Non-aqueous electrolyte secondary battery

Abstract

A positive electrode (11) has a positive electrode core body (30), and a positive electrode mixture layer (31) and a protective layer (33) that are formed on the positive electrode core body (30). A mixture layer non-formation portion (32) at which the positive electrode mixture layer (31) is not formed is provided to a winding start end part (11B) of the positive electrode (11). The protective layer (33) contains an insulating material as the main component, and has a first region (34) that covers the mixture layer non-formation portion (32), and a second region (35) that is disposed between the positive electrode core body (30) and the positive electrode mixture layer (31). The volume resistivity of the protective layer (33) in the second region (35) is less than the volume resistivity of the protective layer (33) in the first region (34).

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] Conventionally, non-aqueous electrolyte secondary batteries have been known that include an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween. Generally, the positive electrode includes a positive electrode core and a positive electrode mixture layer formed on the positive electrode core. Patent Document 1 discloses a non-aqueous electrolyte secondary battery in which a protective layer made primarily of an insulating material is formed between the positive electrode core and the positive electrode mixture layer.

[0003] International Publication No. 2018 / 168272

[0004] With the recent trend toward higher capacities of non-aqueous electrolyte secondary batteries, the volume change of the negative electrode during charge and discharge has become larger. When the volume change of the negative electrode during charge and discharge becomes larger, stress tends to concentrate on the negative electrode near the winding start end of the positive electrode during repeated charge and discharge, which may result in deformation of the negative electrode. As a result of studies by the present inventors, it has become clear that deformation of the negative electrode near the winding start end of the positive electrode can be suppressed by providing a portion where no positive electrode mixture layer is formed at the winding start end of the positive electrode.

[0005] On the other hand, since the positive electrode core has a lower resistance than the positive electrode mixture layer, contact between the positive electrode core in the portion where the mixture layer is not formed and the negative electrode may cause an internal short circuit. Therefore, by covering the portion where the mixture layer is not formed with an insulating protective layer, internal short circuits can be suppressed. Furthermore, from the viewpoint of completely covering the portion where the mixture layer is not formed with the protective layer, it is preferable that a portion of the protective layer be disposed between the positive electrode core and the positive electrode mixture layer. However, if an insulating protective layer is disposed between the positive electrode core and the positive electrode mixture layer, the conduction path between the positive electrode core and the positive electrode mixture layer may be obstructed, resulting in a decrease in battery capacity.

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery including an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound lengthwise with a separator interposed therebetween, the positive electrode having a positive electrode core, and a positive electrode mixture layer and a protective layer formed on the positive electrode core, a mixture layer-free portion where no positive electrode mixture layer is formed is provided at the winding start end of the positive electrode, the protective layer containing an insulating material as a main component, and having a first region covering the mixture layer-free portion and a second region disposed between the positive electrode core and the positive electrode mixture layer, and the volume resistivity of the protective layer in the second region is smaller than the volume resistivity of the protective layer in the first region.

[0007] According to the nonaqueous electrolyte secondary battery of one aspect of the present disclosure, it is possible to suppress internal short circuits while ensuring battery capacity.

[0008] 2 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention, taken along line AA in FIG.

[0009] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that the nonaqueous electrolyte secondary battery according to the present disclosure is not limited to the embodiment described below.

[0010] Fig. 1 is an axial cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in Fig. 1, the nonaqueous electrolyte secondary battery 10 includes an electrode assembly 14 in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween, and a cylindrical outer can 16 with a bottom that houses the electrode assembly 14. The nonaqueous electrolyte secondary battery 10 also includes a nonaqueous electrolyte housed in the outer can 16, and a sealing member 17 that closes the opening of the outer can 16. Hereinafter, for convenience of explanation, the sealing member 17 side of the nonaqueous electrolyte secondary battery 10 will be referred to as the "top" and the bottom side of the outer can 16 will be referred to as the "bottom."

