secondary batteries

Abstract

This secondary battery comprises an electrode body obtained by laminating positive electrodes and negative electrodes with a separator interposed therebetween. The separator includes a first layer and a second layer having lower thermal shrinkage than the first layer, and has a tubular section that is formed into a tube shape and constitutes the outermost surface of the electrode body. The separator is arranged such that, in the tubular section thereof, the first layer faces the inside and the second layer faces the outside.

Description

[Technical Field]

[0001] The present disclosure relates to secondary batteries. [Background technology]

[0002] In recent years, demand for secondary batteries has been increasing in various fields. In particular, lithium-ion secondary batteries using nonaqueous electrolytes are widely used in automotive applications, power storage applications, various electronic devices, and the like, due to their high energy density. Secondary batteries include an electrode assembly including a positive electrode, a negative electrode, and a separator. The electrode assembly has a structure in which a separator is interposed between the positive electrode and the negative electrode, thereby preventing contact between the positive electrode and the negative electrode. In addition, many means have been proposed for more reliably preventing the occurrence of internal short circuits due to contact between the positive electrode and the negative electrode.

[0003] For example, Patent Document 1 proposes a method of providing an adhesive layer on the surface of a separator and thermocompressing an electrode assembly to bond the separator surface and the electrode surface in order to prevent internal short circuits caused by misalignment of the positive and negative electrodes. Patent Document 2 also proposes a secondary battery including a separator in which a porous heat-resistant layer containing inorganic particles is formed on the surface of a substrate in order to prevent internal short circuits caused by conductive foreign matter. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-56142 [Patent Document 2] Japanese Patent Application Publication No. 2018-49758 Summary of the Invention

[0005] Even on the outermost surface of an electrode assembly where no opposing electrode is present, the electrode is covered with a separator to prevent the electrode composite layer from being exposed. However, the edge of the separator may curl up, exposing part of the composite layer. When the electrode composite layer on the outermost surface of the electrode assembly is exposed, the exposed part may fall off and get mixed into the electrode assembly, breaking through the separator and causing a micro-short circuit. In particular, when an electrode assembly is manufactured using a separator containing two or more layers with different thermal shrinkage rates through a thermocompression bonding process, the lifting and curling of the separator edge becomes significant.

[0006] The secondary battery according to the present disclosure is a secondary battery having an electrode body in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, the separator including a first layer and a second layer having a smaller thermal shrinkage rate than the first layer, and having a cylindrical portion formed in a cylindrical shape that constitutes the outermost surface of the electrode body, the separator being arranged in the cylindrical portion so that the first layer faces inward and the second layer faces outward.

[0007] The secondary battery according to the present disclosure can more reliably prevent the end of the separator from curling up, thereby exposing the electrode mixture layer on the outermost surface of the electrode assembly, thereby preventing the occurrence of an internal short circuit due to the electrode mixture layer falling off. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view showing the appearance of a secondary battery as an example of an embodiment. [Figure 2] FIG. 2 is a perspective view of an electrode assembly according to an embodiment. [Figure 3] FIG. 3 is a cross-sectional view of an electrode assembly according to an embodiment. [Figure 4A] FIG. 4A is a cross-sectional view of the electrode body of Comparative Example 1. FIG. [Figure 4B] FIG. 4B is a cross-sectional view of the electrode body of Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the drawings. It is anticipated from the outset that multiple exemplary embodiments and modifications exemplified below may be selectively combined.

[0010] FIG. 1 is a perspective view showing the appearance of a secondary battery 10 according to an embodiment, and FIG. 2 is a perspective view of an electrode assembly 11 constituting the secondary battery 10. FIG. 3 is a cross-sectional view of the electrode assembly 11. In the following, a secondary battery 10 that is a so-called prismatic battery in which the electrode assembly 11 is housed in a prismatic outer can 14 is exemplified, but the outer can of the battery is not limited to the outer can 14 and may be, for example, an outer can made of a laminate sheet including a metal layer and a resin layer. In the following, a stacked electrode assembly 11 in which multiple positive electrodes and multiple negative electrodes are stacked with separators interposed therebetween is exemplified, but the electrode assembly may also be a wound electrode assembly.

