Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure comprises: a wound electrode body in which a belt-like positive electrode and a belt-like negative electrode are wound together, 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 positive electrode mixture layers that are formed on both surfaces of the positive electrode current collector, and a first positive electrode current collector exposed part in which the positive electrode current collector is exposed is formed on one surface of the positive electrode. A positive electrode tab is connected to the first positive electrode current collector exposed part, and a protective tape is adhered so as to cover the first positive electrode current collector exposed part and the positive electrode tab. If the first positive electrode current collector exposed part is divided into a first region to which the positive electrode tab is connected and a second region which is adjacent to the first region, the thickness T1 of the positive electrode in the first region and the thickness T2 of the positive electrode in the second region satisfy the relational expression T2 / T1 > 0.5.

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 breakage of the positive electrode current collector around the positive electrode tab. The technology disclosed in Patent Document 1 does not take into consideration the pressure on the positive electrode tab, and there is still room for improvement.

[0006] An object of the present disclosure is to provide a nonaqueous electrolyte secondary battery in which breakage of a positive electrode current collector due to charge / discharge cycles is suppressed.

[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 positive electrode mixture layers formed on both sides of the positive electrode current collector. One surface of the positive electrode has a first positive electrode current collector exposed portion where the positive electrode current collector is exposed. A positive electrode tab is connected to the first positive electrode current collector exposed portion. Protective tape is attached so as to cover the first positive electrode current collector exposed portion and the positive electrode tab. When the first positive electrode current collector exposed portion is divided into a first region to which the positive electrode tab is connected and a second region adjacent to the first region, a thickness T1 of the positive electrode in the first region and a thickness T2 of the positive electrode in the second region satisfy the relationship T2 / T1>0.5.

[0008] According to the nonaqueous electrolyte secondary battery according to the present disclosure, the safety of the battery can be further improved.

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

[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. The inclusion of a Si-containing compound in the negative electrode mixture layer increases the battery capacity, but also increases the rate at which the negative electrode 12 expands during charging. Therefore, when the negative electrode mixture layer contains a Si-containing compound, the pressure applied to the positive electrode tab 19 tends to increase, making the effect of the thick portion of the positive electrode mixture layer, described below, more pronounced. The proportion of the Si-containing compound relative to the total mass of the negative electrode active material is, for example, 1% by mass or more and 20% by mass or less.

[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 Fig. 3 to Fig. 6. Fig. 3 is a plan view showing the positive electrode 11 according to an example embodiment in a developed state. 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.

[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 containing aluminum or an aluminum alloy as a main component. The thickness of the positive electrode current collector 30 is, for example, 10 μm to 30 μm, and may be 10 μm to 15 μm.

[0024] The thickness of the positive electrode mixture layer 32 is, for example, 10 μm or more and 200 μm or less, and may be 100 μm or more and 150 μm or less on one side of the positive electrode current collector 30. 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 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 first positive electrode current collector exposed portion 34 where the positive electrode current collector 30 is exposed is formed on one surface of the positive electrode 11, and a positive electrode tab 19 is connected to the first positive electrode current collector exposed portion 34. The first 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 in which the positive electrode mixture slurry is not applied to a part of the positive electrode current collector 30.

[0028] In the example shown in Fig. 3, the first positive electrode current collector exposed portion 34 contacts only one end 11a of both ends in the width direction of the positive electrode 11, and does not extend to the other end 11b in the width direction of the positive electrode 11. As a result, the positive electrode mixture layer 32 is present between the other end 11b in the width direction of the positive electrode 11 and the first positive electrode current collector exposed portion 34, thereby increasing the battery capacity of the secondary battery 10. Note that the form of the first positive electrode current collector exposed portion 34 is not limited to the example shown in Fig. 3. The first positive electrode current collector exposed portion 34 may extend from one end 11a to the other end 11b in the width direction of the positive electrode 11 and contact both ends in the width direction of the positive electrode 11.

