Battery

Abstract

The present invention makes it possible to prevent short-circuiting between a positive electrode and a negative electrode when a separator has been damaged by an external force. The battery comprises an electrode assembly having a positive electrode, a negative electrode, and a film separator. The electrode assembly has a flat shape. When the shortest direction of the electrode assembly is defined as a first direction, the separator has a thickness in at least the first direction, and is provided between the positive electrode and the negative electrode in at least the first direction. When, in a plan view taken in the first direction, a direction in which the distance of mutually opposing edges of the separator becomes the shortest is defined as a second direction, and a direction perpendicular to the first direction and the second direction is defined as a third direction, the angle formed, in a plan view taken in the first direction, between the second direction and a direction in which the tensile strength of the separator becomes the minimum is larger than the angle formed between the third direction and the direction in which the tensile strength of the separator becomes the minimum. In a plan view taken in the first direction, the ratio of the length of the separator in the third direction to the length of the separator in the second direction is 2.0 or more.

Description

[Technical Field]

[0001] The present invention relates to a battery. [Background technology]

[0002] In some batteries, a film-like separator stretched uniaxially is provided between the positive and negative electrodes. The film-like separator has a machine direction (MD) that is the stretching direction and a transverse direction (TD) that is the direction perpendicular to the stretching direction.

[0003] Patent Document 1 illustrates a secondary battery in which the short side direction of the battery is the same as the TD of the separator.

[0004] Patent Document 2 discloses a secondary battery in which the TD of the separator is aligned with the short-side direction of the battery case when viewed in a plan view in the stacking direction of the separator, in order to prevent the positive electrode and negative electrode from coming into contact with each other and causing a short circuit due to thermal shrinkage of the separator. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-152199 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-58393 Summary of the Invention [Problem to be solved by the invention]

[0006] In the batteries described in Patent Documents 1 and 2, the short side direction of the battery is the same as the TD of the separator, which may result in insufficient prevention of short-circuiting between the positive electrode and the negative electrode when the separator is damaged by an external force. [Means for solving the problem]

[0007] A battery according to one aspect of the present invention is a battery including an electrode assembly having a positive electrode, a negative electrode, and a film-like separator, wherein the electrode assembly has a flat shape, and when the shortest direction of the electrode assembly is defined as a first direction, the separator has a thickness at least in the first direction and is disposed at least between the positive electrode and the negative electrode in the first direction, and when the direction in which the distance between opposing sides of the separator is smallest in a planar view in the first direction is defined as a second direction, and a direction perpendicular to the first direction and the second direction is defined as a third direction, when viewed in a planar view in the first direction, the angle formed by the direction in which the tensile strength of the separator is smallest and the second direction is larger than the angle formed by the direction in which the tensile strength of the separator is smallest and the third direction, and when viewed in a planar view in the first direction, the ratio of the length of the separator in the third direction to the length of the separator in the second direction is 2.0 or more. [Effects of the Invention]

[0008] According to the present invention, when the separator is damaged by an external force, a short circuit between the positive electrode and the negative electrode can be suppressed. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cutaway perspective view showing an example of a battery according to a first embodiment. [Figure 2] FIG. 2 is an exploded plan view showing components of the electrode assembly according to the first embodiment. [Figure 3] FIG. 3 is a schematic diagram illustrating a method for measuring the tensile strength of a separator. [Figure 4] FIG. 4 is a schematic plan view showing the electrode assembly according to the first embodiment when the separator breaks. [Figure 5] FIG. 5 is a schematic plan view showing a comparative example of the electrode assembly shown in FIG. [Figure 6] FIG. 6 is a schematic plan view showing an electrode assembly according to a modified example of the battery according to the first embodiment. [Figure 7] FIG. 7 is a schematic plan view showing a comparative example of the electrode assembly shown in FIG. [Figure 8] FIG. 8 is a partial cutaway view showing an example of a battery according to the second embodiment. [Figure 9] FIG. 9 is a schematic cross-sectional view taken along line IX-IX in FIG. [Figure 10] FIG. 10 is a schematic plan view showing an electrode assembly according to the second embodiment. [Figure 11] FIG. 11 is a perspective view showing an example of a battery according to the third embodiment. [Figure 12] FIG. 12 is a cross-sectional view taken along line XII-XII in FIG. [Figure 13] FIG. 13 is a cross-sectional view showing a modified example of the battery according to the third embodiment. [Figure 14] FIG. 14 is a schematic plan view showing batteries according to the example and the comparative example. [Figure 15] FIG. 15 is a schematic cross-sectional view illustrating the piercing test. [Figure 16] FIG. 16 is a diagram showing the results of the tensile test of the separators according to Example 1 and Comparative Example 1. [Figure 17] FIG. 17 is a diagram showing the change over time in the potential difference of the battery during the piercing test according to Example 1. [Figure 18] FIG. 18 is an X-ray CT image of the battery according to Example 1 after the puncture test. [Figure 19] FIG. 19 is a diagram showing the change over time in the potential difference of the battery during the piercing test according to Comparative Example 1. As shown in FIG. [Figure 20] FIG. 20 is an X-ray CT image of the battery according to Comparative Example 1 after the puncture test. DETAILED DESCRIPTION OF THE INVENTION

[0010] The following describes embodiments of the present invention. However, the present invention is not limited to these embodiments. Each embodiment is an example, and it goes without saying that partial substitution or combination of the configurations shown in different embodiments is possible.

[0011] (First embodiment) Fig. 1 is a cutaway perspective view showing an example of a battery according to the first embodiment. As shown in Fig. 1, the battery according to the first embodiment includes a positive electrode lead 11A, a negative electrode lead 12A, an electrode assembly 10A, an exterior member 21, and an adhesive 22. The battery 1A according to the first embodiment is a so-called laminated lithium-ion secondary battery in which the electrode assembly 10A is housed in the exterior member 21.

[0012] The positive electrode lead 11A is a terminal drawn from the electrode assembly 10A to the outside of the exterior member 21. In other words, the positive electrode lead 11A is a terminal that serves as the positive electrode of the battery 1A. The positive electrode lead 11A is provided so as to extend in a direction perpendicular to the Z direction described below. The positive electrode lead 11A is made of a conductor.

