Nonaqueous electrolyte secondary battery

Abstract

The present invention provides a nonaqueous electrolyte secondary battery which is capable of achieving high output. A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure includes: a wound electrode body which is obtained by winding a belt-shaped positive electrode and a belt-shaped 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. A positive electrode current collector exposed part, in which the positive electrode current collector is exposed, is formed in the surface of the positive electrode. A positive electrode tab is connected to the positive electrode current collector exposed part. The positive electrode mixture layer has a normal part and thick parts that are thicker than the normal part. The thick parts are close to both ends of the positive electrode current collector exposed part in the longitudinal direction of the positive electrode.

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 strip-shaped positive and negative electrodes are stacked and wound, is housed in an outer casing. Patent Document 1 discloses a secondary battery including a wound electrode assembly, and discloses a technology for improving current collection by providing a positive electrode current collector exposed portion, in which the positive electrode current collector is exposed, at a position approximately in the center of the positive electrode in the longitudinal direction, and connecting a positive electrode tab to this positive electrode current collector exposed portion.

[0003] Japanese Patent Application Laid-Open No. 2008-234855

[0004] In a secondary battery in which a wound electrode assembly is housed in an outer casing, the expansion of the negative electrode during charging can exert a large pressure on the positive electrode tab. In particular, after repeated charge-discharge cycles, the electrode assembly expands, and the pressure on the positive electrode tab becomes even greater.

[0005] In recent years, non-aqueous electrolyte secondary batteries have been required to have increasingly higher output. While reducing the current density can be achieved by thinning the positive electrode mixture layer or thickening the positive electrode tab, both of these approaches increase the pressure on the positive electrode tab, raising concerns about damage to the positive electrode current collector around the positive electrode tab. The technology disclosed in Patent Document 1 does not take into account the pressure on the positive electrode tab, and there is still room for improvement in terms of meeting the demand for higher output.

[0006] An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that can achieve high output.

[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. 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. A positive electrode current collector exposed portion in which the positive electrode current collector is exposed is formed on the surface of the positive electrode. A positive electrode tab is connected to the positive electrode current collector exposed portion. The positive electrode mixture layer has a normal portion and a thick portion that is thicker than the normal portion, and the thick portions are adjacent to both ends of the positive electrode current collector exposed portion in the longitudinal direction of the positive electrode.

[0008] According to the nonaqueous electrolyte secondary battery according to the present disclosure, it is possible to improve output.

[0009] Fig. 3 is an axial cross-sectional view of a cylindrical secondary battery according to an example of an embodiment. Fig. 4 is a perspective view of a wound electrode body provided in the secondary battery shown in Fig. 1. Fig. 5 is a plan view showing the vicinity of a positive electrode tab of a positive electrode according to an example of an embodiment in a developed state. Fig. 6 is a cross-sectional view taken along line A-A in Fig. 3. Fig. 7 is a radial cross-sectional view of an electrode body according to an example of an embodiment, showing an enlarged view of the vicinity of the positive electrode tab.

[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 strip-shaped positive electrode 11 and a strip-shaped 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 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.

[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 circumferential side, and the opposite side is referred to as the outer circumferential side. In the electrode assembly 14, 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 at approximately the center in the longitudinal direction of the positive electrode 11, and a positive electrode tab 19 is connected to this positive electrode current collector exposed portion. In addition, a 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.

[0016] 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.

[0017] In this embodiment, the negative electrode tab 20 is joined to the outer peripheral surface of the negative electrode current collector by, for example, 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 location of the negative electrode tab 20 is not limited to the inner winding end as shown in FIG. 2 , but may be any position in the longitudinal direction γ from the inner winding end to the outer winding end. 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.

[0018] 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.

[0019] 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 negative electrode 12 expands during charging increases. Therefore, when the negative electrode mixture layer contains a Si-containing compound, the pressure applied to the positive electrode tab 19 tends to increase, and the effect of the thick portion of the positive electrode mixture layer, which will be described later, becomes more pronounced.

[0020] 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.

[0021] 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.