[0011] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

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

[0013] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

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

[0015] The positive electrode 11 has a long positive electrode core 30 and a positive electrode mixture layer 31 formed on the positive electrode core 30. The positive electrode core 30 can be made of a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent such as acetylene black, and a binder such as polyvinylidene fluoride (PVdF), and is preferably formed on both sides of the positive electrode core 30. For example, a lithium transition metal composite oxide containing Ni, Co, Mn, Al, or the like is used as the positive electrode active material.

[0016] The thickness of the positive electrode 11 is, for example, 110 μm or more and 270 μm or less. In this embodiment, the thickness of the positive electrode 11 is substantially constant except for a mixture layer non-forming portion 32 (see FIG. 3 ) described later and a region to which the positive electrode lead 20 is connected. The thickness of the positive electrode core 30 is, for example, 10 μm or more and 30 μm or less. The thickness of the positive electrode mixture layer 31 is, for example, 50 μm or more and 120 μm or less on one side of the positive electrode core 30. The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0017] The negative electrode 12 has a long negative electrode core 40 and a negative electrode mixture layer 41 formed on the negative electrode core 40. For the negative electrode core 40, a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper, a copper alloy, stainless steel, nickel, or a nickel alloy, or a film having such a metal disposed on its surface can be used. The negative electrode mixture layer 41 contains a negative electrode active material and a binder such as styrene-butadiene rubber (SBR), and is preferably formed on both sides of the negative electrode core 40.

[0018] The negative electrode active material generally uses a carbon material that reversibly absorbs and releases lithium ions. Suitable examples of the carbon material include natural graphite such as flake graphite, massive graphite, and amorphous graphite, and artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB).

[0019] Furthermore, a silicon-containing material may be used as the negative electrode active material. The silicon-containing material may be any material containing Si, and examples thereof include silicon alloys, silicon compounds, and composite materials containing Si. Among the silicon-containing materials, composite materials containing Si are preferred. A suitable example of a composite material containing Si is SiO ２ Examples of such composite materials include a material in which Si fine particles are dispersed in a silicon dioxide phase, a silicate phase such as lithium silicate, or an amorphous carbon phase. A conductive layer such as a carbon coating may be formed on the particle surface of the composite material.

[0020] The thickness of the negative electrode 12 is, for example, 105 μm or more and 255 μm or less. In this embodiment, the thickness of the negative electrode 12 is substantially constant except for the region where the outer peripheral exposed portion 42 (described later) and the negative electrode lead 21 are connected. The thickness of the negative electrode core 40 is, for example, 5 μm or more and 15 μm or less. The thickness of the negative electrode mixture layer 41 is, for example, 50 μm or more and 120 μm or less on one side of the negative electrode core 40. The negative electrode 12 can be produced, similarly to the positive electrode 11, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode core 40, drying the coating, and then compressing it to form the negative electrode mixture layers 41 on both sides of the negative electrode core 40.

[0021] The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 and a negative electrode lead 21 connected to the negative electrode 12. In this embodiment, the positive electrode lead 20 is connected to the center of the positive electrode 11 in the longitudinal direction, and the negative electrode lead 21 is connected to one end of the negative electrode 12 in the longitudinal direction, which is located on the winding start side of the electrode body 14.

[0022] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes through a through hole in the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0023] In this embodiment, the negative electrode 12 is disposed on the outermost peripheral surface of the electrode assembly 14, and an outer peripheral exposed portion 42 is provided where the surface of the negative electrode core 40 is exposed. At least a portion of the outer peripheral exposed portion 42 abuts against the inner peripheral surface of the outer can 16. By abutting the outer peripheral exposed portion 42 against the inner peripheral surface of the outer can 16, both longitudinal ends of the negative electrode 12 and the outer can 16 are electrically connected, ensuring good current collection. The outer peripheral exposed portion 42 may be provided on a portion of the outermost peripheral surface of the electrode assembly 14, but is preferably provided over the entire outermost peripheral surface of the electrode assembly 14. A separator 13 may be disposed on the outermost peripheral surface of the electrode assembly 14.

