Patent Description:
A secondary battery such as a lithium ion battery comprises an electrode assembly formed by laminating a positive electrode and a negative electrode via a separator. Generally, each electrode plate includes a core configured by metal foil and an active material layer formed on a surface of the core. The active material layer is expanded and contracted by charging/discharging, and a degree of expansion/contraction increases for a battery of a high energy density in particular. Accordingly, a large load acts on a part of the core during the charging/discharging, and local deformation of the electrode plate may occur.

Conventionally, a technology for mitigating effects of the expansion/contraction of the active material layer accompanying the charging/discharging has been proposed. For example, Patent Literature <NUM> discloses an electrode for a secondary battery using a core an elongation percentage of which is <NUM>% or higher. Patent Literature <NUM> describes an effect of suppressing pulverization of active material particles and falling from the core of the active material layer due to expansion/contraction of the active material layer. In addition, Patent Literature <NUM> discloses a manufacturing method of a negative electrode for a secondary battery including a process of heating copper foil to be a negative electrode core to a recrystallization temperature or higher and softening the copper foil.

As a method for suppressing local deformation of an electrode plate, it is an effective technique to increase an elongation percentage of a core as disclosed in Patent Literatures <NUM> and <NUM>, however, when the elongation percentage of the core becomes too high, there is a risk that the electrode plate is brought into contact with an outer housing can or a sealing plate in a case where the electrode plate is expanded by charging/discharging. Accordingly, it is needed to limit a size of the electrode plate and that leads to decline of a capacity. In addition, while predetermined tension acts on the core when manufacturing the electrode plate, when the elongation percentage of the core is high, there are cases where the core is elongated by the tension during manufacture and production stability is damaged.

It is an object of the present disclosure to provide a secondary battery capable of suppressing local deformation of an electrode plate accompanying charging/discharging without causing failures such as decline of a capacity and decline of production stability.

A secondary battery as one aspect of the present disclosure is the secondary battery comprising an electrode assembly including a positive electrode, a negative electrode and a separator, the positive electrode and the negative electrode of which are laminated via the separator, the positive electrode includes a positive electrode core having a longitudinal direction and a width direction and a positive electrode active material layer formed on a surface of the positive electrode core, the negative electrode includes a negative electrode core having a longitudinal direction and a width direction and a negative electrode active material layer formed on a surface of the negative electrode core, and a tensile elongation percentage at a width direction center portion of at least one of the positive electrode core and the negative electrode core is higher than a tensile elongation percentage at both width direction end portions of at least one of the positive electrode core and the negative electrode core.

A secondary battery as another aspect of the present disclosure is the secondary battery comprising an electrode assembly including a positive electrode, a negative electrode and a separator, the positive electrode and the negative electrode of which are laminated via the separator, the positive electrode includes a positive electrode core having a longitudinal direction and a width direction and a positive electrode active material layer formed on a surface of the positive electrode core, the negative electrode includes a negative electrode core having a longitudinal direction and a width direction and a negative electrode active material layer formed on a surface of the negative electrode core, and a number of crystal grains per unit area at a width direction center portion of at least one of the positive electrode core and the negative electrode core is smaller than a number of crystal grains per unit area at both width direction end portions of at least one of the positive electrode core and the negative electrode core.

According to the secondary battery relating to the present disclosure, local deformation of an electrode plate accompanying charging/discharging may be suppressed without causing failures such as decline of a capacity and decline of production stability. When the electrode plate is locally deformed, uniformity of battery reaction is damaged and a cycle characteristic declines for example, however, according to the secondary battery relating to the present disclosure, decline of the cycle characteristic due to the local deformation of the electrode plate may be suppressed.

Hereinafter, an example of the embodiment of a secondary battery relating to the present disclosure will be explained in details with reference to the drawings. Note that it is assumed from the first that embodiments and modifications illustrated below are to be selectively combined. In addition, in the present description, description of "numerical value A-numerical value B" means "numerical value A or higher and numerical value B or lower" unless otherwise mentioned.

