Sealing film, tab lead, secondary battery, and resin composition

Abstract

The present invention provides a sealing film 20 in which the decrease in adhesive strength to the lead conductor 31 at high temperatures is suppressed. [Solution] A single-layer or multi-layer sealing film 20 is placed between a lead conductor 31 connected to the positive or negative electrode and the outer package 40 in a secondary battery 10 covered with an outer package 40 and heat-sealed; the sealing film 20 comprises a layer included in the sealing film 20 that is bonded to the lead conductor 31, and is made of a resin composition containing an acid-modified polyolefin (A), an epoxy group-containing polyolefin (B), and an ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C), wherein the resin composition contains 100 parts by mass of acid-modified polyolefin (A), 1 to 10 parts by mass of epoxy group-containing polyolefin (B), and 1 to 10 parts by mass of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C).

Description

Technical Field

[0001] The present invention relates to a single-layer or multi-layer sealing film that is disposed between a lead conductor connected to a positive electrode or a negative electrode and an exterior package and is thermally sealed in a secondary battery covered with the exterior package. The present invention also relates to a tab lead in which both sides of a part of the lead conductor are covered with the sealing film. The present invention further relates to a secondary battery having a power generation element including a positive electrode, a negative electrode, an electrolyte, and a separator, an exterior package that houses the power generation element and has its peripheral portion thermally sealed, a lead conductor connected to the positive electrode or the negative electrode and drawn out to the outside of the exterior package, and a sealing film disposed between the exterior package and the lead conductor and thermally sealed. The present invention still further relates to a resin composition excellent in adhesive strength at high temperatures.

Background Art

[0002] In recent years, lithium-ion secondary batteries have been mounted in various electronic devices due to their high energy density, and the market has been expanding. At the same time, in order to achieve a decarbonized society, in the mobility field, the EV conversion has been rapidly progressing from engine vehicles using petroleum-based fuels, and the development of EV vehicles equipped with lithium-ion secondary batteries has been progressing, and already many EV vehicles are running. In order to reduce the weight of lithium-ion secondary batteries, consideration has been started to mount a laminated lithium-ion secondary battery (pouch-type battery) using an exterior package in which a metal foil is laminated with a polyolefin or a nylon resin in an EV vehicle.

[0003] Figure 1 shows a schematic diagram of the external appearance of a pouch-type secondary battery 10, and Figure 2 shows a schematic diagram of its A-A' cross-section. As shown in Figures 1 and 2, the secondary battery 10 has a structure in which the positive and negative current collecting electrodes (not shown) inside the pouch surrounded by an outer package 40 are fused with lead conductors 31 using ultrasound, a hot plate, a laser, etc., and the lead conductors 31 are drawn out to the outside of the pouch. In addition, in order to improve the adhesion between the lead conductors 31 and the outer package 40, the heat-fused portion of the lead conductors 31 is covered with a sealing film 20. For this reason, tab leads 30 with the sealing film 20 fused to the lead conductors 31 are prepared in advance (see Figure 4). The lead conductors 31 drawn out to the outside are attached to a battery pack into which the secondary battery 10 is incorporated and connected to power supply terminals.

[0004] The sealing structure of a pouch-type lithium-ion secondary battery is mainly formed by heat-sealing of propylene-based resins. In many cases, as shown in Figure 3, the sealing film 20 has a three-layer structure, and the core layer 22 preferably uses a homopolymer or block copolymer of polypropylene with a high melting point to ensure insulation from the metal foil contained in the outer packaging 40. For the outer adhesive layer 21 that is heat-sealed to the outer packaging 40, a random copolymer with a low melting point is preferably used for the polypropylene, so that heat sealing with the outer packaging 40 is completed at a lower temperature. For the conductor adhesive layer 23 that adheres to the lead conductor 31, acid-modified polypropylene, which has been modified with an unsaturated carboxylic acid such as maleic anhydride to impart polar groups, is preferably used. For the base polymer of this acid-modified polypropylene, a random copolymer with a low melting point is preferably used to increase the adhesive strength with the lead conductor 31. This enhances the adhesion with the lead conductor 31 and improves the reliability of the pouch-type lithium-ion secondary battery.

[0005] On the other hand, lithium-ion secondary batteries used in electric vehicles (EVs) require higher reliability than batteries for mobile devices. For example, improved heat resistance is essential for EVs used in diverse environments. However, since the melting point of random polypropylene bonded to the lead conductor 31 is typically 130-145°C, the adhesive strength with the lead conductor 31 decreases at high temperatures, increasing the risk of electrolyte leakage. Therefore, it is necessary to achieve both adhesive strength and heat resistance when bonding the conductor adhesive layer 23 of the sealing film 20 to the lead conductor 31.

