Secondary battery

Abstract

To suppress the breakage of an anode collector that could occur when the anode inflates.SOLUTION: Provided is a secondary battery that satisfies relational expressions (1) and (2) blow, where, with at least two wrap positions P51, P52 existing in either a cathode or an anode located in the innermost circumference of a power storage element, and a straight line drawn passing through at least two points of one of a cathode collector and an anode collector located on the innermost circumference of the power storage element as seen from the axial direction of winding, distance C1 represents the distance between an edge of a cathode active material layer on the winding start edge side of the cathode and a wrap position close to the edge of the cathode active material layer; distance C2 represents the distance between an edge of a cathode active material layer on the winding end edge side of the cathode and a wrap position close to the edge of the cathode active material layer; and W represents the length in the longitudinal direction of the power storage element. Formula (1): 0.02≤C1 / W≤0.12, formula (2): 0.02≤C2 / W≤0.12.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a secondary battery.

Background Art

[0002] There is known a secondary battery having a wound structure in which a strip-shaped positive electrode and a negative electrode are wound via a strip-shaped separator. Patent Document 1 describes a lithium-ion battery as a secondary battery having such a wound structure. In the lithium-ion battery described in Patent Document 1, the inner peripheral end portion 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.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the lithium-ion battery described in Patent Document 1, due to the expansion and contraction of the power storage element accompanying the charge and discharge cycles, stress concentrates on the negative electrode current collector, and the negative electrode current collector may break.

[0005] An object of the present invention is to provide a secondary battery capable of suppressing breakage of the negative electrode current collector.

Means for Solving the Problems

[0006] To solve the above problems, the present invention provides a power storage element having an oval cylindrical shape in which a positive electrode having a positive electrode active material layer formed on a positive electrode current collector and a negative electrode having a negative electrode active material layer formed on a negative electrode current collector are wound, an exterior body, and At least two folding positions exist on either the positive electrode or the negative electrode located at the innermost circumference of the energy storage element. When viewed from the axial direction of winding, the energy storage element is either the positive electrode current collector or the negative electrode Current collector located at the innermost circumference of the energy storage element. When a straight line passing through at least two locations is drawn, the position where the longest straight line intersects with either the positive electrode current collector or the negative electrode Current collector at least at two locations is the folding position. Let the distance between the end of the positive electrode active material layer on the starting end side of the winding of the positive electrode and the folding position close to the end of the positive electrode active material layer be distance C1, and the distance between the end of the positive electrode active material layer on the ending end side of the winding of the positive electrode and the folding position close to the end of the positive electrode active material layer be distance C2. When the length of the energy storage element in the longitudinal direction is W, the secondary battery satisfies the following relational expressions (1) and (2). 0.02 ≦ C1 / W ≦ 0.12 ··· Expression (1) 0.02 ≦ C2 / W ≦ 0.12 ··· Expression (2)

Advantages of the Invention

[0007] According to the present invention, breakage of the negative electrode current collector can be suppressed.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0009] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The description will be made in the following order. <Problems to be Considered in this Embodiment> <One Embodiment> <Modification Example> 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 and the like.

[0010] <Problems to be Considered in this Embodiment> First, in order to facilitate the understanding of this embodiment, the problems to be considered in this embodiment will be described. In a lithium-ion battery having a winding structure, at the flat portion of the winding structure, there can be portions where the positive electrode active material layer and the negative electrode active material layer face each other and portions where they do not. During charging of the lithium-ion battery, at the portions where the positive electrode active material layer and the negative electrode active material layer face each other, lithium is occluded in the negative electrode active material layer, causing the negative electrode to expand. However, at the non-facing portions, the negative electrode does not expand. For this reason, during charging, the distribution of stress accompanying the expansion of the negative electrode becomes non-uniform, resulting in local stress concentration. In particular, stress concentration occurs near the boundary between the flat portion and the curved portion in the winding structure. There was a problem that the foil of the negative electrode current collector was broken due to the stress concentration. While considering such problems, an embodiment of the present invention will be described in detail below.

