Secondary battery

Abstract

In a secondary battery as an example of an embodiment, an adhesive tape for fixing a winding end is bonded to the outer peripheral surface of a winding-type electrode body. The mass of the positive electrode active material per unit area in at least a first region that overlaps the tape in the radial direction of the electrode body in the outermost peripheral winding outer surface of the positive electrode (11) is less than the mass of the positive electrode active material per unit area in a second region in which the positive electrode mixture layer is present except the first region in the winding outer surface. In the first region, for example, an exposed portion (32) from which the surface of the positive electrode core (30) is exposed is formed.

Description

Technical Field

[0001] The present invention relates to a secondary battery, and more particularly, to a secondary battery including a wound electrode assembly. Background Art

[0002] In the past, secondary batteries having a wound electrode body in which a positive electrode and a negative electrode are wound in a spiral shape with a separator interposed therebetween are widely known (for example, see Patent Documents 1 and 2). A stopper tape for maintaining the winding structure is usually attached to the wound electrode body. For example, Patent Document 1 discloses an electrode body in which stopper tapes for fixing the winding end of the negative electrode are attached to both axial ends of the outer peripheral surface.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: International Publication No. 2018 / 168628

[0006] Patent Document 2: Japanese Patent Application Publication No. 2002-110134 Summary of the invention

[0007] In secondary batteries such as lithium-ion batteries, the wound electrode body expands and contracts with charging and discharging, but if a stop tape is attached to the outer peripheral surface of the electrode body, the stress caused by the expansion of the electrode body is concentrated on the edge of the tape, and sometimes the electrode body is damaged. The inventors have studied that in a wound electrode body in which the negative electrode constitutes the outer peripheral surface, wrinkles of the negative electrode are confirmed at the edge of the stop tape attached to the outer peripheral surface.

[0008] The secondary battery of the present invention is characterized in that it has an electrode body, wherein the electrode body includes a positive electrode, a negative electrode and a separator, and is formed by winding the positive electrode and the negative electrode with the separator interposed therebetween; a tape for fixing the winding end is adhered to the outer peripheral surface of the electrode body; the positive electrode has a positive electrode core and a positive electrode mixture layer; the positive electrode mixture layer includes a positive electrode active material and is formed on the positive electrode core; the mass of the positive electrode active material per unit area of ​​at least the first region of the outermost winding outer surface of the positive electrode that overlaps with the tape in the radial direction of the electrode body is less than the mass of the positive electrode active material per unit area of ​​the second region of the winding outer surface where the positive electrode active material layer exists except the first region.

[0009] According to the secondary battery of the present invention, stress concentrated on the edge portion of the anti-winding tape can be alleviated, and damage to the electrode body can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a cross-sectional view of a secondary battery as an example of an embodiment.

[0011] Figure 2It is a perspective view of an electrode body as an example of an embodiment.

[0012] Figure 3 It is a front view of a positive electrode as an example of an embodiment. DETAILED DESCRIPTION

[0013] As described above, in a wound electrode body with a stop tape attached to the outer circumference, when the volume changes with charging and discharging, stress is easily concentrated on the edge of the tape, so that, for example, wrinkles may be generated in the electrode constituting the outer circumference. The inventors of the present invention have conducted in-depth research to suppress the damage to the electrode body caused by the stop tape, and found that the damage to the electrode body can be highly suppressed by reducing the mass of the positive electrode active material per unit area of ​​the positive electrode in at least the first region of the outermost winding outer surface of the positive electrode that overlaps with the tape in the radial direction of the electrode body. In this case, it can be considered that the stress concentrated on the edge of the stop tape is effectively alleviated.

