Secondary battery

Abstract

This secondary battery is provided with an electrode body formed by laminating a positive electrode and a negative electrode with a separator interposed therebetween. The separator includes a first layer and a second layer having a lesser thermal shrinkage than the first layer, and has a tubular part that is formed in a tubular shape and that constitutes the outermost surface of the electrode body. The tubular part, of the separator, that constitutes the outermost surface of the electrode body has a tape stuck thereto in at least one end portion in the axial direction, the tape pressing the one end portion in the axial direction from one side to the other side in the lamination direction of the electrode body.

Description

[Technical field]

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

[0002] In recent years, the demand for secondary batteries has been increasing in various fields. Among them, lithium ion secondary batteries using non-aqueous electrolytes are widely used in in-vehicle applications, power storage applications, various electronic devices, etc., because they can obtain high energy density. The secondary battery includes 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 an internal short circuit 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, thermocompressing an electrode assembly, and bonding the separator surface and the electrode surface in order to prevent an internal short circuit caused by a misalignment of the stacking positions of a positive electrode and a negative electrode. Patent Document 2 also proposes a secondary battery including a separator having a porous heat-resistant layer containing inorganic particles formed on the substrate surface in order to prevent an internal short circuit caused by conductive foreign matter. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-56142 A [Patent Document 2] JP 2018-49758 A Summary of the Invention

[0005] Even on the outermost surface of the electrode body where there is no opposing electrode, the electrode is covered with a separator so that the electrode composite layer is not exposed, but the end of the separator may be turned up, exposing a part of the composite layer. When the electrode composite layer on the outermost surface of the electrode body is exposed, the exposed part may fall off and mix into the electrode body, breaking through the separator and causing a micro-short circuit. In particular, when the electrode body is manufactured using a separator including two or more layers with different thermal shrinkage rates through a thermocompression bonding process, the separator end is more likely to lift and turn up.

[0006] The secondary battery according to the present disclosure is a secondary battery including 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 tubular portion formed in a cylindrical shape and constituting the outermost surface of the electrode body, and a tape is attached to at least one axial end of the tubular portion of the separator from one side to the other side in the stacking direction of the electrode body to press the one axial end.

[0007] According to the secondary battery of the present disclosure, it is possible to more reliably prevent the end of the separator from curling up and 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 description of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view showing the appearance of a secondary battery according to an embodiment. [Diagram 2] FIG. 2 is a perspective view of an electrode assembly according to an embodiment. [Diagram 3] FIG. 3 is a perspective view of an electrode body according to another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an example of an embodiment of the present disclosure will be described in detail with reference to the drawings. It is assumed from the beginning that the following embodiments and modified examples 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 body 11 constituting the secondary battery 10. In the following, a secondary battery 10 that is a so-called prismatic battery in which the electrode body 11 is housed in a prismatic exterior can 14 is exemplified, but the exterior body of the battery is not limited to the exterior can 14 and may be, for example, an exterior body composed of a laminate sheet including a metal layer and a resin layer. In the following, a laminated electrode body 11 in which a plurality of positive electrodes and a plurality of negative electrodes are laminated in a structure with a separator interposed therebetween is exemplified, but the electrode body may be a wound electrode body.

[0011] 1 and 2, 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 tubular exterior can 14 that houses the electrode assembly 11, and a sealing plate 15 that closes the opening of the exterior can 14. The exterior can 14 is a metal container in the shape of a flattened, approximately rectangular parallelepiped with one axial end open, and the sealing plate 15 has an elongated rectangular shape. The exterior 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 exterior can 14 will be referred to as the "vertical direction" of the secondary battery 10 and each of the components, the sealing plate 15 side will be referred to as the "top", and the bottom side of the exterior can 14 will be referred to as the "bottom". The direction along the longitudinal direction of the sealing plate 15 will be referred to as the "lateral direction" of the secondary battery 10 and each of the components. In addition, the portion of the electrode body 11 excluding a cylindrical portion 43 of a separator 40 described below may be referred to as an "electrode group".

[0013] The secondary battery 10 includes an electrolyte housed in an exterior 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 product in which at least a portion of the hydrogen of these solvents is substituted with a halogen atom such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6.

[0014] The electrode assembly 11 includes a plurality of 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 a separator 40 interposed therebetween. The electrode assembly 11 generally includes one more negative electrode 30 than the positive electrode 20, and the negative electrodes 30 are arranged on both sides of the stacking direction of the electrode group. The separator 40 has a cylindrical portion 43 that is formed in a cylindrical shape and constitutes the outermost surface of the electrode assembly 11. That is, the separator 40 is wound into a cylindrical shape for one or more revolutions on the outermost surface of the electrode assembly 11, and the negative electrodes 30 arranged on both sides of the stacking direction of the electrode group are covered with the separator 40.

