WO2016156966A1 - Battery, method of producing battery, and insulating film - Google Patents

Battery, method of producing battery, and insulating film Download PDF

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Publication number
WO2016156966A1
WO2016156966A1 PCT/IB2016/000386 IB2016000386W WO2016156966A1 WO 2016156966 A1 WO2016156966 A1 WO 2016156966A1 IB 2016000386 W IB2016000386 W IB 2016000386W WO 2016156966 A1 WO2016156966 A1 WO 2016156966A1
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WO
WIPO (PCT)
Prior art keywords
insulating film
battery
battery case
insulator
electrode assembly
Prior art date
Application number
PCT/IB2016/000386
Other languages
French (fr)
Inventor
Keiichiro Kobayashi
Youichi Matsuura
Satoshi Yamada
Yoshitaka Fukagai
Original Assignee
Toyota Jidosha Kabushiki Kaisha
Toray Advanced Film Co., Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Toyota Jidosha Kabushiki Kaisha, Toray Advanced Film Co., Ltd. filed Critical Toyota Jidosha Kabushiki Kaisha
Publication of WO2016156966A1 publication Critical patent/WO2016156966A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
    • C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
    • C08L23/10—Homopolymers or copolymers of propene
    • C08L23/12—Polypropene
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/10—Primary casings; Jackets or wrappings
    • H01M50/131—Primary casings; Jackets or wrappings characterised by physical properties, e.g. gas permeability, size or heat resistance
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/10—Primary casings; Jackets or wrappings
    • H01M50/102—Primary casings; Jackets or wrappings characterised by their shape or physical structure
    • H01M50/103—Primary casings; Jackets or wrappings characterised by their shape or physical structure prismatic or rectangular
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/10—Primary casings; Jackets or wrappings
    • H01M50/116—Primary casings; Jackets or wrappings characterised by the material
    • H01M50/117—Inorganic material
    • H01M50/119—Metals
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/10—Primary casings; Jackets or wrappings
    • H01M50/116—Primary casings; Jackets or wrappings characterised by the material
    • H01M50/121—Organic material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/10—Primary casings; Jackets or wrappings
    • H01M50/116—Primary casings; Jackets or wrappings characterised by the material
    • H01M50/124—Primary casings; Jackets or wrappings characterised by the material having a layered structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention relates to a battery that includes a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including an insulating film and interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly, and the invention relates also to an insulating film used in this battery.
  • JP 2010-287456 A describes a battery in which an insulator provides electrical insulation between a battery case and an electrode assembly.
  • the electrode assembly is accommodated in the insulator obtained by forming an insulating film into a bag shape and then the insulator in which the electrode assembly is accommodated is housed in the battery case, so that electrical insulation is provided between the battery case and the electrode assembly (see, for example, claims, FIG. 2 and FIG. 8 of JP 2010-287456 A).
  • an in-vehicle battery mounted in a vehicle may be used under severe temperature conditions varying from low to high temperatures. Further, for example, when the vehicle is travelling on a rough road, a large impact may be applied to the battery.
  • an insulating film of an insulator interposed between a battery case and an electrode assembly may crack or break, resulting in deterioration of the electrical insulation between the battery case and the electrode assembly.
  • thermal shock test in which the battery temperature is repeatedly varied within a range from -30°C to 50°C as described later in detail (hereinafter, simply referred to as "thermal shock test"), carried out in consideration of the battery use temperature conditions has revealed that insulating films break in some cases.
  • a vehicle provided with a battery may travel on a rough road under an atmosphere of low temperature.
  • a low-temperature impact test carried out in consideration of an impact applied to the battery, for example, when the vehicle runs over a bump while the battery is at a low temperature of -30°C has revealed that an insulating film, especially, a folded portion of the insulating film crazes and then breaks due to the crazing in some cases.
  • the invention provides a highly reliable battery configured such that an insulating film of an insulator interposed between a battery case and an electrode assembly is less likely to crack or break, and the invention also provides an insulating film used in this battery.
  • An aspect of the invention relates to a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including an insulating film.
  • the insulator is interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly.
  • the insulating film contains a polypropylene resin as a main component.
  • the insulating film has a cold-xylene soluble content of 16 wt% or less.
  • the insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
  • the battery according to the above aspect includes the insulator including the insulating film.
  • the insulating film is prevented from crazing and breaking even when the battery is subjected to a thermal shock test and a low-temperature impact test, which will be described later in detail.
  • the battery is highly reliable because the insulating film of the insulator interposed between the battery case and the electrode assembly is less likely to craze and break.
  • the thermal shock test is carried out by using, for example, a liquid-to-liquid thermal shock chamber TSB Series manufactured by ESPEC CORP or a thermal shock chamber TSA series manufactured by ESPEC CORP.
  • the battery is repeatedly cooled and heated within a battery temperature range from -30°C to 50°C. Specifically, the battery temperature is maintained at 50°C for three minutes. The battery temperature is then decreased from 50°C to -30°C at a temperature decrease rate of 3°C/min, and then the battery temperature is maintained at -30°C for three minutes. The battery is then heated from -30°C to 50°C at a temperature increase rate of 3°C/min, and then maintained at 50°C for three minutes.
  • the cooling and heating as described above is regarded as one cycle, and the cycle is repeated 1 ,000 times.
  • the low-temperature impact test is carried out by using, for example, a vibration test system i240/SA3M/C manufactured by IMV CORPORATION. Specifically, a half-sine shock pulse of 10 G or more is applied to the battery prescribed number of times at under an atmosphere of-30°C.
  • the insulating film contains a polypropylene resin as a main component means that the insulating film contains more than 50 wt% of polypropylene resin.
  • polypropylene resin include a homo polypropylene resin which is a propylene homopolymer, a random copolymer resin which is a copolymer of propylene and ethylene and/or butene, and a block copolymer resin prepared by blending an elastomer, such as ethylene-butene rubber, with propylene.
  • Examples of "insulating film” include a film containing, in addition to a polypropylene resin, an ethylene-oc-olefin copolymer, more specifically, at least one of a low-crystallinity ethylene-oc-olefin copolymer and an amorphous ethylene-oc-olefin copolymer.
  • Each of the low-crystallinity ethylene-oc-olefin copolymer and the amorphous ethylene-oc-olefin copolymer is preferably a thermoplastic elastomer.