[0014] In addition, it is preferred that an exposed portion where the surface of the positive electrode core is exposed is provided in at least the first region of the outer peripheral surface of the positive electrode that overlaps with the tape in the radial direction of the electrode body. In this case, the damage suppression effect of the electrode body becomes more significant. In the case where the stop tape is attached to the axial ends of the electrode body, it is preferred that the exposed portion is provided on the outermost winding surface of the positive electrode in the axial direction of the electrode body in a manner that sandwiches the positive electrode mixture layer. In other words, on the outermost winding surface of the positive electrode, the positive electrode mixture layer is also provided in the region that does not overlap with the tape in the radial direction so that the exposed portion is the minimum required area. In this case, high capacity can be ensured and damage to the electrode body can be efficiently suppressed.

[0015] Hereinafter, an example of an embodiment of the secondary battery of the present invention will be described in detail with reference to the accompanying drawings. The embodiment described below is merely an example, and the present invention is not limited to the following embodiment. In addition, the present invention includes a method in which the constituent elements of the embodiment described below are selectively combined.

[0016] Hereinafter, as a secondary battery, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer can 16 is exemplified, but the outer casing of the battery is not limited to a cylindrical outer can. The secondary battery of the present invention may be, for example, a square battery having a square outer can, a coin-shaped battery having a coin-shaped outer can, or a pouch-shaped battery having an outer casing composed of a laminate sheet including a metal layer and a resin layer.

[0017] Figure 1 1 is a diagram schematically showing a cross section of a secondary battery 10 as an example of an embodiment. Figure 1 In the figure, the tape 50 described later is omitted. Figure 1 As shown, the secondary battery 10 includes an electrode body 14, an electrolyte (not shown), and an outer can 16 for storing the electrode body 14 and the electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed therebetween. The outer can 16 is a metal container in a bottomed cylindrical shape with one side open in the axial direction, and the opening of the outer can 16 is sealed by a sealing body 17. In the following, for the sake of convenience of description, the sealing body 17 side of the secondary battery 10 is set as the upper side, and the bottom side of the outer can 16 is set as the lower side.

[0018] The electrolyte can be an aqueous electrolyte, but in the present embodiment, a non-aqueous electrolyte is used. The non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous solvent can use, for example, esters, ethers, nitriles, amides, and mixed solvents of two or more thereof. As an example of a non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents thereof can be cited. The non-aqueous solvent can contain a halogen substitute (for example, fluoroethylene carbonate (FEC) etc.) formed by replacing at least a portion of the hydrogen of these solvents with a halogen atom such as fluorine. The electrolyte salt can use, for example, a lithium salt such as LiPF6.

[0019] The positive electrode 11, the negative electrode 12 and the separator 13 constituting the electrode body 14 are all strip-shaped long bodies, which are alternately stacked in the radial direction of the electrode body 14 by being wound into a spiral shape. In order to prevent the precipitation of lithium, the negative electrode 12 is formed to be one circle larger than the positive electrode 11. In other words, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction and the width direction (short side direction). The separator 13 is formed to be at least one circle larger than the positive electrode 11, and two sheets are arranged in a manner of sandwiching the positive electrode 11. The secondary battery 10 has insulating plates 18 and 19 respectively arranged above and below the electrode body 14.

[0020] The positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 formed on the positive electrode core 30. The positive electrode core 30 can be made of a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a film having the metal disposed on the surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on the exposed portion 32 described later and the exposed portion 33 to which the positive electrode lead 20 is welded (see Figure 3 ) on both sides of the positive electrode core 30 other than the positive electrode core 30. The positive electrode 11 can be manufactured, for example, by coating a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. on the positive electrode core 30, drying the coating film, and then compressing it to form a positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0021] The positive electrode mixture layer 31 contains a particulate lithium metal composite oxide as a positive electrode active material. The lithium metal composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal element constituting the lithium metal composite oxide is, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among them, it is preferred to contain at least one selected from Co, Ni, and Mn. As an example of a suitable composite oxide, a lithium metal composite oxide containing Ni, Co, and Mn and a lithium metal composite oxide containing Ni, Co, and Al can be cited.

