Nonaqueous secondary battery and method for manufacturing the same

Abstract

To provide a non-aqueous secondary battery and a method for manufacturing thereof that can reduce the resistance difference between a mixture layer located on the outer circumferential side and a mixture layer located on the inner circumferential side in the curved portion of an electrode body.SOLUTION: A non-aqueous secondary battery includes an electrode body in which a positive electrode plate and a negative electrode plate 24 are wound in a laminated state with a separator interposed therebetween, and the negative electrode plate 24 includes a negative electrode base material 25, a first negative electrode mixture layer 26A located on the outer peripheral side with respect to the negative electrode base material 25, and a second negative electrode mixture layer 26B located on the inner peripheral side with respect to the negative electrode base material 25, and the BET specific surface area of the negative electrode active material 26C included in the second negative electrode mixture layer 26B is larger than the BET specific surface area of the negative electrode active material 26C contained in the first negative electrode mixture layer 26A.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a nonaqueous secondary battery and a method for manufacturing a nonaqueous secondary battery. [Background technology]

[0002] Electric vehicles and hybrid vehicles are equipped with non-aqueous secondary batteries as their power sources. A lithium ion secondary battery, which is an example of a non-aqueous secondary battery, includes an electrode assembly in which a positive electrode plate, a negative electrode plate, and a separator are stacked and wound (for example, Patent Document 1).

[0003] 13, the electrode body 50 is a wound body in which a positive electrode plate 51 and a negative electrode plate 54 are wound in a stacked state with a separator 57 interposed therebetween. The electrode body 50 has a curved portion 50A in which each layer constituting the electrode body 50 is curved. The positive electrode plate 51 has a positive electrode substrate 52, and positive electrode mixture layers 53A, 53B coated on both sides of the positive electrode substrate 52. The negative electrode plate 54 has a negative electrode substrate 55, and negative electrode mixture layers 56A, 56B coated on both sides of the negative electrode substrate 55. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2011 / 074098 Summary of the Invention [Problem to be solved by the invention]

[0005] As shown in FIG. 14, in the curved portion 50A, the negative electrode mixture layers 56A and 56B of the negative electrode plate 54 have a higher density in the negative electrode mixture layer 56B located on the inner periphery side than in the negative electrode mixture layer 56A located on the outer periphery side. In this case, the negative electrode mixture layer 56B is more likely to have a large resistance than the negative electrode mixture layer 56A because the nonaqueous electrolyte is less likely to penetrate between the negative electrode active material 56C compared to the negative electrode mixture layer 56A. As a result, the negative electrode mixture layer 56B is in a state in which the lithium deposition resistance is deteriorated (lithium is more likely to deposit) compared to the negative electrode mixture layer 56A. The problem of the resistance difference caused by the density difference between the negative electrode mixture layer 56A located on the outer periphery side and the negative electrode mixture layer 56B located on the inner periphery side described above may also occur between the positive electrode mixture layer 53A located on the outer periphery side and the positive electrode mixture layer 53B located on the inner periphery side. [Means for solving the problem]

[0006] A nonaqueous secondary battery for solving the above problems includes an electrode body in which a positive electrode plate and a negative electrode plate are stacked with a separator interposed between them and wound, at least one of the positive electrode plate and the negative electrode plate includes a substrate, a first mixture layer located on the outer periphery of the substrate, and a second mixture layer located on the inner periphery of the substrate, and a BET specific surface area of ​​an active material contained in the second mixture layer is larger than a BET specific surface area of ​​an active material contained in the first mixture layer.

[0007] In the above nonaqueous secondary battery, it is preferable that the electrode body has a curved portion in which each layer constituting the electrode body is curved, and in the curved portion, the density of the second mixture layer is higher than the density of the first mixture layer.

[0008] A method for manufacturing a nonaqueous secondary battery to solve the above problems includes a step of manufacturing a positive electrode plate, a step of manufacturing a negative electrode plate, and a step of manufacturing an electrode body by winding the positive electrode plate and the negative electrode plate in a stacked state with a separator interposed therebetween, wherein at least one of the steps of manufacturing the positive electrode plate and the negative electrode plate includes a step of forming a first mixture layer on a first surface of a substrate, and a step of forming a second mixture layer on a second surface of the substrate opposite to the first surface, wherein in the step of forming the second mixture layer, the second mixture layer is formed on the second surface such that a BET specific surface area of ​​an active material contained in the second mixture layer is larger than a BET specific surface area of ​​an active material contained in the first mixture layer, and in the step of manufacturing the electrode body, the substrate is wound such that the first mixture layer is located on the outer periphery side of the substrate and the second mixture layer is located on the inner periphery side of the substrate.

[0009] According to the above configuration or manufacturing method, in terms of density difference, the second mixture layer located in the curved portion tends to have a higher resistance because it retains less nonaqueous electrolyte than the first mixture layer located in the curved portion. On the other hand, in terms of the BET specific surface area of ​​the active material, the second mixture layer located in the curved portion tends to have a lower resistance because it has a larger reaction area contributing to charging and discharging than the first mixture layer located in the curved portion. Therefore, by making the BET specific surface area of ​​the active material contained in the second mixture layer larger than the BET specific surface area of ​​the active material contained in the first mixture layer, the difference between the resistance of the first mixture layer located in the curved portion and the resistance of the second mixture layer located in the curved portion can be reduced.

[0010] In the method for manufacturing a non-aqueous secondary battery, the step of forming the first mixture layer includes a step of applying a first mixture paste to the first surface and a step of drying the first mixture paste, and the step of forming the second mixture layer includes a step of applying a second mixture paste to the second surface and a step of drying the second mixture paste, and the drying speed of the second mixture paste may be slower than the drying speed of the first mixture paste. When the mixture paste is dried, the binder contained in the mixture paste is unevenly distributed on the surface of the mixture paste due to migration caused by heat. Since the surface side of the mixture layer is a part where the charge and discharge reaction of the battery is more likely to occur than the inside (substrate side) of the mixture layer, the more the migration of the binder progresses, the smaller the BET specific surface area of ​​the active material contained in the mixture layer becomes. In addition, the rate of progress of the migration of the binder has a positive correlation with the drying speed of the mixture paste. Therefore, by making the drying speed of the second mixture paste slower than that of the first mixture paste, the second mixture layer is in a state where the migration of the binder is suppressed compared to the first mixture layer, and as a result, the BET specific surface area of ​​the active material contained in the second mixture layer can be made larger than the BET specific surface area of ​​the active material contained in the first mixture layer.

[0011] In the method for manufacturing a nonaqueous secondary battery, the step of forming the second mixture layer may be performed after the step of forming the first mixture layer. By performing the step of forming the second mixture layer after the step of forming the first mixture layer, when the second mixture layer is formed, the heat capacity is increased by the amount of the first mixture layer that has already been formed. This allows the drying speed of the second mixture paste to be slower than the drying speed of the first mixture paste.

[0012] In the method for manufacturing a nonaqueous secondary battery, the raw material of the second mixture layer may be an active material having a larger BET specific surface area than the active material used as the raw material of the first mixture layer. By changing the specific surface areas of the active materials used as raw materials for the first and second mixture layers, the BET specific surface area of ​​the active material contained in the second mixture layer can be made larger than the BET specific surface area of ​​the active material contained in the first mixture layer.

[0013] In the method for manufacturing a nonaqueous secondary battery, the step of forming the first mixture layer includes a step of kneading a first mixture paste containing raw materials for the first mixture layer and a step of diluting the kneaded first mixture paste, and the step of forming the second mixture layer includes a step of kneading a second mixture paste containing raw materials for the second mixture layer and a step of diluting the kneaded second mixture paste, and the solid content of the second mixture paste in the step of kneading the second mixture paste may be lower than the solid content of the first mixture paste in the step of kneading the first mixture paste. The higher the solid content when kneading the mixture paste, the more uniform the raw materials can be mixed together, but the effective specific surface area of ​​the active material is reduced by covering the active material raw material with a binder. Therefore, by making the solid content when the second mixture paste is kneaded lower than the solid content when the first mixture paste is kneaded, the BET specific surface area of ​​the active material contained in the second mixture layer can be made larger than the BET specific surface area of ​​the active material contained in the first mixture layer.