[0011] <One Embodiment> [Configuration of the Battery] First, with reference to FIGS. 1 to 3, an example of the configuration of a non-aqueous electrolyte secondary battery (hereinafter simply referred to as "battery") according to an embodiment of the present invention will be described. As shown in FIG. 1, the battery has a flat shape. The battery includes a wound electrode body 20 having a flat shape, to which a positive electrode tab (positive electrode lead) 31 and a negative electrode tab (negative electrode lead) 32 are attached, an electrolytic solution (not shown) as an electrolyte, and a case 10 that houses these electrode body 20 and the electrolytic solution. When the battery is viewed in plan from a direction perpendicular to its main surface, the battery has a rectangular shape.

[0012] (Case) A case 10, which is an example of an outer package, is a rectangular parallelepiped-shaped thin battery can made of metal. As the metal, for example, iron (Fe) plated with nickel (Ni) can be used. When a metal case is used, by connecting it to either the positive electrode or the negative electrode, the case itself can also serve as a terminal of the battery, making it easier to miniaturize the battery. The case 10 includes a housing portion 11 and a lid portion 12. The housing portion 11 houses the electrode body 20. The housing portion 11 includes a main surface portion 11A and a wall portion 11B provided at 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 surface and the end surface of the electrode body 20. A positive electrode terminal 13 is provided at a portion of the wall portion 11B that faces one end surface of the electrode body 20 (the end surface from which the positive electrode tab 31 and the negative electrode tab 32 are taken out). The positive electrode tab 31 is connected to the positive electrode terminal 13. The negative electrode tab 32 is connected to the inner surface of the case 10. The lid portion 12 covers the opening of the housing portion 11. The top of the wall portion 11B of the housing portion 11 and the peripheral portion of the lid portion 12 are joined by welding, an adhesive, or the like. Also, the case 10 may be made of a non-rigid material such as a laminated film, but is preferably a metal case mainly composed of metal. The metal case has a certain rigidity and restrains the electrode body 20. Therefore, it is possible to suppress deformation of the battery accompanying expansion and contraction of the electrode body 20 and to suppress breakage of the negative electrode current collector.

[0013] (Positive electrode tab, negative electrode tab) The positive electrode tab 31 and the negative electrode tab 32 are led out from one end surface of the electrode body 20. The positive electrode tab 31 and the negative electrode tab 32 are each made of a metal material such as Al, Cu, Ni, or stainless steel, and are each formed in a thin plate shape or the like.

[0014] Between the case 10 and the positive tab 31 and between the case 10 and the negative tab 32, sealants (adhesive films) 31A and 32A for preventing the intrusion of outside air are inserted, respectively. The sealants 31A and 32A are made of a material having adhesiveness to the positive tab 31 and the negative tab 32, for example, a polyolefin resin such as polyethylene, polypropylene, modified polyethylene or modified polypropylene.

[0015] (Electrode body) The electrode body 20 is an oval cylindrical-shaped power storage element in which a positive electrode having a positive electrode active material layer formed on a positive electrode current collector and a negative electrode having a negative electrode active material layer formed on a negative electrode current collector are wound. The electrode body 20 will be described in detail.

[0016] As shown in FIG. 2, the electrode body 20 has a pair of opposing flat portions 20A and a pair of opposing curved portions 20B provided 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 separators 23A and 23B, insulating members 25B1 and 25B2 provided on the positive electrode 21, and insulating members 26B1 and 26B2 provided on the negative electrode 22. The separators 23A and 23B are alternately provided between the positive electrode 21 and the negative electrode 22. The electrode body 20 has a configuration in which the positive electrode 21 and the negative electrode 22 are laminated via the separator 23A or the separator 23B and wound in the longitudinal direction so as to be flat and spiral. The electrode body 20 is wound such that the positive electrode 21 becomes the innermost peripheral electrode and the negative electrode 22 becomes the outermost peripheral electrode. The negative electrode 22, which is the outermost peripheral electrode, is fixed by a winding tape 24. The positive electrode 21, the negative electrode 22, and the separators 23A and 23B are impregnated with an electrolytic solution.

[0017] (Positive electrode) The positive electrode 21 includes a positive electrode current collector 21A having an inner surface 21S1 and an outer surface 21S2, a positive electrode active material layer 21B1 provided on the inner surface 21S1 of the positive electrode current collector 21A, and a positive electrode active material layer 21B2 provided on the outer surface 21S2 of the positive electrode current collector 21A. In this specification, the "inner surface" means the surface located on the winding center side, and the "outer surface" means the surface located on the side opposite to the winding center. The thickness of the positive electrode current collector 21A is, for example, 3 μm or more and 20 μm or less. The thicknesses of the positive electrode active material layers 21B1 and 21B2 are, for example, 30 μm or more and 100 μm or less.

