Secondary battery

Abstract

Suppressing the possible breakage of the negative electrode current collector during negative electrode expansion. A secondary battery comprises: an elongated cylindrical energy storage element, the elongated cylindrical energy storage element being formed by winding a positive electrode having a positive active material layer formed on a positive current collector and a negative electrode having a negative active material layer formed on a negative current collector; and an outer packaging body, wherein there are at least two fold-back positions on either the positive electrode or the negative electrode located at the innermost periphery of the energy storage element, wherein the distance between the end of the positive active material layer on the winding start end side of the positive electrode and the fold-back position close to the end of the positive active material layer is defined as distance C1, the distance between the end of the positive active material layer on the winding end side of the positive electrode and the fold-back position close to the end of the positive active material layer is defined as distance C2, and the length of the energy storage element in the long side direction is defined as W, satisfying the following relationships (1) and (2): 0.02≤C1 / W≤0.12……Equation (1), 0.02≤C2 / W≤0.12……Equation (2).

Description

Technical Field

[0001] This invention relates to a secondary battery. Background Technology

[0002] Secondary batteries with a wound structure, consisting of a strip-shaped positive electrode and a negative electrode wound together by a strip-shaped separator, are known. Patent Document 1 describes a lithium-ion battery as a secondary battery having this wound structure. In the lithium-ion battery described in Patent Document 1, the inner peripheral end of the positive electrode active material layer is formed in a region that does not overlap with the positive electrode tab in the short axis direction of the wound structure.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent document 1: Japanese Patent Application Publication No. 2007-311139. Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] However, in the lithium-ion battery described in Patent Document 1, the energy storage element expands and contracts with charge and discharge cycles, and the stress is concentrated on the negative electrode current collector, which can lead to the breakage of the negative electrode current collector.

[0008] The purpose of this invention is to provide a secondary battery that can suppress the breakage of the negative electrode current collector.

[0009] Solution for solving the problem

[0010] To address the aforementioned issues, the present invention provides a secondary battery having:

[0011] An elongated cylindrical energy storage element, wherein the elongated cylindrical energy storage element is formed by winding a positive electrode with a positive active material layer formed on a positive current collector and a negative electrode with a negative active material layer formed on a negative current collector; and

[0012] Outer packaging

[0013] There are at least two foldback positions on either the positive or negative terminal located at the innermost periphery of the energy storage element.

[0014] When the distance between the end of the positive electrode active material layer on the beginning end side of the positive electrode winding and the fold-back position close to the end of the positive electrode active material layer is defined as distance C1, the distance between the end of the positive electrode active material layer on the ending end side of the positive electrode winding and the fold-back position close to the end of the positive electrode active material layer is defined as distance C2, and the length of the energy storage element in the long side direction is defined as W, the following relationships (1) and (2) are satisfied:

[0015] 0.02≤C1 / W≤0.12……Equation (1)

[0016] 0.02≤C2 / W≤0.12……Equation (2).

[0017] The effects of the invention

[0018] According to the present invention, the breakage of the negative electrode current collector can be suppressed. Attached Figure Description

[0019] Figure 1 This is an exploded perspective view showing a structural example of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention.

[0020] Figure 2 It is along Figure 1 A sectional view along line II-II.

[0021] Figure 3 This is a diagram used to illustrate the foldback position, etc., involved in one embodiment of the present invention. Detailed Implementation

[0022] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that the description will proceed in the following order.

[0023] <Issues to be considered in this implementation>

[0024] <One implementation method>

[0025] <Variation Example>

[0026] The embodiments described below are preferred specific examples of the present invention, and the content of the present invention is not limited to these embodiments.

[0027] <Issues to be considered in this implementation>

[0028] First, to facilitate understanding of this embodiment, the problems that should be considered in this embodiment will be explained. In a lithium-ion battery with a wound structure, there may be areas where the positive electrode active material layer and the negative electrode active material layer face each other and areas where they do not face each other in the flat portion of the wound structure. When the lithium-ion battery is charged, lithium is embedded in the negative electrode active material layer in the areas where the positive electrode active material layer and the negative electrode active material layer face each other, thereby causing the negative electrode to expand, but in the areas where they do not face each other, the negative electrode does not expand. Therefore, the stress distribution accompanying the expansion of the negative electrode during charging becomes uneven, resulting in local stress concentration. In particular, stress concentration occurs near the boundary between the flat portion and the curved portion in the wound structure. Due to the stress concentration, there is a problem of foil breakage of the negative electrode current collector. Considering this problem, an embodiment of the present invention will be described in detail below.

[0029] <One implementation method>

[0030] [Battery Structure]

[0031] First, refer to Figures 1 to 3 An example of the structure of a non-aqueous electrolyte secondary battery (hereinafter referred to as "battery") according to one embodiment of the present invention will be described. Figure 1 As shown, the battery has a flat shape. The battery includes: a wound electrode body 20, on which a positive electrode tab (positive lead) 31 and a negative electrode tab (negative lead) 32 are mounted, and has a flat shape; an electrolyte (not shown); and a casing 10 that houses the electrode body 20 and the electrolyte. When viewed from above perpendicular to its main surface, the battery has a rectangular shape.

