Non-aqueous electrolyte secondary battery

Abstract

Provided is a non-aqueous electrolyte secondary battery comprising an electrode assembly in which a band-shaped positive electrode (11) and a band-shaped negative electrode are wound along the length direction with a separator therebetween, wherein the positive electrode (11) has a positive electrode core (30), and a positive electrode mixture layer (31) and a protective layer (33) formed on the positive electrode core (30), a non-mixture layer portion (32) in which the positive electrode mixture layer (31) is not formed is disposed at a winding-starting end portion (11B) of the positive electrode (11), the protective layer (33) contains an insulating material as a main component and is formed from the winding-starting end (11A) of the positive electrode (11) toward a winding-ending side to cover the non-mixture layer portion (32), and a portion of the protective layer (33) is disposed between the positive electrode core (30) and the positive electrode mixture layer (31).

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 region in which the positive electrode core is exposed and no positive electrode mixture layer is formed is provided at the end of the positive electrode in the width direction, and a protective layer containing a resin material or the like is provided in part of this region.

[0003] Japanese Patent Application Laid-Open No. 2020-167067

[0004] As the capacity of non-aqueous electrolyte secondary batteries has increased in recent years, 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 when the battery is repeatedly charged and discharged, which may cause deformation of the negative electrode.

[0005] As a result of investigations 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 not forming a positive electrode mixture layer at the winding start end of the positive electrode. On the other hand, it has also become clear that when the above-mentioned structure is adopted, peeling of the positive electrode mixture layer may occur at the winding start end of the positive electrode mixture layer.

[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, the winding start end of the positive electrode having a mixture layer non-forming portion where the positive electrode mixture layer is not formed, the protective layer containing an insulating material as a main component and formed from the winding start end of the positive electrode toward the winding end side so as to cover the mixture layer non-forming portion, and a portion of the protective layer being provided between the positive electrode core and the winding start end of the positive electrode mixture layer.

[0007] According to the nonaqueous electrolyte secondary battery of one aspect of the present disclosure, it is possible to suppress deformation of the negative electrode near the winding start end of the positive electrode, while suppressing peeling of the positive electrode mixture layer.

[0008] Fig. 2 is an axial cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention; Fig. 3 is a plan view of the winding start side of the outer surface of a rolled positive electrode in a developed state according to an embodiment of the present invention; Fig. 4 is a cross-sectional view of the positive electrode according to another embodiment of the present invention taken along line AA in Fig. 2;

[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 studies 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 if a large step is formed at the winding start end 11A of the positive electrode 11, stress is 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] The portion 32 where no mixture layer is formed is covered with a protective layer 33 mainly composed of an insulating material. The positive electrode core 30 has a lower resistance than the positive electrode mixture layer 31. Therefore, contact between the portion 32 where no mixture layer is formed and the negative electrode 12 causes an internal short circuit. Therefore, by covering the portion 32 where no mixture layer is formed with the protective layer 33 mainly composed of an insulating material, contact between the portion 32 where no mixture layer is formed and the negative electrode 12 is prevented, thereby preventing an internal short circuit. The protective layer 33 is formed from the winding start end 11A of the positive electrode 11 toward the winding end, covering the entire portion 32 where no mixture layer is formed. This further prevents an internal short circuit.

[0033] Furthermore, a portion of the protective layer 33 is provided between the positive electrode core 30 and the positive electrode mixture layer 31. More specifically, a portion of the protective layer 33 is provided between the positive electrode core 30 and the winding start end 31B of the positive electrode mixture layer 31. 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. This improves adhesion between the positive electrode core 30 and the positive electrode mixture layer 31, and prevents peeling of the positive electrode mixture layer 31 at the winding start end 31B of the positive electrode mixture layer 31. In other words, if a portion of the protective layer 33 is not provided between the positive electrode core 30 and the positive electrode mixture layer 31, peeling of the positive electrode mixture layer 31 is more likely to occur at the winding start end 31B of the positive electrode mixture layer 31. In particular, when the positive electrode 11 has the mixture layer non-forming portion 32 at the winding start end 11A, peeling of the winding start end 31B of the positive electrode mixture layer 31 is more likely to occur.

[0034] Furthermore, 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 likely to be 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 be concentrated on the negative electrode 12 facing the winding start end 31A of the positive electrode mixture layer 31, which may cause deformation of the negative electrode 12.

