Secondary battery

Abstract

Provided is a secondary battery capable of suppressing the occurrence of wrinkles and cracks in protective layers.　The secondary battery is characterized in that: a positive electrode includes a positive electrode core material and a positive electrode mixture layer disposed on the positive electrode core material; a positive electrode core material exposed part where the positive electrode mixture layer is not disposed is provided at one-side end portion of the positive electrode core material; protective layers containing a water-insoluble polymer and an inorganic filler are disposed on the positive electrode core material exposed part; the protective layers include a first protective layer disposed in a region, in the positive electrode core material exposed part, that is opposite to a negative electrode with a separator interposed therebetween, and include a second protective layer disposed in a region, in the positive electrode core material exposed part, that is located farther from the positive electrode mixture layer as compared to the region where the first protective layer is disposed; the mass ratio of the inorganic filler in the first protective layer is greater than the mass ratio of the inorganic filler in the second protective layer; and the mass ratio of the water-insoluble polymer in the second protective layer is greater than the mass ratio of the water-insoluble polymer in the first protective layer.

Description

secondary battery

[0001] The present disclosure relates to secondary battery technology.

[0002] The secondary battery includes, for example, a wound electrode body in which a separator is disposed between a positive electrode having a positive electrode mixture layer disposed on a positive electrode core material and a negative electrode having a negative electrode mixture layer disposed on a negative electrode core material, and the positive and negative electrodes are wound together while being insulated by the separator; or a stacked electrode body in which a separator is disposed between the positive electrode and the negative electrode, and the positive and negative electrodes are stacked together while being insulated by the separator.

[0003] In secondary batteries such as lithium-ion secondary batteries, the negative electrode mixture layer is designed to be larger than the positive electrode mixture layer so that lithium ions released from the positive electrode active material during charging can be smoothly absorbed into the negative electrode active material. That is, many secondary batteries have a structure in which an exposed portion of the positive electrode core material, where the positive electrode mixture layer is not disposed, faces the negative electrode with the separator interposed therebetween.

[0004] In a case where the exposed portion of the positive electrode core material, on which no positive electrode mixture layer is disposed, faces the negative electrode via the separator, if the battery is dropped, vibrated, or the like, a part of the negative electrode facing the exposed portion of the positive electrode core material or negative electrode active material peeled off from the negative electrode may break through the separator, causing contact between the negative electrode and the exposed portion of the positive electrode core material, which may cause an internal short circuit and cause the secondary battery to generate heat.

[0005] For example, Patent Documents 1 to 4 disclose techniques for suppressing the occurrence of internal short circuits in secondary batteries by providing a protective layer on the exposed portion of the positive electrode core material that faces the negative electrode via the separator.

[0006] JP 2007-95656 A JP 2007-103356 A JP 2012-234822 A JP 2021-184397 A

[0007] However, conventional protective layers are hard and have low flexibility, and therefore, for example, during the battery fabrication process after the protective layer is formed, wrinkles and cracks may occur in the protective layer, which may reduce the insulating properties of the protective layer.

[0008] Therefore, an object of the present disclosure is to provide a secondary battery that can suppress the occurrence of wrinkles and cracks in the protective layer.

[0009] a first protective layer disposed in a region of the exposed portion facing the negative electrode with the separator interposed therebetween; and a second protective layer disposed in a region of the exposed portion that is farther from the positive electrode mixture layer than the region where the first protective layer is disposed. The mass ratio of the inorganic filler in the first protective layer is greater than the mass ratio of the inorganic filler in the second protective layer, and the mass ratio of the water-insoluble polymer in the second protective layer is greater than the mass ratio of the water-insoluble polymer in the first protective layer.

[0010] According to one aspect of the present disclosure, it is possible to provide a secondary battery that can suppress the occurrence of wrinkles and cracks in the protective layer.

[0011] 1 is a cross-sectional view showing an example of the configuration of a secondary battery according to an embodiment; FIG. 2 is a partially enlarged cross-sectional view of an electrode body constituting the secondary battery according to the present embodiment; FIG. 3 is a schematic plan view showing a positive electrode before winding; and FIG. 4 is a schematic cross-sectional view of the positive electrode taken along line AA in FIG.