[0024] The outer can 16 is a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17, sealing the inside of the battery. The outer can 16 has a groove 22 formed, for example, by pressing the side surface from the outside. The groove 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The upper end of the outer can 16 is bent inward and crimped to the periphery of the sealing body 17.

[0025] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective radial centers, with the insulating member 25 interposed between their respective peripheral edges. When abnormal heat generation causes an increase in the internal pressure of the battery, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27. This interrupts the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure further increases, the upper valve body 26 ruptures, and gas is released from the opening of the cap 27.

[0026] The configuration of the winding start side of the positive electrode 11 will be described in detail below with reference to Fig. 2 and Fig. 3. Fig. 2 is a plan view of the winding start side of the outer surface of the wound positive electrode 11 in an unfolded state, and Fig. 3 is a cross-sectional view taken along line AA in Fig. 2.

[0027] 2 and 3 , the positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 formed on the positive electrode core 30. In this embodiment, the positive electrode mixture layer 31 is formed on both sides of the positive electrode core 30.

[0028] The winding start end 11B of the positive electrode 11 is provided with a mixture layer non-forming portion 32 where the positive electrode mixture layer 31 is not formed on the positive electrode core 30. The winding start end 11B of the positive electrode 11 is a region that includes the winding start end 11A of the positive electrode 11 and its vicinity. In other words, the mixture layer non-forming portion 32 is provided over a predetermined length from the winding start end 11A of the positive electrode 11 toward the winding end side. The mixture layer non-forming portion 32 is provided across the entire width of the positive electrode 11. In this embodiment, the mixture layer non-forming portion 32 is provided on both sides of the positive electrode core 30.

[0029] At the winding start end 31A of the positive electrode mixture layer 31, a step is formed by the positive electrode core 30 and the positive electrode mixture layer 31. As a result of investigations by the present inventors, it has been found that if the positive electrode mixture layer 31 is formed up to the winding start end 11A of the positive electrode 11 and this step is formed at the winding start end 11A of the positive electrode 11, deformation of the negative electrode 12 occurs in the negative electrode 12 facing the winding start end 11A of the positive electrode 11 during repeated charge and discharge. This is presumably because, when a large step is formed at the winding start end 11A of the positive electrode 11, stress is more likely to concentrate on the negative electrode 12 facing the winding start end 11A of the positive electrode 11.

[0030] After further investigation, the inventors have found that providing portion 32 without a mixture layer formed thereon at winding starting end 11B of positive electrode 11 can suppress deformation of negative electrode 12 near winding starting end 11A of positive electrode 11. This is presumably because, by adopting the above configuration, winding starting end 31A of positive electrode mixture layer 31 can be separated from winding starting end 11A of positive electrode 11, thereby reducing the step formed at winding starting end 11A of positive electrode 11.

[0031] In the longitudinal direction of the positive electrode 11, the length of the portion 32 where no mixture layer is formed is preferably 0.5 mm or more, and more preferably 1.0 mm or more. By making the length of the portion 32 where no mixture layer is formed 0.5 mm or more, deformation of the negative electrode 12 near the winding start end 11A of the positive electrode 11 can be further suppressed. Furthermore, the length of the portion 32 where no mixture layer is formed is preferably 5.0 mm or less, and more preferably 4.5 mm or less. By making the length of the portion 32 where no mixture layer is formed 5.0 mm or less, the area of ​​the region where the positive electrode mixture layer 31 is formed can be increased, making it easier to achieve a high capacity battery. Therefore, in the longitudinal direction of the positive electrode 11, the length of the portion 32 where no mixture layer is formed is preferably 0.5 mm or more and 5.0 mm or less, and more preferably 1.0 mm or more and 4.5 mm or less.

[0032] 2 and 3 , the positive electrode 11 has a protective layer 33 whose main component is an insulating material. The protective layer 33 is formed from the winding start end 11A of the positive electrode 11 toward the winding end, with a portion of the protective layer 33 formed between the positive electrode core 30 and the positive electrode mixture layer 31. In other words, the protective layer 33 has a first region 34 that covers the mixture layer-free portion 32 and a second region 35 formed between the positive electrode core 30 and the positive electrode mixture layer 31.

