Adhesive film, tab lead, and power-storage device

Abstract

This adhesive film is provided with: a first layer having a first main surface and a second main surface on the opposite side of the first main surface; and a second layer provided on the second main surface. The first main surface is exposed to the outside. The first layer is an acid-modified polyolefin layer and the second layer is a heat resistant resin layer. In a first image obtained by observing the first main surface by using a scanning electron microscope at 10000x magnification after the adhesive film is immersed for 24 hours in hexane at 40°C, the first main surface includes recesses. In the first image, the area proportion of the recesses is 2.0% or more.

Description

Adhesive film, tab lead and electricity storage device

[0001] The present disclosure relates to an adhesive film, a tab lead, and an electricity storage device.

[0002] Electrical components such as batteries and capacitors are required to be smaller and lighter in weight, and for this reason, a technique has been adopted in which a positive electrode, a negative electrode, and an electrolyte are enclosed in a bag-like sealed container.

[0003] Sealed containers are required to have properties that prevent gas permeation and moisture penetration from the outside. For this reason, laminated films, in which a metal layer such as aluminum foil is coated with resin, are used as materials for sealed containers. Sealed containers are formed by heat-sealing the edges of two sheets of laminated film.

[0004] One end of the sealed container is an opening, and the positive electrode, negative electrode, electrolyte, etc. are enclosed within this opening. Furthermore, lead conductors, one end of which is connected to the positive electrode and negative electrode, are arranged so as to extend from the inside to the outside of the sealed container. The opening is then closed by heat sealing (thermal fusion), and the sealed container and the lead conductor are bonded to seal the opening.

[0005] The surface of the sealed container is made of resin, while the lead conductor is made of metal, which are different materials. Therefore, the adhesive strength at the interface between the sealed container and the lead conductor is weak. Therefore, in order to increase the adhesive strength between the sealed container and the lead conductor, a technique of providing an adhesive film between them has been proposed (Patent Document 1, Patent Document 2).

[0006] International Publication No. 2019 / 244971 International Publication No. 2021 / 090951

[0007] The adhesive film of the present disclosure is an adhesive film comprising: a first layer including a first main surface and a second main surface opposite to the first main surface; and a second layer provided on the second main surface, wherein the first main surface is exposed to the outside; the first layer is an acid-modified polyolefin layer; and the second layer is a heat-resistant resin layer. The adhesive film is immersed in hexane at 40°C for 24 hours, and then in a first image obtained by observing the first main surface at 10,000x magnification using a scanning electron microscope, the first main surface includes recesses, and the area ratio of the recesses in the first image is 2.0% or more.

[0008] FIG. 1 is a cross-sectional view of an adhesive film according to embodiment 1. FIG. 2 is an image obtained by immersing an adhesive film in hexane at 40° C. for 24 hours and then binarizing an image obtained by observing the first main surface with a scanning electron microscope at 10,000 magnifications. FIG. 3 is a cross-sectional view of another example of an adhesive film according to embodiment 1. FIG. 4 is a cross-sectional view of another example of an adhesive film according to embodiment 1. FIG. 5 is a cross-sectional view of another example of an adhesive film according to embodiment 1. FIG. 6 is a cross-sectional view of a tab lead according to embodiment 2. FIG. 7 is a front view of an electricity storage device according to embodiment 3. FIG. 8 is a partial cross-sectional view taken along line VIII-VIII′ in FIG. 7.

[0009] [Problems to be Solved by the Present Disclosure] In recent years, there has been an increasing demand for reliability in batteries, and there is also a demand for the adhesive film to have improved adhesion to the lead conductors.

[0010] Therefore, an object of the present disclosure is to provide an adhesive film that has high adhesive strength to lead conductors when used in an electricity storage device.

[0011] Effect of the Present Disclosure According to the present disclosure, it is possible to provide an adhesive film that has high adhesive strength to lead conductors when used in an electricity storage device.

[0012] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be described below. (1) An adhesive film of the present disclosure is an adhesive film comprising: a first layer including a first main surface and a second main surface opposite to the first main surface; and a second layer provided on the second main surface, wherein the first main surface is exposed to the outside, the first layer is an acid-modified polyolefin layer, and the second layer is a heat-resistant resin layer, and the adhesive film is immersed in hexane at 40°C for 24 hours, and then the first main surface is observed at 10,000 times with a scanning electron microscope, and in a first image obtained by observing the first main surface, the first main surface includes recesses, and the area ratio of the recesses in the first image is 2.0% or more.

[0013] According to the present disclosure, it is possible to provide an adhesive film that has high adhesive strength to lead conductors when used in an electricity storage device.

[0014] (2) In the above (1), the area ratio of the recesses may be 2.0% or more and 20% or less. When the area ratio of the recesses is 20% or less, the adhesive film has high adhesive strength to the lead conductors and the material strength of the first layer is good.

[0015] (3) In the above (1) or (2), the first layer may have an average thickness of 20 μm or more and 200 μm or less. If the average thickness of the first layer is 20 μm or more, the first layer is less likely to be too thin and break, and good adhesion to the lead conductor can be ensured. If the average thickness of the first layer is 200 μm or less, the adhesive film can be made thin, and when the adhesive film is placed between the lead conductor and the sealed container and heat-sealed, unnecessary steps do not occur in the sealed portion.

[0016] (4) In any of the above (1) to (3), the acid-modified polyolefin layer may be an acid-modified polypropylene layer. The acid-modified polypropylene layer has excellent adhesion and sealing properties with metal. Therefore, an adhesive film having an acid-modified polypropylene layer as the first layer has particularly excellent adhesion to lead conductors.

