Secondary battery and positive electrode for secondary battery

Abstract

A cylindrical battery (10) is provided with an electrode body (14) in which a positive electrode (11) and a negative electrode (12) are wound with a separator (13) interposed therebetween. A first tape (51) and a second tape (52) are affixed to the positive electrode (11) so as to cover a positive electrode start end (11x) positioned on the winding start side of the electrode body (14). A length (A) in the winding direction of a first portion of the first tape (51) overlapping the winding outer surface of a positive electrode mixture layer (31) is shorter than a length (B) in the winding direction of a second portion of the second tape (52) overlapping the winding inner surface of the positive electrode mixture layer (31).

Description

Secondary battery and positive electrode for secondary battery

[0001] The present disclosure relates to a secondary battery and a positive electrode for a secondary battery, and more particularly, to a secondary battery provided with a wound electrode body and a positive electrode for a secondary battery used in the wound electrode body.

[0002] Some secondary batteries such as cylindrical batteries include a wound electrode body having a structure in which a positive electrode and a negative electrode are wound in a spiral shape with a separator interposed therebetween. In a wound electrode body, generally, at the winding start side of the electrode body, the negative electrode start end, which is the end of the negative electrode, extends closer to the winding center side than the positive electrode start end, which is the end of the positive electrode, and the positive electrode start end is sandwiched between the negative electrodes via a separator.

[0003] The electrodes constituting the wound electrode body have a long core and a mixture layer provided on the core, and are manufactured by cutting the long core to a target length. At this time, cut burrs may occur at the longitudinal ends of the electrodes. Cut burrs are protrusions of an unintended shape, and if this penetrates the separator, there is a risk of internal short circuit of the battery. In particular, since the positive electrode start end is sandwiched between the negative electrodes, the risk of short circuit due to cut burrs is high at the positive electrode start end.

[0004] Patent Document 1 discloses a wound electrode body provided with a protective tape attached to the longitudinal end of the electrode for the purpose of preventing the occurrence of a short circuit caused by the above cut burrs. Patent Document 2 also discloses a wound electrode body provided with a core exposed portion formed at the positive electrode start end and a protective tape attached from the exposed portion over the positive electrode mixture layer.

[0005] Japanese Patent Application Laid-Open No. 2002-42881 International Publication No. 2024 / 135586

[0006] The protective tape of Patent Document 2 is also attached on the positive electrode mixture layer. Further, in the electrode body of Patent Document 2, a core exposed portion is formed at the positive electrode start end. When the ends of the positive electrode core and the positive electrode mixture layer coincide, the protective tape will be attached on the positive electrode mixture layer. When a tape is attached to the positive electrode start end, the influence of cut burrs can be suppressed. However, as a result of the study by the present inventors, it was found that cracks are likely to occur in the positive electrode mixture layer due to the tape attached to the positive electrode mixture layer.

[0007] The purpose of this disclosure is to address cracks in the positive electrode mixture layer while suppressing the effects of cut burrs at the positive electrode starting point. When cracks occur in the positive electrode mixture layer, for example, the battery reaction becomes non-uniform, leading to a decrease in charge-discharge cycle characteristics.

[0008] The secondary battery according to this disclosure comprises a positive electrode, a negative electrode, and a separator, and is an electrode body in which the positive electrode and the negative electrode are wound around the separator, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, and a tape is attached to the positive electrode so as to cover the positive electrode start end, which is the end of the positive electrode at the winding start side of the electrode body, and the tape is attached to the positive electrode such that the length in the winding direction of the first portion that overlaps with the winding outer surface of the positive electrode mixture layer facing radially outward of the electrode body is shorter than the length in the winding direction of the second portion that overlaps with the winding inner surface of the positive electrode mixture layer facing radially inward of the electrode body.

[0009] The positive electrode for a secondary battery according to this disclosure comprises a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, and is used in a wound-type electrode body, and is characterized in that it comprises a tape that is attached so as to cover one longitudinal end of the positive electrode which is the end of the positive electrode at the winding start side of the electrode body, and the tape is attached to the positive electrode such that the length in the winding direction of the first portion that overlaps with the first surface which is the outer surface of the winding of the positive electrode mixture layer facing radially outward of the electrode body is shorter than the length in the winding direction of the second portion that overlaps with the second surface which is the inner surface of the winding of the positive electrode mixture layer facing radially inward of the electrode body.