<FIG> is a sectional view of a secondary battery <NUM> as an example of the embodiment. The secondary battery <NUM> illustrated in <FIG> comprises a bottomed cylindrical outer housing can <NUM> as an outer body, but the outer body is not limited thereto. The secondary battery relating to the present disclosure may be, for example, a rectangular battery comprising an outer housing can in a bottomed rectangular tube shape, a coin shape battery comprising an outer housing can in a coin shape, or a laminated battery comprising an outer body configured by a lamination sheet including a metal layer and a resin layer.

As illustrated in <FIG>, the secondary battery <NUM> comprises a wound type electrode assembly <NUM> for which a positive electrode <NUM> and a negative electrode <NUM> are wound in a helical shape via a separator <NUM>, and the cylindrical outer housing can <NUM> that houses the electrode assembly <NUM>. In addition, the secondary battery <NUM> comprises an electrolyte housed in the outer housing can <NUM> together with the electrode assembly <NUM>. The outer housing can <NUM> is a metallic container in a bottomed cylindrical shape opened on one side in an axial direction, and an opening of the outer housing can <NUM> is closed by a sealing assembly <NUM>. Hereinafter, for convenience of explanation, the side of the sealing assembly <NUM> of the battery is up and a bottom portion side of the outer housing can <NUM> is down.

The electrolyte may be an aqueous electrolyte but is preferably a non-aqueous electrolyte. The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For the non-aqueous solvent, ester, ether, nitrile, amide and a mixed solvent of two or more kinds thereof are used for example. The non-aqueous solvent may contain a halogen substitution product in which at least some hydrogen of the above solvents is replaced by halogen atoms such as fluorine. For the electrolyte salt, a lithium salt such as LiPF<NUM> is used for example. Note that the electrolyte is not limited to a liquid electrolyte and may be a solid electrolyte.

The positive electrode <NUM>, the negative electrode <NUM> and the separator <NUM> configuring the electrode assembly <NUM> are all a belt-like long body, and are alternately laminated in a radial direction of the electrode assembly <NUM> by being wound in the helical shape. The negative electrode <NUM> is formed in a dimension one size larger than the positive electrode <NUM> in order to prevent deposition of lithium. That is, the negative electrode <NUM> is formed longer than the positive electrode <NUM> in a longitudinal direction and a width direction. The two separators <NUM> are formed in a dimension one size larger than at least the positive electrode <NUM>, and are arranged so as to hold the positive electrode <NUM> therebetween for example. The electrode assembly <NUM> includes a positive electrode lead <NUM> connected to the positive electrode <NUM> by welding or the like and a negative electrode lead <NUM> connected to the negative electrode <NUM> by welding or the like.

Above and below the electrode assembly <NUM>, insulating plates <NUM> and <NUM> are arranged respectively. In the example illustrated in <FIG>, the positive electrode lead <NUM> passes through a through-hole of the insulating plate <NUM> and extends to the side of the sealing assembly <NUM>, and the negative electrode lead <NUM> passes through a through-hole of the insulating plate <NUM> and extends to the bottom portion side of the outer housing can <NUM>. The positive electrode lead <NUM> is connected to a lower surface of an internal terminal plate <NUM> of the sealing assembly <NUM> by welding or the like, and a cap <NUM> which is a top plate of the sealing assembly <NUM> electrically connected with the internal terminal plate <NUM> becomes a positive electrode terminal. The negative electrode lead <NUM> is connected to a bottom portion inner surface of the outer housing can <NUM> by welding or the like, and the outer housing can <NUM> becomes a negative electrode terminal.

A gasket <NUM> is provided between the outer housing can <NUM> and the sealing assembly <NUM>, and sealability inside the battery is secured. On the outer housing can <NUM>, a grooved portion <NUM> for which a part of a side face portion is projected to an inner side and which supports the sealing assembly <NUM> is formed. The grooved portion <NUM> is preferably formed in an annular shape along a circumferential direction of the outer housing can <NUM>, and supports the sealing assembly <NUM> by the upper surface. The sealing assembly <NUM> is fixed to an upper portion of the outer housing can <NUM> by the grooved portion <NUM> and an opening end portion of the outer housing can <NUM> caulked to the sealing assembly <NUM>.

