Adhesive film, tab lead, and energy storage device

Abstract

Provided is an adhesive film formed from a first layer including a first main surface and a second layer provided on the first main surface. The first layer is a crosslinked layer chiefly comprising polypropylene. The first layer has a heat collapse residual ratio of 60% or more. The second layer is an acid-modified polyolefin layer. After staining a cross section of the adhesive film with ruthenium tetroxide, the area percentage of the stained portion in a first image obtained by observing the first layer within the cross section at 10000x magnification using a transmission electron microscope is between 20% and 50%, inclusive.

Description

Adhesive films, tab leads, and energy storage devices

[0001] This disclosure relates to adhesive films, tab leads, and energy storage devices.

[0002] Energy storage devices used in small electronic devices have a structure in which a positive electrode and a negative electrode, to which lead conductors are connected, are sealed together with an electrolyte medium in a sealed container.

[0003] The surface of the encapsulation container is made of resin, and the lead conductor is made of metal; they are made of different materials. Therefore, the interface between the encapsulation container and the lead conductor has weak adhesion. To improve the adhesion between the encapsulation container and the lead conductor, a technique has been proposed in which an adhesive film is placed between them (Patent Documents 1 and 2).

[0004] Japanese Patent Publication No. 11-334389 Japanese Patent Publication No. 2011-103245

[0005] The adhesive film of this disclosure comprises a first layer including a first main surface and a second layer provided on the first main surface, wherein the first layer is a crosslinked layer mainly composed of polypropylene, the thermal collapse retention rate of the first layer is 60% or more, the second layer is an acid-modified polyolefin layer, and in the first image obtained by staining a cross-section of the adhesive film with ruthenium tetroxide and observing the first layer within the cross-section at 10,000x magnification using a transmission electron microscope, the area percentage of the stained portion is 20% or more and 50% or less.

[0006] Figure 1 is a cross-sectional view of an adhesive film according to Embodiment 1. Figure 2 is a cross-sectional view of a tab lead according to Embodiment 2. Figure 3 is a front view of an energy storage device according to Embodiment 3. Figure 4 is a partial cross-sectional view of the section IV-IV' in Figure 3.

[0007] [Problems this disclosure aims to solve] In recent years, the demand for battery reliability has been increasing, and adhesive films are required to have improved adhesion not only to lead conductors but also to encapsulation containers.

[0008] Therefore, the present disclosure aims to provide an adhesive film that has high adhesion to a sealed container when used in an energy storage device. The present disclosure also aims to provide a tabrete and an energy storage device that include the adhesive film.

[0009] [Effects of this Disclosure] According to this disclosure, it is possible to provide an adhesive film that has high adhesion to a sealed container when used in an energy storage device. Furthermore, according to this disclosure, it is possible to provide a tabrete and an energy storage device that include the adhesive film.

[0010] [Description of Embodiments of the Disclosure] Embodiments of the Disclosure will be listed and described first. (1) The adhesive film of the Disclosure is an adhesive film comprising a first layer including a first main surface and a second layer provided on the first main surface, wherein the first layer is a crosslinked layer mainly composed of polypropylene, the thermal collapse retention rate of the first layer is 60% or more, the second layer is an acid-modified polyolefin layer, and in the first image obtained by staining a cross-section of the adhesive film with ruthenium tetroxide and observing the first layer within the cross-section at 10,000x magnification with a transmission electron microscope, the area percentage of the stained portion is 20% or more and 50% or less.

[0011] According to this disclosure, it is possible to provide an adhesive film that has high adhesion to a sealed container when used in an energy storage device.

[0012] (2) In (1) above, the first layer may contain ultra-low density polyethylene that does not contain unsaturated bonds. This further improves the adhesion of the adhesive film to the encapsulation container.

[0013] (3) In (2) above, the ultra-low density polyethylene that does not contain unsaturated bonds may include at least one selected from the group consisting of ethylene propylene copolymer, ethylene butene copolymer, ethylene hexene copolymer, and ethylene octene copolymer.