[0014] In the method for manufacturing a nonaqueous secondary battery, the step of forming the first mixture layer includes a step of adjusting the thickness of the first mixture layer by pressing the first mixture layer, and the step of forming the second mixture layer includes a step of adjusting the thickness of the second mixture layer by pressing the second mixture layer, and the amount of pressing the second mixture layer may be greater than the amount of pressing the first mixture layer. The greater the amount of pressing the mixture layer, the greater the BET specific surface area of ​​the active material contained in the mixture layer due to the active material cracking or the like and the formation of a new surface. Therefore, by making the amount of pressing the second mixture layer greater than the amount of pressing the first mixture layer, the BET specific surface area of ​​the active material contained in the second mixture layer can be made greater than the BET specific surface area of ​​the active material contained in the first mixture layer.

[0015] In the above-mentioned method for manufacturing a nonaqueous secondary battery, the step of forming the first mixture layer includes a step of applying a first mixture paste containing raw materials of the first mixture layer to the first surface and a step of drying the first mixture paste, and the step of forming the second mixture layer includes a step of applying a second mixture paste containing raw materials of the second mixture layer to the second surface and a step of drying the second mixture paste, and in the step of applying the second mixture paste, the second mixture paste may be applied so that the basis weight of the second mixture paste is greater than the basis weight of the first mixture paste. By making the basis weight of the second mixture paste greater than the basis weight of the first mixture paste, the amount of pressure applied to the second mixture layer can be greater than the amount of pressure applied to the first mixture layer.

[0016] In the above-described method for manufacturing a nonaqueous secondary battery, the step of forming the first mixture layer includes a step of kneading a first mixture paste containing raw materials for the first mixture layer, a step of diluting the kneaded first mixture paste, a step of applying the diluted first mixture paste to the first surface, and a step of drying the first mixture paste; the step of forming the second mixture layer includes a step of kneading a second mixture paste containing raw materials for the second mixture layer, a step of diluting the kneaded second mixture paste, a step of applying the diluted second mixture paste to the second surface, and a step of drying the second mixture paste; and in the step of diluting the second mixture paste, the second mixture paste may be diluted so that a solid content of the second mixture paste is higher than a solid content of the first mixture paste after it is diluted. When the first and second mixture pastes have the same basis weight, the second mixture layer, which has a higher solid content rate after dilution, tends to have a larger thickness before being pressed. Therefore, the amount of pressure applied to the second mixture layer can be made larger than the amount of pressure applied to the first mixture layer.

[0017] In the method for producing a nonaqueous secondary battery, the raw material of the second mixture layer may have a lower tap density than the raw material of the first mixture layer. When the first mixture layer and the second mixture layer have the same basis weight, the second mixture layer, which has the active material used as the raw material with a lower tap density, tends to have a larger thickness before being pressed. Therefore, the pressing amount of the second mixture layer can be made larger than the pressing amount of the first mixture layer. Effect of the Invention

[0018] According to the present invention, in the curved portion of the electrode body, the difference in resistance between the mixture layer located on the outer periphery side and the mixture layer located on the inner periphery side can be reduced. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a perspective view of a lithium ion secondary battery. [Diagram 2] FIG. 2 is a perspective view showing the electrode body in an expanded state. [Diagram 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is an enlarged cross-sectional view showing a curved portion of the electrode body. [Diagram 5] FIG. 5 is an enlarged cross-sectional view showing a negative electrode plate located at a curved portion of the electrode body. [Figure 6] FIG. 6 is a graph that illustrates the relationship between the density of a negative electrode material mixture layer and the critical current value at which lithium deposition occurs in a negative electrode material mixture layer having a predetermined BET specific surface area. [Figure 7] FIG. 7 is a flowchart showing the steps of manufacturing the negative electrode mixture paste. [Figure 8] FIG. 8 is a flowchart showing the manufacturing process of the negative electrode plate. [Figure 9] FIG. 9 is a flowchart showing the assembly process of a lithium ion secondary battery. [Figure 10] FIG. 10 is a graph showing the change in the BET specific surface area of ​​the negative electrode plate in the manufacturing process of a lithium ion secondary battery. [Figure 11] FIG. 11 is a cross-sectional view that illustrates the state of the negative electrode active material and the negative electrode binder when the drying conditions are changed between the first negative electrode mixture layer and the second negative electrode mixture layer. [Figure 12] FIG. 12 is a cross-sectional view that typically shows a state in which the negative electrode plate shown in FIG. 11 is wound. [Figure 13] FIG. 13 is an enlarged cross-sectional view showing a curved portion of an electrode body in a conventional lithium ion secondary battery. [Figure 14] FIG. 14 is an enlarged cross-sectional view of the negative electrode plate of the electrode assembly shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, one embodiment of the present invention will be described with reference to FIGS. [Lithium-ion secondary battery] As shown in FIG. 1, a lithium ion secondary battery 10, which is an example of a non-aqueous secondary battery, includes a case 11 and an electrode assembly 20. The case 11 includes a storage section 11A and a lid 12. The storage section 11A has a flat, bottomed, rectangular outer shape with an opening on the upper side. The storage section 11A stores the electrode assembly 20 and a non-aqueous electrolyte. The lid 12 closes the opening of the storage section 11A. The case 11 forms a sealed rectangular parallelepiped battery container by attaching the lid 12 to the storage section 11A. The case 11 is made of a metal such as aluminum or an aluminum alloy.

[0021] The lid 12 is provided with a positive external terminal 13A and a negative external terminal 13B. The external terminals 13A and 13B are used for charging and discharging power. The positive electrode side current collector 20A, which is the end of the electrode body 20 on the positive electrode side, is electrically connected to the positive electrode external terminal 13A via the positive electrode side current collector 14A. The negative electrode side current collector 20B, which is the end of the electrode body 20 on the negative electrode side, is electrically connected to the negative electrode external terminal 13B via the negative electrode side current collector 14B. The lid 12 also includes an inlet 15 for injecting a nonaqueous electrolyte. The shapes of the external terminals 13A and 13B are not limited to those shown in FIG. 1 and may be any shape.

[0022] [Electrode body] As shown in FIG. 2, the electrode body 20 is a flat wound body formed by winding a laminate in which a long positive electrode plate 21 and a negative electrode plate 24 are stacked with a separator 27 interposed therebetween. The positive electrode plate 21, the negative electrode plate 24, and the separator 27 are stacked so that their respective longitudinal directions coincide with the longitudinal direction D1. In the laminate before winding, the positive electrode plate 21, the separator 27, the negative electrode plate 24, and the separator 27 are stacked in this order in the stacking direction D3 (see FIG. 3). The electrode body 20 has a structure in which the positive electrode plate 21 and the negative electrode plate 24 stacked with the separator 27 sandwiched therebetween are wound around a winding axis L1 extending along the width direction D2 of the band shape.

[0023] The electrode body 20 includes a flat portion 31, an upper curved portion 32, and a lower curved portion 33. The flat portion 31 includes a pair of flat surfaces 31S facing in opposite directions. The upper curved portion 32 is located at the upper portion of the flat portion 31. The upper curved portion 32 has a shape that bulges upward from the upper end of the flat portion 31. The lower curved portion 33 is located at the lower portion of the flat portion 31. The lower curved portion 33 has a shape that bulges downward from the lower end of the flat portion 31. The electrode body 20 is housed in the case 11 with the winding axis L1 extending parallel to the bottom surface of the housing portion 11A so that the lower curved portion 33 is located on the bottom surface side of the housing portion 11A and the upper curved portion 32 is located on the lid body 12 side. The upper curved portion 32 and the lower curved portion 33 are each an example of a curved portion included in the electrode body 20.

[0024] [Positive plate] As shown in Fig. 3, the positive electrode plate 21 is a positive electrode side electrode plate including a positive electrode substrate 22 and a positive electrode mixture layer 23. The positive electrode substrate 22 is a foil-like member formed in an elongated shape. The positive electrode mixture layer 23 is provided on each of two surfaces of the positive electrode substrate 22 facing in opposite directions. The positive electrode substrate 22 includes a positive electrode side uncoated portion 22A at one end in the width direction D2 where the positive electrode mixture layer 23 is not formed and the positive electrode substrate 22 is exposed.

[0025] A metal foil made of aluminum or an alloy mainly composed of aluminum is used for the positive electrode substrate 22. In the positive electrode substrate 22, the positive electrode side uncoated portion 22A is provided such that, in the wound body state, opposing portions are pressed against each other to form the positive electrode side current collecting portion 20A.