[0015] The electrode body 11 has a laminated structure in which one separator 40 folded in a zigzag pattern is interposed between the positive electrode 20 and the negative electrode 30. A cylindrical portion 43 is formed by the 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 body may be separate, and the electrode body may include a plurality of separators arranged one by one between the positive electrode and the negative electrode, and a single separator constituting the cylindrical portion.

[0016] The electrode body 11 has a plurality of positive electrode tabs 23 and a plurality of negative electrode tabs 33 extending toward the sealing plate 15. For example, the positive electrode tabs 23 are formed by protruding a part of the core body of the positive electrode 20, and similarly, the negative electrode tab 33 is formed by protruding a part of the core body of the negative electrode 30. The positive electrode 20 and the negative electrode 30 are each stacked via a separator 40 such that the positive electrode tabs 23 and the negative electrode tabs 33 face in the same direction, the positive electrode tabs 23 are located at one lateral end of the electrode body 11, and the negative electrode tabs 33 are located at the other lateral end of the electrode body 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 and a gas exhaust valve 17 for opening and discharging gas when an abnormality occurs in the battery.

[0018] The positive electrode 20, the negative electrode 30, and the separator 40 that constitute the electrode assembly 11, in particular 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. For the positive electrode core, a foil of a metal such as aluminum or an aluminum alloy that is stable in the potential range of the positive electrode 20, or a film with the metal disposed on the surface layer, can be used. 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, and a binder, and then drying the coating, and 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, W, etc. Among them, 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 the conductive material contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder 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 carboxymethylcellulose (CMC) or its salts, and 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. For the negative electrode core, a foil of a metal such as copper that is stable in the potential range of the negative electrode 30, a film with the metal disposed on the surface layer, or the like can be used. 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 are graphites such as natural graphite, such as flake graphite, lump graphite, and earthy graphite, and artificial graphite, such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, 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 carbon-based active material and a Si-based active material may be used in combination.

[0024] The binder contained in the negative electrode mixture layer may be, as in the case of the positive electrode 20, a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but is preferably 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 insulation. The separator 40 includes at least two layers having different thermal shrinkage rates, namely, a first layer and a second layer having a smaller thermal shrinkage rate than the first layer. For example, the separator 40 includes a porous resin substrate that is a resin layer as the first layer, and a porous heat-resistant layer containing inorganic particles as the second layer. The heat-resistant layer is formed on one side of the resin substrate. By providing the heat-resistant layer, the separator 40 is less likely to break due to conductive foreign matter, and the shrinkage of the separator 40 when the temperature rises can be suppressed. In order to improve the cost-effectiveness while suppressing the increase in the thickness of the electrode body 11, it is preferable to form a heat-resistant layer only on one side of the resin substrate. The heat shrinkage rate means the degree of shrinkage (change in length) when the separator 40 is heated. The heat shrinkage rate of the second layer is smaller than that of the first layer, for example, at 110°C (the temperature when the electrode body is heated while applying a load, which will be described later).

[0026] Separator 40 may have a third layer. Separator 40 may include a highly heat-resistant resin layer made of a resin having a higher melting point or softening point than the resin constituting the resin substrate, such as an aramid resin, a polyimide, or a polyamideimide, instead of or in addition to the heat-resistant layer.

[0027] The resin substrate functions as a separator by itself. A porous film having ion permeability and insulating properties is used as the resin substrate. The thickness of the resin substrate is, for example, 1 μm to 20 μm, and preferably 5 μm to 15 μm. Examples of the material of the resin substrate 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 is generally 200° C. or lower.

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

[0029] As the inorganic particles, for example, at least one selected from alumina, boehmite, silica, titania, and zirconia can be used. Among them, it is preferable to use alumina or boehmite. The content of the inorganic particles is preferably 85 mass% to 99.9 mass%, more preferably 90 mass% to 99.5 mass%, based on the mass of the heat-resistant layer.

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

[0031] At least one surface of the separator 40 has an adhesive layer formed thereon, which adheres to the surface of the positive electrode 20 or the negative electrode 30, for example. The adhesive layer may be formed on both sides of the separator 40, and in this case, the adhesive layer on one side may have a different configuration from the adhesive layer on the other side. An example of the thickness of the adhesive layer is 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 an adhesive component is dispersed in water to the surface of the separator 40 and drying the coating. The adhesive layer may be formed, for example, in a dot shape.