[0017] (5) In any of the above (1) to (4), the acid-modified polyolefin layer may contain at least one rubber selected from the group consisting of ethylene propylene rubber, ethylene butene rubber, ethylene octene rubber, and propylene butene rubber. These rubber components can be dispersed in small particles in the acid-modified polyolefin layer to form a sea-island structure. This allows the dispersed rubber components to alleviate residual stress generated when the adhesive film and the lead conductor are heat-sealed, further improving the adhesive strength between the adhesive film and the lead conductor.

[0018] (6) In any one of the above (1) to (5), the heat-resistant resin layer may contain homopolypropylene, which allows the shape of the film to be maintained when the adhesive film and the lead conductor are heat-sealed.

[0019] (7) In any of the above (1) to (6), the adhesive film may have an elastic modulus of 600 MPa or more when measured on the first main surface of the first layer using a nanoindenter at 25° C. after immersing the adhesive film in hexane at 40° C. for 24 hours. This prevents the adhesive film from becoming too thin when it is adhered to an enclosed container.

[0020] (8) A tab lead of the present disclosure is a tab lead comprising: a lead conductor; and an adhesive film according to any one of (1) to (7) above attached to a portion of the lead conductor, wherein the first layer of the adhesive film is arranged in contact with the lead conductor.

[0021] In the tab lead of the present disclosure, the adhesive film and the lead conductor are bonded with high adhesive strength, making the tab lead suitable for use in an electricity storage device.

[0022] (9) The presently disclosed energy storage device is 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 an enclosure that seals the battery cell, wherein a portion of the lead conductor is exposed to the outside of the enclosure; and an adhesive film according to any one of (1) to (7) above is disposed between the lead conductor and the enclosure, and the first layer of the adhesive film is disposed in contact with the lead conductor.

[0023] In the electricity storage device of the present disclosure, the adhesive film and the lead conductors are bonded with high adhesive strength, which allows the electricity storage device to have excellent reliability.

[0024] (10) In the above (9), the electrolyte may be a solid electrolyte, and the electricity storage device can have excellent reliability.

[0025] (11) In the above (9), the electrolyte may be a non-aqueous electrolyte, and the electricity storage device can have excellent reliability.

[0026] [Details of the embodiments of the present disclosure] Specific examples of the adhesive film, tab lead, and electricity storage device of the present disclosure will be described below with reference to the drawings. In the drawings of the present disclosure, the same reference symbols represent the same or corresponding parts. Furthermore, dimensional relationships such as length, width, thickness, and depth have been changed as appropriate for clarity and simplification of the drawings, and do not necessarily represent actual dimensional relationships.

[0027] In the present disclosure, the notation in the form of "A to B" means the upper and lower limits of a range (i.e., A or more and B or less), and when no unit is specified for A and a unit is specified only for B, the units of A and B are the same.

[0028] In the present disclosure, when a compound or the like is represented by a chemical formula, unless the atomic ratio is particularly limited, it is intended to include any conventionally known atomic ratio, and should not necessarily be limited to only those within the stoichiometric range.

[0029] In the present disclosure, when one or more numerical values ​​are recited as the lower limit and the upper limit of a numerical range, a combination of any one numerical value recited as the lower limit and any one numerical value recited as the upper limit is also considered to be disclosed. For example, when a1 or more, b1 or more, and c1 or more are recited as the lower limit and a2 or less, b2 or less, and c2 or less are recited as the upper limit, a1 or more and a2 or less, a1 or more and b2 or less, a1 or more and c2 or less, b1 or more and a2 or less, b1 or more and b2 or less, b1 or more and c2 or less, c1 or more and a2 or less, c1 or more and b2 or less, and c1 or more and c2 or less are considered to be disclosed.

[0030] [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 FIG. 1. The adhesive film 1 of Embodiment 1 comprises a first layer 11 including a first main surface 11a and a second main surface 11b opposite the first main surface 11a, and a second layer 12 provided on the second main surface 11b. The first main surface 11a is exposed to the outside. The first layer 11 is an acid-modified polyolefin layer. The second layer 12 is a heat-resistant resin layer. In a first image obtained by immersing the adhesive film 1 in hexane at 40°C for 24 hours and then observing the first main surface 11a with a scanning electron microscope at 10,000x magnification, the first main surface 11a includes recesses. In the first image, the area ratio of the recesses is 2.0% or more.

[0031] The adhesive film of embodiment 1 has high adhesive strength to lead conductors when used in an electricity storage device, and the reason for this is presumed to be as follows.

[0032] (i) The first layer of the adhesive film of embodiment 1 is an acid-modified polyolefin layer. The acid-modified polyolefin layer can be melted by heat during heat sealing between the adhesive film and the lead conductor, and has good adhesive strength to the lead conductor made of metal. Therefore, when the first layer of the adhesive film of embodiment 1 is placed in contact with the lead conductor and heat-sealed, the adhesive film can have high adhesive strength to the lead conductor.

[0033] (ii) During cooling after heat-sealing the adhesive film and the lead conductor, the adhesive film and the lead conductor may shrink, generating stress and causing peeling between the adhesive film and the lead conductor. In the adhesive film of embodiment 1, the first main surface includes a recess in a first image obtained by observing the first main surface at 10,000x magnification using a scanning electron microscope after immersion in hexane at 40°C for 24 hours. It is presumed that the recess was formed by the elution of a rubber component present on the first main surface of the first layer into hexane. The rubber component that elutes into hexane has a particularly low elastic modulus and a high stress relaxation effect. Therefore, peeling between the adhesive film and the lead conductor during cooling is suppressed, and the adhesive film can have high adhesive strength to the lead conductor.