[0011] 1 to 3, a secondary battery 10 includes an electrode assembly 11 formed by stacking a positive electrode 20 and a negative electrode 30 with a separator 40 interposed therebetween, a bottomed rectangular cylindrical outer can 14 that houses the electrode assembly 11, and a sealing plate 15 that closes the opening of the outer can 14. The outer can 14 is a metal container that is a flat, approximately rectangular parallelepiped with one axial end open, and the sealing plate 15 has an elongated rectangular shape. The outer can 14 and the sealing plate 15 are made of, for example, a metal material whose main component is aluminum.

[0012] For ease of explanation, the height direction of the outer can 14 will be referred to as the "vertical direction" of the secondary battery 10 and each component, with the sealing plate 15 side being referred to as the "top" and the bottom side of the outer can 14 being referred to as the "bottom." The direction along the longitudinal direction of the sealing plate 15 will be referred to as the "horizontal direction" of the secondary battery 10 and each component. Furthermore, the portion of the electrode body 11 excluding the tubular portion 43 of the separator 40, which will be described later, may be referred to as the "electrode group."

[0013] The secondary battery 10 includes an electrolyte housed in an outer can 14 together with the electrode assembly 11. The electrolyte may be an aqueous electrolyte, but is preferably a non-aqueous electrolyte. The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous solvent may be, for example, esters, ethers, nitriles, amides, or a mixed solvent of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6.

[0014] The electrode assembly 11 includes multiple positive electrodes 20 and negative electrodes 30, and has a structure in which the positive electrodes 20 and negative electrodes 30 are alternately stacked one by one with separators 40 interposed between them (see FIG. 3). The electrode assembly 11 generally includes one more negative electrode 30 than the positive electrodes 20, and the negative electrodes 30 are arranged on both sides of the electrode assembly in the stacking direction. The separator 40 is formed in a cylindrical shape and has a cylindrical portion 43 that constitutes the outermost surface of the electrode assembly 11. That is, the separator 40 is wound into a cylindrical shape one or more times on the outermost surface of the electrode assembly 11, and the negative electrodes 30 arranged on both sides of the electrode assembly in the stacking direction are covered with the separator 40.

[0015] The electrode assembly 11 has a laminated structure in which a single zigzag-folded separator 40 is interposed between a positive electrode 20 and a negative electrode 30. A cylindrical portion 43 is formed by this single separator 40. Note that the separator interposed between the positive electrode and the negative electrode and the separator constituting the outermost surface of the electrode assembly may be separate entities, and the electrode assembly may include multiple separators, one each disposed between the positive electrode and the negative electrode, and one separator constituting the cylindrical portion.

[0016] The electrode assembly 11 has multiple positive electrode tabs 23 and multiple negative electrode tabs 33 that extend toward the sealing plate 15. For example, the positive electrode tabs 23 are formed by protruding a portion of the core of the positive electrode 20, and similarly, the negative electrode tabs 33 are formed by protruding a portion of the core of the negative electrode 30. The positive electrodes 20 and negative electrodes 30 are each stacked with a separator 40 interposed between them so that the positive electrode tabs 23 and the negative electrode tabs 33 face the same direction, and the positive electrode tabs 23 are located at one lateral end of the electrode assembly 11, and the negative electrode tabs 33 are located at the other lateral end of the electrode assembly 11.

[0017] A positive electrode terminal 12 and a negative electrode terminal 13 are attached to the sealing plate 15. For example, the positive electrode tab 23 is electrically connected to the positive electrode terminal 12 via a positive electrode current collector (not shown), and the negative electrode tab 33 is electrically connected to the negative electrode terminal 13 via a negative electrode current collector (not shown). The positive electrode terminal 12 and the negative electrode terminal 13 are external connection terminals that are electrically connected to other secondary batteries 10, electronic devices, etc., and are attached to the sealing plate 15 via an insulating member. In addition, the sealing plate 15 is generally provided with a liquid injection part 16 for injecting an electrolyte solution and a gas exhaust valve 17 that opens to exhaust gas when an abnormality occurs in the battery.

[0018] The positive electrode 20, negative electrode 30, and separator 40 that constitute the electrode assembly 11, particularly the layer structure and arrangement of the separator 40, will be described in detail below.

[0019] [Positive electrode] The positive electrode 20 has a positive electrode core and a positive electrode composite layer formed on the surface of the positive electrode core. The positive electrode core can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 20, or a film with such a metal disposed on the surface. The positive electrode composite layer contains a positive electrode active material, a conductive material, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 20 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, a binder, etc., onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode composite layer on both sides of the positive electrode core.

