Cylindrical battery

Abstract

A cylindrical battery, which is an exemplary embodiment, comprises an electrode body that has a positive electrode (11), a negative electrode, and a separator and is formed by winding the positive electrode (11) and the negative electrode with the separator interposed therebetween. The positive electrode (11) or the negative electrode is connected to the bottom of an outer can. The positive electrode (11) includes a long positive electrode core body (30) and a positive electrode mixture layer (31) provided on the positive electrode core body (30). Regarding the positive electrode mixture layer (31), where a region including the center of the positive electrode (11) in the width direction is defined as a central region (312), a region including one end in the width direction positioned closer to a sealing body (16) side than the central region (312) is defined as a first region (311), and a region including the other end in the width direction positioned closer to the bottom side of the outer can (15) than the central region (311) is defined as a second region (313), the density of the first region (311) is the same as the density of the central region (312) or greater than the density of the central region (312), and the density of the second region (313) is smaller than the density of the central region (312).

Description

Cylindrical battery

[0001] The present disclosure relates to cylindrical batteries.

[0002] A cylindrical battery generally includes a wound electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, a cylindrical outer can with a bottom that houses the electrode assembly, and a sealing body that closes the opening of the outer can. Patent Document 1 discloses a cylindrical battery that includes a positive electrode in which the amount of positive electrode mixture at the upper and lower ends in the width direction cross section of the positive electrode is larger than the amount of positive electrode mixture at the center, and a negative electrode in which the amount of negative electrode mixture at the upper and lower ends in the width direction cross section of the negative electrode is smaller than the amount of negative electrode mixture at the center.

[0003] Japanese Patent Application Laid-Open No. 2001-15146

[0004] In cylindrical batteries, electrode plates are typically connected to the bottom of an exterior can by welding or the like, and the negative electrode is often connected to the bottom of the exterior can. The inventors' research has revealed that repeated charge and discharge of a cylindrical battery increases the difference in pressure (surface pressure) acting on the surface of the electrode plate in the width direction. This can lead to problems such as deformation of the electrode plate and a decrease in capacity retention during charge and discharge cycles. The difference in surface pressure acting on the electrode plate in the width direction is thought to be caused by the electrode plate being fixed to the bottom of the exterior can. In particular, the greater the area of ​​the electrode plate connected to the bottom of the exterior can and the stronger the electrode plate's constraint against the bottom, the more likely the above problems are to occur.

[0005] The cylindrical battery according to the present disclosure includes an electrode assembly having a positive electrode, a negative electrode, and a separator, the positive electrode and the negative electrode being wound with the separator interposed therebetween; a cylindrical outer can with a bottom that houses the electrode assembly; and a sealing member that closes the opening of the outer can, with the positive electrode or the negative electrode being connected to the bottom of the outer can. The positive electrode includes a long positive electrode core and a positive electrode mixture layer provided on the positive electrode core. Regarding the positive electrode mixture layer, a central region is defined as a region including the widthwise center of the positive electrode, a first region is defined as a region including one widthwise end located closer to the sealing member than the central region, and a second region is defined as a region including the other widthwise end located closer to the bottom of the outer can than the central region. The cylindrical battery is characterized in that the density of the first region is the same as or greater than the density of the central region, and the density of the second region is smaller than the density of the central region.

[0006] The cylindrical battery according to the present disclosure can suppress deformation of the electrode plates and also suppress a decrease in capacity retention rate during charge / discharge cycles.

[0007] It is a cross-sectional view of a cylindrical battery according to a first embodiment. It is a front view of a positive electrode according to a first embodiment. It is a front view of a positive electrode according to a second embodiment. It is a diagram illustrating a method for evaluating electrode plate deformation.

[0008] Hereinafter, an example of an embodiment of a cylindrical battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations formed by selectively combining the components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0009] A cylindrical battery 10 according to the first embodiment will be described in detail with reference to Fig. 1. Fig. 1 is a schematic diagram showing a cross section of a cylindrical battery 10 taken along the axial and radial directions.