[0033] The positive electrode core 30 has a lower resistance than the positive electrode mixture layer 31. Therefore, if the positive electrode core 30 in the mixture layer-free portion 32 comes into contact with the opposing negative electrode 12, an internal short circuit may occur. Therefore, by covering the mixture layer-free portion 32 with the protective layer 33 containing an insulating material as a main component, contact between the mixture layer-free portion 32 and the negative electrode 12 is suppressed, and an internal short circuit can be suppressed.

[0034] Furthermore, by providing a portion of the protective layer 33 between the positive electrode core 30 and the positive electrode mixture layer 31, the mixture layer-free portion 32 can be completely covered with the protective layer 33, further suppressing internal short circuits. If a portion of the protective layer 33 is not provided between the positive electrode core 30 and the positive electrode mixture layer 31 but is provided on the surface of the positive electrode mixture layer 31 so as to cover the winding start end 31A of the positive electrode mixture layer 31, a large step is formed at the winding start end 31A of the positive electrode mixture layer 31. As a result, when charge and discharge are repeated, stress is likely to concentrate on the negative electrode 12 facing the winding start end 31A of the positive electrode mixture layer 31, and deformation of the negative electrode 12 is likely to occur.

[0035] On the other hand, it has been found that if a portion of the protective layer 33 is provided between the positive electrode core 30 and the positive electrode mixture layer 31, the conduction path between the positive electrode core 30 and the positive electrode mixture layer 31 is obstructed, resulting in a decrease in battery capacity. However, in the protective layer 33 of this embodiment, the volume resistivity of the protective layer 33 in the second region 35 formed between the positive electrode core 30 and the positive electrode mixture layer 31 is lower than the volume resistivity of the protective layer 33 in the first region 34 covering the mixture layer non-forming portion 32. This makes it possible to ensure a conduction path between the positive electrode core 30 and the positive electrode mixture layer 31 while suppressing internal short circuits caused by contact between the mixture layer non-forming portion 32 and the negative electrode 12. In other words, it is possible to achieve both suppression of internal short circuits and ensuring battery capacity.

[0036] The volume resistivity of the protective layer 33 in the first region 34 may be larger than the volume resistivity of the protective layer 33 in the second region 35. For example, ７ Ω cm or more, and 1.0 × 10 ８ The volume resistivity of the protective layer 33 in the first region 34 is preferably 1.0×10 ７ When the electrical resistance is Ω·cm or more, an internal short circuit caused by contact between the mixture layer non-forming portion 32 and the negative electrode 12 can be further suppressed.

[0037] The volume resistivity of the protective layer 33 in the second region 35 may be smaller than the volume resistivity of the protective layer 33 in the first region 34, but is preferably equal to or smaller than the volume resistivity of the positive electrode mixture layer 31. By setting the volume resistivity of the protective layer 33 in the second region 35 to be equal to or smaller than the volume resistivity of the positive electrode mixture layer 31, it is possible to further ensure the battery capacity. The volume resistivity of the protective layer 33 in the second region 35 is, for example, 1.0 × 10 ４ Ω cm or less, and 1.0 × 10 ３ The lower limit of the volume resistivity of the protective layer 33 in the second region 35 is not particularly limited, but is, for example, 1.0×10 １ The volume resistivity of the protective layer 33 in the first region 34 and the second region 35 and the volume resistivity of the positive electrode mixture layer 31 can be determined by resistivity measurement based on a two-terminal method.

[0038] In the longitudinal direction of the positive electrode 11, the length of the second region 35 is preferably 0.5 mm or more, and more preferably 1.0 mm or more. By making the length of the second region 35 0.5 mm or more, exposure of the positive electrode core 30 in the mixture layer non-forming portion 32 can be suppressed, and internal short circuits can be further suppressed. Furthermore, the length of the second region 35 is preferably 5.0 mm or less, and more preferably 4.5 mm or less. By making the length of the second region 35 5.0 mm or less, a conductive path between the positive electrode core 30 and the positive electrode mixture layer 31 can be further ensured. Therefore, in the longitudinal direction of the positive electrode 11, the length of the second region 35 is preferably 0.5 mm or more and 5.0 mm or less, and more preferably 1.0 mm or more and 4.5 mm or less.