[0010] The positive electrode and secondary battery according to this disclosure can suppress cracks in the positive electrode mixture layer while suppressing the effect of cut burrs at the positive electrode starting end, thereby reducing the risk of short circuits caused by cut burrs, for example.

[0011] This is a cross-sectional view of a cylindrical battery, which is one example of an embodiment. This is a diagram showing a part of the radial cross-section of an electrode body, which is one example of an embodiment. This is a front view of a positive electrode, which is one example of an embodiment, with the starting end of the positive electrode and its vicinity shown in detail. This is a cross-sectional view along line AA in Figure 3. This is a front view of a positive electrode, which is another example of an embodiment, with the starting end of the positive electrode and its vicinity shown in detail.

[0012] Hereinafter, an example of an embodiment of the secondary battery according to this disclosure will be described in detail with reference to the drawings. However, the secondary battery according to this disclosure is not limited to the embodiments described below. Configurations obtained by appropriately combining the configurations of the multiple embodiments and modified examples described below are included in this disclosure.

[0013] In the following, a cylindrical battery 10 in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 is given as an example, but the battery's outer casing is not limited to a cylindrical outer casing. The secondary battery according to this disclosure only needs to have a wound electrode body, and other embodiments include, for example, a rectangular battery with a rectangular outer casing, and a pouch-type battery with an outer casing made of a laminate sheet including a metal layer and a resin layer.

[0014] Figure 1 is a schematic diagram showing the axial and radial cross-sections of a cylindrical battery 10, which is an example of an embodiment. As shown in Figure 1, the cylindrical battery 10 includes a positive electrode 11, a negative electrode 12, and a separator 13, and comprises an electrode body 14 in which the positive electrode 11 and the negative electrode 12 are wound around the separator 13, and a bottomed cylindrical outer casing 16 that houses the electrode body 14. The cylindrical battery 10 also includes an electrolyte housed in the outer casing 16 and a sealing body 17 that closes the opening of the outer casing 16. The outer casing 16 has grooves 22 formed in its side wall, and the sealing body 17 is supported by the grooves 22 and closes the opening of the outer casing 16. In the following description, for convenience, the side of the cylindrical battery 10 with the sealing body 17 will be considered the top, and the bottom side of the outer casing 16 will be considered the bottom.

[0015] As will be explained in more detail later, a tape is attached to the positive electrode 11 so as to cover the positive electrode start end 11x (see Figure 2, etc., described later), which is the end of the positive electrode 11 at the winding start side of the electrode body 14. Note that the tape is not shown in Figure 1. This tape is a protective tape to suppress the effects of cut burrs on the positive electrode start end 11x.

[0016] The electrolyte may be an aqueous electrolyte, but in this embodiment, a non-aqueous electrolyte is used. The non-aqueous electrolyte has lithium-ion conductivity. The non-aqueous electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte. The cylindrical battery 10 is, for example, a non-aqueous electrolyte secondary battery, and among these, a lithium-ion battery is preferred.

[0017] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0018] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0019] As described above, the electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound in a spiral shape via a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all elongated strip-shaped bodies, and are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal and width directions than the positive electrode 11. The separator 13 is formed to be at least slightly larger in dimensions than the positive electrode 11, and for example, two separators are arranged so as to sandwich the positive electrode 11.

[0020] The positive electrode 11 comprises a long positive electrode core 30 and a positive electrode mixture layer 31 provided on the positive electrode core 30. The positive electrode core 30 can be made of a metal foil that is stable within the potential range of the positive electrode 11, such as aluminum, aluminum alloy, stainless steel, or titanium, or a film with the metal arranged on its surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent such as acetylene black, and a binder such as polyvinylidene fluoride (PVdF), and is preferably formed on both sides of the positive electrode core 30. For example, a lithium transition metal composite oxide containing Ni, Co, Mn, Al, etc., can be used as the positive electrode active material.

[0021] The thickness of the positive electrode 11 is, for example, 160 μm to 200 μm. In this embodiment, the thickness of the positive electrode 11 is substantially constant except for the core body exposed portion to which the positive electrode lead 20 is connected. The thickness of the positive electrode core body 30 is, for example, 10 μm to 30 μm. The thickness of the positive electrode mixture layer 31 is, for example, 70 μm to 100 μm on one side of the positive electrode core body 30. The positive electrode 11 can be manufactured by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core body 30, drying the coating film, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core body 30.