[0022] As the conductive agent contained in the positive electrode mixture layer 31, carbon black such as acetylene black and Ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene and other carbon materials can be exemplified. As the binder contained in the positive electrode mixture layer 31, fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, etc. can be exemplified. In addition, these resins can also be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), etc.

[0023] The negative electrode 12 includes a negative electrode core 40 and a negative electrode mixture layer 41 formed on the negative electrode core 40. The negative electrode core 40 may be made of a foil of a metal such as copper or a copper alloy that is stable within the potential range of the negative electrode 12, or a film in which the metal is arranged on the surface. The negative electrode mixture layer 41 includes a negative electrode active material, a binder, and a conductive agent as required, and is preferably formed on both sides of the negative electrode core 40 except for an exposed portion 42 described later. The negative electrode 12 can be manufactured by coating a negative electrode mixture slurry including a negative electrode active material and a binder on the surface of the negative electrode core 40, drying the coating, and then compressing the coating to form the negative electrode mixture layer 41 on both sides of the negative electrode core 40.

[0024] The negative electrode mixture layer 41 generally contains a carbon material that reversibly absorbs and releases lithium ions as a negative electrode active material. A suitable example of a carbon material is graphite such as natural graphite such as flaky graphite, blocky graphite, and earthy graphite, blocky artificial graphite (MAG), and artificial graphite such as graphitized mesophase carbon microbeads (MCMB). In addition, as a negative electrode active material, a material containing at least one of an element alloyed with Li such as Si and Sn and a material containing the element can be used. Among them, a composite material containing Si is preferred.

[0025] As an example of a suitable composite material containing Si, a material in which Si particles are dispersed in a silicate phase such as SiO2 phase or lithium silicate, or a material in which Si particles are dispersed in an amorphous carbon phase, etc. can be cited. A conductive layer such as a carbon film is formed on the particle surface of the composite material. From the viewpoint of both high capacity and high durability of the battery, it is preferred to use a carbon material and a composite material containing Si in combination as a negative electrode active material.

[0026] As in the case of the positive electrode mixture layer 31, the binder contained in the negative electrode mixture layer 41 may also include fluorine-containing resins, PAN, polyimide, acrylic resins, polyolefins, etc., preferably styrene-butadiene rubber (SBR). In addition, the negative electrode mixture layer 41 preferably includes CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. Among them, it is suitable to use SBR in combination with CMC or its salt, PAA or its salt, etc. The negative electrode mixture layer 41 may include a conductive agent such as CNT.

[0027] The separator 13 can use a porous sheet with ion permeability and insulation. As specific examples of porous sheets, microporous films, woven fabrics, non-woven fabrics, etc. can be cited. As the material of the separator 13, polyolefins such as polyethylene and polypropylene, cellulose, etc. are suitable. The separator 13 can be a single-layer structure or a multi-layer structure. In addition, a resin layer with high heat resistance such as aromatic polyamide resin can also be formed on the surface of the separator 13. A filler layer containing an inorganic filler can be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12.

[0028] The negative electrode 12 is disposed on the outer peripheral surface of the electrode body 14. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode core body 30 by welding or the like. Figure 1 In the example shown, no negative electrode lead is provided, but as in the battery of the embodiment described later, an exposed portion of the negative electrode core may be provided on the inner circumference of the electrode body 14 and a negative electrode lead may be connected to the exposed portion.

[0029] An exposed portion 42 is formed on the outer peripheral surface of the electrode body 14, where the surface of the negative electrode core 40 is exposed. The exposed portion 42 may be formed on a portion of the outer peripheral surface of the electrode body 14, but is preferably formed on the entire outer peripheral surface. The exposed portion 42 may be formed only on a single surface (outer surface) of the negative electrode core 40 facing the outside of the electrode body 14, or may be formed on both surfaces of the negative electrode core 40. The exposed portion 42 is formed, for example, at one end in the longitudinal direction of the negative electrode core 40 (negative electrode 12) located on the outer peripheral surface of the electrode body 14, that is, at the winding end 12Y (see the following description). Figure 2 ) is within the range of about 1 to 2 times the circumference of the electrode body 14.