[0032] (case)

[0033] The casing 10, as an example of the outer packaging, is a thin, rectangular battery can made of metal. For example, nickel-plated iron (Fe) can be used. With a metal casing, the casing itself can serve as a battery terminal by connecting to either the positive or negative electrode, facilitating battery miniaturization. The casing 10 includes a receiving portion 11 and a cover portion 12. The receiving portion 11 houses the electrode body 20. The receiving portion 11 includes a main surface portion 11A and a wall portion 11B disposed around the periphery of the main surface portion 11A. The main surface portion 11A covers the main surface of the electrode body 20, and the wall portion 11B covers the side and end faces of the electrode body 20. A positive terminal 13 is disposed in the wall portion 11B opposite to one end face of the electrode body 20 (the end face on the side where the positive electrode tab 31 and negative electrode tab 32 are removed). The positive electrode tab 31 is connected to the positive terminal 13. The negative electrode tab 32 is connected to the inner surface of the casing 10. The cover 12 covers the opening of the receiving portion 11. The top of the wall portion 11B of the receiving portion 11 and the periphery of the cover 12 are joined by welding or adhesive. Alternatively, the housing 10 can be a non-rigid housing such as a laminate, but it is preferably a metal housing mainly composed of metal. The metal housing has a certain rigidity, constraining the electrode body 20. Therefore, it is possible to suppress the deformation of the battery that accompanies the expansion and contraction of the electrode body 20, and to suppress the breakage of the negative electrode current collector.

[0034] (Positive electrode, negative electrode)

[0035] The positive electrode tab 31 and the negative electrode tab 32 are led out from one end face of the electrode body 20. The positive electrode tab 31 and the negative electrode tab 32 are made of metal materials such as Al, Cu, Ni or stainless steel, and are in the form of thin plates, etc.

[0036] Sealant (adhesive film) 31A and 32A are respectively inserted between the housing 10 and the positive electrode tab 31, and between the housing 10 and the negative electrode tab 32 to prevent the intrusion of external gas. The sealant 31A and 32A are made of a material that has a tight adhesion to the positive electrode tab 31 and the negative electrode tab 32, such as polyethylene, polypropylene, modified polyethylene or modified polypropylene and other polyolefin resins.

[0037] (Electrode)

[0038] The electrode body 20 is an elongated cylindrical energy storage device formed by winding a positive electrode with a positive active material layer formed on the positive current collector and a negative electrode with a negative active material layer formed on the negative current collector. The electrode body 20 will be described in detail below.

[0039] like Figure 2 As shown, the electrode body 20 has a pair of opposing flat portions 20A and a pair of opposing curved portions 20B disposed between the pair of flat portions 20A. The electrode body 20 includes: a strip-shaped positive electrode 21; a strip-shaped negative electrode 22; two strip-shaped diaphragms 23A and 23B; insulating members 25B1 and 25B2 disposed on the positive electrode 21; and insulating members 26B1 and 26B2 disposed on the negative electrode 22. Diaphragms 23A and 23B are alternately disposed between the positive electrode 21 and the negative electrode 22. The electrode body 20 has the following structure: the positive electrode 21 and the negative electrode 22 are stacked via diaphragms 23A or diaphragms 23B and wound into a flat and spiral shape along the long side direction. The electrode body 20 is wound such that the positive electrode 21 is the innermost peripheral electrode and the negative electrode 22 is the outermost peripheral electrode. The negative electrode 22, as the outermost peripheral electrode, is fixed by an anti-winding strip 24. Electrolyte is impregnated in positive electrode 21, negative electrode 22 and separators 23A and 23B.

[0040] (positive electrode)

[0041] The positive electrode 21 comprises: a positive current collector 21A having an inner surface 21S1 and an outer surface 21S2; a positive active material layer 21B1 disposed on the inner surface 21S1 of the positive current collector 21A; and a positive active material layer 21B2 disposed on the outer surface 21S2 of the positive current collector 21A. In this specification, "inner surface" refers to the surface located on the side of the winding center, and "outer surface" refers to the surface located on the side opposite to the winding center. The thickness of the positive current collector 21A is, for example, 3 μm or more and 20 μm or less. The thickness of the positive active material layers 21B1 and 21B2 is, for example, 30 μm or more and 100 μm or less.

[0042] The positive electrode active material layer 21B1 is not provided on the inner surface 21S1 of the outer peripheral end of the positive electrode 21 (hereinafter referred to as the "outer peripheral end"). Instead, a positive electrode current collector exposed portion 21D1 is provided, which is exposed on the inner surface 21S1 of the positive electrode current collector 21A. The positive electrode active material layer 21B2 is not provided on the outer surface 21S2 of the outer peripheral end of the positive electrode 21. Instead, a positive electrode current collector exposed portion 21D2 is provided, which is exposed on the outer surface 21S2 of the positive electrode current collector 21A. A positive electrode tab 31 is connected to the portion of the positive electrode current collector exposed portion 21D2 corresponding to the flat portion 20A. The length of the positive electrode current collector exposed portion 21D1 in the winding direction is approximately the same as the length of the positive electrode current collector exposed portion 21D2 in the winding direction.

[0043] The positive current collector 21A is made of metal foil such as aluminum foil, nickel foil, or stainless steel foil. The positive active material layers 21B1 and 21B2 contain positive active materials capable of lithium insertion and extraction. The positive active material layers 21B1 and 21B2 may further contain at least one of a binder and a conductive agent as needed.

[0044] (Positive electrode active material)

[0045] Lithium-containing compounds, such as lithium oxides, lithium phosphates, lithium sulfides, or lithium-containing intercalation compounds, are suitable as positive electrode active materials, and combinations of two or more of them can also be used. To improve energy density, lithium-containing compounds containing lithium, transition metal elements, and oxygen are preferred. Examples of such lithium-containing compounds include lithium complex oxides with a layered rock salt structure and lithium complex phosphates with an olivine-type structure. More preferably, lithium-containing compounds contain at least one transition metal element selected from the group consisting of Co, Ni, Mn, and Fe. Examples of such lithium-containing compounds include LiNi. 0.50 Co 0.20 Mn 0.30 O2, LiCoO2, LiNiO2, LiNi a Co 1-a O2 (0 < a < 1), LiMn2O4 or LiFePO4, etc.

