invention preferably has an F5 value (the load value when the test specimen is elongated by 5% divided by the cross-sectional area of ​​the test specimen) in at least one in-plane direction of 25 MPa or more. It is more preferable that the breaking strength in at least one of the longitudinal and transverse directions in the film plane be 25 MPa or more, and even more preferable that the F5 values ​​be 25 MPa or more in both directions. By ensuring that the F5 value is 25 MPa or more, when the polyolefin film of the present invention is incorporated into an electrode as a current collector, it is possible to prevent the polyolefin film from breaking when subjected to a slight impact during transportation, thereby preventing a decrease in the performance of the electrode. The F5 value is more preferably 30 MPa or more, and even more preferably 35 MPa or more. Furthermore, by ensuring that the F5 value is 25 MPa or more, it is possible to prevent slight thermal deformation under the tension of the film during transportation in processes in which the polyolefin film of the present invention is subjected to high-temperature heat while held against a roll during transportation, such as a roll-to-roll metal deposition process, which inhibits the formation of conductive paths by conductive particles in the polyolefin film and thus prevents an increase in volume resistivity. Furthermore, when the polyolefin film of the present invention is incorporated into a battery element, even if the battery element is subjected to a large impact such as being dropped or hit, the polyolefin film is prevented from being deformed and broken, thereby preventing a significant deterioration in battery characteristics and the battery element from catching fire due to a short circuit.

[0058] With respect to the F5 value of the polyolefin film of the present invention, when the unwinding direction of the polyolefin film is defined as the longitudinal direction and the direction rotated 90° in-plane relative to the longitudinal direction is defined as the width direction of the polyolefin film, it is more preferable that the F5 value in at least one of the longitudinal direction and the width direction satisfies the above-mentioned preferred range. It is even more preferable that the F5 values ​​in both the longitudinal direction and the width direction satisfy the above-mentioned preferred range.

[0059] The shrinkage rate (hereinafter referred to as "heat shrinkage rate") of the polyolefin film of the present invention when maintained at 120°C for 15 minutes is preferably 20% or less from the viewpoint of vapor deposition processability. When a surface layer (M layer) made of a metal and / or metal compound is formed on the surface of the polyolefin film by a vapor deposition process such as vacuum deposition or vacuum sputtering, the resin film surface may be thermally damaged by high-temperature metal vapor during the vapor deposition process, resulting in a decrease in the mechanical properties of the film (hereinafter referred to as "heat damage"). To prevent heat damage of the film, it is common to perform vapor deposition while charging the surface of the resin film by discharge treatment or the like and applying high transport tension to the film while adhering it to a low-temperature cooled metal roll. However, if the film has a high heat shrinkage rate, the high-temperature metal vapor causes the film to lift off the cooled metal roll, resulting in insufficient cooling of the film and significant heat damage. By setting the heat shrinkage rate of the polyolefin film to 20% or less, heat damage due to insufficient adhesion between the polyolefin film and the cooled metal roll can be prevented. The heat shrinkage rate of the polyolefin film is more preferably 10% or less, and even more preferably 5% or less.

[0060] The water contact angle of the surface of the polyolefin film of the present invention is preferably 75° or more and 100° or less. By setting the water contact angle to 100° or less, when a layer (M layer) made of a metal and / or a metal-based compound is provided on the surface of the polyolefin film, peeling between the polyolefin film surface and the M layer can be suppressed. From the viewpoint of improving the adhesion between the polyolefin film surface and the M layer, the water contact angle of the polyolefin film surface is more preferably 90° or less, and even more preferably 85° or less. In order to set the water contact angle of the surface of the polyolefin film of the present invention within the above range, the polyolefin film surface may be subjected to corona treatment, plasma treatment, flame treatment, ultraviolet irradiation treatment, solvent treatment, etc.

[0061] From the viewpoint of adhesion between the polyolefin film of the present invention and the M layer, it is preferable to add a resin having a functional group, such as a polyester resin or an acid-modified polyolefin resin, to the resin component of the P1 layer when the polyolefin film has a single layer structure (only the P1 layer), or to the resin components of both outermost layers when the polyolefin film has a laminate structure of two or more layers (P1 layer / P2 layer).The addition of these resins can improve adhesion to the M layer.From the viewpoint of compatibility with the polyolefin resin that serves as the base resin, it is more preferable that the resin having an oxygen functional group is an acid-modified polyolefin. Such acid-modified polyolefin resins are preferably polyolefin resins modified with an unsaturated carboxylic acid or a derivative thereof. Examples of such unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid, and citraconic acid. Esters and anhydrides of these acids can also be used. Further examples of the derivatives include methyl acrylate, methyl methacrylate, ethyl acrylate, propyl acrylate, butyl acrylate, butyl methacrylate, vinyl acetate, glycidyl acrylate, glycidyl methacrylate, acrylamide, methacrylamide, and sodium acrylate.

[0062] The polyolefin film of the present invention preferably has a laminated structure of two or more layers (P1 layer / P2 layer) from the viewpoint of achieving both adhesion to a layer made of a metal and / or metal-based compound (hereinafter referred to as an M layer) described below and mechanical strength of the film, and a resin having an oxygen functional group is added only to the outermost layer that is in direct contact with the M layer. This is because, while increasing the amount of resin having an oxygen functional group added improves adhesion to the M layer, mechanical properties decrease as the amount added increases. Note that, when used as a resin current collector for a bipolar battery described below, it is preferable that the M layer be provided on both surfaces of the film. Therefore, from the viewpoint of achieving both adhesion to the film and mechanical strength of the film, it is most preferable that the polyolefin film have a three-layer structure of P2 layer / P1 layer / P2 layer or P2 layer / P1 layer / P3 layer.

[0063] (Method of manufacturing biaxially oriented polyolefin film) Next, an example of a method for producing a biaxially oriented film from the polyolefin film of the present invention will be described, but the present invention should not be construed as being limited to the products obtained by such an example.

[0064] The biaxially oriented polyolefin film of the present invention is preferably obtained by biaxially stretching an unstretched sheet obtained using the above-mentioned resin. The biaxial stretching method can be any of simultaneous biaxial stretching by inflation, simultaneous biaxial stretching by tenter, and sequential biaxial stretching by a roll-type stretching machine and a tenter. Among these, it is preferable to adopt sequential biaxial stretching by a roll-type stretching machine and a tenter in terms of film formation stability, thickness uniformity, and control of high rigidity and dimensional stability of the film.

[0065] Next, one embodiment of the method for producing a biaxially oriented polypropylene film of the present invention will be explained using a two-type, three-layer biaxially oriented polyolefin film as an example, but the biaxially oriented polypropylene film of the present invention is not necessarily limited to this.

[0066] When the polyolefin film of the present invention contains two or more types of conductive particles with different aspect ratios, it is preferable to prepare master pellets containing each type of conductive particle individually and mix them during the production of the polyolefin film. This is because the kneading temperature at which the conductive particles are dispersed and the shear stress during kneading are different when preparing a master pellet by kneading the conductive particles with the polyolefin resin. When preparing a master pellet by kneading multiple types simultaneously, at least one of the conductive particles may aggregate during kneading.

[0067] As a method for preparing a master pellet for dispersing the conductive particles, kneading is preferably carried out using a twin-screw kneader.

[0068] Pellets of the resin composition obtained above or a mixture thereof are fed into a single-screw extruder for the surface layer (I) and / or a single-screw extruder for the base layer (II). Then, melt extrusion is performed at 200 to 280°C, preferably 220 to 275°C, and even more preferably 240 to 270°C, respectively. After removing foreign matter and modified polymers using a filter installed midway through the polymer tube, the mixture is laminated in a layer structure of surface layer (I) / base layer (II) / surface layer (I) using a multi-manifold composite T-die. The mixture is then extruded onto a casting drum and cooled and solidified to obtain a laminated unstretched sheet having a layer structure of surface layer (I) / base layer (II) / surface layer (I). In this case, the lamination thickness ratio is preferably in the range of 1 / 5 / 1 to 1 / 60 / 1 from the viewpoint of balancing surface properties, mechanical strength, and thermal properties.