[0010] As shown in FIG. 1 , the cylindrical battery 10 includes a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11 and the negative electrode 12 are wound with the separator 13 interposed therebetween, forming an electrode assembly 14, and a cylindrical outer can 15 with a bottom that houses the electrode assembly 14. The cylindrical battery 10 also includes an electrolyte housed in the outer can 15 and a sealing body 16 that closes the opening of the outer can 15. The outer can 15 has a groove 21 formed in its side wall, and the sealing body 16 is supported by the groove 21 to close the opening of the outer can 15. The positive electrode 11 or the negative electrode 12 is connected to the bottom of the outer can 15. For ease of explanation, the sealing body 16 side of the cylindrical battery 10 will be referred to as the top, and the bottom side of the outer can 15 will be referred to as the bottom.

[0011] 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 (electrolytic solution) or a solid electrolyte. The cylindrical battery 10 is, for example, a non-aqueous electrolyte secondary battery, and preferably a lithium ion battery.

[0012] The liquid electrolyte (electrolytic solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0013] 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, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, 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. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0014] As described above, the electrode assembly 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the separator 13 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the length direction and width direction (winding axis direction) than the positive electrode 11. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and for example, two separators 13 are arranged to sandwich the positive electrode 11.

[0015] The positive electrode 11 has 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 a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface layer. 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, or the like is used as the positive electrode active material.

[0016] The thickness of the positive electrode 11 is, for example, 160 μm or more and 200 μm or less. In the present embodiment, the thickness of the positive electrode 11 is substantially constant except for the positive electrode core exposed portion 32 to which the positive electrode lead 20 is connected. The thickness of the positive electrode core 30 is, for example, 10 μm or more and 30 μm or less. The thickness of the positive electrode mixture layer 31 is, for example, 70 μm or more and 100 μm or less on one side of the positive electrode core 30. The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0017] In this embodiment, the positive electrode 11 has a positive electrode core exposed portion 32 where the positive electrode mixture layer 31 is not present and the surface of the positive electrode core 30 is exposed. Positive electrode leads 20 that connect the sealing body 16 and the positive electrode core exposed portion 32 are joined to the positive electrode core exposed portion 32. The positive electrode leads 20 are preferably joined directly to the positive electrode core exposed portion 32 by ultrasonic welding or the like.

[0018] The negative electrode 12 has a long negative electrode core 40 and a negative electrode mixture layer 41 provided on the negative electrode core 40. For the negative electrode core 40, a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper, a copper alloy, stainless steel, nickel, or a nickel alloy, or a film having such a metal disposed on its surface can be used. 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. For example, graphite, a Si-containing material, or the like is used as the negative electrode active material.

[0019] The thickness of the negative electrode 12 is, for example, 170 μm or more and 210 μm or less. In the present embodiment, the thickness of the negative electrode 12 is substantially constant except for a negative electrode core exposed portion described later. The thickness of the negative electrode core 40 is, for example, 5 μm or more and 15 μm or less. The thickness of the negative electrode mixture layer 41 is, for example, 70 μm or more and 110 μm or less on one surface side of the negative electrode core 40. The negative electrode 12 can be produced, similarly to 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 40, drying the coating, and then compressing it to form the negative electrode mixture layers 41 on both surfaces of the negative electrode core 40.

[0020] In this embodiment, the negative electrode 12 has a strip-shaped negative electrode core exposed portion, where the surface of the negative electrode core 40 is exposed, formed along the length of the negative electrode 12 at a widthwise end portion located on the bottom side of the outer can 15. The negative electrode core exposed portion is provided, for example, over a range from the end on the winding start side to the end on the winding end side in the longitudinal direction of the long negative electrode 12.

[0021] The negative electrode core exposed portion is connected to the bottom of the outer can 15 directly or via the lower current collector 17. For example, the negative electrode core exposed portion is joined to the upper surface of the lower current collector 17 by welding or the like. Note that the cylindrical battery 10 may not include the lower current collector 17, and the negative electrode core exposed portion may be joined to the inner surface of the bottom of the outer can 15 by welding or the like. The width of the negative electrode core exposed portion is, for example, 2 mm or more and 20 mm or less.