[0032] It is preferable that the adhesive layer does not have adhesiveness at room temperature (25°C) and develops adhesiveness by heating. 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 press process (thermocompression bonding process). Note that in this heat press process, the resin base material is heated and may thermally shrink.

[0033] The separator 40 is preferably disposed so that the heat-resistant layer 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 in contact with the negative electrode 30 and the heat-resistant layer in contact with the positive electrode 20. In this case, oxidation deterioration of the resin substrate of the separator 40 due to the positive electrode potential is suppressed compared to a configuration in which the resin substrate faces the positive electrode 20. In this embodiment, heat-resistant layers are disposed on both sides of all 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 forming 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 means both end surfaces in the stacking direction of the electrode assembly (in this embodiment, the surface of the negative electrode 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 body 11. In this embodiment, the separator 40 is wound around a part of the side surface of the electrode assembly two times, and two sheets of the separator 40 are overlapped. That is, a part of the cylindrical portion 43 is composed of two layers of the separator 40, and the remaining part is composed of one layer of the separator 40.

[0036] A tape 45 for maintaining the shape of the cylindrical portion 43 is attached to the winding end of the separator 40 located on the outermost surface of the electrode body 11. The tape 45 is attached, for example, from the winding end of the second layer separator 40 located on the outside of the cylindrical portion 43 to the first layer separator 40 wound inside it. The cylindrical portion 43 is formed by winding the separator 40 three or more times around the side surface of the electrode group, 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 end of the separator 40 are easily suppressed, but for example, the excess separator 40 absorbs the electrolyte, resulting in a decrease in charge / discharge cycle characteristics.

[0037] At least one axial end of the cylindrical portion 43 of the separator 40 (one widthwise end of the separator 40 in this embodiment) is attached with a tape 46 that presses the axial end from one side to the other in the stacking direction of the electrode body 11. By pressing the axial end of the cylindrical portion 43 with the tape 46, it is possible to prevent the end of the separator 40 from curling up and exposing the composite layer of the negative electrode 30 on the outermost surface of the electrode body 11. This prevents the occurrence of an internal short circuit due to the falling off of the composite layer.

[0038] 2, tape 46 is attached to the upper end, which is one axial end of tubular portion 43, and tape 46 is not attached to the lower end, which is the other axial end. Generally, the width of separator 40 is greater than the width of negative electrode 30 (the width of the portion where negative electrode tab 33 is not formed), and separator 40 is in a state in which there is more surplus at the upper end of tubular portion 43 than at the lower end. In this case, separator 40 is likely to lift or turn up at the upper end of tubular portion 43, so it is preferable to attach tape 46 to at least the upper end.

[0039] The tape 46 is attached to the outermost surface of the electrode body 11 on one side in the stacking direction (hereinafter referred to as the "front surface of the electrode body 11") at the upper end of the tubular portion 43, extends in the stacking direction over the electrode body 11, and is attached to the outermost surface of the electrode body 11 on the other side in the stacking direction (hereinafter referred to as the "rear surface of the electrode body 11"). In the example shown in Fig. 2, the tape 46 is attached across the first and second separators 40 on the front surface of the electrode body 11, similar to the tape 45. Note that the tape 46 may be extended to the lower part of the electrode body 11 along the winding end of the separator 40 and used as the tape 45.

[0040] The tape 46 is attached from the upper end of the front surface of the electrode body 11 to the upper end of the rear surface in a taut state without any flexure. In this case, the upper end of the cylindrical portion 43 is pulled inward, so that lifting and curling are less likely to occur. The tape 46 covers a part of the upper surface of the electrode body 11 and extends in the stacking direction of the electrode body 11. The tape 46 is also disposed at a position equidistant from each of the positive electrode tab 23 and the negative electrode tab 33, for example. The tape 46 is preferably attached so as not to overlap the liquid injection portion 16 in the vertical direction, avoiding the position directly below the liquid injection portion 16.

[0041] The tape 46 may be formed to be wide as long as it does not interfere with the positive electrode tab 23 and the negative electrode tab 33 and does not overlap with the liquid injection portion 16 in the vertical direction. In the example shown in FIG. 2, the width of the tape 46 is narrower than the width of the tape 45, but the tape 46 may be wider than the tape 45. An example of the width of one piece of tape 46 is 10 mm to 20 mm, which is 5% to 30% of the lateral length of the electrode body 11. The length along the vertical direction of the tape 46 attached to the front and rear surfaces of the electrode body 11 is, for example, 5 mm to 15 mm, which is 5% or more of the vertical length of the electrode body 11.