[0013] The negative electrode lead 12A is a terminal drawn from the electrode assembly 10A to the outside of the exterior member 21. In other words, the negative electrode lead 12A is a terminal that serves as the negative electrode of the battery 1A. The negative electrode lead 12A is provided so as to extend in a direction perpendicular to the Z direction described below. The negative electrode lead 12A is made of a conductor.

[0014] The exterior member 21 is a case that houses the electrode assembly 10A. The exterior member 21 includes an insulating layer, a metal layer, and an outermost layer. The exterior member 21 is constructed by laminating the insulating layer, metal layer, and outermost layer in this order from the inside (i.e., the side where the electrode assembly 10A is provided), and then laminating them together. The insulating layer of the exterior member 21 is made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. This reduces the moisture permeability of the battery 1A and improves its airtightness. The metal layer of the exterior member 21 is a metal plate or foil such as aluminum, stainless steel, nickel, or iron. The outermost layer may be made of any material, but is preferably made of a high-strength material such as nylon or the same resin as the insulating layer. This improves the exterior member 21's resistance to tearing and punctures.

[0015] The adhesive 22 is a member that makes the interior of the exterior member 21 airtight. The adhesive 22 is provided so as to surround the positive electrode lead 11A and the negative electrode lead 12A, and seals the gap between the positive electrode lead 11A and the negative electrode lead 12A and the exterior member 21. The material of the adhesive 22 is preferably one that has adhesion to the positive electrode lead 11A and the negative electrode lead 12A. For example, when the positive electrode lead 11A and the negative electrode lead 12A are made of a metal material, the adhesive 22 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. This makes it possible to seal the gap between the exterior member 21 and the positive electrode lead 11A and the negative electrode lead 12A, thereby making the interior of the exterior member 21 airtight.

[0016] FIG. 2 is an exploded plan view showing the components of the electrode assembly according to the first embodiment. The electrode assembly 10A includes a positive electrode, a negative electrode, and a separator 17A. As shown in FIG. 2, the electrode assembly 10A according to the first embodiment is an electrode laminate in which positive electrodes and negative electrodes are alternately stacked with the separator 17A interposed therebetween. The electrode assembly 10A has a flat, generally plate-like shape. The electrode assembly 10A has a generally rectangular parallelepiped shape. In other words, the length of the electrode assembly 10A in one direction is shorter than that in other directions.

[0017] In the following description, the shortest direction of the electrode assembly 10A is referred to as the Z direction. Furthermore, in a plan view in the Z direction, the direction in which the distance between opposing sides of a separator 17A (described later) is shortest is referred to as the Y direction, and the direction perpendicular to the Y and Z directions is referred to as the X direction. That is, the electrode assembly 10A has a flat shape that extends in the Y and Z directions. In the first embodiment, the electrode assembly 10A has a positive electrode, a negative electrode, and a separator 17A stacked in the Z direction. Furthermore, the separator 17A has a thickness in the Z direction, and in a plan view in the Z direction, its length in the X direction is greater than its length in the Y direction. Details of the shape of the separator 17A will be described later.

[0018] (positive electrode) The positive electrode includes a positive electrode current collector layer 13A and a positive electrode active material layer 14A. As shown in Fig. 2, the positive electrode is a laminate in which positive electrode active material layers 14A are stacked on both sides of the positive electrode current collector layer 13A in the Z direction.

[0019] The positive electrode current collector layer 13A is a sheet-like conductive foil, such as an aluminum foil. In the first embodiment, the positive electrode current collector layer 13A has a rectangular shape with a rectangular protrusion 13Aa when viewed in plan in the Z direction. The protrusion 13Aa of the positive electrode current collector layer 13A is connected to the positive electrode lead 11A.

[0020] The positive electrode active material layer 14A is a layer containing a positive electrode active material. The positive electrode active material layer 14A is laminated so as to sandwich the positive electrode current collector layer 13A therebetween. The positive electrode active material layer 14A contains a positive electrode active material, a conductive agent, and a binder. In the first embodiment, the positive electrode active material layer 14A has a rectangular shape when viewed in a plan view in the Z direction. Note that the ingredients contained in the positive electrode active material layer 14A are not limited to those listed above, and the positive electrode active material layer 14A may contain, for example, a dispersant.

[0021] The positive electrode active material is preferably a lithium-containing compound such as a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or a spinel type crystal structure. The lithium-containing phosphate compound is a phosphate compound containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing phosphate compound has, for example, an olivine type crystal structure.

[0022] The binder contained in the positive electrode active material layer 14A may be any material, including at least one of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene-butadiene rubber, fluorine-containing rubber, and ethylene propylene diene. Examples of polymer compounds include polyvinylidene fluoride and polyimide.

[0023] The conductive agent contained in the positive electrode active material layer 14A may be any material, including, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, and ketjen black. However, the positive electrode conductive agent is not limited to these materials, and may be a metal material, a conductive polymer, or the like, as long as it is a conductive material.

[0024] (Negative electrode) The negative electrode includes a negative electrode current collector layer 15A and a negative electrode active material layer 16A. As shown in Fig. 2, the negative electrode is a laminate in which two negative electrode active material layers 16A are stacked on both sides of the negative electrode current collector layer 15A in the Z direction.

[0025] The negative electrode current collector layer 15A is a sheet-like conductive foil, such as a copper foil. In the first embodiment, the negative electrode current collector layer 15A has a rectangular shape with a rectangular protrusion 15Aa when viewed in plan in the Z direction. The protrusion 15Aa of the negative electrode current collector layer 15A is connected to the negative electrode lead 12A.

[0026] The negative electrode active material layer 16A is a layer containing a negative electrode active material. The negative electrode active material layer 16A may further contain a conductive agent and a binder, similar to the positive electrode active material layer 14A.

[0027] The negative electrode active material includes, for example, a material capable of absorbing and desorbing lithium (Li), such as a carbon material, a metal, a semimetal, an alloy or compound of silicon (Si), or an alloy or compound of tin (Sn).

[0028] Examples of carbon materials that can be used as the negative electrode active material include graphite, non-graphitizable carbon, and graphitizable carbon. More specifically, carbon materials include pyrolytic carbons, cokes, glassy carbon fiber, fired organic polymer compounds, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. Here, fired organic polymer compounds are carbonized by firing a polymer compound such as a phenolic resin or a furan resin at an appropriate temperature.

