Non-aqueous electrolyte secondary battery

Abstract

Provided is a non-aqueous electrolyte secondary battery having a reduced risk of a short circuit. A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure comprises: an electrode body in which a first electrode and a second electrode, which are in the form of strips and have mutually different polarities, are wound along the long-axis direction with a separator interposed therebetween; and an exterior body that accommodates the electrode body. The first electrode has a current collector and a mixture layer formed on both surfaces of the current collector, and a current collector exposed part in which the current collector is exposed is formed on the outward-side surface of the first electrode. The mixture layer has a normal part and a thin part that is thinner than the normal part and has a high density. The thin part is close to both ends of the current collector exposed part in the long-axis direction of the first electrode. A protective tape having an adhesive layer is affixed to the first electrode so as to cover the current collector exposed part. The end of the adhesive layer of the protective tape in the long-axis direction of the first electrode is positioned on the thin part.

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] Conventionally, non-aqueous electrolyte secondary batteries have been widely used, in which a wound electrode assembly, in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound longitudinally with a separator interposed therebetween, is housed in an outer casing. Patent Document 1 discloses a technique in which exposed portions where the current collectors are exposed are formed on each of the positive electrode and the negative electrode, leads are welded to these exposed portions, and the leads are then covered with protective tape.

[0003] Japanese Patent Application Laid-Open No. 2004-311282

[0004] The inventors' investigations revealed that, because tensile stress is applied to the mixture layer formed on the outer surface of the current collector, there is a possibility that the mixture layer may peel off from the current collector, starting from the end of the adhesive layer of the protective tape in the longitudinal direction of the electrode, resulting in a short circuit. The technology disclosed in Patent Document 1 does not consider the risk of short circuits, and there is still room for improvement.

[0005] An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery with a reduced risk of short circuit.

[0006] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure includes an electrode assembly in which strip-shaped first and second electrodes having opposite polarities are wound longitudinally with a separator interposed therebetween, and an exterior housing that houses the electrode assembly, wherein the first electrode has a current collector and a mixture layer formed on both sides of the current collector, a current collector exposed portion where the current collector is exposed is formed on the outer surface of the wound first electrode, the mixture layer has a normal portion and a thin portion that is thinner and denser than the normal portion, the thin portion is adjacent to both ends of the current collector exposed portion in the longitudinal direction of the first electrode, a protective tape having an adhesive layer is attached to the first electrode so as to cover the current collector exposed portion, and ends of the adhesive layer of the protective tape in the longitudinal direction of the first electrode are located on the thin portion.

[0007] The nonaqueous electrolyte secondary battery according to the present disclosure reduces the risk of short circuits and is therefore excellent in safety.

[0008] Fig. 3 is an axial cross-sectional view of a cylindrical secondary battery according to an example embodiment. Fig. 4 is a perspective view of a wound electrode body included in the secondary battery shown in Fig. 1. Fig. 5 is a plan view showing a positive electrode according to an example embodiment in a developed state. Fig. 6 is a cross-sectional view of a protective tape according to an example embodiment. Fig. 7 is a cross-sectional view taken along line A-A in Fig. 3.

[0009] 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.

[0010] 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 strip-shaped positive electrode 11 and a negative electrode 12 are wound longitudinally 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 ３etc., 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".

[0011] 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 tab 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 tab 20 extends through a through hole in the insulating plate 18 toward the bottom 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 tab 20 is located near the end of the winding, the negative electrode tab 20 passes outside the insulating plate 18, extends toward the bottom of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0012] 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.

[0013] 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.

[0014] 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 side of the winding, and the opposite side is referred to as the outer side of the winding. In the winding direction γ, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is referred to as the winding direction γ, and the lateral direction of the positive electrode 11 and the negative electrode 12 is referred to as 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. The positive electrode tab 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 assembly 14. In this case, a positive electrode current collector exposed portion (described later) is provided in the longitudinal middle portion of the positive electrode 11, and a positive electrode tab 19 is connected to this positive electrode current collector exposed portion. In addition, the negative electrode tab 20 extends in the axial direction α from the vicinity of the winding axis at the lower end of the electrode body 14. In this embodiment, a case is described in which the first electrode is the positive electrode 11 and the second electrode is the negative electrode 12, but the first electrode may be the negative electrode 12 and the second electrode may be the positive electrode 11. In addition, both the positive electrode 11 and the negative electrode 12 may have the configuration of the first electrode.