The sealing assembly <NUM> has a structure that the internal terminal plate <NUM>, a lower vent member <NUM>, an insulating member <NUM>, an upper vent member <NUM> and the cap <NUM> are laminated in order from the side of the electrode assembly <NUM>. Each member configuring the sealing assembly <NUM> has a disk shape or a ring shape for example, and each member except the insulating member <NUM> is electrically connected to each other. The lower vent member <NUM> and the upper vent member <NUM> are connected at respective center portions, and the insulating member <NUM> is interposed between respective peripheral edge portions. When an internal pressure of the battery rises due to abnormal heat generation, the lower vent member <NUM> is deformed so as to push up the upper vent member <NUM> to the side of the cap <NUM> and is fractured and thus a current path between the lower vent member <NUM> and the upper vent member <NUM> is shut off. When the internal pressure rises further, the upper vent member <NUM> is fractured and gas is discharged from an opening of the cap <NUM>.

Hereinafter, with reference to <FIG>, the positive electrode <NUM>, the negative electrode <NUM> and the separator <NUM> configuring the electrode assembly <NUM>, the negative electrode <NUM> in particular, will be explained in details. <FIG> is a perspective view of the positive electrode <NUM> and the negative electrode <NUM> and illustrates an opposite arrangement of each other.

While details will be described later, in the present embodiment, a tensile elongation percentage at a width direction center portion of a negative electrode core <NUM> is higher than a tensile elongation percentage at both width direction end portions of the negative electrode core <NUM>. In addition, the number of crystal grains per unit area at the width direction center portion of the negative electrode core <NUM> is smaller than the number of the crystal grains per unit area at both width direction end portions of the negative electrode core <NUM>. While the tensile elongation percentage of a positive electrode core <NUM> is practically same in an entire region and the number of the crystal grains per unit area is also practically same in the entire region, a configuration similar to the negative electrode core <NUM> may be applied to the positive electrode core <NUM> and the tensile elongation percentage at the width direction center portion of the positive electrode core <NUM> may be made higher than the tensile elongation percentage at both width direction end portions of the positive electrode core <NUM>. In addition, the number of the crystal grains per unit area at the width direction center portion of the positive electrode core <NUM> may be smaller than the number of the crystal grains per unit area at both width direction end portions of the positive electrode core <NUM>. Alternatively, the configuration may be applied only to the positive electrode core <NUM>.

As illustrated in <FIG>, the positive electrode <NUM> includes the positive electrode core <NUM>, and a positive electrode active material layer <NUM> formed on a surface of the positive electrode core <NUM>. For the positive electrode core <NUM>, foil of a metal which is stable in a potential range of the positive electrode <NUM> such as aluminum and an aluminum alloy or a film for which the metal is arranged on a surface layer or the like can be used. An ideal example of the positive electrode core <NUM> is metal foil configured by the aluminum alloy containing iron. Thickness of the positive electrode core <NUM> is, for example, <NUM> to <NUM>. At the positive electrode <NUM>, for example, the positive electrode active material layer <NUM> is not formed at a longitudinal direction center portion, an exposed portion (not illustrated) where the surface of the positive electrode core <NUM> is exposed is formed, and the positive electrode lead <NUM> is connected to the exposed portion.

It is preferable that the positive electrode active material layer <NUM> includes a positive electrode active material, a conductive agent and a binder and is formed on both surfaces of the positive electrode core <NUM>. The positive electrode active material layer <NUM> has the thickness of <NUM> to <NUM> on one side of the positive electrode core <NUM> for example, and is formed on both surfaces of the positive electrode core <NUM> respectively with the same thickness. The positive electrode <NUM> can be manufactured by applying positive electrode mixture slurry including the positive electrode active material, the conductive agent and the binder or the like onto the positive electrode core <NUM>, drying and then compressing a coated film and forming the positive electrode active material layer <NUM> on both surfaces of the positive electrode core <NUM>.