[0029] Examples of metals and metalloids that can be used as negative electrode active materials include tin, lead (Pb), aluminum, indium (In), silicon, zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, silicon, germanium, tin, and lead are preferred. Silicon and tin are more preferred because they have a high ability to absorb and release lithium and can achieve a high energy density.

[0030] Examples of silicon alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of tin, nickel, copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc, indium, silver, titanium (Ti), germanium, bismuth, antimony, and chromium (Cr) as a second constituent element other than silicon. Examples of silicon compounds that can be used as the negative electrode active material include those containing oxygen (O) or carbon (C), and may contain the above-mentioned second constituent element in addition to silicon.

[0031] Examples of tin alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Examples of tin compounds that can be used as the negative electrode active material include those containing oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

[0032] (electrolyte) The electrolyte is filled inside the exterior member 21. The electrolyte includes an electrolyte salt and a solvent that dissolves the electrolyte salt. Examples of the electrolyte salt include lithium salts such as lithium perchlorate (LiClO), lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium bis(trifluoromethanesulfonyl)imide (LiN(SOCF)), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SOCF)), and lithium hexafluoroarsenate (LiAsF). Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0033] (separator) The separator 17A insulates the positive electrode from the negative electrode. The separator 17A is provided to prevent direct contact between the positive electrode and the negative electrode, and is stacked between the positive electrode and the negative electrode in the Z direction in the electrode assembly 10A. As shown in Fig. 2, in the first embodiment, a plurality of separators 17A are provided, and each separator 17A has a thickness in the Z direction.

[0034] In the first embodiment, separator 17A is rectangular when viewed in a plan view in the Z direction. That is, separator 17A has short sides parallel to the Y direction and long sides parallel to the X direction and longer than the short sides when viewed in a plan view in the Z direction. Hereinafter, the length of separator 17A in the X direction may be referred to as a and the length of separator 17A in the Y direction as b. Note that separator 17A shown in FIG. 2 has right-angled vertices when viewed in a plan view in the Z direction, but this is merely an example, and the vertices may be rounded.

[0035] In the first embodiment, the length a of the separator 17A in the X direction is 2.0 times or more the length b of the separator 17A in the Y direction. By setting the length in this range, when the separator 17A is damaged by an external force, a crack occurring in the separator 17A can reach the long side, thereby preventing a short circuit between the positive electrode and the negative electrode. The effect of preventing a short circuit due to a crack occurring in the separator 17A will be described later.

[0036] The separator 17A is made of a material that is electrically stable, chemically stable against the positive electrode active material, the negative electrode active material, and the electrolyte, and has insulating properties. The separator 17A can be, for example, a polymer nonwoven fabric, a porous film, or a layer made of glass or ceramic fibers. The separator 17A may also be a laminate of multiple layers, or a composite of a porous polyolefin film and a heat-resistant film containing polyimide, glass, or ceramic fibers. The separator 17A may also be a film whose surface is coated with particles such as ceramic particles, but is not limited to this.

[0037] The separator 17A is made of a film synthesized by a uniaxial stretching method or a biaxial stretching method. The separator 17A is preferably made of a film synthesized by a uniaxial stretching method. This allows costs to be reduced. When a film synthesized by a uniaxial stretching method is used as the separator 17A, the separator 17A is more preferably synthesized by a dry stretching method. This allows costs to be further reduced.

[0038] The separator 17A has an MD (Machine Direction) and a TD (Transverse Direction). MD refers to the flow direction of the film, i.e., the direction in which the film runs during production. TD refers to the transverse direction of the film of the separator 17A, i.e., the direction perpendicular to the MD when viewed in a plan view in the thickness direction. In the uniaxial stretching method, the film is stretched only in the MD. On the other hand, in the biaxial stretching method, the film is stretched not only in the MD but also in the TD.

[0039] The MD and TD of the separator 17A can be examined using a scanning electron microscope (SEM), etc. Specifically, in an SEM image of the separator 17A, the direction in which the fibrous structures are oriented can be defined as the MD, and the direction perpendicular to the orientation direction and the thickness direction can be defined as the TD.

[0040] The separator 17A has different tensile strengths in the MD and the tensile strength in the TD, and has anisotropy in tensile strength. This is because the separator 17A is stretched in the MD during production, causing the molecules constituting the separator 17A to be tilted in the MD. Here, the tensile strength of the separator 17A refers to the maximum tensile stress that the separator 17A can withstand. In the first embodiment, the separator 17A has the maximum tensile strength in the MD and the minimum tensile strength in the TD. That is, in the first embodiment, the direction in which the tensile strength of the separator 17A is minimum is the TD. The anisotropy in the tensile strength of the separator 17A is more pronounced in a film produced by a uniaxial stretching method than in a film produced by a biaxial stretching method.

[0041] FIG. 3 is a schematic diagram illustrating a method for measuring the tensile strength of a separator. As shown in FIG. 3, the tensile strength of separator 17A can be measured using an Instron-type universal testing machine. Specifically, the tensile strength can be measured by fixing both ends of a test piece 17T of separator 17A with tensile jigs J1 and J2 and conducting a tensile test under the following conditions. Here, test piece 17T of separator 17A is a film used for separator 17A or a test piece cut out from separator 17A. The width w of test piece 17T refers to the maximum length of test piece 17T in the direction perpendicular to the tensile direction at the start of the test. Width of specimen 17T: 12.5mm Distance c between tension jigs J1 and J2 of test piece 17T: 30 mm Pulling speed: 50mm per minute

[0042] Here, the direction in which the tensile strength of separator 17A is smallest refers to the direction perpendicular to the thickness direction of separator 17A in which the tensile strength of separator 17A is smallest. The direction in which the tensile strength of separator 17A is smallest can be measured by performing the tensile test described above, with the direction perpendicular to the thickness direction of separator 17A as the tensile direction. More specifically, the direction in which the tensile strength of separator 17A is smallest as a result of performing a tensile test with multiple directions perpendicular to the thickness direction of separator 17A as tensile directions can be determined as the direction in which the tensile strength of separator 17A is smallest. Here, the multiple directions perpendicular to the thickness direction of separator 17A that are used as the tensile direction in the tensile test include one direction selected from the directions perpendicular to the thickness direction of separator 17A, a direction perpendicular to the first direction, and multiple directions rotated in 5° increments from the one direction to the perpendicular direction. In other words, by conducting a tensile test while changing the pulling direction of separator 17A in increments of 5°, it is possible to determine the direction in which separator 17A has the smallest tensile strength.