[0022] Furthermore, the negative electrode 12 does not necessarily have to have a strip-shaped negative electrode core exposed portion at the widthwise end on the bottom side of the outer can 15. In this case, a negative electrode core exposed portion in which the negative electrode mixture layer 41 is not present and the surface of the negative electrode core 40 is exposed may be formed at one lengthwise end of the negative electrode 12. The negative electrode core exposed portion is provided at one lengthwise end of the negative electrode 12 located at the winding start side or the winding end side of the electrode body 14. A negative electrode lead is connected to the negative electrode core exposed portion, and the negative electrode lead is connected to the outer can 15.

[0023] An insulating plate 18 is disposed on the electrode body 14. In the example shown in Fig. 1, a positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 22 of the sealing body 16 by welding or the like, and a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the internal terminal plate 22, serves as the positive electrode terminal.

[0024] A lower current collector 17 is disposed below the electrode assembly 14. The lower current collector 17 is a metal member. As described above, the exposed portion of the negative electrode core that constitutes the negative electrode 12 is joined to the upper surface of the lower current collector 17. The lower surface of the lower current collector 17 is welded to the inner surface of the bottom of the outer can 15. This allows the outer can 15 to function as a negative electrode terminal. The shape of the lower current collector 17 is not particularly limited, and may, for example, have a generally circular shape when viewed from above.

[0025] The outer can 15 is a cylindrical metal container with a bottom. A gasket 27 is provided between the outer can 15 and the sealing body 16 to seal the inside of the battery. The outer can 15 has a groove 21 that supports the sealing body 16, formed, for example, by pressing the side surface from the outside. The groove 21 is preferably formed in an annular shape along the circumferential direction of the outer can 15, and supports the sealing body 16 on its top surface. The upper end of the outer can 15 is bent inward and crimped to the periphery of the sealing body 16.

[0026] The sealing body 16 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26 are stacked. Each member constituting the sealing body 16 has, for example, a disk or ring shape, and each member except for the insulating member 24 is electrically connected to one another. The lower valve body 23 and the upper valve body 25 are connected at their respective centers, and the insulating member 24 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 23 deforms and breaks, pushing the upper valve body 25 toward the cap 26, thereby interrupting the current path between the lower valve body 23 and the upper valve body 25. When the internal pressure further increases, the upper valve body 25 breaks, and gas is discharged from the opening 26a of the cap 26.

[0027] The positive electrode mixture layer 31 of the positive electrode 11 will be described in detail below with further reference to Fig. 2. Fig. 2 is a front view illustrating the positive electrode 11 of the first embodiment.

[0028] As described above, the positive electrode 11 has an elongated positive electrode core 30 and a positive electrode mixture layer 31 provided on the positive electrode core 30. The positive electrode mixture layer 31 is divided into a central region 312 that is a region including the center in the width direction of the positive electrode 11, a first region 311 that is a region including one end in the width direction located closer to the sealing member 16 than the central region 312, and a second region 313 that is a region including the other end in the width direction located closer to the bottom of the outer can 15 than the central region 312. As will be described in detail later, the density of the first region 311 is the same as or higher than the density of the central region 312, and the density of the second region 313 is lower than the density of the central region 312.

[0029] The width L311 of the first region 311 may be 5% to 30% or less, or may be 10% to 25% or less, of the overall width L11 of the positive electrode 11. An example of the width L311 of the first region 311 is 15% of the width L11 of the positive electrode 11.

[0030] The width L313 of the second region 313 may be 5% to 30% or 10% to 25% of the overall width L11 of the positive electrode 11. An example of the width L313 of the second region 313 is 15% of the width L11 of the positive electrode 11. An example of the ratio of the width L311, the width L312, and the width L313 is 15:70:15. The overall width L11 of the positive electrode 11 is, for example, 70 mm.