  • thermoplastic elastomer of ethylene-oc-olefin copolymer When an insulating film contains a thermoplastic elastomer of ethylene-oc-olefin copolymer, the low-temperature impact resistance of the polypropylene resin is improved.
  • thermoplastic elastomer of low-crystallinity ethylene-oc-olefin copolymer include ethylene-butene rubber (EBR), and examples of the thermoplastic elastomer of amorphous ethylene-oc-olefin copolymer include ethylene-propylene rubber (EPR).
  • the "cold-xylene soluble content" of the insulating film means the 20°C xylene soluble content (CXS) (wt%).
  • the cold-xylene soluble content is determined in the following method. Specifically, a sample (1 g) of the insulating film is completely dissolved in boiling xylene (100 mL), and the solution is cooled to 20°C and is left for four hours. After that, the mixture is separated into a precipitate and a filtrate, and the filtrate is dried and solidified, and dried under a reduced pressure and at 70°C, to obtain a residue. The weight of the obtained residue is measured, and the ratio of the residue to the sample (1 g) is obtained and used as the 20°C xylene soluble content (wt%).
  • the "yield strength (N/10 mm) under an atmosphere of 60°C" is determined in the following method by using a tensile tester obtained by attaching a thermostatic chamber TLF-R3T-F-G-A for Tensilon (manufactured by Mita Sangyo Co., Ltd.) to Tensilon RTG-1210 (manufactured by A&D Company, Limited).
  • the insulating film is cut into a cut piece that is 100 mm in the film machine direction and 10 mm in the width direction, the cut piece is held with chucks (distance between the chucks: 50 mm), and the cut piece is stretched at a tensile rate of 300 mm/min under a measurement atmosphere of 60°C, to measure the tensile load.
  • the tensile load-strain curve obtained by the measurement the tensile load at the first peak of the tensile load is determined as the yield strength (N/10 mm).
  • the “battery case” may have any shape, such as a rectangular shape or a cylindrical shape.
  • the “electrode assembly” may be in any form.
  • the electrode assembly may be, for example, a flat or cylindrical rolled electrode assembly obtained by rolling a positive-electrode plate having a strip shape and a negative-electrode plate having a strip shape with a separator interposed therebetween, or a stacked electrode assembly obtained by stacking multiple rectangular positive-electrode plates and multiple rectangular negative-electrode plates with separators interposed therebetween.
  • the insulating film may contain a thermoplastic elastomer of ethylene-oc-olefin copolymer.
  • the insulating film may be a non-oriented film, and the insulator may be obtained through welding of the insulating film.
  • the non-oriented film By using the non-oriented film, the insulator is formed easily through welding. Thus, the battery is highly reliable.
  • the insulator may be a bursiform insulator that surrounds the electrode assembly.
  • the insulator is the bursiform insulator.
  • the electrode assembly is reliably surrounded by the bursiform insulator to reliably provide electrical insulation between the electrode assembly and the battery case.
  • Another aspect of the invention relates to an insulating film used in a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including the insulating film and interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly.
  • the insulating film contains a polypropylene resin as a main component.
  • the insulating film has a cold-xylene soluble content of 16 wt% or less.
  • the insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
  • the battery is further highly reliable because the insulating film is further less likely to crack and break.
  • the insulating film described above may contain a thermoplastic elastomer of ethylene-a-olefin copolymer.
  • the insulator of the battery may be obtained through welding of the insulating film, and the insulating film may be a non-oriented film. By using the non-oriented film, the insulator is formed easily through welding. When the thus configured insulator is used in the battery, the battery is highly reliable.
  • Another aspect of the invention relates to a method of producing a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including an insulating film and interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly.
  • the method includes forming the insulating film using a polypropylene resin as a main component.
  • the insulating film has a cold-xylene soluble content of 16 wt% or less.
  • the insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
  • the insulating film may contain a thermoplastic elastomer of ethylene-a-olefin copolymer.
  • thermoplastic elastomer of ethylene-a-olefin copolymer may be a thermoplastic elastomer of amorphous ethylene-a-olefin copolymer.
  • the insulating film may be a non-oriented film, and the insulator may be obtained through welding of the insulating film.
  • FIG. 1 is a vertical sectional view of a lithium-ion secondary battery according to an embodiment of the invention.
  • FIG. 2 is a perspective view of a bursiform (bag-shaped) insulator according to the embodiment.
  • FIG. 1 illustrates a lithium-ion secondary battery (non-aqueous electrolyte secondary battery) 1 (hereinafter, simply referred to as "battery 1" where appropriate) according to the present embodiment.
  • FIG. 2 illustrates a bursiform (bag-shaped) insulator (insulator) 30 according to the present embodiment.
  • the battery 1 is a rectangular sealed lithium-ion secondary battery to be mounted in, for example, vehicles, such as a hybrid vehicle and an electric vehicle.
  • the battery 1 includes, for example, a battery case 10, an electrode assembly 20 accommodated in the battery case 10, the bursiform insulator 30 interposed between the battery case 10 and the electrode assembly 20, a positive-electrode terminal 40, and a negative-electrode terminal 41.
  • the positive-electrode terminal 40 and the negative-electrode terminal 41 are supported by the battery case 10.
  • a non-aqueous electrolyte 17 is retained in the battery case 10.
  • the battery case 10 has a rectangular parallelepiped shape, and is made of metal (aluminum, in the present embodiment).
  • the battery case 10 includes a case body 11 and a case lid 13.
  • the case body 11 is in the form of a rectangular parallelepiped that is open only at the upper side UW.
  • the case lid 13 is in the form of a rectangular plate, and is welded to the case body 11 so as to close an opening 1 lh of the case body 1 1.
  • the case lid 13 is provided with a safety valve 14 that breaks when the internal pressure of the battery case 10 reaches a prescribed value.
  • the case lid 13 has a through-hole (injection hole) 13h that provides communication between the inside and outside of the battery case 10.
  • the through-hole 13h is hermetically sealed with a rivet 15.
  • the positive-electrode terminal 40 and the negative-electrode terminal 41 are fixed to the case lid 13 with an insulating resin member 47 interposed therebetween.
  • the positive-electrode terminal 40 is connected to a positive-electrode current collector 21m of a positive-electrode plate 21 of the electrode assembly 20, and the negative-electrode terminal 41 is connected to a negative-electrode current collector 25m of a negative-electrode plate 25 of the electrode assembly 20, in the battery case 10.