For the positive electrode active material, for example, lithium transition metal composite oxide is used. Examples of metal elements contained in the lithium transition metal composite oxide are Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta and W. Among them, it is preferable to contain at least one of the group consisting of Ni, Co, Mn. Examples of ideal composite oxide are lithium transition metal composite oxide containing Ni, Co and Mn, and lithium transition metal composite oxide containing Ni, Co and Al.

An example of the conductive agent included in the positive electrode active material layer <NUM> is a carbon material such as carbon black, acetylene black, Ketjen black and graphite. Examples of the binder included in the positive electrode active material layer <NUM> are fluorocarbon resin such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resin, acrylic resin and polyolefin resin. In addition, the resin and a cellulose derivative such as carboxymethyl cellulose (CMC) or the salt thereof and polyethylene oxide (PEO) or the like may be used together.

The negative electrode <NUM> includes the negative electrode core <NUM>, and a negative electrode active material layer <NUM> formed on a surface of the negative electrode core <NUM>. For the negative electrode core <NUM>, foil of a metal which is stable in a potential range of the negative electrode <NUM> such as copper or a film for which the metal is arranged on a surface layer or the like can be used. An ideal example of the negative electrode core <NUM> is metal foil a main component of which is copper, and may be copper foil configured practically by copper only. The thickness of the negative electrode core <NUM> is thinner than the thickness of the positive electrode core <NUM> and is <NUM> to <NUM>, for example. At the negative electrode <NUM>, for example, the negative electrode active material layer <NUM> is not formed at both longitudinal direction end portions, an exposed portion <NUM> where the surface of the negative electrode core <NUM> is exposed is formed, and the negative electrode lead <NUM> (not illustrated in <FIG>) is connected to the exposed portion <NUM>.

The negative electrode core <NUM> may be either of rolled copper foil manufactured by hot rolling an ingot of high purity and electrolytic copper foil manufactured by electroplating. The electrolytic copper foil is manufactured while controlling a crystal grain diameter by adjusting a type, a concentration and a deposition speed or the like of an additive. Generally, the copper foil is easily elongated when the crystal grain diameter of the copper foil is large, and the copper foil is not easily elongated when the crystal grain diameter is small. The crystal grains of the copper foil can be confirmed by a scanning electron microscope (SEM).

The negative electrode core <NUM> is, as described above, a belt-like long body, and the tensile elongation percentage at the width direction center portion is higher than the tensile elongation percentage at both width direction end portions. The tensile elongation percentage at the width direction center portion of the negative electrode core <NUM> is preferably <NUM> times of the tensile elongation percentage at both width direction end portions or higher, is more preferably two times or higher, and is further more preferably three times or higher. In the present embodiment, a first region <NUM> is formed so that the tensile elongation percentage at the width direction center portion of the negative electrode core <NUM> becomes high. At the negative electrode core <NUM>, the first region <NUM> which is a high elongation region and a second region <NUM> where the tensile elongation percentage is lower than that in the first region <NUM> are present. The second region <NUM> is arranged in a range including both width direction end portions of the negative electrode core <NUM> such that the tensile elongation percentage at the width direction center portion of the negative electrode core <NUM> becomes higher than the tensile elongation percentage at both width direction end portions.

The first region <NUM> is preferably formed along the longitudinal direction of the negative electrode core <NUM>. In the example illustrated in <FIG>, the first region <NUM> is formed in a fixed width over the entire length of the negative electrode core <NUM>. The first region <NUM> is formed in a belt shape centering on the width direction center of the negative electrode core <NUM>, for example. The width of the first region <NUM> is preferably <NUM>% or less of the entire width of the negative electrode core <NUM>, is more preferably <NUM>% or less, and is especially preferably <NUM>% or less. A lower limit value of the width of the first region <NUM> is preferably <NUM>%, is more preferably <NUM>%, and is especially preferably <NUM>%.