[0020] A lithium transition metal composite oxide is used as the positive electrode active material. Metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0021] Examples of conductive materials contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins may also be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, or polyethylene oxide (PEO).

[0022] [Negative electrode] The negative electrode 30 has a negative electrode core and a negative electrode composite layer formed on the surface of the negative electrode core. The negative electrode core can be made of a foil of a metal such as copper that is stable within the potential range of the negative electrode 30, or a film with such a metal disposed on the surface. The negative electrode composite layer contains a negative electrode active material and a binder, and is preferably formed on both sides of the negative electrode core. The negative electrode 30 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode composite layer on both sides of the negative electrode core.

[0023] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0024] The binder contained in the negative electrode mixture layer can be, as in the case of the positive electrode 20, a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). The negative electrode mixture layer preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof.

[0025] [Separator] The separator 40 is made of a porous sheet having ion permeability and insulating properties. The separator 40 includes a first layer and a second layer having a smaller thermal shrinkage rate than the first layer. In this embodiment, the first layer is a porous resin layer, and the second layer is a porous heat-resistant layer containing inorganic particles. Both the first and second layers may be resin layers, or the separator may have a third layer. The second layer may be a highly heat-resistant resin layer made of, for example, a resin having a higher melting point or softening point than the resin constituting the first resin layer, such as an aramid resin, polyimide, or polyamideimide.

[0026] 3, the separator 40 includes a porous resin substrate 41 (first layer) and a porous heat-resistant layer 42 (second layer) formed on one surface of the resin substrate 41. By providing the heat-resistant layer 42, for example, the separator 40 becomes less susceptible to breakage due to conductive foreign matter, and shrinkage of the separator 40 when the temperature rises can be suppressed. To improve cost-effectiveness while suppressing an increase in the thickness of the electrode body 11, it is preferable to form the heat-resistant layer 42 on only one surface of the resin substrate 41.

[0027] The resin substrate 41 functions as a separator even when used alone. A porous film having ion permeability and insulating properties is used as the resin substrate 41. The thickness of the resin substrate 41 is, for example, 1 μm to 20 μm, and preferably 5 μm to 15 μm. Examples of materials for the resin substrate 41 include olefin resins such as polyethylene, polypropylene, ethylene-propylene copolymers, and copolymers with ethylene, propylene, and other α-olefins. The melting point of the resin substrate 41 is generally 200° C. or lower.

[0028] The heat-resistant layer 42 is composed mainly of inorganic particles. The heat-resistant layer 42 is preferably composed of insulating inorganic particles and a binder that binds the particles to each other and to the resin substrate 41. The heat-resistant layer 42 has ion permeability and insulating properties, similar to the resin substrate 41. The thickness of the heat-resistant layer 42 is, for example, 1 μm to 10 μm, and preferably 1 μm to 6 μm.

[0029] The inorganic particles may be at least one selected from the group consisting of alumina, boehmite, silica, titania, and zirconia. Among these, alumina or boehmite is preferably used. The content of the inorganic particles is preferably 85% by mass to 99.9% by mass, and more preferably 90% by mass to 99.5% by mass, relative to the mass of the heat-resistant layer 42.

[0030] The binder constituting heat-resistant layer 42 can be a resin similar to the binder contained in the positive electrode composite layer and the negative electrode composite layer, such as a fluororesin such as PVdF or SBR. The content of the binder is preferably 0.1 mass % to 15 mass %, and more preferably 0.5 mass % to 10 mass %, relative to the mass of heat-resistant layer 42. Heat-resistant layer 42 is formed, for example, by applying a slurry containing inorganic particles and a binder to one surface of resin substrate 41 and drying the coating.

[0031] An adhesive layer is formed on at least one surface of the separator 40, for example, to adhere to the surface of the positive electrode 20 or the negative electrode 30. The adhesive layer may be formed on both surfaces of the separator 40, in which case the adhesive layer on one surface may have a different configuration from the adhesive layer on the other surface. The thickness of the adhesive layer is, for example, 0.1 μm to 1 μm, or 0.2 μm to 0.9 μm. The adhesive layer is formed, for example, by applying an emulsion adhesive in which adhesive components are dispersed in water to the surface of the separator 40 and drying the coating. The adhesive layer may be formed, for example, in the form of dots.