  • the electrode assembly 20 has a flat shape, and is housed in the battery case 10 after being accommodated in the bursiform insulator 30, which will be described later in detail.
  • the electrode assembly 20 is obtained by rolling the positive-electrode plate 21 having a strip-shape and the negative-electrode plate 25 having a strip-shape, which are placed on each other with a separator 29 having a strip shape interposed therebetween, and then compressing this rolled assembly into a flat shape.
  • the positive-electrode plate 21 includes a positive-electrode current collector foil 22, which is an aluminum foil having a strip shape, and a positive-electrode active material layer 23 having a strip shape and provided in a region of the main surfaces of the positive-electrode current collector foil 22.
  • the region is a part of the main surface in the width direction of the positive-electrode current collector foil 22, and the region extends in the longitudinal direction of the positive-electrode current collector foil 22.
  • the positive-electrode active material layer 23 contains a positive-electrode active material, a conductive agent, and a binder.
  • One end portion of the positive-electrode current collector foil 22 in its width direction is not provided with the positive-electrode active material layer 23 in the thickness direction of the positive-electrode current collector foil 22, and thus the one end portion serves as the positive-electrode current collector 21m at which the positive-electrode current collector foil 22 is exposed.
  • the positive-electrode terminal 40 is connected to the positive-electrode current collector 21m.
  • the negative-electrode plate 25 includes a negative-electrode current collector foil 26, which is a copper foil having a strip shape, and a negative-electrode active material layer 27 having a strip shape and provided in a region of the main surfaces of the negative-electrode current collector foil 26.
  • the region is a part of the main surface in the width direction of the negative-electrode current collector foil 26, and the region extends in the longitudinal direction of the negative-electrode current collector foil 26.
  • the negative-electrode active material layer 27 contains a negative-electrode active material, a binder, and a thickner.
  • the separator 29 is a porous membrane made of resin (more specifically, made of polypropylene (PP) and polyethylene (PE)), and has a strip shape.
  • the bursiform insulator 30 is obtained by folding an insulating film 31 that has been cut into a prescribed shape, and then welding multiple portions of the folded insulating film 31 to turn the insulating film 31 into a bag shape having an opening 3 Oh only at the upper side UW.
  • the bursiform insulator 30 is housed in the battery case 10 with the electrode assembly 20 surrounded by the bursiform insulator 30.
  • the bursiform insulator 30 is interposed between the battery case 10 and the electrode assembly 20 to provide electrical insulation between the battery case 10 and the electrode assembly 20 (to electrically insulate the battery case 10 and the electrode assembly 20 from each other).
  • the insulating film 31 constituting the bursiform insulator 30 contains a polypropylene resin as a main component.
  • the polypropylene resin include a homo polypropylene resin which is a propylene homopolymer, a random copolymer resin which is a copolymer of propylene and ethylene or butene, and a block copolymer resin prepared by blending an ethylene elastomer with propylene.
  • the insulating film 31 further contains a thermoplastic elastomer of ethylene-a-olefin copolymer, specifically a thermoplastic elastomer of amorphous ethylene- -olefin copolymer, and more specifically ethylene-propylene rubber (EPR).
  • the insulating film 31 has a "cold-xylene soluble content" of 16 wt% or less (8 wt%, in the present embodiment) as measured in the above-described method. Further, the insulating film 31 is configured such that the "yield strength under an atmosphere of 60°C" as measured in the above-described method is within a range from 2.5 N/10 mm to 7.4 N/10 mm (4.5 N/10 mm, in the present embodiment).
  • the insulating film 31 is produced by the following common method.
  • a resin composition obtained by blending a thermoplastic elastomer of ethylene-a-olefin copolymer (specifically, ethylene-propylene rubber) with a polypropylene resin is melted in consideration of the melting point and melt flow rate (generally 1 g to 20 g/10 min) of the resin, and the molten resin composition is extruded through a T-die or a ring die and then cooled to be formed into a film.
  • the extruded film may be oriented, but is preferably kept in the non-oriented state in order to achieve sufficient welding quality when the insulating film 31 is turned into the bursiform insulator 30.
  • the insulating film 31 according to the present embodiment is a non-oriented film obtained by extruding a molten resin composition through a T-die and cooling the resin composition while the resin composition is cast on a cooling drum.
  • the insulating film 31 has a birefringence (i.e., an index of orientation) of 1 10 2 or less.
  • the thickness of the insulating film 31 may be adjusted by controlling the amount of resin composition to be extruded and the drawing speed.
  • the kind or blending amount of polypropylene resin to be used may be changed as needed or the kind or blending amount of thermoplastic elastomer of ethylene-a-olefin copolymer to be used may be changed as needed.
  • Examples 1 to 6 six kinds of batteries were prepared. These batteries have the same configurations as those of the battery 1 of the above-described embodiment, except that bursiform insulators 30 of these batteries were made of insulating films 31 that are different from each other only in cold-xylene soluble content. Specifically, the cold-xylene soluble contents of the insulating films 31 were as follows: 2 wt% (Example 1), 4 wt% (Example 2), 8 wt% (Example 3), 9 wt% (Example 4), 12 wt% (Example 5), and 15 wt% (Example 6).
  • the battery of Example 3 includes the same insulating film 31 as that of the battery 1 of the above-described embodiment.
  • a battery of Comparative Example 1 has the same configurations as those of the battery 1 of the above-described embodiment, except that an insulating film having a cold-xylene soluble content of 17 wt% was used.
  • each of the batteries of Examples 1 to 6 and Comparative Example 1 was subjected to the above-described thermal shock test. After the test, each battery was disassembled, and the condition of the insulating film of the bursiform insulator was checked. Specifically, each insulating film was visually observed to determine whether breakage had occurred in the insulating film. The results are shown in Table 1.
  • the reason why an insulating film having a cold-xylene soluble content of 16 wt% or less is prevented from breaking even if the battery is subjected to the thermal shock test is considered to be as follows.
  • the insulating film 31 contains a thermoplastic elastomer as described above.
  • a thermoplastic elastomer with low crystallinity is easily dissolved in cold xylene, whereas a thermoplastic elastomer with high crystallinity is less likely to be dissolved in cold xylene.
  • the cold-xylene soluble content indicates the rate of content of a thermoplastic elastomer with low crystallinity.