[0015] The negative electrode 12 includes a strip-shaped negative electrode current collector, a negative electrode mixture layer formed on both sides of the negative electrode current collector, and a negative electrode current collector exposed portion where the negative electrode current collector is exposed. The thickness of the negative electrode current collector is, for example, 5 μm to 30 μm. The thickness of the negative electrode mixture layer is, for example, 10 μm to 150 μm on one side of the negative electrode current collector. The negative electrode current collector can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode mixture layer includes, for example, a negative electrode active material, a binder, and the like. The negative electrode 12 is fabricated, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and a solvent such as water to the surface of the negative electrode current collector, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0016] In this embodiment, the negative electrode tab 20 is joined to the surface of the negative electrode current collector on the inner side of the winding, for example, by ultrasonic welding. One end of the negative electrode tab 20 is located in the negative electrode current collector exposed portion, and the other end extends downward from the lower end of the negative electrode current collector exposed portion. The position of the negative electrode tab 20 is not limited to the winding start end as shown in FIG. 2 , but may be anywhere in the longitudinal direction γ of the negative electrode 12. The negative electrode current collector exposed portion is provided, for example, by intermittent application in which the negative electrode mixture slurry is not applied to a part of the negative electrode current collector.

[0017] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly absorb and release lithium ions, and generally, a carbon material such as graphite is used. The graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads.

[0018] As the negative electrode active material, metals that can be alloyed with Li, such as Si and Sn, metal compounds containing Si, Sn, etc., and lithium-titanium composite oxides may be used. For example, SiO ｘ Si-containing compounds represented by (0.5≦x≦1.6), Li ２ｙ SiO （２＋ｙ）A Si-containing compound in which Si fine particles are dispersed in a lithium silicate phase represented by (0<y<2), or a Si-containing compound in which Si is dispersed in a carbon material, may be used in combination with graphite. When the negative electrode mixture layer contains a Si-containing compound, the battery capacity can be increased, but the rate at which the electrode body 14 expands and contracts during charging increases. Therefore, when the negative electrode mixture layer contains a Si-containing compound, the positive electrode mixture layer is more likely to peel off at the end of the adhesive layer of the protective tape on the outside of the winding, and the effect of the thin portion of the positive electrode mixture layer described below becomes more pronounced.

[0019] Examples of binders contained in the negative electrode mixture layer include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0020] 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.

[0021] Next, the positive electrode 11 will be described in detail with reference to Figures 3 to 5. Figure 3 is a plan view showing the positive electrode 11 according to an example embodiment in a developed state, illustrating the outer surface of the wound positive electrode 11. The positive electrode 11 has a strip-shaped positive electrode current collector 30 and positive electrode mixture layers 32 formed on both sides of the positive electrode current collector 30.

[0022] For example, a foil of a metal such as aluminum, or a film having such a metal disposed on the surface thereof, is used for the positive electrode current collector 30. 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.

[0023] 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 the positive electrode active material, the conductive agent, the binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) to both sides of the positive electrode current collector 30, followed by drying and rolling.

[0024] 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.

[0025] 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 may be used alone or in combination of two or more.

[0026] A positive electrode current collector exposed portion 34, where the positive electrode current collector 30 is exposed, is formed on the outer surface of the wound positive electrode 11. In the example shown in Fig. 3 , a positive electrode current collector exposed portion 34 is formed on the inner surface of the wound positive electrode 11 at a position overlapping the positive electrode current collector exposed portion 34 formed on the outer surface of the wound positive electrode 11 in the thickness direction of the positive electrode 11, and a positive electrode tab 19 is connected to 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, and is provided, for example, by intermittent application of the positive electrode mixture slurry to a part of the positive electrode current collector 30.

[0027] In the example shown in Figure 3, the positive electrode current collector exposed portion 34 contacts only one end 11a of both ends in the short side direction of the positive electrode 11, and does not extend to the other end 11b in the short side direction of the positive electrode 11. The form of the positive electrode current collector exposed portion 34 is not limited to the example shown in Figure 3. The positive electrode current collector exposed portion 34 may extend from one end 11a to the other end 11b in the short side direction of the positive electrode 11 and contact both ends in the short side direction of the positive electrode 11. When the positive electrode current collector exposed portion 34 is in the form of the example shown in Figure 3, tensile stress is likely to be applied to the end of the adhesive layer of the protective tape 40 in the longitudinal direction of the positive electrode 11, and the effect of the present disclosure of suppressing peeling of the mixture layer becomes more pronounced.

[0028] In the longitudinal direction of the positive electrode 11, the positive electrode mixture layer 32 is adjacent to both ends of the positive electrode current collector exposed portion 34. In the example shown in Fig. 3, in order to improve current collection performance, the positive electrode current collector exposed portion 34 is formed in the middle portion in the longitudinal direction of the positive electrode 11. Note that the position of the positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11 is not particularly limited.