[0006] Patent Document 1 describes a tab lead in which a sealing film is fused to a lead conductor, and states that the side of the sealing film facing the lead conductor contains a heat-adhesive polyolefin resin having epoxy functional groups. The examples described therein describe an example using a polypropylene resin in which epoxy groups have been introduced by reacting maleic anhydride groups contained in polypropylene resin. However, even when using polypropylene with such functional groups, the adhesive strength at high temperatures was insufficient. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] WO2014 / 091544A1 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The present invention was made to solve the above problems, and aims to provide a sealing film in which the decrease in adhesive strength to lead conductors at high temperatures is suppressed. Another objective of the present invention is to provide tab leads coated with the sealing film. Another objective of the present invention is to provide a secondary battery using the tab leads. Furthermore, another objective of the present invention is to provide a resin composition suitably used in the sealing film. [Means for solving the problem]

[0009] The above problems are solved by the following invention. [1] A single-layer or multi-layer sealing film that is placed between a lead conductor connected to the positive or negative electrode and the outer packaging and heat-sealed in a secondary battery covered by an outer packaging; The layer contained in the sealing film and bonded to the lead conductor is a layer made of a resin composition containing an acid-modified polyolefin (A), an epoxy group-containing polyolefin (B), and an ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C). A sealing film comprising 100 parts by mass of acid-modified polyolefin (A), 1 to 10 parts by mass of epoxy group-containing polyolefin (B), and 1 to 10 parts by mass of copolymer (C). [2] The sealing film according to [1], wherein the resin composition further comprises 1 to 10 parts by mass of phenoxy resin (D) per 100 parts by mass of acid-modified polyolefin (A). [3] The sealing film according to [1] or [2], wherein the resin composition further comprises 1 to 10 parts by mass of an ethylene-vinyl acetate copolymer saponified product (E) per 100 parts by mass of an acid-modified polyolefin (A). [4] The sealing film according to any one of [1] to [3], wherein the sealing film is a multilayer film having an intermediate layer and two surface layers disposed on both sides thereof, and the surface layer on the side to be bonded to the lead conductor is a layer made of the resin composition. [5] A tab lead in which both sides of a portion of the lead conductor are covered with a sealing film as described in any one of items [1] to [4]. [6] A secondary battery comprising a power generation element including a positive electrode, a negative electrode, an electrolyte and a separator; an outer package housing the power generation element and having a heat-sealed periphery; lead conductors connected to the positive electrode or the negative electrode and leading out to the outside of the outer package; and a heat-sealed sealing film disposed between the outer package and the lead conductors; A secondary battery in which the sealing film is a sealing film according to any one of [1] to [4], and a layer made of the resin composition is bonded to the lead conductor. [7] A resin composition comprising an acid-modified polyolefin (A), an epoxy group-containing polyolefin (B), and an ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C), wherein the composition contains 1 to 10 parts by mass of epoxy group-containing polyolefin (B) and 1 to 10 parts by mass of copolymer (C) per 100 parts by mass of acid-modified polyolefin (A). [8] The resin composition according to [7], further comprising 1 to 10 parts by mass of phenoxy resin (D) per 100 parts by mass of acid-modified polyolefin (A). [9] The resin composition according to [7] or [8], further comprising 1 to 10 parts by mass of an ethylene-vinyl acetate copolymer saponified product (E) per 100 parts by mass of an acid-modified polyolefin (A). [Effects of the Invention]

[0010] The sealing sheet of the present invention suppresses the decrease in adhesive strength to lead conductors at high temperatures. Therefore, the sealing performance of pouch-type secondary batteries used at high temperatures is improved. Consequently, the reliability of pouch-type secondary batteries used in applications where operating temperatures may be high is improved, and secondary batteries suitable for automotive applications and the like can be provided. [Brief explanation of the drawing]

[0011] [Figure 1] This is the appearance of the pouch-type rechargeable battery 10. [Figure 2] This is a cross-sectional view taken along line A-A' in Figure 1. [Figure 3] This is a cross-sectional view of a three-layer film, which is a preferred embodiment of the sealing film 20 of the present invention. [Figure 4] This is the appearance of the tab lead 30 of the present invention. [Figure 5] This diagram shows a sample used for measuring adhesive strength. [Figure 6] This diagram illustrates a method for measuring adhesive strength.

Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. As shown in FIGS. 1 and 2, the sealing film 20 of the present invention is a single-layer or multi-layer sealing film 20 that is disposed between a lead conductor 31 connected to a positive electrode or a negative electrode and the exterior package 40 and heat-sealed in a secondary battery 10 covered with the exterior package 40. The feature of the present invention is that the layer contained in the sealing film 20 and adhered to the lead conductor 31 is a layer made of a resin composition containing an acid-modified polyolefin (A), an epoxy group-containing polyolefin (B), and an ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic anhydride copolymer (C), and the resin composition contains 1 to 10 parts by mass of the epoxy group-containing polyolefin (B) and 1 to 10 parts by mass of the copolymer (C) with respect to 100 parts by mass of the acid-modified polyolefin (A).

[0013] Conventionally, in the sealing film 20 used for the tab lead 30, the layer adhered to the lead conductor 31 often contains an acid-modified polyolefin (A). Thereby, the adhesive force between the metal lead conductor 31 and the sealing film 20 is improved. However, when the use environment is at a high temperature, the adhesive force tends to be insufficient. Therefore, the present inventor studied, and although the epoxy group-containing polyolefin (B) was blended with the acid-modified polyolefin (A), the improvement of the adhesive force at high temperatures was insufficient.

[0014] In contrast, when the present inventor further intensively studied, it was found that by blending both the epoxy group-containing polyolefin (B) and the ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic anhydride copolymer (C) with the acid-modified polyolefin (A), the adhesive strength at high temperatures was greatly improved, and the present invention was completed. Hereinafter, the present invention will be described in more detail.