[0046] In addition to MnO2, V2O5, and V6O2, other cathode active materials capable of lithium insertion and extraction can also be used. 13 Inorganic compounds that do not contain lithium, such as NiS and MoS.

[0047] The positive electrode active material capable of lithium insertion and extraction can be any material other than those described above. Furthermore, two or more of the positive electrode active materials exemplified above can be mixed in any combination.

[0048] (Adhesive)

[0049] As an adhesive, at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, polyacrylonitrile, styrene-butadiene rubber, carboxymethyl cellulose, and copolymers based on one of these resin materials can be used.

[0050] (Conductive agent)

[0051] As a conductive agent, at least one carbon material selected from the group consisting of graphite, carbon fiber, carbon black, acetylene black, Ketjen black, carbon nanotubes, and graphene can be used. It should be noted that the conductive agent can be any material that is conductive and is not limited to carbon materials. For example, metallic materials or conductive polymers can also be used as conductive agents. Furthermore, the shape of the conductive agent can be, for example, granular, flake-like, hollow, needle-like, or cylindrical, but is not particularly limited to these shapes.

[0052] (negative electrode)

[0053] The negative electrode 22 comprises: a negative electrode current collector 22A having an inner surface 22S1 and an outer surface 22S2; a negative electrode active material layer 22B1 disposed on the inner surface 22S1 of the negative electrode current collector 22A; and a negative electrode active material layer 22B2 disposed on the outer surface 22S2 of the negative electrode current collector 22A. The thickness of the negative electrode current collector 22A is, for example, 3 μm or more and 20 μm or less. The thickness of the negative electrode active material layers 22B1 and 22B2 is, for example, 30 μm or more and 100 μm or less.

[0054] On the inner surface 22S1 of the outer peripheral end of the negative electrode 22, a negative electrode active material layer 22B1 is not provided, but a negative electrode current collector exposed portion 22D1 is provided, which is exposed on the inner surface 22S1 of the positive electrode current collector 21A. On the outer surface 22S2 of the outer peripheral end of the negative electrode 22, a negative electrode active material layer 22B2 is not provided, but a negative electrode current collector exposed portion 22D2 is provided, which is exposed on the outer surface 22S2 of the negative electrode current collector 22A. A negative electrode tab 32 is connected to the portion of the negative electrode current collector exposed portion 22D1 corresponding to the flat portion 20A. It should be noted that the positive electrode tab 31 and the negative electrode tab 32 are provided on one side of the same flat portion 20A.

[0055] The length of the exposed portion 22D2 of the negative electrode current collector in the winding direction is about one circumference longer than the length of the exposed portion 22D1 of the negative electrode current collector in the winding direction. That is, at the outer peripheral end of the negative electrode 22, there is a single-sided active material layer forming portion, for example, about one circumference, in which only the negative electrode active material layer 22B1 of the negative electrode active material layer 22B1 and the negative electrode active material layer 22B2 is formed on the negative electrode current collector 22A.

[0056] On the outermost periphery of the negative electrode 22, the exposed portions of both the inner surface 22S1 and the outer surface 22S2 of the negative electrode current collector 22A (i.e., the portions where the exposed portions 22D1 and 22D2 of the negative electrode current collector are located on both sides of the positive electrode 21) are provided for approximately one circumference. Thus, the exposed portion 22D2 of the negative electrode current collector is in electrical contact with the inner surface of the housing 10. Therefore, by electrically connecting the negative electrode 22 and the housing 10, the resistance can be further reduced.

[0057] The negative current collector 22A is made of metal foil such as copper foil, nickel foil, or stainless steel foil. In this embodiment, copper foil is used as the negative current collector 22A. The copper foil used as the negative current collector 22A has an impurity content (e.g., sulfur content) of less than 20 ppm (parts per million) and an elongation of 7% or more after heat treatment at 200°C. The elongation after heat treatment at 200°C refers to the elongation measured at room temperature after heating at 200°C for 3 hours. For example, a test was conducted using the Autograph AG-IS manufactured by Shimadzu Corporation, using copper foil with a test specimen size of ASTM-D638-V (dimensions: maximum width 9.53 mm, minimum width 3.15 mm, length orthogonal to the width 63.50 mm), a test speed of 1 mm / min, and an elongation of 7% or more after heating at 200°C for 3 hours and measuring at room temperature.

[0058] The negative electrode active material layers 22B1 and 22B2 contain negative electrode active materials capable of lithium insertion and extraction. The negative electrode active material layers 22B1 and 22B2 may further contain at least one of a binder and a conductive agent, as needed.

[0059] (Negative electrode active material)

[0060] Examples of carbon materials that can be used as negative electrode active materials include, for example, difficult-to-graphitize carbon, easily-graphitize carbon, graphite, pyrolytic carbon, coke, glassy carbon, sintered organic polymer compounds, carbon fibers, or activated carbon. Among these, coke includes pitch coke, needle coke, or petroleum coke. Sintered organic polymer compounds refer to materials carbonized by sintering phenolic resins or furan resins at appropriate temperatures, and some of these are classified as difficult-to-graphitize carbon or easily-graphitize carbon. These carbon materials exhibit very small changes in crystal structure during charge and discharge, resulting in high charge-discharge capacity and good cycle characteristics, and are therefore preferred. Graphite, in particular, has a large electrochemical equivalent, enabling high energy density, and is therefore preferred. Furthermore, difficult-to-graphitize carbon exhibits excellent cycle characteristics and is therefore preferred. Additionally, materials with low charge-discharge potentials, specifically those close to lithium metal, can easily achieve high energy density in batteries, and are therefore preferred.

[0061] (Adhesive)

[0062] As a binder, the same binder as the positive electrode active material layers 21B1 and 21B2 can be used.