[0018] On the inner surface 21S1 of the end portion on the outer peripheral side of the winding of the positive electrode 21 (hereinafter simply referred to as the "outer peripheral side end portion"), no positive electrode active material layer 21B1 is provided, and a positive electrode current collector exposed portion 21D1 where the inner surface 21S1 of the positive electrode current collector 21A is exposed is provided. On the outer surface 21S2 of the outer peripheral side end portion of the positive electrode 21, no positive electrode active material layer 21B2 is provided, and a positive electrode current collector exposed portion 21D2 where the outer surface 21S2 of the positive electrode current collector 21A is exposed is provided. The 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, for example, substantially the same as the length of the positive electrode current collector exposed portion 21D2 in the winding direction.

[0019] The positive electrode current collector 21A is composed of, for example, a metal foil such as an aluminum foil, a nickel foil, or a stainless steel foil. The positive electrode active material layers 21B1 and 21B2 contain a positive electrode active material capable of occluding and releasing lithium. The positive electrode active material layers 21B1 and 21B2 may further contain at least one of a binder and a conductive agent as necessary.

[0020] (Positive Electrode Active Material) As the positive electrode active material, for example, lithium-containing compounds such as lithium oxide, lithium phosphate, lithium sulfide, or an intercalation compound containing lithium are suitable, and two or more of these may be mixed and used. To increase the energy density, a lithium-containing compound containing lithium, a transition metal element, and oxygen is preferable. Examples of such lithium-containing compounds include lithium composite oxides having a layered rock salt structure and lithium composite phosphates having an olivine structure. As the lithium-containing compound, it is more preferable that it contains at least one selected from the group consisting of Co, Ni, Mn, and Fe as the transition metal element. 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.

[0021] In addition to these, as the positive electrode active material capable of occluding and releasing lithium, inorganic compounds not containing lithium such as MnO2, V2O5, V6O 13 , NiS, and MoS can also be used.

[0022] The positive electrode active material capable of occluding and releasing lithium may be other than those described above. Also, the positive electrode active materials exemplified above may be mixed in two or more kinds in any combination.

[0023] (Binder) As the binder, for example, at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, polyacrylonitrile, styrene-butadiene rubber, carboxymethyl cellulose, and copolymers mainly composed of one of these resin materials can be used.

[0024] (Conductive agent) As the conductive agent, for example, at least one carbon material selected from the group consisting of graphite, carbon fiber, carbon black, acetylene black, ketjen black, carbon nanotube, graphene, etc. can be used. Note that the conductive agent only needs to be a material having conductivity and is not limited to carbon materials. For example, a metal material or a conductive polymer material, etc. may be used as the conductive agent. Further, examples of the shape of the conductive agent include granular, flaky, hollow, needle-like or cylindrical, etc., but it is not particularly limited to these shapes.

[0025] (Negative electrode) The negative electrode 22 includes a negative electrode current collector 22A having an inner surface 22S1 and an outer surface 22S2, a negative electrode active material layer 22B1 provided on the inner surface 22S1 of the negative electrode current collector 22A, and a negative electrode active material layer 22B2 provided 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 thicknesses of the negative electrode active material layers 22B1 and 22B2 are, for example, 30 μm or more and 100 μm or less.

[0026] On the inner surface 22S1 of the outer peripheral side end portion of the negative electrode 22, a negative electrode current collector exposed portion 22D1 where the inner surface 22S1 of the positive electrode current collector 21A is exposed is provided without the negative electrode active material layer 22B1. On the outer surface 22S2 of the outer peripheral side end portion of the negative electrode 22, a negative electrode current collector exposed portion 22D2 where the outer surface 22S2 of the negative electrode current collector 22A is exposed is provided without the negative electrode active material layer 22B2. The negative electrode tab 32 is connected to a portion of the negative electrode current collector exposed portion 22D1 corresponding to the flat portion 20A. Note that the positive electrode tab 31 and the negative electrode tab 32 are provided on the side of the same flat portion 20A.