  • the thermoplastic elastomer with low crystallinity has flexibility and is easily decomposed.
  • the insulating film breaks more easily. Specifically, it is considered that when the cold-xylene soluble content exceeds 16 wt%, the insulating film breaks in the thermal shock test.
  • batteries of Examples 7 to 11 five kinds of batteries were prepared. These batteries have the same configurations as those of the battery 1 of the above-described embodiment, except that bursiform insulators 30 of these batteries were made of insulating films 31 that are different from each other only in yield strength under an atmosphere of 60°C. Specifically, the insulating films are configured such that the yield strengths under an atmosphere of 60°C were as follows: 2.9 N/10 mm (Example 7), 3.5 N/10 mm (Example 8), 4.5 N/10 mm (Example 9), 5.7 N/10 mm (Example 10), and 7.0 N/10 mm (Example 11).
  • the battery of Example 9 includes the same insulating film 31 as that of each of the battery 1 of the above-described embodiment the battery of Example 3.
  • a battery of Comparative Example 2 has the same configurations as those of the battery 1 of the above-described embodiment, except that the yield strength under an atmosphere of 60°C is 2.1 N/10 mm.
  • a battery of Comparative Example 3 has the same configurations as those of the battery 1 of the above-described embodiment, except that the yield strength under an atmosphere of 60°C is 7.8 N/10 mm.
  • each of the batteries of Examples 7 to 11 and Comparative Examples 2 and 3 was subjected to the above-described low-temperature impact test. After the test, each battery was disassembled, and the condition of the insulating film of the bursiform insulator was checked. Specifically, each insulating film was visually observed to determine whether crazing has occurred in the insulating film. The results are shown in Table 1.
  • the yield strength under an atmosphere of 60°C needs to be within a range from 2.5 N/10 mm to 7.4 N/10 mm.
  • the yield strength under an atmosphere of 60°C which is a relatively high temperature, is used as the condition for the insulating film for the following reason.
  • insulating films become hard, and thus the difference in yield strength among different kinds of insulating films becomes small.
  • high temperatures such as 60°C, the insulating films become soft, and thus the difference in yield strength among the different kinds of insulating films becomes large.
  • an insulating film configured such that the yield strength under an atmosphere of 60°C is set to a prescribed value
  • an insulating film configured such that the yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm
  • the thickness of an insulating film is preferably within a range from 30 ⁇ to 80 ⁇ .
  • the battery 1 includes the bursiform insulator 30 including the insulating film 31, the insulating film 31 is prevented from crazing and breaking even when the battery 1 is subjected to the thermal shock test and low-temperature impact test.
  • the battery 1 is highly reliable because the insulating film 31 of the bursiform insulator 30 interposed between the battery case 10 and the electrode assembly 20 is less likely to craze and break.
  • the bursiform insulator 30 has a bag shape, the electrode assembly 20 is reliably surrounded by the bursiform insulator 30, so that the electrode assembly 20 and the battery case 10 are reliably insulated from each other.
  • the invention is not limited to the above-described embodiment and the above-described embodiment may be modified as needed within the scope of the invention.
  • multiple portions of the insulating film 31 are welded together to form the bursiform insulator 30.
  • the method of forming the bursiform insulator 30 from the insulating film 31 is not limited to this.
  • multiple portions of the insulating film 31 may be fixed together with the use of an adhesive tape or an adhesive agent to form the bursiform insulator 30.
  • the insulating film 31 contains, in addition to a polypropylene resin, a thermoplastic elastomer of ethylene-a-olefin copolymer (specifically, ethylene-propylene rubber).
  • a thermoplastic elastomer of ethylene-a-olefin copolymer specifically, ethylene-propylene rubber
  • the insulating film 31 is not limited to this.
  • An insulating film that contains a polypropylene resin but does not contain a thermoplastic elastomer of ethylene-a-olefin copolymer may be used.
  • the insulating film may be produced by melting only the polypropylene resin, and extruding the molten resin through a T-die or a ring die and cooling the extruded resin.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Health & Medical Sciences (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Organic Chemistry (AREA)
  • Cell Separators (AREA)
  • Secondary Cells (AREA)
  • Sealing Battery Cases Or Jackets (AREA)

Abstract

A battery (1) includes a battery case (10) made of metal, an electrode assembly (20) accommodated in the battery case (10), and an insulator (30) including an insulating film (31) and interposed between the battery case (10) and the electrode assembly (20) to provide electrical insulation between the battery case (10) and the electrode assembly (20). The insulating film (31) contains a polypropylene resin as a main component. The insulating film (31) has a cold-xylene soluble content of 16 wt% or less. The insulating film (31) is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.

Description

BATTERY, METHOD OF PRODUCING BATTERY, AND INSULATING FILM
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No. 2015-069597 filed on
March 30, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The invention relates to a battery that includes a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including an insulating film and interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly, and the invention relates also to an insulating film used in this battery.
2. Description of Related Art
[0003] In some batteries including a battery case made of metal and an electrode assembly accommodated in the battery case, an insulator including an insulating film is interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly. For example, Japanese Patent Application Publication No. 2010-287456 (JP 2010-287456 A) describes a battery in which an insulator provides electrical insulation between a battery case and an electrode assembly. According to JP 2010-287456 A, the electrode assembly is accommodated in the insulator obtained by forming an insulating film into a bag shape and then the insulator in which the electrode assembly is accommodated is housed in the battery case, so that electrical insulation is provided between the battery case and the electrode assembly (see, for example, claims, FIG. 2 and FIG. 8 of JP 2010-287456 A).
[0004] However, under some battery use conditions, an in-vehicle battery mounted in a vehicle may be used under severe temperature conditions varying from low to high temperatures. Further, for example, when the vehicle is travelling on a rough road, a large impact may be applied to the battery. When the battery is used under severe temperature conditions or when a large impact is applied to the battery as described above, an insulating film of an insulator interposed between a battery case and an electrode assembly may crack or break, resulting in deterioration of the electrical insulation between the battery case and the electrode assembly.