[0012] The drawings referred to in the following description of the embodiments are schematic, and the dimensional ratios of the components depicted in the drawings may differ from those of the actual components.

[0013] Fig. 1 is a cross-sectional view showing an example of the configuration of a secondary battery according to an embodiment. The secondary battery 10 shown in Fig. 1 includes an electrode assembly 14 having a positive electrode 11, a negative electrode 12, and a separator 13 disposed between the positive electrode 11 and the negative electrode 12. This electrode assembly 14 is a wound type electrode assembly in which the positive electrode 11 and the negative electrode 12 are wound with the separator 13 interposed therebetween. Note that this embodiment is not limited to a wound type electrode assembly, and may also be a stacked type electrode assembly in which a separator is disposed between the positive electrode and the negative electrode and these are stacked.

[0014] The positive electrode 11 includes a positive electrode core material 15 and positive electrode mixture layers 16 formed on both sides of the positive electrode core material 15. A positive electrode core material exposed portion 27 where the positive electrode mixture layer 16 is not disposed is formed on one side end of the positive electrode core material 15 on the side of one end 26 in the winding axis direction of the electrode body 14. A protective layer (not shown in FIG. 1 ), which will be described later, is disposed on the positive electrode core material exposed portion 27.

[0015] The negative electrode 12 includes a negative electrode core material 17 and a negative electrode mixture layer 18 formed on both sides of the negative electrode core material 17. A negative electrode core material exposed portion 30 where the negative electrode mixture layer 18 is not disposed is formed at one end of the negative electrode core material 17 on the side of the other end 29 in the winding axis direction of the electrode body 14.

[0016] The secondary battery 10 also includes a battery case 19, a negative electrode current collector 22, a sealing plate 24, an insulating gasket 25, and a positive electrode current collector 28. The electrode assembly 14 is housed in the battery case 19 together with an electrolyte (not shown). The battery case 19 is a substantially cylindrical member with one end closed by a bottom 20 and the other end open. The bottom 20 has a through-hole 21 in its center. The through-hole 21 is sealed by a protrusion 23 of the negative electrode current collector 22, and the edge of the through-hole 21 and the surface of the protrusion 23 are joined to each other by seam welding or the like. The protrusion 23 of the negative electrode current collector 22 also serves as a negative electrode terminal. The other end of the battery case 19 is sealed by a sealing plate 24 and an insulating gasket 25 attached to the periphery of the sealing plate 24.

[0017] The positive electrode core exposed portion 27 is electrically connected to the sealing plate 24, which is also used as a positive electrode terminal, via a positive electrode current collector plate 28 that is arranged on one end 26 side of the winding axis direction of the electrode body 14 inside the battery case 19. On the other hand, the negative electrode core exposed portion 30 is electrically connected to the negative electrode current collector plate 22 that is arranged on the other end 29 side of the winding axis direction of the electrode body 14 inside the battery case 19.

[0018] The positive electrode core material exposed portion 27 is bent toward the center of the electrode body 14, and a tip portion 31 of the bent positive electrode core material exposed portion 27 is joined to the positive electrode current collector plate 28. When the positive electrode core material exposed portion 27 is bent toward the center of the electrode body 14, a part of the tip portion 31 of the positive electrode core material exposed portion 27 on the outer peripheral surface side of the positive electrode core material 15, which is the outer peripheral side of the winding, becomes a joint with the positive electrode current collector plate 28. On the other hand, the tip portion 31 of the positive electrode core material exposed portion 27 on the inner peripheral surface side of the positive electrode core material 15, which is the inner peripheral side of the winding, faces one end 26 of the electrode body 14 in the winding axis direction. Similarly, the negative electrode core material exposed portion 30 is bent toward the center of the electrode body 14, and a tip portion 32 of the bent negative electrode core material exposed portion 30 is joined to the negative electrode current collector plate 22. 1 has a so-called tabless structure in which current is collected directly from the positive electrode core material 15 and the negative electrode core material 17. However, the secondary battery 10 of this embodiment is not limited to the tabless structure, and may have a structure in which leads are joined to the positive electrode core material 15 and the negative electrode core material 17 and current is collected via the leads. Note that in the tabless structure, the tip of the positive electrode core material exposed portion 27 may be joined to the positive electrode current collector plate 28 and the tip of the negative electrode core material exposed portion 30 may be joined to the negative electrode current collector plate 22 without bending the positive electrode core material exposed portion 27 and the negative electrode core material exposed portion 30.