[0027] The length of the negative electrode current collector exposed portion 22D2 in the winding direction is approximately one turn longer than the length of the negative electrode current collector exposed portion 22D1 in the winding direction. That is, on the outer peripheral side end portion of the negative electrode 22, a single-sided active material layer forming portion in which only the negative electrode active material layer 22B1 of the negative electrode active material layers 22B1 and 22B2 is formed on the negative electrode current collector 22A is provided, for example, for about one turn.

[0028] At the outermost periphery of the negative electrode 22, a portion where both the inner surface 22S1 and the outer surface 22S2 of the negative electrode current collector 22A are exposed (that is, a portion where the negative electrode current collector exposed portions 22D1 and 22D2 are provided on both surfaces of the positive electrode 21) is provided, for example, over approximately one turn. As a result, the negative electrode current collector exposed portion 22D2 and the inner surface of the case 10 are in electrical contact. Therefore, the negative electrode 22 and the case 10 can be electrically connected, and the resistance can be further reduced.

[0029] The negative electrode current collector 22A is composed of, for example, a metal foil such as a copper foil, a nickel foil, or a stainless steel foil. In this embodiment, a copper foil is used as the negative electrode current collector 22A. As the copper foil of the negative electrode current collector 22A, a copper foil in which impurities (for example, sulfur components) contained in the copper foil are 20 ppm (parts per million) or less and the elongation rate after heat treatment at 200 °C is 7% or more is used. The elongation rate after heat treatment at 200 °C means the elongation rate measured at room temperature after heating at 200 °C for 3 hours. For example, a test using an autograph AG-IS manufactured by Shimadzu Corporation is performed, the measurement sample size is ASTM-D638-V (the size is a maximum width of 9.53 mm, a minimum width of 3.15 mm, and a length of 63.50 mm orthogonal to the width), the test speed is 1 mm / min, and a copper foil with an elongation rate of 7% or more as a result of measurement at room temperature after heating at 200 °C for 3 hours is used.

[0030] The negative electrode active material layers 22B1 and 22B2 contain a negative electrode active material capable of occluding and releasing lithium. The negative electrode active material layers 22B1 and 22B2 may further contain at least one of a binder and a conductive agent as required.

[0031] (Negative electrode active material) Examples of the negative electrode active material include carbon materials such as graphitization-resistant carbon, graphitization-prone carbon, graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, carbon fibers, or activated carbon. Among these, cokes include pitch coke, needle coke, or petroleum coke. The fired organic polymer compound refers to a material obtained by firing a polymer material such as a phenol resin or a furan resin at an appropriate temperature to carbonize it, and some of them are classified as graphitization-resistant carbon or graphitization-prone carbon. These carbon materials are preferred because they have very little change in crystal structure during charge and discharge, can obtain a high charge-discharge capacity, and can obtain good cycle characteristics. In particular, graphite is preferred because it has a large electrochemical equivalent and can obtain a high energy density. Also, graphitization-resistant carbon is preferred because excellent cycle characteristics can be obtained. Furthermore, those with a low charge-discharge potential, specifically those with a charge-discharge potential close to that of lithium metal, are preferred because they can easily achieve a high energy density of the battery.

[0032] (Binder) As the binder, the same ones as those for the positive electrode active material layers 21B1 and 21B2 can be used.

[0033] (Conductive agent) As the conductive agent, the same ones as those for the positive electrode active material layers 21B1 and 21B2 can be used.

[0034] (Separator) The separators 23A and 23B isolate the positive electrode 21 and the negative electrode 22 and prevent a short circuit of the current due to contact between the two electrodes while allowing lithium ions to pass through. The separators 23A and 23B are composed of, for example, a porous membrane made of polytetrafluoroethylene, a polyolefin resin (such as polypropylene (PP) or polyethylene (PE)), an acrylic resin, a styrene resin, a polyester resin, or a nylon resin, or a resin blend of these, and may have a structure in which two or more of these porous membranes are laminated.