[0022] The negative electrode 12 comprises a long negative electrode core 40 and a negative electrode mixture layer 41 provided on the negative electrode core 40. The negative electrode core 40 can be made of a metal foil that is stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with the metal arranged on its surface. The negative electrode mixture layer 41 contains a negative electrode active material and a binder such as styrene-butadiene rubber (SBR), and is preferably formed on both sides of the negative electrode core 40. Examples of negative electrode active materials include graphite and Si-containing materials.

[0023] The thickness of the negative electrode 12 is, for example, 170 μm to 210 μm. In this embodiment, the thickness of the negative electrode 12 is substantially constant, except for the core body exposed portion described later. The thickness of the negative electrode core body 40 is, for example, 5 μm to 15 μm. The thickness of the negative electrode mixture layer 41 is, for example, 70 μm to 110 μm on one side of the negative electrode core body 40. The negative electrode 12 can be manufactured in the same way as the positive electrode 11 by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode core body 40, drying the coating film, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core body 40.

[0024] The electrode body 14 has a positive electrode lead 20 connected to a positive electrode 11 and a negative electrode lead 21 connected to a negative electrode 12. In this embodiment, the positive electrode mixture layer 31 is absent in the longitudinal center of the positive electrode 11, and a core exposed portion is formed where the surface of the positive electrode core 30 is exposed, to which the positive electrode lead 20 is connected. On the other hand, the negative electrode lead 21 is provided at one longitudinal end of the negative electrode 12, which is located on the winding start side of the electrode body 14. At one longitudinal end of the negative electrode 12, the negative electrode mixture layer 41 is absent, and a first core exposed portion 42 (see Figure 2 described later) is formed where the surface of the negative electrode core 40 is exposed, to which the negative electrode lead 21 is connected.

[0025] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends through a through-hole in the insulating plate 19 towards the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 becomes the negative electrode terminal.

[0026] A negative electrode 12 is positioned on the outermost surface of the electrode body 14, and a second core exposure portion 43 is provided where the surface of the negative electrode core body 40 is exposed. The core exposure portion 43 is in contact with the inner surface of the outer casing 16. By the core exposure portion 43 contacting the inner surface of the outer casing 16, both longitudinal ends of the negative electrode 12 and the outer casing 16 are electrically connected, ensuring good current collection. The core exposure portion 43 may be provided on a part of the outermost surface of the electrode body 14, but preferably it is provided over the entire outermost surface. Note that a winding stopper tape may be attached to the outermost surface of the electrode body 14.

[0027] The outer casing 16 is a bottomed cylindrical metal container. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to seal the inside of the battery. The outer casing 16 has a grooved portion 22 that supports the sealing body 17, which is formed, for example, by pressing the side surface from the outside. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The upper end of the outer casing 16 is bent inward and crimped to the peripheral edge of the sealing body 17.

[0028] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0029] The electrode body 14, and in particular the protective tape attached to the positive electrode 11, will be described in detail below with reference to Figures 2 to 5. Figure 2 is a radial cross-sectional view of the electrode body 14 at the winding start end. In Figure 2, the separator 13 is omitted for clarity. Figure 3 is a front view of the positive electrode 11, showing the positive electrode start end 11x and its vicinity in an enlarged view. Figure 4 is a cross-sectional view taken along line AA in Figure 3.

[0030] As shown in Figure 2, the electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are alternately arranged in the radial direction of the electrode body 14 via a separator 13. The negative electrode 12 is longer than the positive electrode 11, and a portion of the negative electrode 12 overlaps the electrode body 14 radially without the positive electrode 11 passing between them. The negative electrode 12 extends towards the beginning of the winding of the electrode body 14 beyond the position facing the positive electrode start end 11x, which is the end of the positive electrode 11 at the beginning of the winding of the electrode body 14. As a result, the positive electrode start end 11x is sandwiched between the negative electrode 12 via the separator 13. On the beginning of the winding of the negative electrode 12, there is a non-opposing region that has been wound one or more times without facing the positive electrode 11.

[0031] A negative electrode mixture layer 41 may be formed in the non-opposing region of the negative electrode 12, but in this embodiment, most of the non-opposing region, excluding the vicinity of the positive electrode start end 11x, is the core body exposed portion 42. A negative electrode lead 21 is connected to the core body exposed portion 42 near the negative electrode start end 12x, which is the end of the negative electrode 12 that starts winding. In the example shown in Figure 2, the negative electrode lead 21 is welded to the outer surface of the core body exposed portion 42, but it may also be welded to the inner surface of the core body exposed portion 42. In this specification, the surface of the electrode body 14 facing radially outward is referred to as the "outer surface of the winding," and the surface of the electrode body 14 facing radially inward is referred to as the "inner surface of the winding."