[0015] The sealing film 20 of the present invention may be single-layer or multi-layer, but the layer in contact with the metal lead conductor 31 is made of a resin composition containing an acid-modified polyolefin (A), an epoxy group-containing polyolefin (B), and an ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C). This resin composition has excellent adhesion to metals at high temperatures and is useful in applications other than sealing films.

[0016] Acid-modified polyolefin (A) is a polyolefin containing a carboxyl group or a carboxylic anhydride group. Examples include polyolefins copolymerized with unsaturated carboxylic acids such as (meth)acrylic acid and unsaturated carboxylic acid anhydrides such as maleic anhydride. In this specification, (meth)acrylic refers to a material containing methacrylic and acrylic. The form of the copolymer is not limited and may be a random copolymer, block copolymer, or graft copolymer. Specific examples of acid-modified polyolefin (A) include a random copolymer of ethylene and (meth)acrylic acid, a random copolymer of polypropylene and (meth)acrylic acid, polyethylene graft-modified with maleic anhydride, and polypropylene graft-modified with maleic anhydride. Among these, polyolefin graft-modified with maleic anhydride, and particularly polypropylene graft-modified with maleic anhydride, are preferred. The acid value of acid-modified polyolefin (A) is preferably 0.1 to 10 mg·KOH / g. Acid-modified polyolefin (A) may contain other monomer units other than olefins (ethylene, propylene), unsaturated carboxylic acids, and unsaturated carboxylic acid anhydrides, to the extent that they do not hinder the effects of the present invention. The content of these other monomer units is usually less than 5 mol%, and preferably less than 1 mol%. Furthermore, the MFR (230°C, 21.18N) of acid-modified polyolefin (A) is preferably 1 to 15 g / 10 min, and more preferably 1.5 to 10 g / 10 min. Multiple types of acid-modified polyolefin (A) may be used, in which case the average values ​​of the acid value, MFR, and monomer unit content are adopted.

[0017] The epoxy group-containing polyolefin (B) is a polyolefin containing an epoxy group, and examples thereof include polyolefins obtained by copolymerizing an unsaturated monomer containing an epoxy group. The form of the copolymer at this time is not limited, and it may be any copolymer such as a random copolymer, a block copolymer, or a graft copolymer. Specific examples of the epoxy group-containing polyolefin (B) include a random copolymer of ethylene and glycidyl (meth) acrylate, a random copolymer of propylene and glycidyl (meth) acrylate, and the like. The preferred content of the unsaturated monomer unit containing an epoxy group in all monomer units is 2 to 25% by mass, and the preferred content of the olefin unit is 75 to 98% by mass. The epoxy group-containing polyolefin (B) may contain other monomer units other than the olefin (ethylene, propylene) and the unsaturated monomer containing an epoxy group as long as the effects of the present invention are not inhibited. The content of the other monomer units is usually less than 5% by mass, preferably less than 1% by mass. Further, the MFR (190 ° C, 21.18 N) of the epoxy group-containing polyolefin (B) is preferably 1 to 50 g / 10 min. A plurality of types of the epoxy group-containing polyolefin (B) may be used. In that case, the MFR, monomer unit content, etc. are average values.

[0018] The ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) is a random copolymer of ethylene, an unsaturated carboxylic acid ester, and an unsaturated dicarboxylic acid anhydride copolymer. As the unsaturated carboxylic acid ester, alkyl (meth)acrylates are preferred, specifically methyl methacrylate, ethyl methacrylate, methyl acrylate, and ethyl acrylate, with methyl methacrylate being particularly preferred. As the unsaturated dicarboxylic acid anhydride, maleic anhydride is a preferred example. The preferred content of ethylene units in the total monomer units is 65-90% by mass, the preferred content of unsaturated carboxylic acid ester units is 5-30% by mass, and the preferred content of unsaturated dicarboxylic acid anhydride units is 1-10% by mass. The ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) may also contain other monomer units other than ethylene, unsaturated carboxylic acid ester, and unsaturated dicarboxylic acid anhydride, to the extent that they do not impair the effects of the present invention. The content of these other monomer units is usually less than 5% by mass, and preferably less than 1% by mass. Furthermore, the MFR (190°C, 21.18N) of the ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) is preferably 2 to 100 g / 10 min. Multiple types of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) may be used, in which case the average values ​​of the MFR and monomer unit content are adopted.

[0019] The resin composition of the present invention contains 1 to 10 parts by mass of epoxy group-containing polyolefin (B) and 1 to 10 parts by mass of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) per 100 parts by mass of acid-modified polyolefin (A).

[0020] In the resin composition of the present invention, the content of epoxy group-containing polyolefin (B) is 1 to 10 parts by mass per 100 parts by mass of acid-modified polyolefin (A). Including a certain amount or more of epoxy group-containing polyolefin (B) increases the adhesion strength to metal. The content of epoxy group-containing polyolefin (B) is preferably 2 parts by mass or more, and more preferably 3 parts by mass or more. On the other hand, if the content of epoxy group-containing polyolefin (B) is too high, the fluidity of the resin composition may decrease, so it is preferably 8 parts by mass or less.