[0026] The positive electrode mixture layer 23 is a hardened body of a liquid positive electrode mixture paste. The positive electrode mixture paste includes a positive electrode active material, a positive electrode solvent, a positive electrode conductive agent, and a positive electrode binder. The positive electrode mixture layer 23 is formed by drying the positive electrode mixture paste and evaporating the positive electrode solvent. Therefore, the positive electrode mixture layer 23 includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder.

[0027] The positive electrode active material is a lithium-containing composite metal oxide capable of absorbing and releasing lithium ions, which are charge carriers in the lithium-ion secondary battery 10. The lithium-containing composite oxide is an oxide containing lithium and a metal element other than lithium. The metal element other than lithium is at least one selected from the group consisting of nickel, cobalt, manganese, vanadium, magnesium, molybdenum, niobium, titanium, tungsten, aluminum, and iron contained in the lithium-containing composite oxide as iron phosphate.

[0028] For example, the lithium-containing composite oxide is lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or lithium manganese oxide (LiMn2O4). For example, the lithium-containing composite oxide is a ternary lithium-containing composite oxide (NCM) containing nickel, cobalt, and manganese, which is lithium nickel cobalt manganese oxide (LiNiCoMnO2). For example, the lithium-containing composite oxide is lithium iron phosphate (LiFePO4).

[0029] The positive electrode solvent is an NMP (N-methyl-2-pyrrolidone) solution, which is an example of an organic solvent. The positive electrode conductive agent is, for example, carbon black such as acetylene black (AB) or Ketjen black, carbon fiber such as carbon nanotube (CNT) or carbon nanofiber, or graphite. The positive electrode binder is an example of a resin component contained in the positive electrode mixture paste. The positive electrode binder is, for example, at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), and styrene butadiene rubber (SBR). The mass ratio of the positive electrode binder is, for example, 0.1 mass% or more and 20 mass% or less with respect to the mass of the positive electrode mixture layer 23.

[0030] The positive electrode plate 21 may have an insulating layer at the boundary between the positive electrode uncoated portion 22A and the positive electrode mixture layer 23. The insulating layer contains an inorganic component having insulating properties and a resin component functioning as a binder. The inorganic component is at least one selected from the group consisting of powdered boehmite, titania, and alumina. The resin component is at least one selected from the group consisting of PVDF, PVA, and acrylic.

[0031] [Negative plate] The negative electrode plate 24 is a negative electrode side plate including a negative electrode substrate 25 and a negative electrode mixture layer 26. The negative electrode substrate 25 is a foil-like member formed in an elongated shape. The negative electrode mixture layer 26 is provided on each of two surfaces of the negative electrode substrate 25 that face in opposite directions. The negative electrode substrate 25 includes a negative electrode side uncoated portion 25A where the negative electrode mixture layer 26 is not formed and the negative electrode substrate 25 is exposed at one end in the width direction D2, which is located opposite to the positive electrode side uncoated portion 22A.

[0032] A metal foil made of copper or an alloy mainly composed of copper is used for the negative electrode substrate 25. In the negative electrode uncoated portion 25A, when wound, opposing portions are pressed against each other to form the negative electrode current collecting portion 20B.

[0033] The negative electrode mixture layer 26 is a hardened body of the liquid negative electrode mixture paste. The negative electrode mixture paste includes a negative electrode active material 26C (see FIG. 5), a negative electrode solvent, a negative electrode conductor, a negative electrode thickener, and a negative electrode binder 26D (see FIG. 11). The negative electrode mixture layer 26 is formed by drying the negative electrode mixture paste and evaporating the negative electrode solvent. Therefore, the negative electrode mixture layer 26 includes the negative electrode active material 26C, the negative electrode conductor, the negative electrode thickener, and the negative electrode binder 26D.

[0034] The negative electrode active material 26C is a material capable of absorbing and releasing lithium ions. For example, carbon materials such as graphite, non-graphitizable carbon, easily graphitizable carbon, and carbon nanotubes are used as the negative electrode active material 26C. The negative electrode active material 26C may be composite particles in which graphite particles are coated with an amorphous carbon layer.

[0035] The negative electrode solvent is, for example, water. The negative electrode conductive agent may be, for example, the same as the positive electrode conductive agent. The negative electrode thickener may be, for example, carboxymethyl cellulose (CMC). The mass ratio of the negative electrode thickener is, for example, 0.1 mass% or more and 20 mass% or less with respect to the mass of the negative electrode mixture layer 26. The negative electrode binder 26D is at least one selected from the group consisting of PVDF, PVA, and SBR. The mass ratio of the negative electrode binder 26D is, for example, 0.1 mass% or more and 20 mass% or less with respect to the mass of the negative electrode mixture layer 26.

[0036] [Separator] The separator 27 prevents contact between the positive electrode plate 21 and the negative electrode plate 24, and retains a nonaqueous electrolyte between the positive electrode plate 21 and the negative electrode plate 24. When the electrode body 20 is immersed in a nonaqueous electrolyte, the nonaqueous electrolyte permeates the separator 27 from the ends toward the center.

[0037] The separator 27 is a nonwoven fabric made of polypropylene, etc. As the separator 27, for example, a porous polymer membrane such as a porous polyethylene membrane, a porous polyolefin membrane, a porous polyvinyl chloride membrane, an ion-conductive polymer electrolyte membrane, etc. can be used.

[0038] [Nonaqueous electrolyte] The non-aqueous electrolyte is a composition in which a supporting salt is contained in a non-aqueous solvent. The non-aqueous solvent is, for example, one or more materials selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The supporting salt is, for example, one or more lithium compounds (lithium salts) selected from LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, etc.

[0039] In this embodiment, ethylene carbonate is used as the non-aqueous solvent. Lithium bis(oxalato)borate (LiBOB) is added to the non-aqueous electrolyte as a lithium salt as an additive. For example, LiBOB is added to the non-aqueous electrolyte so that the concentration of LiBOB in the non-aqueous electrolyte is 0.001 to 0.1 [mol / L].

[0040] [Configuration of curved section] 4, the upper curved portion 32 of the electrode body 20 has a structure in which each layer constituting the electrode body 20 is curved. The lower curved portion 33 has a structure in which the upper curved portion 32 is inverted upside down.

[0041] The positive electrode substrate 22 has a first surface 22B and a second surface 22C. The first surface 22B and the second surface 22C are two surfaces facing in directions opposite to each other, and are surfaces on which the positive electrode mixture layer 23 is provided. The first surface 22B faces the outer periphery side in the direction in which the electrode body 20 is wound. The second surface 22C faces the inner periphery side in the direction in which the electrode body 20 is wound.

[0042] Of the positive electrode mixture layers 23 provided on the first surface 22B and the second surface 22C, the positive electrode mixture layer 23 provided on the first surface 22B is the first positive electrode mixture layer 23A. The positive electrode mixture layer 23 provided on the second surface 22C is the second positive electrode mixture layer 23B. In the following, when there is no need to distinguish between the first positive electrode mixture layer 23A and the second positive electrode mixture layer 23B, they are simply referred to as the positive electrode mixture layer 23.

[0043] The negative electrode substrate 25 has a first surface 25B and a second surface 25C. The first surface 25B and the second surface 25C are two surfaces facing in directions opposite to each other, and are surfaces on which the negative electrode mixture layer 26 is provided. The first surface 25B faces the outer periphery side in the direction in which the electrode body 20 is wound. The second surface 25C faces the inner periphery side in the direction in which the electrode body 20 is wound.

[0044] Of the negative electrode mixture layers 26 provided on the first surface 25B and the second surface 25C, the negative electrode mixture layer 26 provided on the first surface 25B is the first negative electrode mixture layer 26A. The negative electrode mixture layer 26 provided on the second surface 25C is the second negative electrode mixture layer 26B. In the following description, when there is no need to distinguish between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B, they are simply referred to as the negative electrode mixture layer 26.

[0045] The first positive electrode mixture layer 23A is an example of a first mixture layer located on the outer periphery side with respect to the positive electrode substrate 22. The first negative electrode mixture layer 26A is an example of a first mixture layer located on the outer periphery side with respect to the negative electrode substrate 25. The second positive electrode mixture layer 23B is an example of a second mixture layer located on the inner periphery side with respect to the positive electrode substrate 22. The second negative electrode mixture layer 26B is an example of a second mixture layer located on the inner periphery side with respect to the negative electrode substrate 25.

