Nonaqueous electrolyte secondary battery

Abstract

Provided is a nonaqueous electrolyte secondary battery in which deformation of a negative electrode is suppressed, while maintaining the battery capacity. This nonaqueous electrolyte secondary battery is provided with: a wound electrode body which is obtained by winding a band-like positive electrode and a band-like negative electrode, with a separator being interposed therebetween; and an outer package which houses the electrode body. The positive electrode has a positive electrode current collector and a positive electrode mixture layer that is formed on the surface of the positive electrode current collector. The positive electrode mixture layer has a first inclined region in which the thickness gradually decreases toward the winding start end, and a second inclined region in which the thickness gradually decreases toward the winding termination end. In the longitudinal direction of the positive electrode, the length L1 of the first inclined region and the length L2 of the second inclined region satisfy the relational expression L1 > L2.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery including a wound electrode assembly.

[0002]

[0003] Conventionally, non-aqueous electrolyte secondary batteries have been widely used, in which a wound electrode assembly, in which strip-shaped positive and negative electrodes are wound, is housed in an outer casing. The positive and negative electrodes each have a mixture layer containing an active material on the surface of a metal current collector. Generally, the mixture layer is formed by applying a liquid mixture slurry to the surface of the current collector and drying it. However, depending on the viscosity of the mixture slurry, the mixture layer may become locally thick around the end point of application of the mixture slurry.

[0003] Patent Document 1 discloses a technique for adjusting the amount of coating at the start and end of the coating area where the composite slurry is applied, thereby providing a sloped shape so that the thickness of the composite layer gradually decreases, in order to prevent damage to the electrode due to a local increase in thickness.

[0004] JP 2010-108678 A

[0005] In a secondary battery in which a wound electrode assembly is housed in an outer casing, pressure acts on the electrode assembly from the outer casing when the electrode assembly expands with charge / discharge cycles. This can cause internal short circuits in the electrode assembly due to bending or other deformation of the negative electrode facing the inner end of the positive electrode. The technology disclosed in Patent Document 1 does not take into account deformation of the negative electrode when the electrode assembly expands, and there is still room for improvement in optimizing the slope shape. Patent Document 1 also discloses a technology in which the starting and ending ends have the same slope shape, but does not consider the possibility that providing a slope shape reduces the volume of the mixture layer and decreases battery capacity.

[0006] An object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that suppresses deformation of the negative electrode while maintaining battery capacity.

[0007] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure includes a wound electrode assembly in which strip-shaped positive and negative electrodes are wound with a separator interposed therebetween, and an exterior housing that houses the electrode assembly, wherein the positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector, and the positive electrode mixture layer has a first inclined region that gradually becomes thinner toward the winding start end and a second inclined region that gradually becomes thinner toward the winding end, and wherein a length L1 of the first inclined region and a length L2 of the second inclined region satisfy the relationship L1 > L2 in the longitudinal direction of the positive electrode.

[0008] According to the nonaqueous electrolyte secondary battery according to the present disclosure, deformation of the negative electrode can be suppressed while maintaining the battery capacity.

[0009] Fig. 2 is an axial cross-sectional view of a cylindrical secondary battery according to an example of an embodiment. Fig. 3 is a perspective view of a wound electrode body provided in the secondary battery shown in Fig. 1. Fig. 4 is a cross-sectional view showing the positional relationship between a positive electrode and a negative electrode constituting an electrode body according to an example of an embodiment in a developed state.

[0010] Hereinafter, an example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the cylindrical secondary battery. Furthermore, when the following description includes multiple embodiments and modified examples, it is assumed from the beginning that the characteristic portions of those embodiments and modified examples can be appropriately combined and used.

[0011] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. The secondary battery 10 shown in FIG. 1 includes an electrode assembly 14 and a nonaqueous electrolyte (not shown) housed in an outer casing 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. Examples of nonaqueous solvents (organic solvents) for the nonaqueous electrolyte include carbonates, lactones, ethers, ketones, esters, and the like. Two or more of these solvents can be mixed together. When two or more solvents are mixed together, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferably used. For example, ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC) can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) can be used as the chain carbonate. The electrolyte salt of the non-aqueous electrolyte is LiPF ６ , LiBF ４ , LiCF ３ SO ３ etc., and mixtures thereof can be used. The amount of electrolyte salt dissolved in the non-aqueous solvent can be, for example, 0.5 mol / L or more and 2.0 mol / L or less. For convenience of explanation, the following description will be given with the sealing body 16 side as the "upper" and the bottom side of the exterior body 15 as the "lower".