[0063] (Conductive agent)

[0064] As a conductive agent, the same conductive agent as the positive electrode active material layers 21B1 and 21B2 can be used.

[0065] (Diaphragm)

[0066] Separators 23A and 23B isolate the positive electrode 21 and the negative electrode 22, allowing lithium ions to pass through while preventing short circuits caused by contact between the two electrodes. Separators 23A and 23B may be composed of, for example, porous membranes made of polytetrafluoroethylene, polyolefin resins (such as polypropylene (PP) or polyethylene (PE)), acrylic resins, styrene resins, polyester resins, or nylon resins, or resins obtained by blending these resins, or may be a structure in which two or more of these porous membranes are stacked.

[0067] Among these, porous membranes made of polyolefins are preferred because they exhibit excellent short-circuit prevention and improve battery safety through the circuit-breaking effect. Polyethylene, in particular, is preferred because it exhibits a circuit-breaking effect within a temperature range of 100°C to 160°C and also demonstrates excellent electrochemical stability. Low-density polyethylene, high-density polyethylene, and linear polyethylene are preferred because they have suitable melting temperatures and are readily available. Furthermore, materials copolymerized or blended with chemically stable resins of polyethylene or polypropylene can also be used. Alternatively, the porous membrane may have a structure consisting of three or more layers: a polypropylene layer, a polyethylene layer, and another polypropylene layer stacked sequentially. For example, a three-layer structure of PP / PE / PP is desirable, with a PP:PE mass ratio [wt%] of PP:PE = 60:40 to 75:25. Alternatively, from a cost perspective, a single-layer substrate of 100wt% PP or 100wt% PE can be manufactured. The manufacturing methods for membranes 23A and 23B can be wet or dry processes.

[0068] Nonwoven fabrics can also be used as diaphragms 23A and 23B. As the fibers constituting the nonwoven fabric, aramid fibers, glass fibers, polyolefin fibers, polyethylene terephthalate (PET) fibers, or nylon fibers can be used. Alternatively, nonwoven fabrics can be made by blending two or more of these fibers.

[0069] (electrolyte)

[0070] The electrolyte is a so-called non-aqueous electrolyte, containing an organic solvent (the non-aqueous solvent) and an electrolyte salt dissolved in that organic solvent. To improve battery characteristics, the electrolyte may contain known additives. It should be noted that an electrolyte layer containing the electrolyte and a polymeric compound that serves as a retainer of the electrolyte can be used instead of the electrolyte. In this case, the electrolyte layer may be gel-like.

[0071] As an organic solvent, cyclic carbonates such as ethylene carbonate or propylene carbonate can be used, and it is preferable to use one or more of ethylene carbonate and propylene carbonate, especially a mixture of both. This is because it can further improve the cycling characteristics.

[0072] In addition to these cyclic carbonates, it is preferable to use a mixture of chain carbonates such as diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, or methyl propyl carbonate as an organic solvent. This is because it results in higher ionic conductivity.

[0073] As an organic solvent, it is also preferable to contain 2,4-difluoroanisole or vinylene carbonate. This is because 2,4-difluoroanisole can further improve the discharge capacity, and vinylene carbonate can further improve the cycling characteristics. Therefore, if they are used in combination, both the discharge capacity and cycling characteristics can be further improved, which is preferred.

[0074] In addition, examples of organic solvents include butylene carbonate, γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, methyl acetate, methyl propionate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile, N,N-dimethylformamide, N-methylpyrrolidone, N-methyloxazolidinone, N,N-dimethylimidazolinone, nitromethane, nitrobenzene, sulfolane, dimethyl sulfoxide, or trimethyl phosphate.

[0075] It should be noted that compounds in which at least a portion of the hydrogen in these organic solvents is replaced by fluorine can sometimes improve the reversibility of the electrode reaction, depending on the type of electrode combination, and are therefore sometimes preferred.

[0076] As electrolyte salts, examples include lithium salts, which can be used alone or in combination with two or more. Examples of lithium salts include LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiAlCl4, LiSiF6, LiCl, lithium difluoro[oxalate-O,O']borate, lithium bis(oxalate-borate), or LiBr. Among these, LiPF6 exhibits high ionic conductivity and further improves cycling characteristics, making it a preferred choice.

[0077] (Insulating components)

[0078] Insulating components 25B1, 25B2, 26B1, and 26B2 are, for example, rectangular films with an adhesive surface on one side. More specifically, insulating components 25B1, 25B2, 26B1, and 26B2 have a substrate and an adhesive layer disposed on the substrate. It should be noted that, in this specification, adhesion (pressure-sensitive adhesion) is defined as a type of adhesion. According to this definition, an adhesive layer is considered a type of adhesive layer. Furthermore, the definition of film also includes sheets. Insulating components 25B1, 25B2, 26B1, and 26B2 are, for example, made of insulating tape. Materials used for insulating components 25B1, 25B2, 26B1, and 26B2 include, for example, polyethylene terephthalate (PET), polyimide (PI), polyethylene (PE), or polypropylene (PP).

[0079] (Insulating component installed on the positive terminal)

[0080] Insulating component 25B1 covers the stepped portion at the boundary between the exposed positive current collector 21D1 and the positive active material layer 21B1, and the exposed positive current collector 21D1. Insulating component 25B2 covers the stepped portion at the boundary between the exposed positive current collector 21D2 and the positive active material layer 21B2, and the exposed positive current collector 21D2. It should be noted that insulating component 25B2 covers the positive electrode tab 31 in addition to the exposed positive current collector 21D2. The boundaries between the exposed positive current collector 21D1 and the positive active material layer 21B1, and between the exposed positive current collector 21D2 and the positive active material layer 21B2, are formed parallel to the winding axis direction of the electrode body 20.