[0021] In the resin composition of the present invention, the content of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) is 1 to 10 parts by mass per 100 parts by mass of acid-modified polyolefin (A). By including a certain amount or more of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C), the adhesive strength to metals at high temperatures, particularly at temperatures exceeding 100°C, is increased. The content of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) is preferably 2 parts by mass or more, and more preferably 3 parts by mass or more. On the other hand, if the content of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) is too high, the fluidity of the resin composition may decrease, so it is preferable that it be 8 parts by mass or less.

[0022] The resin composition of the present invention may further contain phenoxy resin (D), which improves the adhesive strength at room temperature. Here, phenoxy resin (D) is a polyhydroxy polyether synthesized from bisphenol and epichlorohydrin, and is a thermoplastic resin synthesized by the alternating bonding of bisphenol and epichlorohydrin while hydrogen chloride is eliminated and the epoxy ring opens. Bisphenol A is preferred as the bisphenol used in this case. The preferred MFR (200°C, 21.18N) of phenoxy resin (D) is 1 to 100 g / 10 min. However, since the adhesive strength at high temperatures tends to decrease when phenoxy resin (D) is included, it is necessary to consider the amount of phenoxy resin (D) while taking into account the required heat resistance temperature depending on the application.

[0023] A preferred content of phenoxy resin (D) is 1 to 10 parts by mass per 100 parts by mass of acid-modified polyolefin (A). Including a certain amount or more of phenoxy resin (D) increases the adhesive strength to metal at room temperature. The content of phenoxy resin (D) is more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more. On the other hand, if the amount of phenoxy resin (D) is too high, the adhesive strength at high temperatures tends to decrease, and the fluidity of the resin composition may decrease. The content of phenoxy resin (D) is more preferably 8 parts by mass or less.

[0024] The resin composition of the present invention may further contain ethylene-vinyl acetate copolymer saponified product (E), which improves the adhesive strength at room temperature, although the effect is smaller than that of the phenoxy resin (D). Here, ethylene-vinyl acetate copolymer saponified product (E) is a thermoplastic resin obtained by hydrolyzing (saponifying) vinyl acetate units contained in a random copolymer of ethylene and vinyl acetate to obtain vinyl alcohol units. Therefore, it contains ethylene units, vinyl acetate units, and vinyl alcohol units. The preferred content of ethylene units in ethylene-vinyl acetate copolymer saponified product (E) is 80 to 95 mol%, and the preferred total content of vinyl acetate units and vinyl alcohol units is 5 to 15 mol%. Furthermore, the ratio of vinyl alcohol units to the total of vinyl acetate units and vinyl alcohol units, i.e., the degree of saponification, is 20 to 100 mol%. Note that when the degree of saponification is 100 mol%, no vinyl acetate units are included. The preferred MFR (230°C, 21.18N) of ethylene-vinyl acetate copolymer saponified product (E) is 1 to 50 g / 10 min. The inclusion of ethylene-vinyl acetate copolymer saponified product (E) tends to reduce adhesive strength at high temperatures, but this tendency is less pronounced than with the phenoxy resin (D). Therefore, it is necessary to consider the formulation of ethylene-vinyl acetate copolymer saponified product (E) while taking into account the required heat resistance temperature depending on the application.

[0025] A preferred content of ethylene-vinyl acetate copolymer saponified product (E) is 1 to 10 parts by mass per 100 parts by mass of acid-modified polyolefin (A). Including a certain amount or more of ethylene-vinyl acetate copolymer saponified product (E) increases the adhesion strength to metal at room temperature. The content of ethylene-vinyl acetate copolymer saponified product (E) is more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more. On the other hand, if the amount of ethylene-vinyl acetate copolymer saponified product (E) is too high, the adhesion strength at high temperatures tends to decrease, and the fluidity of the resin composition may decrease. The content of ethylene-vinyl acetate copolymer saponified product (E) is more preferably 8 parts by mass or less.

[0026] The resin composition of the present invention may contain other resins besides acid-modified polyolefin (A), epoxy group-containing polyolefin (B), ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C), phenoxy resin (D), and ethylene-vinyl acetate copolymer saponified product (E), to the extent that the effects of the present invention are not hindered. The content of such other resins is usually less than 20% by mass, preferably less than 5% by mass, more preferably less than 1% by mass, and even more preferably substantially free of other resins, based on the total mass of the resin composition.

[0027] The resin composition of the present invention may contain fillers to the extent that the effects of the present invention are not inhibited. Since too much filler may reduce adhesive strength, the filler content is usually less than 50% by mass, preferably less than 10% by mass, and more preferably less than 1% by mass. In addition, various additives such as plasticizers, lubricants, release agents, colorants, antioxidants, and ultraviolet absorbers may be contained to the extent that the effects of the present invention are not inhibited. The content of these additives is usually less than 5% by mass, and preferably less than 1% by mass.

[0028] The MFR (Melting Free Filtration Rate) (230°C, 21.18N) of the resin composition of the present invention is preferably 0.5 to 10 g / 10 min. If the MFR is too high or too low, melt extrusion molding may become difficult. The MFR is more preferably 1 to 8 g / 10 min, and even more preferably 1.5 to 6 g / 10 min.