[0029] A protective tape 40 is attached to the outer surface of the wound positive electrode 11 so as to cover the positive electrode current collector exposed portion 34. In other words, the protective tape 40 is attached to the outer surface of the wound positive electrode 11 so as to cover the entire positive electrode current collector exposed portion 34 and straddle the positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11. This prevents contact between the positive electrode current collector exposed portion 34 and the negative electrode 12 even if the separator 13 is damaged. The length of the protective tape 40 in the longitudinal direction of the positive electrode 11 is, for example, 10 mm or more and 30 mm or less.

[0030] 4 , the protective tape 40 has a base material layer 42 and an adhesive layer 44 formed on the surface of the base material layer 42. The protective tape 40 is adhered to the surface of the positive electrode mixture layer 32 by the adhesive layer 44. In the example shown in FIG. 4 , the end of the base material layer 42 and the end of the adhesive layer 44 overlap in the thickness direction of the protective tape 40, but this is not limited to this example, and the end of the adhesive layer 44 may be located either inside or outside the protective tape 40 relative to the end of the base material layer 42.

[0031] The base layer 42 may be made of any insulating resin, such as polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyimide (PI), polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). Among these, PI, which has a relatively high hardness, is preferred for the purpose of protecting the exposed portion of the positive electrode current collector. The thickness of the base layer 42 is, for example, 5 μm to 50 μm, and may be 10 μm to 25 μm.

[0032] The adhesive layer 44 is a portion for adhering the protective tape 40 to the positive electrode 11. The adhesive layer 44 is formed, for example, over the entire surface of one of the substrate layers 42. The thickness of the adhesive layer 44 is, for example, 1 μm or more and 30 μm or less, and may be 1 μm or more and 10 μm or less. The adhesive layer 44 may contain at least one of a rubber-based polymer and an acrylic-based polymer. The rubber-based polymer and the acrylic-based polymer have adhesive properties, and therefore can adhere the protective tape 40 to the surface of the positive electrode 11. The adhesive layer 44 may further contain, for example, a silicone-based polymer. In the protective tape 40, a heat-resistant layer containing inorganic particles such as metal oxide may be provided between the substrate layer 42 and the adhesive layer 44.

[0033] The positive electrode mixture layer 32 has a normal portion 32a and a thin portion 32b that is thinner and denser than the normal portion 32a. The thin portion 32b is located close to both ends of the positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11. The thin portion 32b is also formed in a region from the end of the positive electrode current collector exposed portion 34 to the other end 11b of the positive electrode 11 in the lateral direction of the positive electrode 11. In this specification, the normal portion 32a refers to a portion of the positive electrode mixture layer 32 other than the thin portion 32b provided in the vicinity of the positive electrode current collector exposed portion 34. The thin portion 32b can be obtained, for example, by rolling a coating of positive electrode mixture slurry applied to the surface of the positive electrode current collector 30 with a stronger pressure than the other portions. The density of the normal portion 32a is, for example, 3.5 g / cm ３ 3.7g / cm or more ３ The density of the thin-walled portion 32b is, for example, 3.7 g / cm ３ 3.9g / cm or more ３ The density of the normal portion 32 a is measured, for example, at a position adjacent to the thin portion 32 b. The density of the thin portion 32 b is measured, for example, near the end of the adhesive layer 44 of the protective tape 40.

[0034] A protective tape 40 is attached so as to cover the exposed positive electrode current collector portion 34, and the end of the adhesive layer 44 of the protective tape 40 in the longitudinal direction of the positive electrode 11 is located on the thin portion 32b. This makes it possible to suppress peeling of the positive electrode mixture layer 32, thereby reducing the risk of short circuiting and improving the safety of the battery.

[0035] Next, the cross-sectional structure of the positive electrode 11 will be described with reference to FIG. 5 . FIG. 5 is a cross-sectional view taken along line A-A in FIG. 3 . In the example shown in FIG. 5 , the positive electrode mixture layer 32 is formed on both sides of the positive electrode current collector 30, and the positive electrode current collector exposed portion 34, the normal portion 32a, and the thin portion 32b are provided at opposing positions on both sides of the positive electrode current collector 30. The positive electrode mixture layers 32 provided on the inner and outer sides of the positive electrode current collector 30 may have the same shape. Note that the configuration of the positive electrode 11 is not limited to the example shown in FIG. 5 . On the inner side of the winding, the thin portion 32b may not be formed, and the positive electrode mixture layer 32 may be composed of only the normal portion 32a.