[0030] In the secondary battery 10, the exposed portion 42 of the negative electrode 12 is in contact with the inner surface of the outer can 16, so that the negative electrode 12 is electrically connected to the outer can 16. The exposed portion 42 is in contact with the inner surface of the outer can 16 over the entire circumference of the outer circumference of the electrode body 14, for example. The positive electrode lead 20 extends toward the sealing body 17 through the through hole of the insulating plate 18, and is connected to the lower surface of the internal terminal plate 23, which is the bottom plate of the sealing body 17, by welding or the like. In this embodiment, the sealing body 17 serves as the positive electrode external terminal, and the outer can 16 serves as the negative electrode external terminal.

[0031] As described above, the outer can 16 is a metal container in a cylindrical shape with a bottom and an opening on one side in the axial direction. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness of the battery interior and the insulation between the outer can 16 and the sealing body 17. The outer can 16 is provided with a groove 21 that extends inwardly from a part of the side surface and supports the sealing body 17. The groove 21 is preferably formed in an annular shape along the circumference of the outer can 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the upper part of the outer can 16 through the groove 21 and the open end of the outer can 16 that is caulked relative to the sealing body 17.

[0032] 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 cover 27 are stacked in order from the electrode body 14 side. The components constituting the sealing body 17 have, for example, a disc shape or a ring shape, and the components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and an insulating member 25 is sandwiched between their respective peripheral portions. If an abnormality occurs in the battery and the internal pressure rises, the lower valve body 24 is deformed and broken in a manner that pushes the upper valve body 26 toward the cover 27 side, thereby cutting off 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 breaks, and the gas is discharged from the opening of the cover 27.

[0033] Below, refer to Figure 2 and Figure 3 The electrode body 14 , particularly the positive electrode 11 and the tape 50 , will be described in detail.

[0034] Figure 2 is a three-dimensional diagram of the electrode body 14. Figure 2 As shown, a tape 50 is attached to the outer peripheral surface of the electrode body 14. The tape 50 is a winding stopper used to fix the winding end 12Y and maintain the winding structure of the electrode body 14. In the present embodiment, as described above, the outer peripheral surface of the electrode body 14 is formed by the exposed portion 42 of the negative electrode 12. The tape 50 is attached to the exposed portion 42 in a manner that spans the winding end 12Y that is one end in the longitudinal direction of the negative electrode 12, and the winding end 12Y is fixed to the outer peripheral surface of the electrode body 14.

[0035] The tape 50 is formed into a long and thin strip. The tape 50 can also be attached in a manner that its length direction is along the axial direction of the electrode body 14, but it is preferably attached in a manner that the length direction is along the circumferential direction. The tape 50 crosses the winding end 12Y along the circumferential direction of the electrode body 14 and is attached to a length range of more than 50% of the circumference of the outer peripheral surface, preferably 60% to 90%. In addition, the tape 50 is preferably attached to both axial ends of the electrode body 14. In this case, for example, the axial ends of the electrode body 14 are prevented from hitting the edge of the outer can 16 and rolling up, and the storage of the electrode body 14 into the outer can 16 becomes smoother.

[0036] The tape 50 preferably has a width corresponding to 5% to 25% of the axial length of the electrode body 14. Usually, the width of the tape 50 is constant over the entire length. The tape 50 is, for example, only attached to a range of 25% or less of the width (vertical length) of the negative electrode 12 from the upper and lower ends of the exposed portion 42 (negative electrode 12). The tape 50 may also be attached to the upper and lower ends of the exposed portion 42 with a predetermined gap between them in consideration of the bonding error, but the gap between the tape 50 and the upper and lower ends of the exposed portion 42 is, for example, set to be less than 1 mm.