[0046] 5, in the upper curved portion 32 of the electrode body 20, the second negative electrode mixture layer 26B has a smaller distance from the center of the curved shape than the first negative electrode mixture layer 26A. Therefore, the second negative electrode mixture layer 26B is compressed more strongly than the first negative electrode mixture layer 26A when the electrode body 20 is wound, and has a higher density than the first negative electrode mixture layer 26A. Therefore, the second negative electrode mixture layer 26B is more difficult to penetrate by the nonaqueous electrolyte than the first negative electrode mixture layer 26A, and therefore has a higher resistance and a slower rate of accepting lithium ions during charging.

[0047] If the rate at which lithium ions are accepted becomes excessively slow, there is a risk that the lithium ions released from the positive electrode mixture layer 23 will precipitate as metallic lithium on the surface of the negative electrode mixture layer 26. If metallic lithium is precipitated, the amount of lithium ions that contribute to the charge / discharge reaction will decrease, leading to a deterioration in battery performance.

[0048] The density of the first negative electrode mixture layer 26A is, for example, 1.00 g / cm 3 More than 1.40g / cm 3 The density of the second negative electrode mixture layer 26B is, for example, 1.01 g / cm 3 More than 1.43g / cm 3 The density difference between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B is, for example, 0.01 g / cm 3 More than 0.03g / cm 3 The density of the second negative electrode mixture layer 26B is, for example, 1.01 times or more and 1.02 times or less the density of the first negative electrode mixture layer 26A.

[0049] The negative electrode active material 26C contained in the second negative electrode mixture layer 26B has a larger BET specific surface area than the negative electrode active material 26C contained in the first negative electrode mixture layer 26A. The BET specific surface area is a value measured by the BET (Brunauer, Emmett, Teller) method for a sample cut out from a part of the negative electrode mixture layer 26. An example of an adsorption gas used in measuring the BET specific surface area is nitrogen gas. The BET specific surface area represents the size of an effective specific surface area per unit weight. The "effective specific surface area" here refers to the specific surface area of ​​the reaction surface of the negative electrode active material 26C that contributes to the charge / discharge reaction. For example, the area of ​​a portion of the surface of the negative electrode active material 26C that does not contribute to the charge / discharge reaction because the surface is covered with the negative electrode binder 26D is not included in the BET specific surface area.

[0050] Therefore, in the second negative electrode mixture layer 26B, the BET specific surface area of ​​the negative electrode active material 26C is larger than that of the first negative electrode mixture layer 26A, so that the resistance is lower and the rate at which lithium ions are accepted during charging is higher.

[0051] The BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A is, for example, 3.8 cm 2 / g or more 4.2cm 2 The BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B is, for example, 4.3 cm 2 / g or more 4.7cm 2 The difference in the BET specific surface area of ​​the negative electrode active material 26C between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B is, for example, 0.3 cm 2 / g or more 0.7cm 2 / g or less. The BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B is, for example, 1.02 to 1.24 times the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A.

[0052] From the above, in terms of density difference, the second negative electrode mixture layer 26B is likely to have a higher resistance in the upper curved portion 32 than the first negative electrode mixture layer 26A because the amount of nonaqueous electrolyte held therein is smaller. On the other hand, in terms of BET specific surface area, the second negative electrode mixture layer 26B is likely to have a lower resistance in the upper curved portion 32 than the first negative electrode mixture layer 26A because the BET specific surface area of ​​the negative electrode active material 26C is larger. Therefore, by increasing the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B compared to the negative electrode active material 26C contained in the first negative electrode mixture layer 26A, the resistance difference between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B in the upper curved portion 32 can be reduced. That is, the lithium precipitation resistance can be made uniform by reducing the difference in the rate at which lithium ions are accepted between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B located in the upper curved portion 32. Also in the lower curved portion 33, similar to the upper curved portion 32, the difference in resistance and the difference in the rate at which lithium ions are accepted between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B are reduced.

[0053] In addition, similarly to the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B, the positive electrode active material contained in the second positive electrode mixture layer 23B may be configured to have a larger BET specific surface area than the positive electrode active material contained in the first positive electrode mixture layer 23A. The second positive electrode mixture layer 23B located in the upper curved portion 32 has a higher density than the first positive electrode mixture layer 23A, so that the nonaqueous electrolyte is less likely to penetrate therein. Therefore, the second positive electrode mixture layer 23B has a higher resistance and releases lithium ions at a slower rate during charging than the first positive electrode mixture layer 23A.

[0054] Therefore, by increasing the BET specific surface area of ​​the positive electrode active material contained in the second positive electrode mixture layer 23B compared to the positive electrode active material contained in the first positive electrode mixture layer 23A, the resistance difference between the first positive electrode mixture layer 23A and the second positive electrode mixture layer 23B in the upper curved portion 32 can be reduced. That is, the difference in the rate of releasing lithium ions between the first positive electrode mixture layer 23A and the second positive electrode mixture layer 23B located in the upper curved portion 32 is reduced. This makes it possible to make the lithium precipitation resistance in the second negative electrode mixture layer 26B facing the first positive electrode mixture layer 23A and the lithium precipitation resistance in the first negative electrode mixture layer 26A facing the second positive electrode mixture layer 23B more uniform.

[0055] In addition, the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B may have different raw materials or different manufacturing conditions in order to differentiate the BET specific surface areas of the negative electrode active material 26C. Similarly, the first positive electrode mixture layer 23A and the second positive electrode mixture layer 23B may have different raw materials or different manufacturing conditions in order to differentiate the BET specific surface areas of the positive electrode active material.

[0056] [Lithium precipitation resistance] Hereinafter, with reference to FIG. 6, the relationship between the density of the negative electrode mixture layer 26 and the BET specific surface area of ​​the negative electrode active material 26C and the lithium deposition resistance will be described.

[0057] In the graph 40 shown in FIG. 6, the horizontal axis represents the density of the negative electrode mixture layer 26, and the vertical axis represents the critical current value at which metallic lithium precipitates during charging. Note that a higher critical current value indicates a higher resistance to lithium precipitation, and a lower critical current value indicates a lower resistance to lithium precipitation. A straight line 41 in the graph 40 represents the relationship between the critical current value and the density of the negative electrode mixture layer 26 when the BET specific surface area of ​​the negative electrode active material 26C is a first specific surface area. A straight line 42 in the graph 40 represents the relationship between the critical current value and the density of the negative electrode mixture layer 26 when the BET specific surface area of ​​the negative electrode active material 26C is a second specific surface area larger than the first specific surface area.

[0058] As shown by the lines 41 and 42, when the BET specific surface area of ​​the negative electrode active material 26C is the same, the higher the density of the negative electrode mixture layer 26, the lower the critical current value, i.e., the lower the lithium precipitation resistance. When the density of the negative electrode mixture layer 26 is the same, the smaller the BET specific surface area of ​​the negative electrode active material 26C, the lower the critical current value, i.e., the lower the lithium precipitation resistance.

[0059] For example, in the upper curved portion 32, the density of the first negative electrode mixture layer 26A located on the outer periphery side is set to a first density ρ1, and the density of the second negative electrode mixture layer 26B located on the inner periphery side is set to a second density ρ2 (ρ2>ρ1). If the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A is the first specific surface area, the critical current value in the first negative electrode mixture layer 26A is the first current value I1. Note that point 43 in the graph 40 is a point on the straight line 41, and indicates the critical current value when the density of the negative electrode mixture layer 26 is the first density ρ1.

[0060] At this time, when the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B is the first specific surface area, which is the same as that of the first negative electrode mixture layer 26A, the critical current value in the second negative electrode mixture layer 26B becomes the second current value I2 smaller than the first current value I1. That is, the second negative electrode mixture layer 26B is in a state in which the lithium deposition resistance is lower than that of the first negative electrode mixture layer 26A. Note that a point 44 indicated by a dashed line in the graph 40 is a point on the straight line 41, and indicates the critical current value when the density of the negative electrode mixture layer 26 is the second density ρ2.