[0032] Preferably, the adhesive layer has no adhesiveness at room temperature (25°C) and becomes adhesive when heated. One example of the adhesive constituting the adhesive layer is an adhesive containing acrylic resin as a main component. The electrode body 11 is manufactured, for example, by laminating the negative electrode 30 / separator 40 with adhesive layer / positive electrode 20 / separator 40 with adhesive layer in this order, and then performing a heat pressing process (thermocompression bonding process). Note that in this heat pressing process, the resin base material 41 is heated and may thermally shrink.

[0033] The separator 40 is preferably disposed so that the heat-resistant layer 42 faces the positive electrode 20. That is, the separator 40 is disposed between the positive electrode 20 and the negative electrode 30 with the resin substrate 41 in contact with the negative electrode 30 and the heat-resistant layer 42 in contact with the positive electrode 20. In this case, oxidation degradation of the resin substrate 41 of the separator 40 due to the positive electrode potential is suppressed compared to a configuration in which the resin substrate 41 faces the positive electrode 20. In the example shown in FIG. 3, heat-resistant layers 42 are disposed on both sides of all of the positive electrodes 20.

[0034] The separator 40 is folded zigzag and interposed between the positive electrode 20 and the negative electrode 30, and is formed into a cylindrical shape to form the outermost surface of the electrode assembly 11. The cylindrical portion 43 of the separator 40 that forms the outermost surface of the electrode assembly 11 is formed by rolling the separator 40 into a cylindrical shape one or more times along the side surface of the electrode assembly, and covers the entire side surface so that the side surface of the electrode assembly is not exposed. Here, the side surface of the electrode assembly refers to a surface along the up-down direction of the electrode assembly 11, and includes both end surfaces in the stacking direction of the electrode assembly (in this embodiment, the surfaces of the negative electrodes 30 arranged at both ends in the stacking direction of the electrode assembly where there is no opposing positive electrode 20), and a surface along the stacking direction of the electrode assembly.

[0035] The separator 40 is attached so as to cover the entire composite layer of the negative electrode 30 arranged on the outermost side in the stacking direction. That is, the separator 40 is wound in a cylindrical shape around the side surface of the electrode assembly to form a cylindrical portion 43 so that the composite layer of the negative electrode 30 is not exposed on the outermost surface of the electrode assembly 11. In the example shown in FIG. 3 , the separator 40 is wound twice around a portion of the side surface of the electrode assembly, resulting in two overlapping separators 40. That is, a portion of the cylindrical portion 43 is made up of two layers of separator 40, and the remaining portion is made up of a single layer of separator 40.

[0036] The cylindrical portion 43 is formed by wrapping the separator 40 around the side surface of the electrode group three or more times, and may be composed of three or more layers of separator 40, but is preferably composed of one or two layers of separator 40. When the number of layers of separator 40 constituting the cylindrical portion 43 is increased, lifting and curling of the ends of the separator 40 is easily suppressed, but for example, the excess separator 40 may absorb the electrolyte, resulting in a decrease in charge / discharge cycle characteristics.

[0037] The separator 40 is arranged in the cylindrical portion 43 so that the first layer having a large thermal shrinkage rate faces the inside of the electrode body 11, and the second layer having a smaller thermal shrinkage rate than the first layer faces the outside of the electrode body 11. In this embodiment, the separator 40 is arranged so that the resin base material 41 faces the inside and the heat-resistant layer 42 faces the outside. Here, the thermal shrinkage rate means the degree of shrinkage (change in length) when the separator 40 is heated.

[0038] In this embodiment, the heat-resistant layer 42 of the separator 40 is disposed on the outermost surface of the electrode assembly 11. The heat-resistant layer 42 has a smaller thermal shrinkage rate than the resin substrate 41 at, for example, 110°C (the temperature at which the electrode assembly is heated while a load is applied, as described later). The separator 40 is disposed between the positive electrode 20 and the negative electrode 30 in a zigzag manner so that the heat-resistant layer 42 faces the positive electrode 20, and is wound into a cylindrical shape one or more times, covering the side surface of the electrode assembly, to form a cylindrical portion 43 in which the resin substrate 41 faces inward and the heat-resistant layer 42 faces outward.