[0037] The adhesive tape 50 has a base material made of an insulating resin and an adhesive layer formed on one side of the base material. The adhesive tape 50 is preferably an insulating tape that is substantially non-conductive. The adhesive tape 50 may have a layer structure of three or more layers, and the base material may be composed of two or more layers of the same or different types of laminated films. The adhesive tape 50 may contain inorganic fillers such as titanium dioxide, aluminum oxide, silicon dioxide, and zirconium oxide, and may also be provided with a layer containing inorganic fillers different from the base material and the adhesive layer.

[0038] As the resin constituting the base material of the adhesive tape 50, polyesters such as polyethylene terephthalate (PET), polypropylene (PP), polyimide (PI), polyphenylene sulfide (PPS), polyetherimide (PEI), polyamide, etc. can be exemplified. The adhesive layer is formed, for example, by applying an adhesive on one side of the base material. The adhesive constituting the adhesive layer can be a hot melt type that exhibits adhesiveness by heating or a thermosetting type that solidifies by heating, and preferably has adhesiveness at room temperature from the viewpoint of productivity, etc. As the adhesive constituting the adhesive layer, acrylic adhesives, synthetic rubber adhesives, etc. can be exemplified.

[0039] The thickness of the tape 50 is, for example, 20 μm to 100 μm, preferably 30 μm to 70 μm. The thickness of the positive electrode mixture layer 31 on one side of the positive electrode core 30 is preferably greater than the thickness of the tape 50 (the thickness of the positive electrode mixture layer 31 ≥ the thickness of the tape 50), and more preferably greater than the thickness of the tape 50. In this case, the effect of the exposed portion 32 described later becomes more significant. The thickness of the positive electrode mixture layer 31 on one side of the positive electrode core 30 is, for example, 50 μm to 130 μm, preferably 60 μm to 110 μm.

[0040] Figure 3 1 is a front view of the positive electrode 11 after it is unfolded, showing the surface of the positive electrode 11 facing the radial outer side of the electrode body 14 (the outer surface of the winding). Figure 3 As shown, there is no positive electrode mixture layer 31 in the longitudinal center of the positive electrode 11, and an exposed portion 33 is formed where the surface of the positive electrode core 30 is exposed. The exposed portion 33 is the portion where the positive electrode lead 20 is welded, and is formed with a constant width over the entire width of the positive electrode 11. Figure 3 In the example shown, protective tapes 34 and 35 covering the exposed portion 33 and the positive electrode lead 20 are attached to the longitudinal center of the positive electrode 11. The protective tapes 34 and 35 can be the same as the tape 50 for winding prevention.

[0041] The mass of the positive electrode active material per unit area in the first region of the outermost winding outer surface of the positive electrode 11, which overlaps with the tape 50 in the radial direction of the electrode body 14, is less than the mass of the positive electrode active material per unit area in the second region of the winding outer surface where the positive electrode mixture layer 31 exists except the first region. In the conventional secondary battery, a homogeneous positive electrode mixture layer is formed over the entire region of the positive electrode 11 (see Experimental Example 3 described later). In contrast, in the secondary battery 10, there is a region where the mass of the positive electrode active material per unit area decreases in at least a portion of the outermost winding outer surface of the positive electrode 11.

[0042] If the amount of positive electrode active material per unit area is reduced in the first region of the positive electrode 11, the volume change of the region caused by charge and discharge becomes smaller, and the stress acting on the edge of the tape 50 is relieved. As a result, it is considered that the damage to the electrode body 14 is effectively suppressed. It should be noted that the results of the inventor's research show that even if the amount of positive electrode active material per unit area is reduced in the region other than the outermost winding outer surface of the positive electrode 11, the damage suppression effect of the electrode body 14 is small or almost no effect is obtained.