[0061] Therefore, by making the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B larger than the first specific surface area, the critical current value in the second negative electrode mixture layer 26B can be made closer to the critical current value in the first negative electrode mixture layer 26A. Therefore, when the density of the second negative electrode mixture layer 26B is the second density ρ2, the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B should be determined so that the critical current value in the second negative electrode mixture layer 26B approaches the first current value I1.

[0062] 6 illustrates a case where the critical current value is the first current value I1 when the density of the second negative electrode mixture layer 26B is the second density ρ2 and the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B is the second specific surface area. Point 45 shown in graph 40 is a point on line 42, and indicates the critical current value when the density of the negative electrode mixture layer 26 is the second density ρ2.

[0063] As a reference example, in graph 40, the first density ρ1 is set to 1.174 g / cm 3 , the second density ρ2 is 1.189 g / cm 3 , the first specific surface area is 4.00 cm 2 / g, the difference between the first current value I1 and the second current value I2 is about 11.3 A. In this case, the second specific surface area, which is the target value for reducing the difference between the first current value I1 and the second current value I2, is 4.33 cm 2 / g.

[0064] [Method of manufacturing lithium-ion secondary batteries] The method for manufacturing the lithium ion secondary battery 10 includes steps S1-1 to S1-3 shown in FIG. 7, steps S2-1 to S2-7 shown in FIG. 8, and steps S3-1 to S3-4 shown in FIG.

[0065] 7 are part of the process of manufacturing the negative electrode plate 24, and are processes of manufacturing a negative electrode mixture paste. The process of manufacturing the positive electrode plate 21 includes a process of manufacturing a positive electrode mixture paste. Although the process of manufacturing the positive electrode mixture paste differs from the process of manufacturing the negative electrode mixture paste in terms of raw materials, manufacturing conditions, etc., the basic process flow is the same as that of the process of manufacturing the negative electrode mixture paste, and therefore detailed description thereof will be omitted.

[0066] In the process of producing the negative electrode mixture paste, first, in step S1-1, the raw materials of the negative electrode mixture layer 26 are mixed. Next, in step S1-2, the raw materials are mixed together in a state where the solid content is higher than the solid content of the final negative electrode mixture paste, thereby producing the negative electrode mixture paste. Finally, in step S1-3, the solid content of the negative electrode mixture paste is adjusted by diluting the kneaded negative electrode mixture paste with a negative electrode solvent. Through the above steps, the negative electrode mixture paste is produced.

[0067] In addition, when the first negative electrode mixture paste containing the raw materials of the first negative electrode mixture layer 26A and the second negative electrode mixture paste containing the raw materials of the second negative electrode mixture layer 26B have different raw materials or manufacturing conditions, the first negative electrode mixture paste and the second negative electrode mixture paste are manufactured separately. Similarly, when the first positive electrode mixture paste containing the raw materials of the first positive electrode mixture layer 23A and the second positive electrode mixture paste containing the raw materials of the second positive electrode mixture layer 23B have different raw materials or manufacturing conditions, the first positive electrode mixture paste and the second positive electrode mixture paste are manufactured separately. The first positive electrode mixture paste and the first negative electrode mixture paste are each an example of a first mixture paste. The second positive electrode mixture paste and the second negative electrode mixture paste are each an example of a second mixture paste.

[0068] 8 are part of the process of manufacturing the negative electrode plate 24, and are processes of forming the negative electrode mixture layer 26 on the negative electrode substrate 25 using the negative electrode mixture paste. The process of manufacturing the positive electrode plate 21 includes a process of forming the positive electrode mixture layer 23 on the positive electrode substrate 22. Although the process of forming the positive electrode mixture layer 23 on the positive electrode substrate 22 differs from the process of forming the negative electrode mixture layer 26 on the negative electrode substrate 25 in terms of raw materials, manufacturing conditions, etc., the basic process flow is the same, and therefore detailed description thereof will be omitted.

[0069] In the process of forming the negative electrode mixture layer 26 on the negative electrode substrate 25, first, in step S2-1, the first negative electrode mixture paste is applied to the first surface 25B of the negative electrode substrate 25 so as to form the negative electrode side uncoated portion 25A at both ends in the width direction D2. Next, in step S2-2, the first negative electrode mixture paste is dried to evaporate the negative electrode solvent, thereby forming the first negative electrode mixture layer 26A. At this time, for example, the first negative electrode mixture paste is dried so that dry air is applied to the first negative electrode mixture paste from the side opposite to the negative electrode substrate 25. Then, in step S2-3, the first negative electrode mixture layer 26A is pressed by a press roll to adjust the thickness of the first negative electrode mixture layer 26A.

[0070] Next, in step S2-4, the second negative electrode mixture paste is applied to the second surface 25C of the negative electrode substrate 25 so as to form the negative electrode side uncoated portion 25A at both ends in the width direction D2. Next, in step S2-5, the second negative electrode mixture paste is dried to evaporate the negative electrode solvent, thereby forming the second negative electrode mixture layer 26B. At this time, for example, the second negative electrode mixture paste is dried so that dry air hits the second negative electrode mixture paste from the side opposite to the negative electrode substrate 25. Then, in step S2-6, the second negative electrode mixture layer 26B is pressed by a press roll to adjust the thickness of the second negative electrode mixture layer 26B. Finally, in step S2-7, the negative electrode substrate 25 is cut at the center in the width direction D2. Through the above steps, two negative electrode plates 24 are manufactured at once.

[0071] Steps S3-1 to S3-4 shown in FIG. 9 are steps of manufacturing the electrode assembly 20 using the positive electrode plate 21, the negative electrode plate 24, and the separator 27, and steps of housing the electrode assembly 20 inside the case 11. In the step of manufacturing the electrode assembly 20, first, in step S3-1, the positive electrode plate 21 and the negative electrode plate 24 are stacked with the separator 27 interposed therebetween, then wound, and further pressed flat. At this time, the positive electrode substrate 22 is wound so that the first positive electrode mixture layer 23A is located on the outer periphery side with respect to the positive electrode substrate 22, and the second positive electrode mixture layer 23B is located on the inner periphery side with respect to the positive electrode substrate 22. Similarly, the negative electrode substrate 25 is wound so that the first negative electrode mixture layer 26A is located on the outer periphery side with respect to the negative electrode substrate 25, and the second negative electrode mixture layer 26B is located on the inner periphery side with respect to the negative electrode substrate 25.

[0072] Next, in step S3-2, the positive electrode uncoated portion 22A is pressure-welded to form the positive electrode current collector 20A. Similarly, the negative electrode uncoated portion 25A is pressure-welded to form the negative electrode current collector 20B. Through the above procedure, the electrode body 20 is manufactured.

[0073] Next, in step S3-3, the electrode body 20 is sealed in the case 11. At this time, the positive electrode side current collector 20A is electrically connected to the positive electrode external terminal 13A via the positive electrode side current collector 14A. The negative electrode side current collector 20B is electrically connected to the negative electrode external terminal 13B via the negative electrode side current collector 14B. The top of the housing portion 11A is closed by the lid 12.

[0074] Next, in step S3-4, moisture is removed from the electrode body 20 by a heat treatment, and then a nonaqueous electrolyte is injected into the case 11. Through the above procedure, the lithium ion secondary battery 10 is manufactured.

[0075] [Method for controlling BET specific surface area] 10 to 12, a method for controlling the BET specific surface area of ​​the negative electrode active material 26C contained in the negative electrode mixture layer 26 will be described below. The BET specific surface area of ​​the positive electrode active material contained in the positive electrode mixture layer 23 can be controlled in the same manner as the BET specific surface area of ​​the negative electrode active material 26C contained in the negative electrode mixture layer 26. The BET specific surface area of ​​the negative electrode active material 26C contained in the negative electrode mixture layer 26 changes as the material undergoes a manufacturing process.

[0076] [BET specific surface area in raw material state] 10 represents the BET specific surface area of ​​the negative electrode active material 26C in the state of a raw material. The BET specific surface area of ​​the negative electrode active material 26C in the state of a raw material has a positive correlation with the BET specific surface area of ​​the negative electrode active material 26C contained in the negative electrode mixture layer 26.

[0077] Therefore, for example, the raw material of the second negative electrode mixture layer 26B may be a negative electrode active material 26C having a larger BET specific surface area than the negative electrode active material 26C used as the raw material of the first negative electrode mixture layer 26A. This makes it possible to make the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B larger than the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A.