[0029] In the longitudinal direction of the positive electrode 11, the positive electrode mixture layer 32 is adjacent to both ends of the first positive electrode current collector exposed portion 34. In the example shown in Fig. 3, in order to improve current collection performance, the first positive electrode current collector exposed portion 34 is formed in approximately the center in the longitudinal direction of the positive electrode 11. Note that the position of the first 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 first positive electrode current collector exposed portion 34 in the longitudinal direction of the positive electrode 11.

[0030] A protective tape 40 is attached to the surface of the positive electrode 11 so as to cover the first positive electrode current collector exposed portion 34 and the positive electrode tab 19. The protective tape 40 covers the entire surface of the first positive electrode current collector exposed portion 34, and, for example, an end of the protective tape 40 in the longitudinal direction of the positive electrode 11 is located on the surface of the positive electrode mixture layer 32. Furthermore, as in the example shown in FIG. 3 , the protective tape 40 may protrude outward beyond one end 11 a in the lateral direction of the positive electrode 11. It is preferable that the protective tape 40 does not have an adhesive layer 44, which will be described later, in this protruding portion.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] Next, the thickness of the positive electrode 11 near the positive electrode tab 19 will be described with reference to Figures 5 and 6. Figure 5 is a cross-sectional view taken along line A-A in Figure 3. Line A-A is parallel to the longitudinal direction of the positive electrode 11. The thickness of the positive electrode tab 19 is, for example, 20 µm or more and 150 µm or less, and may be 40 µm or more and 120 µm or less.

[0035] 5 , a positive electrode mixture layer 32 is formed on the other surface of the positive electrode 11 at a position that overlaps with the first positive electrode current collector exposed portion 34 in the thickness direction of the positive electrode 11. That is, the positive electrode mixture layer 32 is formed on the entire surface of the pair of surfaces of the positive electrode 11 that is not connected to the positive electrode tab 19.

[0036] When the first positive electrode current collector exposed portion 34 is divided into a first region 50 to which the positive electrode tab 19 is connected and a second region 52 adjacent to the first region 50, the thickness T1 of the positive electrode 11 in the first region 50 and the thickness T2 of the positive electrode 11 in the second region 52 satisfy the relationship T2 / T1 > 0.5. By reducing the difference in thickness of the positive electrode 11 near the positive electrode tab 19 so as to satisfy T2 / T1 > 0.5, the pressure applied to the positive electrode tab 19 can be made equal to or lower than the threshold value at which fracture of the positive electrode current collector occurs.

[0037] 5 , the thickness T1 of the first region 50 is the sum of the thickness of the protective tape 40, the thickness of the positive electrode tab 19, the thickness of the positive electrode current collector 30, and the thickness of the positive electrode mixture layer 32, and the thickness T2 of the second region 52 is the sum of the thickness of the protective tape 40, the thickness of the positive electrode current collector 30, and the thickness of the positive electrode mixture layer 32. The thickness T1 of the first region 50 is, for example, the thickness measured at a position halfway across the width of the first region in a cross section taken along the longitudinal direction of the positive electrode 11. The thickness T2 of the second region 52 is, for example, the thickness measured at a position halfway across the width of the second region in a cross section taken along the longitudinal direction of the positive electrode 11.

[0038] In the longitudinal direction of the positive electrode 11, the length of the first region 50 is, for example, 1 mm or more and 10 mm or less, and the length of the second region 52 is, for example, 1 mm or more and 10 mm or less. In the longitudinal direction of the positive electrode 11, the ratio of the length of the first region 50 to the length of the second region 52 is, for example, in the range of 5:1 to 5:8.

[0039] Fig. 6 is a diagram of another example of an embodiment corresponding to Fig. 5. In the following, only parts that are different from the embodiment shown in Fig. 5 will be described, and parts that are not described can have the same configuration as the embodiment shown in Fig. 5.