[0036] The thickness T1 of the normal portion 32a and the thickness T2 of the thin portion 32b satisfy the relationship T2 / T1≦0.97, for example. The lower limit of T2 / T1 is, for example, 0.9 from the viewpoint of manufacturing the positive electrode 11. The thickness of the normal portion 32a and the thickness of the thin portion 32b are both, for example, 50 μm or more and 150 μm or less.

[0037] The thickness T1 of the normal portion 32a is approximately constant, and the surface of the positive electrode 11 in the portion corresponding to the normal portion 32a is approximately parallel to the surface of the positive electrode current collector 30. The thickness T1 of the normal portion 32a is measured, for example, at a position adjacent to the thin portion 32b. The thin portion 32b may have an inclined portion whose thickness increases toward the normal portion 32a near the portion where it contacts the normal portion 32a. The thickness of the thin portion 32b is approximately constant except for the inclined portion, and the surface of the positive electrode 11 in the portion corresponding to the thin portion 32b is approximately parallel to the surface of the positive electrode current collector 30. The thickness T2 of the thin portion 32b is measured, for example, near the end of the adhesive layer 44 of the protective tape 40.

[0038] In the example shown in Fig. 5 , the normal portion 32a and the thin portion 32b formed on the surface of the inside of the winding of the positive electrode current collector 30 have approximately the same thickness as the normal portion 32a and the thin portion 32b formed on the surface of the outside of the winding of the positive electrode current collector 30. Note that the form of the positive electrode 11 is not limited to the example shown in Fig. 5 , and the ratio of the normal portion 32a to the thin portion 32b on the inside of the winding does not have to satisfy the above relationship.

[0039] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0040] Example 1 [Preparation of Positive Electrode] Aluminum-containing lithium nickel cobalt oxide was used as the positive electrode active material. This positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 100:2:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was intermittently applied to both sides of a positive electrode current collector made of 15 μm thick aluminum foil, and a 9 mm wide gap was formed at a position overlapping the positive electrode current collector in the thickness direction, forming a positive electrode current collector exposed portion. After drying, the positive electrode current collector was cut to a size of 59.0 mm wide and 840 mm long, with a positive electrode current collector exposed portion measuring 9 mm in length and 20 mm in width at the longitudinal middle of the positive electrode current collector, and the coating was rolled. When rolling the coating, a stronger pressure was applied to a region extending 5.5 mm in the longitudinal direction of the positive electrode current collector from the end of the positive electrode mixture layer that contacted the exposed portion of the positive electrode current collector and the positive electrode 11 in the longitudinal direction than to other regions, forming a thin-walled portion in this region. A protective tape with a PI substrate layer was then applied to cover the exposed portion of the positive electrode current collector on the outer side of the winding, with the end of the adhesive layer of the protective tape in the longitudinal direction of the positive electrode positioned on the thin-walled portion. An aluminum positive electrode tab measuring 68 mm in length, 5 mm in width, and 100 μm in thickness was welded to the exposed portion of the positive electrode current collector on the inner side of the winding, and the same protective tape as the outer side of the winding was applied so as to overlap the outer side of the winding. In this manner, the positive electrodes shown in FIGS. 3 to 5 were produced. The thickness T1 of the normal portion was 77.5 μm, the thickness T2 of the thin-walled portion was 75 μm, and the T2 / T1 ratio was 0.97.

[0041] [Preparation of Negative Electrode] A mixture of graphite and SiO in a mass ratio of 94:6 was used as the negative electrode active material. This negative electrode active material, styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were mixed in a mass ratio of 100:1:1, and water was used as a dispersion medium to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was intermittently applied to both sides of a negative electrode current collector made of copper foil with a thickness of 8 μm, and the coating was dried and rolled. The thickness of the negative electrode mixture layer was 116 μm. Then, the negative electrode current collector was cut into a size of 59.3 mm wide and 943 mm long so that the negative electrode current collector exposed portions were present at both ends of the longitudinal direction of the negative electrode current collector. A Ni / Cu negative electrode tab was welded to the exposed portion of the negative electrode current collector at the start of winding to prepare a negative electrode. The exposed portion of the negative electrode current collector at the winding end contacts the exterior body, so that the negative electrode is connected to the exterior body at both ends in the longitudinal direction.

[0042] [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 3:7, and lithium hexafluorophosphate (LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.3 mol / L of ammonium hydroxide in water.

[0043] [Test Cell Fabrication] A wound electrode assembly was fabricated by spirally winding a positive electrode and a negative electrode with a separator interposed therebetween. Insulating plates were placed on the top and bottom of the electrode assembly, and the electrode assembly was housed in a cylindrical metal outer can with a bottom. The negative electrode tab was welded to the bottom of the outer can, and the positive electrode tab was welded to a sealing member. After pouring a nonaqueous electrolyte into the outer can, the opening of the outer can was sealed with a sealing member via a gasket to fabricate an 18650-type test cell.