[0081] Insulating component 25B1 is disposed in the region opposite to the positive current collector exposed portion 21D1 and the negative active material layer 22B2, and in the region opposite to the positive current collector exposed portion 21D1 and the negative current collector exposed portion 22D2. Insulating component 25B2 is disposed in the region opposite to the positive current collector exposed portion 21D2 and the negative active material layer 22B1, and in the region opposite to the positive current collector exposed portion 21D2 and the negative current collector exposed portion 22D1.

[0082] The positive electrode 21 has a positive current collector exposed portion 21D3 where the outer peripheral end of the positive current collector exposed portion 21D1 is not covered by the insulating member 25B1, and a positive current collector exposed portion 21D4 where the outer peripheral end of the positive current collector exposed portion 21D2 is not covered by the insulating member 25B2.

[0083] (Insulating component installed on the negative terminal)

[0084] The insulating component 26B1 covers the portion of the negative current collector exposed portion 22D1 where the negative electrode tab 32 is provided, as well as the portion opposite to the positive current collector exposed portion 21D4. The insulating component 26B1 may also cover approximately the entire portion of the negative current collector exposed portion 22D1 corresponding to a flat portion 20A.

[0085] The insulating component 26B2 covers the stepped portion at the boundary 22P between the negative current collector exposed portion 22D2 and the negative active material layer 22B2 (i.e., the boundary 22P between the single-sided active material layer forming portion and the negative active material layer 22B2) and the negative current collector exposed portion 22D2. The boundary 22P between the negative current collector exposed portion 22D2 and the negative active material layer 22B2 is formed parallel to the winding axis direction of the electrode body 20. The insulating component 26B2 preferably also covers the portion of the negative current collector exposed portion 22D2 opposite to the positive current collector exposed portion 21D3. The positive current collector exposed portion 21D3 is located on the outer periphery of the winding of the electrode body 20 relative to the boundary 22P, and the negative electrode tab 32 is located on the outer periphery of the winding of the electrode body 20 relative to the positive current collector exposed portion 21D3. The positive current collector exposed portion 21D3 is, for example, located on the flat portion 20A opposite to the flat portion 20A on which the boundary 22P is provided.

[0086] (Return to position)

[0087] There are at least two foldback positions on either the positive or negative terminal located at the innermost periphery of the energy storage element. For example, as... Figure 3 As shown, there are two fold-back positions P51 and P52 on the positive electrode 21 located at the innermost periphery of the electrode body 20 according to this embodiment. Depending on the winding structure of the electrode body 20, the negative electrode 22 may be present at the innermost periphery and there may be a fold-back position of the negative electrode 22 at the innermost periphery.

[0088] Furthermore, the positive electrode 21 constituting the electrode body 20 has a winding start end, which serves as the starting point of the winding structure, and a winding end, which serves as the ending point of the winding structure. An end 41A of the positive electrode active material layer 21B1 is located at the winding start end side of the positive electrode 21. An end 41B of the positive electrode active material layer 21B1 is located at the winding end side of the positive electrode 21. The distance (distance along the long axis of the electrode body 20) between the end 41A of the positive electrode active material layer 21B1 and the fold-back position P51 near the end 41A of the positive electrode active material layer 21B1 is defined as C1 (mm). The distance (distance along the long axis of the electrode body 20) between the end 41B of the positive electrode active material layer 21B2 at the winding end side of the positive electrode 21 and the fold-back position P52 near the end 41B of the positive electrode active material layer 21B2 is defined as distance C2 (mm). It should be noted that, as in this embodiment, when the positive electrode active material layer is formed on both sides of the positive electrode current collector 21A, the distance C1 or the distance C2 is defined by the end of the positive electrode active material layer near the folded-back position.

[0089] Furthermore, the length of the long side (major axis) of the electrode body 20 is set as W (mm). In this case, the battery satisfies the following relationships (1) and (2).

[0090] 0.02≤C1 / W≤0.12……Equation (1)

[0091] 0.02≤C² / W≤0.12……Equation (2)

[0092] The distances C1 and C2 can also be equal (C1 = C2).

[0093] like Figure 2 As shown, in the electrode body 20 of this embodiment, the positive electrode tab 31 and the negative electrode tab 32 are connected to the outermost periphery of the electrode body 20. Specifically, the positive electrode tab 31 is connected to the positive current collector 21A located at the outermost periphery, and the negative electrode tab 32 is connected to the negative current collector 22A located at the outermost periphery.

[0094] More specifically, the positive electrode tab 31 and the negative electrode tab 32 are located on the outermost flat portion ( Figure 2 The flat portion 20A on the upper side of the above-mentioned positive electrode active material layer 21B1 and the end 41B of the positive electrode active material layer 21B2 are located on the flat portion opposite to the flat portion on the side where the positive electrode tab 31 and the negative electrode tab 32 are located. Figure 2 (The flat portion 20A on the lower side of the middle).

[0095] [Battery manufacturing method]

[0096] Next, an example of a battery manufacturing method according to one embodiment of the present invention will be described.

[0097] (The manufacturing process of the positive electrode)

[0098] The positive electrode 21 is fabricated as follows. First, for example, a positive electrode active material, a binder, and a conductive agent are mixed to prepare a positive electrode mixture, and this positive electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) to prepare a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry is coated on both sides of the positive electrode current collector 21A, the solvent is dried, and it is compressed and molded using a roller press or the like to form positive electrode active material layers 21B1 and 21B2, thus obtaining the positive electrode 21. At this time, the coating position of the positive electrode mixture slurry is adjusted so that positive electrode current collector exposed portions 21D1 and 21D2 are formed at one end of the positive electrode 21.