[0040] In the example shown in FIG. 6 , on the other surface of the positive electrode 11, a second positive electrode current collector exposed portion 35 where the positive electrode current collector 30 is exposed is formed at a position overlapping with the first positive electrode current collector exposed portion 34 in the thickness direction of the positive electrode 11, and a protective tape 40 is attached so as to cover the second positive electrode current collector exposed portion 35.

[0041] In the example shown in FIG. 6 , the thickness T1 of the first region 50 is the sum of the thickness of the protective tape 40 on the first positive electrode current collector exposed portion 34 side, the thickness of the positive electrode tab 19, the thickness of the positive electrode current collector 30, and the thickness of the protective tape 40 on the second positive electrode current collector exposed portion 35 side, and the thickness T2 of the second region 52 is the sum of the thickness of the protective tape 40 on the first positive electrode current collector exposed portion 34 side, the thickness of the positive electrode current collector 30, and the thickness of the protective tape 40 on the second positive electrode current collector exposed portion 35 side. The protective tape 40 in FIG. 6 has a configuration similar to that of the protective tape 40 in FIGS. 3 to 5 . The protective tape 40 on the first positive electrode current collector exposed portion 34 side and the protective tape 40 on the second positive electrode current collector exposed portion 35 side are the same in the example shown in FIG. 6 , but are not limited to this example and may have different thicknesses or materials.

[0042] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0043] 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 applied to both sides of a positive electrode current collector made of 15 μm thick aluminum foil. The coating was dried, rolled, and then cut to a predetermined electrode size to prepare a positive electrode. At this time, an exposed positive electrode current collector portion was formed on only one surface of the positive electrode current collector by intermittent application. The thickness of the positive electrode mixture layer was 150 μm on both the side to which the positive electrode tab was connected (front side) and the side to which the positive electrode tab was not connected (back side). Thereafter, a 90 μm thick aluminum positive electrode tab was welded to the exposed portion of the positive electrode current collector, and a 30 μm thick protective tape was attached to cover the exposed portion of the positive electrode current collector and the positive electrode tab. The protective tape had a 25 μm thick substrate layer and a 5 μm thick adhesive layer. In this manner, the positive electrode shown in FIGS. 3 and 5 was fabricated. The thickness T1 of the first region was 285 μm, the thickness T2 of the second region was 195 μm, and the T2 / T1 ratio was 0.68.

[0044] [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 the 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, and then cut to a predetermined electrode size to prepare a negative electrode. At this time, a negative electrode current collector exposed portion was prepared at the end of the winding start side of the negative electrode current collector. The thickness of the negative electrode mixture layer was 120 μm. Then, a Ni / Cu negative electrode tab was welded to the negative electrode current collector exposed portion to prepare a negative electrode.

[0045] [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.

[0046] [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.

[0047] Example 2 A test cell was produced in the same manner as in Example 1, except that the thickness of the positive electrode tab used in producing the positive electrode was changed to 120 μm. The thickness T1 of the first region was 315 μm, the thickness T2 of the second region was 195 μm, and T2 / T1 was 0.62.

[0048] Example 3 A test cell was produced in the same manner as in Example 1, except that in the production of the positive electrode, exposed positive electrode current collector portions were formed on both sides of the positive electrode current collector, protective tape was also attached to the back side, the thickness of the positive electrode tab used was changed to 40 μm, and a positive electrode of the embodiment shown in Figures 3 and 6 was produced. The thickness T1 of the first region was 115 μm, the thickness T2 of the second region was 75 μm, and T2 / T1 was 0.65.

[0049] Example 4 A test cell was fabricated in the same manner as in Example 1, except that the thickness of the protective tape used in fabricating the positive electrode was changed to 15 μm. In the protective tape, the thickness of the base layer was 10 μm and the thickness of the adhesive layer was 5 μm. The thickness T1 of the first region was 270 μm, the thickness T2 of the second region was 180 μm, and the T2 / T1 ratio was 0.67.