At the width direction center portion of the negative electrode core <NUM>, the negative electrode active material layer <NUM> is expanded and pulled to both sides in the width direction during charging, for example. In particular, for an region Z opposite to an edge of the positive electrode active material layer <NUM>, since volume is changed by charging/discharging at the part opposite to the positive electrode active material layer <NUM> of the negative electrode active material layer <NUM> but the volume is not changed at the part not opposite to the positive electrode active material layer <NUM>, larger stress easily acts in the region Z compared to the other region. By forming the first region <NUM> which is the high elongation region at the width direction center portion of the negative electrode core <NUM>, local deformation of the negative electrode <NUM> can be efficiently suppressed. Note that, when the tensile elongation percentage of the entire negative electrode core <NUM> is increased, as described above, failures such as decline of a capacity and decline of production stability are concerned.

The tensile elongation percentage of the negative electrode core <NUM> is measured by a following method based on IPC-TM-<NUM>.

The tensile elongation percentage measured by the method described above in the first region <NUM> is preferably <NUM>% or higher and is more preferably <NUM>% or higher. An upper limit value of the tensile elongation percentage in the first region <NUM> is preferably <NUM>%, and is more preferably <NUM>%. The tensile elongation percentage measured by the method described above in the second region <NUM> is lower than <NUM>% for example, and is preferably <NUM>% or lower. The lower limit value of the tensile elongation percentage in the second region <NUM> is <NUM>% for example. When the tensile elongation percentages in the first region <NUM> and the second region <NUM> are within the range, the local deformation of the negative electrode <NUM> can be suppressed without causing the failures such as the capacity decline.

For the negative electrode core <NUM>, the number of the crystal grains per unit area at the width direction center portion is smaller than the number of crystal grains per unit area at both width direction end portions. The number of the crystal grains per unit area at the width direction center portion of the negative electrode core <NUM> is preferably <NUM>/<NUM> or less of the number of the crystal grains per unit area at both width direction end portions, is more preferably <NUM>/<NUM> or less, and is especially preferably <NUM>/<NUM> or less.

The crystal grains of the negative electrode core <NUM> can be confirmed by a SEM as described above. The number of the crystal grains per unit area is obtained by measuring the crystal grains present in a range of a predetermined square in a sectional SEM image of the negative electrode core <NUM>. The number of the crystal grains at the width direction center portion of the negative electrode core <NUM> can be measured by matching the center of the predetermined square with the width direction center of the negative electrode core <NUM>, for example. The number of the crystal grains at both width direction end portions of the negative electrode core <NUM> can be measured by matching one side of the predetermined square with one end or the other end in the width direction of the negative electrode core <NUM>, for example. At the time, a size of the square is preferably determined so as to include at least one crystal grain at the width direction center portion of the negative electrode core <NUM>.

In the present embodiment, the number per unit area of the crystal grains in the first region <NUM> which is the high elongation region is smaller than the number per unit area of the crystal grains in the second region <NUM> which is a low elongation region. Since the first region <NUM> is present in the range including the width direction center portion of the negative electrode core <NUM> and the second region <NUM> is present in the range including both width direction end portions of the negative electrode core <NUM>, the tensile elongation percentage at the width direction center portion is higher than the tensile elongation percentage at both width direction end portions for the negative electrode core <NUM>.

The first region <NUM> is formed by locally heat-treating a part including the width direction center portion of the negative electrode core <NUM>. A heat treatment method of the negative electrode core <NUM> is not limited in particular, however, it is preferable to apply contact type heating means. For example, a part of the negative electrode core <NUM> can be locally heat-treated by using a heat roller or a hot plate which holds the negative electrode core <NUM> from both sides in a thickness direction. A heat treatment temperature is different depending on the metal configuring the negative electrode core <NUM>, and in the case of the copper foil, it is preferably <NUM> to <NUM> and is more preferably <NUM> to <NUM>. Note that it is also possible to perform heat treatment of the negative electrode core <NUM> by heating the negative electrode core <NUM> together with the negative electrode active material layer <NUM> after the negative electrode active material layer <NUM> is formed.