[0032] As shown in Figures 2 to 4, a first tape 51 and a second tape 52 are attached to the positive electrode 11 as tapes covering the positive electrode start end 11x. As described above, the positive electrode 11 is manufactured by cutting a long positive electrode core 30 to the desired length, but at this time, cut burrs, which are unintended protrusions, may occur at the longitudinal end of the positive electrode 11. Since the positive electrode start end 11x is sandwiched between the negative electrode 12, cut burrs on the positive electrode start end 11x increase the risk of short circuits. The first and second tapes 51 and 52 are protective tapes that cover the positive electrode start end 11x and reduce the risk of short circuits caused by cut burrs.

[0033] The protective tape is attached to the positive electrode 11 such that the length (A) of the first portion overlapping with the positive electrode mixture layer 31 located on the outer surface of the winding of the positive electrode 11 is shorter than the length (B) of the second portion overlapping with the positive electrode mixture layer 31 located on the inner surface of the winding of the positive electrode 11. The length in the winding direction can be rephrased as the length along the longitudinal direction of the positive electrode 11, or the length along the circumferential direction of the electrode body 14. The protective tape can be formed by folding a single tape to create the first and second portions, but from the viewpoint of improving the productivity of the positive electrode 11, it is preferable that the tape includes a first tape 51 constituting the first portion and a second tape 52 constituting the second portion. In this embodiment, the positive electrode starting end 11x, which may generate cut burrs, is sandwiched between the first and second tapes 51 and 52.

[0034] The first and second tapes 51 and 52 have the same configuration except that they differ in length. The first and second tapes 51 and 52 each have, for example, a tape base material and an adhesive layer provided on one side of the tape base material. Furthermore, the first and second tapes 51 and 52 extend beyond the positive electrode start end 11x toward the winding start side of the electrode body 14, and also extend in the width direction from both ends in the width direction of the positive electrode 11. The extended portions of the first and second tapes 51 and 52 are joined to each other via the adhesive layer. In this embodiment, the positive electrode start end 11x is wrapped by two protective tapes, which more effectively reduces the risk of short circuits caused by cut burrs.

[0035] The thickness of the first and second tapes 51 and 52 is such that the cut burrs do not penetrate the tape, for example, 15 μm to 70 μm, preferably 20 μm to 50 μm. The thickness of the tape substrate is greater than the thickness of the adhesive layer, for example, 10 μm to 65 μm, or 15 μm to 45 μm.

[0036] The above tape substrate is composed of a single-layer or multi-layer resin substrate. Examples of resins constituting the tape substrate include polyester such as polyethylene terephthalate (PET), polypropylene (PP), polyimide (PI), polyphenylene sulfide (PPS), polyetherimide (PEI), and polyamide. The first and second tapes 51 and 52 may contain inorganic particles such as titania, alumina, silica, and zirconia, and a layer containing inorganic particles may be provided separately from the tape substrate and adhesive layer.

[0037] The adhesive constituting the above adhesive layer may be a hot-melt type that exhibits adhesive properties upon heating or a thermosetting type that hardens upon heating, but from the viewpoint of productivity, it is preferable to have adhesive properties at room temperature. Examples of adhesives include acrylic adhesives and synthetic rubber adhesives. An example of the thickness of the adhesive layer is 5 μm to 30 μm. The adhesive layer can be provided on only a part of one side of the tape substrate, but in this embodiment, it is provided over the entire area of ​​one side of the tape substrate.

[0038] The first tape 51 includes a first portion attached to the positive electrode mixture layer 31 located on the outer surface of the winding of the positive electrode 11. In this embodiment, the positive electrode mixture layer 31 is formed over the entire outer and inner surfaces of the winding of the positive electrode core 30, excluding the core body exposed portion to which the positive electrode lead 20 is connected. The first portion of the first tape 51 extends for a predetermined length from the positive electrode start end 11x toward the winding end of the electrode body 14 and across the entire width of the positive electrode 11. As described above, the first tape 51 extends from the positive electrode start end 11x toward the winding start of the electrode body 14 and extends from both ends in the width direction of the positive electrode 11. That is, the first tape 51 includes a first portion attached to the positive electrode mixture layer 31 and a portion extending from the end of the positive electrode 11.