[0042] In this embodiment, each of the positive electrode 20 and the negative electrode 30 has a generally rectangular shape when viewed from the front. The tape 46 is attached at a position overlapping the center of the long side of the positive electrode 20 and the negative electrode 30. Since the separator 40 is most likely to lift and turn over at the center of the long side of the front and rear surfaces of the electrode body 11, such an attachment form of the tape 46 is effective in suppressing the separator 40 from lifting and turning over.

[0043] As shown in Fig. 3, the tapes 46 may be attached to both lateral sides of the electrode body 11, one on each side, at least at one axial end of the cylindrical portion 43. In the example shown in Fig. 3, the tape 46 is attached between one lateral end of the electrode body 11 and the positive electrode tab 23, near the one lateral end. In addition, the tape 46 is attached between the other lateral end of the electrode body 11 and the negative electrode tab 33, near the other lateral end.

[0044] In the example shown in Fig. 3, no tape 46 is attached between the positive electrode tab 23 and the negative electrode tab 33, but in addition to the two tapes 46 on both sides of the electrode body 11 in the lateral direction, a tape 46 may be attached to the lateral center of the cylindrical portion 43. The number of tapes 46 is not particularly limited, and four or more tapes may be attached, but three or less tapes are preferable from the viewpoint of productivity. In addition, the tape 46 may be attached to both the upper and lower ends of the cylindrical portion 43, and the tape 46 may be attached in a different manner at the upper end and the lower end of the cylindrical portion 43.

[0045] The tapes 45 and 46 are, for example, adhesive tapes including an insulating resin base material and an adhesive layer. The same tape can be used for the tapes 45 and 46. The thickness of the tapes 45 and 46 is, for example, 10 μm to 60 μm, and preferably 15 μm to 40 μm. The resin base material may be any material that is durable against the electrolyte, and may be made of, for example, a polyester such as polyethylene terephthalate, polypropylene, polyimide, polyphenylene sulfide, polyetherimide, polyamide, or the like.

[0046] It is expected that the separator 40 will undergo thermal shrinkage in the above-mentioned heat pressing process, similar to conventional separators. Conventional separators are prone to floating and curling at the axial end of the tubular portion due to thermal shrinkage, but the separator 40 uses the tape 46 to suppress such floating and curling, and can highly prevent the composite layer of the negative electrode 30 from being exposed on the outermost surface of the electrode body 11. The separator 40 may undergo thermal shrinkage not only during the above-mentioned heat pressing process, but also due to heat generation during use of the secondary battery 10.

[0047] The separator 40 is preferably arranged in the cylindrical portion 43 such 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 such that the resin base material faces the inside and the heat-resistant layer faces the outside. In this case, the heat-resistant layer functions as a rigid layer that maintains the shape of the separator 40, and the axial end of the cylindrical portion 43 is prevented from warping outward and curling up. By arranging in this manner, the axial end of the cylindrical portion 43 is further prevented from floating and curling up.

[0048] <Example> The present disclosure will be described in further detail below with reference to examples, but the present disclosure is not limited to these examples.

[0049] <Example 1> [Preparation of positive electrode] 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 solid content mass ratio of 97:2:1, and a positive electrode composite slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil with a thickness of 13 μm, leaving a portion that would become a positive electrode tab, and the coating was dried and compressed, and then cut to a predetermined electrode size to obtain a positive electrode (76 mm × 139 mm) in which a positive electrode composite layer (thickness: 62 μm on one side) was formed on both sides of the positive electrode core. In addition, a positive electrode tab with a width of 20 mm from which a part of the core protrudes was formed on the positive electrode.

[0050] [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 solid content mass ratio of 98:1:1, and a negative electrode composite slurry was prepared using water as a dispersion medium. Next, the negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil with a thickness of 8 μm, leaving a portion that would become a negative electrode tab, and the coating was dried and compressed, and then cut to a predetermined electrode size to obtain a negative electrode (78 mm x 143 mm) in which a negative electrode composite layer (thickness: 76 μm on one side) was formed on both sides of the negative electrode core. In addition, a negative electrode tab with a width of 18 mm was formed with a part of the core protruding.

[0051] [Making the separator] A 12 μm thick polyethylene porous substrate was used as the resin substrate, and a slurry containing alumina particles and PVdF was applied to one side of the substrate to form a 4 μm thick heat-resistant layer, resulting in a two-layer separator (width: 81 mm) consisting of a porous resin substrate and a porous heat-resistant layer. In addition, an adhesive mainly composed of acrylic resin was applied in dots on both sides of the separator to form an adhesive layer.

[0052] [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 atm). LiPF6 was dissolved in the mixed solvent to a concentration of 1 mol / L to prepare a non-aqueous electrolyte.