[0039] It is expected that separator 40 will undergo thermal shrinkage during the heat-pressing process, similar to conventional separators. Conventional separators are prone to lifting and curling at the axial end of the tubular portion due to thermal shrinkage, but separator 40 suppresses such lifting and curling, effectively preventing the composite layer of negative electrode 30 from being exposed on the outermost surface of electrode assembly 11. In separator 40, heat-resistant layer 42, which has a small thermal shrinkage rate or does not substantially shrink, is disposed on the outside of tubular portion 43. Therefore, heat-resistant layer 42 functions as a rigid layer that maintains the shape of separator 40, and prevents the axial end of tubular portion 43 from warping outward and curling up.

[0040] Furthermore, in the separator 40, the resin base material 41, which has a large thermal shrinkage rate, is disposed inside the cylindrical portion 43, so that, for example, the resin base material 41 thermally shrinks and the axial end of the cylindrical portion 43 comes into close contact with the surface of the negative electrode 30 (the side surface of the electrode group). That is, lifting of the axial end of the cylindrical portion 43 is suppressed. Note that the separator 40 may thermally shrink not only during the above-mentioned heat pressing process but also due to heat generation during use of the secondary battery 10.

[0041] <Example> The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0042] Example 1 [Preparation of positive electrode] A lithium-nickel-cobalt-manganese composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 97:2:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to both sides of a 13 μm-thick aluminum foil positive electrode core, leaving a portion that would become the positive electrode tab. The coating was then dried and compressed, and the positive electrode was cut to the specified electrode size to obtain a positive electrode (76 mm × 139 mm) with a positive electrode composite layer (thickness: 62 μm per side) formed on both sides of the positive electrode core. The positive electrode also had a 20 mm-wide positive electrode tab protruding from a portion of the core.

[0043] [Preparation of negative electrode] Graphite was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a solids mass ratio of 98:1:1, and water was used as the dispersion medium to prepare a negative electrode composite slurry. Next, the negative electrode composite slurry was applied to both sides of a negative electrode core made of 8 μm thick copper foil, leaving a portion that would become the negative electrode tab. The coating was then dried and compressed, and cut to the specified electrode size to obtain a negative electrode (78 mm × 143 mm) with a negative electrode composite layer (thickness: 76 μm per side) formed on both sides of the negative electrode core. The negative electrode also had a 18 mm wide negative electrode tab protruding from a portion of the core.

[0044] [Separator fabrication] A 12-μm-thick porous polyethylene substrate was used as the resin substrate, and a 4-μm-thick heat-resistant layer was formed on one side of the substrate by applying a slurry containing alumina particles and PVdF. This resulted in a two-layer separator (width: 81 mm) consisting of a porous resin substrate and a porous heat-resistant layer. In addition, an adhesive whose main component was acrylic resin was applied in dots on both sides of the separator to form an adhesive layer.

[0045] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4 (25°C, 1 atmosphere). LiPF6 was dissolved in the mixed solvent to a concentration of 1 mol / L to prepare a non-aqueous electrolyte solution.

[0046] [Preparation of electrode body] Thirty-five positive electrodes and 36 negative electrodes were alternately stacked one by one with the zigzag-folded separator interposed therebetween to produce an electrode assembly. The separator was then wrapped around the side of the electrode assembly and the end of the wrapping was secured with tape to obtain a laminate (electrode assembly before thermocompression bonding) in which the entire side of the electrode assembly was covered with the separator. Between the positive and negative electrodes, the separator was positioned so that the heat-resistant layer faced the positive electrode. In addition, in the cylindrical portion of the separator formed in a cylindrical shape covering the side of the electrode assembly, the resin substrate faced inward and the heat-resistant layer faced outward.

[0047] The laminate was heated on a hot plate at 110° C. for 43 seconds while applying a load of 20 kN to the laminate, to obtain an electrode assembly.

[0048] [Secondary battery production] The positive electrode tabs extending from the electrode assembly were connected to a positive electrode terminal via a current collector, and the negative electrode tabs were similarly connected to a negative electrode terminal via a current collector. The positive electrode terminal and the negative electrode terminal were each fixed to a sealing plate via an insulating member. The electrode assembly was housed in a bottomed, rectangular cylindrical outer can, and the sealing plate was laser welded to the periphery of the opening of the outer can. The nonaqueous electrolyte was poured through the filling hole in the sealing plate, which was then sealed with a blind rivet, resulting in a nonaqueous electrolyte secondary battery with external dimensions of 148 mm wide x 91 mm high x 26.5 mm thick.