[0039] The second tape 52 includes a second portion attached to the positive electrode mixture layer 31 located on the inner surface of the winding of the positive electrode 11. The second portion of the second tape 52, like the first portion of the first tape 51, extends for a predetermined length from the positive electrode start end 11x toward the winding end of the electrode body 14 and spans the entire width of the positive electrode 11. The second tape 52 includes a second portion that extends beyond the positive electrode start end 11x toward the winding end of the electrode body 14 and extends from both ends in the width direction of the positive electrode 11 and is attached to the positive electrode mixture layer 31, and a portion that extends from the end of the positive electrode 11.

[0040] The first and second tapes 51 and 52 have a winding length of one turn or less from the positive electrode starting end 11x to the winding end side of the electrode body 14, more preferably 0.5 turns or less, and particularly preferably 0.3 turns or less. That is, the predetermined length of the portion attached to the positive electrode mixture layer 31 is preferably one turn or less from the positive electrode starting end 11x, more preferably 0.5 turns or less, and particularly preferably 0.3 turns or less. In this case, the capacity reduction caused by the protective tape is not substantially a problem.

[0041] On the other hand, if the first and second tapes 51 and 52 are too short, problems such as difficulty in attaching to the positive electrode 11 and easy peeling of the tape from the positive electrode 11 are assumed. For this reason, the first and second tapes 51 and 52 preferably have a winding direction length of, for example, at least 0.1 turn or at least 0.5 mm from the positive electrode start end 11x to the winding end side of the electrode body 14. In addition, from the viewpoint of improving productivity and the like, the total length of the first and second tapes 51 and 52 is preferably 1.5 mm or more, or 2.0 mm or more. The width of the first and second tapes 51 and 52 is preferably larger than the width of the positive electrode 11 and smaller than the width of the separator 13.

[0042] The first and second tapes 51 and 52 effectively suppress the influence of burrs at the positive electrode start end 11x, while making cracks likely to occur in the positive electrode mixture layer 31. When pulled by the tape, stress acts on the positive electrode mixture layer 31, making cracks likely to occur. As a result of the study by the inventors, it was found that cracks in the positive electrode mixture layer 31 do not occur uniformly on the winding outer surface and the winding inner surface, but the occurrence frequency is high on the winding outer surface. Since the vicinity of the positive electrode start end 11x has a small winding radius and a large circumferential difference between the winding outer surface and the winding inner surface, in particular, the stress acting on the positive electrode mixture layer 31 on the winding outer surface becomes large, and cracks are likely to occur in the positive electrode mixture layer 31 on the winding outer surface.

[0043] As described above, the winding direction length (A) of the first portion of the first tape 51 is shorter than the winding direction length (B) of the second portion of the second tape 52, preferably 95% or less, or 90% or less of the winding direction length (B). In this case, it was found that cracks in the positive electrode mixture layer 31 can be effectively suppressed. By setting the winding direction length (A) of the first portion > the winding direction length (B) of the second portion, the stress acting on the winding outer surface is reduced, and as a result, cracks in the positive electrode mixture layer 31 are considered to be suppressed. The winding direction length (A) of the first portion of the first tape 51 is more preferably 85% or less of the winding direction length (B) of the second portion of the second tape 52, and particularly preferably 80% or less of the winding direction length (B). In this case, the crack suppression effect of the positive electrode mixture layer 31 becomes more prominent.

[0044] The winding length (A) of the first portion of the first tape 51 varies slightly depending on the size of the electrode body 14, but is preferably 1.0 mm to 8.0 mm, more preferably 1.5 mm to 5.0 mm, or more preferably 2.0 mm to 3.5 mm. If the winding length (A) of the first portion is within this range, the crack suppression effect of the positive electrode mixture layer 31 becomes more pronounced while preventing capacity reduction and tape peeling. On the other hand, even if the winding length (A) of the first portion is within this range, if the condition that winding length (A) of the first portion > winding length (B) of the second portion is not met, cracks are likely to occur in the positive electrode mixture layer 31 on the outer surface of the winding (see Comparative Examples 1 and 2 described later). The winding length (B) of the second portion of the second tape 52 is, for example, 2.0 mm to 10.0 mm, or 2.5 mm to 5.0 mm.