[0005] Specifically, a thermal shock test, in which the battery temperature is repeatedly varied within a range from -30°C to 50°C as described later in detail (hereinafter, simply referred to as "thermal shock test"), carried out in consideration of the battery use temperature conditions has revealed that insulating films break in some cases. A vehicle provided with a battery may travel on a rough road under an atmosphere of low temperature. A low-temperature impact test carried out in consideration of an impact applied to the battery, for example, when the vehicle runs over a bump while the battery is at a low temperature of -30°C (hereinafter, simply referred to as "low-temperature impact test") has revealed that an insulating film, especially, a folded portion of the insulating film crazes and then breaks due to the crazing in some cases.
SUMMARY OF THE INVENTION
[0006] The invention provides a highly reliable battery configured such that an insulating film of an insulator interposed between a battery case and an electrode assembly is less likely to crack or break, and the invention also provides an insulating film used in this battery.
[0007] An aspect of the invention relates to a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including an insulating film. The insulator is interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly. The insulating film contains a polypropylene resin as a main component. The insulating film has a cold-xylene soluble content of 16 wt% or less. The insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
[0008] The battery according to the above aspect includes the insulator including the insulating film. Thus, the insulating film is prevented from crazing and breaking even when the battery is subjected to a thermal shock test and a low-temperature impact test, which will be described later in detail. In other words, the battery is highly reliable because the insulating film of the insulator interposed between the battery case and the electrode assembly is less likely to craze and break.
[0009] The thermal shock test is carried out by using, for example, a liquid-to-liquid thermal shock chamber TSB Series manufactured by ESPEC CORP or a thermal shock chamber TSA series manufactured by ESPEC CORP. Specifically, in consideration of the battery use temperature conditions, the battery is repeatedly cooled and heated within a battery temperature range from -30°C to 50°C. Specifically, the battery temperature is maintained at 50°C for three minutes. The battery temperature is then decreased from 50°C to -30°C at a temperature decrease rate of 3°C/min, and then the battery temperature is maintained at -30°C for three minutes. The battery is then heated from -30°C to 50°C at a temperature increase rate of 3°C/min, and then maintained at 50°C for three minutes. The cooling and heating as described above is regarded as one cycle, and the cycle is repeated 1 ,000 times.
[0010] The low-temperature impact test is carried out by using, for example, a vibration test system i240/SA3M/C manufactured by IMV CORPORATION. Specifically, a half-sine shock pulse of 10 G or more is applied to the battery prescribed number of times at under an atmosphere of-30°C.
[0011] The description "the insulating film contains a polypropylene resin as a main component" means that the insulating film contains more than 50 wt% of polypropylene resin. Examples of "polypropylene resin" include a homo polypropylene resin which is a propylene homopolymer, a random copolymer resin which is a copolymer of propylene and ethylene and/or butene, and a block copolymer resin prepared by blending an elastomer, such as ethylene-butene rubber, with propylene.
[0012] Examples of "insulating film" include a film containing, in addition to a polypropylene resin, an ethylene-oc-olefin copolymer, more specifically, at least one of a low-crystallinity ethylene-oc-olefin copolymer and an amorphous ethylene-oc-olefin copolymer. Each of the low-crystallinity ethylene-oc-olefin copolymer and the amorphous ethylene-oc-olefin copolymer is preferably a thermoplastic elastomer. When an insulating film contains a thermoplastic elastomer of ethylene-oc-olefin copolymer, the low-temperature impact resistance of the polypropylene resin is improved. Specific examples of the thermoplastic elastomer of low-crystallinity ethylene-oc-olefin copolymer include ethylene-butene rubber (EBR), and examples of the thermoplastic elastomer of amorphous ethylene-oc-olefin copolymer include ethylene-propylene rubber (EPR).
[0013] The "cold-xylene soluble content" of the insulating film means the 20°C xylene soluble content (CXS) (wt%). The cold-xylene soluble content is determined in the following method. Specifically, a sample (1 g) of the insulating film is completely dissolved in boiling xylene (100 mL), and the solution is cooled to 20°C and is left for four hours. After that, the mixture is separated into a precipitate and a filtrate, and the filtrate is dried and solidified, and dried under a reduced pressure and at 70°C, to obtain a residue. The weight of the obtained residue is measured, and the ratio of the residue to the sample (1 g) is obtained and used as the 20°C xylene soluble content (wt%).
[0014] The "yield strength (N/10 mm) under an atmosphere of 60°C" is determined in the following method by using a tensile tester obtained by attaching a thermostatic chamber TLF-R3T-F-G-A for Tensilon (manufactured by Mita Sangyo Co., Ltd.) to Tensilon RTG-1210 (manufactured by A&D Company, Limited). Specifically, the insulating film is cut into a cut piece that is 100 mm in the film machine direction and 10 mm in the width direction, the cut piece is held with chucks (distance between the chucks: 50 mm), and the cut piece is stretched at a tensile rate of 300 mm/min under a measurement atmosphere of 60°C, to measure the tensile load. In the tensile load-strain curve obtained by the measurement, the tensile load at the first peak of the tensile load is determined as the yield strength (N/10 mm).
[0015] The "battery case" may have any shape, such as a rectangular shape or a cylindrical shape. The "electrode assembly" may be in any form. The electrode assembly may be, for example, a flat or cylindrical rolled electrode assembly obtained by rolling a positive-electrode plate having a strip shape and a negative-electrode plate having a strip shape with a separator interposed therebetween, or a stacked electrode assembly obtained by stacking multiple rectangular positive-electrode plates and multiple rectangular negative-electrode plates with separators interposed therebetween.
[0016] In the battery described above, the insulating film may contain a thermoplastic elastomer of ethylene-oc-olefin copolymer. The thus configured battery is further highly reliable because the insulating film of the insulator is further less likely to crack and break.
[0017] In the battery described above, the insulating film may be a non-oriented film, and the insulator may be obtained through welding of the insulating film. By using the non-oriented film, the insulator is formed easily through welding. Thus, the battery is highly reliable.
[0018] In the battery described above, the insulator may be a bursiform insulator that surrounds the electrode assembly. In this battery, the insulator is the bursiform insulator. Thus, the electrode assembly is reliably surrounded by the bursiform insulator to reliably provide electrical insulation between the electrode assembly and the battery case.
[0019] Another aspect of the invention relates to an insulating film used in a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including the insulating film and interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly. The insulating film contains a polypropylene resin as a main component. The insulating film has a cold-xylene soluble content of 16 wt% or less. The insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
[0020] When the insulator is formed of the insulating film and the insulator is used in the battery, the battery is further highly reliable because the insulating film is further less likely to crack and break. [0021] The insulating film described above may contain a thermoplastic elastomer of ethylene-a-olefin copolymer. When the insulator is formed of the insulating film and the insulator is used in the battery, the battery is further highly reliable because the insulating film is further less likely to crack and break.
