Patent Description:
There have widely been used non-aqueous electrolyte secondary batteries which house a winding-shaped electrode assembly having strip-shaped positive and negative electrodes wound with intervening separators in a metal-made battery case. PATENT LITERATURE <NUM> discloses a non-aqueous electrolyte secondary battery which provides restraint on thermal runaway of the battery due to impact from the outside in the charging state by using separators which satisfy predetermined tensile breaking strengths. PATENT LIETRATURE <NUM> discloses an electrode assembly and non-aqueous electrolyte battery. PATENT LITERATURE <NUM> discloses a nonaqueous electrolyte secondary battery.

Now, it is an important problem, on a non-aqueous electrolyte secondary battery, to secure electric insulation between the positive electrode and the negative electrode. The insulation between the positive electrode and the negative electrode is evaluated, for example, with a breakdown test to measure a leak current flowing between the positive electrode and the negative electrode under application of high voltage to the electrode assembly that is still to be impregnated with an electrolytic solution. The breakdown test is performed in order to reject a defective product that is insufficient in terms of the insulation between the positive electrode and the negative electrode in the production process of the non-aqueous electrolyte secondary batteries. Accordingly, securing the insulation between the positive electrode and the negative electrode results in reduction of the fraction defective in the production process. The technology disclosed in PATENT LITERATURE <NUM> does not consider such insulation between the positive electrode and the negative electrode, and still has room for improvement.

It is an advantage of the present disclosure to provide a non-aqueous electrolyte secondary battery capable of enhancing insulation between a positive electrode and a negative electrode and restraining thermal runaway of the battery due to impact from the outside in the charging state.

A non-aqueous electrolyte secondary battery which is an aspect of the present disclosure comprises a winding-shaped electrode assembly having a positive electrode and a negative electrode wound with a first separator and a second separator intervening therebetween, characterized in that the first separator is provided on an outer winding side of the positive electrode, the second separator is provided on an inner winding side of the positive electrode, and a piercing strength of the second separator is higher than a piercing strength of the first separator.

According to the non-aqueous electrolyte secondary battery according to the present disclosure, there may be enhanced the insulation between the positive electrode and the negative electrode and be restrained the thermal runaway of the battery due to the impact from the outside in the charging state.

Hereafter, an example of embodiments of a non-aqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Specific shapes, materials, numerical values, directions and the like in the following description are exemplary illustrations for facilitating understanding of the present disclosure, and can be properly modified to meet the specifications of a nonaqueous electrolyte secondary battery. Moreover, it has been originally supposed that, in the case where the following description includes a plurality of embodiments and modifications, their characteristic portions are properly combined and used.

<FIG> is a longitudinal sectional view of a non-aqueous electrolyte secondary battery <NUM> which is an example of embodiments. As exemplarily shown in <FIG>, the non-aqueous electrolyte secondary battery <NUM> has an electrode assembly <NUM> and a nonaqueous electrolyte (not shown) which are housed in a battery case <NUM>. For a nonaqueous solvent (organic solvent) of the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, and the like can be used, and two kinds or more of these solvents can be mixed and used. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, mixed solvents of the cyclic carbonates and the chain carbonates, and the like can be used. For an electrolyte salt of the non-aqueous electrolyte, LiPF<NUM>, LiBF<NUM>, LiCF<NUM>SO<NUM>, and the like, and mixtures of these can be used. The amount of the dissolved electrolyte salt relative to and in the non-aqueous solvent can be set, for example, to <NUM> to <NUM> mol/L.

An exterior member <NUM> and a sealing assembly <NUM> constitute the battery case <NUM>. Insulating plates <NUM> and <NUM> are provided on and below on the electrode assembly <NUM>, respectively. A positive electrode lead <NUM> extends to the sealing assembly <NUM> side through a through hole of the insulating plate <NUM> and is welded onto a lower surface of a filter <NUM> which is a bottom plate of the sealing assembly <NUM>. In the non-aqueous electrolyte secondary battery <NUM>, a cap <NUM> which is a top board of the sealing assembly <NUM> electrically connected to the filter <NUM> is a positive electrode terminal. Meanwhile, a negative electrode lead <NUM> extends to the bottom part side of the exterior member <NUM> through a through hole of the insulating plate <NUM> and is welded onto an inner surface of the bottom part of the exterior member <NUM>. In the non-aqueous electrolyte secondary battery <NUM>, the exterior member <NUM> is a negative electrode terminal. When the negative electrode lead <NUM> is installed in the vicinity of the outer winding end, the negative electrode lead <NUM> extends to the bottom part side of the battery case <NUM> through the outside of the insulating plate <NUM> and is welded onto an inner surface of the bottom part of the battery case <NUM>.