[0019] 2 is a partially enlarged cross-sectional view of an electrode body constituting a secondary battery according to this embodiment. As shown in FIG. 2 , the positive electrode core exposed portion 27 disposed at one end of the positive electrode core 15 includes a positive electrode core exposed portion 27a on one surface of the positive electrode core 15 and a positive electrode core exposed portion 27b on the other surface of the positive electrode core 15. Hereinafter, one surface of the positive electrode core 15 will be referred to as the inner circumferential surface of the positive electrode core 15, which is the inner circumferential surface of the positive electrode core 15, and the positive electrode core exposed portion 27a on one surface of the positive electrode core 15 will be referred to as the inner circumferential surface-side positive electrode core exposed portion 27a. Furthermore, the other surface of the positive electrode core 15 will be referred to as the outer circumferential surface of the positive electrode core 15, which is the outer circumferential surface of the positive electrode core 15, and the positive electrode core exposed portion 27b on the other surface of the positive electrode core 15 will be referred to as the outer circumferential surface-side positive electrode core exposed portion 27b. The inner peripheral surface of the positive electrode core material 15 is the surface of the positive electrode core material 15 that is located radially inward of the wound positive electrode 11 when the positive electrode 11 is wound, and the outer peripheral surface of the positive electrode core material 15 is the surface of the positive electrode core material 15 that is located radially outward of the wound positive electrode 11.

[0020] 2, a protective layer 40 is disposed on each of the positive electrode core material exposed portion 27a on the inner peripheral surface side and the positive electrode core material exposed portion 27b on the outer peripheral surface side. The protective layer 40 contains a water-insoluble polymer and an inorganic filler, and has a first protective layer 42 and a second protective layer 44. The volume resistivity of the protective layer 40 (the volume resistivity of each of the first protective layer 42 and the second protective layer 44) is set to, for example, 10 ７ It is preferable that the resistance is Ω·cm or more, and 10 ９ It is more preferable that the resistivity is Ω·cm or more.

[0021] The first protective layer 42 is disposed in a region of the positive electrode core material exposed portion (27a, 27b) that faces the negative electrode 12 with the separator 13 interposed therebetween. The second protective layer 44 is disposed alongside the first protective layer 42 in a region of the positive electrode core material exposed portion (27a, 27b) that is located farther from the positive electrode mixture layer 16 than the region where the first protective layer 42 is disposed. The first protective layer 42 and the second protective layer 44 are preferably adjacent to each other, but may be disposed with a gap therebetween. The mass ratio of the inorganic filler in the first protective layer 42 is greater than the mass ratio of the inorganic filler in the second protective layer 44, and the mass ratio of the water-insoluble polymer in the second protective layer 44 is greater than the mass ratio of the water-insoluble polymer in the first protective layer 42.

[0022] Conventionally, protective layers have a hard single-layer structure containing a large amount of inorganic filler to ensure high insulation. During secondary battery fabrication, the positive electrode mixture layer and protective layer are rolled using a rolling roller or the like. This rolling can cause wrinkles in the protective layer, which is composed of a hard single layer, resulting in a decrease in the insulating properties of the protective layer. However, in the present embodiment, a second protective layer 44 is provided on the outside of the first protective layer 42. The second protective layer 44 has a lower inorganic filler ratio and a higher water-insoluble polymer ratio, making it more flexible than the first protective layer 42. This allows the second protective layer 44 to mitigate stress applied to the first protective layer 42 during rolling. As a result, it is possible to prevent wrinkles from forming in the protective layer 40 during rolling. Furthermore, the positive electrode core material exposed portion 27 may be folded to connect the positive electrode current collector plate 28. However, in the past, folding the protective layer 40 at the folding position could cause cracks in the protective layer, resulting in a decrease in the insulating properties of the protective layer. On the other hand, in the present embodiment, for example, by positioning the folding position at the highly flexible second protective layer 44, it is possible to suppress the occurrence of cracks in the protective layer 40. Note that by positioning the folding position at the second protective layer 44, the second protective layer 44 remains at the tip 31 of the folded positive electrode core material exposed portion 27, and therefore the second protective layer 44 can protect the surface of the tip 31 that faces the one end 26 of the electrode body 14 in the winding axis direction from contact. For example, when the secondary battery 10 is charged and discharged, the negative electrode 12 may protrude from the one end 26 of the electrode body 14 and come into contact with the tip 31. However, as described above, by positioning the folding position at the second protective layer 44, the second protective layer 44 may be able to protect the negative electrode 12 from contact with the tip 31.