[0022] 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 vicinity of the positive electrode tab 19 of the positive electrode 11 according to an example of the embodiment in a developed state. 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] A positive electrode current collector exposed portion 34, where the positive electrode current collector 30 is exposed, is formed on the surface 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 portion of the positive electrode current collector 30. The width W34 of the positive electrode current collector exposed portion 34 is, for example, 5 mm or more and 15 mm or less. The width W19 of the positive electrode tab 19 is not particularly limited as long as it is smaller than the width W34 of the positive electrode current collector exposed portion 34, but is, for example, 1 mm or more and 10 mm or less.

[0028] The positive electrode mixture layer 32 is adjacent to both ends of the positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11. In this embodiment, in order to improve current collection performance, the positive electrode current collector exposed portion 34 is formed in approximately the center in the longitudinal direction of the positive electrode 11. The position of the positive electrode current collector exposed portion 34 is not particularly limited, as long as the positive electrode mixture layer 32 is adjacent to both ends of the positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11.

[0029] The positive electrode mixture layer 32 has thick portions 38 that are thicker than the normal portions 36, and the thick portions 38 are located close to both ends of the positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11. This reduces the pressure applied to the positive electrode tab 19, thereby preventing damage to the positive electrode current collector 30. In this specification, the normal portions 36 refer to the portions of the positive electrode mixture layer 32 other than the thick portions 38 that are located close to the positive electrode current collector exposed portion 34.

[0030] The entire surface of the thick portion 38 is covered with a protective tape 40. This prevents the thick portion 38 from releasing more non-aqueous electrolyte than the normal portion 36, thereby suppressing deposition of the non-aqueous electrolyte on the opposing negative electrode 12 and improving the reliability of the battery. For example, as shown in FIG. 3 , the protective tape 40 is attached to the surface of the positive electrode 11 so as to straddle the positive electrode current collector exposed portion 34 and cover the entire surface of the thick portion 38.

[0031] The protective tape 40 has, for example, a base layer and an adhesive layer formed on the surface of the base layer. The thickness of the protective tape 40 is, for example, 20 μm or more and 70 μm or less. The base layer is mainly composed of a resin such as polypropylene (PP). The adhesive layer is mainly composed of an acrylic adhesive or a synthetic rubber adhesive. Note that the protective tape 40 is not limited to a two-layer structure and may have, for example, a three-layer structure in which an inorganic particle-containing layer is formed between the base layer and the adhesive layer.

[0032] Next, the cross-sectional structure of the positive electrode 11 will be described with reference to Fig. 4. Fig. 4 is a cross-sectional view taken along line A-A in Fig. 3. In this embodiment, 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 36, and the thick portion 38 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 circumferential sides of the positive electrode current collector 30 may have the same shape.

[0033] The thickness T36 of the normal portion 36 and the thickness T38 of the thick portion 38 are the sum of the thickness of the positive electrode mixture layer 32 provided on both sides of the positive electrode current collector 30 at the corresponding portions and the thickness of the positive electrode current collector 30. The thickness T36 of the normal portion 36 is substantially constant, and the surface of the positive electrode 11 at the portion corresponding to the normal portion 36 is substantially parallel to the surface of the positive electrode current collector 30. The thickness T38 of the thick portion 38 is substantially constant except for an inclined portion formed in the portion in contact with the normal portion 36, and the surface of the positive electrode 11 at the portion corresponding to the thick portion 38 is substantially parallel to the surface of the positive electrode current collector 30. In this specification, the thickness of the thick portion 38 means the thickness of the thick portion 38 excluding the inclined portion. Note that the thick portion 38 may have an inclined portion in the portion in contact with the positive electrode current collector exposed portion 34. Since the thick portion 38 only needs to be close to the positive electrode current collector exposed portion 34, a part of the inclined portion may be thinner than the normal portion.

[0034] The positive electrode tab 19 is connected to the outer peripheral surface of the positive electrode current collector 30. The configuration of the positive electrode 11 is not limited to the example shown in Fig. 3, and the positive electrode mixture layer 32 may be formed on only one surface of the positive electrode current collector 30, or the positive electrode tab 19 may be connected to the inner peripheral surface of the positive electrode current collector 30. The positive electrode mixture layer 32 is preferably provided on at least the side of the surface of the positive electrode current collector 30 to which the positive electrode tab 19 is connected.