[0045] The first and second tapes 51 and 52 are arranged radially from the winding center Z of the electrode body 14 so as not to overlap with the negative electrode lead 21. The first and second tapes 51 and 52 have a winding length of 0.5 turns or less, or 0.3 turns or less, from the positive electrode start end 11x to the winding end side of the electrode body 14, and are arranged radially opposite the negative electrode lead 21 and the electrode body 14 with the winding center Z in between. The winding length of the negative electrode lead 21 is 0.5 turns or less of the electrode body 14. If the negative electrode lead 21 and the first and second tapes 51 and 52 overlap radially, the step difference becomes large, making it easy for problems such as electrode plate deformation to occur. By arranging the first and second tapes 51 and 52 on the opposite side of the negative electrode lead 21 with the winding center Z of the electrode body 14 in between, problems such as electrode plate deformation can be suppressed.

[0046] As described above, the first and second tapes 51 and 52 cover the positive electrode 11 over the entire width of the positive electrode start end 11x and are adhered to the positive electrode 11 in a state of protruding from both ends in the width direction of the positive electrode 11. The lengths of the first and second tapes 51 and 52 along the longitudinal direction of the positive electrode 11 are different from each other, but the lengths along the width direction of the positive electrode 11 are the same, and both ends in the width direction of each tape coincide with each other. However, when there is a portion where cut burrs are likely to occur in a part of the positive electrode start end 11x, the tape may be adhered so as to cover only that portion of the positive electrode start end 11x. In this case, it is easy to suppress capacity reduction, cracks in the positive electrode active material layer 31, etc., and the influence of cut burrs can be efficiently suppressed.

[0047] FIG. 5 is a diagram showing a modified example of the first and second tapes 51 and 52. It is assumed that the positive electrode lead 20 extends from the first end X1 in the width direction of the positive electrode 11, and the positive electrode 11 is manufactured by being cut from the second end X2 side in the width direction toward the first end X1 side. In the example shown in FIG. 5, for example, since cut burrs are likely to occur on the first end X1 side rather than on the second end X2 side of the positive electrode 11, the tape is adhered at least from the center in the width direction to the first end X1. The first and second tapes 51 and 52 are not adhered to the second end X2 of the positive electrode 11 and its vicinity, and are adhered in a state of protruding from the first end X1 in the width direction. Further, the tape may be adhered so as to cover only a part of the positive electrode start end 11x, such as only the central portion in the width direction of the positive electrode 11 or only both end portions in the width direction, according to the portion where cut burrs are likely to occur, or may be adhered in a plurality of divided portions.

[0048] The positive electrode 11 having the above configuration is manufactured by adhering a protective tape so as to cover one end in the longitudinal direction that becomes the positive electrode start end 11x. The protective tape is adhered to the positive electrode 11 such that the winding direction length (A) of the first portion that overlaps the first surface of the positive electrode active material layer 31 that becomes the outer winding surface of the positive electrode 11 is shorter than the winding direction length (B) of the second portion that overlaps the second surface of the positive electrode active material layer 31 that becomes the inner winding surface of the positive electrode 11. As described above, it is preferable to use the first tape 51 that constitutes the first portion and the second tape 52 that constitutes the second portion for the protective tape.

[0049] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0050] <Example 1> [Preparation of the 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 were mixed in a solid content mass ratio of 98:1:1, and a positive electrode mixture slurry was prepared using N-methylpyrrolidone (NMP) as the dispersion medium. The slurry was applied to both sides of a long aluminum foil positive electrode core with a thickness of 15 μm. After drying and compressing the coating, it was cut to a predetermined length to obtain a positive electrode (thickness: 180 μm) in which a positive electrode mixture layer was formed on both sides of the positive electrode core. A core exposure area without a positive electrode mixture layer was provided in the longitudinal center of the positive electrode, and an aluminum positive electrode lead was ultrasonically welded to this exposed area.

[0051] Two tapes were attached to the positive electrode so as to cover one longitudinal end that serves as the starting end of the positive electrode. Each tape consisted of a tape base material and an adhesive layer provided on one side of the tape base material, and was attached to both sides of the positive electrode 11 in the state shown in Figure 3. At this time, the winding length (A) of the portion of the first tape attached to the first surface of the positive electrode mixture layer was 3 mm, and the winding length (B) of the portion of the second tape attached to the second surface of the positive electrode mixture layer was 5 mm, with A / B being 60%.