[0014] (4) In any of (1) to (3) above, the acid-modified polyolefin layer may be an acid-modified polypropylene layer. This improves the adhesion and sealing properties of the adhesive film to the lead conductor.

[0015] (5) The tab lead of the present disclosure comprises a lead conductor and an adhesive film according to any one of (1) to (4) above attached to a part of the lead conductor, wherein the second layer of the adhesive film is disposed in contact with the lead conductor.

[0016] When the tab lead of this disclosure is used in an energy storage device, the first layer is positioned in contact with an adhesive film. In this energy storage device, the adhesive film and the encapsulation container are bonded with high adhesive strength. The tab lead is suitable for energy storage devices.

[0017] (6) The energy storage device of the present disclosure comprises a battery cell including a positive electrode, a negative electrode, and an electrolyte sandwiched between the positive electrode and the negative electrode, lead conductors electrically connected to the positive electrode and the negative electrode, and a sealed container for sealing the battery cell, wherein a portion of the lead conductors is exposed to the outside of the sealed container, an adhesive film according to any one of (1) to (4) above is disposed between the lead conductors and the sealed container, and the first layer of the adhesive film is disposed in contact with the sealed container.

[0018] In the energy storage device of this disclosure, the adhesive film and the encapsulation container are bonded together with high adhesive strength. Therefore, the energy storage device can have excellent reliability.

[0019] (7) In (6) above, the electrolyte may be a solid electrolyte. This allows the high adhesion of the adhesive film to the encapsulation container to be maintained for a long period of time, further improving the reliability of the energy storage device.

[0020] (8) In (6) above, the electrolyte may be a non-aqueous electrolyte. The energy storage device can have excellent reliability.

[0021] [Details of Embodiments of the Disclosure] Specific examples of the adhesive film, tab lead, and energy storage device of the Disclosure will be described below with reference to the drawings. In the drawings of the Disclosure, the same reference numerals indicate the same or corresponding parts. In addition, dimensional relationships such as length, width, thickness, and depth have been modified as appropriate for clarity and simplification of the drawings and do not necessarily represent actual dimensional relationships.

[0022] In this disclosure, the notation "A to B" means A or greater and B or less. If no unit is specified for A, and only a unit is specified for B, then the unit for A and the unit for B are the same.

[0023] In this disclosure, when compounds and the like are represented by chemical formulas, unless otherwise specified, the atomic ratios should include all conventionally known atomic ratios and should not necessarily be limited to those within the stoichiometric range.

[0024] In this disclosure, if one or more numerical values ​​are listed as the lower limit and upper limit of a numerical range, any combination of any one numerical value listed as the lower limit and any one numerical value listed as the upper limit shall also be disclosed.

[0025] In this disclosure, “equipment,” “includes,” “possesses,” and variations thereof are open-ended terms. Open-ended terms may or may not include additional elements in addition to the essential elements. The statement “consists of” is a closed term. However, even a configuration expressed in closed terms may include additional elements that are usually incidental or irrelevant to the subject technology.

[0026] [Embodiment 1: Adhesive Film] An adhesive film according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 1") will be described with reference to Figure 1. The adhesive film 1 of Embodiment 1 is an adhesive film comprising a first layer 11 including a first main surface 11a and a second layer 12 provided on the first main surface 11a. The first layer 11 is a crosslinked layer mainly composed of polypropylene. The thermal collapse retention rate of the first layer 11 is 60% or more. The second layer 12 is an acid-modified polyolefin layer. After staining the cross-section of the adhesive film 1 with ruthenium tetroxide, the first layer 11 in the cross-section is observed at 10,000x magnification with a transmission electron microscope to obtain a first image, in which the area percentage of the stained area is 20% or more and 50% or less.

[0027] <Structure of the Adhesive Film> As shown in Figure 1, the adhesive film 1 consists of a first layer 11 and a second layer 12 provided on the first main surface 11a of the first layer 11. The first layer 11 may include a first main surface 11a and a second main surface 11b opposite to the first main surface 11a. The second layer 12 may include a third main surface 12a in contact with the first main surface 11a and a fourth main surface 12b opposite to the third main surface 12a. In an energy storage device comprising the adhesive film 1, the second main surface 11b of the first layer 11 is arranged in contact with the sealing container, and the fourth main surface 12b of the second layer 12 is arranged in contact with the lead conductor.