[0029] The method for producing the resin composition of the present invention is not particularly limited, but a preferred method is to dry blend the raw material pellets (A) to (E) and then melt-knead them using a twin-screw extruder or the like. The obtained molten resin composition may be used to produce single-layer or multi-layer films directly, or pellets may be produced first, and then single-layer or multi-layer films may be produced thereafter.

[0030] The sealing film 20 of the present invention may be a single-layer film made of the resin composition, or it may be a multilayer film including the resin composition layer and other layers. Preferably, the sealing film 20 is a multilayer film having an intermediate layer and two surface layers arranged on both sides thereof, and the surface layer on the side that is bonded to the lead conductor 31 is a layer made of the resin composition. Specifically, a preferred embodiment is a three-layer multilayer film structure in which an exterior adhesive layer 21, a core layer 22, and a conductor adhesive layer 23 are stacked in this order, as shown in Figure 3. In this embodiment, the conductor adhesive layer 23 is made of the resin composition. For the core layer 22, polypropylene homopolymer or block copolymer with a relatively high melting point is preferably used to ensure insulation between the metal foil contained in the exterior package 40 and the lead conductor 31. For the exterior adhesive layer 21, polypropylene random copolymer with a low melting point is preferably used, considering adhesion to the innermost layer of the exterior package 40. In addition, the same resin composition as the conductor adhesive layer 23 may be used for the exterior adhesive layer 21 in order to eliminate the need to manage the front and back sides of the sealing film 20.

[0031] The thickness of the sealing film 20 of the present invention is preferably 50 to 500 μm. If the thickness of the sealing film 20 is less than 50 μm, it is not easy to sufficiently prevent short circuits between the lead conductor 31 and the metal layer in the outer package 40, and there is a risk of liquid leakage at high temperatures. The thickness of the sealing film 20 is more preferably 80 μm or more, and even more preferably 100 μm or more. On the other hand, if the thickness of the sealing film 20 exceeds 500 μm, the weight and cost increase. The thickness of the sealing film 20 is more preferably 400 μm or less, and even more preferably 300 μm or less. When the sealing film 20 is a multilayer film, the thickness ratio of each layer is not particularly limited, but the thickness ratio of each layer is preferably in the range of 0.2 to 5, and more preferably in the range of 0.5 to 2.

[0032] The method for manufacturing the single-layer or multi-layer sealing film 20 of the present invention is not particularly limited. The film can be manufactured by melting the resin composition of the present invention or polypropylene resin, etc., using a uniscrew or twin-screw extruder and extruding it from a T-die. When manufacturing a multi-layer film, co-extrusion molding can be performed using a T-die such as a multi-manifold type or a feed block type. Alternatively, a single-layer or multi-layer film can be manufactured by inflation molding. Furthermore, a multi-layer film may be formed by casting a pre-formed film from an extruder through a T-die. In particular, co-extrusion molding using a multi-manifold type T-die that can accurately control the thickness of each layer is preferred.

[0033] The secondary battery 10 of the present invention uses the sealing film 20. Specifically, the secondary battery 10 comprises a power generation element including a positive electrode, a negative electrode, an electrolyte, and a separator; an outer package 40 that houses the power generation element and whose peripheral edge is heat-sealed; a lead conductor 31 connected to the positive electrode or the negative electrode and drawn out to the outside of the outer package 40; and a sealing film 20 that is disposed between the outer package 40 and the lead conductor 31 and is heat-sealed.

[0034] The lead conductor 31 used in the secondary battery 10 of the present invention is a metal tape-shaped member connected to the positive or negative electrode, and can be charged and discharged through it. Aluminum is mainly used for the lead conductor 31 connected to the positive electrode. For the lead conductor 31 connected to the negative electrode, nickel or copper with nickel plating, or a copper-nickel clad material is mainly used. To improve the corrosion resistance of these metal materials, or to improve adhesion to the sealing film 20, they may be treated with hexavalent chromate, trivalent chromate, zirconium-based or manganese-based surface treatment agents, or organic surface treatment agents mainly composed of polyvinyl alcohol or polyacrylic acid. The thickness of the lead conductor 31 is usually 0.03 to 1 mm, and its width is 2 to 100 mm.

[0035] As shown in Figure 4, the tab lead 30 used in the secondary battery 10 of the present invention has a portion of the lead conductor 31 on both sides covered with a sealing film 20. At this time, the layer made of the resin composition of the present invention is superimposed and covered in a direction that is in contact with the lead conductor 31. Since the sealing film 20 is heat-sealed to the lead conductor 31 in advance, the space between the lead conductor 31 and the sealing film 20 can be reliably sealed, and the positioning when heat-sealing the outer packaging 40 becomes easier. One end of the lead conductor 31 is connected to the positive or negative electrode, and the other end is connected to the charge / discharge device, so the middle part is covered with the sealing film 20. Then, the outer packaging 40 is heat-sealed at the position where it overlaps with the sealing film 20.

[0036] The method for manufacturing the tab lead 30 involves placing a sealing film 20 above and below a predetermined position on the lead conductor 31, and then heating and pressurizing the fusion portion from above and below. Furthermore, by placing a buffer material such as silicone rubber or a Teflon sheet between the press head and the sealing film 20 during pressurization, the resin can be efficiently transferred to the end of the lead conductor 31. The fusion conditions when manufacturing the tab lead 30 are adjusted according to the melting point of the resin used in the sealing film 20, the MFR, etc., but generally, it is preferable to heat at a temperature 10 to 100°C higher than the melting point of the resin contained in the surface layer.