[0012] The opening of the exterior body 15 is closed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of the filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 to the bottom side of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that when the negative electrode lead 20 is located near the end of the winding, the negative electrode lead 20 passes outside the insulating plate 18, extends to the bottom side of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0013] The exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between the exterior body 15 and the sealing body 16 to ensure the airtightness of the interior of the secondary battery 10. The exterior body 15 has a grooved portion 21 that supports the sealing body 16, formed, for example, by pressing the side surface from the outside. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the exterior body 15, and supports the sealing body 16 on its upper surface.

[0014] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may rupture, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may rupture, allowing gas to be released from the opening 26a of the cap 26.

[0015] Next, the electrode assembly 14 will be described with reference to FIG. 2 . FIG. 2 is a perspective view of the electrode assembly 14. 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 formed in strip shapes and are spirally wound around a winding axis, resulting in an alternating stacked state in the radial direction β of the electrode assembly 14. In the radial direction β, the winding axis side is referred to as the inner winding side, and the opposite side is referred to as the outer winding side. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction γ, and the width direction of the positive electrode 11 and the negative electrode 12 is the axial direction α. ​​In the winding direction γ, the winding axis side is referred to as the winding start side, and the opposite side is referred to as the winding end side. Furthermore, for the positive electrode 11 and the negative electrode 12, and the first inclined region and the second inclined region described below, the ends on the winding start side are referred to as the winding start end, and the ends on the winding end side are referred to as the winding end end. The positive electrode lead 19 extends in the axial direction α from approximately the center in the radial direction from the center to the outermost periphery at the upper end of the electrode body 14. The negative electrode lead 20 extends in the axial direction α from near the winding axis at the lower end of the electrode body 14.

[0016] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0017] Next, the positive electrode 11 and the negative electrode 12 that constitute the electrode assembly 14 will be described in detail with reference to Fig. 3. Fig. 3 is a cross-sectional view showing the positional relationship between the positive electrode 11 and the negative electrode 12 that constitute the electrode assembly 14 according to one example of the embodiment in a developed state, and shows a cross section along the longitudinal direction of the positive electrode 11 and the negative electrode 12.

[0018] The positive electrode 11 has a strip-shaped positive electrode current collector 30 and a positive electrode mixture layer 32 formed on the surface of the positive electrode current collector 30. In this embodiment, the positive electrode mixture layer 32 is formed on both sides of the positive electrode current collector 30. The positive electrode current collector 30 may be, for example, a foil of a metal such as aluminum, or a film having such a metal disposed on its surface. A suitable positive electrode current collector 30 is a foil of a metal whose main component is aluminum or an aluminum alloy. The thickness of the positive electrode current collector 30 is, for example, 10 μm or more and 30 μm or less.

[0019] The positive electrode mixture layer 32 is formed on the entire surface of the positive electrode current collector 30, except for the positive electrode current collector exposed portion 34. The positive electrode current collector exposed portion 34 is a portion of the surface of the positive electrode current collector 30 that is not covered with the positive electrode mixture layer 32. The positive electrode mixture layer 32 preferably contains a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 is produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) to both surfaces of the positive electrode current collector 30, followed by drying and rolling. The positive electrode current collector exposed portion 34 is provided, for example, by intermittent application in which the positive electrode mixture slurry is not applied to a portion of the positive electrode current collector 30.

[0020] As the positive electrode active material, a lithium-containing transition metal oxide containing a transition metal element such as Co, Mn, or Ni can be used. The lithium-containing transition metal oxide is not particularly limited, but may be any of the following oxides represented by the general formula: Li １＋ｘ MO ２ (wherein, −0.2<x≦0.2, and M contains at least one of Ni, Co, Mn, and Al) is preferred.

[0021] Examples of the conductive agent contained in the positive electrode mixture layer 32 include carbon black (CB) such as acetylene black (AB) and Ketjen black, and carbon materials such as graphite. Examples of the binder contained in the positive electrode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. These materials may be used alone or in combination of two or more.

[0022] The positive electrode mixture layer 32 has a first inclined region 36 that gradually decreases in thickness toward the winding start end 11s and a second inclined region 38 that gradually decreases in thickness toward the winding end 11e. This reduces the pressure exerted on the negative electrode 12 by the vicinity of the winding start end 11s and the winding end 11e of the positive electrode 11 when the electrode assembly expands, thereby suppressing deformation of the negative electrode 12. Furthermore, in the longitudinal direction of the positive electrode 11, the length L1 of the first inclined region 36 and the length L2 of the second inclined region 38 satisfy the relationship L1 > L2. The pressure exerted on the negative electrode 12 near the winding end 11e is smaller than that near the winding start end 11s, making it less likely for the negative electrode to deform. Therefore, by making L2 shorter than L1, it is possible to suppress deformation of the negative electrode 12 while suppressing a decrease in battery capacity due to the inclined regions.