[0028] The average thickness of the adhesive film can be set appropriately depending on the application. The average thickness of the adhesive film may be 50 μm to 250 μm, 60 μm to 210 μm, or 90 μm to 170 μm.

[0029] In this disclosure, the average thickness of the adhesive film is measured by the following procedure: The adhesive film is cut along the direction normal to the largest surface area on the outer surface of the adhesive film using a microtome or the like to expose the cross-section. The cross-section is observed at 250x magnification using a digital microscope, and the thickness of the adhesive film is measured at five locations. The average of the five thicknesses is calculated. This average corresponds to the average thickness of the adhesive film. In this disclosure, the average thicknesses of the first and second layers described later are measured in the same manner.

[0030] <First Layer> <<Components of the First Layer>> The first layer is a cross-linked layer with polypropylene as the main component. Examples of polypropylene include homopolypropylene, block polypropylene, random polypropylene, acid-modified polypropylene, etc.

[0031] It can be confirmed that the first layer contains polypropylene by performing imaging IR analysis on only the first layer of the adhesive film using a Fourier transform infrared spectrophotometer.

[0032] When the spectrum obtained by imaging IR analysis has clear peaks (2955 cm －１ : CH ２ antisymmetric stretching, 2840 cm －１ : CH ２ symmetric stretching, 1465 cm －１ : CH ２ in-plane bending (scissors)) other than those of polyethylene, and clear peaks (2955 cm ３ : CH －１ antisymmetric stretching, 1375 cm ３ : CH －１ symmetric bending) derived from the CH ３ group of polypropylene, it is determined that the first layer contains polypropylene as the main component.

[0033] The first layer may contain ultra-low density polyethylene containing no unsaturated bonds as an elastomer component.

[0034] Ultra-low density polyethylene containing no unsaturated bonds may contain at least one selected from the group consisting of ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-hexene copolymer, and ethylene-octene copolymer. Ultra-low density polyethylene containing no unsaturated bonds may consist of at least one selected from the group consisting of ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-hexene copolymer, and ethylene-octene copolymer.

[0035] In the first layer, the percentage of ultra-low density polyethylene containing no unsaturated bonds with respect to the total of polypropylene and ultra-low density polyethylene containing no unsaturated bonds may be 20% by volume or more and 50% by volume or less.

[0036] The first layer may not contain polybutadiene. Polybutadiene is prone to react at unsaturated bond portions and during film formation, unsaturated bonds are likely to crosslink, easily forming foreign substances such as crosslinked gels. With the long-term use of the adhesive film, destruction is likely to occur starting from the foreign substances, the adhesive strength decreases, and the long-term reliability decreases. Also, since the appearance becomes defective due to the foreign substances, the yield decreases. If the first layer does not contain polybutadiene, the generation of foreign substances during film formation can be suppressed, so the long-term reliability of the adhesive film is improved and the yield is also improved.

[0037] The first layer can contain various additives such as crosslinking aids, antioxidants, flame retardants, ultraviolet absorbers, light stabilizers, heat stabilizers, lubricants, colorants, etc. The types and amounts of these additives can be the same as those of conventionally known types and amounts. Examples of crosslinking aids include trimethylolpropane methacrylate, trimethylolpropane methacrylate, pentaerythritol triacrylate, ethylene glycol dimethacrylate, triallyl cyanurate, and triallyl isocyanurate. Examples of antioxidants include phenolic antioxidants, sulfur-based antioxidants, and amine-based antioxidants.

[0038] <<Thermal collapse remaining rate>> The thermal collapse remaining rate of the first layer is 60% or more. When the thermal collapse remaining rate of the first layer is 60% or more, the first layer has excellent heat resistance, is difficult to melt during heat fusion with the encapsulation container, and can prevent short circuits between the metal layer of the encapsulation container and the lead conductor due to the melting of the adhesive film. The upper limit of the thermal collapse remaining rate of the first layer is not particularly limited, but for example, it may be 99% or less. The thermal collapse remaining rate of the first layer may be 60% or more and 99% or less, or may be 70% or more and 95% or less. Note that the fact that the thermal collapse remaining rate of the first layer is 60% or more indicates that the first layer is a crosslinked layer.