[0069] To suppress the formation of β-crystals during crystallization, the surface temperature of the casting drum is preferably 10 to 40°C, more preferably 15 to 30°C, even more preferably 19 to 27°C, and particularly preferably 20 to 25°C. The layer structure may be a two-layer laminate structure of a surface layer (I) and a base layer (II). The method for adhering the film to the casting drum may be any of electrostatic application, adhesion methods utilizing the surface tension of water, air knife methods, press roll methods, and underwater casting methods. From the viewpoint of achieving uniform adhesion in the width direction, the electrostatic application method or air knife method is preferred.

[0070] The resulting laminated unstretched sheet is introduced into the longitudinal stretching process. In the longitudinal stretching process, longitudinal stretching is preferably performed in two stages in a preheating process before stretching. In the first half of the preheating process, the laminated unstretched sheet is preferably heated by contacting it with multiple metal rolls maintained at a temperature below the softening temperature of the polyolefin resin used in the film. In the second half of the preheating process, heating is preferably performed at a temperature higher than the preheating temperature in the first half of the process, more preferably at a heating temperature 10°C higher than the preheating temperature in the first half of the process, and even more preferably at a heating temperature 20°C higher than the preheating temperature in the first half of the process. By setting the preheating temperature and stretching temperature within the above ranges, the resin can be appropriately softened in the preheating process, allowing deformation control during stretching, and as a result, the crystalline structure of the film can be suitably controlled.

[0071] Thereafter, in the stretching step, the laminated unstretched sheet is stretched in the longitudinal direction between rolls with different peripheral speeds by 4.0 to 6.0 times, more preferably 4.5 to 5.8 times, and even more preferably 4.8 to 5.5 times. The longitudinal direction refers to the direction in which the film runs during the production process (the winding direction when wound into a roll). A longitudinal stretching ratio of 4.0 times or more increases the mechanical strength in the longitudinal direction, thereby suppressing film deformation even under high tension and making it easier to maintain the flatness of the biaxially oriented polyolefin film. Furthermore, a longitudinal stretching ratio of 6.0 times or less suppresses thermal shrinkage caused by excessive strain during stretching, making it easier to maintain the flatness of the biaxially oriented polyolefin film.

[0072] Next, by performing a relaxation treatment in the longitudinal direction, an increase in the heat shrinkage rate of the biaxially oriented polyolefin film can be suppressed. From the viewpoint of improving the handleability of a film with good flatness, the temperature of the relaxation treatment in the longitudinal direction is preferably equal to or lower than the softening temperature of the resin added to the film, more preferably equal to or lower than the softening temperature of the polyolefin resin minus 10°C, and even more preferably equal to or lower than the softening temperature of the polyolefin resin minus 20°C. From the viewpoint of good film transportability, the relaxation rate in the relaxation treatment in the longitudinal direction is preferably 2.0% or more and less than 15.0%, and the lower limit of the relaxation treatment in the longitudinal direction is more preferably 3.0%, and even more preferably 5.0%. The upper limit of the relaxation treatment in the longitudinal direction is more preferably 12.0%, and even more preferably 10.0%.

[0073] Next, the uniaxially oriented film is guided into a tenter with both widthwise ends held with clips, preheated, and then transversely stretched 7.0 to 12 times in the widthwise direction. The lower limit of the stretching ratio is preferably 7.5 times, more preferably 8.0 times, from the viewpoint of uniform stretchability of the film. The upper limit of the stretching ratio is more preferably 11 times, from the viewpoint of stable stretching without film rupture. The widthwise direction refers to the direction perpendicular to the longitudinal direction within the film plane.

[0074] From the viewpoint of uniformly stretching the film and suppressing the formation of a sharp protrusion structure, the preheating temperature is preferably the melting point of the polyolefin resin used in the film minus 15°C or less. The upper limit of the preheating temperature is more preferably the melting point of the polyolefin resin minus 20°C, and even more preferably the melting point of the polyolefin resin minus 25°C.

[0075] From the viewpoint of uniform stretching of the film, the stretching temperature is preferably equal to or lower than the melting point of the polyolefin resin. From the viewpoint of stable stretching without melting and rupturing the film, the lower limit of the stretching temperature is more preferably the melting point of the polyolefin resin minus 5°C, and even more preferably the melting point of the polyolefin resin minus 10°C. By setting the preheating and stretching temperatures within the above ranges, the uniaxially oriented film can be uniformly stretched laterally, and a biaxially oriented film having a uniform thickness can be obtained.

[0076] In the subsequent heat treatment and relaxation treatment steps, to ensure uniform relaxation and shrinkage of the film, both widthwise ends of the biaxially stretched film are tensely held with clips, and the film is heat-set at 140°C to 180°C, more preferably 150°C to 173°C, while being relaxed at a relaxation rate of 5.0 to 20%, more preferably 7 to 15%, and even more preferably 9 to 12% in the width direction. Furthermore, while both widthwise ends are tensely held with clips, the film is cooled at 80 to 100°C and then guided outside the tenter, and the clips on both widthwise ends are released. Subsequently, in the winding step, both surfaces of the film are subjected to plasma treatment using atmospheric pressure glow discharge, after which the film edges are slit and the biaxially oriented polyolefin film product roll is wound up.

[0077] (M layer: layer made of metal and / or metal-based compound) The polyolefin film of the present invention preferably has at least a layer of metal and / or metal-based compound, the polyolefin film, and a layer of metal and / or metal-based compound in this order, and more preferably has a layer of metal and / or metal-based compound (M layer) provided on both sides of the polyolefin film.

[0078] The method for providing the M layer in the present invention is not particularly limited, but may include methods such as deposition, sputtering, or electroplating under vacuum conditions or reduced pressure conditions in which an inert gas such as argon gas is sealed (hereinafter these methods may be collectively referred to as deposition methods), methods in which a metal foil or a foil of a metal compound is bonded to a polyolefin film directly or via an adhesive layer, and methods in which a metal layer is provided by an electrochemical reaction using a solution containing a metal salt (electrolytic plating method, electroless plating method).Of these, deposition methods are preferred from the viewpoint of continuously forming the M layer on a film using a polyolefin film roll.

[0079] When using the vacuum deposition method, a preferred embodiment is to place a polyolefin roll in a vacuum chamber in advance, and then adhere the heated and vaporized metal and / or metal compound to the surface of the polyolefin film while the unwound film is in close contact with a cooling roll, forming an M layer, and then winding the film up again into a film roll.

[0080] Here, the inside of the vacuum chamber is 9.0 × 10 -3 Under vacuum conditions of 9.0 x 10 Pa or less, or by sealing in an inert gas such as argon gas. -3 Pa or more 1×10 -1 Any of the conditions under which the pressure is reduced to or below Pa can be suitably used. The M layer may also be formed by successively carrying out two or more vapor deposition steps, such as providing a first M layer by sputtering and then providing a second M layer by vacuum deposition.

[0081] Vacuum deposition methods include induction heating deposition, resistance heating deposition, laser beam deposition, and electron beam deposition, among which electron beam deposition, laser beam deposition, and induction heating deposition, which have a large amount of heat generated by the deposition source, are preferably used. The amount of heat generated by the deposition source must be increased until an M layer of the desired thickness is formed, and the substrate surface temperature must be sufficiently high. However, since this is difficult to measure, it is possible to determine whether the amount of heat is sufficient by confirming that the M layer after deposition has the desired thickness.

[0082] However, if the heat output of the vapor deposition source is increased to the required level, the temperature of the resin film will rise if the cooling function is controlled as in conventional vacuum deposition, resulting in thermal damage that can reduce the mechanical properties of the resin film and even melt the resin film. Therefore, it is preferable to perform vapor deposition while controlling the cooling function so that the film can be uniformly cooled and the temperature does not rise too much during vapor deposition. Specifically, it is preferable to uniformly cool the film from the backside of the deposition surface using a cooling mechanism consisting of a metal plate or metal roll sufficiently cooled by a refrigerant. To achieve uniform cooling, it is essential to ensure close contact between the resin film and the cooling mechanism without creating any gaps. Improved adhesion reduces thermal damage to the resin film surface and suppresses deterioration of the resin film's mechanical properties.