[0099] Next, the positive electrode tab 31 is installed on the positive current collector exposed portion 21D2 located at one end of the positive electrode 21 by welding. Then, the insulating components 25B1 and 25B2 are respectively attached to the positive current collector exposed portions 21D1 and 21D2 located at one end of the positive electrode 21.

[0100] (The manufacturing process of the negative electrode)

[0101] The negative electrode 22 is manufactured as follows. First, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and this negative electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to make a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry is coated on both sides of the negative electrode current collector 22A, the solvent is dried, and it is compressed and molded by a roller press or the like to form negative electrode active material layers 22B1 and 22B2, thus obtaining the negative electrode 22. At this time, the coating position of the negative electrode mixture slurry is adjusted so that negative electrode current collector exposed portions 22D1 and 22D2 are formed at one end of the negative electrode 22.

[0102] Next, the negative electrode tab 32 is installed on the negative current collector exposed portion 22D1 located at one end of the negative electrode 22 by welding. Then, the insulating components 26B1 and 26B2 are respectively attached to the positive current collector exposed portions 21D1 and 21D2 located at the other end of the negative electrode 22.

[0103] (Winding process)

[0104] The positive electrode 21, negative electrode 22, and separators 23A and 23B are wound to a predetermined length using a winding core to fabricate the electrode body 20. It should be noted that the positive electrode 21 and negative electrode 22 are cut to a predetermined length.

[0105] (Bending process of the negative end)

[0106] Using a clamp (not shown), the outer peripheral end of the negative electrode 22 is tilted in a predetermined direction (e.g., downward). The tilted outer peripheral end of the negative electrode 22 includes the boundary 22P between the exposed negative current collector 22D2 and the negative active material layer 22B2. Since the insulating member 26B2 covers the boundary 22P, the rigidity of the negative electrode 22 at the boundary 22P is improved, and bending of the outer peripheral end of the negative electrode 22 from the boundary 22P is suppressed. Therefore, the shedding of the negative active material from the portion of the negative active material layer 22B1 located on the back side of the boundary 22P is suppressed. Thus, the occurrence of minor short circuits caused by the shedding of the negative active material is suppressed. It should be noted that the outer peripheral end of the negative electrode 22 can also be tilted using a mechanism other than the clamp.

[0107] By pre-installing the negative electrode tab 32 on the outer peripheral end of the negative electrode 22, the negative electrode tab 32 can function as a counterweight when the outer peripheral end of the negative electrode 22 is tilted. Therefore, the outer peripheral end of the negative electrode 22 can be easily tilted. Therefore, in the "diaphragm cutting process," which is a subsequent process to the "bending process of the negative electrode end," it is possible to prevent the negative electrode 22 from being cut together with the diaphragms 23A and 23B.

[0108] (Diaphragm cutting process)

[0109] After supporting the diaphragms 23A and 23B above the electrode body 20 using a support member (not shown), the diaphragms 23A and 23B are cut using a cutter. After cutting, the outer peripheral end of the negative electrode 22, which serves as the outermost peripheral electrode, is fixed by an anti-winding band 24. Thus, the electrode body 20 is obtained.

[0110] It should be noted that, in the wound state, the negative electrode 22 is attracted to the diaphragm 23A by electrostatic attraction. When the diaphragms 23A and 23B are cut in this state, the negative electrode 22 is also cut along with the diaphragms 23A and 23B, and the negative electrode 22 may become shorter than the specified length. As described above, by tilting the outer peripheral end of the negative electrode 22, cutting the diaphragms 23A and 23B can prevent the negative electrode 22 from being cut along with the diaphragms 23A and 23B.

[0111] (Sealing process)

[0112] The electrode body 20 is sealed by the housing 10 as follows: First, the electrode body 20 and electrolyte are housed in the housing 11. Next, the positive electrode tab 31 is connected to the positive terminal 13 provided on the housing 10, and the negative electrode tab 32 is connected to the inner side of the housing 10. Then, the opening of the housing 11 is covered with the cover 12, and the periphery of the housing 11 and the cover 12 are joined by welding or adhesive. Thus, a battery is obtained.

[0113] [Effect]

[0114] In this embodiment, the following effects can be obtained.

[0115] The ranges of distances C1 and C2 are set to the ranges described in the embodiment, that is, the ranges that satisfy both relations (1) and (2). Therefore, in each of the two flat portions, the positive electrode active material layer of the positive electrode and the positive electrode active material layer of the negative electrode can be positioned opposite each other over a wider range. Thus, the expansion of the negative electrode during charging is generated uniformly in all directions, and localized stress concentration in the electrode body can be suppressed. Furthermore, the breakage of the negative electrode current collector caused by localized stress concentration can be suppressed.

[0116] In addition, by having positive and negative tabs on the outermost periphery, the deformation of the positive and negative electrodes becomes significant due to the presence of steps in each lead. However, by making the distances C1 and C2 satisfy the relationships (1) and (2) respectively, it is not easy to cause breakage.

[0117] Furthermore, the two ends of the positive electrode active material layer are located on the flat portions opposite to the flat portions connecting the positive and negative electrode tabs. As a result, the deformed portions of the positive and negative electrodes caused by the steps appear symmetrical when viewed from above. This disperses deformation and further suppresses breakage.

[0118] In addition, by making the distance C1 = C2, the breakage of the negative electrode can be effectively suppressed.

[0119] Furthermore, the copper foil used as the negative electrode current collector can be used to suppress copper foil breakage by using copper foil with impurities (e.g., sulfur content) of less than 20 ppm and an elongation of more than 7% after heat treatment at 200°C, so that the copper foil elongates when it expands.

[0120] Example

[0121] The present invention will be specifically described below through embodiments, but the present invention is not limited to these embodiments.