[0043] In the first embodiment, the MD tensile strength of the separator 17A is 1.05 times or more its TD tensile strength. By setting the tensile strength in this range, when the separator 17A is damaged by an external force, a crack occurs in the separator 17A in a fixed direction, thereby reliably preventing a short circuit between the positive electrode and the negative electrode. The direction of the crack that occurs in the separator 17A will be described in detail later.

[0044] In the first embodiment, the direction in which the tensile strength of the separator 17A is minimum, i.e., the TD of the separator 17A, is the direction along the X direction. Here, the direction along the X direction includes a direction completely parallel to the X direction as well as a direction substantially parallel to the X direction. Two directions being substantially parallel means, for example, that the angle between the two directions is between 0° and 10°. This can prevent a short circuit between the positive electrode and the negative electrode when the separator 17A is damaged by an external force. This point will be described in detail below with reference to FIGS. 4 and 5.

[0045] FIG. 4 is a schematic plan view showing the electrode assembly according to the first embodiment when the separator breaks. In the electrode assembly 10A1 shown in FIG. 4, the TD is the same as the X direction. As shown in FIG. 4, when an external force is applied to the electrode assembly 10A1 so as to pierce the electrode assembly 10A1 in the Z direction, a crack T1 occurs in the separator 17A1. The crack T1 propagates depending on the anisotropy of the tensile strength of the separator 17A1, so that the crack T1 propagates in the MD, tearing the separator 17A1 in the TD, which has weaker tensile strength. At this time, the crack T1 propagates in the short direction, i.e., the Y direction, so that the end E1 of the crack T1 reaches the long side of the separator 17A1. As a result, the positive electrode and the negative electrode break together with the separator 17A1 before coming into contact in the Z direction. This prevents a short circuit between the positive electrode and the negative electrode when the separator 17A1 is damaged by an external force.

[0046] FIG. 5 is a schematic plan view showing a comparative example of the electrode assembly shown in FIG. 4. In the electrode assembly 10A2 shown in FIG. 5, the TD is the same as the Y direction. As shown in FIG. 5, when an external force is applied to the electrode assembly 10A2 so as to pierce it in the Z direction, a crack T2 occurs in the separator 17A2. As in FIG. 4, the crack T2 propagates in the MD, tearing the separator 17A2 in the TD, which has weak tensile strength. At this time, the crack T2 propagates in the longitudinal direction, i.e., the X direction, so that the end E2 of the crack T2 does not reach the short side of the separator 17A2. As a result, the electrode assembly 10A2 is compressed in a crushed manner, causing the positive and negative electrodes to come into contact in the Z direction and resulting in a short circuit.

[0047] 2, the TD of the separator 17A, i.e., the direction in which the tensile strength is smallest, is the same for the multiple separators 17A. As a result, when the separator 17A is broken by an external force, the crack tends to spread in the Y direction, which makes it possible to prevent a short circuit between the positive electrode and the negative electrode.

[0048] In the first embodiment, the separators 17A are not welded to each other. As a result, when the separators 17A are damaged by an external force, the cracks tend to spread in the Y direction, which makes it possible to prevent a short circuit between the positive electrode and the negative electrode.

[0049] Although the battery 1A according to the first embodiment has been described above, the battery according to the first embodiment is not limited to the battery 1A shown in FIG.

[0050] Fig. 6 is a schematic plan view showing an electrode assembly according to a modified example of the battery according to the first embodiment. As shown in Fig. 6, in electrode assembly 10A3, in a plan view in the Z direction, the TD of separator 17A3 does not have to be oriented along the X direction. It is sufficient that the angle β3 between the TD of separator 17A3 and the Y direction is larger than the angle α3 between the TD of separator 17A3 and the X direction. Even in this case, when separator 17A3 is damaged by an external force, cracks are likely to reach the long sides of separator 17A3. Therefore, the positive electrode and the negative electrode are broken together with separator 17A3 before coming into contact in the Z direction, thereby preventing a short circuit between the positive electrode and the negative electrode.

[0051] Fig. 7 is a schematic plan view showing a comparative example of the electrode assembly shown in Fig. 6. As shown in Fig. 7, in the electrode assembly 10A4, when viewed in plan in the Z direction, if the angle β4 between the TD of the separator 17A4 and the Y direction is equal to or smaller than the angle α4 between the TD of the separator 17A4 and the X direction, cracks are less likely to reach the long sides of the separator 17A4. As a result, when the separator 17A4 is damaged by an external force, the electrode assembly 10A4 is compressed in a crushed manner, which may cause the positive and negative electrodes to come into contact in the Z direction and cause a short circuit.

[0052] Alternatively, the electrode assembly may include a gel electrolyte layer made of a polymer compound that retains the electrolyte solution instead of the electrolyte solution. In this case, the electrolyte layer is provided between the separator 17A and the positive electrode or the negative electrode. The polymer compound that forms the gel of the electrolyte layer is not particularly limited as long as it absorbs a solvent and gels. Examples of the polymer compound that forms the gel of the electrolyte layer include fluorine-based polymer compounds such as copolymers of polyvinylidene fluoride or vinylidene fluoride with hexafluoropropylene, ether-based polymer compounds such as polyethylene oxide or crosslinked polyethylene oxide, and polymer compounds containing polyacrylonitrile, polypropylene oxide, or polymethyl methacrylate as a monomer. In terms of stability against redox reactions, the polymer compound that forms the gel of the electrolyte layer is preferably a fluorine-based polymer compound, and more preferably a copolymer containing vinylidene fluoride and hexafluoropropylene as components. The copolymer may further contain, as a component, a monoester of an unsaturated dibasic acid such as monomethyl maleate, a halogenated ethylene such as trifluorochloroethylene, a cyclic carbonate of an unsaturated compound such as vinylene carbonate, an epoxy group-containing acrylic vinyl monomer, etc. This allows for excellent cycle characteristics to be obtained.