[0022] The insulator of the battery may be obtained through welding of the insulating film, and the insulating film may be a non-oriented film. By using the non-oriented film, the insulator is formed easily through welding. When the thus configured insulator is used in the battery, the battery is highly reliable.
[0023] Another aspect of the invention relates to a method of producing a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including an insulating film and interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly. The method includes forming the insulating film using a polypropylene resin as a main component. The insulating film has a cold-xylene soluble content of 16 wt% or less. The insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
[0024] In the method, the insulating film may contain a thermoplastic elastomer of ethylene-a-olefin copolymer.
[0025] In the method, the thermoplastic elastomer of ethylene-a-olefin copolymer may be a thermoplastic elastomer of amorphous ethylene-a-olefin copolymer.
[0026] In the method, the insulating film may be a non-oriented film, and the insulator may be obtained through welding of the insulating film. BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
FIG. 1 is a vertical sectional view of a lithium-ion secondary battery according to an embodiment of the invention; and
FIG. 2 is a perspective view of a bursiform (bag-shaped) insulator according to the embodiment. DETAILED DESCRIPTION OF EMBODIMENTS
[0028] Hereinafter, an embodiment of the invention will be described with reference to the accompanying drawings. FIG. 1 illustrates a lithium-ion secondary battery (non-aqueous electrolyte secondary battery) 1 (hereinafter, simply referred to as "battery 1" where appropriate) according to the present embodiment. FIG. 2 illustrates a bursiform (bag-shaped) insulator (insulator) 30 according to the present embodiment. In this specification, description will be provided using the upper side and the lower side in FIG. 1 as an upper side UW and a lower side DW of the battery 1, respectively. The battery 1 is a rectangular sealed lithium-ion secondary battery to be mounted in, for example, vehicles, such as a hybrid vehicle and an electric vehicle. The battery 1 includes, for example, a battery case 10, an electrode assembly 20 accommodated in the battery case 10, the bursiform insulator 30 interposed between the battery case 10 and the electrode assembly 20, a positive-electrode terminal 40, and a negative-electrode terminal 41. The positive-electrode terminal 40 and the negative-electrode terminal 41 are supported by the battery case 10. A non-aqueous electrolyte 17 is retained in the battery case 10.
[0029] The battery case 10 has a rectangular parallelepiped shape, and is made of metal (aluminum, in the present embodiment). The battery case 10 includes a case body 11 and a case lid 13. The case body 11 is in the form of a rectangular parallelepiped that is open only at the upper side UW. The case lid 13 is in the form of a rectangular plate, and is welded to the case body 11 so as to close an opening 1 lh of the case body 1 1. The case lid 13 is provided with a safety valve 14 that breaks when the internal pressure of the battery case 10 reaches a prescribed value. The case lid 13 has a through-hole (injection hole) 13h that provides communication between the inside and outside of the battery case 10. The through-hole 13h is hermetically sealed with a rivet 15. The positive-electrode terminal 40 and the negative-electrode terminal 41, each including an extended terminal member 43 and a bolt 44, are fixed to the case lid 13 with an insulating resin member 47 interposed therebetween. The positive-electrode terminal 40 is connected to a positive-electrode current collector 21m of a positive-electrode plate 21 of the electrode assembly 20, and the negative-electrode terminal 41 is connected to a negative-electrode current collector 25m of a negative-electrode plate 25 of the electrode assembly 20, in the battery case 10.
[0030] Next, the electrode assembly 20 will be described. The electrode assembly 20 has a flat shape, and is housed in the battery case 10 after being accommodated in the bursiform insulator 30, which will be described later in detail. The electrode assembly 20 is obtained by rolling the positive-electrode plate 21 having a strip-shape and the negative-electrode plate 25 having a strip-shape, which are placed on each other with a separator 29 having a strip shape interposed therebetween, and then compressing this rolled assembly into a flat shape. The positive-electrode plate 21 includes a positive-electrode current collector foil 22, which is an aluminum foil having a strip shape, and a positive-electrode active material layer 23 having a strip shape and provided in a region of the main surfaces of the positive-electrode current collector foil 22. The region is a part of the main surface in the width direction of the positive-electrode current collector foil 22, and the region extends in the longitudinal direction of the positive-electrode current collector foil 22. The positive-electrode active material layer 23 contains a positive-electrode active material, a conductive agent, and a binder. One end portion of the positive-electrode current collector foil 22 in its width direction is not provided with the positive-electrode active material layer 23 in the thickness direction of the positive-electrode current collector foil 22, and thus the one end portion serves as the positive-electrode current collector 21m at which the positive-electrode current collector foil 22 is exposed. As described above, the positive-electrode terminal 40 is connected to the positive-electrode current collector 21m.
[0031] The negative-electrode plate 25 includes a negative-electrode current collector foil 26, which is a copper foil having a strip shape, and a negative-electrode active material layer 27 having a strip shape and provided in a region of the main surfaces of the negative-electrode current collector foil 26. The region is a part of the main surface in the width direction of the negative-electrode current collector foil 26, and the region extends in the longitudinal direction of the negative-electrode current collector foil 26. The negative-electrode active material layer 27 contains a negative-electrode active material, a binder, and a thickner. One end portion of the negative-electrode current collector foil 26 in its width direction is not provided with the negative-electrode active material layer 27 in the thickness direction of the negative-electrode current collector foil 26, and thus the one end portion serves as the negative-electrode current collector 25m at which the negative-electrode current collector foil 26 is exposed. As described above, the negative-electrode terminal 41 is connected to the negative-electrode current collector 25m. The separator 29 is a porous membrane made of resin (more specifically, made of polypropylene (PP) and polyethylene (PE)), and has a strip shape.
[0032] Next, the bursiform insulator 30 will be described (see FIG. 2 as well).
The bursiform insulator 30 is obtained by folding an insulating film 31 that has been cut into a prescribed shape, and then welding multiple portions of the folded insulating film 31 to turn the insulating film 31 into a bag shape having an opening 3 Oh only at the upper side UW. The bursiform insulator 30 is housed in the battery case 10 with the electrode assembly 20 surrounded by the bursiform insulator 30. Thus, the bursiform insulator 30 is interposed between the battery case 10 and the electrode assembly 20 to provide electrical insulation between the battery case 10 and the electrode assembly 20 (to electrically insulate the battery case 10 and the electrode assembly 20 from each other).