[0023] Examples of inorganic fillers contained in the protective layer 40 include oxides such as alumina, silica, zirconia, titania, magnesia, ceria, yttria, zinc oxide, iron oxide, barium titanium oxide, and alumina-silica composite oxide; nitrides such as silicon nitride, titanium nitride, boron nitride, and aluminum nitride; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate; covalently bonded crystals such as silicon and diamond; silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, boehmite, apatite, mullite, spinel, and olivine; and compounds containing at least one of these. Examples of inorganic fillers include SnO ２ The particles may be made electrically insulating by treating the surfaces of conductive particles such as oxides such as tin-indium oxide (ITO), carbonaceous materials such as carbon black and graphite with an electrically insulating material (for example, the material of the inorganic filler).

[0024] The water-insoluble polymer contained in the protective layer 40 is preferably one that dissolves at 20°C in less than 1 part by mass per 100 parts by mass of water, for example. Examples include fluorine-containing polymers such as polyvinylidene fluoride (PVDF), copolymers of vinylidene fluoride and hexafluoropropylene, copolymers of ethylene and vinyl alcohol, polyacrylonitrile, polyphosphazene, polysiloxane, polyvinyl acetate, polymethyl methacrylate, polystyrene, polycarbonate, polyamide, polyimide, polyamideimide, crosslinked polymers of cellulose and chitosan pyrrolidone carboxylate, chitin or chitosan derivatives, etc. Chitosan derivatives include polymer compounds obtained by glycerylating chitosan and crosslinked chitosan. Among these, the water-insoluble polymer preferably includes a fluorine-containing polymer, and particularly preferably includes polyvinylidene fluoride (PVDF), for example, in terms of electrochemical stability.

[0025] The mass ratio of the water-insoluble polymer in the first protective layer 42 is preferably 2% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 25% by mass or less. The mass ratio of the water-insoluble polymer in the second protective layer 44 is preferably 50% by mass or more and 100% by mass or less, and more preferably 75% by mass or more and 100% by mass or less. When the mass ratio of the water-insoluble polymer satisfies the above range, the insulating properties of the first protective layer 42 may be further improved, and the flexibility of the second protective layer 44 may be further improved. As a result, internal short circuits due to contact between the negative electrode 12 and the exposed positive electrode core material portion 27 may be less likely to occur, and the occurrence of wrinkles and cracks in the protective layer 40 during the battery fabrication process may be further suppressed. The first protective layer 42 and the second protective layer 44 may contain components other than the water-insoluble polymer and the inorganic filler.

[0026] FIG. 3 is a schematic plan view showing the positive electrode before winding. FIG. 4 is a schematic cross-sectional view of the positive electrode taken along line A-A in FIG. 3. The thicknesses of the first protective layer 42 and the second protective layer 44 are preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 40 μm or less, in terms of insulation and flexibility. Furthermore, the width (W1) of the first protective layer 42 and the width (W2) of the second protective layer 44 are preferably 2 mm or more, and more preferably 2 mm or more and 5 mm or less, in terms of reducing the likelihood of an internal short circuit due to contact between the negative electrode 12 and the exposed positive electrode core material portion 27 (27a, 27b). The width (W1) of the first protective layer 42 and the width (W2) of the second protective layer 44 are the lengths of the first protective layer 42 and the second protective layer 44 in the direction in which they are arranged.