[0039] As described above, the protective layer 33 contains an insulating material as a main component. Here, the main component means the component that has the highest mass ratio among the components constituting the protective layer 33. The content of the insulating material is preferably 80 mass % or more, and more preferably 90 mass % or more, of the total mass of the protective layer 33.

[0040] The insulating material preferably includes at least a resin material. The resin material functions, for example, as a binder that bonds the positive electrode core 30 and the positive electrode mixture layer 31 together. When the insulating material includes a resin material, peeling of the protective layer 33 from the mixture layer non-forming portion 32 is suppressed, thereby further suppressing internal short circuits. Furthermore, when the insulating material includes a resin material, peeling of the positive electrode mixture layer 31 near the winding start end 31B of the positive electrode mixture layer 31 is further suppressed. The winding start end 31B of the positive electrode mixture layer 31 is a region that includes the winding start end 31A of the positive electrode mixture layer 31 and its vicinity.

[0041] The resin material contained in the insulating material constituting the protective layer 33 is preferably a polymer material, and examples thereof include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), polyimide-based resins, polyamide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0042] The insulating material may be composed of only a resin material, or may further contain an inorganic material. Examples of inorganic materials include metal oxides, metal nitrides, metal fluorides, and metal carbides. Examples of metal oxides include aluminum oxide, titanium oxide, magnesium oxide, zirconium oxide, nickel oxide, silicon oxide, and manganese oxide. Examples of metal nitrides include titanium nitride, boron nitride, aluminum nitride, magnesium nitride, and silicon nitride. Examples of metal fluorides include aluminum fluoride, lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, and barium fluoride. Examples of metal carbides include silicon carbide, boron carbide, titanium carbide, and tungsten carbide. Furthermore, examples of inorganic materials include zeolite (M ２／ｎ O.Al ２ O ３ xSiO ２ ・yH２ O, M is a metal element, n is the valence of M, x≧2, y≧0), porous aluminosilicates such as talc (Mg ３ Si ４ O １０ (OH) ２ ), layered silicates such as barium titanate (BaTiO ３ ), strontium titanate (SrTiO ３ These may be used alone or in combination of two or more.

[0043] The inorganic material is, for example, composed of particulate inorganic particles. The average particle size of the inorganic particles is, for example, 0.05 μm or more and 2 μm or less. The average particle size of inorganic particles means the particle size at which the cumulative frequency of the smallest particle size in a volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of inorganic particles can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II manufactured by Microtrac-Bell) using water as a dispersion medium.

[0044] When the insulating material contains the inorganic material, the content of the inorganic material relative to the total mass of the protective layer 33 is, for example, 60% by mass or more and 99% by mass or less, and preferably 70% by mass or more and 95% by mass or less. When the insulating material contains the inorganic material, the protective layer 33 is more easily formed on the surface of the positive electrode core 30.

[0045] The protective layer 33 may further contain a conductive agent. When the protective layer 33 contains a conductive agent, the volume resistivity of the protective layer 33 can be reduced. Examples of the conductive agent contained in the protective layer 33 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, and other carbon materials. These may be used alone or in combination of two or more.

[0046] Here, the content of the conductive agent relative to the total mass of the protective layer 33 in the second region 35 may be greater than the content of the conductive agent relative to the total mass of the protective layer 33 in the first region 34. This allows the volume resistivity of the protective layer 33 in the second region 35 to be smaller than the volume resistivity of the protective layer 33 in the first region 34. Alternatively, the conductive agent may be contained substantially only in the second region 35. The content of the conductive agent relative to the total mass of the protective layer 33 in the second region 35 may be, for example, 0.1 mass% or more and 20 mass% or less, or 0.1 mass% or more and 15 mass% or less.