[0035] Among them, the porous film made of polyolefin is preferable because it has an excellent short-circuit prevention effect and can improve the safety of the battery by the shutdown effect. In particular, polyethylene can obtain a shutdown effect within the range of 100°C or higher and 160°C or lower, and also has excellent electrochemical stability. Therefore, it is preferable as a material constituting the separators 23A and 23B. Among them, low-density polyethylene, high-density polyethylene, and linear polyethylene are preferably used because they have an appropriate melting temperature and are easily available. In addition, a resin having chemical stability can be used as a material copolymerized or blended with polyethylene or polypropylene. Alternatively, the porous film may have a structure of three or more layers in which a polypropylene layer, a polyethylene layer, and a polypropylene layer are sequentially laminated. For example, it is desirable to have a three-layer structure of PP / PE / PP, and the mass ratio [wt%] of PP to PE is PP:PE = 60:40 to 75:25. Alternatively, from the perspective of cost, a single-layer base material of 100 wt% of PP or 100 wt% of PE can also be used. The method for producing the separators 23A and 23B may be wet or dry.

[0036] As the separators 23A and 23B, a nonwoven fabric may be used. As the fibers constituting the nonwoven fabric, aramid fibers, glass fibers, polyolefin fibers, polyethylene terephthalate (PET) fibers, nylon fibers, or the like can be used. Also, a nonwoven fabric may be formed by mixing two or more of these fibers.

[0037] (Electrolyte solution) The electrolyte solution is a so-called non-aqueous electrolyte solution, and includes an organic solvent (non-aqueous solvent) and an electrolyte salt dissolved in this organic solvent. The electrolyte solution may contain known additives in order to improve battery characteristics. Instead of the electrolyte solution, an electrolyte layer including the electrolyte solution and a polymer compound serving as a holding body for holding this electrolyte solution may be used. In this case, the electrolyte layer may be in a gel state.

[0038] As the organic solvent, cyclic carbonates such as ethylene carbonate or propylene carbonate can be used, and it is preferable to use one of ethylene carbonate and propylene carbonate, particularly preferably a mixture of both. This is because the cycle characteristics can be further improved.

[0039] As the organic solvent, in addition to these cyclic carbonates, it is preferable to mix and use chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate or methyl propyl carbonate. This is because high ionic conductivity can be obtained.

[0040] Furthermore, as the organic solvent, it is 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 cycle characteristics. Therefore, mixing and using them is preferable because the discharge capacity and cycle characteristics can be further improved.

[0041] In addition to these, examples of the organic solvent 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-methylpyrrolidinone, N-methyloxazolidinone, N,N-dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, dimethyl sulfoxide or trimethyl phosphate.

[0042] Note that a compound in which at least a part of the hydrogen of these organic solvents is substituted with fluorine may be preferable in some cases because it may improve the reversibility of the electrode reaction depending on the type of the electrodes to be combined.

[0043] Examples of the electrolyte salt include lithium salts, which may be used alone or in combination of two or more. Examples of the lithium salt include LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiAlCl4, LiSiF6, LiCl, lithium difluoro[oxalato-O,O']borate, lithium bisoxalate borate, or LiBr. Among them, LiPF6 is preferred because it can provide high ionic conductivity and further improve the cycle characteristics.

[0044] (Insulating member) The insulating members 25B1, 25B2, 26B1, and 26B2 have, for example, a rectangular film shape and have an adhesive surface on one side. More specifically, the insulating members 25B1, 25B2, 26B1, and 26B2 include a base material and an adhesive layer provided on the base material. In this specification, pressure sensitive adhesion is defined as a type of adhesion. According to this definition, the pressure sensitive adhesive layer is regarded as a type of adhesive layer. Also, the film is defined to include a sheet. As the insulating members 25B1, 25B2, 26B1, and 26B2, for example, insulating tapes are used. Examples of the material of the insulating members 25B1, 25B2, 26B1, and 26B2 include polyethylene terephthalate (PET), polyimide (PI), polyethylene (PE), or polypropylene (PP).

[0045] (Insulating member provided on the positive electrode) The insulating member 25B1 covers the step portion at the boundary between the positive electrode current collector exposed portion 21D1 and the positive electrode active material layer 21B1, and the positive electrode current collector exposed portion 21D1. The insulating member 25B2 covers the step portion at the boundary between the positive electrode current collector exposed portion 21D2 and the positive electrode active material layer 21B2, and the positive electrode current collector exposed portion 21D2. Note that the insulating member 25B2 also covers the positive electrode tab 31 together with the positive electrode current collector exposed portion 21D2. The boundaries between the positive electrode current collector exposed portion 21D1 and the positive electrode active material layer 21B1, and between the positive electrode current collector exposed portion 21D2 and the positive electrode active material layer 21B2 are formed parallel to the winding axis direction of the electrode body 20.