The exterior member <NUM> is a bottomed cylindrical metal-made container. A gasket <NUM> is provided between the exterior member <NUM> and the sealing assembly <NUM> and the sealability of the interior of the battery case is secured. The exterior member <NUM> has a projecting portion <NUM> which is formed, for example, by pressing its lateral surface part from the outside and which supports the sealing assembly <NUM>. The projecting portion <NUM> is preferably formed into an annular shape along the circumferential direction of the exterior member <NUM> and supports the sealing assembly <NUM> on its upper surface.

The sealing assembly <NUM> has the filter <NUM>, a lower vent member <NUM>, an insulating member <NUM>, an upper vent member <NUM> and the cap <NUM> which are laminated sequentially from the electrode assembly <NUM> side. The members constituting the sealing assembly <NUM> have disc shapes or ring shapes, for example, and the members except the insulating member <NUM> are electrically connected to one another. The lower vent member <NUM> and the upper vent member <NUM> are connected to each other at their center parts, and the insulating member <NUM> interposes between their peripheral edges. When the internal pressure of the battery rises due to abnormal heat generation, the lower vent member <NUM> fractures, for example, this causes the upper vent member <NUM> to expand to the cap <NUM> side and to be separated from the lower vent member <NUM>, and thereby, electric connection between these is interrupted. When the internal pressure further rises, the upper vent member <NUM> fractures, and gas is discharged from an opening of the cap <NUM>.

Hereafter, the electrode assembly <NUM> will be described in detail with reference to <FIG> is a perspective view of the electrode assembly <NUM> in a winding shape of the non-aqueous electrolyte secondary battery <NUM> shown in <FIG>. The electrode assembly <NUM> has a winding-shaped structure in which a positive electrode <NUM> and a negative electrode <NUM> are wound with a first separator 13a and a second separator 13b intervening therebetween. All of the positive electrode <NUM>, the negative electrode <NUM>, the first separator 13a and the second separator 13b are formed into strip shapes, which are wound into a spiral shape around the winding axis which is the center of the winding of the electrode assembly <NUM> into a state where they are alternately laminated in a radial direction β of the electrode assembly <NUM>. On the electrode assembly <NUM>, the longitudinal direction of the positive electrode <NUM> and the negative electrode <NUM> is a winding direction y, and the width direction of the positive electrode <NUM> and the negative electrode <NUM> is a winding axis direction α. Hereinafter, the longitudinal direction of the positive electrode <NUM> and the negative electrode <NUM> may be referred to as a longitudinal direction γ. Moreover, an inner winding side means the winding axis side in the radial direction β, and an outer winding side means the outside of the electrode assembly <NUM> in the radial direction β.

The positive electrode <NUM> has a strip-shaped positive electrode current collector, and a positive electrode active material layer formed on at least one of surfaces of the positive electrode current collector. In other words, the positive electrode <NUM> has a positive electrode active material layer including a positive electrode active material on at least one of its surfaces. The positive electrode active material layer is preferably formed on both sides of the positive electrode current collector. For the positive electrode current collector, there is used, for example, foil of a metal such as aluminum, a film having the metal disposed on its surface layer, or the like. A preferable positive electrode current collector is foil of metal the main component of which is aluminum or aluminum alloy. The thickness of the positive electrode current collector is <NUM> to <NUM>, for example.

The positive electrode active material layer is preferably formed in the whole region, on both sides of the positive electrode current collector, except a positive electrode exposed part mentioned later. The positive electrode active material layer preferably includes the positive electrode active material, a conductive agent, and a binder. The positive electrode <NUM> is produced, for example, by applying positive electrode slurry including the positive electrode active material, the conductive agent, the binder, and a solvent such as N-methyl-<NUM>-pyrrolidone (NMP) on both sides of the positive electrode current collector, and after that, drying and rolling it.