[0023] L1 preferably satisfies 1 mm≦L1≦20 mm, more preferably 1 mm≦L1≦10 mm, and further preferably 2 mm≦L1≦8 mm. The lower limit of L2 is, for example, 0.5 mm.

[0024] In a cross section taken along the longitudinal direction of the positive electrode 11, for example, the inclination angle D1 at the tip 36a of the first inclined region 36 and the inclination angle D2 at the tip 38a of the second inclined region 38 satisfy the relationship D1<D2. In other words, the inclination angles D1 and D2 are the angle D1 between a tangent C1 to the tip 36a of the first inclined region 36 and the positive electrode current collector 30, and the angle D2 between a tangent C2 to the tip 38a of the second inclined region 38 and the positive electrode current collector 30, respectively. Here, D1 refers to the angle between C1 and the positive electrode current collector 30 that is 90° or less, and the same applies to D2.

[0025] D1 satisfies, for example, 1°≦D1≦30°, and D2 satisfies, for example, 3°≦D2≦60°.

[0026] As described above, the positive electrode mixture layer 32 has a flat region in the longitudinal direction, with a substantially uniform thickness in the center, and sloped regions at both ends that are continuous from the flat region and gradually decrease in thickness toward the ends. In this embodiment, both the first sloped region 36 and the second sloped region 38 have a convex shape. In other words, for example, the first sloped region 36 has a shape that protrudes beyond the line segment connecting the tip 36a and the slope starting point 36b where the slope begins. The slope angle is greatest at the tip 36a, gradually decreases as the tip 36a approaches the slope starting point 36b, and is approximately 0° at the slope starting point 36b. For example, a positive electrode mixture slurry with a higher viscosity is more likely to have a convex shape. When the first sloped region 36 and the second sloped region 38 have such a shape, the pressure applied to the negative electrode 12 when the electrode assembly 14 expands can be reduced. The shapes of the first inclined region 36 and the second inclined region 38 are not limited to the example of this embodiment, and for example, the inclination angle may be substantially the same from the tip 36a to the inclination start point 36b.

[0027] In this embodiment, both the positive electrode mixture layer 32 on the inside side of the winding and the positive electrode mixture layer 32 on the outside side of the winding have the first inclined region 36 and the second inclined region 38. In this case, it is preferable that the first inclined region 36 and the second inclined region 38 have the same shape on the inside side of the winding and the outside side of the winding. Alternatively, only one of the positive electrode mixture layer 32 on the inside side of the winding and the positive electrode mixture layer 32 on the outside side of the winding may have the first inclined region 36 and the second inclined region 38, and the other may not have the first inclined region 36 and the second inclined region 38.

[0028] In this embodiment, the positive electrode 11 has a positive electrode current collector exposed portion 34 where the positive electrode current collector 30 is exposed at the winding start end 11s and the winding end 11e, and a protective tape 50 is attached to the positive electrode current collector exposed portion 34. This makes it possible to suppress deformation of the negative electrode 12 caused by contact between the winding start end 11s and the winding end 11e of the positive electrode 11 and the negative electrode 12. This is presumably because the thicknesses of the winding start end 11s and the winding end 11e of the positive electrode 11 are reduced and further elasticity is imparted to the winding start end 11s and the winding end 11e by the protective tape 50, thereby reducing the pressure that the winding start end 11s and the winding end 11e of the positive electrode 11 apply to the negative electrode 12.

[0029] The protective tape 50 includes, for example, a base layer and an adhesive layer formed on the surface of the base layer. The thickness of the protective tape is, for example, 20 μm or more and 70 μm or less, and preferably 25 μm or more and 60 μm or less. The base layer may include an organic material and an inorganic material. The main component of the organic material contained in the base layer is preferably a resin that has excellent insulating properties, electrolyte resistance, heat resistance, puncture strength, etc. Specifically, the main component of the base layer is preferably a resin such as polypropylene (PP). The thickness of the base layer is, for example, 10 μm or more and 45 μm or less, and preferably 15 μm or more and 35 μm or less. The adhesive layer is a layer that imparts adhesiveness to the positive electrode 11 to the protective tape. The adhesive layer is composed of, for example, an acrylic adhesive or a synthetic rubber adhesive. The thickness of the adhesive layer is, for example, 5 μm or more and 30 μm or less.