[0050] Example 5 A test cell was fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, a protective tape was attached to the surface of the back-side positive electrode mixture layer at a position overlapping with the exposed portion of the positive electrode current collector. The thickness T1 of the first region was 315 μm, the thickness T2 of the second region was 225 μm, and T2 / T1 was 0.71.

[0051] Example 6 A test cell was fabricated in the same manner as in Example 1, except that the thickness of the positive electrode mixture layer was changed to 100 μm and the thickness of the protective tape used was changed to 15 μm. In the protective tape, the thickness of the base layer was 10 μm and the thickness of the adhesive layer was 5 μm. The thickness T1 of the first region was 220 μm, the thickness T2 of the second region was 130 μm, and T2 / T1 was 0.59.

[0052] Example 7 A test cell was fabricated in the same manner as in Example 1, except that the thickness of the positive electrode current collector used in fabricating the positive electrode was changed to 13 μm. The thickness T1 of the first region was 283 μm, the thickness T2 of the second region was 193 μm, and T2 / T1 was 0.68.

[0053] <Comparative Example 1> A test cell was produced in the same manner as in Example 1, except that in the production of the positive electrode, exposed positive electrode current collector portions were formed on both sides of the positive electrode current collector, protective tape was also attached to the back side, the thickness of the positive electrode tab used was changed to 120 μm, and a positive electrode of the embodiment shown in Figures 3 and 6 was produced. The thickness T1 of the first region was 195 μm, the thickness T2 of the second region was 75 μm, and T2 / T1 was 0.38.

[0054] Comparative Example 2 A test cell was produced in the same manner as in Example 1, except that in producing the positive electrode, exposed positive electrode current collector portions were formed on both sides of the positive electrode current collector and protective tape was also attached to the back side, thereby producing a positive electrode having the configuration shown in Figures 3 and 6. The thickness T1 of the first region was 165 µm, the thickness T2 of the second region was 75 µm, and T2 / T1 was 0.45.

[0055] Comparative Example 3 A test cell was prepared in the same manner as in Example 1, except that in preparing a positive electrode, exposed positive electrode current collector portions were formed on both sides of the positive electrode current collector, protective tape was also attached to the back side, the thickness of the protective tape used was changed to 37.5 μm, and a positive electrode of the configuration shown in FIGS. 3 and 6 was prepared. In the protective tape, the thickness of the base layer was 32.5 μm and the thickness of the adhesive layer was 5 μm. The thickness T1 of the first region was 180 μm, the thickness T2 of the second region was 90 μm, and T2 / T1 was 0.50.

[0056] Comparative Example 4 A test cell was fabricated in the same manner as in Example 1, except that in fabricating a positive electrode, exposed positive electrode current collector portions were formed on both sides of the positive electrode current collector, protective tape was also attached to the back side, the thickness of the protective tape used was changed to 20 μm, and a positive electrode of the configuration shown in FIGS. 3 and 6 was fabricated. In the protective tape, the thickness of the base layer was 15 μm and the thickness of the adhesive layer was 5 μm. The thickness T1 of the first region was 145 μm, the thickness T2 of the second region was 55 μm, and T2 / T1 was 0.38.

[0057] Comparative Example 5 A test cell was produced in the same manner as in Example 1, except that the thickness of the positive electrode mixture layer was 45 μm in the production of the positive electrode. The thickness T1 of the first region was 180 μm, the thickness T2 of the second region was 90 μm, and T2 / T1 was 0.50.

[0058] Comparative Example 6 A test cell was produced in the same manner as in Example 1, except that the thickness of the positive electrode mixture layer was 30 μm in the production of the positive electrode. The thickness T1 of the first region was 165 μm, the thickness T2 of the second region was 75 μm, and T2 / T1 was 0.45.