[0083] For example, if the metal roll of the cooling mechanism has scratches, the scratched area will create a gap, preventing the resin film from cooling, and increasing thermal damage to the resin film. Furthermore, if foreign matter gets into the resin film and the metal roll of the cooling mechanism, the foreign matter will prevent the resin film from cooling, increasing thermal damage. If the heat output of the vapor deposition source is increased to the required level, scratches on the metal roll and the inclusion of foreign matter, which are tolerated in conventional vacuum vapor deposition methods, will become a problem, so even stricter management of scratches and inclusion of foreign matter in the metal roll is required.

[0084] When the M layer in the present invention is to have a desired metal layer thickness, a method of forming it in a single vapor deposition (a set of unwinding, vapor deposition, and winding is defined as one vapor deposition) is preferred from the viewpoints of productivity, resistance characteristics, and grade and quality. However, for example, thin film vapor deposition to form a 50 nm thick aluminum vapor deposition layer in a single vapor deposition may be repeated 20 times (the above set is repeated 20 times) to form an aluminum metal layer with a total thickness of 1 μm.

[0085] Examples of metal elements constituting the M layer include gold, silver, copper, zinc, lead, nickel, iron, aluminum, titanium, cobalt, manganese, cadmium, and palladium, and the M layer may be a metal layer consisting of a simple metal element, or a layer consisting of a metal compound containing the metal element mixed with oxygen, nitrogen, fluorine, carbon, boron, chlorine, sulfur, or phosphorus. The metal elements of the M layer provided on both sides of the polyolefin film may be the same or different.

[0086] When used as a resin current collector for a bipolar battery, the M layers present on both sides of the laminate film preferably have an M layer using copper element on one side and an M layer using aluminum element on the other side.

[0087] In the polyolefin film of the present invention, the laminate structure of the polyolefin film and the layer made of metal and / or metal-based compound preferably has at least a layer made of metal and / or metal-based compound, the polyolefin film, and a layer made of metal and / or metal-based compound, in this order. Specific examples of the structure include the following. When the polyolefin film is a single layer, a structure of M layer / P1 layer / M layer or M layer / P1 layer / M' layer, which includes a layer (M' layer) made of metal and / or metal-based compound consisting of a metal element different from that of the M layer, is preferred.

[0088] When the polyolefin film has two or more layers, the structure is preferably M layer / P1 layer / P2 layer / M layer, M layer / P1 layer / P2 layer / M' layer, M layer / P2 layer / P1 layer / P2 layer / M layer, or M layer / P2 layer / P1 layer / P2 layer / M' layer. When used as a resin current collector for a bipolar battery, from the viewpoint of reducing the contact resistance at the interface between the active material layer and the resin current collector and suppressing a deterioration in battery characteristics, it is more preferred that the outermost layer on both sides of the polyolefin film of the present invention be the P1 layer, and that layers made of metal and / or metal-based compounds be provided on both surfaces of the P1 layer.

[0089] The polyolefin film of the present invention may also be provided with an anchor layer or an undercoat layer to improve adhesion with the M layer and electrochemical stability when used as a resin current collector for a bipolar battery, provided that the effects of the present invention are not lost. The thickness of the anchor layer or undercoat layer (U layer) is preferably 0.01 μm or more and 1.0 μm or less. A thickness of the U layer of 0.01 μm or more can improve adhesion between the polyolefin film and the M layer and electrochemical stability when used as a resin current collector for a bipolar battery. Furthermore, a thickness of the U layer of 1.0 μm or less can prevent the bipolar battery from becoming too large when the polyolefin film of the present invention is used as a resin current collector for a bipolar battery.

[0090] The thickness of the M layer in the polyolefin film of the present invention is not particularly limited, but is preferably 0.1 μm or more, because this minimizes the deterioration of electrical properties due to uneven thickness of the metal layer when used as a resin current collector. The thickness of the M layer is more preferably 0.2 μm or more, and even more preferably 0.5 μm or more. Furthermore, by setting the thickness of the M layer to 5 μm or less, it is possible to minimize the increase in battery weight when used as a resin current collector for a battery, which is preferable. The thickness of the M layer is more preferably 3 μm or less.

[0091] In the polyolefin film of the present invention, the adhesion strength between the outermost layer of the polyolefin film and the M layer is preferably 0.5 N / 15 mm or more on both sides. When the adhesion strength between the outermost layer of the polyolefin film and the M layer is 0.5 N / 15 mm or more on both sides, when the polyolefin film having the M layer is incorporated as a current collector in an energy storage device, if the current collector is deformed due to stress such as impact or heat, the interface between the polyolefin at the edge and the M layer will peel off, and other battery components will penetrate into the interface, reducing the efficiency of the charge / discharge reaction of the energy storage device and reducing the battery characteristics. The adhesion strength on both sides is more preferably 0.7 N / 15 mm or more, and even more preferably 1.0 N / 15 mm or more.

[0092] The polyolefin film of the present invention has M layers on both surfaces of the polyolefin film, and the volume resistivity in the penetration direction of the polyolefin film having the M layers (hereinafter, sometimes referred to as penetration resistivity) measured by the method described below is 1.0 × 10 0 Ωcm or more 1.0×10 8 It is preferable that the penetration resistivity is 1.0×10 Ωcm or less. 0 When the polyolefin film of the present invention is incorporated into an electricity storage element as a resin current collector for a bipolar battery, the polyolefin film can suppress thermal runaway caused by a short circuit when the electricity storage element is damaged, and fire from the electricity storage element, by acting as a resistance. 8 When the polyolefin film of the present invention is incorporated into a bipolar energy storage element as a resin current collector for a bipolar battery, an increase in the internal resistance of the bipolar energy storage element and a decrease in battery output can be suppressed by the polyolefin film having a through resistance of 5.0×10 Ωcm or less. 4 It is more preferable that the resistance is Ωcm or less.

[0093] [Resin current collector] The resin current collector of the present invention preferably has the polyolefin film and an M layer on each of both surfaces of the polyolefin film. By using the polyolefin film with a good volume resistivity, the resin current collector of the present invention has excellent conductivity in the thickness direction, which is expressed as through-hole resistivity. Furthermore, a surface treatment may be performed to improve adhesion between the M layer and the polyolefin film. This allows the current collector to be suitably used as a current collector for a bipolar battery, as described below.

[0094] [Bipolar battery current collector] In the current collector for a bipolar battery of the present invention, it is preferable that the current collector has a configuration in which different M layers are provided on both surfaces of the polyolefin film.

[0095] By using the current collector, which has excellent conductivity in the thickness direction, when incorporated into a bipolar battery, it is possible to achieve excellent current value stability, as described below, and suppress variation in battery characteristics. In addition, it has excellent mechanical properties, and bipolar batteries incorporating the current collector for bipolar batteries exhibit excellent durability against external stresses such as deformation and impact. Furthermore, by controlling the shape of the conductive particles contained in the polyolefin film, stable battery operation can be achieved even in high-temperature environments.

[0096] When the bipolar battery current collector of the present invention is used as a lithium ion battery, it is more preferable that one side of the M layer is made of aluminum and the other side is made of copper as the metal element.

[0097] [electrode] The bipolar battery electrode of the present invention comprises an electrode assembly including a positive electrode and a negative electrode, the negative electrode having a negative electrode active material layer on one surface side of a resin current collector for a bipolar battery and a positive electrode active material layer on the opposite surface side.

[0098] [Secondary battery] Examples of secondary batteries include lithium secondary batteries, lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-iron batteries, silver oxide-zinc batteries, manganese dioxide-lithium secondary batteries, lithium cobalt oxide-carbonate secondary batteries, and vanadium-lithium secondary batteries.

[0099] Among these, secondary batteries are preferred because they can be used for a long period of time, and lithium secondary batteries are more preferred because they achieve high energy density by using organic solvents.

[0100] The battery case may be, for example, an aluminum case, an iron case with a nickel-plated inner surface, or a case made of an aluminum laminate film.