[0027] When bending the positive electrode core material exposed portion 27 to join it to the positive electrode current collector plate 28, it is preferable to bend it between the first protective layer 42 and the second protective layer 44 and join the bent tip end 31 to the positive electrode current collector plate 28. When the first protective layer 42 and the second protective layer 44 are adjacent to each other, it is preferable to bend it at the boundary between the first protective layer 42 and the second protective layer 44. By positioning the bending position between (or at) the first protective layer 42 and the second protective layer 44, it is easy to bend the positive electrode core material exposed portion 27 and it is also possible to further suppress the occurrence of cracks in the protective layer 40. Furthermore, because the second protective layer 44 remains at the tip end 31 of the positive electrode core material exposed portion 27, even if the negative electrode 12 protrudes from one end 26 of the electrode body 14 due to, for example, charge and discharge of the secondary battery 10, the second protective layer 44 may be able to protect the negative electrode 12 from contact with the tip end 31.

[0028] In the case of a wound electrode body 14, the ratio (B / A) of the total width (B) of the first protective layer 42 and the second protective layer 44 arranged on the outer peripheral surface of the positive electrode core material exposed portion 27b to the total width (A) of the first protective layer 42 and the second protective layer 44 arranged on the inner peripheral surface of the positive electrode core material exposed portion 27a is preferably 0.3 or more and 0.6 or less. Setting B / A within the above range ensures a large space in the outer peripheral surface of the positive electrode core material exposed portion 27b where no protective layer 40 is arranged, facilitating bonding of this space to the positive electrode current collector 28 and improving the bonding strength and electronic conductivity between the positive electrode 11 and the positive electrode current collector 28. When B / A satisfies the above range, when bending the positive electrode core material exposed portion 27 toward the center of the electrode body 14, it is preferable to bend the positive electrode core material exposed portion 27 toward the center between (or at the boundary between) the first protective layer 42 and the second protective layer 44 arranged on the inner peripheral surface of the positive electrode core material exposed portion 27a. This suppresses the occurrence of cracks in the protective layer 40 disposed in the positive electrode core material exposed portion 27a on the inner circumferential surface side, thereby further suppressing the occurrence of internal short circuits due to contact with the negative electrode 12. Meanwhile, on the side of the positive electrode core material exposed portion 27b on the outer circumferential surface side, any location of the second protective layer 44 may become a bending position, but because the second protective layer 44 is flexible as described above, the occurrence of cracks in the protective layer 40 due to bending is minimized.

[0029] The materials used for the positive electrode 11, the negative electrode 12, the separator 13, and the electrolyte will be described below.

[0030] [Positive Electrode] The positive electrode core material 15 may be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface. The positive electrode mixture layer 16 includes, for example, a positive electrode active material, a binder, a conductive material, etc. The protective layer 40 is as described above, and therefore a description thereof will be omitted.

[0031] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M: at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3). These may be used alone or in combination of two or more. In terms of increasing the capacity of the secondary battery 10, the positive electrode active material is preferably Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z It is preferable that the lithium-nickel composite oxide contains a lithium-nickel composite oxide such as (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B; 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3).

[0032] Examples of the conductive material include carbon particles such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more.

[0033] Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, carboxymethyl cellulose (CMC) or salts thereof (CMC-Na, CMC-K, CMC-NH4, etc., or partially neutralized salts), polyethylene oxide (PEO), etc. These may be used alone or in combination of two or more.