[0043] The region with less mass of positive electrode active material per unit area on the outermost winding surface of the positive electrode 11 can be formed, for example, by making the thickness of the positive electrode mixture layer 31 thinner than the second region, but is preferably set as an exposed portion 32 exposed on the surface of the positive electrode core 30. That is, the exposed portion 32 without the positive electrode mixture layer 31 is formed in the first region of the positive electrode 11. The second region is a region on the outermost winding surface of the positive electrode 11 except the first region where the positive electrode mixture layer 31 is formed, and in this embodiment, it is a region other than the exposed portion 33. It should be noted that the mass of the positive electrode active material per unit volume in the second region is substantially uniform.

[0044] The exposed portion 32 is a region where the positive electrode active material is substantially absent, and is formed, for example, by not applying the positive electrode mixture slurry to the positive electrode core 30 or by peeling off a portion of the positive electrode mixture layer 31 from the positive electrode core 30. The exposed portion 32 may be formed over the entire area of ​​the outer peripheral surface of the positive electrode 11, but from the perspective of increasing the capacity, it is preferably formed only in the first region and its vicinity that overlaps with the tape 50 in the radial direction of the electrode body 14. Considering the positional offset between the exposed portion 32 and the tape 50, the exposed portion 32 is formed in a range slightly wider than the first region.

[0045] One end of the positive electrode 11 in the longitudinal direction is located on the winding core side of the electrode body 14 and becomes the winding start end 11X, and the other end in the longitudinal direction is located on the outer peripheral surface side of the electrode body 14 and becomes the winding end end 11Y. Therefore, a predetermined length range from the other end in the longitudinal direction of the positive electrode 11 becomes the outermost periphery of the positive electrode 11 in the electrode body 14, and a part of the winding outer surface becomes the first region overlapping with the tape 50 in the radial direction of the electrode body 14. In the present embodiment, the tape 50 is attached to both ends in the axial direction of the electrode body 14, so that both ends in the width direction of the positive electrode 11 become the first region, and the exposed portion 32 is formed at least in this region.

[0046] On the outer surface of the winding of the outermost circumference of the positive electrode 11, an exposed portion 32 is formed in the axial direction of the electrode body 14 in such a manner as to sandwich the positive electrode mixture layer 31. In other words, within a predetermined length range from the winding end 11Y of the positive electrode 11, the exposed portion 32, the positive electrode mixture layer 31, and the exposed portion 32 are sequentially formed along the width direction of the positive electrode 11. Each exposed portion 32 has, for example, the same front view shape and size as each other. The area sandwiched by each exposed portion 32 is an area that does not overlap with the tape 50, so by providing the positive electrode mixture layer 31 in this area, the capacity reduction can be suppressed to the minimum required.

[0047] Preferably, the exposed portion 32 is formed continuously from the winding end 11Y to the winding start end 11X side on the outermost winding surface of the positive electrode 11. The length L of the exposed portion 32 along the length direction of the positive electrode 11 is preferably longer than the length of the tape 50, and may be a length exceeding the circumference of the outer peripheral surface of the positive electrode 11. The length L of the exposed portion 32 slightly varies depending on the length of the tape 50, and is, for example, a length equivalent to 90% to 110% of the circumference of the outer peripheral surface of the positive electrode 11. In this case, it is possible to efficiently suppress damage to the electrode body 14 while ensuring high capacity.

[0048] The width W of the exposed portion 32 is preferably longer than the width of the tape 50. For example, considering the positional deviation between the positive electrode 11 and the tape 50, the width W of the exposed portion 32 is set to a length equivalent to 105% to 120% of the width of the tape 50. By forming the exposed portion 32 within the minimum required range, it is possible to effectively suppress damage to the electrode body 14 while ensuring high capacity. The width W of the exposed portion 32 is constant over the entire length of the exposed portion 32, and the exposed portion 32 has a rectangular shape when viewed from the front.

[0049] Example

[0050] Hereinafter, the present invention will be further described by way of experimental examples, but the present invention is not limited to these experimental examples.