[0035] In the longitudinal direction of the positive electrode 11, the length of the region of the protective layer 33 provided between the positive electrode core 30 and the positive electrode mixture layer 31 (hereinafter referred to as "region A") is preferably 0.5 mm or more, and more preferably 1.0 mm or more. By making the length of region A 0.5 mm or more, peeling of the positive electrode mixture layer 31 at the winding start end 31B of the positive electrode mixture layer 31 can be more reliably suppressed. Furthermore, the length of region A is preferably 5.0 mm or less, and more preferably 4.5 mm or less. By making the length of region A 5.0 mm or less, low resistance of the positive electrode 11 can be achieved. Therefore, in the longitudinal direction of the positive electrode 11, the length of region A 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.

[0036] 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. Alternatively, the protective layer 33 may be composed essentially of the insulating material alone.

[0037] 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. When the insulating material includes a resin material, peeling of the positive electrode mixture layer 31 at the winding start end 31B of the positive electrode mixture layer 31 can be further suppressed.

[0038] 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.

[0039] 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 (M２／ｎ O.Al ２ O ３ xSiO ２ ・yH ２ 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.

[0040] 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.

[0041] 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.

[0042] The protective layer 33 may further contain a conductive agent in addition to the insulating material. 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.

[0043] The thickness of the protective layer 33 may be, for example, 3 μm or more and 40 μm or less, or 5 μm or more and 30 μm or less, from the viewpoint of suppressing internal short circuits and suppressing peeling of the positive electrode mixture layer 31. 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 direction of the positive electrode 11.

[0044] 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 and, if necessary, an inorganic material, 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, the positive electrode mixture slurry is applied to overlap a portion of the protective layer 33 while forming the mixture layer non-forming portion 32, and 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. The mixture layer non-forming portion 32 and the protective layer 33 may be formed at the winding end portion 11B of the positive electrode 11 in addition to the winding start end portion 11B.

[0045] As described above, the positive electrode 11 of this embodiment has a mixture layer-free portion 32 at the winding start end 11B of the positive electrode 11, where the positive electrode mixture layer 31 is not formed on the positive electrode core 30. The mixture layer-free portion 32 is covered with a protective layer 33 mainly composed of an insulating material. This prevents deformation of the negative electrode 12 near the winding start end 11A of the positive electrode 11, even when repeated charge and discharge cycles are performed. On the other hand, if the mixture layer-free portion 32 is provided at the winding start end 11B of the positive electrode 11, peeling of the positive electrode mixture layer 31 at the winding start end 31B of the positive electrode 11 is likely to occur. Therefore, a portion of the protective layer 33 is interposed between the positive electrode core 30 and the positive electrode mixture layer 31. This prevents peeling of the positive electrode mixture layer 31 at the winding start end 31B of the positive electrode mixture layer 31. That is, the positive electrode 11 having the configuration of this embodiment can suppress deformation of the negative electrode 12 near the winding start end 11A of the positive electrode 11 while suppressing peeling of the positive electrode mixture layer 31.

[0046] Next, a modified example of the positive electrode 11 will be described with reference to Fig. 4. Fig. 4 is a view of the modified positive electrode 11 corresponding to Fig. 3.

[0047] 4 , the positive electrode mixture layer 31 of this embodiment has an inclined portion 34 in which the thickness of the positive electrode mixture layer 31 decreases toward the winding start end 31A of the positive electrode mixture layer 31. By providing the inclined portion 34, it is possible to reduce the step formed at the winding start end 31A of the positive electrode mixture layer 31. This further alleviates stress concentration on the negative electrode 12, and further suppresses deformation of the negative electrode 12, even when charge and discharge are repeated.

[0048] The length of the inclined portion 34 is, for example, 0.5 mm to 5 mm, or may be 1 mm to 3 mm in the longitudinal direction of the positive electrode 11. In this case, peeling of the positive electrode mixture layer 31 can be suppressed, and deformation of the negative electrode 12 at the winding start side of the positive electrode 11 can be further suppressed.

[0049] The inclined portion 34 can be formed, for example, by decreasing the amount of applied positive electrode mixture slurry toward the winding start end 31A of the positive electrode mixture layer 31.

[0050] 4, the inclined portion 34 is provided on both sides of the positive electrode mixture layer 31, but the inclined portion 34 may be provided on only one side of the positive electrode mixture layer 31. Furthermore, in the example shown in FIG. 4, the thickness of the positive electrode mixture layer 31 decreases linearly toward the winding start end 31A of the positive electrode mixture layer 31, but the thickness may decrease nonlinearly toward the winding start end 31A of the positive electrode mixture layer 31. Furthermore, the inclined portion 34 may be provided on the winding end side of the positive electrode mixture layer 31 in addition to the winding start side of the positive electrode mixture layer 31.

[0051] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0052] 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.

[0053] [Preparation of Slurry for Protective Layer] A slurry for the protective layer was prepared by adding an appropriate amount of NMP to polyvinylidene fluoride (PVDF) as a resin material so that the solid content concentration was 30% by mass.