[0034] An example of a method for manufacturing the positive electrode 11 will be described. First, a slurry for a positive electrode mixture layer containing a positive electrode active material, a binder, and a conductive material is prepared. A slurry for a first protective layer and a slurry for a second protective layer containing a water-insoluble polymer and an inorganic filler are also prepared. Here, the mass ratio of the inorganic filler in the slurry for the first protective layer is greater than the mass ratio of the inorganic filler in the slurry for the second protective layer, and the mass ratio of the water-insoluble polymer in the slurry for the second protective layer is greater than the mass ratio of the water-insoluble polymer in the slurry for the first protective layer. The slurry for the positive electrode mixture layer is then applied to one or both sides of the positive electrode core material 15, excluding the positive electrode core material exposed portion 27 at one end of the positive electrode core material 15, and the slurry for the first protective layer and the slurry for the second protective layer are applied in parallel to the uncoated positive electrode core material exposed portion 27. The order in which these slurries are applied is not particularly limited. For example, these slurries may be applied simultaneously, or the positive electrode mixture layer slurry may be applied first, followed by the first and second protective layer slurries. The coating of the positive electrode mixture layer 16 and the coating of the protective layer 40 applied to the positive electrode core material 15 are dried and then rolled using a rolling roller or the like. This allows the positive electrode 11 of this embodiment to be obtained. As described above, in this embodiment, the second protective layer 44, which has a lower inorganic filler ratio and a higher water-insoluble polymer ratio and is more flexible than the first protective layer 42, is provided on the outer side of the first protective layer 42. Therefore, the second protective layer 44 can alleviate stress applied to the first protective layer 42 during rolling, thereby preventing wrinkles from forming in the protective layer 40 during rolling. Furthermore, conventionally, when a slurry containing a large amount of inorganic filler is applied to the positive electrode core material 15, the edges of the coating film drip, resulting in a thinner edge of the formed protective layer 40, which may result in insufficient insulation at the edges of the protective layer 40. However, since the second protective layer slurry has a high proportion of water-insoluble polymer and high viscosity, as in the present embodiment, by applying the second protective layer slurry to the outside of the coating film made of the first protective layer slurry to form a coating film of the second protective layer slurry, dripping at the edges of the coating film made of the first protective layer 42 can be suppressed. Furthermore, the edges of the coating film made of the highly viscous second protective layer slurry are less likely to drip.Therefore, the thickness of the end portions of the obtained protective layer 40 (the end portions of the first protective layer 42 and the second protective layer 44) is unlikely to become thin, and sufficient insulation properties can be ensured.

[0035] [Negative Electrode] A foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film having such a metal disposed on the surface layer, can be used for the negative electrode core material 17. The negative electrode mixture layer 18 includes, for example, a negative electrode active material, a binder, etc.

[0036] Examples of the negative electrode active material include metallic lithium, lithium alloys such as lithium-aluminum alloys, lithium-lead alloys, lithium-silicon alloys, and lithium-tin alloys, carbon materials such as graphite, coke, and organic sintered bodies, and SnO ２ , SnO, TiO ２ These may be used alone or in combination of two or more.

[0037] As in the case of the positive electrode 11, examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, carboxymethyl cellulose (CMC) or salts thereof (CMC-Na, CMC-K, CMC-NH, etc., or partially neutralized salts), polyethylene oxide (PEO), etc. These may be used alone or in combination of two or more.

[0038] An example of a method for manufacturing the negative electrode 12 will be described. First, a negative electrode mixture layer slurry containing a negative electrode active material, a binder, and the like is applied to one or both surfaces of the negative electrode core material 17, excluding an exposed portion at one end of the negative electrode core material 17. After the negative electrode mixture layer slurry is applied, the coating of the negative electrode mixture layer 18 is dried and rolled using a rolling roller or the like. In this way, the negative electrode 12 can be obtained.

[0039] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0040] [Electrolyte] The electrolyte has, for example, ion conductivity (for example, lithium ion conductivity). The electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0041] The liquid electrolyte (electrolytic solution) contains, for example, 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

[0042] Furthermore, examples of the solid electrolyte that can be used include solid or gel-like polymer electrolytes, inorganic solid electrolytes, and the like. 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. For example, a polymer material that absorbs a non-aqueous solvent and gels is used as the matrix polymer. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. For example, the inorganic solid electrolyte can be a material known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, and the like). While the above-exemplified electrolytes are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may be an aqueous electrolyte.