[0053] As described above, the battery 1A according to the first embodiment is a battery including an electrode assembly 10A having a positive electrode, a negative electrode, and a film-like separator 17A. The electrode assembly 10A is flat. When the shortest direction of the electrode assembly 10A is defined as a first direction, the separator 17A has a thickness at least in the first direction and is disposed between the positive electrode and the negative electrode at least in the first direction. When the direction in which the distance between opposing sides of the separator 17A is shortest in a plan view along the first direction is defined as a second direction, and the direction perpendicular to the first and second directions is defined as a third direction, the angle between the direction in which the tensile strength of the separator 17A is shortest (TD) and the second direction is larger than the angle between the direction in which the tensile strength of the separator 17A is shortest and the third direction. When viewed in a plan view along the first direction, the ratio of the length of the separator 17A in the third direction to the length of the separator 17A in the second direction is 2.0 or greater.

[0054] As a result, when an external force is applied, a crack T1 propagates in the direction (MD) perpendicular to the direction in which the tensile strength of separator 17A is minimum, so as to tear separator 17A in the direction in which the tensile strength of separator 17A is minimum, as viewed in a plan view in the first direction. Because the angle between the direction in which the tensile strength of separator 17A is minimum (TD) and the second direction is larger than the angle between the direction in which the tensile strength of separator 17A is minimum and the third direction, end E1 of crack T1 is more likely to reach the long side of separator 17A than the short side of separator 17A. As a result, the positive electrode and the negative electrode are fractured together with separator 17A before they come into contact in the Z direction, thereby preventing a short circuit between the positive electrode and the negative electrode.

[0055] In a preferred embodiment, the direction in which the tensile strength of separator 17A is minimized is the direction along the third direction. As a result, when an external force is applied, crack T1 propagates in the short-side direction in a plan view in the first direction, so that end E1 of crack T1 more reliably reaches the long side of separator 17A. This more reliably prevents short circuits between the positive electrode and the negative electrode.

[0056] In addition, separator 17A has, in plan view in the first direction, short sides parallel to the second direction and long sides parallel to the third direction and longer than the short sides. Even in this case, the positive electrode and the negative electrode break together with separator 17A before coming into contact in the Z direction, and therefore, it is possible to prevent a short circuit between the positive electrode and the negative electrode when separator 17A is damaged by an external force.

[0057] Furthermore, the electrode assembly 10A is an electrode stack in which a positive electrode and a negative electrode are stacked in the first direction with a separator 17A interposed therebetween. Even in this case, the positive electrode and the negative electrode are broken together with the separator 17A before they come into contact in the Z direction, and therefore, it is possible to prevent a short circuit between the positive electrode and the negative electrode when the separator 17A is broken by an external force.

[0058] In a preferred embodiment, the electrode stack has multiple separators 17A, and the multiple separators 17A all have the same direction in which their tensile strength is at its minimum. This makes it easier for cracks to spread in a direction perpendicular to the direction in which their tensile strength is at its minimum when the separator 17A is damaged by an external force, thereby further preventing short circuits between the positive electrode and the negative electrode.

[0059] In a desirable embodiment, the ratio of the tensile strength of separator 17A in the direction in which the tensile strength of separator 17A is smallest to the tensile strength of separator 17A in the direction perpendicular to the direction in which the tensile strength of separator 17A is smallest (MD) is 1.05 or more. This causes cracks to occur in separator 17A in a certain direction when separator 17A is damaged by an external force, thereby further preventing a short circuit between the positive electrode and the negative electrode.

[0060] (Second embodiment) Fig. 8 is a partial cutaway view showing an example of a battery according to the second embodiment. As shown in Fig. 8, the battery 1B according to the second embodiment differs from the first embodiment in that the electrode assembly 10B is a wound electrode body. The battery 1B according to the second embodiment will be described below with reference to the drawings. Note that a description of the same configuration as the first embodiment will be omitted.

[0061] 9 is a schematic cross-sectional view taken along line IX-IX in FIG. 8. As shown in FIG. 9, an electrode assembly 10B according to the second embodiment includes a positive electrode including a positive electrode current collector layer 13B and a positive electrode active material layer 14B, a negative electrode including a negative electrode current collector layer 15B and a negative electrode active material layer 16B, a separator 17B, and a protective material 18. As shown in FIG. 9, the electrode assembly 10B is a wound electrode body formed by winding a laminate in which a positive electrode and a negative electrode are stacked with a separator 17B interposed therebetween around a positive electrode lead 11B and a negative electrode lead 12B extending in a direction perpendicular to the Z direction. In the example shown in FIG. 9, the laminate of the electrode assembly 10B including a positive electrode, a negative electrode, and a separator 17B is wound in a direction perpendicular to the X direction. That is, in the electrode assembly 10B, the positive electrode, the negative electrode, and the separator 17B are stacked in a direction parallel to at least the Z direction, and the separator 17B has a thickness at least in a direction parallel to the Z direction. Note that the winding direction of the electrode assembly 10B shown in Fig. 9 is merely an example, and the electrode assembly 10B may be wound in a direction perpendicular to the Y direction.

[0062] The electrode assembly 10B has a flat shape, similar to the electrode assembly 10A according to the first embodiment. That is, the electrode assembly 10B has a plate-like shape, with its length in the Z direction being shorter than those in the X and Y directions. In other words, the length of the electrode assembly 10B in the direction perpendicular to the winding axis is not constant, and therefore the battery 1B is not a so-called cylindrical battery. In a cylindrical battery, the surface perpendicular to the Z direction is curved, and therefore, even if an external force is applied in a piercing manner in the Z direction, cracks in the separator do not propagate in the MD direction, which may cause a short circuit between the positive and negative electrodes.

[0063] The electrode assembly 10B has a structure in which, from the outside, i.e., the protective material 18 side, a negative electrode current collector layer 15B, a negative electrode active material layer 16B, a separator 17B, a positive electrode active material layer 14B, a positive electrode current collector layer 13B, a positive electrode active material layer 14B, a separator 17B, and a negative electrode active material layer 16B are stacked in this order. The electrode assembly 10B has no layers other than the negative electrode current collector layer 15B, the separator 17B, and the positive electrode current collector layer 13B near the positive electrode lead 11B and the negative electrode lead 12B. This structure allows the positive electrode current collector layer 13B to be connected to the positive electrode lead 11B, and the negative electrode current collector layer 15B to be connected to the negative electrode lead 12B.