[0034] <First Layer> In the adhesive film 1 of embodiment 1, the first layer 11 has a first main surface 11 a and a second main surface 11 b, and the first main surface 11 a is exposed to the outside. The first main surface 11 a of the first layer 11 of the adhesive film 1 is the surface that comes into contact with and is bonded to a lead conductor in a tab lead or an electricity storage device.

[0035] The first layer is an acid-modified polyolefin layer. Acid-modified polyolefins are polyolefins containing acid-modified groups that have been 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. An acid-modified polypropylene layer is preferred for the first layer because of its excellent adhesion and sealing properties with metals. A maleic anhydride-modified polypropylene layer is preferred for the first layer because of its extremely excellent adhesion and sealing properties with metals. The acid-modified polyolefin layer may contain an unmodified polyolefin together with the acid-modified polyolefin as a resin component, as long as the effects of the present disclosure are not impaired. Hereinafter, acid-modified polyolefins and unmodified polyolefins will be collectively referred to as polyolefins.

[0036] The presence of an acid-modified group in the first layer can be confirmed by extracting only the first layer from the adhesive film and subjecting the first layer to transmission light analysis using a Fourier transform infrared spectrophotometer. Examples of acid-modified groups include maleic acid, acrylic acid, methacrylic acid, and maleic anhydride. The content of the acid-modified group in the first layer can be 0.01% by mass or more and 1.0% by mass or less. The presence of a polyolefin in the first layer can be confirmed by extracting only the first layer from the adhesive film and subjecting the first layer to reflection light analysis using a Fourier transform infrared spectrophotometer, or by subjecting the first layer to analysis using gas chromatography mass spectrometry.

[0037] The adhesive film of embodiment 1 is immersed in hexane at 40°C for 24 hours, and then the first principal surface is observed at 10,000x magnification using a scanning electron microscope (measurement conditions: accelerating voltage 2 kV, pretreatment: carbon coating). In a first image obtained by this observation, the first principal surface includes recesses, and the area ratio of the recesses in the first image is 2.0% or more. This indicates that the first layer has high adhesive strength to the lead conductor.

[0038] In the first image of the adhesive film of embodiment 1, the lower limit of the area ratio of the recesses is 2.0% or more, or may be 2.2% or more, 2.6% or more, 3.1% or more, 5.0% or more, or 7.0% or more, from the viewpoint of improving adhesive strength. The upper limit of the area ratio of the recesses may be 20% or less, or 15% or less, from the viewpoint of increasing the material strength of the first layer. The area ratio of the recesses may be 2.0% or more and 20% or less, 2.2% or more and 20% or less, 2.6% or more and 20% or less, 5.0% or more and 15% or less, or 7.0% or more and 15% or less.

[0039] In the present disclosure, the area ratio of recesses in the first image is measured by the following procedure.

[0040] An adhesive film to be measured is prepared. If a first layer of the adhesive film is bonded to a lead conductor and the first main surface of the first layer is not exposed to the outside, the adhesive film and the lead conductor are immersed in an acid such as dilute hydrochloric acid to dissolve the lead conductor, thereby preparing an adhesive film with an exposed first main surface. The type of acid is selected appropriately depending on the material of the lead conductor.

[0041] The adhesive film to be measured is immersed in hexane at 40° C. for 24 hours, then removed from the hexane and allowed to dry naturally.

[0042] The first principal surface is observed at 10,000x magnification using a scanning electron microscope to obtain a first image. The first image is then binarized using the image processing software ImageJ (https: / / imagej.nih.gov / ij / index.html). Specifically, the first image is acquired as a grayscale image (JPEG) digital file and processed according to the binarization procedure and parameters below, outputting pixels with a gradation equal to or greater than a threshold (bright) as 1 and pixels with a gradation less than the threshold (dark) as 0. Pixels with a gradation less than the threshold (dark) are defined as recesses.

[0043] <Binarization processing> 1. Spike noise removal (Despeckle) 2. Island contour removal (Remove Outliers radius = 50 pixels, threshold = 0, which = Bright) 3. Sea contour removal (Remove Outliers radius = 2 pixels, threshold = 0, which = Dark) 4. Automatic binarization (Yen method)

[0044] A rectangular measurement field of 8 μm × 11 μm is set in the binarized image. Using the image processing software ImageJ, the percentage (S2 / S1) × 100 of the area S2 of the recesses represented by pixels with a gradation below the threshold (dark) to the area S1 of the entire measurement field is calculated.

[0045] The percentage (S2 / S1) x 100 is measured for the first images acquired in five mutually non-overlapping regions. The average of the percentages (S2 / S1) x 100 for the five measurement fields of view is calculated. In the present disclosure, this average corresponds to the area ratio of the recesses in the first image.

[0046] As far as the applicant has measured, it has been confirmed that, as long as measurements are taken on the same sample, there is almost no variation in the measurement results even if the acquisition area of ​​the first image is arbitrarily set.

[0047] Figure 2 shows an image obtained by performing the above-described binarization process using image processing software ImageJ (https: / / imagej.nih.gov / ij / index.html) on an image obtained by observing the first principal surface of the adhesive film of embodiment 1 at 10,000x magnification using a scanning electron microscope after immersion in hexane at 40°C for 24 hours. Figure 2 confirms the presence of recesses, represented by pixels with a gradation (dark) below the threshold, on the first surface after hexane immersion.