[0052] [Fabrication of the negative electrode] A mixture of graphite powder and a Si-containing material in a mass ratio of 95:5 was used as the negative electrode active material. The negative electrode active material, a dispersion of styrene-butadiene rubber, and sodium carboxymethylcellulose were mixed in a solid content mass ratio of 98:1:1, and a negative electrode mixture slurry was prepared using water as the dispersion medium. This slurry was applied to both sides of a long copper foil negative electrode core with a thickness of 8 μm. After drying and compressing the coating, it was cut to a predetermined length to obtain a negative electrode (thickness: 190 μm) with negative electrode mixture layers formed on both sides of the negative electrode core. First and second core body exposed portions were provided from both ends in the longitudinal direction of the negative electrode, within a predetermined length range where the negative electrode mixture layer was absent. A nickel negative electrode lead was ultrasonically welded to the first core body exposed portion.

[0053] [Fabrication of Electrode Body] The positive electrode, the negative electrode, and a polyethylene separator were wound in a spiral shape using a cylindrical winding core member to obtain a wound electrode body. At this time, the positive electrode was positioned such that one longitudinal end of the positive electrode to which the first and second tapes were attached was located on the winding start side of the electrode body, and the first tape was located on the outside of the winding than the second tape. The negative electrode was also positioned such that the first core body exposed portion of the negative electrode to which the negative electrode lead was attached was located on the winding start side of the electrode body. After forming the wound structure of the electrode body, the winding core member was removed to obtain a wound electrode body in which a cavity was formed in the winding core portion.

[0054] [Preparation of non-aqueous electrolyte] 100 parts by mass of a mixed solvent prepared by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:3 (25°C) is mixed with 5 parts by mass of vinylene carbonate (VC) to form LiPF ６ A non-aqueous electrolyte was prepared by dissolving it at a concentration of 1.5 mol / L.

[0055] [Fabrication of Cylindrical Battery] After placing insulating plates above and below the electrode body, the negative electrode lead was welded to the inner surface of the bottom of a bottomed cylindrical outer can, and the positive electrode lead was welded to the internal terminal plate of the sealing body, thereby housing the electrode body inside the outer can. Subsequently, a non-aqueous electrolyte was injected into the outer can using a reduced pressure method, and the opening of the outer can was sealed with the sealing body via a gasket to obtain a cylindrical battery. The exposed portion of the second core of the negative electrode forms the outermost surface of the electrode body and is in contact with the inner surface of the outer can.

[0056] <Examples 2-4, Comparative Examples 1, 2> Positive electrodes and cylindrical batteries were manufactured in the same manner as in Example 1, except that the lengths A and B of the portions of the first and second tapes that are attached to the positive electrode mixture layer were changed to the values ​​shown in Table 1.

[0057] The performance of each cylindrical battery in the examples and comparative examples was evaluated using the following method, and the evaluation results regarding cracks in the positive electrode mixture layer, along with the lengths A and B of the portions of the first and second tapes that are attached to the positive electrode mixture layer, are shown in Table 1.

[0058] [Evaluation of 0.2C Capacity] Each battery in the examples and comparative examples was charged at a constant current of 0.2C at a temperature of 25°C until the battery voltage reached 4.2V. Then, it was discharged at a constant current of 0.2C until the battery voltage reached 2.5V. The battery capacity at this time was measured as the 0.2C capacity. The 0.2C capacity was substantially the same for each battery in the examples and comparative examples, and it was confirmed that the length of the first and second tapes does not affect the battery capacity as long as they are within the range shown in Table 1.

[0059] [Evaluation of Cracks in the Compound Layer] The longitudinal length and depth of cracks in the positive electrode compound layer were confirmed from the X-ray CT image of the positive electrode, and the compound layer cracks were evaluated according to the following criteria. The evaluation criteria for ○, △, and × shown in Table 1 are as follows: ○: Crack length is 0.5 mm or less, and the crack does not reach the positive electrode core (positive electrode core is not exposed) △: Crack length exceeds 0.5 mm, and the crack does not reach the positive electrode core ×: Crack length exceeds 0.5 mm, and the crack reaches the positive electrode core (positive electrode core is exposed)

[0060]

[0061] As shown in Table 1, the positive electrodes of the examples are all less prone to cracking in the composite layer compared to the positive electrodes of the comparative examples. That is, it is understood that when the winding direction length (A) of the portion of the first tape attached to the positive electrode composite layer is shorter than the winding direction length (B) of the portion of the second tape attached to the positive electrode composite layer, cracking in the positive electrode composite layer can be effectively suppressed. As is clear from the results of the comparative examples, even if the length (A) is 3.0 mm or 4.0 mm, similar to that of the examples, if the length (B) is less than or equal to the length (A) and A / B is 100% or more, cracking in the positive electrode composite layer cannot be sufficiently suppressed.