[0039] In the present disclosure, the thermal collapse remaining rate of the first layer is measured by the following procedure. Prepare a sample consisting of the entire adhesive film composed of the first layer and the second layer. Next, place the sample into a TMA apparatus. Heat the sample while applying a load of 0.1 MPa (probe diameter 0.9 mmφ) to the probe, hold it at 200°C for 1 minute, release the load, and then take out the sample after cooling it to room temperature. In the sample after heating and pressurization, measure the thickness Th (μm) of the first layer in the pressurized part and the thickness Tr (μm) of the first layer in the non-pressurized part around the pressurized part in the same manner as the method for measuring the average thickness of the above adhesive film. (Th / Tr) × 100 is defined as the thermal collapse remaining rate (%) of the first layer.

[0040] ≪Area percentage of the stained part of the first layer≫ After staining the cross-section of the adhesive film with ruthenium tetroxide, in the first image obtained by observing the first layer in the cross-section with a transmission electron microscope at a magnification of 10,000 times, the area percentage of the stained part is 20% or more and 50% or less. The cross-section of the adhesive film is a cross-section in the thickness direction along the TD of the first layer. TD (Transverse Direction) means a direction perpendicular to the direction (MD: Machine Direction) in which the insulating film is conveyed in the manufacturing process of the insulating film.

[0041] The stained part is presumed to be a region corresponding to the amorphous part of the non-crystalline material. The non-stained part is presumed to be a region corresponding to the crystalline part of the crystalline material. When the area percentage of the stained part is 20% or more, the amorphous part of the polymer material increases and the impact resistance improves. When the area percentage of the stained part is 50% or less, the peel strength between the first layer and the encapsulating container is good. In the first image, the area percentage of the stained part may be 22% or more and 48% or less, or may be 25% or more and 45% or less.

[0042] In this disclosure, the area percentage of the stained area is measured by performing image processing on the first image. The image processing is performed using the image analysis software ImageJ. Specifically, the first image is acquired as a grayscale image (JPEG) digital file, and processed according to the following binarization procedure and parameters. Pixels with a gradation above the threshold (bright) are output as 1, and pixels with a gradation below the threshold (dark) are output as 0, defining these as the stained area and the unstained area, respectively. Bright pixels are the unstained area and correspond to the crystalline portion. Dark pixels are the stained area and correspond to the non-crystalline portion.

[0043] <Binarization Process> 1. Spike noise removal (Despect) 2. Island contour removal (Remove Outliers radius=4 threshold=1 which=Bright) 3. Sea contour removal (Remove Outliers radius=4 threshold=1 which=Dark) 4. Spike noise removal (Despect) 5. Gaussian blur in the X-axis direction (sample short side) (threshold=3 pixels) 6. Contrast enhancement (saturated=0.2) 7. Island contour removal (Remove Outliers radius=4 threshold=1 which=Bright) 8. 9. Removing the outline of Kaibu (Remove Outliers radius=4 threshold=1 which=Dark)

[0044] Using the image analysis software ImageJ, the area percentage of the stained area relative to the total area of ​​the first image is calculated. The above measurement is performed on three non-overlapping first images. The average of the area percentages of the stained areas of the three first images is calculated. In this disclosure, this average corresponds to the area percentage of the stained area in the first image.

[0045] ≪Average Thickness of the First Layer≫ The average thickness of the first layer may be 15 μm or more and 150 μm or less, 20 μm or more and 110 μm or less, or 30 μm or more and 60 μm or less. If the average thickness of the first layer is 15 μm or more, good adhesion to the lead conductor can be ensured. If the average thickness of the first layer is 150 μm or less, the energy storage device including the adhesive film can be made smaller.