[0064] The protective material 18 is a member that protects the inside of the electrode assembly 10 B. The protective material 18 is provided so as to be wrapped around the electrode assembly 10 B. The protective material 18 is, for example, an insulating tape.

[0065] FIG. 10 is a schematic plan view showing an electrode assembly according to a second embodiment. As shown in FIG. 10, in the second embodiment, the direction in which the tensile strength of separator 17B is minimized, i.e., TD, is along the X direction. As a result, when separator 17B is damaged by an external force, a crack generated in electrode assembly 10B propagates in the short direction, i.e., the Y direction, regardless of the winding direction, and the end of the crack reaches the long side of separator 17B. As a result, the positive electrode and the negative electrode are broken together with separator 17B before coming into contact in the Z direction, and therefore, when separator 17B is damaged by an external force, a short circuit between the positive electrode and the negative electrode can be prevented.

[0066] Although the battery 1B according to the second embodiment has been described above, the battery according to the second embodiment is not limited to the battery 1B shown in FIG. 8. For example, in the battery 1B according to the second embodiment, the electrolyte is filled in the exterior member 21 as in the first embodiment, but this is not limiting, and instead, a gel electrolyte layer made of a polymer compound that holds an electrolytic solution may be provided. That is, instead of the electrolyte, an electrolyte layer may be provided between the separator 17B and the positive electrode or the negative electrode.

[0067] As described above, in the battery according to the second embodiment, the electrode assembly 10B is an electrode winding body in which a positive electrode and a negative electrode are stacked and wound with the separator 17B interposed therebetween. Even in this case, the positive electrode and the negative electrode are broken together with the separator 17B before they come into contact in the Z direction, and therefore, it is possible to prevent a short circuit between the positive electrode and the negative electrode when the separator 17B is broken by an external force.

[0068] (Third embodiment) FIG. 11 is a perspective view showing an example of a battery according to the third embodiment. FIG. 12 is a cross-sectional view taken along line XII-XII in FIG. 11. As shown in FIGS. 11 and 12, the battery 1C according to the third embodiment differs from the first embodiment in that the electrode assembly 10C is housed in a battery case 31. That is, the battery 1C according to the third embodiment is a so-called prismatic lithium-ion secondary battery. The battery 1C according to the third embodiment will be described below with reference to the drawings. Note that descriptions of configurations similar to those of the first and second embodiments will be omitted.

[0069] The battery 1C includes an electrode assembly 10C, a battery case 31, electrodes 32 and 33, and seals 34, 35, and 36.

[0070] The battery case 31 is a can that houses the electrode assembly 10C. The battery case 31 is a flat rectangular parallelepiped can made of a conductor.

[0071] The electrode 32 is a terminal extending from the inside of the battery case 31 to the outside. The electrode 32 is the positive terminal of the battery 1C. The electrode 32 is made of a conductor. The electrode 32 is connected to the protrusion 13Ca of the positive electrode current collector of the electrode assembly 10C.

[0072] The electrode 33 is a terminal extending from the inside to the outside of the battery case can 31. The electrode 33 is the negative terminal of the battery 1C. The electrode 33 is made of a conductor. The electrode 33 is connected to the protrusion 15Ca of the negative electrode current collector of the electrode assembly 10C.

[0073] The sealant 34 is provided to surround the electrode 32 and seals the inside of the battery case 31. The sealant 34 is made of an insulator, which can prevent electrical conduction between the battery case 31 and the electrode 32.

[0074] The sealant 35 is provided to surround the electrode 33 and seals the inside of the battery case 31. The sealant 35 is made of an insulator, which can prevent electrical conduction between the battery case 31 and the electrode 33.

[0075] The sealant 36 is provided to cover the inside of the battery case 31. The sealant 35 is made of an insulator, which prevents electrical conduction between the electrode assembly 10C and the battery case 31.

[0076] The electrode assembly 10C is an electrode stack in which positive electrodes and negative electrodes are alternately stacked with separators interposed between them. The electrode assembly 10C is an electrode stack similar to the electrode assembly 10A according to the first embodiment.

[0077] In the separator 17C according to the third embodiment, as in the first embodiment, the direction in which the tensile strength is smallest, i.e., the TD, is the direction along the X direction when viewed in plan in the Z direction, which can prevent a short circuit between the positive electrode and the negative electrode when the separator is damaged by an external force.

[0078] Although the battery 1C according to the third embodiment has been described above, the battery according to the third embodiment is not limited to the battery 1C shown in Fig. 12. For example, the electrode assembly may be a wound electrode body.

[0079] FIG. 13 is a cross-sectional view showing a modified example of the battery according to the third embodiment. In a battery 1D according to the modified example of the third embodiment, an electrode assembly 10D is a wound electrode body wound around a positive electrode lead 11D and a negative electrode lead 12D extending in a direction perpendicular to the Z direction. The positive electrode lead 11D and the negative electrode lead 12D are connected to electrodes 32 and 33, respectively. The electrode assembly 10D is a wound electrode body having a configuration similar to that of the electrode assembly 10B according to the second embodiment. In the example shown in FIG. 13, the laminate of the electrode assembly 10D, which includes a positive electrode, a negative electrode, and a separator, is wound in a direction perpendicular to the X direction. That is, in the electrode assembly 10D, the positive electrode, the negative electrode, and the separator are stacked in at least a direction parallel to the Z direction, and the separator 17B has a thickness at least in a direction parallel to the Z direction. In the electrode assembly 10D, as in the first embodiment, the separator has a direction in which its tensile strength is minimized, i.e., the TD, which is the direction along the X direction in a plan view of the Z direction. Therefore, even in this case, short-circuiting between the positive electrode and the negative electrode can be prevented when the separator is damaged by an external force. Note that the winding direction of the electrode assembly 10B shown in FIG. 13 is merely an example, and the electrode assembly may be wound in a direction perpendicular to the Y direction.

[0080] (Example) Examples will be described below, but the present invention is not limited to the examples described below.