[0101] The shape of the battery case can be a pouch type, a cylindrical type, a square type, a coin type, etc. Among these, the pouch type is preferred because it can achieve a high energy density and can be freely designed in shape at low cost.

[0102] The positive electrode is a current collector laminated with a cathode material consisting of an active material, a binder resin, and a conductive additive. Examples of active materials include layered lithium-containing transition metal oxides such as LiCoO2, LiNiO2, and Li(NiCoMn)O2, spinel-type manganese oxides such as LiMn2O4, and iron-based compounds such as LiFePO4. Resins with high oxidation resistance can be used as the binder resin. Specific examples include fluorine-containing resins, acrylic resins, and styrene-butadiene resins. Examples of conductive additives include carbon materials such as carbon black and graphite. Metal foils are preferred as current collectors, with aluminum foil being particularly popular.

[0103] The negative electrode is made by laminating a negative electrode material consisting of an active material and a binder resin on a current collector. Active materials include carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon, lithium alloy materials such as tin and silicon, metal materials such as lithium, and lithium titanate (Li4Ti5O 12 ) and the like. Examples of binder resins include fluorine-containing resins, acrylic resins, and styrene-butadiene resins. Metal foils are suitable as current collectors, and copper foils are often used in particular.

[0104] In the secondary battery of the present invention, it is preferable to use a bipolar battery electrode having a bipolar battery resin current collector made of a polyolefin film having a copper layer on one side and an aluminum layer on the other side, which are formed by vacuum deposition or the like. In particular, a preferred example of a bipolar battery electrode is one having a configuration in which at least two layers of the positive electrode active material layer and the negative electrode active material layer are laminated together with an electrolyte interposed therebetween.

[0105] The secondary battery may also contain an electrolytic solution, in which case it is preferable to include a separator interposed between the positive electrode and the negative electrode. Preferred examples include secondary batteries made of a solid electrolyte that does not contain an electrolytic solution.

[0106] When the secondary battery of the present invention contains an electrolytic solution, the electrolytic solution serves as a site for transferring ions between a positive electrode and a negative electrode in an electrochemical element such as a secondary battery, and is configured by dissolving the electrolyte in an organic solvent.

[0107] Examples of the electrolyte include LiPF6, LiBF4, and LiClO4, but LiPF6 is preferably used from the viewpoints of solubility in organic solvents and ionic conductivity.

[0108] Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and two or more of these organic solvents may be mixed and used.

[0109] A method for producing a lithium secondary battery, which is preferably used among the energy storage elements, will be described below.

[0110] The method for producing a lithium secondary battery involves first dispersing an active material and a conductive additive in a binder resin solution to prepare an electrode coating solution, then coating the coating solution on a current collector and drying the solvent to obtain a positive electrode and a negative electrode. The thickness of the coating film after drying is preferably 50 μm to 500 μm. Furthermore, it is preferable to apply pressure to the active material layer formed on the current collector, preferably by a roll press method, to densify it and thin the current collector.

[0111] A lithium secondary battery separator is placed between the obtained positive electrode and negative electrode so as to be in contact with the active material layer of each electrode, and the resulting battery is enclosed in an exterior material such as an aluminum laminate film. After injecting an electrolyte, a negative electrode lead and a safety valve are installed, and the exterior material is sealed.

[0112] The lithium secondary battery obtained in this manner has high adhesion to the electrode, has excellent battery characteristics, and can be produced at low cost.

[0113] The bipolar battery electrode of the present invention may be used as a secondary battery by connecting multiple electrodes in series to meet the application and required battery capacity of the secondary battery. In this case, it is preferable to connect multiple electrodes in series to form a secondary battery equipped with voltage management, temperature management, and safety devices, or to connect multiple module units, each of which has multiple electrodes connected in series and housed in a case, in series or parallel to form a secondary battery equipped with voltage management, temperature management, and safety devices. A preferred example of a secondary battery is one in which the electrodes are connected to each other with tab lead wires (current extraction wires) and housed in a resin or metal module case.

[0114] [Electric vehicle] The secondary battery produced by the above-mentioned method or the like is preferably installed in an electric vehicle due to its excellent battery characteristics and durability. An electric vehicle is a vehicle in which some or all of the driving energy required for running is supplied from a secondary battery. Examples of electric vehicles include BEVs (Battery Electric Vehicles) equipped only with secondary battery cells, and HEVs (Hybrid Electric Vehicles) and PHEVs (Plug-in Hybrid Electric Vehicles) equipped with both a fossil fuel such as gasoline and a secondary battery cell. The secondary battery of the present invention can be suitably used in any of these applications.

[0115] [Electric flying object] The secondary battery produced by the above-described method or the like is preferably mounted on an electric flying vehicle due to its excellent battery characteristics and durability. An electric flying vehicle is a flying vehicle in which some or all of the driving energy required during flight is supplied from a secondary battery. Specific examples include electric aircraft such as drones, stratospheric communication platform aircraft (HAPS), air metro systems, and air taxis. The secondary battery of the present invention can be suitably used in any of these applications.

[0116] [Characteristics evaluation method] A. Film thickness (i) Polyolefin film thickness T The total thickness of the polyolefin film is measured using a dial gauge in accordance with JIS K7130 (1992) A-2 method, with 10 films stacked together, at any five points. The average value is divided by 10 to obtain the polyolefin film thickness T (μm).

[0117] (ii) Lamination thickness (T P1 , T P2 , T M ) A cross section of the polyolefin film of the present invention is cut out with a microtome in a direction parallel to the film width direction. After the cross section is subjected to a sputtering treatment using platinum palladium, it is observed at a magnification of 5,000 to 20,000 times using a scanning electron microscope (JEOL Ltd., JSM-6700) under an accelerating voltage of 3 kV, and the thickness ratios of the layers (P1 layer, P2 layer) and the metal and / or metal compound layer (M layer) constituting the laminated polyolefin film are determined. The thickness of each layer (T P1 , T P2 , T M ) is calculated.

[0118] Constituent elements of the BM layer After the surface of the polyolefin film of the present invention is subjected to a sputtering treatment using platinum-palladium, the metal elements are identified using a scanning electron microscope (JEOL Ltd., JSM-6700) and an energy dispersive X-ray spectroscopy (EDX) detector (Oxford Ltd., AZtecLiv Standard UltimMax65).

[0119] Measurements are performed by changing the acceleration voltage from 0.5 kV to 30 kV, and the detected elements are considered to be the elements that make up the metal and / or metal compound layer (M layer). In this case, platinum and palladium are excluded, and if only platinum or palladium is detected through measurements at all acceleration voltages, they are considered to be the elements that make up the metal and / or metal compound layer (M layer).

[0120] C. Particle dispersion evaluation As in Section B above, cross-sectional images taken at a magnification of 10,000 times using a scanning electron microscope (JEOL Ltd., JSM-6700) are analyzed using image analysis software (National Institutes of Health, ImageJ) as follows.

[0121] (Image analysis conditions) (i) Pretreatment After capturing the cross-sectional image using the software, execute the "8-bit" command under "Type" in the "Image" menu to perform 8-bit grayscale processing. Next, use the "Straight" menu to draw a straight line of a known distance, and set the scale of the captured image using the "Set Scales" command under the "Analyze" menu. After that, use the "Subtract" command under "Math" in the "Process" menu to subtract "Value" by 30. Use the "Enhance Contrast" command from the "Process" menu to set "Saturated pixels" to 5%, check "Normalize", and press the "OK" button. Use the "Gaussian Blur" command under "Filters" in the "Process" menu to perform blurring with a "Sigma (Radius)" of 1.

[0122] (ii) Binarization From the "Image" menu, select "Adjust" and then "Threshold," check only "Dark background" and "Don't reset range," then press "Auto" and then "Apply."

[0123] (iii) Noise reduction From the "Process" menu, select "Noize" and then the "Remove Outliers" command, set "Saturated pixels" to 1 pixel, "Threshold" to 50, and "Which Outliers" to Bright, then press the "OK" button.