Examples of the positive electrode active material include lithium-containing composite oxides containing transition metal elements such as Co, Mn, and Ni. Although the lithium-containing composite oxides are not specially limited, they are preferably composite oxides represented by the general formula Li<NUM>+xMO<NUM> wherein -<NUM><x≤<NUM>, and M includes at least one of the group consisting of Ni, Co, Mn, and Al.

Examples of the conductive agent include carbon materials such as carbon black (CB), acetylene black (AB), Ketjen black, and graphite, and the like. Examples of the binder can include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, polyolefin-based resins, and the like. Moreover, there may be used, together with these resins, carboxymethylcellulose (CMC) or its salt, polyethylene oxide (PEO), and the like. These may be used singly or in combinations of two or more thereof.

There is provided, on the positive electrode <NUM>, the positive electrode exposed part from which a surface of the positive electrode current collector is exposed. The positive electrode exposed part is a portion to which the positive electrode lead <NUM> is connected and is a portion where the surface of the positive electrode current collector is not covered by the positive electrode active material layer. The positive electrode lead <NUM> is joined to the positive electrode exposed part, for example, by ultrasonic welding. A structure material of the positive electrode lead <NUM> is not specially limited as long as it has conductivity. The positive electrode lead <NUM> is preferably composed of metal the main component of which is aluminum.

The positive electrode exposed part can be provided, for example, in a center part of the positive electrode <NUM> in the longitudinal direction γ. Although the positive electrode exposed part may be formed shifting toward an end part of the positive electrode <NUM> in the longitudinal direction y, it is preferably provided at a position at substantially the same distances from both ends thereof in the longitudinal direction y, preferably, in view of current collectability. By connecting the positive electrode lead <NUM> to the positive electrode exposed part provided at such a position, the positive electrode lead <NUM> is to be arranged to protrude from an end face of the electrode assembly <NUM> in the winding axis direction α at the middle position thereof in the radial direction β as shown in <FIG> after the winding is made as the electrode assembly <NUM>. The positive electrode exposed part is provided, for example, by intermittent application by which a part of the positive electrode current collector is not coated with the positive electrode slurry.

The negative electrode <NUM> has a strip shaped negative electrode current collector, and a negative electrode active material layer formed on at least one of surfaces of the negative electrode current collector. The negative electrode active material layer is preferably formed on both sides of the negative electrode current collector. For the negative electrode current collector, there is used, for example, foil of a metal such as copper, a film having the metal disposed on its surface layer, or the like. A preferable negative electrode current collector is foil of metal the main component of which is copper or copper alloy. The thickness of the negative electrode current collector is <NUM> to <NUM>, for example.

The negative electrode active material layer is preferably formed in the whole region, on both sides of the negative electrode current collector, except a negative electrode exposed part mentioned later. The negative electrode active material layer preferably includes a negative electrode active material and a binder. The negative electrode <NUM> is produced, for example, by applying negative electrode slurry including the negative electrode active material, the binder, water, and the like on both sides of the negative electrode current collector, and after that, drying and rolling it.

The negative electrode active material is not specially limited as long as it can reversibly store and release lithium ions and, for example, can employ carbon materials such as natural graphite and artificial graphite, metals alloyed with lithium, such as Si and Sn, alloys and oxides including these, and the like. For the binder included in the negative electrode active material layer, there are used, for example, fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, polyolefin-based resins, and the like, similarly to the case of the positive electrode <NUM>. In the case where the negative electrode slurry is prepared with an aqueous solvent, there can be used styrene-butadiene rubber (SBR), CMC or its salt, polyacrylic acid or its salt, polyvinyl alcohol, and the like. These may be used singly or in combinations of two or more thereof.

There is provided, on the negative electrode <NUM>, the negative electrode exposed part from which a surface of the negative electrode current collector is exposed. The negative electrode exposed part is a portion to which the negative electrode lead <NUM> is connected and is a portion where the surface of the negative electrode current collector is not covered by the positive electrode active material layer. The negative electrode lead <NUM> is joined to the positive electrode exposed part, for example, by ultrasonic welding. A structure material of the negative electrode lead <NUM> is not specially limited as long as it has conductivity. The negative electrode lead <NUM> is preferably composed of metal the main component of which is nickel or copper, or of metal including both nickel and copper.