[0078] [Solid content when kneaded tightly] Point P2 shown in Fig. 10 represents the BET specific surface area of ​​the negative electrode active material 26C in the state of the negative electrode mixture paste after the raw materials are kneaded in step S1-2. When the raw materials are kneaded together in step S1-2, the surface of the negative electrode active material 26C is covered with the negative electrode thickener (CMC) and the negative electrode binder 26D, thereby reducing the BET specific surface area.

[0079] In step S1-2, the higher the solid content of the negative electrode mixture paste when the raw materials are kneaded together, the stronger the raw materials are rubbed together, and therefore the more the negative electrode thickener and negative electrode binder 26D adhere to the surface of the negative electrode active material 26C. That is, the higher the solid content of the negative electrode mixture paste when the raw materials are kneaded together, the greater the decrease in the BET specific surface area of ​​the negative electrode active material 26C caused by kneading.

[0080] Therefore, for example, the solid content rate when the second negative electrode mixture paste is kneaded may be lower than the solid content rate when the first negative electrode mixture paste is kneaded. This makes it possible to reduce the area of ​​the surface of the negative electrode active material 26C covered by the negative electrode binder 26D in the second negative electrode mixture layer 26B. Therefore, the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B can be made larger than the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A.

[0081] [Drying conditions] Point P3 shown in Fig. 10 represents the BET specific surface area of ​​the negative electrode active material 26C in a state after step S2-2 or step S2-5 and after the negative electrode mixture paste is dried. When the negative electrode mixture paste is dried, the negative electrode binder 26D contained in the negative electrode mixture paste is unevenly distributed on the surface of the negative electrode mixture paste (the side opposite to the negative electrode substrate 25) due to migration caused by heat. The rate at which the migration of the negative electrode binder 26D progresses is positively correlated with the drying rate of the negative electrode mixture paste.

[0082] The surface side of the negative electrode mixture layer 26 is a portion where the charge / discharge reaction of the battery is more likely to occur than the inside (negative electrode substrate 25 side) of the negative electrode mixture layer 26. Therefore, as the migration of the negative electrode binder 26D progresses, the BET specific surface area of ​​the negative electrode active material 26C contained in the negative electrode mixture layer 26 becomes smaller.

[0083] 11, for example, the rate at which the second negative electrode mixture paste dries is set lower than the rate at which the first negative electrode mixture paste dries, thereby relatively suppressing the migration of the negative electrode binder 26D in the second negative electrode mixture layer 26B. This makes it possible to reduce the amount of reduction in the BET specific surface area of ​​the negative electrode active material 26C associated with the migration of the negative electrode binder 26D in the second negative electrode mixture layer 26B. Therefore, the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B can be made larger than the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A.

[0084] 12, in the upper curved portion 32 of the electrode body 20 in which the negative electrode plate 24 shown in FIG. 11 is wound, the density of the second negative electrode mixture layer 26B is higher than the density of the first negative electrode mixture layer 26A as described above. On the other hand, since the BET specific surface area of ​​the negative electrode active material 26C of the second negative electrode mixture layer 26B is larger than that of the first negative electrode mixture layer 26A, the lithium deposition resistance of the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B can be made uniform in the upper curved portion 32.

[0085] In step S2-5, the drying speed of the second negative electrode mixture paste is slower by the heat capacity of the first negative electrode mixture layer 26A already formed on the negative electrode substrate 25, compared to step S2-2 in which only the first negative electrode mixture paste is applied onto the negative electrode substrate 25. In other words, the drying speed of the second negative electrode mixture paste can be slowed down by the simple method of performing the step of forming the second negative electrode mixture layer 26B after the step of forming the first negative electrode mixture layer 26A.

[0086] [Pressing conditions] Returning to Fig. 10, point P4 shown in Fig. 10 represents the BET specific surface area of ​​the negative electrode active material 26C in a state after step S2-3 or step S2-6 and after the negative electrode mixture layer 26 is pressed. When the negative electrode active material 26C is pressed in step S2-3 or step S2-6, the surface of the negative electrode active material 26C is cracked, etc., and a new surface is formed, thereby increasing the BET specific surface area. At this time, the greater the amount of pressure applied to the negative electrode mixture layer 26, the greater the increase in the BET specific surface area of ​​the negative electrode active material 26C.

[0087] Therefore, for example, the amount of pressure applied when pressing the second negative electrode mixture layer 26B may be greater than the amount of pressure applied when pressing the first negative electrode mixture layer 26A. This allows the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B to be greater than the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A.

[0088] For example, the second negative electrode mixture paste may be applied so that the basis weight of the second negative electrode mixture paste is greater than the basis weight of the first negative electrode mixture paste. As a result, for example, when the required thicknesses of the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B are equal, the pressing amount of the second negative electrode mixture layer 26B becomes greater than the pressing amount of the first negative electrode mixture layer 26A by the difference in basis weight between the second negative electrode mixture paste and the first negative electrode mixture paste.

[0089] For example, the second negative electrode mixture paste may be diluted so that the solid content rate of the second negative electrode mixture paste after dilution in step S1-3 is higher than that of the first negative electrode mixture paste. When the basis weights of the first negative electrode mixture paste and the second negative electrode mixture paste are the same, the second negative electrode mixture layer 26B, which has a higher solid content rate of the second negative electrode mixture paste after dilution, is more likely to have a larger thickness before being pressed. As a result, for example, when the required thickness values ​​of the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B are the same, the pressing amount of the second negative electrode mixture layer 26B is larger than the pressing amount of the first negative electrode mixture layer 26A by the difference in thickness between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B before pressing.

[0090] For example, a negative electrode active material 26C having a lower tap density than the negative electrode active material 26C serving as the raw material of the first negative electrode mixture layer 26A may be used as the raw material of the second negative electrode mixture layer 26B. When the basis weights of the first negative electrode mixture paste and the second negative electrode mixture paste are the same, the second negative electrode mixture layer 26B containing the negative electrode active material 26C having a lower tap density is more likely to have a larger thickness before being pressed. As a result, for example, when the required thickness values ​​of the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B are the same, the second negative electrode mixture layer 26B is pressed by a larger amount than the first negative electrode mixture layer 26A by the difference in thickness between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B before being pressed.

[0091] The above-mentioned method for controlling the BET specific surface area of ​​the negative electrode active material 26C may be a single method or a combination of multiple methods. When the pressing amount of the second negative electrode mixture layer 26B is made larger than the pressing amount of the first negative electrode mixture layer 26A as a method for controlling the BET specific surface area of ​​the negative electrode active material 26C, the above-mentioned method for controlling the pressing amount may be a single method or a combination of multiple methods.

[0092] [Effects of the embodiment] According to the above embodiment, the following effects can be obtained. (1) The BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B is configured to be larger than the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A. This makes it possible to reduce the resistance difference caused by the density difference between the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B in the upper curved portion 32 and the lower curved portion 33. Therefore, by making uniform the rate at which the first negative electrode mixture layer 26A accepts lithium ions and the rate at which the second negative electrode mixture layer 26B accepts lithium ions during charging, it is possible to make uniform the lithium precipitation resistance of the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B.

[0093] Similarly, if the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B is larger than that of the first positive electrode mixture layer 23A, the resistance difference due to the density difference between the first positive electrode mixture layer 23A and the second positive electrode mixture layer 23B can be reduced in the upper curved portion 32 and the lower curved portion 33. Therefore, the speed at which the first positive electrode mixture layer 23A releases lithium ions and the speed at which the second positive electrode mixture layer 23B releases lithium ions during charging can be made uniform. This makes it possible to uniformize the lithium precipitation resistance of the second negative electrode mixture layer 26B facing the first positive electrode mixture layer 23A and the first negative electrode mixture layer 26A facing the second positive electrode mixture layer 23B.

[0094] (2) By making the drying speed of the second negative electrode mixture paste slower than that of the first negative electrode mixture paste, the second negative electrode mixture layer 26B is in a state where the migration of the negative electrode binder 26D is suppressed compared to that of the first negative electrode mixture layer 26A. As a result, the BET specific surface area of ​​the negative electrode active material 26C contained in the second negative electrode mixture layer 26B can be made larger than the BET specific surface area of ​​the negative electrode active material 26C contained in the first negative electrode mixture layer 26A. In this case, the same negative electrode mixture paste can be used as the first negative electrode mixture paste and the second negative electrode mixture paste, so that the manufacturing cost can be reduced. Similarly, by making the drying speed of the second positive electrode mixture paste slower than that of the first positive electrode mixture paste, the BET specific surface area of ​​the positive electrode active material can be made larger in the second positive electrode mixture layer 23B compared to the first positive electrode mixture layer 23A.