[0124] (iv) Particle analysis In the "Analyze" menu, select the "Set Measurements" command, check "Area" and "Feret's diameter," and click the "OK" button. Next, in the "Analyze" menu, select the "Analyze Particles" command, set each item as follows, and click the "OK" button to display the analysis results. Size: 0-Infinity Circularity: 0.00-1.00 Show:Nothing Check "Display Results", "Clear Results", "Exclude on edges" and "include holes".

[0125] From the results obtained, the volume-based average dispersed particle diameter, aspect ratio, and area of ​​each particle are calculated. The aspect ratio is calculated by dividing the "Feret" of each particle in the Results by the "MinFeret." The area is the "Area" value of each particle.

[0126] (Volume-based average dispersed particle diameter) Using the "Feret" of each particle detected above, the volume-based average dispersed diameter is calculated according to the following formula 2. The volume-based average dispersed diameters are calculated for five different fields of view, and their average value is taken as the volume-based average dispersed diameter R (μm) of the particles in the sample. R(μm)=(Σ(Feret) 2 × number) / (Σ(Feret) × number) Equation 2

[0127] (aspect ratio and area of ​​each particle) A count histogram is created by plotting the aspect ratios obtained for the five different fields of view on the horizontal axis in 0.5 intervals in the range of 1 to 10, and in 10 intervals in the range of 10 or more. If a maximum value exists in the region of the number histogram where the aspect ratio is 1 to less than 5, the minimum value in the range showing that maximum value (for example, if the maximum value is in the range of 2 to 2.5, 2 is used as the maximum value) is taken as the aspect ratio of the conductive particles (conductive particles C1) existing in the region where the aspect ratio is 1 to less than 5. If multiple maximum peaks exist in the region where the aspect ratio is 1 to less than 5, the weighted average of those vertical axis values ​​is taken as the aspect ratio of the conductive particles (conductive particles C1) existing in the region where the aspect ratio is 1 to less than 5.

[0128] Similarly, if a maximum value exists in a region where the aspect ratio is 5 or more, that maximum value is taken as the aspect ratio of the conductive particle (conductive particle C2) existing in the region where the aspect ratio is 5 or more. If multiple maximum peaks exist in the region where the aspect ratio is 5 or more, the weighted average of those vertical axis values ​​is taken as the aspect ratio of the conductive particle (conductive particle C2) existing in the region where the aspect ratio is 5 or more.

[0129] For each of the five different fields of view, the aspect ratios of the conductive particles (conductive particles C1) present in the region with an aspect ratio of 1 or more but less than 5, and the conductive particles (conductive particles C2) present in the region with an aspect ratio of 5 or more are calculated, and the average value of the five fields of view in each region is taken as the aspect ratio of the conductive particles C1 and C2 in the polyolefin film.

[0130] The sum (SC1) of the areas of conductive particles (conductive particles C1) present in the region with an aspect ratio of 1 or more and less than 5 confirmed in the number histogram is determined. Next, the sum (SC2) of the areas of conductive particles (conductive particles C2) present in the region with an aspect ratio of 5 or more is determined to calculate SC1 / SC2. Note that the area ratio can be determined using the same measurement method regardless of whether the conductive particles are metal particles, metal oxide particles, conductive resin particles, or carbon materials.

[0131] D. Volume resistivity measurement Ten samples of 10 cm x 10 cm are taken from different locations on the polyolefin film of the present invention, and after conditioning for one day in an environment of 23°C temperature and 65% humidity, the volume resistivity is measured according to the following method.

[0132] (i) Resistivity meter measurement Measurements are taken using a low resistivity meter (Mitsubishi Chemical, Loresta-GP series MCP-T360) in volume resistivity measurement mode (unit: Ωcm). The sample film thickness is input into the low resistivity meter and the measurement is taken. Measurements are taken for all 10 samples, and the average value is taken as the volume resistivity of the sample.

[0133] If the volume resistivity measured using the low resistivity meter is outside the upper limit of measurement, manually measure using a high resistivity meter (Hirester UP Series MCP-HT450) according to the following procedure. After placing the sample on the attached register table, press the attached URS probe against it and determine the volume resistivity after holding it for 1 minute under a voltage of 10V. Measurements are taken for all 10 samples, and the average value is taken as the volume resistivity of the sample.

[0134] If the volume resistivity is still outside the upper limit of the measurement range, the voltage setting is changed to 500 V, 1000 V, and the value under the lowest voltage condition among the conditions under which a measurement was obtained is taken as the volume resistivity of the sample. The measurement is performed in an environment of 23°C and 65% RH.

[0135] (ii) Evaluation of in-plane variation in volume resistivity The maximum, minimum and average values ​​of the 10 measured values ​​obtained by measuring the 10 samples measured in the previous section (i) are used to calculate the value obtained by the following formula (3), which is the in-plane variation value of the volume resistivity of the sample. (In-plane variation value) = 100 × (maximum value - minimum value) / (average value) Equation (3).

[0136] E.DSC measurement The polyolefin film of the present invention or its raw material is measured using a differential scanning calorimeter (DSC) such as a Thermo Plus Evo2 series DSC Vesta manufactured by Rigaku Corporation. Approximately 5 mg of a sample is placed on an aluminum tray and heated from room temperature to 300°C at a heating rate of 20°C / min, and held for 5 minutes (first run measurement). The endothermic peak of melting observed at this time is the melting peak, and the peak temperature is the melting point. The average value of the melting points measured three times is taken as the melting point of the sample. The half-width of the melting peak is the spectral width at half the peak height at the melting point, and the half-width of the three measurements is taken as the half-width of the melting peak of the sample.

[0137] F. Carbon content and carbon species identification (Identification of carbon species) The carbon particles were analyzed by Raman spectroscopy to identify the G band, which is generally called the graphite structure (sp 2 bond) at 1580 cm -1 The diamond structure (sp 3 bond) at 1350 cm -1 Peaks in the vicinity are observed, and identification is performed from the peak intensity ratio. When identifying, a comparison is made with a publicly known Raman spectrum database and peak intensity data obtained by measuring commercially available carbon materials. The carbon material is observed with a transmission electron microscope (TEM) at 50,000 to 500,000 magnifications, and the resulting shape is observed. This, combined with the Raman spectroscopy results from the previous section, identifies the type of carbon material present in the sample. When multiple carbon materials are observed, the number of each carbon material observed with the TEM is converted into a volume ratio, which is taken as the abundance ratio of each carbon material.

[0138] G. Mechanical Property Evaluation of Polyolefin Films The F5 value of the polyolefin film (the load value when the test piece elongates by 5% divided by the cross-sectional area of ​​the test piece) was measured. A rectangular sample 150 mm long and 10 mm wide was cut from the polyolefin film, with the unwinding direction of the roll being the longitudinal direction of the polyolefin film, and the long side being the longitudinal direction. According to the following method specified in ASTM-D882, an Instron-type tensile tester (Orientec AMF / RTA-100) was used to set a 10 mm wide sample film so that the chuck distance was 50 mm, and a tensile test was performed at a tensile speed of 300 mm / min, and the F5 value was read. Five measurements were taken, and the average value was used as the F5 value in the longitudinal direction of the polyolefin film.

[0139] In the same manner as in the longitudinal direction measurement, the direction rotated 90° in-plane from the longitudinal direction of the polyolefin film is defined as the width direction, and the F5 value of a rectangular sample 150 mm long and 10 mm wide is measured so that the width direction is the long side. Measurements are made five times, and the average value is defined as the F5 value in the width direction of the polyolefin film.

[0140] When the longitudinal and transverse directions of a polyolefin film are unknown, the F5 value is measured in a total of four directions: a specific direction and directions rotated 45°, 90°, and 135° in-plane from the specific direction. The maximum F5 value obtained in the four directions is taken as the F5 value of the polyolefin film whose longitudinal direction and longitudinal direction are unknown.