The negative electrode exposed part is provided, for example, in an inner end part of the negative electrode <NUM> in the longitudinal direction γ. In this case, as shown in <FIG>, the negative electrode lead <NUM> is arranged to protrude from an end face of the electrode assembly <NUM> in the winding axis direction α at a center part thereof in the radial direction β. Herein, the inner end part and an outer end part mean the end part of the positive electrode <NUM> on the inner winding side, and the end part of the negative electrode <NUM> on the outer winding side, respectively. The negative electrode exposed part is provided, for example, by intermittent application by which a part of the negative electrode current collector is not coated with the negative electrode slurry. The arrangement position of the negative electrode lead <NUM> is not limited to that in the example shown in <FIG> but the negative electrode lead <NUM> may be provided in the outer end part of the negative electrode <NUM>. Moreover, the negative electrode leads <NUM> may be provided in both the inner end part and the outer end part. In this case, the current collectability can be improved. The terminal end part of the negative electrode <NUM> may be electrically connected to the battery case <NUM> not using the negative electrode lead <NUM>, by bringing an exposed part of the negative electrode <NUM> at the terminal end part into contact with the inner peripheral surface of the battery case <NUM>.

As shown in <FIG>, the first separator 13a is provided on the outer winding side of the positive electrode <NUM>, and the second separator 13b is provided on the inner winding side of the positive electrode <NUM>. The first separator 13a and the second separator 13b intervene between the positive electrode <NUM> and the negative electrode <NUM>, and thereby, make the positive electrode <NUM> and the negative electrode <NUM> physically and electrically separated from each other. Moreover, the first separator 13a and the second separator 13b protect the positive electrode <NUM> and the negative electrode <NUM> upon reception of impact from the outside.

For the first separator 13a and the second separator 13b, there are used porous sheets having ion permeability and insulation properties. Specific examples of the porous sheets include microporous thin films, woven fabric, nonwoven fabric, and the like. For the materials of the bases of the first separator 13a and the second separator 13b, there can be used polyolefin-based resins such as polyethylene and polypropylene. The thicknesses of the first separator 13a and the second separator 13b are <NUM> to <NUM>, for example, and are preferably <NUM> to <NUM>. The first separator 13a and the second separator 13b have melting points, for example, of about <NUM> to about <NUM>.

The piercing strength of the second separator 13b is higher than the piercing strength of the first separator 13a. This can enhance the insulation between the positive electrode <NUM> and the negative electrode <NUM>. The reason is considered as follows. When the positive electrode <NUM> and the negative electrode <NUM> are wound into spiral shapes, their active material layers that are on the respective inner winding sides are subject to compressive stress. Since the positive electrode <NUM> includes a hard active material, there is a case where the positive electrode active material layer cannot absorb the compressive stress. Therefore, the second separator 13b which is on the inner winding side of the positive electrode <NUM> is more subject to stress from a positive electrode mixture layer than the first separator 13a which is on the outer winding side thereof. Accordingly, enhancing the piercing strength of the second separator 13b leads to effective means for enhancing the insulation between the positive electrode <NUM> and the negative electrode <NUM>. The piercing strengths are measured in conformity to the JIS standard, JIS Z-<NUM>. A fixed piercing strength measurement sample (separator) is pricked with a semicircular needle having <NUM> of diameter and <NUM> of tip diameter at <NUM>±<NUM>/min of speed to measure the maximum stress until it is pierced by the needle. The measurements are performed on five piercing strength measurement samples which are sampled at random to set an average value of the measurement results as the value of the piercing strength of each separator.

Moreover, by providing the first separator 13a on the outer winding side of the positive electrode <NUM> and providing the second separator 13b on the inner winding side of the positive electrode <NUM>, there are restrained the separators from fracturing in the case where the non-aqueous electrolyte secondary battery <NUM> receives the impact from the outside. Therefore, the thermal runaway can be restrained even when the impact from the outside is exerted on the non-aqueous electrolyte secondary battery <NUM> in the charging state. According to the invention, the first separator 13a lower in piercing strength has a higher coefficient of extension in the winding axis direction α than the second separator 13b. It is therefore inferred that the difference in coefficient of extension between the first separator 13a and the second separator 13b contributes to the restraint of the thermal runaway. Accordingly, by arranging the first separator 13a lower in piercing strength on the outer winding side of the positive electrode <NUM> and arranging the second separator 13b higher in piercing strength on the inner winding side of the positive electrode <NUM>, there can be enhanced the insulation between the positive electrode <NUM> and the negative electrode <NUM> and be obtained the nonaqueous electrolyte secondary battery <NUM> that is strong against impact from the outside.