[0035] The thickness T19 of the positive electrode tab 19 is smaller than the total thickness of the positive electrode mixture layer 32 in the portion corresponding to the thick portion 38 and larger than the total thickness of the positive electrode mixture layer 32 in the portion corresponding to the normal portion 36. Here, the total thickness of the positive electrode mixture layer 32 refers to the sum of the thicknesses of the positive electrode mixture layers 32 formed on the inner and outer peripheral sides when the positive electrode mixture layers 32 are formed on both sides of the positive electrode current collector 30. When the positive electrode mixture layer 32 is formed on only one side of the positive electrode current collector 30, the total thickness refers to the thickness of the positive electrode mixture layer 32. When the positive electrode 11 has such a configuration, the effect of the thick portion 38 in suppressing breakage of the positive electrode current collector 30 becomes more pronounced. The positive electrode 11 receives pressure from both the inner and outer peripheral sides of the negative electrode 12 facing it. However, the presence of the thick portion 38 near the positive electrode current collector exposed portion 34 reduces the pressure received from the negative electrode 12. In the example shown in FIG. 4 , the sum of the thickness T19 of the positive electrode tab 19 and the thickness T30 of the positive electrode current collector 30 (T19 + T30) is smaller than the thickness T38 of the portion corresponding to the thick portion 38 and is larger than the thickness T36 of the portion corresponding to the normal portion 36.

[0036] Next, the cross-sectional structure of the positive electrode 11 will be further described with reference to Fig. 5. Fig. 5 is a radial cross-sectional view of an electrode body 14 according to an example of an embodiment, and is an enlarged view of the vicinity of the positive electrode tab 19. Fig. 5 does not show the negative electrode 12 and separator 13, and only shows the cross-sectional structure of the positive electrode 11.

[0037] At least a part of the thick portion 38 may be located within 0.25 revolutions from the end 19a of the positive electrode tab 19 in the longitudinal direction of the positive electrode 11. That is, in Fig. 5 , the central angle with respect to an arc centered on the winding central axis O and passing through the end 19a of the positive electrode tab 19 and the end 38a of the thick portion 38 may be 90° or less. Note that the same relationship as above also holds for the end 19a of the positive electrode tab 19 and the end 38a of the thick portion 38 that are located closer to the winding end than the winding central axis O in Fig. 5 .

[0038] It is preferable that the thick portions 38 do not overlap in the radial direction of the electrode body 14. Furthermore, it is more preferable that the other end 38b of the thick portions 38 is located within 0.25 circumference from the center of the positive electrode tab 19. This allows the cross-sectional shape of the outermost periphery of the electrode body 14 to be approximately circular, which is preferable from the perspective of increasing capacity, etc. In the thick portion 38 in Figure 5, the end 38a represents the end closer to the positive electrode tab 19, and the end 38b represents the end farther from the positive electrode tab 19.

[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, forming 9 mm-wide gaps that would serve as exposed portions of the positive electrode current collector. After drying, the positive electrode current collector was cut to a size of 59.0 mm wide and 840 mm long, and the 9 mm-wide exposed portion of the positive electrode current collector was present in the longitudinal center of the positive electrode current collector, and the coating was rolled. When rolling the coating, the pressure was weaker in a region of 5.5 mm in the longitudinal direction of the positive electrode current collector from the end of the positive electrode mixture layer in contact with the exposed portion of the positive electrode current collector than in other regions, forming a thick portion in this region. Then, 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, and a protective tape with a PI substrate layer was applied to cover the thick portion. Protective tape was also applied to the surface to which the positive electrode tab was not connected, overlapping the protective tape. In this way, the positive electrode shown in FIGS. 3 to 5 was produced. The total thickness of the positive electrode mixture layer in the portion corresponding to the normal portion was 95 μm, and the total thickness of the positive electrode mixture layer in the portion corresponding to the thick portion was 98 μm.