[0141] G-2. Shrinkage rate when kept at 120℃ for 15 minutes The thermal shrinkage of polyolefin film is measured. A rectangular sample 150 mm long and 10 mm wide is cut from the polyolefin film, with the unwinding direction of the roll being the longitudinal direction of the polyolefin film, and the long side is in the longitudinal direction. The initial length in the longitudinal direction (L0) is measured using a universal projector. The sample is hung in a gear oven maintained at 120°C with a 2.1 g weight attached, and held for 15 minutes. The sample is then removed, and the length in the longitudinal direction of the sample (L1) is measured using a universal projector. The thermal shrinkage in the longitudinal direction is calculated using the following formula: Heat shrinkage rate (%) = 100 × (L1 - L0) / L0 Formula (5)

[0142] The above measurement is carried out five times using different samples, and the average value is taken as the thermal shrinkage rate in the longitudinal direction of the polyolefin film.

[0143] In the same manner as in the measurement in the longitudinal direction, the direction rotated 90° in-plane from the longitudinal direction of the polyolefin film is defined as the width direction, and a rectangular sample 150 mm long and 10 mm wide is cut out so that the width direction is the long side, and the heat shrinkage of the sample is measured. Measurements are made five times, and the average value is defined as the heat shrinkage in the width direction of the polyolefin film.

[0144] When the longitudinal and width directions of a polyolefin film are unknown, the heat shrinkage is measured in a specific direction and in four directions rotated at 45°, 90°, and 135° in-plane from the specific direction. The maximum value of the heat shrinkage in the four directions is taken as the heat shrinkage of the polyolefin film whose longitudinal and width directions are unknown.

[0145] G-3. Adhesion between M layer and polyolefin film A sample with an M layer formed on the surface of a polyolefin film is cut into a rectangle of 15 mm wide x 80 mm long. A Nitto Cellophane Tape No. 29 (15 mm wide) is attached to the M layer of the cut sample as a peeling tape, and the tape is pressed tightly with a 2 kg rubber roller. The sample is then conditioned at 23°C and 65% RH for one day.

[0146] After humidity conditioning, the sample is subjected to a 180° peel test under the following conditions using an adhesive and film peeling analyzer (VPA-2) manufactured by Kyowa Interface Science Co., Ltd. The side of the sample opposite to the tape-attached side is fixed to the device, and the edge of the peeling tape is fixed to the load cell of the device and measurements are taken.

[0147] (Peel test conditions) Peeling angle: 180° Tape width: 15mm Peeling speed: 25mm / min Starting peel force: 0N Measurement distance: 50mm

[0148] Of the obtained peel force waveforms, the average value of the peel force (N / 15 mm) in the section where the travel distance is 15 mm to 35 mm is calculated. The peel force is measured for three different samples, and the average value is taken as the adhesion between the M layer and the polyolefin film.

[0149] H. Penetration resistivity A 25 mm square sample of the laminated polyolefin film of the present invention having an M layer on both surfaces is taken as an evaluation sample. The sample is sandwiched between circular copper electrodes with a diameter of 20 mm and a thickness of 10 mm, each equipped with an electrode tab, and a pressure of 1 MPa is applied in the thickness direction of the sample. The electrode tabs are connected to a milliohmmeter GOM-805 manufactured by Texio Co., Ltd., and the resistance value is read one minute after it is displayed under conditions of DC +6.25 V. The obtained resistance value is multiplied by the electrode area (3.14 cm). 2 ) and divided by the thickness (cm) of the sample to obtain the penetration resistivity (Ωcm). Similar measurements were performed on three different samples to obtain the penetration resistivity, and the average value was used as the penetration resistivity of the sample.

[0150] H-2.Water contact angle The film is left for 24 hours in an atmosphere of room temperature 23°C and relative humidity 65%. Then, under the same atmosphere, the contact angle of pure water on the film surface is measured at five different points using a contact angle diameter DropMaster DM-501 manufactured by Kyowa Interface Science Co., Ltd. The average value of the three measurements excluding the maximum and minimum values ​​of the five measurements is taken as the water contact angle of the polyolefin film.

[0151] [Evaluation method for application characteristics] I. Discharge stability In the manufacturing process of the polyolefin film of the present invention, the unstretched film is obtained by melt extruding from a die, cooling using an electrostatic application method on a cast drum, and molding into a sheet, and a 10 m unstretched film roll is collected.

[0152] The thickness of the unstretched film obtained is measured every 50 cm in the winding direction of the unstretched film. The average of all measurements is the average thickness of the unstretched film, the standard deviation of all measurements is the standard deviation of the thickness of the unstretched film, and the percentage of the value obtained by dividing the standard deviation by the average thickness is the thickness unevenness value (%) of the unstretched film, and the discharge stability is evaluated as follows. A: The thickness unevenness value (%) of the unstretched film is 10 or less. B: The thickness unevenness value (%) of the unstretched film is greater than 10 and 15 or less. C: The thickness unevenness value (%) of the unstretched film is greater than 15 and less than 20. D: The thickness unevenness value (%) of the unstretched film is greater than 20. The ejection stability is preferably rated as A to C, with A being the best.

[0153] J. Film Formability The polyolefin film of the present invention is wound up to 1000 m at a winding speed of 20 m / min or more. The film formability is evaluated based on the number of times the film breaks during collection of the film roll, as follows. A: The film breaks less than twice. B: The film broke 3 or more times but not more than 4 times. C: The film broke 5 or more times but not more than 6 times. D: The film broke 7 or more times. The film formability is preferably evaluated as A to C, with A being the best.

[0154] K. Electrical properties and processing suitability evaluation (i) Conductivity evaluation The polyolefin film of the present invention was subjected to the volume resistivity measurement in accordance with the above "D. Volume resistivity measurement". The film is evaluated as follows: A: Volume resistivity is 1.0 x 10 0 Ωcm or more 1.0×10 2 Ωcm or less B: Volume resistivity is 1.0 x 10 2 Higher than 1.0×10 Ωcm 4 Ωcm or less C: Volume resistivity is 1.0 x 10 4 Higher than 1.0×10 Ωcm 8 Ωcm or less D: Volume resistivity is 1.0 × 10 8 Higher than Ωcm. The conductivity is preferably rated as A to C, with A being the best.

[0155] (ii) Conductivity evaluation after tension Ten 10 cm x 10 cm pieces of the polyolefin film of the present invention were cut out. Both ends of the cut-out sample were sandwiched between two rubber-laminated metal plates to secure the sample. The distance between the metal plates securing the ends, i.e., the exposed portion of the polyolefin film, was set to 50 mm. Then, the metal plate holding the cut-out sample was fixed to the chuck of an Instron-type tensile tester (AMF / RTA-100, manufactured by Orientec Co., Ltd.), and a tensile test was performed to deform the polyolefin film by 2%. Specifically, the initial sample length of the rubber-laminated metal plate was set to 50 mm, and the tensile test was performed at a tensile speed of 300 mm / min until the sample length was elongated by 1 mm. The volume resistivity of the stretched portion was then measured in the same manner as in "D. Volume Resistivity Measurement" above. Using the volume resistivity R1 (Ωcm) obtained and the volume resistivity R0 (Ωcm) obtained in section (i) above, the increase rate of volume resistivity calculated by the following formula (6) was used to evaluate the following: Increase rate of volume resistivity (%) = 100 × (R1 - R0) / R0 Equation (6) A: The increase in volume resistivity is 30% or less. B: The increase rate of volume resistivity is more than 30% and 50% or less. C: The increase rate of volume resistivity is more than 50% and 100% or less. D: The increase in volume resistivity exceeds 100%. The conductivity after tension is preferably rated as A to C, with A being the best.

[0156] (iii) Conductivity evaluation after bending Ten pieces of 10 cm x 10 cm are cut out from the polyolefin film of the present invention. A load of 300 g is applied to both ends of each 10 cm x 10 cm sample, and the sample is then attached to a SUS cylinder with a diameter of 3 cm and a length of 15 cm. The sample attached to the cylinder is placed in an oven heated to 50°C and heat-treated for one hour.