It is preferable that the negative electrode active material layer <NUM> includes a negative active material and a binder and is formed on both surfaces of the negative electrode core <NUM>. The negative electrode active material layer <NUM> has the thickness of <NUM> to <NUM> on one side of the negative electrode core <NUM> for example, and is formed on both surfaces of the negative electrode core <NUM> respectively with the same thickness. The negative electrode <NUM> can be manufactured by applying negative electrode mixture slurry including the negative electrode active material and the binder or the like onto the negative electrode core <NUM> the width direction center portion of which is locally heat-treated, drying and then compressing a coated film and forming the negative electrode active material layer <NUM> on both surfaces of the negative electrode core <NUM>.

The negative electrode active material includes a carbon-based active material which reversibly occludes and releases lithium ions, for example. The ideal carbon-based active material is graphite, such as natural graphite such as flaky graphite, massive graphite and earthy graphite and artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, for the negative electrode active material, a Si-based active material configured by at least one of Si and a Si-containing compound may be used or the carbon-based active material and the Si-based active material may be used together. A lithium metal may be also used for the negative electrode active material, and in that case, the negative electrode active material layer <NUM> can be formed without using the binder.

For the binder included in the negative electrode active material layer <NUM>, similarly to the case of the positive electrode <NUM>, fluorocarbon resin, PAN, polyimide resin, acrylic resin and polyolefin resin or the like can be used, however, it is preferable to use styrenebutadiene rubber (SBR). In addition, the negative electrode active material layer <NUM> preferably includes CMC or the salt thereof, polyacrylic acid (PAA) or the salt thereof, and polyvinyl alcohol (PVA) or the like, further. Among them, it is ideal to use SBR, CMC or the salt thereof and PAA or the salt thereof together.

For the separator <NUM>, a porous sheet having ion permeability and an insulation property is used. Specific examples of the porous sheet are a microporous thin film, woven fabric and non-woven fabric. Ideal materials of the separator <NUM> are polyolefin such as polyethylene, polypropylene and a copolymer of ethylene and α olefin, and cellulose or the like. The separator <NUM> may be either of a single layer structure and a laminated structure. On the surface of the separator <NUM>, a heat resistant layer including inorganic particles or a heat resistant layer configured by highly heat resistant resin such as aramid resin, polyimide and polyamide-imide or the like may be formed.

The present disclosure will be explained further by examples hereinafter, however, the present disclosure is not limited by the examples.

Both width direction ends of the copper foil of <NUM> thickness cut into <NUM> in width × <NUM> in length were lifted to slacken the copper foil. The width direction center portion of the slackened copper foil was pressed to a hot plate heated to <NUM> using a heat resistant jig of <NUM> width from above. At the time, force of lifting the copper foil was adjusted so that the part where the copper foil was in contact with the hot plate was only the part pressed by the heat resistant jig and a fold was not made at the part where an edge of the heat resistant jig was in contact. The state was continued for five minutes and the heat treatment of the copper foil was ended. Thus, the copper foil relating to Example <NUM> for which the high elongation region having the high elongation percentage is formed at the width direction center portion over the entire length in the longitudinal direction was obtained. Note that the heat treatment was performed in a dry box where a dew point was -<NUM> or lower.

The copper foil before the heat treatment of Example <NUM> was defined as the copper foil relating to Comparative Example <NUM>.

Except that the heat treatment was performed by pressing the entire copper foil to the hot plate heated to <NUM>, the copper foil relating to Comparative Example <NUM> was obtained similarly to Example <NUM>.