According to the invention, the first separator 13a has <NUM> N or more and less than <NUM> N of piercing strength, and <NUM>% or more and <NUM>% or less of coefficient of extension in the winding axis direction α, preferably <NUM> N or more and <NUM> N or less of piercing strength, and <NUM>% or more and <NUM>% or less of coefficient of extension in the winding axis direction α. According to the invention, the second separator 13b has <NUM> N or more and <NUM> N or less of piercing strength, and <NUM>% or more and <NUM>% or less of coefficient of extension in the winding axis direction α, preferably <NUM> N or more and <NUM> N or less of piercing strength, and <NUM>% or more and <NUM>% or less of coefficient of extension in the winding axis direction α.

The coefficients of extension in the winding axis direction α are measured in conformity to the JIS standard, JIS K-<NUM>. The first separator 13a and the second separator 13b are cut to have <NUM> to <NUM> of widths and <NUM> or more of lengths to produce samples for measuring coefficients of extension. Each sample for measuring a coefficient of extension is attached to grippers such that its longitudinal direction coincides with the axis of the tester, there are measured tensile force and its extension during pulling the sample for measuring the coefficient of extension at a constant speed to measure the coefficient of extension in the winding axis direction α.

Moreover, when the negative electrode lead <NUM> is provided at the inner end part of the negative electrode <NUM> in the longitudinal direction y, there is a case where, at a site on the outer winding side of the negative electrode lead <NUM>, the insulation between the positive electrode <NUM> and the negative electrode <NUM> lowers. Accordingly, when the negative electrode lead <NUM> is provided at the inner end part of the negative electrode <NUM> in the longitudinal direction y, there is significant the effect, of the present embodiment, of improving the insulation between the positive electrode <NUM> and the negative electrode <NUM>.

The first separator 13a and the second separator 13b each is produced by extruding and shaping a polyolefin-based resin into a sheet shape, and after that, stretching it in the flow direction (MD: Machine Direction) and in the direction (TD: Transverse Direction) perpendicular to the flow direction simultaneously or sequentially to make it into a thin film. The stretching can make molecules in the polyolefin-based resin oriented, which leads to its crystallization and can improve its piercing strength. Meanwhile, a polyolefin-based resin has a predefined amount by which it can be stretched, and the stretching of those into thin films causes the coefficients of extension of the first separator 13a and the second separator 13b after made into the thin films to be made small. In other words, the first separator 13a and the second separator 13b have trade-off relation between the coefficients of extension and the piercing strengths.

While the present disclosure will be hereinafter further described with examples, the present disclosure is not limited to these examples.

As the positive electrode active material, there was used aluminum-containing lithium cobalt nickelate represented by LiNi<NUM>Co<NUM>Al<NUM>O<NUM>. <NUM> parts by mass of LiNi<NUM>Co<NUM>Al<NUM>O<NUM>, <NUM> part by mass of acetylene black, and <NUM> parts by mass of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-<NUM>-pyrrolidone (NMP) was added to those to prepare the positive electrode slurry. Next, the positive electrode slurry was applied onto both sides of a long strip-shaped positive electrode current collector composed of aluminum foil with <NUM> of thickness, and the coating film was heated and dried at <NUM> to <NUM>. The dried coating film was compressed to have <NUM> of thickness using rolls, and after that, was cut to have <NUM> of width and <NUM> of length to produce the positive electrode having the positive electrode active material layers formed on both sides of the positive electrode current collector.

<NUM> parts by mass of graphite, <NUM> parts by mass of Si oxide, <NUM> part by mass of carboxymethylcellulose (CMC), and <NUM> part by mass of styrene-butadiene rubber were mixed, and an appropriate amount of water was added to those to prepare the negative electrode slurry. Next, the negative electrode slurry was applied onto both sides of a long strip-shaped negative electrode current collector composed of copper foil with <NUM> of thickness, and the coating film was dried. The dried coating film was compressed to have <NUM> of thickness using rolls, and after that, was cut to have <NUM> of width and <NUM> of length to produce the negative electrode having the negative electrode active material layers formed on both sides of the negative electrode current collector.