[0031] The density of the first region 311 is the same as or greater than the density of the central region 312, and the density of the second region 313 is less than the density of the central region 312. For example, the density of the first region 311 may be greater than or equal to 100% and less than 130%, or may be greater than or equal to 115% and less than 125% of the density of the central region 312. An example of the density of the first region 311 is 120% of the density of the central region 312. The density of the second region 313 may be greater than 70% and less than 100%, or may be greater than or equal to 75% and less than 95% of the density of the central region 312. An example of the density of the second region 313 is 80% of the density of the central region 312.

[0032] If the density of the first region 311 is excessively higher than the density of the central region 312, and if the density of the second region 313 is excessively lower than the density of the central region 312, it may be difficult to satisfy the performance requirements of the electrode body 14, such as peel strength. An example of an excessively high density of the first region 311 is when it is 130% or more of the density of the central region 312. An example of an excessively low density of the second region 313 is when it is 70% or less of the density of the central region 312.

[0033] The average density of the central region 312 is preferably higher than 3.30 g / cc, particularly preferably 3.45 g / cc or higher, and may be 3.55 g / cc or higher. While there is no particular upper limit to the density of the central region 312, from the viewpoint of improving rapid charging performance, it is preferably 3.65 g / cc or lower. An example of a suitable range for the average density of the central region 312 is 3.45 g / cc or higher and 3.65 g / cc or lower, or 3.45 g / cc or higher and 3.55 g / cc or lower.

[0034] The density of the positive electrode mixture layer 31 mainly depends on the packing density of the positive electrode active material. Therefore, it can be said that the packing density of the positive electrode active material in the first region 311 is the same as or higher than the packing density of the positive electrode active material in the central region 312. Similarly, it can be said that the packing density of the positive electrode active material in the second region 313 is lower than the packing density of the positive electrode active material in the central region 312. The thickness of the positive electrode mixture layer 31 is, for example, 70 μm or more and 100 μm or less on one side of the positive electrode core 30. It is preferable that the thicknesses of the first region 311, the central region 312, and the second region 313 are substantially the same from the viewpoint of uniformity of the battery reaction, etc.

[0035] The positive electrode mixture layer 31 is formed by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core 30, drying the coating, and then compressing it. The first region 311, the central region 312, and the second region 313 can be formed, for example, by adjusting the amount of positive electrode mixture slurry applied per unit area.

[0036] As a specific example, when forming the first region 311, the amount of positive electrode mixture slurry applied is set to 100% or more and less than 110% of the amount applied when forming the central region 312. It is preferably 100%. Note that, for example, an error of about ±2.5% may occur in the amount applied relative to the target value. When forming the second region 313, the amount of positive electrode mixture slurry applied is set to 75% or more and 85% or less of the amount applied when forming the central region 312. Then, the entire positive electrode mixture layer 31 is compressed under the same conditions to make the thickness uniform.

[0037] Alternatively, two types of positive electrode mixture slurries may be used, such that the same type of positive electrode mixture slurry as that used in the central region 312 is used to form the first region 311, and a positive electrode mixture slurry containing a positive electrode active material that is more likely to be packed loosely than the central region 312 is used to form the second region 313. Alternatively, three types of positive electrode mixture slurries may be used, such that the first region 311 is formed using a positive electrode mixture slurry containing a positive electrode active material that is more likely to be packed tightly than the central region 312. The second region 313 may be formed using a positive electrode mixture slurry containing a positive electrode active material that is more likely to be packed loosely than the central region 312. The positive electrode mixture slurry is applied by, for example, a die coating method.

[0038] The densities of the central region 312, the first region 311, and the second region 313 may be lower in the intra-winding region 314 located on the inner side of the electrode assembly 14 than in the extra-winding region 315 located on the outer side of the electrode assembly 14. For example, the intra-winding region 314 is a region that extends from the inner end of the winding to one-third of the total length of the positive electrode 11, and the extra-winding region 315 is a region that extends from the outer end of the winding to two-thirds of the total length of the positive electrode 11. That is, the densities of the central region 312, the first region 311, and the second region 313 may be lower in the intra-winding region 314 that extends to one-third of the total length of the positive electrode 11 located on the inner side of the winding of the electrode assembly 14 than in the region that extends to two-thirds of the total length of the positive electrode 11 located on the outer side of the winding of the electrode assembly 14. The density of each region in the intra-winding region 314 is, for example, more than 70% but less than 100% of the density of each region in the extra-winding region 315. The inner winding region 314 has a higher surface pressure than the outer winding region 315, so by reducing the density of the inner winding region 314 compared to the outer winding region 315, the surface pressure in the radial direction can be made uniform.