[0157] After the heat treatment, the sample is removed from the cylinder and the volume resistivity of the central portion of the sample is measured in the same manner as in "D. Volume Resistivity Measurement" above. Using the volume resistivity R2 (Ωcm) thus obtained and the volume resistivity R0 (Ωcm) obtained in (i) above, the increase rate of volume resistivity is calculated using the following formula (7) to perform the following evaluation. Increase rate of volume resistivity (%) = 100 × (R2 - R0) / R0 Equation (7) A: The increase in volume resistivity is 30% or less. B: The increase rate of volume resistivity is more than 30% and 50% or less. C: The increase rate of volume resistivity is more than 50% and 100% or less. D: The increase in volume resistivity exceeds 100%. The conductivity after bending is preferably rated as A to C, with A being the best.

[0158] L. Bipolar Battery Evaluation (i) Preparation of resin current collector for bipolar battery A resin current collector for a bipolar battery is produced by vacuum vapor deposition on both sides of the polyolefin film of the present invention to produce a film having a copper layer on one side and an aluminum layer on the opposite side.

[0159] Specifically, a roll of the polyolefin film of the present invention is placed in a roll-type vacuum deposition apparatus (ULVAC, EWC-060), and an aluminum ingot is heated by induction heating deposition using a carbon crucible to form an aluminum metal layer by vacuum deposition. Vacuum deposition is performed by adjusting the conveyance speed and output conditions so that the aluminum metal layer has a predetermined thickness. The roll of polyolefin film with the aluminum metal layer on one side is then placed again in the roll-type vacuum deposition apparatus (ULVAC, EWC-060), and a copper metal layer is formed on the surface of the polyolefin film opposite the aluminum metal layer by vacuum deposition by heating a copper ingot by induction heating deposition using a carbon crucible. Vacuum deposition is performed by adjusting the conveyance speed and output conditions so that the copper metal layer has a predetermined thickness.

[0160] (ii) Active material for positive electrode, active material for negative electrode A positive electrode active material slurry is prepared by mixing 85 mass% of LiMn2O4 as a positive electrode active material, 5 mass% of acetylene black as a conductive additive, 10 mass% of polyvinylidene fluoride (PVDF) as a binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent.

[0161] In addition, as a positive electrode to be placed at the end of a bipolar battery, the positive electrode active material slurry is applied to an aluminum foil with a thickness of 30 μm and similarly cured by thermal polymerization to prepare a terminal positive electrode in which a positive electrode is formed on aluminum.

[0162] A negative electrode active material slurry is prepared by mixing 90 parts by mass of hard carbon as a negative electrode active material, 5 parts by mass of acetylene black as a conductive additive, 10 parts by mass of PVDF as a binder, and an appropriate amount of NMP as a slurry viscosity adjusting solvent.

[0163] In addition, as the negative electrode to be placed at the end of the bipolar battery, the negative electrode slurry is applied to a 30 μm copper foil and similarly cured by thermal polymerization to prepare a terminal negative electrode in which the negative electrode is formed on the copper foil.

[0164] (iii) Preparation of battery evaluation cells <Positive electrode-resin current collector assembly> The positive electrode active material slurry is applied onto the surface of the aluminum layer of the polyolefin film having the metal layer with a doctor blade, and then the coating of the positive electrode active material slurry is pressed to make the thickness of the current collector 30 μm.

[0165] <Preparation of electrolytic layer> An electrolyte slurry is prepared using PEO (polyethylene oxide) (64.5 mass%) as an ion-conductive polymer, Li(C2F5SO2)2N (35.5 mass%) as a supporting salt, and acetonitrile as a viscosity adjusting solvent.

[0166] An electrolyte slurry is poured between glass plates with a 50 μm gap between them and dried to create a 40 μm thick electrolyte layer.

[0167] <Fabrication of bipolar battery evaluation cells> The fabricated terminal positive electrode, negative electrode, terminal negative electrode, electrolyte layer, and positive electrode-current collector assembly are cut into a size of 120 mm x 70 mm. The terminal positive electrode, negative electrode, positive electrode-current collector assembly, and electrolyte layer are stacked twice in sequence, and finally the terminal negative electrode is attached to form a three-layer bipolar battery.

[0168] An Al tab and a Ni tab are welded to the terminal positive electrode and terminal negative electrode, respectively, and the completed battery is sealed in an aluminum laminate under high vacuum to complete a bipolar battery evaluation cell.

[0169] (iv) Current measurement during charging and discharging The bipolar battery evaluation cell was set at 10 mV (Li + Charge and discharge operations are performed up to a maximum capacity (1000kJ / Li), and the current value generated during this process is measured.

[0170] (v) Current stability Sampling is performed from 10 different locations on a polyolefin film having metal layers on both sides, and 10 bipolar battery evaluation cells are fabricated according to the above items (i) to (iii).

[0171] Next, charge and discharge are performed according to the procedure (iv) above, and the current flowing through each of the 10 bipolar battery evaluation cells is recorded. The current stability is evaluated as follows based on the fluctuation of the current from the average value of the obtained current values ​​throughout the charge and discharge operations. A: The current value halves compared to the average value less than once. B: The current value is reduced to half of the average value two or more times but less than three times. C: The current value is reduced to half of the average value four or more times but less than five times. D: The current value is half of the average value six or more times, or the current value is zero one or more times. In terms of current value stability, A to C are good, with A being the best among them.

[0172] (vi) Variation in battery characteristics Regarding the current values ​​during charging and discharging performed in (v) above, the variation calculated by the following formula (8) using the average value of the 10 obtained current values ​​and the difference between the maximum and minimum of the 10 current values ​​is taken as the variation in battery characteristics, and evaluation is performed as follows. (Battery characteristic variation) = 100 × (maximum value - minimum value) / (average value) Equation (8) A: Battery characteristics variation is less than 10%. B: Variation in battery characteristics is greater than 10% and less than 20%. C: Variation in battery characteristics is greater than 20% and less than 30%. D: The battery characteristics variation is greater than 30%. In terms of battery characteristic variations, A to C are good, with A being the best among them.

[0173] (viii) Battery high temperature stability Sampling is performed from 10 different locations on a polyolefin film having metal layers on both sides, and 10 bipolar battery evaluation cells are fabricated according to the above items (i) to (iii).

[0174] Next, charge / discharge operations are performed according to the above item (iv), and the current value flowing through each of the 10 bipolar battery evaluation cells is recorded. Thereafter, the 10 bipolar battery evaluation cells are placed in a constant temperature oven with air circulation, and the temperature is raised from 25°C to 70°C at a rate of 5°C / min. Heating is continued for 30 minutes, and then charge / discharge operations are performed according to the above item (iv), and the current value flowing through each of the 10 bipolar battery evaluation cells is recorded. The evaluation was carried out based on the decrease (%) in the current value before and after the heat treatment as follows. A: The reduction in current value after heat treatment is 20% or less. B: The decrease in current value after heat treatment is greater than 20% and less than or equal to 40%. C: The reduction in current value after heat treatment is greater than 40% and less than 50%. D: The decrease in current value after heat treatment is more than 50%. In terms of high temperature stability of the battery, A to C are good, and among them, A is the best.

[0175] (ix) Battery heat deformation durability Sampling is performed from 10 different locations on a polyolefin film having metal layers on both sides, and 10 bipolar battery evaluation cells are fabricated according to the above items (i) to (iii).

[0176] Next, in accordance with the heating test described in JIS8715-2 (2019), the 10 bipolar battery evaluation cells were fully charged and placed in a constant temperature oven with a circulating airflow, and the temperature was raised from 25°C to 85°C at a rate of 5°C / min. After holding the bipolar battery evaluation cells at 85°C for 3 hours, they were removed from the oven and evaluated as follows: A: Less than 1 in 10 cells ignited or exploded. B: Two to three out of ten cells ignited or exploded. C: Four to five out of ten cells ignited or exploded. D: More than 6 out of 10 cells ignited or exploded. In terms of battery thermal deformation durability, A to C are good, with A being the best among them.