[0030] Preferably, the positive electrode lead 19 is not joined to the positive electrode current collector 30 at the positive electrode current collector exposed portion 34 formed at the winding start end of the positive electrode 11. In this embodiment, a positive electrode current collector exposed portion 34 is also provided at approximately the center of the positive electrode 11 in the longitudinal direction γ, and the positive electrode lead 19 is joined to the positive electrode current collector 30 at this positive electrode current collector exposed portion 34. This improves current collection performance. Furthermore, the positive electrode current collector exposed portion 34 to which the positive electrode lead is joined is preferably provided on both sides of the positive electrode 11 so as to overlap in the thickness direction of the positive electrode 11. The positive electrode current collector exposed portion 34 to which the positive electrode lead 19 is joined is preferably covered with a protective tape 50. In the width direction α, the width of the protective tape 50 may be wider than the width of the positive electrode 11. This allows the entire positive electrode 11 in the width direction α to be covered, thereby more reliably suppressing internal short circuits. The protective tape 50 used on the positive electrode current collector exposed portion 34 to which the positive electrode lead 19 is joined may be the same as or different from the protective tape 50 used on the positive electrode current collector exposed portion 34 provided at the winding start end 11s and the winding end 11e in terms of shape, size, material, etc.

[0031] The negative electrode 12 has a strip-shaped negative electrode current collector 40 and a negative electrode mixture layer 42 formed on the surface of the negative electrode current collector 40. In this embodiment, the negative electrode mixture layer 42 is formed on both sides of the negative electrode current collector 40. The negative electrode current collector 40 may be, for example, a foil of a metal such as copper, or a film having such a metal disposed on its surface. The thickness of the negative electrode current collector 40 is, for example, 5 μm or more and 30 μm or less.

[0032] The negative electrode mixture layer 42 is formed on the entire surface of each of the negative electrode current collectors 40, except for the negative electrode mixture layer 44. The negative electrode current collector exposed portion 44 is a portion of the surface of the negative electrode current collector 40 that is not covered with the negative electrode mixture layer 42. The negative electrode mixture layer 42 preferably contains a negative electrode active material and a binder. The negative electrode 12 is produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, water, and the like to both surfaces of the negative electrode current collector 40, followed by drying and rolling. The negative electrode current collector exposed portion 44 is provided, for example, by intermittent application in which the negative electrode mixture slurry is not applied to a portion of the negative electrode current collector 40.

[0033] The negative electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions. For example, carbon materials such as natural graphite and artificial graphite, metals that can be alloyed with lithium such as Si and Sn, or alloys or oxides containing these, can be used.

[0034] The binder contained in the negative electrode mixture layer 42 may be, for example, the same resin as that used in the positive electrode 11, but when preparing the negative electrode mixture slurry in an aqueous solvent, it is preferable to use styrene-butadiene rubber (SBR), CMC or a salt thereof, polyacrylic acid or a salt thereof, polyvinyl alcohol, etc. These may be used alone or in combination of two or more.

[0035] As shown in Fig. 3 , the length in the longitudinal direction γ of the negative electrode 12 is greater than the length in the longitudinal direction γ of the positive electrode 11, the winding start end of the negative electrode 12 is located closer to the winding start side than the winding start end 11s of the positive electrode 11, and the winding end end of the negative electrode 12 is located closer to the winding end side than the winding end end of the positive electrode 11. Although not shown in Fig. 3 , the length in the width direction (axial direction) α of the negative electrode 12 is greater than the length in the width direction α of the positive electrode 11. This positions at least the portion of the positive electrode 11 where the positive electrode mixture layer 32 is formed opposite the portion of the negative electrode 12 where the negative electrode mixture layer 42 is formed, with the separator 13 interposed between them, thereby preventing lithium deposition on the negative electrode 12.

[0036] The negative electrode 12 has a negative electrode current collector exposed portion 44 near the winding start end. The negative electrode current collector exposed portion 44 is provided, for example, on both sides of the negative electrode 12 so as to overlap in the thickness direction of the negative electrode 12. The negative electrode lead 20 is joined to the negative electrode current collector 40 at the negative electrode current collector exposed portion 44 on the outside of the winding. The arrangement position of the negative electrode lead 20 is not limited to the example of this embodiment, and the negative electrode lead 20 may be provided only near the winding end of the negative electrode 12. Alternatively, the negative electrode lead 20 may be provided near both the winding start end and the winding end of the negative electrode 12. In this case, current collection performance is improved. The negative electrode current collector exposed portion 44 may be formed at the winding end of the negative electrode 12 and this negative electrode current collector exposed portion 44 may be brought into contact with the inner circumferential surface of the exterior body 15 (see FIG. 1 ), thereby electrically connecting the winding end of the negative electrode 12 to the exterior body 15 without using the negative electrode lead 20.