[0037] The outer packaging 40 used in the secondary battery 10 of the present invention is made of a multilayer film including at least a metal layer and a sealant resin layer. Preferably, it is further preferable to have a surface resin layer outside the metal layer. The surface resin layer may be printed or otherwise treated as appropriate, or the surface resin layer may be composed of multiple resin layers. The overall thickness of the outer packaging 40 is usually 50 to 500 μm.

[0038] The resin constituting the sealant resin layer of the outer packaging 40 can be any resin that can be heat-sealed, and thermoplastic resins are usually used. Preferably, polyolefins, and especially polypropylene, are used. The thickness of the sealant resin layer is usually 10 to 200 μm. The resin constituting the surface resin layer is not particularly limited, and polyamide resins, polyester resins, polyolefin resins, polystyrene resins, thermoplastic polyimide resins, phenoxy resins, epoxy resins, acetal resins, fluororesins, etc., can be used depending on the application. The thickness of the surface resin layer is usually 5 to 400 μm. The metal constituting the metal layer is not particularly limited, but aluminum is preferred considering processability, flexibility, cost, etc. Metal foil with a thickness of 5 to 100 μm may be used, or a vapor-deposited film with a thickness of 0.1 to 2 μm may be used. An adhesive layer may be provided to bond each of the above layers, and the thickness of the adhesive layer is usually 5 to 100 μm.

[0039] In the secondary battery 10 of the present invention, the power generation elements, including a positive electrode, a negative electrode, an electrolyte, and a separator, are housed and sealed by heat sealing the peripheral edge of the outer package 40. The type of secondary battery 10 is not limited, but a lithium-ion battery is preferred. A so-called pouch type, in which the power generation elements are housed by placing two outer packages 40 facing each other, is preferred.

[0040] The secondary battery 10 is manufactured by placing an outer package 40 containing the power generation element on top of the portion of the tab lead 30 covered with the sealing film 20, and then heating and pressurizing the periphery of the outer package 40 from both sides. Alternatively, instead of manufacturing the tab lead 30 in advance, the lead conductor 31 and the sealing film 20 can be placed on top of each other and heat-sealed simultaneously when sealing the outer package 40.

[0041] The secondary battery 10 obtained in this way exhibits strong adhesion between the sealing film 20 and the lead conductor 31 even at high temperatures, suppressing leakage even when used at high temperatures. Moreover, it can prevent short circuits between the metal layer in the outer package 40 and the lead conductor 31, resulting in a secondary battery 10 with excellent sealing and insulation properties. For this reason, it is particularly suitable for applications where weight reduction is a critical requirement and where the operating environment is harsh, such as on-board batteries for electric vehicles. [Examples]

[0042] The present invention will be described in detail below using examples. The resins used in the following examples and comparative examples are as follows. • Acid-modified polyolefin (A) Admer QF551, a maleic anhydride-modified propylene random copolymer manufactured by Mitsui Chemicals: MFR 5.8g / 10 min (230℃, 21.18N), acid value 1mg·KOH / g • Epoxy group-containing polyolefin (B) Sumitomo Chemical's ethylene-glycidyl methacrylate copolymer "Bondfast BF-3C": Random copolymer of 81% ethylene by mass and 19% glycidyl methacrylate by mass, MFR 3g / 10min (190℃, 21.18N) • Ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) Lexpearl ET330H, an ethylene-methyl acrylate-maleic anhydride copolymer manufactured by Nippon Polyethylene Co., Ltd.: A ternary copolymer consisting of 81.5% by mass of ethylene, 16.0% by mass of methyl acrylate, and 2.5% by mass of maleic anhydride. MFR 10g / 10min (190℃, 21.18N) • Phenoxy resin (D) Huntsman phenoxy resin "PKHH" (synthesized from bisphenol A and epichlorohydrin): MFR 4g / 10 min (200℃, 21.18N) • Ethylene-vinyl acetate copolymer saponified product (E) Tosoh Corporation's "Mersen H-6051K": Ethylene 80 mol%, vinyl acetate 0 mol%, vinyl alcohol 20 mol%. MFR 5.5g / 10 min (190℃, 21.18N) • Propylene random copolymer Prime Polymer's propylene random copolymer "Prime PolyPro E330GV": MFR 2.4g / 10min (190℃, 21.18N) • Propylene block copolymer Sun Allomer PC480A, a propylene block copolymer manufactured by Sun Allomer Co., Ltd.: MFR 2.0g / 10 min (230℃, 21.18N)

[0043] [Example 1] (Preparation of resin compositions) 100 parts by mass of pellets of maleic anhydride-modified propylene random copolymer "Admer QF551" were dry-blended with 5 parts by mass of pellets of ethylene-glycidyl methacrylate copolymer "Bondfast BF-3C" and 5 parts by mass of pellets of ethylene-methyl acrylate-maleic anhydride copolymer "Rexpearl ET330H", and then fed into a twin-screw extruder. The mixture was melt-kneaded in the twin-screw extruder at 200°C, extruded into strands, and then cut with a pelletizer to obtain pellets of the resin composition.