[0047] The thickness of the protective layer 33 may be, for example, 3 μm to 40 μm, or 5 μm to 30 μm, from the viewpoint of suppressing peeling of the positive electrode mixture layer while suppressing internal short circuits. Furthermore, the thickness of the protective layer 33 is preferably 50% or less, and more preferably 30% or less, of the maximum thickness of the positive electrode mixture layer 31. The protective layer 33 has, for example, a substantially uniform thickness along the length of the positive electrode 11.

[0048] The protective layer 33 can be formed by applying a protective layer slurry containing an insulating material to the surface of the positive electrode core 30 and drying the resulting coating. The protective layer slurry is prepared by mixing a resin material, an inorganic material as needed, and a conductive agent, and then adding an appropriate amount of water, N-methyl-2-pyrrolidone (NMP), or the like. The solids concentration of the protective layer slurry is, for example, 3% by mass or more and 50% by mass or less. Examples of methods for applying the protective layer slurry include gravure coating, spraying, die coating, roll coating, reverse roll coating, screen printing, and inkjet printing. The method for drying the resulting coating is not particularly limited, and may include, for example, natural drying, ventilation drying using warm air, heat drying, reduced pressure / vacuum drying, and combinations thereof. After forming the protective layer 33, a positive electrode mixture slurry is applied so as to overlap a portion of the protective layer 33, the coating is dried, and compressed, thereby forming the protective layer 33 between the positive electrode core 30 and the positive electrode mixture layer 31. At this time, the second region 35 of the protective layer 33 is compressed due to the compression during the formation of the positive electrode mixture layer 31. If the protective layer 33 contains a conductive agent, a conductive path of the conductive agent is more likely to be formed when the second region 35 is compressed. As a result, the volume resistivity of the compressed second region 35 becomes smaller than the volume resistivity of the uncompressed first region 34.

[0049] The protective layer 33 may be formed from two types of protective layer slurries with different conductive agent contents. The protective layer slurries may include, for example, a first protective layer slurry that does not contain a conductive agent and a second protective layer slurry that contains a conductive agent. The first protective layer slurry is applied to the surface of the positive electrode core 30 near the winding start end 11A of the positive electrode 11, and the second protective layer slurry is applied to the winding end side of the area where the first protective layer slurry was applied, and the resulting coatings are dried. Then, while forming the composite layer non-forming portion 32, the positive electrode composite slurry is applied so as to overlap the coating formed by the second protective layer slurry, and the coating is dried and compressed, thereby forming the protective layer 33 having the first region 34 and the second region 35. The composite layer non-forming portion 32 and the protective layer 33 may be formed on the winding end side of the positive electrode 11 in addition to the winding start side.

[0050] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0051] Example 1 Preparation of Positive Electrode Mixture Slurry Lithium nickel oxide (LiNi) containing cobalt and aluminum was used as the positive electrode active material. ０．８８ Co ０．０９ Al ０．０３ O ２ 100 parts by mass of this positive electrode active material, 1 part by mass of acetylene black (AB) as a conductive agent, and 1 part by mass of polyvinylidene fluoride (PVDF) as a binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode mixture slurry.

[0052] [Preparation of Slurry for Protective Layer] Alumina (α-Al) was used as an inorganic material having an average particle size of 0.7 μm. ２ O ３ The first protective layer slurry was prepared by mixing the inorganic particles of alumina (α-Al 2 O 3 ) with polyvinylidene fluoride (PVDF) as a resin material in a solid content mass ratio of 84:16, and then adding an appropriate amount of NMP to adjust the solid content concentration to 30 mass %. ２ O ３ ) particles, polyvinylidene fluoride (PVDF) as a resin material, and acetylene black (AB) as a conductive agent were mixed in a solid content mass ratio of 80:16:4, and then an appropriate amount of NMP was added to make the solid content concentration 30 mass % to prepare a slurry for the second protective layer.