[0048] The adhesive film of embodiment 1 may have an elastic modulus of 600 MPa or more when measured on the first main surface of the first layer using a nanoindenter at 25°C after immersion in hexane at 40°C for 24 hours. This prevents the adhesive film from becoming too thin when adhered to an enclosed container.

[0049] In the present disclosure, the modulus of elasticity at 25°C of the first main surface of the first layer is obtained by immersing the adhesive film in hexane at 40°C for 24 hours and then measuring the modulus of elasticity of the first main surface using a nanoindenter. The modulus of elasticity is measured in an area of ​​the first main surface where no recesses are formed. The nanoindenter used is a TriboIndenter TI980 manufactured by HYSITRON. The nanoindenter uses a regular triangular pyramid indenter (Berkovich indenter) with a diamond tip. Using the nanoindenter, the indenter is pressed perpendicularly into the first main surface of the first layer under the following measurement conditions, and the load-displacement curve is measured to calculate the modulus of elasticity. (1) Indentation time: 3 seconds (2) Holding time: 0 seconds (3) Unloading time: 0 seconds (4) Load application rate: 8 mN / second (5) Indentation load: 0.5 mN to 5 mN (adjust as appropriate so that the indentation size is approximately 10 μm to 20 μm). (6) Indentation depth reaching time: 5 seconds (7) Load holding time: 0 seconds (8) Indentation depth unloading time: 5 seconds

[0050] In the adhesive film of embodiment 1, the acid-modified polyolefin layer may contain a rubber component. At least a portion of the rubber component is exposed on the first main surface of the first layer. At least a portion of the rubber component exposed on the first main surface is soluble in hexane at 40°C. Examples of the rubber component include ethylene propylene rubber, ethylene butene rubber, ethylene octene rubber, and propylene butene rubber.

[0051] In the adhesive film of embodiment 1, the acid-modified polyolefin layer may contain at least one selected from the group consisting of ethylene propylene rubber, ethylene butene rubber, ethylene octene rubber, and propylene butene rubber. Among these, ethylene propylene rubber is preferred because of its excellent dispersibility in acid-modified polyolefins, especially acid-modified polypropylene. When the acid-modified polyolefin layer contains ethylene propylene rubber, the adhesive strength between the adhesive film and the lead conductor is further improved.

[0052] The acid-modified polyolefin layer of embodiment 1 can be composed of a polyolefin and a rubber component. The acid-modified polyolefin layer of embodiment 1 can contain impurities in addition to the polyolefin and the rubber component, as long as the effects of the present disclosure are not impaired.

[0053] From the viewpoint of suppressing peeling between the adhesive film and the lead conductor, the content of the rubber component in the acid-modified polyolefin layer may be 5% by volume or more and 40% by volume or less, 10% by volume or more and 35% by volume or less, or 10% by volume or more and 30% by volume or less. When the acid-modified polyolefin layer contains two or more types of rubber, the content of the rubber component means the total content of all the rubbers.

[0054] The rubber component content of the acid-modified polyolefin layer is measured by the following procedure: The adhesive film is cut along an imaginary plane perpendicular to the first main surface to prepare a thin section exposing the cross section of the acid-modified polyolefin layer. The thin section is stained with a heavy metal (such as ruthenium tetroxide) and observed under a transmission microscope. In the transmission microscope image, the area percentage A (area %) of the stained portion relative to the entire area of ​​the transmission microscope image is measured. The area percentage A (area %) is measured in five non-overlapping regions, and the average B (area %) of these area percentages A is calculated. In the present disclosure, the average B (area %) is considered to be the volume-based rubber component content (volume %) of the acid-modified polyolefin layer.

[0055] As far as the applicant has measured, it has been confirmed that, as long as measurements are taken on the same sample, there is almost no variation in the measurement results of the rubber component content of the acid-modified polyolefin layer, even if the position at which the above-mentioned thin section is obtained and the observation area in the transmission microscope image are arbitrarily set.

[0056] The melt flow rate of the rubber component in the acid-modified polyolefin layer may be 0.2 g / 10 min or more and 30 g / 10 min or less, or 1.8 g / 10 min or more and 5.4 g / 10 min or less. This makes it easier for the rubber component to be exposed to the first main surface of the first layer. In the present disclosure, the melt flow rate is measured in accordance with ISO 1133-1:2011. The measurement temperature is 230°C and the load is 2.16 kg.

[0057] In the adhesive film of embodiment 1, the average thickness of the first layer may be 20 μm or more and 200 μm or less. The lower limit of the average thickness of the first layer may be 20 μm or more, 25 μm or more, or 30 μm or more, from the viewpoint of ensuring good adhesion to the lead conductor. The upper limit of the average thickness of the first layer may be 200 μm or less, 150 μm or less, 120 μm or less, or 100 μm or less, from the viewpoint of miniaturizing an electricity storage device including the adhesive film. The average thickness of the first layer may be 20 μm or more and 150 μm or less, 25 μm or more and 120 μm or less, or 30 μm or more and 100 μm or less.

[0058] In the present disclosure, the method for measuring the average thickness of the first layer is as follows: The adhesive film is cut along the normal direction of the first main surface of the adhesive film using a microtome or the like to expose a cross section. The cross section is observed at 500x magnification using a digital microscope, and the thickness of the first layer is measured at five locations. The average of the thicknesses at the five locations is calculated. This average corresponds to the average thickness of the first layer. In the present disclosure, the average thicknesses of the second layer 12, other layers, and adhesive film described below are also measured in a similar manner.