[0095] (3) By performing the step of forming the second negative electrode mixture layer 26B after the step of forming the first negative electrode mixture layer 26A, when the second negative electrode mixture layer 26B is formed, the heat capacity is increased by the amount of the already formed first negative electrode mixture layer 26A. Even by such a simple method, the drying speed of the second negative electrode mixture paste can be made slower than the drying speed of the first negative electrode mixture paste. Similarly, by performing the step of forming the second positive electrode mixture layer 23B after the step of forming the first positive electrode mixture layer 23A, the drying speed of the second positive electrode mixture paste can be made slower than the drying speed of the first positive electrode mixture paste.

[0096] (4) By using, as the raw material of the second negative electrode mixture layer 26B, the negative electrode active material 26C having a larger BET specific surface area than the raw material of the first negative electrode mixture layer 26A, the BET specific surface area of ​​the negative electrode active material 26C can be made larger in the second negative electrode mixture layer 26B than in the first negative electrode mixture layer 26A. Similarly, by using, as the raw material of the second positive electrode mixture layer 23B, a positive electrode active material having a larger BET specific surface area than the raw material of the first positive electrode mixture layer 23A, the BET specific surface area of ​​the positive electrode active material can be made larger in the second positive electrode mixture layer 23B than in the first positive electrode mixture layer 23A.

[0097] (5) By setting the solid content rate when the second negative electrode mixture paste is kneaded lower than the solid content rate when the first negative electrode mixture paste is kneaded, the BET specific surface area of ​​the negative electrode active material 26C can be made larger in the second negative electrode mixture layer 26B than in the first negative electrode mixture layer 26A. Similarly, by setting the solid content rate when the second positive electrode mixture paste is kneaded lower than the solid content rate when the first positive electrode mixture paste is kneaded, the BET specific surface area of ​​the positive electrode active material can be made larger in the second positive electrode mixture layer 23B than in the first positive electrode mixture layer 23A.

[0098] (6) By applying a larger amount of pressure to the second negative electrode mixture layer 26B than to the first negative electrode mixture layer 26A, the BET specific surface area of ​​the negative electrode active material 26C can be made larger in the second negative electrode mixture layer 26B than in the first negative electrode mixture layer 26A. Similarly, by applying a larger amount of pressure to the second positive electrode mixture layer 23B than to the first positive electrode mixture layer 23A, the BET specific surface area of ​​the positive electrode active material can be made larger in the second positive electrode mixture layer 23B than in the first positive electrode mixture layer 23A.

[0099] (7) By applying the second negative electrode mixture paste so that the basis weight of the second negative electrode mixture paste is greater than the basis weight of the first negative electrode mixture paste, the amount of pressure applied to the second negative electrode mixture layer 26B can be greater than the amount of pressure applied to the first negative electrode mixture layer 26A. In this case, the same negative electrode mixture paste can be used as the first negative electrode mixture paste and the second negative electrode mixture paste, so that the manufacturing cost can be reduced. Similarly, by applying the second positive electrode mixture paste so that the basis weight of the second positive electrode mixture paste is greater than the basis weight of the first positive electrode mixture paste, the amount of pressure applied to the second positive electrode mixture layer 23B can be greater than that of the first positive electrode mixture layer 23A.

[0100] (8) By diluting the second negative electrode mixture paste so that the solid content of the second negative electrode mixture paste is higher than the solid content of the first negative electrode mixture paste after dilution, the amount of pressure applied to the second negative electrode mixture layer 26B can be increased compared to the first negative electrode mixture layer 26A. Similarly, by diluting the second positive electrode mixture paste so that the solid content of the second positive electrode mixture paste is higher than the solid content of the first positive electrode mixture paste after dilution, the amount of pressure applied to the second positive electrode mixture layer 23B can be increased compared to the first positive electrode mixture layer 23A.

[0101] (9) By using, as the raw material of the second negative electrode mixture layer 26B, the negative electrode active material 26C having a lower tap density than the raw material of the first negative electrode mixture layer 26A, it is possible to increase the amount of pressure applied to the second negative electrode mixture layer 26B compared to the first negative electrode mixture layer 26A. Similarly, by using, as the raw material of the second positive electrode mixture layer 23B, a positive electrode active material having a lower tap density than the raw material of the first positive electrode mixture layer 23A, it is possible to increase the amount of pressure applied to the second positive electrode mixture layer 23B compared to the first positive electrode mixture layer 23A.

[0102] [Example of change] The above embodiment can be modified as follows: The following modified examples can be combined as long as they are not technically inconsistent.

[0103] The tap density of the negative electrode active material 26C used as the raw material of the second negative electrode mixture layer 26B may be higher than or the same as the tap density of the negative electrode active material 26C used as the raw material of the first negative electrode mixture layer 26A. In this case, the BET specific surface area of ​​the negative electrode active material 26C of the second negative electrode mixture layer 26B may be larger than that of the first negative electrode mixture layer 26A by another method. Similarly, the tap density of the positive electrode active material used as the raw material of the second positive electrode mixture layer 23B may be higher than or the same as the tap density of the positive electrode active material used as the raw material of the first positive electrode mixture layer 23A. In this case, the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B may be larger than that of the first positive electrode mixture layer 23A by another method.

[0104] The solid content rate of the second negative electrode mixture paste after dilution may be lower than or the same as the solid content rate of the first negative electrode mixture paste after dilution. In this case, the BET specific surface area of ​​the negative electrode active material 26C may be larger in the second negative electrode mixture layer 26B than in the first negative electrode mixture layer 26A by another method. Similarly, the solid content rate of the second positive electrode mixture paste after dilution may be lower than or the same as the solid content rate of the first positive electrode mixture paste after dilution. In this case, the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B may be larger than in the first positive electrode mixture layer 23A by another method.

[0105] The basis weight of the second negative electrode mixture paste may be smaller than or equal to the basis weight of the first negative electrode mixture paste. In this case, the second negative electrode mixture layer 26B may be configured to have a larger BET specific surface area of ​​the negative electrode active material 26C than the first negative electrode mixture layer 26A by another method. Similarly, the basis weight of the second positive electrode mixture paste may be smaller than or equal to the basis weight of the first positive electrode mixture paste. In this case, the second positive electrode mixture layer 23B may be configured to have a larger BET specific surface area of ​​the positive electrode active material than the first positive electrode mixture layer 23A by another method.

[0106] The amount of pressure applied to the second negative electrode mixture layer 26B may be smaller than or the same as the amount of pressure applied to the first negative electrode mixture layer 26A. In this case, the BET specific surface area of ​​the negative electrode active material 26C may be larger in the second negative electrode mixture layer 26B than in the first negative electrode mixture layer 26A by another method. Similarly, the amount of pressure applied to the second positive electrode mixture layer 23B may be smaller than or the same as the amount of pressure applied to the first positive electrode mixture layer 23A. In this case, the BET specific surface area of ​​the positive electrode active material in the second positive electrode mixture layer 23B may be larger than in the first positive electrode mixture layer 23A by another method.

[0107] The solid content rate when the second negative electrode mixture paste is kneaded may be higher than or the same as the solid content rate when the first negative electrode mixture paste is kneaded. In this case, the BET specific surface area of ​​the negative electrode active material 26C may be larger in the second negative electrode mixture layer 26B than in the first negative electrode mixture layer 26A by another method. Similarly, the solid content rate when the second positive electrode mixture paste is kneaded may be higher than or the same as the solid content rate when the first positive electrode mixture paste is kneaded. In this case, the BET specific surface area of ​​the positive electrode active material in the second positive electrode mixture layer 23B may be larger than in the first positive electrode mixture layer 23A by another method.

[0108] The BET specific surface area of ​​the negative electrode active material 26C used as the raw material of the second negative electrode mixture layer 26B may be smaller than or equal to the BET specific surface area of ​​the negative electrode active material 26C used as the raw material of the first negative electrode mixture layer 26A. In this case, the BET specific surface area of ​​the negative electrode active material 26C of the second negative electrode mixture layer 26B may be larger than that of the first negative electrode mixture layer 26A by another method. Similarly, the BET specific surface area of ​​the positive electrode active material used as the raw material of the second positive electrode mixture layer 23B may be smaller than or equal to the BET specific surface area of ​​the positive electrode active material used as the raw material of the first positive electrode mixture layer 23A. In this case, the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B may be larger than that of the first positive electrode mixture layer 23A by another method.