[0046] <Second Layer> The second layer is an acid-modified polyolefin layer. Acid-modified polyolefins are polyolefins containing acid-modified groups, modified with carboxylic acids such as maleic acid, acrylic acid, methacrylic acid, and maleic anhydride. Examples of acid-modified polyolefins include maleic acid-modified polypropylene, acrylic acid-modified polypropylene, maleic anhydride-modified polypropylene, acrylic acid-modified polyethylene, maleic anhydride-modified polyethylene, and acrylic acid-modified ethylene acrylate. If the second layer is an acid-modified polypropylene layer, it has excellent adhesion and sealing properties to metals. If the second layer is a maleic anhydride-modified polypropylene layer, it has extremely excellent adhesion and sealing properties to metals. The acid-modified polyolefin layer may also contain unmodified polyolefin as a resin component, as long as it does not impair the effects of this disclosure. Hereinafter, acid-modified polyolefins and unmodified polyolefins will be collectively referred to as polyolefins.

[0047] The presence of acid-modifying groups in the second layer can be confirmed by separating only the second layer from the adhesive film and performing transmitted light analysis on the second layer using a Fourier transform infrared spectrophotometer. Examples of acid-modifying groups include maleic acid, acrylic acid, methacrylic acid, and maleic anhydride. The content of acid-modifying groups in the second layer can be between 0.01% by mass and 1.0% by mass.

[0048] The presence of polyolefin in the second layer can be confirmed by removing only the second layer from the adhesive film and performing reflected light analysis on the second layer using a Fourier transform infrared spectrophotometer.

[0049] The second layer may contain various additives such as flame retardants, ultraviolet absorbers, light stabilizers, heat stabilizers, lubricants, and colorants. The types and amounts of these additives can be the same as those used in conventionally known acid-modified polyolefin layers.

[0050] ≪Average Thickness of the Second Layer≫ The average thickness of the second layer may be 35 μm or more and 200 μm or less, 40 μm or more and 160 μm or less, or 50 μm or more and 110 μm or less. If the average thickness of the second layer is 35 μm or more, good adhesion with the lead conductor can be ensured. If the average thickness of the second layer is 200 μm or less, the energy storage device including the adhesive film can be made smaller.

[0051] <Method for producing an adhesive film> The adhesive film of Embodiment 1 is produced, for example, by the following method. First, polypropylene resin and elastomer components are prepared as raw materials for the first layer, and acid-modified polyolefin is prepared as raw material for the second layer.

[0052] Specific examples of the polypropylene resin and elastomer component used as raw materials for the first layer are as described in Embodiment 1. The MFR of the polypropylene resin is 7. The MFR of the elastomer component is 2. Specific examples of the acid-modified polyolefin used as raw materials for the second layer are as described in Embodiment 1.

[0053] If at least one of the first and second layers contains either an additive or a crosslinking aid, then the additive and the crosslinking aid are prepared. An example of a crosslinking aid is trimethylolpropane methacrylate.

[0054] The raw materials for the first layer and the raw materials for the second layer are mixed using known mixing equipment such as an open roll, a pressure kneader, a single-screw kneader, or a twin-screw kneader to obtain the mixed raw materials for the first layer and the mixed raw materials for the second layer.

[0055] The mass ratio of polypropylene resin to elastomer component in the first layer of mixed raw materials is polypropylene resin:elastomer component = 55:45 to 84:16.

[0056] The mixed raw materials for the first and second layers are extruded using a single-layer T-die extruder to obtain a single-layer film consisting of the first layer and a single-layer film consisting of the second layer. The single-layer film consisting of the first layer is irradiated with an electron beam at an acceleration voltage of 200 kV in an oxygen-free atmosphere to achieve an absorbed dose of 150 kGy using an electron beam irradiator to crosslink the polypropylene resin and elastomer components. Subsequently, the single-layer film consisting of the first layer is laminated with the single-layer film consisting of the second layer by thermal lamination at 180°C to obtain an adhesive film. The film formation conditions in the T-die extruder are, for example, a die lip opening of 0.3 mm thickness to 1.0 mm thickness and a take-up speed of 2 m / min to 20 m / min. The thickness of the first layer after extrusion is adjusted to 15 μm to 200 μm.