[0039] In this specification, the density of the positive electrode mixture layer 31 is measured by cutting out each region of the positive electrode mixture layer 31 with a jig and measuring the weight. For example, the first region 311, the central region 312, and the second region 313 are each cut out into a disk shape at a plurality of locations and the weights are measured to obtain an average density in each region.

[0040] The second embodiment will be described in detail using Figure 3. Figure 3 is a front view illustrating the positive electrode 11 of a cylindrical battery 10 according to the second embodiment. The second embodiment differs from the first embodiment only in that a plurality of positive electrode substrate exposed portions 32 are formed on the positive electrode 11. In the second embodiment, the same components as those in the first embodiment are assigned the same reference numerals as those in the first embodiment, and their description will be omitted. Furthermore, in the second embodiment, description of the same effects and modifications as those in the first embodiment will be omitted.

[0041] In the second embodiment, as shown in Fig. 3 , the positive electrode 11 has a plurality of positive electrode substrate exposed portions 32 in the first region 311. Furthermore, a positive electrode lead 20 is connected to each of the plurality of positive electrode substrate exposed portions 32. The plurality of positive electrode leads 20 all extend in the same direction and extend from one widthwise end of the positive electrode 11. In the second embodiment, one lengthwise end of each positive electrode lead 20 is joined to the positive electrode 11, and the other lengthwise end is joined to an internal terminal plate 22 of the sealing body 16, and the positive electrode 11 and the internal terminal plate 22 are electrically connected via the positive electrode lead 20.

[0042] The number of positive electrode leads 20 is not particularly limited, but is, for example, 3 to 15 or 6 to 10. An example of the number of positive electrode leads 20 is 8. The positive electrode leads 20 may be provided at equal intervals along the length direction of the positive electrode 11. Furthermore, the positive electrode leads 20 do not have to be provided at equal intervals, and may be provided at varying intervals so that multiple positive electrode leads 20 are stacked in the wound electrode body 14. Providing multiple positive electrode leads 20 in the positive electrode 11 can reduce resistance.

[0043] The above embodiment can be modified as appropriate within the scope of the present disclosure. For example, the positive electrode 11 may be provided with a positive electrode core exposed portion 32, where the positive electrode core 30 is exposed, at the widthwise end located on the sealing member 16 side. In this case, the positive electrode core exposed portion 32 is wound so as to extend from the upper end of the electrode assembly 14. The extended positive electrode core exposed portion 32 is connected by welding or the like to an upper current collector plate placed on the electrode assembly. By joining the positive electrode core exposed portion 32 to the upper current collector plate, the contact area between the positive electrode core exposed portion 32 and the upper current collector plate increases, thereby reducing the internal resistance of the positive electrode 11.

[0044] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0045] Example 1 Preparation of Positive Electrode Mixture Slurry Lithium nickel oxide (LiNi) containing cobalt and aluminum was used as the positive electrode active material. ０．８８ Co ０．０９ Al ０．０３ O ２ The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 98:1:1, and N-methylpyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry.

[0046] [Preparation of Positive Electrode] The positive electrode mixture slurry was applied to both sides of a positive electrode core made of a 15 μm-thick long aluminum foil by die coating, and a first coating film was formed in a region including one widthwise end of the positive electrode core, a central coating film in a region including the center in the widthwise direction, and a second coating film in a region including the other widthwise end, and the coating films were dried. The first coating film was formed with the same coating amount as when forming the central coating film, and the second coating film was formed with 90% of the coating amount when forming the central coating film. Note that the first coating film became the first region, the central coating film became the central region, and the second coating film became the second region.