[0059] As far as the applicant has measured, it has been confirmed that, as long as measurements are taken on the same sample, there is almost no variation in the measurement results even if the cutting points of the adhesive film and the points at which the thickness is measured are arbitrarily set.

[0060] In the present disclosure, the first layer may contain various additives such as a flame retardant, an ultraviolet absorber, a light stabilizer, a heat stabilizer, a lubricant, a colorant, etc. The types and amounts of these additives may be similar to those used in conventionally known acid-modified polyolefin layers.

[0061] <Second Layer> In the adhesive film of embodiment 1, the second layer is a heat-resistant resin layer. The second layer functions as a support for the adhesive film. The second layer is disposed between the first layer and the enclosure in the electricity storage device.

[0062] Examples of heat-resistant resins constituting the second layer include polyolefin resins, polyethylene terephthalate, polyamides, and fluororesins. Among these, polyolefin resins are preferred due to their excellent adhesion to the acid-modified polyolefin layer constituting the first layer. Examples of polyolefin resins include polyethylene, polypropylene, ionomer resins, and acid-modified polyolefins. In particular, the second layer preferably contains homopolypropylene, a type of polypropylene, and more preferably consists of homopolypropylene. Homopolypropylene has excellent support properties because it allows for the thermocompression bonding temperature to be adjusted so that the adhesive film and the lead conductor do not melt during thermocompression bonding. The heat-resistant resin constituting the second layer may be a crosslinked product obtained by irradiation with ionizing radiation such as accelerated electron beams or gamma rays.

[0063] The heat-resistant resin constituting the second layer may be a cross-linked polypropylene, which does not melt even when the temperature for thermocompression bonding between the adhesive film and the lead conductor is increased, and therefore functions well as a support.

[0064] The heat-resistant resin constituting the second layer may be polyethylene naphthalate (PEN), or may be polyester, in which case the second layer may be a polyester nonwoven fabric.

[0065] In the adhesive film of embodiment 1, the average thickness of the second layer may be 20 μm or more and 150 μm or less, 30 μm or more and 120 μm or less, or 40 μm or more and 100 μm or less.

[0066] In the present disclosure, the second layer may contain various additives such as a flame retardant, an ultraviolet absorber, a light stabilizer, a heat stabilizer, a lubricant, a colorant, etc. The types and amounts of these additives may be the same as those used in conventionally known heat-resistant resin layers.

[0067] <Other Layers> The adhesive film of embodiment 1 may include other layers in addition to the first and second layers. Examples of the other layers include a layer (hereinafter also referred to as the "third layer") that improves adhesion to an aluminum foil-containing enclosure, and a layer (hereinafter also referred to as the "fourth layer") that traps hydrofluoric acid, which corrodes lead conductors. The third layer is required to have strong adhesion to the enclosure of the energy storage device. Depending on the material of the inner surface of the enclosure (the surface in contact with the adhesive film), if the inner surface of the enclosure is polyolefin, the third layer is preferably also polyolefin. The other layer may be a single layer, or two or more layers may be combined. The position of the other layer is appropriately selected depending on the type of the other layer.

[0068] When a third layer is included as another layer, as shown in Figure 3, the adhesive film 1 can include a first layer 11 including a first main surface 11a and a second main surface 11b opposite to the first main surface 11a, a second layer 12 provided on the second main surface 11b, and a third layer 17 provided on the main surface of the second layer 12 opposite to the main surface facing the first layer 11.

[0069] When a fourth layer is included as another layer, as shown in Figure 4, the adhesive film 1 can include a first layer 11 including a first main surface 11a and a second main surface 11b opposite to the first main surface 11a, a fourth layer 18 provided in contact with the second main surface 11b, and a second layer 12 provided on the main surface of the fourth layer 18 opposite to the main surface facing the first layer 11.

[0070] When the adhesive film 1 includes a third layer and a fourth layer as other layers, as shown in Figure 5, the adhesive film 1 can include a first layer 11 including a first main surface 11a and a second main surface 11b opposite the first main surface 11a, a fourth layer 18 provided in contact with the second main surface 11b, a second layer 12 provided on the main surface of the fourth layer 18 opposite the main surface facing the first layer 11, and a third layer 17 provided on the main surface of the second layer 12 opposite the main surface facing the fourth layer 18.

[0071] In the adhesive film of embodiment 1, the average thickness of the third layer may be 20 μm or more and 100 μm or less, 30 μm or more and 70 μm or less, or 35 μm or more and 60 μm or less. In the adhesive film of embodiment 1, the average thickness of the fourth layer may be 20 μm or more and 100 μm or less, 30 μm or more and 70 μm or less, or 35 μm or more and 60 μm or less.

[0072] In the present disclosure, the other layers may contain various additives such as flame retardants, ultraviolet absorbers, light stabilizers, heat stabilizers, lubricants, colorants, etc. The types and amounts of these additives may be similar to those used in other conventionally known layers.

[0073] <Adhesive Film> The lower limit of the average thickness of the adhesive film of embodiment 1 may be 40 μm or more, 50 μm or more, 70 μm or more, 80 μm or more, 100 μm or more, or 120 μm or more. The upper limit of the average thickness of the adhesive film may be 500 μm or less, 300 μm or less, or 250 μm or less. The average thickness may be 40 μm or more to 500 μm or less, 80 μm or more to 500 μm or less, 100 μm or more to 300 μm or less, or 120 μm or more to 250 μm or less.