[0059] Comparative Example 7 A test cell was fabricated in the same manner as in Example 1, except that the thickness of the positive electrode mixture layer was changed to 50 μm and the thickness of the positive electrode current collector used was changed to 10 μm. The thickness T1 of the first region was 180 μm, the thickness T2 of the second region was 90 μm, and T2 / T1 was 0.50.

[0060] [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 100 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. For each of the examples and comparative examples, evaluation was performed on 10 test cells, and the number of test cells in which fracture was confirmed was used for evaluation. The evaluation results are shown in Table 1.

[0061] [Charge-Discharge Cycle] At an ambient temperature of 25°C, the test cell was charged at a constant current of 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 1 / 50 C, and allowed to stand for 10 minutes, after which it was discharged at a constant current of 5 C until the battery voltage reached 2.5 V, and allowed to stand for 30 minutes, completing one charge-discharge cycle.

[0062]

[0063] 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 breakage of the positive electrode tab can be suppressed by satisfying the relationship T2 / T1 > 0.5 near the positive electrode tab.

[0064] The present disclosure is further described by the following embodiments. Configuration 1: A non-aqueous 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 positive electrode mixture layers formed on both sides of the positive electrode current collector, a first positive electrode current collector exposed portion in which the positive electrode current collector is exposed is formed on one surface of the positive electrode, a positive electrode tab is connected to the first positive electrode current collector exposed portion, and a protective tape is attached so as to cover the first positive electrode current collector exposed portion and the positive electrode tab, and when the first positive electrode current collector exposed portion is divided into a first region to which the positive electrode tab is connected and a second region adjacent to the first region, a thickness T1 of the positive electrode in the first region and a thickness T2 of the positive electrode in the second region satisfy the relationship T2 / T1 > 0.5. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the positive electrode mixture layer is formed on the other surface of the positive electrode at a position overlapping the first positive electrode current collector exposed portion in the thickness direction of the positive electrode.Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein a second positive electrode current collector exposed portion, in which the positive electrode current collector is exposed, is formed on the other surface of the positive electrode at a position overlapping the first positive electrode current collector exposed portion in the thickness direction of the positive electrode, and the protective tape is attached so as to cover the second positive electrode current collector exposed portion.Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the first positive electrode current collector exposed portion contacts only one of both end portions in the width direction of the positive electrode.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the first positive electrode current collector exposed portion contacts both end portions in the width direction of the positive electrode. Configuration 6: The nonaqueous electrolyte secondary battery of 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 a surface of the negative electrode current collector, and the negative electrode mixture layer contains a Si-containing compound.

[0065] 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 First positive electrode current collector exposed portion, 35 Second positive electrode current collector exposed portion, 40 Protective tape, 42 Base layer, 44 Adhesive layer, 50 First region, 52 Second region

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 positive electrode mixture layers formed on both sides of the positive electrode current collector; one surface of the positive electrode has a first positive electrode current collector exposed portion where the positive electrode current collector is exposed; a positive electrode tab is connected to the first positive electrode current collector exposed portion, and protective tape is affixed so as to cover the first positive electrode current collector exposed portion and the positive electrode tab; and wherein, when the first positive electrode current collector exposed portion is divided into a first region to which the positive electrode tab is connected and a second region adjacent to the first region, a thickness T1 of the positive electrode in the first region and a thickness T2 of the positive electrode in the second region satisfy the relationship T2 / T1>0.

5.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode mixture layer is formed on the other surface of the positive electrode at a position overlapping the first positive electrode current collector exposed portion in the thickness direction of the positive electrode.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein a second positive electrode current collector exposed portion, in which the positive electrode current collector is exposed, is formed on the other surface of the positive electrode at a position overlapping with the first positive electrode current collector exposed portion in the thickness direction of the positive electrode, and the protective tape is attached so as to cover the second positive electrode current collector exposed portion.

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

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first positive electrode current collector exposed portion contacts both ends of the positive electrode in the short side direction.

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 the surface of the negative electrode current collector, and the negative electrode mixture layer contains a Si-containing compound.

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