[0047] In this example, the ratio of the width of the first region to the width of the central region to the width of the second region was adjusted to 15:70:15. The average width of the first region was 10 mm, the average width of the central region was 50 mm, and the average width of the second region was 10 mm. Eight positive electrode substrate exposed portions where no positive electrode mixture layer was present were provided in the first region, and aluminum positive electrode leads were welded to the positive electrode substrate exposed portions.

[0048] Next, the coating (positive electrode mixture layer) was compressed using a roller, and the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core. The average thickness of the positive electrode mixture layer was 70 μm on one side of the positive electrode core, and the first region, central region, and second region had substantially the same thickness. The density of the first region was the same as that of the central region, and the density of the second region was 90% of that of the central region. According to the above measurement method, the average density of the first region was 3.55 g / cc, the average density of the central region was 3.55 g / cc, and the average density of the second region was 3.20 g / cc.

[0049] [Negative Electrode Fabrication] 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 carboxymethyl cellulose were mixed in a solids mass ratio of 98:1:1, and a negative electrode mixture slurry was prepared using water as a dispersion medium. The slurry was applied to both sides of a negative electrode core made of a long copper foil with a thickness of 8 μm, and the coating was dried and compressed to obtain a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode core. A strip-shaped negative electrode core exposed portion was provided at the lower end of the width direction of the negative electrode, where the negative electrode mixture layer was not present from the start end to the end of the winding along the length of the negative electrode.

[0050] [Fabrication of Electrode Assembly] The positive electrode, the negative electrode, and a polyethylene separator were spirally wound around a cylindrical core member, and a stop tape was attached to the outermost peripheral surface to obtain a wound electrode assembly. After forming the wound structure of the electrode assembly, the core member was removed to obtain a wound electrode assembly with a cavity formed in the core portion.

[0051] [Preparation of non-aqueous electrolyte] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:3 (25°C), and LiPF ６ was dissolved in a concentration of 1.5 mol / L to prepare a non-aqueous electrolyte solution.

[0052] [Fabrication of Cylindrical Battery] An insulating plate was placed on the electrode assembly, and a negative electrode current collector was placed underneath. The exposed portion of the negative electrode core extending from the lower end of the electrode assembly was welded to the upper surface of the lower current collector, and the lower surface of the lower current collector was welded to the inner surface of the bottom of the outer can. Furthermore, a positive electrode lead was welded to the internal terminal plate of the sealing member, and the electrode assembly was housed in the outer can. A nonaqueous electrolyte was then injected into the outer can under reduced pressure, and the opening of the outer can was sealed with the sealing member via a gasket, thereby obtaining a cylindrical battery.

[0053] Example 2 An electrode assembly and a cylindrical battery were produced in the same manner as in Example 1, except that in producing the positive electrode, the density of the second region was set to 80% of the density of the central region.

[0054] Example 3 An electrode assembly and a cylindrical battery were fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the density of the first region was set to 110% of the density of the central region.

[0055] Example 4 An electrode body and a cylindrical battery were produced in the same manner as in Example 1, except that in producing the positive electrode, the density of the first region was set to 120% of the density of the central region, and the density of the second region was set to 80% of the density of the central region.

[0056] Comparative Example 1 An electrode assembly and a cylindrical battery were fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the density of the second region was set to 100% of the density of the central region.

[0057] Comparative Example 2 An electrode assembly and a cylindrical battery were fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the density of the second region was set to 110% of the density of the central region.

[0058] <Comparative Example 3> An electrode body and a cylindrical battery were produced in the same manner as in Example 1, except that in producing the positive electrode, the density of the first region was set to 90% of the density of the central region, and the density of the second region was set to 100% of the density of the central region.

[0059] <Comparative Example 4> An electrode body and a cylindrical battery were produced in the same manner as in Example 1, except that in producing the positive electrode, the density of the first region was set to 110% of the density of the central region, and the density of the second region was set to 100% of the density of the central region.

[0060] <Comparative Example 5> An electrode body and a cylindrical battery were produced in the same manner as in Example 1, except that in producing the positive electrode, the density of the first region was set to 120% of the density of the central region and the density of the second region was set to 100% of the density of the central region.