[0051] <Experimental Example 1>

[0052] [Production of positive electrode]

[0053] As the positive electrode active material, LiNi 0.88 Co 0.09 Al 0.03 Lithium metal composite oxide shown as O2. The positive electrode active material, acetylene black and polyvinylidene fluoride are mixed in a solid content mass ratio of 100:1:0.9, and N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry is applied to both sides of a positive electrode core formed of aluminum foil with a thickness of 15 μm in such a way that the welding part of the positive electrode lead remains. After the coating film is dried and compressed, it is cut into a specified electrode size to produce a positive electrode (thickness 0.144 mm, width 62.6 mm, length 703 mm) having a positive electrode mixture layer formed on both sides of the positive electrode core. It should be noted that an exposed portion of the welding part of the positive electrode lead is provided in the central part in the longitudinal direction of the positive electrode, and the positive electrode lead is welded to the exposed portion.

[0054] Next, on one side of the positive electrode, a predetermined range of the positive electrode mixture layer was peeled off from one end in the longitudinal direction of the positive electrode to form an exposed portion A where the surface of the positive electrode core was exposed. The exposed portion A was formed into a rectangular shape with a length of 64 mm and a width of 10 mm at both ends in the width direction of the positive electrode.

[0055] [Production of negative electrode]

[0056] As the negative electrode active material, a mixed powder of 95 parts by mass of graphite powder and 5 parts by mass of a Si-containing composite material (a composite material in which Si particles are dispersed in a SiO2 phase) is used. The negative electrode active material, sodium salt of carboxymethyl cellulose (CMC-Na) and a dispersion of styrene-butadiene rubber (SBR) are mixed at a solid component mass ratio of 100:1:1, and water is used as a dispersion medium to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to both sides of a negative electrode core formed of a copper foil with a thickness of 8 μm. After the coating is dried and compressed, it is cut into a specified electrode size to produce a negative electrode (thickness 0.192 mm, width 64 mm, length 816 mm) with a negative electrode mixture layer formed on both sides of the negative electrode core.

[0057] It should be noted that an exposed portion to be a welding portion of a negative electrode lead is provided at one end in the longitudinal direction of the negative electrode, and the negative electrode lead is welded to the exposed portion. In addition, an exposed portion B is formed on one side of the negative electrode to expose the surface of the negative electrode core body from the other end in the longitudinal direction of the negative electrode to a length of about 1.5 circumferences of the electrode body.

[0058] [Preparation of non-aqueous electrolyte]

[0059] To a non-aqueous solvent prepared by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:3 (25°C), vinylene carbonate was added to a concentration of 5% by mass and LiPF6 was added to a concentration of 1.5M to prepare a non-aqueous electrolyte.

[0060] [Battery production]

[0061] The positive electrode and the negative electrode are wound into a spiral shape with a polyethylene separator interposed therebetween, thereby producing a wound electrode body. At this time, the positive electrode and the negative electrode are wound with the separator interposed therebetween in a manner that the exposed portion A of the positive electrode faces the radial outer side of the electrode body and is located at the outermost periphery of the positive electrode, and the exposed portion B of the negative electrode faces the radial outer side of the electrode body and forms the outer peripheral surface of the electrode body. Then, a stopper tape (length 60 mm, width 9 mm) is attached and fixed to the winding end end at both axial ends of the outer peripheral surface of the electrode body to maintain the winding structure. It should be noted that the exposed portion A of the positive electrode is arranged at a position overlapping with the tape in the radial direction of the electrode body, and there is no positive electrode mixture layer within the range of the outer peripheral surface of the positive electrode overlapping with the tape.

[0062] Next, after insulating plates are arranged above and below the electrode body, the negative electrode lead is welded to the inner surface of the bottom of the outer can, the positive electrode lead is welded to the internal terminal plate of the sealing body, and the electrode body is stored in the outer can. Then, a non-aqueous electrolyte is injected into the outer can in a decompressed manner, and the opening of the outer can is sealed with a sealing body through a gasket to produce a cylindrical non-aqueous electrolyte secondary battery with a battery capacity of 4600 mAh. It should be noted that the exposed portion B of the negative electrode is in contact with the inner peripheral surface of the outer can.