[0044] (MFR measurement) The MFR of the resin composition obtained as described above was measured. The measurement was performed at 230°C and under a load of 21.18 N, in accordance with JIS K7210. As a result, the MFR was 3.6 g / 10 min, which is 62.1% of the MFR (5.8 g / 10 min) of maleic anhydride-modified propylene random copolymer "Admer QF551" measured under the same conditions. This indicates a degree of viscosity increase that does not pose a problem for melt molding the film.

[0045] (Preparation of sealing film) A three-layer co-extruded film manufacturing apparatus equipped with three extruders was used to manufacture the sealing film 20. Propylene random copolymer pellets "Prime Polypro E330GV" were supplied to the extruder for the outer material adhesive layer 21, and propylene block copolymer pellets "Sun Allomer PC480A" were supplied to the extruder for the core layer 22. Pellets of the resin composition were supplied to the extruder for the conductor adhesive layer 23. The extrusion temperature in all three extruders was set to 220°C, and the temperature of the multi-manifold type T-die was set to 230°C to co-extrude the three types of three-layer sealing film 20. In this way, a multilayer film with a three-layer structure of 60 μm for the outer material adhesive layer, 65 μm for the core layer, and 75 μm for the conductor adhesive layer, with a total thickness of 200 μm, was obtained, and then it was cut to a length of 60 mm and a width of 12 mm to obtain the sealing film 20. A cross-sectional view of the sealing film 20 is shown in Figure 3.

[0046] (Preparation of tab leads) A lead conductor 31 with a length of 43 mm and a width of 45 mm was fabricated by slitting a 0.4 mm thick aluminum plate (A1050). The lead conductor 31 was sandwiched between two sealing films 20, with the conductor adhesive layers 23 of the two sealing films 20 positioned to contact the lead conductor 31. Furthermore, it was sandwiched above and below with a 1 mm thick fluororesin cushioning sheet and heat-pressed at 175°C for 30 seconds to obtain the tab lead 30 shown in Figure 4.

[0047] (Measurement of adhesive strength) As the outer packaging 40, we used outer packaging material for lithium-ion secondary batteries manufactured by Dai Nippon Printing Co., Ltd. The outer packaging 40 is a sheet with a total thickness of 150 μm, consisting of layers from the outside in: polyamide film (10 μm) / adhesive (15 μm) / aluminum foil (50 μm) / adhesive (50 μm) / propylene random copolymer film (25 μm). This was cut into pieces 50 mm in length and 15 mm in width, and these pieces were placed on top of each other so that the propylene random copolymer film side of the outer packaging 40 and the adhesive layer 21 of the sealing film 20 were in contact, with their edges at approximately right angles. The pieces were then heat-pressed at 170°C for 10 seconds to obtain the adhesive strength measurement sample shown in Figure 5.

[0048] The obtained adhesive strength samples were set in the chucks (50, 50') of a tensile testing machine, as shown in Figure 6, and the adhesive strength was measured at a tensile speed of 10 mm / min. The tensile testing machine used was a Shimadzu Autograph AG-Xplus 50kN equipped with a constant temperature chamber capable of measurements at high temperatures. Adhesion strength was measured at room temperature (23°C) and at high temperatures (100°C and 120°C). When measuring adhesive strength at high temperatures, the sample was attached to the chuck and maintained at 100°C and 120°C in the constant temperature chamber for 10 minutes before measurement began. Adhesion strengths of 80 N / 15 mm were obtained at 23°C, 54 N / 15 mm at 100°C, and 42 N / 15 mm at 120°C, indicating that practical adhesive strength can be maintained even at high temperatures.

[0049] [Example 2] In Example 1, the resin composition was prepared in the same manner as in Example 1, except that the amount of ethylene-glycidyl methacrylate copolymer "Bondfast BF-3C" pellets was reduced to 3 parts by mass per 100 parts by mass of ethylene-methyl acrylate-maleic anhydride copolymer "Admer QF551" pellets, and the amount of ethylene-methyl acrylate-maleic anhydride copolymer "Rexpearl ET330H" pellets was also reduced to 3 parts by mass. A three-layer sealing film of three types was obtained. Using the obtained sealing film, a tab lead was manufactured in the same manner as in Example 1, and the adhesive strength was obtained. The results are summarized in Table 1.

[0050] [Example 3] A resin composition was prepared in the same manner as in Example 1, except that 5 parts by mass of phenoxy resin "PKHH" pellets were added to the three types of pellets dry-blended in Example 1, and a 3-layer sealing film of 3 types was obtained. Tab leads were manufactured using the obtained sealing film in the same manner as in Example 1, and the adhesive strength was obtained. The results are summarized in Table 1.

[0051] [Example 4] A resin composition was prepared in the same manner as in Example 1, except that 5 parts by mass of ethylene-vinyl acetate copolymer saponified "Mersen H6051" pellets were added to the three types of pellets dry-blended in Example 1, and a 3-layer sealing film of 3 types was obtained. Using the obtained sealing film, a tab lead was manufactured in the same manner as in Example 1, and the adhesive strength was obtained. The results are summarized in Table 1.