[0122] [Examples 1-4]

[0123] (The manufacturing process of the positive electrode)

[0124] The positive electrode is prepared as follows. First, 91 parts by mass of lithium cobalt composite oxide (LiCoO2) as the positive electrode active material, 6 parts by mass of graphite as a conductive agent, and 3 parts by mass of polyvinylidene fluoride as a binder are mixed to prepare a positive electrode mixture. Then, the positive electrode mixture is dispersed in N-methyl-2-pyrrolidone to prepare a paste-like positive electrode mixture slurry.

[0125] Next, a 19μm thick strip of aluminum foil is prepared as the positive electrode current collector. A positive electrode slurry is coated onto both sides of the aluminum foil and allowed to dry. Then, it is compressed using a roller press to form a positive electrode active material layer, thus obtaining the positive electrode. At this point, the coating position of the positive electrode slurry is adjusted so that a positive electrode current collector is exposed on both sides of one end of the positive electrode. Next, aluminum positive electrode tabs are welded onto the exposed positive electrode current collector on the outer side of the outer peripheral end formed at one end of the positive electrode. Finally, insulating tape is applied to the exposed positive electrode current collector on both sides of one end of the positive electrode (see reference). Figure 2 ).

[0126] (The manufacturing process of the negative electrode)

[0127] The negative electrode is prepared as follows. First, 97 parts by mass of artificial graphite powder as the negative electrode active material and 3 parts by mass of polyvinylidene fluoride as a binder are mixed to prepare a negative electrode mixture. Then, the negative electrode mixture is dispersed in N-methyl-2-pyrrolidone to prepare a paste-like negative electrode mixture slurry.

[0128] Next, a 6μm thick strip of copper foil is prepared as the negative electrode current collector. A negative electrode slurry is coated onto both sides of the copper foil and allowed to dry. Then, it is compressed using a roller press to form a negative electrode active material layer, thus obtaining the negative electrode. At this point, the coating position of the negative electrode slurry is adjusted so that a negative electrode current collector is exposed on both sides of one end of the negative electrode. Next, nickel negative electrode tabs are welded onto the exposed negative electrode current collector portions formed on both sides of one end of the negative electrode, specifically on the inner side of the outer peripheral end after winding. Finally, insulating tape is applied to the exposed negative electrode current collector portions formed on both sides of one end of the negative electrode (see reference). Figure 2 ).

[0129] (Electrolyte preparation process)

[0130] The electrolyte was prepared as follows. First, ethylene carbonate (EC) and propylene carbonate (PC) were mixed in a mass ratio of EC:PC = 1:1 to prepare a mixed solvent. Next, lithium hexafluorophosphate (LiPF6), as the electrolyte salt, was dissolved in this mixed solvent at a concentration of 1.0 mol / kg to prepare the electrolyte.

[0131] (Battery manufacturing process)

[0132] The battery is manufactured as follows. First, a flat, wound electrode body is obtained by winding a positive electrode, a negative electrode, and two separators using a winding core. A microporous polyethylene membrane with a thickness of 25 μm is used as the separator. Next, the outer peripheral end of the negative electrode is tilted using a clamp. Then, the separator is supported above the electrode body by a support member, and then cut using a cutter. Next, the outer peripheral end of the negative electrode, serving as the outermost electrode, is fixed using an anti-winding band. This yields the electrode body. Next, the electrode body and electrolyte are housed in a metal can housing, and the opening of the housing is covered with a cap. The metal can is sealed by joining the periphery of the housing and the cap. This yields the target battery.

[0133] It should be noted that the length of the long side of the electrode body is set to 25mm. Then, in the positive electrode manufacturing process, the starting and ending positions of the winding of the positive electrode current collector are appropriately adjusted. In addition, by adjusting the coating position of the positive electrode slurry, the positions of the ends of the positive electrode active material layer on the starting and ending ends of the winding of the positive electrode are appropriately adjusted. The above adjustments satisfy the relationships (1) and (2).

[0134] [Comparative Examples 1-4]

[0135] Except for adjusting to not satisfy relations (1) and (2), the battery is obtained in the same manner as in Example 1.

[0136] (Fracturing rate)

[0137] The fracture rate was evaluated as follows. The batteries were overcharged to 150% SOC (State of Charge), and then disassembled. At this point, the fracture of the copper foil in the negative electrode current collector was visually confirmed. The fracture rate was calculated as the ratio of the total number of fractured batteries to the total number of batteries manufactured (evaluated). It should be noted that 100 batteries were manufactured.

[0138] (Fragmentation rate after cyclic charge and discharge)

[0139] The fracture rate after cyclic charge-discharge was evaluated as follows. At 40°C, a charge-discharge cycle of 1C (Capacity / 1C) was defined as one cycle, and 10,000 cycles were performed. The batteries were then disassembled after the cycles. Visual inspection was performed to confirm the fracture of the copper foil in the negative electrode current collector. The ratio of the total number of fractured batteries to the number of batteries manufactured was used as the fracture rate after cyclic charge-discharge. It should be noted that 100 batteries were manufactured.

[0140] Table 1 shows the structure and evaluation results of the batteries in Examples 1-4 and Comparative Examples 1-4.

[0141] [Table 1]

[0142] W = 25mm

[0143]

[0144] The following information can be obtained from Table 1.

[0145] In the batteries of Examples 1 to 4 where C1 / W and C2 / W satisfy equations (1) and (2), the breakage rate is 0%. In contrast, in the batteries of Comparative Examples 1 to 4 where C1 / W and C2 / W do not satisfy equations (1) and (2), the breakage rate is 20% or more.

[0146] Furthermore, in the batteries of Examples 1 to 4, the breakage rate after cyclic charge-discharge cycles was 10% or less. In contrast, in the batteries of Comparative Examples 1 to 4, the breakage rate after cyclic charge-discharge cycles was 60% or more.