[0057] [Embodiment 2: Tab Lead] A tab lead according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 2") will be described with reference to Figure 2. The tab lead 20 of Embodiment 2 comprises a lead conductor 3 and an adhesive film 1 of Embodiment 1 attached to a part of the lead conductor 3. In the tab lead 20, the second layer 12 of the adhesive film 1 is arranged in contact with the lead conductor 3. In an energy storage device equipped with the tab lead 20, the second main surface 11b of the first layer 11 is arranged in contact with the sealing container, and the fourth main surface 12b of the second layer 12 is arranged in contact with the lead conductor.

[0058] In Embodiment 2, metals such as aluminum, nickel, copper, and nickel-plated copper are used as lead conductors. Aluminum is often used for the positive electrode, and aluminum, nickel, or nickel-plated copper is often used for the negative electrode. The shape of the lead conductor is not particularly limited, but a flat metal plate with a thickness of 50 μm to 2 mm, a width of 1 mm to 200 mm, and a length of 20 mm to 200 mm is preferably used.

[0059] [Embodiment 3: Energy Storage Device] An energy storage device according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 3") will be described with reference to Figures 3 and 4. Figure 3 is a schematic front view showing one embodiment of the energy storage device 30 of Embodiment 3. Figure 4 is a partial cross-sectional view taken along section IV-IV' of Figure 3. The energy storage device 30 of Embodiment 3 includes a battery cell 15 comprising a positive electrode 10, a negative electrode 14, and an electrolyte 13 sandwiched between the positive electrode 10 and the negative electrode 14, lead conductors 3 electrically connected to the positive electrode 10 and the negative electrode 14, and a sealed container 2 that seals the battery cell 15. A portion of the lead conductors 3 is exposed to the outside of the sealed container 2, and the adhesive film 1 of Embodiment 1 is placed between the lead conductors 3 and the sealed container 2. The first layer 11 of the adhesive film is placed in contact with the sealed container 2.

[0060] As shown in Figure 4, the sealing container 2 consists of a three-layer laminate film 8 comprising a metal layer 5 and a first resin layer 6 and a second resin layer 7 covering the metal layer 5. The metal layer 5 is formed from a metal such as aluminum foil. As the first resin layer 6 located on the outside of the sealing container 2, polyamide resins such as 6,6-nylon, 6-nylon, polyester resins, polyimide resins, etc., can be used. Furthermore, it is preferable to use an insulating resin that does not dissolve in a non-aqueous electrolyte and melts when heated for the second resin layer 7 located on the inside of the sealing container 2 and in contact with the first layer 11 of the adhesive film 1. Examples of such insulating resins include polyolefin resins, acid-modified polyolefin resins, and acid-modified styrene elastomers. The sealing container may also include a polypropylene layer as the second resin layer 7 in contact with the first layer 11. The polypropylene layer may be an acid-modified polypropylene layer. Examples of acid-modified polypropylene layers include maleic acid-modified polypropylene layers, acrylic acid-modified polypropylene layers, and maleic anhydride-modified polypropylene layers.

[0061] The sealing container 2 is made by overlapping two laminate films 8 and heat-sealing three sides other than the side through which the lead conductor 3 passes. At the outer circumference of the sealing container 2, the two metal layers 5 are bonded together via a second resin layer 7.

[0062] The lead conductor 3 is bonded (heat-sealed) to the sealing container (laminate film) via the adhesive film 1 at the sealing portion 9. Inside the energy storage device 30, a positive electrode 10, a negative electrode 14, and an electrolyte 13 are further sealed. Figure 4 shows a lead conductor connected to the negative electrode 14. A lead conductor connected to the positive electrode 10 also exists, but is not shown in Figure 4. In Embodiment 3, the electrolyte 13 may be a solid electrolyte. In Embodiment 3, the electrolyte 13 may be a non-aqueous electrolyte. For example, LiPF ６ LiBF ４ Examples include lithium salts containing fluorine, dissolved in diethyl carbonate (DEC), dimethyl carbonate (DMC), propylene carbonate (PC), etc.