[0081] FIG. 14 is a schematic plan view showing batteries according to examples and comparative examples. FIG. 15 is a schematic cross-sectional view illustrating a puncture test. The battery 1E shown in FIG. 14 is shown in FIG. 15 as a cross section taken along line XV-XV in FIG. 14. For the test, the battery 1E shown in FIGS. 14 and 15 was fabricated. The battery 1E is a laminated stacked battery. As shown in FIGS. 14 and 15, the battery 1E includes a positive electrode lead 11E, a negative electrode lead 12E, an electrode assembly 10E, a housing member 21E, and an adhesive 22. The electrode assembly 10E is housed within the housing member 21E. The electrode assembly 10E has a structure in which a positive electrode having a positive electrode current collector layer 13E and a positive electrode active material layer 14E, and a negative electrode having a negative electrode current collector layer 15E and a negative electrode active material layer 16E are stacked with a separator 17E interposed therebetween.

[0082] The separator 17E according to Example 1 and Comparative Example 1 is a porous polyolefin film manufactured by a uniaxial stretching method. The separator 17E is rectangular when viewed in plan in the Z direction, with a length a in the X direction of 25 mm and a length b in the Y direction of 7 mm. That is, in Example 1 and Comparative Example 1, the length a in the X direction of the separator 17E is 3.5 times the length b in the Y direction of the separator 17E.

[0083] Here, a tensile test was carried out under the following conditions for the films used in the separators 17E according to Example 1 and Comparative Example 1. The tensile test was carried out in the air. Measurement equipment: Instron type universal material testing machine 5564 (Instron) Test piece width: 12.5 mm Distance between the tensile jigs of the test specimen c: 30 mm Pulling speed: 50mm per minute Pulling direction: MD or TD

[0084] Fig. 16 is a diagram showing the results of a tensile test on the separators according to Example 1 and Comparative Example 1. As a result of the tensile test, the load-displacement curve shown in Fig. 16 was obtained. In the load-displacement curve in Fig. 16, the maximum value of the load corresponds to the tensile strength, and it can be seen that the tensile strength in the MD of separator 17E is 14 times the tensile strength in the TD.

[0085] The battery 1E is further provided with pressure plates K on both sides of the electrode assembly 10E in the Z direction. The pressure plates K are provided inside the exterior member 21E. The pressure plates K have a circular recess Ka with a diameter of 6 mm in plan view in the Z direction at the center in the X direction. The recess Ka is a recess that guides a pin L used in a puncture test into the electrode assembly 10E during a puncture test.

[0086] A puncture test was conducted on the battery 1E according to the example and comparative example described above. As shown in FIG. 15 , in the puncture test, a pin L was punctured in the Z direction toward a recess Ka in a pressing plate K of the battery 1E placed on a stand M at a speed of 200 mm / s. The pin L had a spherical tip with a diameter of 3 mm. The change over time in the potential difference between the positive electrode lead 11E and the negative electrode lead 12E before and after puncture was measured to check for the presence or absence of a short circuit between the positive and negative electrodes of the battery 1E. After that, X-ray CT (Computed Tomography) was performed on the battery 1E to observe any cracks that had occurred in the electrode assembly 10E.

[0087] In the battery according to Example 1, the TD of all separators is in the same direction as the X direction. That is, the TD of all separators in the battery according to Example 1 is parallel to the long side of the separator.

[0088] FIG. 17 is a diagram showing the change over time in the potential difference of the battery during the puncture test according to Example 1. In the puncture test according to Example 1, the pin L came into contact with the electrode assembly at 3110 ms, and the pin L penetrated the electrode assembly at 3132 ms. As shown in FIG. 17, in the battery according to Example 1, the potential difference hardly changed before and after puncture. Therefore, it can be seen that in the battery according to Example 1, the positive electrode and the negative electrode were not short-circuited even when an external force was applied in the Z direction.

[0089] Fig. 18 is an X-ray CT image of the battery after the puncture test according to Example 1. The X-ray CT image according to Fig. 18 is a planar image of the electrode assembly after the puncture test according to Example 1 in the Z direction. As shown in Fig. 18, it can be seen that the cracks that occurred in the electrode assembly due to the puncture test reached both ends of the electrode assembly in the Y direction. From this, it is thought that, at the time of puncture, the positive electrode and the negative electrode of the electrode assembly were fractured together with the separator before coming into contact in the Z direction, and therefore, the positive electrode and the negative electrode were not short-circuited.

[0090] In the battery 1E according to Comparative Example 1, the TD of all separators is the same direction as the Y direction. That is, the TD of all separators in the battery 1E according to the Example is parallel to the short side of the separator.

[0091] FIG. 19 is a diagram showing the change over time in the potential difference of the battery during the puncture test according to Comparative Example 1. In the puncture test according to Comparative Example 1, the pin L came into contact with the electrode assembly at 2332 ms, and the pin L penetrated the electrode assembly at 2350 ms. As shown in FIG. 19, in the battery according to Comparative Example 1, the potential difference decreased after the electrode assembly was penetrated. Therefore, it can be seen that in the battery according to Comparative Example 1, when an external force is applied in the Z direction, the positive electrode and the negative electrode are short-circuited.

[0092] Fig. 20 is an X-ray CT image of the battery after the puncture test according to Comparative Example 1. The X-ray CT image according to Fig. 20 is a planar image of the electrode assembly after the puncture test according to Comparative Example 1 in the Z direction. As shown in Fig. 20, it can be seen that the cracks that occurred in the electrode assembly due to the puncture test did not reach both ends of the electrode assembly in the X direction. From this, it is thought that the electrode assembly is compressed in a crushed manner during puncture, causing the positive electrode and negative electrode to come into contact in the Z direction and resulting in a short circuit between the positive electrode and the negative electrode.

[0093] In Example 2, a battery 1E was fabricated in the same manner as in Example 1, except that the length b of the separator 17E in the Y direction was set to 12 mm, and a puncture test was performed. That is, in the battery 1E according to Example 2, the TD of the separator is all in the same direction as the X direction and parallel to the long side of the separator. Also, in Example 2, the length a of the separator 17E in the X direction is 2.1 times the length b of the separator 17E in the Y direction. In the battery according to Example 2, the potential difference was almost unchanged before and after puncture. Therefore, it can be seen that the positive electrode and negative electrode of the battery according to Example 2 were not short-circuited even when an external force was applied in the Z direction.