[0147] In addition, as shown in Example 4, when C1 = C2, the fracture rate is also 0%, and the fracture rate after cyclic charge and discharge is also a low value of 8%.

[0148] In addition, as in Comparative Examples 1 to 3, in batteries that only satisfy either of the relations (1) and (2), the breakage rate is as high as 21% to 32%, and the breakage rate after cyclic charging and discharging is as high as 69% to 90%.

[0149] [Examples 5-11]

[0150] Next, a battery was fabricated with C1 / W = 0.10 and C2 / W = 0.10, satisfying equations (1) and (2). The battery was fabricated using the same method as in Example 1. The same evaluation as in Example 1 was performed while changing the sulfur content in the copper foil of the negative electrode current collector and the elongation of the copper foil.

[0151] Table 2 shows the structure and evaluation results of the batteries in Examples 5 to 11.

[0152] [Table 2]

[0153] (C1 / W, C2 / W = 0.10)

[0154]

[0155] The following information can be obtained from Table 2.

[0156] In the batteries of Examples 5 to 11 where C1 / W and C2 / W satisfy the relationships (1) and (2), the breakage rate can be reduced to 0%. In addition, the breakage rate after cyclic charge and discharge can be reduced to 12% or less.

[0157] Furthermore, in Examples 7 to 10, where the copper foil in the negative electrode current collector contains a copper foil sulfur content of 20 ppm or less and a copper foil elongation of 7% or more, the breakage rate can be reduced to a single digit (8% or less).

[0158] [Comparative Examples 5-11]

[0159] Next, a battery was fabricated with C1 / W = 0.40 and C2 / W = 0.48, which did not satisfy equations (1) and (2). The battery was fabricated using the same method as in Example 1. The same evaluation as in Example 1 was performed while changing the sulfur content in the copper foil of the negative electrode current collector and the elongation of the copper foil.

[0160] Table 3 shows the structure and evaluation results of the batteries in Comparative Examples 5 to 11.

[0161] [Table 3]

[0162] (C1 / W = 0.40, C2 / W = 0.48)

[0163]

[0164] The following information can be obtained from Table 3.

[0165] In Comparative Examples 5-11, where C1 / W and C2 / W do not satisfy equations (1) and (2), the breakage rate is high, exceeding 69%. Furthermore, the breakage rate after cyclic charge-discharge is 100% for all of them. Thus, in batteries where C1 / W and C2 / W do not satisfy equations (1) and (2), even with changes in the copper foil sulfur composition and copper foil elongation, the breakage rate and the breakage rate after cyclic charge-discharge remain high.

[0166] <Variation Example>

[0167] The embodiments and examples of the present invention have been described in detail above. However, the present invention is not limited to the embodiments and examples described above, and various modifications can be made based on the technical concept of the present invention.

[0168] For example, the structures, methods, processes, shapes, materials, and values ​​listed in the above embodiments and examples are merely examples, and different structures, methods, processes, shapes, materials, and values ​​can be used as needed. Furthermore, the structures, methods, processes, shapes, materials, and values ​​in the above embodiments and examples can be combined with each other as long as they do not depart from the spirit of the invention.

[0169] Furthermore, the chemical formulas of the compounds exemplified in the above embodiments are representative; they are simply general names of the same compounds and are not limited to the stated valence numbers, etc. Additionally, in the numerical ranges described in stages in the above embodiments, the upper or lower limit of a certain stage's numerical range can be replaced with the upper or lower limit of other stages' numerical ranges. Furthermore, unless otherwise specified, the materials exemplified in the above embodiments can be used individually or in combination of two or more.

[0170] Symbol Explanation

[0171] 10. Shell; 20. Electrode body; 20A. Flat portion; 21. Positive electrode; 21A. Positive current collector; 21B1, 21B2. Positive active material layer; 22. Negative electrode; 22A. Negative current collector; 22B1, 22B2. Negative active material layer; 23A, 23B. Separator; 31. Positive electrode tab; 32. Negative electrode tab; 41A, 41B. End; P51, P52. Fold-back position.

Claims

1. A secondary battery, comprising: An elongated cylindrical energy storage element, wherein the elongated cylindrical energy storage element is formed by winding a positive electrode with a positive active material layer formed on a positive current collector and a negative electrode with a negative active material layer formed on a negative current collector; and Outer packaging There are at least two foldback positions on either the positive or negative terminal located at the innermost periphery of the energy storage element. The first and second intersection points of the positive current collector located at the innermost circumference and the long axis of the elongated cylindrical energy storage element, counted from the winding direction, are respectively called the first fold-back position and the second fold-back position. The distance between the end of the positive active material layer on the winding start end side of the positive electrode and the second fold-back position is defined as distance C1. The distance between the end of the positive active material layer on the winding end side of the positive electrode and the first fold-back position is defined as distance C2. The length of the energy storage element in the long side direction is defined as W. The following relationships (1) and (2) are satisfied, wherein the long axis refers to an imaginary line that is parallel to the long side direction and passes through the winding center when viewed from the direction perpendicular to the circumference of the elongated cylindrical energy storage element: 0.02≤C1 / W≤0.12……Equation (1) 0.02≤C2 / W≤0.12……Equation (2).

2. The secondary battery according to claim 1, wherein, The positive and negative electrodes are connected to the outermost periphery of the energy storage element.

3. The secondary battery according to claim 2, wherein, The positive electrode tab and the negative electrode tab are located on the outermost flat portion. The ends of the positive electrode active material layer on the beginning and ending sides of the positive electrode winding are located on the flat side opposite to the flat side where the positive electrode tab and the negative electrode tab are located.

4. The secondary battery according to any one of claims 1 to 3, wherein, The negative electrode current collector contains less than 20 ppm of sulfur and has an elongation of more than 7%.

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