[0074] The adhesive film of embodiment 1 is produced, for example, by the following method. First, an acid-modified polyolefin and a rubber component are prepared as raw materials for the first layer, and a heat-resistant resin is prepared as raw material for the second layer. If at least one of the first and second layers contains an additive, the additive is prepared. The raw materials for the first and second layers are mixed using a known mixing device such as an open roll, a pressure kneader, a single-screw kneader, or a twin-screw kneader to obtain a mixed raw material.

[0075] The mixed raw materials for the first and second layers can be extruded using a co-extrusion T-die extruder to obtain an adhesive film in which the first and second layers are laminated. The film formation conditions for the T-die extruder are, for example, a die lip opening of 0.3 to 1.0 mm and a take-up speed of 2 to 20 m / min. The thickness of the first layer after extrusion is adjusted to 20 to 200 μm.

[0076] By adjusting the amount of rubber component in the raw material of the first layer, the melt flow rate of the rubber component, and the film-forming conditions, the rubber component is more likely to be exposed on the first main surface of the first layer. For example, when the amount of rubber component is small, the higher the take-up speed, the more likely the rubber component is exposed on the first main surface. When the melt flow rate of the rubber component is small, the higher the take-up speed, the more likely the rubber component is exposed on the first main surface. By adjusting the film-forming conditions for the first layer in the T-die extruder within the above range, the rubber component is exposed on the first main surface of the first layer, which can contribute to stress relaxation at the interface between the lead conductor and the adhesive film. Furthermore, by adjusting the film-forming conditions for the first layer in the T-die extruder within the above range, the rubber component dissolves in hexane, making it possible to measure the amount of rubber component in the first layer present at the interface between the first layer and the lead conductor. The relationship between these film-forming conditions and the amount of rubber component exposed on the first main surface of the first layer was newly discovered by the present inventors.

[0077] Differences Between the Present Invention and Patent Documents 1 and 2 The present inventors developed the adhesive film of the present disclosure by focusing on the condition of the surface in contact with the lead conductor, i.e., the first main surface of the first layer, in order to obtain an adhesive film with high adhesion to the lead conductor. Specifically, by having a predetermined amount or more of a rubber component soluble in hexane at 40°C present on the first main surface of the first layer made of an acid-modified polyolefin layer, high adhesion strength to the lead conductor of the first main surface is achieved. Rubber components soluble in hexane at 40°C or higher have a particularly low elastic modulus and a high stress relaxation effect. Therefore, peeling between the adhesive film of embodiment 1 and the lead conductor due to stress generated during the cooling process is suppressed. The present inventors have newly discovered that high adhesion strength of the adhesive film to the lead conductor can be achieved by focusing on the proportion of the rubber component soluble in hexane at 40°C present on the first main surface of the first layer.

[0078] Patent Documents 1 and 2 disclose that the polyolefin layer disposed on the metal terminal (corresponding to the lead conductor) side of an adhesive film for metal terminals has a sea-island structure. In Patent Documents 1 and 2, the sea-island structure is observed in a cross section of the polyolefin layer in a direction parallel to the TD and in the thickness direction. Therefore, Patent Documents 1 and 2 do not disclose or suggest information regarding the state of the first main surface that contacts the lead conductor, which is the focus of the adhesive film of the present disclosure. With regard to adhesion to the lead conductor, the state of the first main surface that contacts the lead conductor is more important than the state of the cross section of the polyolefin layer in a direction parallel to the TD and in the thickness direction. The state of the cross section of the polyolefin layer in a direction parallel to the TD and in the thickness direction specified in Patent Documents 1 and 2 does not affect adhesion to the lead conductor.

[0079] In Patent Documents 1 and 2, polyolefin layers are dyed with ruthenium tetroxide to observe the sea-island structure. In this case, the sea portion (sea part) and the island portion (island part) are dyed differently based on the difference in dyeing speed caused by differences in crystallinity. The distinction between the sea portion and the island part by dyeing merely indicates the difference in crystallinity between the sea portion and the island part, and it is unclear whether the island parts in Patent Documents 1 and 2 are soluble in hexane. Therefore, the island parts shown in Figure 9 of Patent Document 1 and Figures 8 to 13 of Patent Document 2 do not represent a rubber component soluble in hexane. Therefore, Patent Documents 1 and 2 do not disclose or suggest a rubber component soluble in hexane.

[0080] [Embodiment 2: Tab Lead] A tab lead according to an embodiment of the present disclosure (hereinafter also referred to as "Embodiment 2") will be described with reference to Fig. 6. The tab lead of Embodiment 2 includes a lead conductor 3 and the adhesive film of Embodiment 1 attached to a portion of the lead conductor 3. In the tab lead, a first layer of the adhesive film is disposed in contact with the lead conductor 3.

[0081] In the tab lead of embodiment 2, the adhesive film and the lead conductor are bonded with high adhesive strength, making the tab lead particularly suitable for electricity storage devices including all-solid-state batteries.

[0082] In the second embodiment, metals such as aluminum, nickel, copper, and nickel-plated copper are used as the lead conductor. Aluminum is often used for the positive electrode, and aluminum, nickel, and nickel-plated copper are often used for the negative electrode. The shape of the lead conductor is not particularly limited, but a flat metal plate having 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.