[0062] Furthermore, the positive electrode of Example 3, with an A / B ratio of 75%, showed even greater suppression of cracks in the positive electrode mixture layer compared to the positive electrode of Example 4, with an A / B ratio of 93%. Our investigations revealed that the crack suppression effect becomes more pronounced when the A / B ratio is 90% or less, particularly 80% or less.

[0063] This disclosure is further illustrated by the following embodiments. Configuration 1: A secondary battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprising a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, the positive electrode having a tape attached to the positive electrode so as to cover the positive electrode start end, which is the end of the positive electrode at the winding start side of the electrode body, the tape being attached to the positive electrode such that the length in the winding direction of a first portion that overlaps with the outer surface of the positive electrode mixture layer facing radially outward of the electrode body is shorter than the length in the winding direction of a second portion that overlaps with the inner surface of the positive electrode mixture layer facing radially inward of the electrode body. Configuration 2: The secondary battery according to Configuration 1, wherein the tape comprises a first tape constituting the first portion and a second tape constituting the second portion. Configuration 3: The secondary battery according to Configuration 1 or 2, wherein the winding length of the first portion of the tape is 80% or less of the winding length of the second portion. Configuration 4: The secondary battery according to any one of Configurations 1 to 3, wherein the tape has a winding length of one turn or less from the positive electrode start end to the end of the electrode body. Configuration 5: The secondary battery according to any one of Configurations 1 to 3, wherein the tape has a winding length of 0.5 turns or less from the positive electrode start end to the end of the electrode body. Configuration 6: The secondary battery according to any one of Configurations 1 to 5, wherein the electrode body has a lead connected to the negative electrode at the winding start end of the electrode body, and the tape is arranged radially from the winding center of the electrode body so as not to overlap with the lead. Configuration 7: A positive electrode for a secondary battery used in a wound-type electrode body, comprising a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, wherein a tape is attached to cover one longitudinal end of the positive electrode at the winding start side of the electrode body, and the tape is attached to the positive electrode such that the length in the winding direction of a first portion that overlaps with a first surface which is the outer surface of the winding of the positive electrode mixture layer facing radially outward of the electrode body is shorter than the length in the winding direction of a second portion that overlaps with a second surface which is the inner surface of the winding of the positive electrode mixture layer facing radially inward of the electrode body.

[0064] 10 Cylindrical battery, 11 Positive electrode, 11x Positive electrode start end, 12 Negative electrode, 12x Negative electrode start end, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18 Upper insulating plate, 19 Lower insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core body, 31 Positive electrode mixture layer, 40 Negative electrode core body, 41 Negative electrode mixture layer, 42, 43 Core body exposed section, 51 First tape, 52 Second tape, Z Winding center

Claims

1. A secondary battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode having a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, a tape being attached to the positive electrode so as to cover the positive electrode start end, which is the end of the positive electrode at the winding start side of the electrode body, and the tape being attached to the positive electrode such that the length in the winding direction of a first portion that overlaps with the outer surface of the positive electrode mixture layer facing radially outward of the electrode body is shorter than the length in the winding direction of a second portion that overlaps with the inner surface of the positive electrode mixture layer facing radially inward of the electrode body.

2. The secondary battery according to claim 1, wherein the tape includes a first tape constituting the first part and a second tape constituting the second part.

3. The secondary battery according to claim 1 or 2, wherein the winding length of the first portion of the tape is 80% or less of the winding length of the second portion.

4. The secondary battery according to claim 1 or 2, wherein the tape has a winding length of one turn or less from the positive electrode starting end to the end of the electrode body winding.

5. The secondary battery according to claim 1 or 2, wherein the tape has a winding length of 0.5 turns or less from the positive electrode starting end to the end of the electrode body winding.

6. The secondary battery according to claim 1 or 2, wherein the electrode body has a lead connected to the negative electrode at a point closer to the winding start end of the electrode body than to the positive electrode start end, and the tape is arranged radially from the winding center of the electrode body so as not to overlap with the lead.

7. A positive electrode for a secondary battery used in a wound-type electrode body, comprising a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, wherein a tape is attached to cover one longitudinal end of the positive electrode at the winding start side of the electrode body, and the tape is attached to the positive electrode such that the length in the winding direction of a first portion that overlaps with a first surface which is the outer surface of the winding of the positive electrode mixture layer facing radially outward of the electrode body is shorter than the length in the winding direction of a second portion that overlaps with a second surface which is the inner surface of the winding of the positive electrode mixture layer facing radially inward of the electrode body.

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