[0063] This embodiment will be described in more detail by reference to examples. However, this embodiment is not limited by these examples.

[0064] [Preparation of adhesive films]

[0065] As the raw material for the first layer, polypropylene resin (random polypropylene, melting point 140°C, MFR 7), elastomer components, a crosslinking aid (trimethylolpropane methacrylate), and a phenolic antioxidant were mixed using a twin-screw kneader in the mass ratios shown in Tables 1 and 2 to obtain the raw material for the first layer. The component names, melting points, and MFRs of the elastomer components used in each sample are shown in Tables 1 and 2.

[0066]

[0067]

[0068] Maleic anhydride-modified random polypropylene (melting point 140°C, MFR7) was prepared as the raw material for the second layer. The raw material for the second layer was also kneaded using a twin-screw kneader.

[0069] The mixed raw materials for the first and second layers were extruded in a single-layer T-die extruder under conditions of an extrusion temperature of 220°C to obtain single-layer films consisting of the first and second layers, respectively. The single-layer film consisting of the first layer was irradiated with an electron beam at an acceleration voltage of 200 kV in an oxygen-free atmosphere to an absorbed dose of 150 kGy using an electron beam irradiator to crosslink the polypropylene resin and elastomer components. Subsequently, the single-layer film consisting of the first layer was laminated with the single-layer film consisting of the second layer by thermal lamination at 180°C to obtain an adhesive film. This obtained adhesive films for each sample with the first and second layers laminated. In all samples, the take-up speed was 8 m / min and the die lip opening was 0.4 mm. The average thicknesses of the first and second layers in the obtained adhesive films are shown in Tables 1 and 2.

[0070] [Measurement of Adhesive Film] <Area Percentage of Stained Area in the First Layer> After staining the cross-section of the adhesive film with ruthenium tetroxide, the area percentage of the stained area in the first image obtained by observing the first layer within the cross-section at 10,000x magnification using a transmission electron microscope was measured. The specific measurement method is as described in Embodiment 1. The results are shown in Tables 1 and 2.

[0071] <<Remaining thermal deformation rate of the first layer>> For the first layer of the adhesive film of each sample, the remaining thermal deformation rate was measured after applying a load of 0.1 MPa at 200°C and holding for 1 minute. The specific measurement method is as described in Embodiment 1. The results are shown in Tables 1 and 2.

[0072] ≪Film Formation Properties of the First Layer≫ The presence or absence of cross-linked gel was visually confirmed for the single-layer film consisting of the first layer of each sample. Cross-linked gel was confirmed in samples 1-4. Cross-linked gel was not confirmed in the samples other than samples 1-4. In samples 1-4, the cross-linked gel portion was taken and pressed under the conditions of 200°C, 0.5 MPa, and 10 seconds to confirm whether the cross-linked gel portion could be thermally melted. Furthermore, the IR spectrum of the cross-linked gel portion and the surrounding area were confirmed to match using the FT-IR method. In samples 1-4, the cross-linked gel portion that was visually confirmed did not thermally melt when pressed, and the spectrum matched that of the surrounding area using the FT-IR method.

[0073] [Adhesion test between adhesive film and encapsulation container] For samples other than samples 1-4, in which no cross-linked gel was confirmed in the first layer, the adhesion test was performed according to the following procedure.

[0074] As a encapsulation container, a three-layer sheet was prepared, consisting of a polyamide layer (33 μm thick, corresponding to the first resin layer), a metal layer made of aluminum foil (40 μm thick), and a maleic acid-modified polypropylene layer (80 μm thick, corresponding to the second resin layer), laminated in the order described above. The size of the encapsulation container was 500 mm in length and 100 mm in width. The first layer of adhesive film (10 mm in length, 55 mm in width) for each sample was placed in contact with the maleic acid-modified polypropylene layer of the encapsulation container and pressed to bond, thereby preparing the first test sample. The pressing conditions were as described in Press Condition 1 and Press Condition 2 below. Press Condition 1: 200°C, surface pressure 2.0 MPa, 5 seconds Press Condition 2: 200°C, surface pressure 2.0 MPa, 3 seconds