[0044] Example 2 A test cell was produced in the same manner as in Example 1, except that in producing the positive electrode, the pressure applied to the portion corresponding to the thin-walled portion when rolling the coating film was higher than the pressure applied in Example 1. In the obtained positive electrode, the thickness T1 of the normal portion was 77.5 μm, the thickness T2 of the thin-walled portion was 73 μm, and T2 / T1 was 0.94.

[0045] Comparative Example A test cell was produced in the same manner as in Example 1, except that, in producing the positive electrode, the same pressure as that applied to the portion corresponding to the normal portion in Example 1 was applied to the entire surface of the coating film when rolling the coating film. The total thickness of the positive electrode mixture layer over the entire surface of the obtained positive electrode was 77.5 μm. In other words, the thickness T1 of the normal portion and the thickness T2 of the thin portion were both 77.5 μm, and T2 / T1 was 1.

[0046] [Evaluation of Positive Electrode Current Collector Fracture] The test cells of the examples and comparative examples were charged at a constant current of 3000 mA (1 C) at an ambient temperature of 25 ° C. until the battery voltage reached 4.2 V, then charged at a constant voltage of 4.2 V until the current value reached 100 mA (1 / 30 C) and allowed to stand for 30 minutes. Thereafter, the cells were discharged at a constant current of 15000 mA (5 C) until the battery voltage reached 2.5 V and allowed to stand for 60 minutes. Next, the interior of each test cell was observed using an X-ray CT scanner to evaluate the presence or absence of peeling of the positive electrode mixture layer near the end of the adhesive layer of the protective tape. Table 1 shows the evaluation results.

[0047]

[0048] As shown in Table 1, peeling of the positive electrode mixture layer was not observed in the test cells of the examples, but peeling of the positive electrode mixture layer was observed near the edge of the adhesive layer of the protective tape in the test cells of the comparative examples. This shows that peeling of the positive electrode mixture layer can be suppressed by providing a thin portion that is denser and thinner than a normal portion, near the exposed portion of the positive electrode current collector.

[0049] The present disclosure will be further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including an electrode assembly in which strip-shaped first and second electrodes having opposite polarities are wound longitudinally with a separator interposed therebetween, and an exterior housing that houses the electrode assembly, wherein the first electrode has a current collector and a mixture layer formed on both sides of the current collector, a current collector exposed portion where the current collector is exposed is formed on the outer surface of the wound first electrode, the mixture layer has a normal portion and a thin portion that is thinner and denser than the normal portion, the thin portion is adjacent to both ends of the current collector exposed portion in the longitudinal direction of the first electrode, a protective tape having an adhesive layer is attached to the first electrode so as to cover the current collector exposed portion, and ends of the adhesive layer in the longitudinal direction of the first electrode are located on the thin portion. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the thickness T1 of the normal portion and the thickness T2 of the thin portion satisfy the relationship T2 / T1≦0.97. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the current collector exposed portion contacts only one of both ends in the short direction of the first electrode. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the first electrode is a positive electrode. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the current collector exposed portion is formed in a middle portion in the long direction of the first electrode.

[0050] REFERENCE SIGNS LIST 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Exterior body, 16 Sealing body, 17, 18 Insulating plate, 19 Positive electrode tab, 20 Negative electrode tab, 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, 32a Normal portion, 32b Thin portion, 34 Positive electrode current collector exposed portion, 40 Protective tape, 42 Base material layer, 44 Adhesive layer

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode assembly in which strip-shaped first and second electrodes having opposite polarities are wound longitudinally with a separator interposed therebetween, and an exterior housing that houses the electrode assembly, wherein the first electrode has a current collector and a mixture layer formed on both sides of the current collector, and a current collector exposed portion where the current collector is exposed is formed on the outer surface of the wound first electrode, the mixture layer has a normal portion and a thin portion that is thinner and denser than the normal portion, and the thin portion is adjacent to both ends of the current collector exposed portion in the longitudinal direction of the first electrode, a protective tape having an adhesive layer is attached to the first electrode so as to cover the current collector exposed portion, and ends of the adhesive layer in the longitudinal direction of the first electrode are located on the thin portion.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thickness T1 of the normal portion and the thickness T2 of the thin portion satisfy the relationship T2 / T1≦0.

97.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the current collector exposed portion is in contact with only one of both ends in the short side direction of the first electrode.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first electrode is a positive electrode.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the current collector exposed portion is formed in a longitudinally intermediate portion of the first electrode.

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