[0083] [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 FIGS. 7 and 8. FIG. 7 is a front view schematically illustrating an energy storage device 30 according to Embodiment 3. FIG. 8 is a partial cross-sectional view taken along line VIII-VIII' in FIG. 7. The energy storage device 30 according to Embodiment 3 includes a battery cell 15 including 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 an enclosure 2 that seals the battery cell 15. A portion of the lead conductor 3 is exposed to the outside of the enclosure 2, and the adhesive film 1 according to Embodiment 1 is disposed between the lead conductor 3 and the enclosure 2. A first layer 11 of the adhesive film is disposed in contact with the lead conductor 3.

[0084] In the electricity storage device of embodiment 3, the adhesive film 1 and the lead conductor 3 are bonded with high adhesive strength. Therefore, the electrolyte does not escape from the sealed container, and the electricity storage device can have excellent reliability.

[0085] As shown in FIG. 8 , the sealed container 2 comprises a three-layer laminate film 8 consisting of a metal layer 5 and first and second resin layers 6 and 7 covering the metal layer 5. The metal layer 5 is formed from a metal such as aluminum foil. The first resin layer 6, located on the outside of the sealed container 2, can be made of a polyamide resin such as 6,6-nylon or 6-nylon, a polyester resin, or a polyimide resin. The second resin layer 7, located on the inside of the sealed container 2, is preferably made of an insulating resin that is insoluble in nonaqueous electrolytes and melts when heated. Examples of such insulating resins include polyolefin resins, acid-modified polyolefin resins, and acid-modified styrene-based elastomers. The sealed container 2 is fabricated by overlapping two laminate films 8 and heat-sealing the three edges of the laminate film 8, excluding the edge through which the lead conductor 3 passes. At the outer periphery of the sealed container 2, the two metal layers 5 are bonded via the second resin layer 7.

[0086] The lead conductor 3 is bonded (thermally fused) to the sealing container (laminate film) via the adhesive film 1 at a seal portion 9. A positive electrode 10, a negative electrode 14, and an electrolyte 13 are further enclosed inside the electricity storage device 30. FIG. 8 shows the lead conductor connected to the negative electrode 14. A lead conductor connected to the positive electrode 10 also exists, but is not shown in FIG. 8. In the third embodiment, the electrolyte 13 may be a solid electrolyte. This maintains high adhesive strength of the adhesive film to the lead conductor 3 for a long period of time, further improving the reliability of the electricity storage device 30. In the third embodiment, the electrolyte 13 may be a non-aqueous electrolyte. Examples of non-aqueous electrolytes include LiPF ６ , LiBF ４ Examples of suitable lithium salts include those obtained by dissolving fluorine-containing lithium salts such as those mentioned above in diethyl carbonate (DEC), dimethyl carbonate (DMC), propylene carbonate (PC), or the like.

[0087] [Supplementary Note 1] In the adhesive film of the present disclosure, the acid-modified polyolefin layer may contain a rubber component, and the rubber component content of the acid-modified polyolefin layer may be 5% by volume or more and 40% by volume or less. In the adhesive film of the present disclosure, the acid-modified polyolefin layer may contain a rubber component, and the rubber component content of the acid-modified polyolefin layer may be 10% by volume or more and 35% by volume or less. In the adhesive film of the present disclosure, the acid-modified polyolefin layer may contain a rubber component, and the rubber component content of the acid-modified polyolefin layer may be 10% by volume or more and 30% by volume or less.

[0088] [Supplementary Note 2] In the adhesive film of the present disclosure, the content of acid-modified groups in the first layer may be 0.01% by mass or more and 1.0% by mass or less.

[0089] [Preparation of adhesive film] The following materials were prepared for the first layer: Random PP: random polypropylene with a melting point of 139°C and a melt flow rate (MFR) of 5 g / 10 min; Maleic anhydride-modified random PP: maleic anhydride-modified random polypropylene with a melting point of 157°C, a MFR of 10 g / 10 min, and an acid modification rate of 1.2 mass%; Ethylene propylene rubber [A]: melting point of 50°C or less, density of 0.869 g / cm ３ , MFR 5.4 g / 10 min Ethylene propylene rubber [B]: melting point 50 ° C or less, density 0.869 g / cm ３ , ethylene propylene rubber with MFR 1.8g / 10min

[0090] The raw materials for the first layer were mixed using a twin-screw kneader in the mass ratios shown in Table 1 to obtain mixed raw materials PP32 to PP36. For example, the mass ratio of PP32 was random PP: maleic anhydride-modified random PP: ethylene propylene rubber [A] = 90:5:5.

[0091]

[0092] As the raw material for the second layer, homopolypropylene (hereinafter also referred to as "PP2") with a melting point of 166°C and an MFR of 7 g / 10 min was prepared. As the raw material for the third layer, random PP (hereinafter also referred to as "PP1") with a melting point of 142°C and an MFR of 6.7 g / 10 min was prepared. The raw materials for the second and third layers were also kneaded using a twin-screw kneader.

[0093] The raw materials for the first layer, second layer, and third layer were extruded using a co-extrusion T-die extruder to obtain adhesive films for each sample in which the first layer, second layer, and third layer were laminated in the above order. The film-forming conditions (take-up speed and die lip opening) are as shown in the "Film-forming conditions" column in Table 2. The average thicknesses of the first layer, second layer, and third layer, and the average total thickness of the adhesive film obtained are as shown in Table 2.

[0094] [Measurement of Adhesive Film] <Area Ratio of Recesses> After immersing the adhesive film of each sample in hexane at 40°C for 24 hours, the exposed surface on the first layer side (corresponding to the first main surface) was observed at 10,000x magnification using a scanning electron microscope to obtain a first image. In the first image, the area ratio of recesses was measured. The specific measurement method is as described in embodiment 1. The results are shown in the "Recess Area Ratio" column of "First Layer" under "Adhesive Film" in Table 2. Table 2 also shows the elastic modulus (25°C) measured using a nanoindenter at locations other than the recesses on the exposed surface of the first layer of the same sample.