[0043] The present disclosure will be further described by the following embodiments. Configuration 1: A secondary battery including an electrode assembly having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode includes a positive electrode core material and a positive electrode mixture layer disposed on the positive electrode core material, one side end of the positive electrode core material has an exposed portion where the positive electrode mixture layer is not disposed, and a protective layer including a water-insoluble polymer and an inorganic filler is disposed on the exposed portion, the protective layer having a first protective layer disposed in a region of the exposed portion that faces the negative electrode with the separator interposed therebetween, and a second protective layer disposed in a region of the exposed portion that is farther from the positive electrode mixture layer than the region where the first protective layer is disposed, wherein a mass ratio of the inorganic filler in the first protective layer is greater than a mass ratio of the inorganic filler in the second protective layer, and a mass ratio of the water-insoluble polymer in the second protective layer is greater than a mass ratio of the water-insoluble polymer in the first protective layer. Configuration 2: The secondary battery according to Configuration 1, wherein the mass ratio of the water-insoluble polymer in the first protective layer is 2 mass% or more and 30 mass% or less, and the mass ratio of the water-insoluble polymer in the second protective layer is 50 mass% or more and 100 mass% or less.Configuration 3: The secondary battery according to Configuration 1 or 2, wherein the thicknesses of the first protective layer and the second protective layer are each 5 μm or more and 50 μm or less.Configuration 4: The secondary battery according to any one of Configurations 1 to 3, wherein the widths of the first protective layer and the second protective layer are each 2 mm or more.Configuration 5: The secondary battery according to any one of Configurations 1 to 4, wherein the exposed portion is folded between the first protective layer and the second protective layer, and the folded tip is connected to a positive electrode current collector plate. Configuration 6: The secondary battery according to any one of Configurations 1 to 5, wherein the electrode body is a wound electrode body in which the positive electrode and the negative electrode are wound with the separator interposed therebetween, and a ratio (A / B) of a total width (B) of the first protective layer and the second protective layer disposed on the exposed portion provided on the outer peripheral surface side of the positive electrode core material, which is the inner peripheral side of the winding, to a total width (A) of the first protective layer and the second protective layer disposed on the exposed portion provided on the inner peripheral surface side of the positive electrode core material, which is the inner peripheral side of the winding, is 0.3 or more and 0.6 or less.

[0044] REFERENCE SIGNS LIST 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 positive electrode core material, 16 positive electrode mixture layer, 17 negative electrode core material, 18 negative electrode mixture layer, 19 battery case, 20 bottom, 21 through hole, 22 negative electrode current collector plate, 23 convex portion, 24 sealing plate, 25 insulating gasket, 26 one end, 27, 27a, 27b positive electrode core material exposed portion, 28 positive electrode current collector plate, 29 other end, 30 negative electrode core material exposed portion, 31 tip portion, 32 tip portion, 40 protective layer, 42 first protective layer, 44 second protective layer.

Claims

1. A secondary battery comprising an electrode assembly having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode includes a positive electrode core material and a positive electrode mixture layer disposed on the positive electrode core material, one side end of the positive electrode core material has an exposed portion where the positive electrode mixture layer is not disposed, and a protective layer containing a water-insoluble polymer and an inorganic filler is disposed on the exposed portion, the protective layer having a first protective layer disposed in a region of the exposed portion that faces the negative electrode via the separator, and a second protective layer disposed in a region of the exposed portion that is farther from the positive electrode mixture layer than the region where the first protective layer is disposed, wherein a mass ratio of the inorganic filler in the first protective layer is greater than a mass ratio of the inorganic filler in the second protective layer, and a mass ratio of the water-insoluble polymer in the second protective layer is greater than a mass ratio of the water-insoluble polymer in the first protective layer.

2. A secondary battery as described in claim 1, wherein the mass ratio of the water-insoluble polymer in the first protective layer is 2 mass% or more and 30 mass% or less, and the mass ratio of the water-insoluble polymer in the second protective layer is 50 mass% or more and 100 mass% or less.

3. The secondary battery according to claim 1 or 2, wherein the thickness of the first protective layer and the second protective layer is 5 μm or more and 50 μm or less, respectively.

4. The secondary battery according to claim 1 or 2, wherein the width of each of the first protective layer and the second protective layer is 2 mm or more.

5. The secondary battery according to claim 1 or 2, wherein the exposed portion is folded between the first protective layer and the second protective layer, and the folded tip is connected to the positive electrode current collector plate.

6. The secondary battery according to claim 1 or 2, wherein the electrode body is a wound type electrode body in which the positive electrode and the negative electrode are wound with the separator interposed therebetween, and a ratio (A / B) of a total width (B) of the first protective layer and the second protective layer disposed on the exposed portion provided on the outer peripheral surface side of the positive electrode core material which is the outer peripheral side of the winding to a total width (A) of the first protective layer and the second protective layer disposed on the exposed portion provided on the inner peripheral surface side of the positive electrode core material which is the inner peripheral side of the winding is 0.3 or more and 0.6 or less.