[0033] The insulating film 31 constituting the bursiform insulator 30 contains a polypropylene resin as a main component. Specific examples of the polypropylene resin include a homo polypropylene resin which is a propylene homopolymer, a random copolymer resin which is a copolymer of propylene and ethylene or butene, and a block copolymer resin prepared by blending an ethylene elastomer with propylene. The insulating film 31 further contains a thermoplastic elastomer of ethylene-a-olefin copolymer, specifically a thermoplastic elastomer of amorphous ethylene- -olefin copolymer, and more specifically ethylene-propylene rubber (EPR). The insulating film 31 has a "cold-xylene soluble content" of 16 wt% or less (8 wt%, in the present embodiment) as measured in the above-described method. Further, the insulating film 31 is configured such that the "yield strength under an atmosphere of 60°C" as measured in the above-described method is within a range from 2.5 N/10 mm to 7.4 N/10 mm (4.5 N/10 mm, in the present embodiment).
[0034] The insulating film 31 is produced by the following common method. A resin composition obtained by blending a thermoplastic elastomer of ethylene-a-olefin copolymer (specifically, ethylene-propylene rubber) with a polypropylene resin is melted in consideration of the melting point and melt flow rate (generally 1 g to 20 g/10 min) of the resin, and the molten resin composition is extruded through a T-die or a ring die and then cooled to be formed into a film. The extruded film may be oriented, but is preferably kept in the non-oriented state in order to achieve sufficient welding quality when the insulating film 31 is turned into the bursiform insulator 30. The insulating film 31 according to the present embodiment is a non-oriented film obtained by extruding a molten resin composition through a T-die and cooling the resin composition while the resin composition is cast on a cooling drum. The insulating film 31 has a birefringence (i.e., an index of orientation) of 1 10 2 or less.
[0035] The thickness of the insulating film 31 may be adjusted by controlling the amount of resin composition to be extruded and the drawing speed. In order to obtain the insulating film 31 that has a cold-xylene soluble content of 16 wt% or less and that is configured such that the yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm, the kind or blending amount of polypropylene resin to be used may be changed as needed or the kind or blending amount of thermoplastic elastomer of ethylene-a-olefin copolymer to be used may be changed as needed.
Examples and Comparative Examples
[0036] Next, the results of tests carried out to verify the advantageous effects of the invention will be described. As batteries of Examples 1 to 6, six kinds of batteries were prepared. These batteries have the same configurations as those of the battery 1 of the above-described embodiment, except that bursiform insulators 30 of these batteries were made of insulating films 31 that are different from each other only in cold-xylene soluble content. Specifically, the cold-xylene soluble contents of the insulating films 31 were as follows: 2 wt% (Example 1), 4 wt% (Example 2), 8 wt% (Example 3), 9 wt% (Example 4), 12 wt% (Example 5), and 15 wt% (Example 6). The battery of Example 3 includes the same insulating film 31 as that of the battery 1 of the above-described embodiment. A battery of Comparative Example 1 has the same configurations as those of the battery 1 of the above-described embodiment, except that an insulating film having a cold-xylene soluble content of 17 wt% was used.
[0037] Next, each of the batteries of Examples 1 to 6 and Comparative Example 1 was subjected to the above-described thermal shock test. After the test, each battery was disassembled, and the condition of the insulating film of the bursiform insulator was checked. Specifically, each insulating film was visually observed to determine whether breakage had occurred in the insulating film. The results are shown in Table 1.
[0038]
Table 1
Figure imgf000013_0001
[0039] As can be seen from Table 1, breakage occurred in the insulating film of the battery of Comparative Example 1. In contrast to this, no breakage occurred in the insulating film of each of all the batteries of Examples 1 to 6. Thus, it is considered that when an insulating film has a cold-xylene soluble content of 16 wt% or less, the insulating film is prevented from breaking even if the battery is subjected to the thermal shock test.
[0040] The reason why an insulating film having a cold-xylene soluble content of 16 wt% or less is prevented from breaking even if the battery is subjected to the thermal shock test is considered to be as follows. The insulating film 31 contains a thermoplastic elastomer as described above. A thermoplastic elastomer with low crystallinity is easily dissolved in cold xylene, whereas a thermoplastic elastomer with high crystallinity is less likely to be dissolved in cold xylene. Thus, the cold-xylene soluble content indicates the rate of content of a thermoplastic elastomer with low crystallinity. The thermoplastic elastomer with low crystallinity has flexibility and is easily decomposed. Thus, if the rate of content of the thermoplastic elastomer with low crystallinity increases, the insulating film breaks more easily. Specifically, it is considered that when the cold-xylene soluble content exceeds 16 wt%, the insulating film breaks in the thermal shock test.
[0041] Next, as batteries of Examples 7 to 11, five kinds of batteries were prepared. These batteries have the same configurations as those of the battery 1 of the above-described embodiment, except that bursiform insulators 30 of these batteries were made of insulating films 31 that are different from each other only in yield strength under an atmosphere of 60°C. Specifically, the insulating films are configured such that the yield strengths under an atmosphere of 60°C were as follows: 2.9 N/10 mm (Example 7), 3.5 N/10 mm (Example 8), 4.5 N/10 mm (Example 9), 5.7 N/10 mm (Example 10), and 7.0 N/10 mm (Example 11). The battery of Example 9 includes the same insulating film 31 as that of each of the battery 1 of the above-described embodiment the battery of Example 3. A battery of Comparative Example 2 has the same configurations as those of the battery 1 of the above-described embodiment, except that the yield strength under an atmosphere of 60°C is 2.1 N/10 mm. A battery of Comparative Example 3 has the same configurations as those of the battery 1 of the above-described embodiment, except that the yield strength under an atmosphere of 60°C is 7.8 N/10 mm.
[0042] Next, each of the batteries of Examples 7 to 11 and Comparative Examples 2 and 3 was subjected to the above-described low-temperature impact test. After the test, each battery was disassembled, and the condition of the insulating film of the bursiform insulator was checked. Specifically, each insulating film was visually observed to determine whether crazing has occurred in the insulating film. The results are shown in Table 1.