[0063] <Experimental Example 2>

[0064] A cylindrical nonaqueous electrolyte secondary battery was prepared in the same manner as in Experimental Example 1 except that the width of the exposed portion A of the positive electrode was changed to 5 mm. In this case, a portion of the positive electrode mixture layer formed on the outer peripheral surface of the positive electrode overlapped the anti-winding tape in the radial direction of the electrode body.

[0065] <Experimental Example 3>

[0066] A cylindrical nonaqueous electrolyte secondary battery was produced in the same manner as in Experimental Example 1 except that the exposed portion A of the positive electrode was not formed.

[0067] [Cycle test]

[0068] The secondary battery of each experimental example was charged at a constant current of 1380 mA (0.3 It) at a temperature of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the termination current reached 92 mA (0.02 It). After pausing for 20 minutes, the battery was discharged at a constant current of 2300 mA (0.5 It) until the battery voltage reached 2.75 V. This charge and discharge was repeated 100 times.

[0069] The battery subjected to the above charge and discharge cycles 100 times was disassembled, and the presence or absence of wrinkles on the negative electrode core on the outer peripheral surface of the electrode assembly was checked.

[0070] [Table 1]

[0071]

[0072] As shown in Table 1, in the battery of Experimental Example 1 in which a wide exposed portion A was provided on the outer peripheral surface of the positive electrode so that the positive electrode mixture layer and the anti-rolling tape did not overlap in the radial direction of the electrode body, wrinkles of the negative electrode core were not confirmed on the outer peripheral surface of the electrode body. On the other hand, in the batteries of Experimental Examples 2 and 3 in which the positive electrode mixture layer and the anti-rolling tape on the outer peripheral surface of the positive electrode overlapped in the radial direction of the electrode body, wrinkles of the negative electrode core were confirmed.

[0073] In the batteries of Experimental Examples 2 and 3, it is believed that due to the influence of the height difference formed at the edge of the tape, stress was concentrated at the edge during charge and discharge, resulting in wrinkles in the negative electrode core. In contrast, in the battery of Experimental Example 1, it is believed that the influence of the height difference was alleviated and the stress was dispersed, resulting in the prevention of wrinkles in the negative electrode core.

[0074] Description of Reference Numerals

[0075] 10 secondary battery, 11 positive electrode, 11X winding start end, 11Y, 12Y winding end end, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 slot, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cover, 28 gasket, 30 positive electrode core, 31 positive electrode mixture layer, 32, 33, 42 exposed portion, 34, 35 protective tape, 40 negative electrode core, 41 negative electrode mixture layer, 50 tape

Claims

1. A secondary battery comprising an electrode body, wherein the electrode body comprises a positive electrode, a negative electrode and a separator, wherein the positive electrode and the negative electrode are wound with the separator interposed therebetween. An adhesive tape for fixing the winding end is attached to the outer peripheral surface of the electrode body. The positive electrode comprises a positive electrode core and a positive electrode mixture layer, wherein the positive electrode mixture layer contains a positive electrode active material and is formed on the positive electrode core. The mass of the positive electrode active material per unit area of ​​at least a first region of the outermost wound outer surface of the positive electrode that overlaps with the tape in the radial direction of the electrode body is less than the mass of the positive electrode active material per unit area of ​​a second region of the wound outer surface where the positive electrode mixture layer exists other than the first region.

2. The secondary battery according to claim 1, wherein An exposed portion where the surface of the positive electrode substrate is exposed is formed in the first region.

3. The secondary battery according to claim 2, wherein: The adhesive tape is attached to both axial ends of the electrode body.

4. The secondary battery according to any one of claims 1 to 3, wherein The thickness of the tape is less than or equal to the thickness of the positive electrode mixture layer in the second region.

Images