In order to simulate a state where the copper foil as the negative electrode core was locally loaded by expansion/contraction of the negative electrode active material layer accompanying charging/discharging of the battery, a pressurization test was performed by a following method using a universal tester (the autograph described above). The pressurization test was performed for the individual copper foil of Example <NUM>, Comparative Example <NUM> and Comparative Example <NUM>.

A jig of <NUM> square manufactured by SUS Corporation was fixed to an upper part and a base manufactured by SUS Corporation was fixed to a lower part respectively horizontally to ground. The heat-treated copper foil was placed on the base and a fluororubber sheet of <NUM> thickness cut into <NUM> square was put on the copper foil. At the time, the copper foil and the rubber sheet were arranged so as to match the centers of the jig, the rubber sheet and the copper foil. In the state, the jig at the upper part was lowered until the load of <NUM> kN was applied to the jig at a speed of <NUM>/sec by the universal tester. Thereafter, the jig was elevated until no more load was applied to the jig at the same speed, and presence/absence of fracture of the copper foil was confirmed. A tape (<NUM> in width and <NUM> in thickness) made of polypropylene was stuck to one surface of the copper foil and the present test was performed in the state of turning the tape to the base side. A level difference of the tape was provided in order to simulate the region Z illustrated in <FIG> where the local load is applied to the electrode assembly. The tape was stuck so as to position one width direction end at the width direction center of the copper foil.

By the method described above, the tensile elongation percentages of the individual copper foil of Comparative Example <NUM> and Comparative Example <NUM> were measured. Note that the width direction center portion of the copper foil of Example <NUM> and the copper foil of Comparative Example <NUM> were heat-treated under a same condition. In addition, both width direction end portions of the copper foil of Example <NUM> were not heat-treated similarly to the copper foil of Comparative Example <NUM>. Accordingly, for the individual tensile elongation percentages at the width direction center portion and both width direction end portions in Table <NUM>, measured results of Comparative Example <NUM> and Comparative Example <NUM> were used respectively.

From the sectional SEM image of the copper foil, the numbers of the crystal grains present in the range of the predetermined square of the individual copper foil of Example <NUM>, Comparative Example <NUM> and Comparative Example <NUM> were measured.

As illustrated in Table <NUM>, the copper foil of Example <NUM> for which the high elongation region is formed at the width direction center portion has fracture resistance similarly to the case where the entire copper foil is heat-treated (Comparative Example <NUM>). That is, since the high elongation region absorbs the local load, the local deformation accompanying the charging/discharging is suppressed for the negative electrode including the copper foil of Example <NUM>. In the copper foil of Comparative Example <NUM> without the presence of the high elongation region, the fracture occurs in the pressurization test. Further, for the copper foil of Example <NUM>, since the tensile elongation percentage at both width direction end portions is low, the elongation of the copper foil due to tension during manufacture and the charging/discharging is suppressed compared to Comparative Example <NUM> in which the tensile elongation percentage of the entire copper foil is high. Thus, the failures such as the decline of the production stability and the decline of a battery capacity due to regulation of the size of the electrode plate can be avoided.

Claim 1:
A secondary battery (<NUM>), comprising an electrode assembly (<NUM>) including a positive electrode (<NUM>), a negative electrode (<NUM>) and a separator (<NUM>), the positive electrode (<NUM>) and the negative electrode (<NUM>) of which are laminated via the separator (<NUM>), wherein
the positive electrode (<NUM>) includes a positive electrode core (<NUM>) having a longitudinal direction and a width direction, and a positive electrode active material layer (<NUM>) formed on a surface of the positive electrode core (<NUM>),
the negative electrode (<NUM>) includes a negative electrode core (<NUM>) having a longitudinal direction and a width direction, and a negative electrode active material layer (<NUM>) formed on a surface of the negative electrode core (<NUM>), and
a tensile elongation percentage at a width direction center portion of at least one of the positive electrode core (<NUM>) and the negative electrode core (<NUM>) is higher than a tensile elongation percentage at both width direction end portions of at least one of the positive electrode core (<NUM>) and the negative electrode core (<NUM>).