[0043]
Table 2
Figure imgf000015_0001
[0044] As can be seen from Table 2, crazing occurred in each of the insulating films of the batteries of Comparative Example 2 and 3. In contrast to this, no crazing occurred in the insulating film of each of the batteries of Examples 7 to 11. Thus, it is considered that when an insulating film configured such that the yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm is used, it is possible to obtain a battery configured such that the insulating film is prevented from crazing even when the battery is subjected to the low-temperature impact test.
[0045] The reason why an insulating film is prevented from crazing in the low-temperature impact test when the yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm is considered to be as follows. It was found that generation of crazing in an insulating film upon application of an impact to a battery at a low temperature is related with the yield strength determined by a tensile test. Specifically, when the yield strength is excessively low, more particularly, when the yield strength is less than 2.5 N/10 mm, the insulating film easily yields and thus stretches and becomes thinner. Thus, when an impact is applied to the battery at a low temperature, crazing is easily generated in a portion near an electrode assembly, to which contact pressure is applied by contact between the insulating film and the electrode assembly. On the other hand, when the yield strength is excessively high, more particularly, when the yield strength exceeds 7.4 N/10 mm, the stiffness of the insulating film is excessively high. Thus, when an impact is applied to the battery at a low temperature, crazing is easily generated in the insulating film. In view of this, the yield strength under an atmosphere of 60°C needs to be within a range from 2.5 N/10 mm to 7.4 N/10 mm.
[0046] The yield strength under an atmosphere of 60°C, which is a relatively high temperature, is used as the condition for the insulating film for the following reason. At low temperatures, insulating films become hard, and thus the difference in yield strength among different kinds of insulating films becomes small. On the other hand, at high temperatures, such as 60°C, the insulating films become soft, and thus the difference in yield strength among the different kinds of insulating films becomes large. In view of this, by using an insulating film configured such that the yield strength under an atmosphere of 60°C is set to a prescribed value, more specifically, by using an insulating film configured such that the yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm, it is possible to obtain a battery configured such that the insulating film is prevented from crazing in the low-temperature impact test. The thicker an insulating film is, the higher the yield strength is. However, when an insulating film is thick, the thickness of an electrode assembly housed in a battery case needs to be reduced. In view of this, the thickness of an insulating film is preferably within a range from 30 μπι to 80 μιη.
[0047] As described above, because the battery 1 includes the bursiform insulator 30 including the insulating film 31, the insulating film 31 is prevented from crazing and breaking even when the battery 1 is subjected to the thermal shock test and low-temperature impact test. Thus, the battery 1 is highly reliable because the insulating film 31 of the bursiform insulator 30 interposed between the battery case 10 and the electrode assembly 20 is less likely to craze and break. Because the bursiform insulator 30 has a bag shape, the electrode assembly 20 is reliably surrounded by the bursiform insulator 30, so that the electrode assembly 20 and the battery case 10 are reliably insulated from each other.
[0048] While the example embodiment of the invention has been described, the invention is not limited to the above-described embodiment and the above-described embodiment may be modified as needed within the scope of the invention. For example, in the above-described embodiment, multiple portions of the insulating film 31 are welded together to form the bursiform insulator 30. However, the method of forming the bursiform insulator 30 from the insulating film 31 is not limited to this. For example, multiple portions of the insulating film 31 may be fixed together with the use of an adhesive tape or an adhesive agent to form the bursiform insulator 30.
[0049] In the above-described embodiment, the insulating film 31 contains, in addition to a polypropylene resin, a thermoplastic elastomer of ethylene-a-olefin copolymer (specifically, ethylene-propylene rubber). However, the insulating film 31 is not limited to this. An insulating film that contains a polypropylene resin but does not contain a thermoplastic elastomer of ethylene-a-olefin copolymer may be used. In this case, the insulating film may be produced by melting only the polypropylene resin, and extruding the molten resin through a T-die or a ring die and cooling the extruded resin.

Claims

1. A battery comprising:
a battery case made of metal;
an electrode assembly accommodated in the battery case; and
an insulator including an insulating film, the insulator being interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly, wherein
the insulating film contains a polypropylene resin as a main component,
the insulating film has a cold-xylene soluble content of 16 wt% or less, and the insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
2. The battery according to claim 1, wherein the insulating film contains a thermoplastic elastomer of ethylene-cc-olefin copolymer.
3. The battery according to claim 2, wherein the thermoplastic elastomer of ethylene-a-olefin copolymer is a thermoplastic elastomer of amorphous ethylene-a-olefin copolymer.
4. The battery according to any one of claims 1 to 3, wherein the insulating film is a non-oriented film, and the insulator is obtained through welding of the insulating film.
5. An insulating film used in a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including the insulating film, the insulator being interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly,
the insulating film comprising a polypropylene resin as a main component, wherein the insulating film has a cold-xylene soluble content of 16 wt% or less, and the insulating film is configured such that a yield strength under an atmosphere of 60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
6. The insulating film according to claim 5, wherein the insulating film contains a thermoplastic elastomer of ethylene-a-olefin copolymer.
7. The insulating film according to claim 6, wherein the thermoplastic elastomer of ethylene-a-olefin copolymer is a thermoplastic elastomer of amorphous ethylene-a-olefin copolymer.
8. The insulating film according to any one of claims 5 to 7, wherein the insulator of the battery is obtained through welding of the insulating film, and the insulating film is a non-oriented film.
9. A method of producing a battery including a battery case made of metal, an electrode assembly accommodated in the battery case, and an insulator including an insulating film, the insulator being interposed between the battery case and the electrode assembly to provide electrical insulation between the battery case and the electrode assembly,
the method comprising forming the insulating film using a polypropylene resin as a main component, wherein:
the insulating film has a cold-xylene soluble content of 16 wt% or less; and the insulating film is configured such that a yield strength under an atmosphere of
60°C is within a range from 2.5 N/10 mm to 7.4 N/10 mm.
10. The method according to claim 9, wherein the insulating film contains a thermoplastic elastomer of ethylene-a-olefin copolymer.
11. The method according to claim 10, wherein the thermoplastic elastomer of ethylene- -olefin copolymer is a thermoplastic elastomer of amorphous ethylene-a-olefin copolymer.
12. The method according to any one of claims 9 to 11, wherein the insulating film is a non-oriented film, and the insulator is obtained through welding of the insulating film.
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