[0075] For the first test sample of each specimen, the adhesive film was pulled away from the encapsulation container at a 180° angle, and the 180° peel strength was measured. The tensile speed was 50 mm / min. The results are shown in the "Adhesion to Encapsulation Container" column, "Press Condition 1" and "Press Condition 2" in Tables 1 and 2. In Tables 1 and 2, "A" is indicated if the peel strength is 60 N / cm or higher, and "B" is indicated if the peel strength is less than 60 N / cm. In the above adhesion test, if the results for both "Press Condition 1" and "Press Condition 2" are "A", the adhesive film is judged to have high adhesion to the encapsulation container.

[0076] The adhesive films of Samples 1 to 4 correspond to the examples. It was confirmed that the adhesive films of these samples exhibit high adhesion to the encapsulation container.

[0077] The adhesive films of Samples 1-1 to 1-4 are comparative examples. It was confirmed that the adhesive films of Samples 1-1 to 1-3 had insufficient adhesion to the encapsulation container. The adhesive film of Sample 1-4 has a cross-linked gel in the first layer, and with long-term use of the adhesive film, it is prone to fracture starting from foreign matter, reducing adhesive strength and long-term reliability. In addition, the appearance is poor due to the foreign matter, resulting in a decrease in yield.

[0078] While embodiments and examples of this disclosure have been described above, it is intended from the outset that the configurations of each of the embodiments and examples described above may be combined or modified in various ways as appropriate. The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than the embodiments and examples described above, and all modifications within the scope of the claims are intended to be included in the meaning of equivalences.

[0079] 1 Adhesive film, 2 Encapsulation container, 3 Lead conductor, 5 Metal layer, 6 First resin layer, 7 Second resin layer, 8 Laminate film, 9 Sealing portion, 10 Positive electrode, 11 First layer, 11a First main surface, 11b Second main surface, 12 Second layer, 12a Third main surface, 12b Fourth main surface, 13 Electrolyte, 14 Negative electrode, 15 Battery cell, 30 Energy storage device.

Claims

1. An adhesive film comprising a first layer including a first main surface and a second layer provided on the first main surface, wherein the first layer is a crosslinked layer mainly composed of polypropylene, the thermal collapse retention rate of the first layer is 60% or more, the second layer is an acid-modified polyolefin layer, and in the first image obtained by staining a cross-section of the adhesive film with ruthenium tetroxide and observing the first layer within the cross-section at 10,000x magnification using a transmission electron microscope, the area percentage of the stained portion is 20% or more and 50% or less.

2. The adhesive film according to claim 1, wherein the first layer comprises ultra-low density polyethylene that does not contain unsaturated bonds.

3. The adhesive film according to claim 2, wherein the ultra-low density polyethylene that does not contain unsaturated bonds comprises at least one selected from the group consisting of ethylene propylene copolymer, ethylene butene copolymer, ethylene hexene copolymer, and ethylene octene copolymer.

4. The adhesive film according to any one of claims 1 to 3, wherein the acid-modified polyolefin layer is an acid-modified polypropylene layer.

5. A tab lead comprising a lead conductor and an adhesive film according to any one of claims 1 to 4 attached to a part of the lead conductor, wherein the second layer of the adhesive film is disposed in contact with the lead conductor.

6. An energy storage device comprising: a battery cell including a positive electrode, a negative electrode, and an electrolyte sandwiched between the positive electrode and the negative electrode; lead conductors electrically connected to the positive electrode and the negative electrode, respectively; and a sealing container for sealing the battery cell, wherein a portion of the lead conductors is exposed to the outside of the sealing container, and an adhesive film according to any one of claims 1 to 4 is disposed between the lead conductors and the sealing container, and the first layer of the adhesive film is disposed in contact with the sealing container.

7. The energy storage device according to claim 6, wherein the electrolyte is a solid electrolyte.

8. The energy storage device according to claim 6, wherein the electrolyte is a non-aqueous electrolyte.

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