[0061] Each battery of the examples and comparative examples was evaluated by the following method, and the evaluation results are shown in Table 1. The density of the positive electrode mixture layer shown in Table 1 indicates the ratio of the density of each region when the density of the central region is set to 100%.

[0062] [Evaluation of electrode plate deformation (buckling)] Each battery of the examples and comparative examples was charged at a constant current of 0.5 C in a temperature environment of 25° C. until the battery voltage reached 4.2 V. Thereafter, the battery was discharged at a constant current of 0.7 C until the battery voltage reached 2.5 V. After 500 cycles of this charge / discharge, the battery was placed in a charged state, and the vicinity of the winding core of the electrode body was observed using an X-ray CT device (Shimadzu Corporation, SMX-225CT FPD HR).

[0063] 4, when deformation (buckling) of the electrode plate (at least one of the positive electrode 11 and the negative electrode 12) was confirmed near the starting end of the positive electrode in the CT image of the electrode body, where the angle α was 150° or less, it was determined that deformation had occurred. In Examples 1 to 4 and Comparative Examples 1 to 5, evaluation was performed on 10 batteries each.

[0064] [Evaluation of Capacity Retention Rate] Each battery of the Examples and Comparative Examples was charged at a constant current of 1 C in a temperature environment of 25° C. until the battery voltage reached 4.2 V. Thereafter, the battery was discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This charge / discharge cycle was counted as one cycle, and 500 cycles were repeated. The discharge capacity at the first cycle and the discharge capacity at the 500th cycle were determined, and the capacity retention rate was calculated according to the following formula: Capacity retention rate (%) = (Discharge capacity at 500th cycle / Discharge capacity at first cycle) × 100

[0065] When the capacity retention rate calculated above was below 90%, it was determined that the capacity was not maintained and was counted. That is, the numerator of the capacity retention rate in Table 1 is the number of times the capacity retention rate after 500 cycles was below 90%. In Examples 1 to 4 and Comparative Examples 1 to 5, evaluation was performed on 10 batteries each.

[0066]

[0067] As shown in Table 1, Examples 1 to 4 were able to both suppress buckling and suppress a decrease in capacity retention. In particular, Example 4, in which the density of the first region was 120% of that of the central region and the density of the second region was 80% of that of the central region, more reliably suppressed deformation of the negative electrode and a decrease in capacity retention. On the other hand, Comparative Examples 1 to 5 were unable to both suppress buckling and a decrease in capacity retention.

[0068] The present disclosure will be further described by the following embodiments. Configuration 1: A cylindrical battery comprising: an electrode assembly having a positive electrode, a negative electrode, and a separator, the positive electrode and the negative electrode being wound with the separator interposed therebetween; a bottomed cylindrical outer can containing the electrode assembly; and a sealing body that closes an opening of the outer can, wherein the positive electrode or the negative electrode is connected to the bottom of the outer can, wherein the positive electrode includes an elongated positive electrode core and a positive electrode mixture layer provided on the positive electrode core, and wherein, in the positive electrode mixture layer, a region including the center in the width direction of the positive electrode is defined as a central region, a region including one end in the width direction that is located closer to the sealing body than the central region is defined as a first region, and a region including the other end in the width direction that is located closer to the bottom of the outer can than the central region is defined as a second region, the cylindrical battery has a density that is the same as or greater than the density of the central region, and a density of the second region that is smaller than the density of the central region. Configuration 2: The cylindrical battery according to Configuration 1 or 2, wherein the negative electrode includes a long negative electrode core and a negative electrode mixture layer provided on the negative electrode core, and a strip-shaped negative electrode core exposed portion in which the surface of the negative electrode core is exposed is formed along the length direction of the negative electrode at a widthwise end portion of the negative electrode located on the bottom side of the outer can, and the negative electrode core exposed portion is connected to the bottom of the outer can directly or via a negative electrode current collector plate.Configuration 3: The cylindrical battery according to any one of Configurations 1 to 3, wherein the density of the second region is 90% or less of the density of the central region.Configuration 4: The cylindrical battery according to any one of Configurations 1 to 4, wherein the density of the first region is 110% or more of the density of the central region. Configuration 5: The cylindrical battery according to any one of Configurations 1 to 5, wherein the positive electrode has a plurality of positive electrode core exposed portions in which the surface of the positive electrode core is exposed in a part of the first region, and the positive electrode core exposed portions have a positive electrode lead connecting the sealing body and the positive electrode core exposed portions.Configuration 6: The cylindrical battery according to any one of Configurations 1 to 5, wherein the width of the first region is 5% to 30% of the overall width of the positive electrode.Configuration 7: The cylindrical battery according to any one of Configurations 1 to 6, wherein the width of the second region is 5% to 30% of the overall width of the positive electrode.Configuration 8: The cylindrical battery of any one of Configurations 1 to 7, wherein the densities of the central region, the first region, and the second region are lower in a region of 1 / 3 of the total length of the positive electrode located on the inner side of the electrode assembly than in a region of 2 / 3 of the total length of the positive electrode located on the outer side of the electrode assembly.