[0109] The step of forming the first negative electrode mixture layer 26A may be performed after the step of forming the second negative electrode mixture layer 26B. For example, the drying speed of the second negative electrode mixture paste may be slower than the drying speed of the first negative electrode mixture paste by changing drying conditions such as air volume and temperature. Similarly, the step of forming the first positive electrode mixture layer 23A may be performed after the step of forming the second positive electrode mixture layer 23B. For example, the drying speed of the second positive electrode mixture paste may be slower than the drying speed of the first positive electrode mixture paste by changing drying conditions such as air volume and temperature.

[0110] The drying speed of the second negative electrode mixture paste may be faster than or the same as the drying speed of the first negative electrode mixture paste. In this case, the BET specific surface area of ​​the negative electrode active material 26C may be larger in the second negative electrode mixture layer 26B than in the first negative electrode mixture layer 26A by another method. Similarly, the drying speed of the second positive electrode mixture paste may be faster than or the same as the drying speed of the first positive electrode mixture paste. In this case, the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B may be larger than in the first positive electrode mixture layer 23A by another method.

[0111] · If the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B is larger than that of the first positive electrode mixture layer 23A, the BET specific surface area of ​​the negative electrode active material 26C of the second negative electrode mixture layer 26B may be smaller than that of the first negative electrode mixture layer 26A, or may be the same. Conversely, if the BET specific surface area of ​​the negative electrode active material 26C of the second negative electrode mixture layer 26B is larger than that of the first negative electrode mixture layer 26A, the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B may be smaller than that of the first positive electrode mixture layer 23A, or may be the same. Note that the BET specific surface area of ​​the positive electrode active material of the second positive electrode mixture layer 23B may be larger than that of the first positive electrode mixture layer 23A, and the BET specific surface area of ​​the negative electrode active material 26C of the second negative electrode mixture layer 26B may be larger than that of the first negative electrode mixture layer 26A. In this case, it is possible to uniformize the rate at which the first negative electrode mixture layer 26A and the second negative electrode mixture layer 26B accept lithium ions during charging, and to uniformize the rate at which the first positive electrode mixture layer 23A and the second positive electrode mixture layer 23B release lithium ions. Therefore, it is possible to make the lithium precipitation resistance of the negative electrode mixture layer 26 more uniform.

[0112] The lithium ion secondary battery 10 may be another non-aqueous secondary battery, for example, a nickel-metal hydride battery. The lithium ion secondary battery 10 may be mounted on an automatic transport vehicle, a special vehicle for loading and unloading, an electric vehicle, a hybrid vehicle, a computer, or other electronic device, or may be part of other systems. For example, it may be mounted on a moving object such as a ship or an aircraft, or may be a power supply system that supplies power from a power plant via a substation to a building or home in which a secondary battery is installed. [Explanation of symbols]

[0113] 10...Lithium-ion secondary battery 20...Electrode body 21…Positive electrode plate 22...Positive electrode substrate 22B…Side 1 22C…Second side 23...Positive electrode mixture layer 24...Negative electrode plate 25...Negative electrode substrate 25B…Side 1 25C…Second side 26...Negative electrode mixture layer 26A: First negative electrode mixture layer 26B: second negative electrode mixture layer 26C…Negative electrode active material 26D…Negative electrode binder 27…Separator 32…Upper curved section 33…Lower curved part

Claims

1. The battery includes an electrode assembly in which a positive electrode plate and a negative electrode plate are stacked with a separator interposed therebetween and wound, At least one of the positive electrode plate and the negative electrode plate includes a substrate, a first mixture layer located on an outer circumferential side with respect to the substrate, and a second mixture layer located on an inner circumferential side with respect to the substrate, the BET specific surface area of ​​the active material contained in the second mixture layer is larger than the BET specific surface area of ​​the active material contained in the first mixture layer; The electrode body includes a curved portion in which each layer constituting the electrode body is curved, In the curved portion, the density of the second mixture layer is higher than the density of the first mixture layer. Non-aqueous secondary battery.

2. The method includes a step of manufacturing a positive electrode plate, a step of manufacturing a negative electrode plate, and a step of manufacturing an electrode body by winding the positive electrode plate and the negative electrode plate in a stacked state with a separator interposed therebetween, At least one of the steps of manufacturing the positive electrode plate and the step of manufacturing the negative electrode plate is forming a first mixture layer on a first surface of a substrate; forming a second mixture layer on a second surface of the substrate opposite to the first surface; In the step of forming the second mixture layer, the second mixture layer is formed on the second surface such that a BET specific surface area of ​​an active material contained in the second mixture layer is larger than a BET specific surface area of ​​an active material contained in the first mixture layer; In the step of manufacturing the electrode body, the base material is wound so that the first mixture layer is located on the outer periphery side of the base material and the second mixture layer is located on the inner periphery side of the base material, thereby forming a curved portion in the electrode body in which each layer constituting the electrode body is curved; In the curved portion, the density of the second mixture layer is higher than the density of the first mixture layer. A method for manufacturing a non-aqueous secondary battery.

3. The step of forming the first mixture layer includes a step of applying a first mixture paste to the first surface and a step of drying the first mixture paste, The step of forming the second mixture layer includes a step of applying a second mixture paste to the second surface and a step of drying the second mixture paste, The second mixture paste dries at a slower rate than the first mixture paste. The method for producing the nonaqueous secondary battery according to claim 2 .

4. The step of forming the second mixture layer is performed after the step of forming the first mixture layer. The method for producing the nonaqueous secondary battery according to claim 3 .

5. As a raw material for the second mixture layer, an active material having a larger BET specific surface area than an active material used as a raw material for the first mixture layer is used. A method for producing the nonaqueous secondary battery according to claim 2 .

6. The step of forming the first mixture layer includes a step of kneading a first mixture paste containing raw materials of the first mixture layer, and a step of diluting the kneaded first mixture paste, The step of forming the second mixture layer includes a step of kneading a second mixture paste containing raw materials of the second mixture layer, and a step of diluting the kneaded second mixture paste, A solid content rate of the second mixture paste in the step of kneading the second mixture paste is lower than a solid content rate of the first mixture paste in the step of kneading the first mixture paste. A method for producing the nonaqueous secondary battery according to claim 2 .

7. The step of forming the first mixture layer includes a step of pressing the first mixture layer to adjust a thickness of the first mixture layer, The step of forming the second mixture layer includes a step of pressing the second mixture layer to adjust a thickness of the second mixture layer, The amount of pressure applied to the second mixture layer is greater than the amount of pressure applied to the first mixture layer. A method for producing the nonaqueous secondary battery according to claim 2 .

8. The step of forming the first mixture layer includes a step of applying a first mixture paste containing raw materials of the first mixture layer to the first surface, and a step of drying the first mixture paste, The step of forming the second mixture layer includes a step of applying a second mixture paste containing raw materials of the second mixture layer to the second surface, and a step of drying the second mixture paste, In the step of applying the second mixture paste, the second mixture paste is applied so that the basis weight of the second mixture paste is greater than the basis weight of the first mixture paste. The method for producing the nonaqueous secondary battery according to claim 7 .

9. The step of forming the first mixture layer includes a step of kneading a first mixture paste containing raw materials of the first mixture layer, a step of diluting the kneaded first mixture paste, a step of applying the diluted first mixture paste to the first surface, and a step of drying the first mixture paste, The step of forming the second mixture layer includes a step of kneading a second mixture paste containing raw materials of the second mixture layer, a step of diluting the kneaded second mixture paste, a step of applying the diluted second mixture paste to the second surface, and a step of drying the second mixture paste, In the step of diluting the second mixture paste, the second mixture paste is diluted so that a solid content rate of the second mixture paste is higher than a solid content rate of the first mixture paste after dilution. The method for producing the nonaqueous secondary battery according to claim 7 .

10. As a raw material for the second mixture layer, an active material having a lower tap density than an active material used as a raw material for the first mixture layer is used. The method for producing the nonaqueous secondary battery according to claim 7 .

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