[0095] The second and third layers were also immersed in hexane at 40° C. for 24 hours, and then the elastic modulus of each surface was measured using a nanoindenter at 25° C. The results are shown in Table 2.

[0096] <Rubber Component Content of First Layer (Acid-Modified Polyolefin Layer)> The rubber component content of the first layer (acid-modified polyolefin layer) of each sample adhesive film was measured. The specific measurement method is as described in Embodiment 1. The results are shown in the "Rubber Component Content" column of "First Layer" under "Adhesive Film" in Table 2.

[0097]

[0098] [Evaluation] <Peeling test> A flat conductor made of an aluminum plate with a length of 100 mm, a width of 45 mm, and an average thickness of 0.4 mm was used as the substrate of the lead conductor. Chromium chloride hexahydrate (5.0 g / L) and potassium formate (170 g / L) were mixed with pure water to obtain a surface treatment solution. The lead conductor was immersed in the surface treatment solution at 45°C, and a current density of 10 A / dm was applied. ２ Cathodic electrolysis was performed at 400°C for 10 seconds. This resulted in a lead conductor with a conductor coating. A first layer of each adhesive film (10 mm long, 55 mm wide) was placed on one main surface of the lead conductor and pressed at 210°C for 10 seconds to form a test sample. For each test sample, the conductor was cut 10 mm from the widthwise end, leaving the insulating film on the surface, and the conductor was bent. The adhesive film was pulled 180° from the lead conductor to peel it off, and the 180° peel strength was measured. The pulling speed was 100 mm / min. Measurements were performed 1 hour after adhesion. The results are shown in the "Peel Test" column in Table 2. In the peel test, an adhesive film with a peel strength of 18 N / cm or more was considered to have high adhesion to the lead conductor. The unit N / cm, used to express peel strength, represents the force (N) required to peel off a 1 cm wide film.

[0099] <Consideration 1> The adhesive films of Samples 2 to 8 correspond to Examples. It was confirmed that the adhesive films of these samples had high adhesiveness to the lead conductors.

[0100] The adhesive film of Sample 1 corresponds to a comparative example. It was confirmed that the adhesive strength of the adhesive film to the lead conductor was insufficient for Sample 1. The area ratio of the recesses in Sample 1 was less than 2.0%. This is presumably because the take-up speed during film formation was high relative to the amount of rubber component in the raw material of the first layer, making it difficult for the rubber component to appear on the first main surface of the first layer.

[0101] Although the embodiments and examples of the present disclosure have been described above, it is intended from the beginning that the configurations of the above-described embodiments and examples may be appropriately combined and modified in various ways. The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims, not by the above-described embodiments and examples, and is intended to include meanings equivalent to the claims and all modifications within the scope of the claims.

[0102] REFERENCE SIGNS LIST 1 adhesive film, 2 encapsulation container, 3 lead conductor, 5 metal layer, 6 first resin layer, 7 second resin layer, 8 laminate film, 9 seal portion, 10 positive electrode, 11 first layer, 11a first main surface, 11b second main surface, 12 second layer, 13 electrolyte, 14 negative electrode, 15 battery cell, 17 third layer, 18 fourth layer, 30 electricity storage device.

Claims

1. An adhesive film comprising a first layer including a first main surface and a second main surface opposite to the first main surface, and a second layer provided on the second main surface, wherein the first main surface is exposed to the outside, the first layer is an acid-modified polyolefin layer, and the second layer is a heat-resistant resin layer, and the adhesive film is immersed in hexane at 40°C for 24 hours, and then in a first image obtained by observing the first main surface at 10,000 times magnification using a scanning electron microscope, the first main surface includes recesses, and in the first image, the area ratio of the recesses is 2.0% or more.

2. The adhesive film according to claim 1, wherein the area ratio of the recesses is 2.0% or more and 20% or less.

3. An adhesive film according to claim 1 or 2, wherein the average thickness of said first layer is from 20 μm to 200 μm.

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

5. An adhesive film described in any one of claims 1 to 4, wherein the acid-modified polyolefin layer contains at least one rubber selected from the group consisting of ethylene propylene rubber, ethylene butene rubber, ethylene octene rubber and propylene butene rubber.

6. An adhesive film according to any one of claims 1 to 5, wherein the heat-resistant resin layer contains homopolypropylene.

7. An adhesive film according to any one of claims 1 to 6, wherein the elastic modulus of the first main surface of the first layer measured using a nanoindenter at 25°C after immersing the adhesive film in hexane at 40°C for 24 hours is 600 MPa or more.

8. A tab lead comprising: a lead conductor; and an adhesive film according to any one of claims 1 to 7 attached to a portion of the lead conductor, wherein the first layer of the adhesive film is positioned in contact with the lead conductor.

9. 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 each of the positive electrode and the negative electrode; and an enclosure that seals the battery cell, wherein a portion of the lead conductor is exposed to the outside of the enclosure, and an adhesive film described in any one of claims 1 to 7 is disposed between the lead conductor and the enclosure, and the first layer of the adhesive film is disposed in contact with the lead conductor.

10. The electricity storage device according to claim 9, wherein the electrolyte is a solid electrolyte.

11. The electricity storage device according to claim 9, wherein the electrolyte is a non-aqueous electrolyte.

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