[0177] (x) Battery impact durability Sampling is performed from 10 different locations on the polyolefin having metal layers on both sides, and 10 bipolar battery evaluation cells are fabricated according to the above items (i) to (iii). Next, in accordance with the impact test described in JIS8715-2 (2019), 10 bipolar battery evaluation cells were placed on a flat concrete floor in a discharged state to 50% of their rated capacity. A 15.8 mm diameter SUS316 rod, longer than the maximum dimension of the bipolar battery evaluation cells, was placed across the center of the bipolar battery evaluation cells, and a 9.1 kg weight was dropped onto the rod from a height of 610 mm. The 10 bipolar battery evaluation cells were evaluated in the following state. A: Less than 1 in 10 cells ignited or exploded. B: Two to three out of ten cells ignited or exploded. C: Four to five out of ten cells ignited or exploded. D: More than 6 out of 10 cells ignited or exploded. The battery impact durability is graded A to C, with A being the best. [Example]

[0178] The present invention will be described below with reference to examples, but the present invention is not necessarily limited to these examples.

[0179] (Polyolefin resin) The polyolefin resins used in the production of the polypropylene films in the Examples and Comparative Examples are listed below. Polyolefin-1: Polypropylene "Sumitomo Noblen" (registered trademark) FS2011DG3 manufactured by Sumitomo Chemical Co., Ltd. Polyolefin-2: Sumitomo Chemical Co., Ltd. polypropylene "Sumitomo Noblen" (registered trademark) S131 Polyolefin-3: Modified polyolefin "Admer" (registered trademark) QF500 manufactured by Mitsui Chemicals, Inc.

[0180] (Examples 1 to 10, Comparative Examples 1 to 3) Polyolefin-1, polyolefin-2, polyolefin-3, Ketjen black (aspect ratio 1.2), and carbon nanotubes (aspect ratio 250) were mixed to the final content ratios shown in the table and fed into the extruders P1 and P2. For the monolayer structure, the same raw materials were fed into each extruder. Each resin mixture was melt-extruded, and after removing impurities with a 20 μm cutoff sintered filter, the mixture was laminated in a feedblock-type A / B / A composite T-die to a thickness ratio of 1:8:1 (surface layer (I) / base layer (II) / surface layer (I)). The resulting molten sheet was extruded onto a casting drum with a surface temperature controlled at 20°C and adhered to the casting drum with an air knife. The sheet on the casting drum was then cooled by spraying 15°C compressed air at an air velocity of 140 m / s with an air knife to obtain an unstretched sheet. Next, the unstretched sheet was brought into contact with a 95°C metal roll in the first half of the preheating process, and heated to 120°C in the second half of the preheating process. It was then stretched 4.2 times in the longitudinal direction between 147°C rolls with a peripheral speed difference. Next, a 5.5% relaxation treatment was performed in the longitudinal direction using a metal roll heated to 95°C to obtain a uniaxially stretched film. The resulting uniaxially stretched film was then introduced into a tenter-type stretching machine with both widthwise ends held with clips. It was preheated at 137°C for 2 seconds, stretched 9.0 times in the widthwise direction at 152°C, and then heat-treated at 155°C while providing 10% relaxation in the widthwise direction. It was then cooled to 100°C, introduced to the outside of the tenter, the clips at both widthwise ends of the film were released, and both surfaces of the film were subjected to plasma treatment using atmospheric pressure glow discharge. It was then wound around a core to obtain a 5 μm-thick biaxially stretched polyolefin film. The physical properties and evaluation results of the resulting film are shown in Table 1.

[0181] Metal layers M and M' were formed on both surfaces of the obtained polyolefin film by vacuum deposition so that the thicknesses of the metal layers were as shown in the table, thereby obtaining a polyolefin film having a metal layer. Resin current collectors for bipolar batteries were produced using the metal species as shown in the table. As described above in the [Bipolar Battery Evaluation] section, the polyolefin film having a metal layer was incorporated as a resin current collector into a bipolar battery evaluation cell, and bipolar battery evaluation was performed. The evaluation results are shown in the table.

[0182] [Table 1-1]

[0183] [Table 1-2]

[0184] [Table 2]

[0185] [Table 3-1]

[0186] [Table 3-2]

[0187] [Table 4]

[0188] [Table 5]

Claims

1. A polyolefin film containing conductive particles, which satisfies the following (1) and (2): (1) The composite material contains a carbon material and / or metal particles in a total amount of 1.0 mass % or more and 40 mass % or less. (2) The conductive particles include at least conductive particles having an aspect ratio of 1 or more and less than 5, and conductive particles having an aspect ratio of 5 or more.

2. 2. The polyolefin film according to claim 1, wherein the conductive particles (C1) have an aspect ratio of 1 or more but less than 5, and the conductive particles (C2) have an aspect ratio of 5 or more, and the ratio of the area (SC1) occupied by C1 to the area (SC2) occupied by C2 contained in the polyolefin film satisfies formula 1. 5≧SC1 / SC2≧0.1 ...Formula 1

3. 2. The polyolefin film according to claim 1, wherein the F5 value (the load value when the test piece is elongated by 5% divided by the cross-sectional area of ​​the test piece) in at least one in-plane direction is 25 MPa or more.

4. 4. The polyolefin film according to claim 3, wherein the F5 values ​​in both the longitudinal direction and the width direction of the film are 25 MPa or more.

5. The volume resistivity at 23°C and 65% RH is 10 ０ Ω・cm or more 10 ８ The polyolefin film according to claim 1, having a modulus of less than Ω·cm.

6. 2. The polyolefin film according to claim 1, wherein the conductive particles contained therein have a volume-based average dispersed diameter of 0.1 μm or more and 10 μm or less.

7. A polyolefin film containing conductive particles, which satisfies the following (3) and (4): (3) The volume resistivity at 23°C and 65% RH is 10 ０ Ω・cm or more 5.0×10 ６ It is Ω·cm or less. (4) The F5 value (the load value when the test piece is elongated by 5% divided by the cross-sectional area of ​​the test piece) in at least one direction in the plane is 25 MPa or more.

8. 8. The polyolefin film according to claim 1, which has a shrinkage rate of 20% or less when maintained at 120°C for 15 minutes.

9. 8. The polyolefin film according to claim 1, wherein the half-value width of the heat of fusion peak measured by a differential scanning calorimeter (DSC) is 10°C or more and 40°C or less.

10. The polyolefin film according to claim 1 or 7, comprising at least two or more layers.

11. 8. The polyolefin film according to claim 1, wherein the water contact angle of at least one surface is 75° or more and 100° or less.

12. A laminate comprising the polyolefin film according to claim 1 or 7, and a layer of a metal and / or a metal-based compound on at least one surface of the polyolefin film.

13. The laminate according to claim 12, wherein the layer made of a metal and / or a metal-based compound contains aluminum element.

14. A laminate comprising the polyolefin film according to claim 1 or 7, and layers of metal and / or metal-based compounds on both surfaces of the film.

15. 15. The laminate according to claim 14, wherein the layer of metal and / or metal-based compound on one surface contains elemental aluminum, and the layer of metal and / or metal-based compound on the opposite surface contains elemental copper.

16. A laminated film having a layer of a metal and / or a metal-based compound on at least one surface of a polyolefin film containing conductive particles, which satisfies the following (5), and has a penetration resistivity of 1.0 × 10 ０ Ωcm or more 1.0 x 10 ８ A laminated film having a resistivity of Ωcm or less. (5) The composite material contains a carbon material and / or metal particles in a total amount of 1.0% by mass or more and 40% by mass or less.

17. A resin current collector comprising the polyolefin film according to claim 1 or 7.

18. A resin current collector for a bipolar battery, comprising the polyolefin film according to claim 1 or 7.

19. 20. A bipolar battery electrode comprising the resin current collector for a bipolar battery according to claim 18, which comprises a negative electrode active material layer on one surface side thereof and a positive electrode active material layer on the opposite surface side thereof.

20. A secondary battery comprising the bipolar battery electrode according to claim 19.

21. 20. A secondary battery comprising the bipolar battery electrode according to claim 19, wherein at least two positive electrode active material layers and two negative electrode active material layers are stacked with an electrolyte interposed therebetween.

22. An electric vehicle equipped with the secondary battery according to claim 21.

23. An electric flying object equipped with the secondary battery according to claim 21.

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