[0053] [Preparation of electrode body] The 35 positive electrodes and the 36 negative electrodes were alternately stacked one by one through the zigzag-folded separator to prepare an electrode group, and then the separator was wrapped around the side of the electrode group and the end of the winding was fixed with adhesive tape to obtain a laminate (electrode body before thermocompression bonding) in which the entire side of the electrode group was covered with the separator. Note that, between the positive electrode and the negative electrode, the separator was arranged so that the heat-resistant layer faces the positive electrode side. In addition, a 15 mm wide adhesive tape was attached to the cylindrical part of the separator covering the side of the electrode group, pressing the upper end of the cylindrical part without bending from the front to the rear of the laminate. As shown in FIG. 2, the tape was attached so as to overlap the central part of the long side direction of the positive electrode and the negative electrode (the central part of the horizontal direction of the laminate), and the length of the tape attached to the front and rear of the laminate along the vertical direction was set to 10 mm.

[0054] The laminate was heated for 43 seconds on a hot plate at 110° C. while applying a load of 20 kN to the laminate to obtain an electrode assembly.

[0055] [Preparation of secondary battery] A plurality of positive electrode tabs extending from the electrode body were connected to a positive electrode terminal via a current collector, and a plurality of negative electrode tabs were connected to a negative electrode terminal via a current collector. The positive electrode terminal and the negative electrode terminal were fixed to a sealing plate via an insulating member. After the electrode body was housed in a bottomed rectangular cylindrical outer can, the sealing plate was laser welded to the periphery of the opening of the outer can. The nonaqueous electrolyte was poured from the liquid pouring hole of the sealing plate, and the liquid pouring hole was sealed with a blind rivet to obtain a nonaqueous electrolyte secondary battery with external dimensions of 148 mm wide x 91 mm high x 26.5 mm thick.

[0056] <Example 2> As shown in FIG. 3, an electrode assembly and a secondary battery were obtained in the same manner as in Example 1, except that a piece of tape was attached to each side of the electrode assembly in the lateral direction from the front to the rear to press down the upper end of the separator.

[0057] <Comparative Example 1> Except for not using tape attached from the front to the back of the electrode body, implementation In the same manner as in Example 1, an electrode assembly and a secondary battery were obtained.

[0058] For each of the electrode assemblies of the examples and comparative examples, the separator lifting, curling over 90°, and exposure of the negative electrode mixture layer on the outermost surface of the electrode assembly were evaluated by the following methods. The evaluation results are shown in Table 1.

[0059] [Evaluation of separator lift, curling over 90°, and exposure of negative electrode composite layer] Each electrode body of the examples and comparative examples was placed on a desk with the flat surface on which one longitudinal end of the separator was located facing downward, and the axial end of the tubular 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.

[0060] [Table 1]

[0061] As shown in Table 1, in the electrode body of Example 1, the upper end of the separator was hardly lifted, and no curling occurred, and it was confirmed that the negative electrode plate composite layer was not exposed on the outermost surface of the electrode body. In the electrode body of Example 2, lifting and curling occurred in the lateral center part away from the tape, but the extent of the lifting and curling was small and did not lead to exposure of the negative electrode composite layer, and the effect of suppressing exposure of the negative electrode composite layer was confirmed as in the case of Example 1. On the other hand, in the electrode body of Comparative Example 1, the upper end of the separator was lifted and curled up by 90° or more, and the negative electrode composite layer was exposed on the outermost surface of the electrode body. [Explanation of symbols]

[0062] 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 tab 30 negative electrode 33 Negative tab 40 Separator 43 Cylindrical section 45,46 Tape

Claims

1. A secondary battery having an electrode assembly in which a positive electrode and a negative electrode are laminated 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 formed in a cylindrical shape and constituting an outermost surface of the electrode assembly; At least one tape is attached to the cylindrical portion of the separator, the tape pressing the end of the cylindrical portion in the axial direction from one side to the other side in the stacking direction of the electrode body, and pressing a winding end of the cylindrical portion in at least the axial center portion, the first layer is a resin layer, the second layer is a heat-resistant layer containing inorganic particles, The separator is disposed in the cylindrical portion such that the resin layer faces inward and the heat-resistant layer faces outward.

2. The electrode body includes a plurality of positive electrodes and a plurality of negative electrodes each having a generally rectangular shape when viewed from the front, The secondary battery according to claim 1 , wherein the tape is attached to a position overlapping with a central portion of the positive electrode and the negative electrode in a long side direction.

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

4. The secondary battery according to claim 1 , wherein the separator is disposed so that the heat-resistant layer faces the positive electrode.

5. 5. 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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