There were prepared two kinds of separators made of polyolefin-based resins. As the first separator, there was used a separator A having <NUM> of thickness, <NUM> N of piercing strength, and <NUM>% of coefficient of extension in the winding axis direction. Moreover, as the second separator, there was used a separator B having <NUM> of thickness, <NUM> N of piercing strength, and <NUM>% of coefficient of extension in the winding axis direction.

To <NUM> parts by mass of a mixed solvent composed of ethylene carbonate (EC) and dimethylmethyl carbonate (DMC) (EC:DMC=<NUM>:<NUM> by volume), <NUM> parts by mass of vinylene carbonate (VC) was added. LiPF<NUM> was dissolved in the mixed solvent so as to be in <NUM> mol/L of concentration to prepare the electrolytic solution.

Ten winding-shaped electrode assemblies were produced each by laminating, and then winding, the second separator, the positive electrode, the first separator and the negative electrode in this order from the inner winding side. To each of the electrode assemblies, <NUM> V of AC voltage was applied in an atmosphere at <NUM>, and it was examined whether <NUM> mA or more of current flowed therein. The electrode assembly(assemblies) that allowed <NUM> mA or more of current to flow was(were) taken as being defective and the other(s) was(were) taken as being good to calculate the fraction defective of those.

Two winding-shaped electrode assemblies were produced each by laminating, and then winding, the second separator, the positive electrode, the first separator and the negative electrode in this order from the inner winding side. Insulating plates were arranged on and below on the electrode assembly, and the electrode assembly was housed in the battery case. Next, a negative electrode lead was welded to the bottom part of the battery case, and a positive electrode lead was welded to a sealing assembly having a vent valve of internal pressure operation type. After that, after the electrolytic solution was injected into the interior of the battery case in a reduced pressure scheme, the opening end part of the battery case was sealed in such a manner that the opening end part of the battery case is crimped onto the sealing assembly across the gasket, for producing two cylinder-shaped secondary batteries. After the produced batteries were charged up to <NUM> V through constant current charging at <NUM> mA (<NUM> of hour rate) in an atmosphere at <NUM>, they were charged through constant voltage charging at <NUM> V with <NUM> mA of cut-off current. After that, on one of the batteries, the test was conducted in conformity to the items for the T6 impact test under the UN test conditions for transport (causing <NUM> of weight to drop from <NUM> of height onto a metal-made round rod with <NUM> of diameter placed above on the center of the battery). On the other of the batteries, the test was conducted similarly to the one battery except that <NUM> of weight was caused to drop from <NUM> of height. It was examined whether or not firing from each battery and/or blowup of each battery arose within six hours after the tests. The case of no firing or blowup was taken as being good, and the other cases were taken as being defective.

Winding-shaped electrode assemblies were produced similarly to Example <NUM> except that the separators A were used for the first separator and the second separator.

Winding-shaped electrode assemblies were produced similarly to Example <NUM> except that the separators B were used for the first separator and the second separator.

Winding-shaped electrode assemblies were produced similarly to Example <NUM> except that the separator B was used for the first separator and the separator A was used for the second separator.

The evaluation results for the example and the comparative examples are presented in Table <NUM>.

Claim 1:
A non-aqueous electrolyte secondary battery (<NUM>), comprising:
a winding-shaped electrode assembly (<NUM>) having a positive electrode (<NUM>) and a negative electrode (<NUM>) wound with a first separator (13a) and a second separator (13b) intervening therebetween; and
a battery case (<NUM>) housing the electrode assembly (<NUM>), wherein
the first separator (13a) is provided on an outer winding side of the positive electrode (<NUM>),
the second separator (13b) is provided on an inner winding side of the positive electrode (<NUM>), and
characterized in that the first separator (13a) has <NUM> N or more and less than <NUM> N of piercing strength, and <NUM>% or more and <NUM>% or less of coefficient of extension in a winding axis direction, and
the second separator (13b) has <NUM> N or more and <NUM> N or less of piercing strength, and <NUM>% or more and <NUM>% or less of coefficient of extension in the winding axis direction, wherein the piercing strength and the coefficient of extension are measured according to the description,
wherein the first separator (13a) has a higher coefficient of extension in the winding axis direction (α) than the second separator (13b).