[0046] The insulating member 25B1 is provided in a region where the positive electrode current collector exposed portion 21D1 faces the negative electrode active material layer 22B2, and in a region where the positive electrode current collector exposed portion 21D1 faces the negative electrode current collector exposed portion 22D2. The insulating member 25B2 is provided in a region where the positive electrode current collector exposed portion 21D2 faces the negative electrode active material layer 22B1, and in a region where the positive electrode current collector exposed portion 21D2 faces the negative electrode current collector exposed portion 22D1.

[0047] The positive electrode 21 has a positive electrode current collector exposed portion 21D3 where the outer peripheral side end portion of the positive electrode current collector exposed portion 21D1 is exposed without being covered by the insulating member 25B1, and a positive electrode current collector exposed portion 21D4 where the outer peripheral side end portion of the positive electrode current collector exposed portion 21D2 is exposed without being covered by the insulating member 25B2.

[0048] (Insulating member provided on the negative electrode) The insulating member 26B1 covers the portion of the negative electrode current collector exposed portion 22D1 where the negative electrode tab 32 is provided and the portion facing the positive electrode current collector exposed portion 21D4. The insulating member 26B1 may cover substantially the entire portion corresponding to one flat portion 20A of the negative electrode current collector exposed portion 22D1.

[0049] The insulating member 26B2 covers a stepped portion at the boundary 22P between the negative electrode current collector exposed portion 22D2 and the negative electrode active material layer 22B2 (i.e., the boundary 22P between the single-sided active material layer forming portion and the negative electrode active material layer 22B2) and the negative electrode current collector exposed portion 22D2. The boundary 22P between the negative electrode current collector exposed portion 22D2 and the negative electrode active material layer 22B2 is formed parallel to the winding axis direction of the electrode body 20. The insulating member 26B2 preferably also covers a portion of the negative electrode current collector exposed portion 22D2 that faces the positive electrode current collector exposed portion 21D3. The positive electrode current collector exposed portion 21D3 is located on the outer circumferential side of the winding of the electrode body 20 with respect to the boundary 22P, and the negative electrode tab 32 is located on the outer circumferential side of the winding of the electrode body 20 with respect to the positive electrode current collector exposed portion 21D3. The positive electrode current collector exposed portion 21D3 is located, for example, on the flat portion 20A opposite to the flat portion 20A where the boundary 22P is provided.

[0050] (Folding position) There are at least two folding positions in either the positive electrode or the negative electrode located at the innermost circumference of the energy storage element. For example, as shown in FIG. 3, there are two folding positions P51 and P52 in the positive electrode 21 located at the innermost circumference of the electrode body 20 according to the present embodiment. Depending on the winding structure of the electrode body 20, the negative electrode 22 may be present at the innermost circumference, and folding positions of the negative electrode 22 may exist.

[0051] Further, the positive electrode 21 constituting the electrode body 20 has a winding start end portion that is the starting point of the winding structure and a winding end end portion that is the ending point of the winding structure. On the winding start end portion side of the positive electrode 21, an end portion 41A of the positive electrode active material layer 21B1 exists. Also, on the winding end end portion side of the positive electrode 21, an end portion 41B of the positive electrode active material layer 21B1 exists. The distance (distance in the long axis direction of the electrode body 20) between the end portion 41A of the positive electrode active material layer 21B1 and a folding position P51 close to the end portion 41A of the positive electrode active material layer 21B1 is defined as C1 (mm). Also, the distance (distance in the long axis direction of the electrode body 20) between the end portion 41B of the positive electrode active material layer 21B2 on the winding end end portion side of the positive electrode 21 and a folding position P52 close to the end portion 41B of the positive electrode active material layer is defined as distance C2 (mm). Note that, as in the present embodiment, when the positive electrode active material layers are formed on both surfaces of the positive electrode current collector 21A, the distance C1 or the distance C2 is defined by the end portion of the positive electrode active material layer closer to the folding position.