[0052] [Example 5] Two types of three-layer sealing films were obtained in the same manner as in Example 1, except that the resin composition prepared in Example 1 was supplied to both the extruder for the conductor adhesive layer 23 and the extruder for the exterior material adhesive layer 21. Tab leads were manufactured using the obtained sealing films in the same manner as in Example 1, and the adhesive strength was obtained. The results are summarized in Table 1.

[0053] [Comparative Example 1] In Example 1, a three-layer sealing film was obtained in the same manner as in Example 1, except that pellets of the ethylene-methyl acrylate-maleic anhydride copolymer "Rexpearl ET330H" were not incorporated. Using the obtained sealing film, tab leads were manufactured in the same manner as in Example 1, and the adhesive strength was obtained. The results are summarized in Table 1.

[0054] [Comparative Example 2] In Example 1, a 3-layer sealing film was obtained in the same manner as in Example 1, except that 5 parts by mass of phenoxy resin "PKHH" pellets were added instead of 5 parts by mass of ethylene-methyl acrylate-maleic anhydride copolymer "Rexpearl ET330H" pellets. Tab leads were manufactured using the obtained sealing film in the same manner as in Example 1, and the adhesive strength was obtained. The results are summarized in Table 1.

[0055] [Comparative Example 3] In Example 1, a sealing film was obtained in the same manner as in Example 1, except that 5 parts by mass of pellets of ethylene-vinyl acetate copolymer saponified product "Mersen H6051" were added instead of 5 parts by mass of pellets of ethylene-methyl acrylate-maleic anhydride copolymer "Rexpar ET330H". A three-layer sealing film of three types was obtained using the obtained sealing film in the same manner as in Example 1, and the adhesive strength was obtained. The results are summarized in Table 1.

[0056] [Comparative Example 4] In Example 1, three types of three-layer sealing films were obtained in the same manner as in Example 1, except that pellets of ethylene-glycidyl methacrylate copolymer "Bondfast BF-3C" were not included, nor were pellets of ethylene-methyl acrylate-maleic anhydride copolymer "Rexpearl ET330H" included, and only pellets of maleic anhydride-modified propylene random copolymer "Admer QF551" were supplied to the extruder for the conductor adhesive layer. Tab leads were manufactured using the obtained sealing films in the same manner as in Example 1, and adhesive strength was obtained. The results are summarized in Table 1.

[0057] [Table 1] [Explanation of Symbols]

[0058] 10 Secondary battery 20 sealing film 21 Exterior material adhesive layer 22 core layers 23 Conductor adhesive layer 30 Tablead 31 Lead conductors 40 Exterior Packages 50, 50' Chuck

Claims

1. A single-layer or multi-layer sealing film, in a secondary battery covered with an outer package, which is placed between a lead conductor connected to the positive or negative electrode and the outer package and heat-sealed; The layer contained in the sealing film and bonded to the lead conductor is a layer made of a resin composition containing an acid-modified polyolefin (A), an epoxy group-containing polyolefin (B), and an ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C). A sealing film comprising 100 parts by mass of acid-modified polyolefin (A), 1 to 10 parts by mass of epoxy group-containing polyolefin (B), and 1 to 10 parts by mass of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C).

2. The sealing film according to claim 1, wherein the resin composition further comprises 1 to 10 parts by mass of phenoxy resin (D) per 100 parts by mass of acid-modified polyolefin (A).

3. The sealing film according to claim 1, wherein the resin composition further comprises 1 to 10 parts by mass of ethylene-vinyl acetate copolymer saponified product (E) per 100 parts by mass of acid-modified polyolefin (A).

4. The sealing film according to claim 1, wherein the sealing film is a multilayer film having an intermediate layer and two surface layers disposed on both sides thereof, and the surface layer on the side that is bonded to the lead conductor is a layer made of the resin composition.

5. A tab lead in which both sides of a portion of the lead conductor are covered with a sealing film according to any one of claims 1 to 4.

6. A secondary battery comprising a power generation element including a positive electrode, a negative electrode, an electrolyte, and a separator; an outer package housing the power generation element and having a heat-sealed periphery; lead conductors connected to the positive electrode or the negative electrode and leading out to the outside of the outer package; and a heat-sealed sealing film disposed between the outer package and the lead conductors; A secondary battery in which the sealing film is the sealing film according to any one of claims 1 to 4, and the layer made of the resin composition is bonded to the lead conductor.

7. A resin composition comprising an acid-modified polyolefin (A), an epoxy group-containing polyolefin (B), and an ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C), wherein the composition contains 1 to 10 parts by mass of epoxy group-containing polyolefin (B) and 1 to 10 parts by mass of ethylene-unsaturated carboxylic acid ester-unsaturated dicarboxylic acid anhydride copolymer (C) per 100 parts by mass of acid-modified polyolefin (A).

8. The resin composition according to claim 7, wherein the resin composition further comprises 1 to 10 parts by mass of phenoxy resin (D) per 100 parts by mass of acid-modified polyolefin (A).

9. The resin composition according to claim 7 or 8, wherein the resin composition further comprises 1 to 10 parts by mass of ethylene-vinyl acetate copolymer saponified product (E) per 100 parts by mass of acid-modified polyolefin (A).

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