[0053] [Preparation of Positive Electrode] The first protective layer slurry and the second protective layer slurry were applied sequentially from one end side of both sides of a positive electrode core made of aluminum foil with a thickness of 15 μm and a width of 62.6 mm, and the coating was dried to form a protective layer. Then, as shown in FIG. 2, the positive electrode mixture slurry was applied to both sides of the positive electrode core so as to overlap the protective layer formed by the second protective layer slurry while forming a mixture layer non-forming portion. After drying the coating, the positive electrode was cut to a predetermined electrode size and rolled using a roller to prepare a positive electrode. In the longitudinal direction of the positive electrode, the length of the mixture layer non-forming portion (first region of the protective layer) was 3.0 mm, and the length of the second region of the protective layer was 2.5 mm. When the volume resistivity of the protective layer and the positive electrode mixture layer was confirmed by a two-terminal method, it was confirmed that the volume resistivity of the second region was smaller than that of the first region and equivalent to that of the positive electrode mixture layer.

[0054] Thereafter, one positive electrode core exposed portion, where no positive electrode mixture layer was formed on either side of the positive electrode core, was provided in the longitudinal center of the positive electrode, and a 120 μm thick aluminum positive electrode lead was fixed to the positive electrode core exposed portion by ultrasonic welding.

[0055] [Preparation of Negative Electrode] A mixture of graphite and a silicon-containing material in a mass ratio of 95:5 was used as the negative electrode active material. 100 parts by mass of this negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethyl cellulose (CMC) as a thickener were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. Next, this negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, dried, cut to a predetermined electrode size, and rolled using a roller to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. In addition, a negative electrode core exposed portion in which a negative electrode mixture layer was not formed on both sides of the negative electrode core was formed at one end in the longitudinal direction of the negative electrode, and a nickel negative electrode lead was fixed to the negative electrode core exposed portion by ultrasonic welding.

[0056] [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 (LiPF６ A non-aqueous electrolyte was prepared by dissolving 1.5 mol / L of ammonium hydroxide in water.

[0057] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] A wound electrode assembly was fabricated by spirally winding the positive electrode and negative electrode with a separator interposed therebetween. The protective layer was positioned at the start of winding of the positive electrode. Insulating plates were placed on the top and bottom of the electrode assembly, and the electrode assembly was housed in an outer can. The negative electrode lead was welded to the bottom of the cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member. After injecting the non-aqueous electrolyte into the outer can, the opening of the outer can was sealed with a sealing member via a gasket, completing the fabrication of a non-aqueous electrolyte secondary battery.

[0058] [Evaluation of Discharge Capacity] The fabricated nonaqueous electrolyte secondary battery was charged to 4.2 V at a constant current of 0.2 C in a temperature environment of 25° C., and then charged to 0.02 C at a constant voltage of 4.2 V. Thereafter, it was discharged to 2.5 V at a constant current of 0.2 C, and the discharge capacity was measured.

[0059] Example 2 In preparing a slurry for a protective layer, alumina (α-Al) was used as an inorganic material having an average particle size of 0.7 μm. ２ O ３ A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that a slurry for the second protective layer was prepared by mixing cellulose acylate particles, polyvinylidene fluoride (PVDF) as a resin material, and acetylene black (AB) as a conductive agent in a solid content mass ratio of 83.9:16:0.1, and then adding an appropriate amount of NMP so that the solid content concentration was 30 mass%. The volume resistivities of the protective layer and the positive electrode mixture layer were confirmed by the above-mentioned method, and it was confirmed that the volume resistivity of the second region was smaller than that of the first region and larger than that of the positive electrode mixture layer.

[0060] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the slurry for the first protective layer was applied instead of the slurry for the second protective layer in the fabrication of the positive electrode. That is, the protective layer of the positive electrode in Comparative Example 1 did not contain a conductive agent throughout. Therefore, the volume resistivity of the second region was equivalent to that of the first region and was much greater than the volume resistivity of the positive electrode mixture layer.

[0061] Table 1 shows the discharge capacities of Examples 1 and 2 and Comparative Example 1. The discharge capacities in Table 1 are expressed as relative values, with the discharge capacity of Example 1 being set at 100, and a larger value indicates a higher capacity.