[0054] [Preparation of Positive Electrode] The above-mentioned protective layer slurry was applied to a portion of both sides of a positive electrode core made of aluminum foil having 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 above-mentioned positive electrode mixture slurry was applied to both sides of the positive electrode core so as to overlap a portion of the protective layer while forming a mixture layer non-forming portion, and the coating was dried. After that, it was cut into a predetermined electrode size and rolled using a roller to prepare a positive electrode. The length of the region of the protective layer provided between the positive electrode core and the positive electrode mixture layer was 2.5 mm.

[0055] [Evaluation of peeling of positive electrode mixture layer] The prepared positive electrode was bent 180° along the outer peripheral surface of a round bar with a diameter of 3 mm. Then, after performing the bending test five times, the surface of the positive electrode was visually inspected to evaluate the presence or absence of peeling of the end of the positive electrode mixture layer. Note that when the positive electrode was bent along the outer peripheral surface of the round bar, the end of the positive electrode mixture layer was in contact with the outer peripheral surface of the round bar.

[0056] Example 2 In preparing a slurry for a protective layer, alumina (α-Al) having an average particle size of 0.7 μm was used as an inorganic material. ２ O ３ A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the protective layer slurry was prepared by mixing the cellulose acylate particles and polyvinylidene fluoride (PVDF) as a resin material in a solid content mass ratio of 83.3:16.7, and then adding an appropriate amount of NMP so that the solid content concentration was 30 mass %.

[0057] Example 3 A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that in fabricating the positive electrode, the positive electrode mixture slurry was applied to the winding start side of the positive electrode mixture layer so as to form an inclined portion as shown in Fig. 4. The length of the inclined portion in the longitudinal direction of the positive electrode was 3.0 mm.

[0058] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the protective layer slurry was not applied in the preparation of the positive electrode mixture layer. That is, in the positive electrode of Comparative Example 1, the portion where the mixture layer was not formed was not covered with a protective layer, and the positive electrode core was exposed.

[0059] Table 1 shows the evaluation results of Examples 1 to 3 and Comparative Example 1. Table 1 also shows the material that constitutes the protective layer and whether or not there is an inclined portion.

[0060]

[0061] As shown in Table 1, the positive electrodes of Examples 1 to 3 did not experience peeling of the positive electrode mixture layer, but the positive electrode of Comparative Example 1 did experience peeling of the positive electrode mixture layer. Therefore, it can be said that peeling of the positive electrode mixture layer can be suppressed by covering the portion where the mixture layer is not formed with a protective layer and providing a portion of this protective layer between the positive electrode core and the positive electrode mixture layer. Furthermore, because the positive electrodes of Examples 1 to 3 and Comparative Example 1 have a portion where the mixture layer is not formed, batteries incorporating these positive electrodes are prevented from deforming the negative electrode near the winding start end of the positive electrode, even when repeatedly charged and discharged.

[0062] 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 the winding start end of the positive electrode, the protective layer contains an insulating material as a main component and is formed from the winding start end toward the winding end of the positive electrode so as to cover the mixture layer-free portion, and a portion of the protective layer is provided between the positive electrode core and the positive electrode mixture layer. Aspect 2: The nonaqueous electrolyte secondary battery according to Aspect 1, in which the insulating material includes a resin material. Aspect 3: The nonaqueous electrolyte secondary battery according to Aspect 1 or 2, in which the insulating material includes an inorganic material. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the positive electrode mixture layer has an inclined portion in which the thickness of the positive electrode mixture layer decreases toward the winding start end of the positive electrode mixture layer.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the length of the portion without the mixture layer in the longitudinal direction of the positive electrode is 0.5 mm or more and 5.0 mm or less.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the length of a region of the protective layer provided between the positive electrode core and the positive electrode mixture layer in the longitudinal 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 thickness of the protective layer is 50% or less of the maximum thickness of the positive electrode mixture layer.Configuration 8: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein the thickness of the protective layer is 5 μm or more and 25 μm or less.

[0063] 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, 32B winding start end, 32 mixture layer non-forming portion, 33 protective layer, 34 inclined portion, 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 its main component and is formed from the winding start end toward the winding end of the positive electrode so as to cover the mixture layer-free portion, and a portion of the protective layer is provided between the positive electrode core and the positive electrode mixture layer.

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

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

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode mixture layer has a sloped portion in which the thickness of the positive electrode mixture layer decreases toward the winding start end of the positive electrode mixture layer.

5. 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.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the length of a region of the protective layer provided between the positive electrode core and the positive electrode mixture layer in the longitudinal direction of the positive electrode is 0.5 mm or more and 5.0 mm or less.

7. 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.

8. 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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