[0094] In Comparative Example 2, a battery 1E was fabricated in the same manner as in Comparative Example 1, except that the length b of the separator 17E in the Y direction was 12 mm, and a puncture test was performed. That is, in the battery 1E according to Comparative Example 2, the TD of the separator is all in the same direction as the Y direction and parallel to the short side of the separator. Also, in Comparative Example 2, the length a of the separator 17E in the X direction is 2.1 times the length b of the separator 17E in the Y direction. In the battery according to Comparative Example 2, the potential difference decreased after the puncture test. Therefore, it can be seen that in the battery according to Comparative Example 2, the positive electrode and negative electrode are short-circuited when an external force is applied in the Z direction.

[0095] In Comparative Example 3, a battery 1E was fabricated in the same manner as in Example 1, except that the length b of the separator 17E in the Y direction was 17 mm, and a puncture test was performed. That is, in the battery 1E according to Example 3, the TD of the separator is all in the same direction as the X direction and parallel to the long side of the separator. Also, in Comparative Example 3, the length a of the separator 17E in the X direction is 1.5 times the length b of the separator 17E in the Y direction. In the battery according to Comparative Example 3, the potential difference decreased after the puncture test. Therefore, it can be seen that in the battery according to Comparative Example 3, the positive electrode and negative electrode are short-circuited when an external force is applied in the Z direction.

[0096] The above results show that the batteries according to Examples 1 and 2, in which the length a of the separator 17E in the X direction is 2.0 times or more the length b of the separator 17E in the Y direction, can suppress short-circuiting between the positive electrode and the negative electrode, compared to the battery according to Comparative Example 3, in which the length a of the separator 17E in the X direction is less than 2.0 times the length b of the separator 17E in the Y direction.

[0097] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit thereof, and the present disclosure also includes equivalents thereof.

[0098] Regarding the claims, the present disclosure may take the following forms. (1) A battery comprising an electrode assembly having a positive electrode, a negative electrode, and a film-like separator, The electrode assembly has a flat shape, When the shortest direction of the electrode assembly is defined as a first direction, the separator has a thickness at least in the first direction and is disposed between the positive electrode and the negative electrode at least in the first direction, When viewed in a plane in the first direction, a direction in which the distance between opposing sides of the separator is smallest is defined as a second direction, and a direction perpendicular to the first direction and the second direction is defined as a third direction, an angle formed between the direction in which the tensile strength of the separator is smallest and the second direction is larger than an angle formed between the direction in which the tensile strength of the separator is smallest and the third direction, A battery in which, when viewed in a plan view in the first direction, the ratio of the length of the separator in the third direction to the length of the separator in the second direction is 2.0 or more. (2) The battery according to (1) above, wherein the direction in which the tensile strength of the separator is smallest is a direction along the third direction. (3) The battery according to (1) or (2), wherein the separator has, when viewed in a plane in the first direction, a short side parallel to the second direction and a long side parallel to the third direction and longer than the short side. (4) The battery according to any one of (1) to (3), wherein the electrode assembly is an electrode stack in which the positive electrode and the negative electrode are stacked in the first direction with the separator interposed therebetween. (5) The electrode stack has a plurality of separators, The battery according to (4) above, wherein the plurality of separators all have the same direction in which their tensile strengths are minimum. (6) The battery according to any one of (1) to (3), wherein the electrode assembly is an electrode winding body in which the positive electrode and the negative electrode are stacked and wound with the separator interposed therebetween. (7) The battery according to any one of (1) to (6), wherein the ratio of the tensile strength of the separator in the direction in which the tensile strength of the separator is smallest to the tensile strength of the separator in the direction perpendicular to the direction in which the tensile strength of the separator is smallest is 1.05 or more. [Explanation of symbols]

[0099] 1A~1E battery 10A~10E, 10A1~10A4 electrode assembly 11A, 11B, 11D, 11E Positive lead 12A, 12B, 12D, 12E negative lead 13A, 13B, 13E Positive electrode current collector layer 13Aa, 13Ca protrusion 14A, 14B, 14E Positive electrode active material layer 15A, 15B, 15E Negative electrode current collector layer 15Aa, 15Ca protrusion 16A, 16B, 16E Negative active material layer 17A, 17B, 17C, 17E, 17A1 to 17A4 separator 17T test piece 18 Protective Materials 21, 21E Exterior parts 22 Adhesive 31 Battery container 32, 33 electrode 34, 35, 36 Sealing material E1, E2 ends J1, J2 tension jig K Retaining plate Ka dent L-pin M units T1, T2 cracks w width α3, α4, β3, β4 angles

Claims

1. A battery comprising an electrode assembly having a positive electrode, a negative electrode, and a film-like separator, The electrode assembly has a flat shape, When the shortest direction of the electrode assembly is defined as a first direction, the separator has a thickness at least in the first direction and is disposed between the positive electrode and the negative electrode at least in the first direction, When viewed in a plane in the first direction, a direction in which a distance between opposing sides of the separator is smallest is defined as a second direction, and a direction perpendicular to the first direction and the second direction is defined as a third direction, an angle formed between the direction in which the tensile strength of the separator is smallest and the second direction is larger than an angle formed between the direction in which the tensile strength of the separator is smallest and the third direction, a ratio of a length of the separator in the third direction to a length of the separator in the second direction in a plan view in the first direction being 2.0 or greater.

2. The battery according to claim 1 , wherein the direction in which the tensile strength of the separator is minimum is along the third direction.

3. The battery according to claim 1 , wherein the separator has, in a plan view in the first direction, a short side parallel to the second direction and a long side parallel to the third direction and longer than the short side.

4. The battery according to claim 1 or 2, wherein the electrode assembly is an electrode stack in which the positive electrode and the negative electrode are stacked in the first direction with the separator interposed therebetween.

5. The electrode stack has a plurality of separators, The battery according to claim 4 , wherein the plurality of separators all have the same direction in which their tensile strengths are minimum.

6. 3. The battery according to claim 1, wherein the electrode assembly is an electrode winding body in which the positive electrode and the negative electrode are stacked and wound with the separator interposed therebetween.

7. 3. The battery according to claim 1, wherein a ratio of the tensile strength of the separator in a direction perpendicular to the direction in which the tensile strength of the separator is minimum to the tensile strength of the separator in a direction in which the tensile strength of the separator is minimum is 1.05 or more.

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