[0037] An insulating protective tape 50 is attached to the negative electrode current collector exposed portion 44 on the outer side of the wound negative electrode 12 so as to straddle the negative electrode lead 20. The protective tape 50 suppresses internal short circuits in the negative electrode lead 20. In the width direction α, the width of the protective tape 50 may be wider than the width of the negative electrode 12. This allows the entire negative electrode 12 in the width direction α to be covered, thereby more reliably suppressing internal short circuits. Note that the shape, size, material, etc. of the protective tape 50 used in the negative electrode current collector exposed portion 44 to which the negative electrode lead 20 is joined may be the same as or different from the shape, size, material, etc. of the protective tape 50 used in the positive electrode current collector exposed portion 34 provided on the positive electrode 11.

[0038] As described above, the nonaqueous electrolyte secondary battery 10 according to the present disclosure includes the positive electrode 11 having the first inclined region 36 and the second inclined region 38 with optimal shapes, and therefore can suppress deformation of the negative electrode 12 while maintaining the battery capacity.

[0039] The present disclosure is further described by the following embodiments. Configuration 1: A nonaqueous electrolyte secondary battery including a wound electrode assembly in which strip-shaped positive and negative electrodes are wound with a separator interposed therebetween, and an exterior housing that houses the electrode assembly, wherein the positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector, the positive electrode mixture layer has a first inclined region that gradually becomes thinner toward the winding start end and a second inclined region that gradually becomes thinner toward the winding end end, and a length L1 of the first inclined region and a length L2 of the second inclined region in the longitudinal direction of the positive electrode satisfy the relationship L1 > L2. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein L1 satisfies 1 mm ≦ L1 ≦ 20 mm. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein, in a cross section along the longitudinal direction of the positive electrode, an inclination angle D1 at a tip of the first inclined region and an inclination angle D2 at a tip of the second inclined region satisfy the relationship D1<D2. Configuration 4: The nonaqueous electrolyte secondary battery according to Configuration 3, wherein D1 satisfies 1°≦D1≦30°. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the positive electrode has a positive electrode current collector exposed portion at the winding start end and the winding end, where the positive electrode current collector is exposed, and a protective tape is attached to the positive electrode current collector exposed portion.

[0040] REFERENCE SIGNS LIST 10 secondary battery, 11 positive electrode, 11s winding start end, 11e winding end end, 12 negative electrode, 13 separator, 14 electrode body, 15 outer casing, 16 sealing body, 17, 18 insulating plate, 19 positive electrode lead, 20 negative electrode lead, 21 grooved portion, 22 filter, 23 lower valve body, 24 insulating member, 25 upper valve body, 26 cap, 26a opening, 27 gasket, 30 positive electrode current collector, 32 positive electrode mixture layer, 34 positive electrode current collector exposed portion, 36 first inclined region, 38 second inclined region, 40 negative electrode current collector, 42 negative electrode mixture layer, 44 negative electrode current collector exposed portion, 50 protective tape

Claims

1. A non-aqueous electrolyte secondary battery comprising a wound electrode assembly in which strip-shaped positive and negative electrodes are wound with a separator interposed therebetween, and an exterior housing that houses the electrode assembly, wherein the positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector, the positive electrode mixture layer has a first inclined region that is gradually thinner toward the start end of the winding, and a second inclined region that is gradually thinner toward the end end of the winding, and in the longitudinal direction of the positive electrode, a length L1 of the first inclined region and a length L2 of the second inclined region satisfy the relationship L1>L2.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein L1 satisfies 1 mm≦L1≦20 mm.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein in a cross section along the longitudinal direction of the positive electrode, an inclination angle D1 at an end of the first inclined region and an inclination angle D2 at an end of the second inclined region satisfy the relationship D1 < D2.

4. The nonaqueous electrolyte secondary battery according to claim 3, wherein D1 satisfies 1°≦D1≦30°.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode has a positive electrode current collector exposed portion at the winding start end and the winding end, where the positive electrode current collector is exposed, and a protective tape is attached to the positive electrode current collector exposed portion.

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