[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 prepared 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 nonaqueous electrolyte was poured into the outer can, the opening of the outer can was sealed with a sealing member via a gasket, producing an 18650-type test cell. The circumference of the electrode assembly, including the location of the positive electrode tab, was approximately 40 mm. The end of the thick portion coinciding with the end of the exposed portion of the positive electrode current collector was 4.5 mm from the center of the positive electrode tab, and the length of the thick portion was 5.5 mm. Therefore, the other end of the thick portion was located approximately 0.25 revolutions from the center of the positive electrode tab 19.

[0044] Example 2 A positive electrode and a test cell were produced in the same manner as in Example 1, except that in producing the positive electrode, when the coating film was rolled, the pressure applied to the portion corresponding to the thick portion was weaker than the pressure applied in Example 1. In the obtained positive electrode, the total thickness of the positive electrode mixture layer in the portion corresponding to the normal portion was 95 μm, and the total thickness of the positive electrode mixture layer in the portion corresponding to the thick portion was 100 μm.

[0045] <Comparative Example> A positive electrode and a test cell were 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 when rolling the coating film. The total thickness of the positive electrode mixture layer on the entire surface of the obtained positive electrode was 95 μm.

[0046] [Evaluation of fracture of positive electrode current collector] The test cells of the examples and comparative examples were subjected to the following charge-discharge cycle 200 times. After the charge-discharge cycle, the test cells were disassembled and visually inspected for fracture of the positive electrode current collector near the positive electrode tab. The evaluation results are shown in Table 1.

[0047] [Charge-Discharge Cycle] At an ambient temperature of 25°C, the test cell was charged at a constant current of 3000 mA (1 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 test cell was 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, completing one charge-discharge cycle.

[0048]

[0049] As shown in Table 1, no breakage occurred in the test cells of the examples, but breakage was observed in the positive electrode current collector near the positive electrode tab in the test cells of the comparative examples. This shows that by providing a thick portion that is thicker than the normal portion near the exposed portion of the positive electrode current collector, breakage of the positive electrode tab can be suppressed even in nonaqueous electrolyte secondary batteries that support higher output.

[0050] The present disclosure is further described by the following embodiments. Aspect 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, a positive electrode current collector exposed portion where the positive electrode current collector is exposed is formed on the surface of the positive electrode, and a positive electrode tab is connected to the positive electrode current collector exposed portion, and the positive electrode mixture layer has a normal portion and a thick portion that is thicker than the normal portion, and the thick portion is adjacent to both ends of the positive electrode current collector exposed portion in the longitudinal direction of the positive electrode. Aspect 2: The nonaqueous electrolyte secondary battery according to Aspect 1, wherein at least a portion of the thick portion is located within 0.25 revolutions from an end of the positive electrode tab in the longitudinal direction of the positive electrode. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the thickness of the positive electrode tab is smaller than the total thickness of the positive electrode mixture layer in a portion corresponding to the thick portion and larger than the total thickness of the positive electrode mixture layer in a portion corresponding to the normal portion.Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the entire surface of the thick portion is covered with a protective tape.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the thick portions do not overlap in the radial direction of the electrode body.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, and the negative electrode mixture layer contains a Si-containing compound.

[0051] 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, 34 exposed portion of positive electrode current collector, 36 normal portion, 38 thick portion, 40 protective tape, O winding center axis

Claims

1. A nonaqueous 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 a surface of the positive electrode current collector, a positive electrode current collector exposed portion at which the positive electrode current collector is exposed is formed on the surface of the positive electrode, and a positive electrode tab is connected to the positive electrode current collector exposed portion, the positive electrode mixture layer has a normal portion and a thick portion that is thicker than the normal portion, and the thick portions are adjacent to both ends of the positive electrode current collector exposed portion in the longitudinal direction of the positive electrode.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein at least a portion of said thick portion is disposed within a range of 0.25 circumference from an end of said positive electrode tab in the longitudinal direction of said positive electrode.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thickness of the positive electrode tab is smaller than the total thickness of the positive electrode mixture layer in a portion corresponding to the thick portion and is greater than the total thickness of the positive electrode mixture layer in a portion corresponding to the normal portion.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the entire surface of the thick portion is covered with a protective tape.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thick portions do not overlap in a radial direction of the electrode body.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector, and the negative electrode mixture layer contains a Si-containing compound.

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