[0069] REFERENCE SIGNS LIST 10 Cylindrical battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Outer can, 16 Sealing body, 17 Lower current collector plate, 18 Insulating plate, 20 Positive electrode lead, 21 Grooved portion, 22 Internal terminal plate, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cap, 26a Through hole, 27 Gasket, 30 Positive electrode core, 31 Positive electrode mixture layer, 32 Positive electrode core exposed portion, 40 Negative electrode core, 41 Negative electrode mixture layer, 42 Negative electrode core exposed portion, 311 First region, 312 Central region, 313 Second region, 314 Inner winding region, 315 Outer winding region

Claims

1. A cylindrical battery comprising: an electrode assembly having a positive electrode, a negative electrode, and a separator, the electrode assembly being formed by winding the positive electrode and the negative electrode with the separator interposed therebetween; a bottomed cylindrical outer can containing the electrode assembly; and a sealing body that closes the opening of the outer can, wherein the positive electrode or the negative electrode is connected to the bottom of the outer can, wherein the positive electrode includes a long positive electrode core and a positive electrode mixture layer provided on the positive electrode core, and wherein, in the positive electrode mixture layer, a region including the center of the positive electrode in the width direction is defined as a central region, a region including one end in the width direction that is located closer to the sealing body than the central region is defined as a first region, and a region including the other end in the width direction that is closer to the bottom of the outer can than the central region is defined as a second region, wherein the density of the first region is the same as or greater than the density of the central region, and the density of the second region is smaller than the density of the central region.

2. The cylindrical battery according to claim 1, wherein the negative electrode includes an elongated negative electrode core and a negative electrode mixture layer provided on the negative electrode core, and a strip-shaped negative electrode core exposed portion in which the surface of the negative electrode core is exposed is formed along the length of the negative electrode at a widthwise end of the negative electrode located on the bottom side of the outer can, and the negative electrode core exposed portion is connected to the bottom of the outer can directly or via a negative electrode current collector plate.

3. The cylindrical battery according to claim 1, wherein the density of said second region is 90% or less of the density of said central region.

4. The cylindrical battery according to claim 1, wherein the density of said first region is 110% or more of the density of said central region.

5. The cylindrical battery according to claim 1, wherein the positive electrode has a plurality of positive electrode core exposed portions in which the surface of the positive electrode core is exposed in part of the first region, and the positive electrode core exposed portions have a positive electrode lead connecting the sealing body and the positive electrode core exposed portions.

6. The cylindrical battery according to claim 1, wherein the width of the first region is 5% to 30% of the overall width of the positive electrode.

7. The cylindrical battery according to claim 1, wherein the width of the second region is 5% to 30% of the overall width of the positive electrode.

8. A cylindrical battery according to any one of claims 1 to 7, wherein the densities of the central region, the first region, and the second region are lower in a region of one-third of the total length of the positive electrode located on the inner side of the electrode assembly than in a region of two-thirds of the total length of the positive electrode located on the outer side of the electrode assembly.

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