[0049] <Comparative Example 1> As shown in FIG. 4A, the electrode body and secondary battery were obtained in the same manner as in Example 1, except that the separator 40 was wrapped around the side surface of the electrode assembly so that the resin base material 41 faced the outside of the electrode body and the heat-resistant layer 42 faced the inside of the electrode body in the cylindrical portion of the separator 40 that constitutes the outermost surface of the electrode body.

[0050] <Comparative Example 2> As shown in FIG. 4B, an electrode assembly and a secondary battery were obtained in the same manner as in Comparative Example 1, except that separator 40 was wound around the side surface of the electrode assembly by one more turn than in the electrode assembly of Comparative Example 1.

[0051] For each electrode assembly of the Examples and Comparative Examples, separator lifting, curling of 90° or more, and exposure of the negative electrode composite layer on the outermost surface of the electrode assembly were evaluated using the following methods. The evaluation results are shown in Table 1. The amount of separator used shown in Table 1 is the mass ratio of the separator in each electrode assembly when the mass of the separator in the electrode assembly of Example 1 is taken as the reference (1.00).

[0052] [Evaluation of separator lift, curling over 90°, and negative electrode composite layer exposure] Each electrode body of the examples and comparative examples was placed on a desk with the plane where one longitudinal end of the separator was located facing downward, and the axial end of the cylindrical part of the separator that constituted the outermost surface of the electrode body was observed to check for any floating of the separator, curling up by 90° or more (curving outward), and whether or not the negative electrode plate composite layer was exposed on the outermost surface of the electrode body.

[0053] [Table 1]

[0054] As shown in Table 1, in the electrode assembly of Example 1, the separator edge did not lift, and the negative electrode composite layer was not exposed on the outermost surface of the electrode assembly. On the other hand, in the electrode assembly of Comparative Example 1, in which the heat-resistant layer of the separator faced inward, the separator edge lifted and curled up by more than 90°, exposing the negative electrode composite layer on the outermost surface of the electrode assembly. In the electrode assembly of Comparative Example 2, in which another separator was wound around the electrode assembly of Comparative Example 1, the negative electrode composite layer was covered by two layers of separator, so the separator edge did not lift or curl up by more than 90°, exposing the negative electrode composite layer. However, because the electrode assembly of Comparative Example 2 used a large amount of separator, the amount of electrolyte held in the portion of the separator interposed between the positive and negative electrodes was relatively reduced, and a tendency for the charge / discharge cycle characteristics to deteriorate was observed. [Explanation of symbols]

[0055] 10 Secondary battery 11 Electrode body 12 Positive terminal 13 Negative terminal 14 Outer can 15 Sealing plate 16 Injection section 17 Gas exhaust valve 20 positive electrode 23 Positive electrode tab 30 negative electrode 33 Negative electrode tab 40 Separator 41 Resin substrate 42 Heat-resistant layer 43 Cylindrical part

Claims

1. A secondary battery having an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, the separator includes a first layer and a second layer having a smaller thermal shrinkage rate than the first layer, and has a cylindrical portion that is disposed along a side surface of the electrode assembly and is formed in a cylindrical shape to constitute the outermost surface of the electrode assembly; the separator is disposed in the cylindrical portion such that the first layer faces inward and the second layer faces outward; the first layer is a resin layer, the second layer is a highly heat-resistant resin layer made of a resin having a melting point or a softening point higher than that of the resin constituting the first layer, the thickness of the first layer is 1 μm to 20 μm; the melting point of the first layer is 200°C or less; A secondary battery, wherein the second layer has a thickness of 1 μm to 10 μm.

2. The secondary battery according to claim 1 , wherein the separator is disposed so that the second layer faces the positive electrode.

3. 3. The secondary battery according to claim 1, wherein the electrode assembly includes a plurality of the positive electrodes and a plurality of the negative electrodes, has a laminated structure in which one zigzag-folded separator is interposed between the positive electrodes and the negative electrodes, and the cylindrical portion is formed by the one separator.

4. 4. The secondary battery according to claim 1, wherein an adhesive layer for adhering to the positive electrode or the negative electrode is formed on at least one surface of the separator.

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