[0062]

[0063] As shown in Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 and 2 have higher capacities than the nonaqueous electrolyte secondary battery of Comparative Example 1. Furthermore, it was confirmed that the nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Example 1 do not suffer from deformation of the negative electrode near the winding start end of the positive electrode or internal short circuits due to contact between the portion without the mixture layer and the negative electrode, even after repeated charge and discharge. This is because the winding start end of the positive electrode is provided with a portion without the mixture layer, and the portion without the mixture layer is covered with a protective layer.

[0064] The present disclosure is further described by the following embodiments. Aspect 1: A nonaqueous electrolyte secondary battery including an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound lengthwise with a separator interposed therebetween, the positive electrode having a positive electrode core and a positive electrode mixture layer and a protective layer formed on the positive electrode core, a mixture layer-free portion where the positive electrode mixture layer is not formed is provided at a winding start end of the positive electrode, the protective layer containing an insulating material as a main component and having a first region covering the mixture layer-free portion and a second region disposed between the positive electrode core and the positive electrode mixture layer, the volume resistivity of the protective layer in the second region being lower than the volume resistivity of the protective layer in the first region. Aspect 2: The nonaqueous electrolyte secondary battery according to Aspect 1, wherein the volume resistivity of the protective layer in the second region is equal to or lower than the volume resistivity of the positive electrode mixture layer. Aspect 3: The nonaqueous electrolyte secondary battery according to Aspect 1 or Aspect 2, wherein the insulating material includes a resin material. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the insulating material includes an inorganic material.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the protective layer further includes a conductive agent, and the content of the conductive agent in the second region is greater than the content of the conductive agent in the first region.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the length of the portion without a mixture layer formed in the length direction of the positive electrode is 0.5 mm or more and 5.0 mm or less.Configuration 7: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the length of the second region in the length direction of the positive electrode is 0.5 mm or more and 5.0 mm or less.Configuration 8: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein the thickness of the protective layer is 50% or less of the maximum thickness of the positive electrode mixture layer. Configuration 9: The nonaqueous electrolyte secondary battery according to any one of configurations 1 to 8, wherein the protective layer has a thickness of 5 μm or more and 25 μm or less.

[0065] REFERENCE SIGNS LIST 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 11A winding start end, 11B winding start end, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18 insulating plate, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode core, 31 positive electrode mixture layer, 31A winding start end, 31B winding start end, 32 mixture layer non-forming portion, 33 protective layer, 34 first region, 35 second region, 40 negative electrode core, 41 negative electrode mixture layer, 42 outer periphery exposed portion.

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound lengthwise with a separator interposed therebetween, wherein the positive electrode has a positive electrode core, and a positive electrode mixture layer and a protective layer formed on the positive electrode core, a mixture layer-free portion where the positive electrode mixture layer is not formed is provided at the winding start end of the positive electrode, the protective layer contains an insulating material as a main component, and has a first region covering the mixture layer-free portion and a second region disposed between the positive electrode core and the positive electrode mixture layer, and the volume resistivity of the protective layer in the second region is smaller than the volume resistivity of the protective layer in the first region.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the volume resistivity of the protective layer in the second region is equal to or lower than the volume resistivity of the positive electrode mixture layer.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the insulating material includes a resin material.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the insulating material includes an inorganic material.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the protective layer further contains a conductive agent, and the content of the conductive agent in the second region is greater than the content of the conductive agent in the first region.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the length of the portion of the positive electrode where the mixture layer is not formed is 0.5 mm or more and 5.0 mm or less in the longitudinal direction of the positive electrode.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the length of the second region in the longitudinal direction of the positive electrode is 0.5 mm or more and 5.0 mm or less.

8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thickness of said protective layer is 50% or less of the maximum thickness of said positive electrode mixture layer.

9. The nonaqueous electrolyte secondary battery according to claim 1, wherein the protective layer has a thickness of 5 μm or more and 25 μm or less.

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