[0052] Also, let the length of the electrode body 20 in the longitudinal direction (long axis direction) be W (mm). In this case, the battery satisfies the following relational expressions (1) and (2). 0.02 ≦ C1 / W ≦ 0.12 ··· Expression (1) 0.02 ≦ C2 / W ≦ 0.12 ··· Expression (2) The distances C1 and C2 may be equal (C1 = C2) in length.

[0053] As shown in FIG. 2, in the electrode body 20 according to the present 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 electrode current collector 21A located on the outermost periphery, and the negative electrode tab 32 is connected to the negative electrode current collector 22A located on the outermost periphery.

[0054] More specifically, the positive electrode tab 31 and the negative electrode tab 32 are located on the outermost flat portion (the upper flat portion 20A in FIG. 2). And the end portion 41A of the positive electrode active material layer 21B1 and the end portion 41B of the positive electrode active material layer 21B2 described above are located on the flat portion on the side opposite to the flat portion where the positive electrode tab 31 and the negative electrode tab 32 are located (the lower flat portion 20A in FIG. 2).

[0055] [Method for manufacturing battery] Next, an example of a method for manufacturing a battery according to an embodiment of the present invention will be described.

[0056] [Positive electrode manufacturing process] The positive electrode 21 is manufactured 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, this positive electrode mixture slurry is applied to both sides of the positive electrode current collector 21A, the solvent is dried, and compression molding is performed using a roll press or the like to form positive electrode active material layers 21B1 and 21B2, thereby obtaining the positive electrode 21. At this time, the application 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.

[0057] Next, the positive electrode tab 31 is attached to the positive electrode current collector exposed portion 21D2 provided at one end of the positive electrode 21 by welding. Next, insulating members 25B1 and 25B2 are respectively bonded to the positive electrode current collector exposed portions 21D1 and 21D2 provided at one end of the positive electrode 21.

[0058] [Negative electrode manufacturing process] 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 prepare a paste-like negative electrode mixture slurry. Next, this negative electrode mixture slurry is applied to both sides of the negative electrode current collector 22A, the solvent is dried, and compression molding is performed using a roll press or the like to form negative electrode active material layers 22B1 and 22B2, thereby obtaining the negative electrode 22. At this time, the application 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.

[0059] Next, a negative electrode tab 32 is attached to a negative electrode current collector exposed portion 22D1 provided at one end of the negative electrode 22 by welding. Next, insulating members 26B1 and 26B2 are respectively bonded to positive electrode current collector exposed portions 21D1 and 21D2 provided at the other end of the negative electrode 22.

[0060] (Winding step) The electrode body 20 is manufactured by winding the positive electrode 21, the negative electrode 22, and the separators 23A and 23B around a winding core to a specified length. Note that the positive electrode 21 and the negative electrode 22 are previously cut to a specified length.

[0061] (Bending step of the negative electrode end portion) Using a jig (not shown), the outer peripheral side end portion of the negative electrode 22 is tilted in a predetermined direction (for example, downward). The outer peripheral side end portion of the negative electrode 22 tilted in this way includes a boundary 22P between the negative electrode current collector exposed portion 22D2 and the negative electrode active material layer 22B2. Since the insulating member 26B2 covers the boundary 22P, the rigidity of the negative electrode 22 at the boundary 22P can be increased, and the outer peripheral side end portion of the negative electrode 22 can be suppressed from bending starting from the boundary 22P. Therefore, it is possible to suppress the negative electrode active material from falling off from the portion located on the back side of the boundary 22P in the negative electrode active material layer 22B1. Thus, it is possible to suppress the occurrence of a minute short circuit due to the falling off of the negative electrode active material. Note that the outer peripheral side end portion of the negative electrode 22 may be tilted by means other than the jig.

[0062] Since the negative electrode tab 32 is previously attached to the outer peripheral side end portion of the negative electrode 22, the negative electrode tab 32 can function as a weight when tilting the outer peripheral side end portion of the negative electrode 22. Therefore, the outer peripheral side end portion of the negative electrode 22 can be easily tilted. Thus, in the "separator cutting step", which is a subsequent step to the "bending step of the negative electrode end portion", it is possible to suppress the negative electrode 22 from being cut together with the separators 23A and 23B.

[0063] (Separator cutting step) After supporting the separators 23A and 23B above the electrode body 20 with a support member (not shown), the separators 23A and 23B are cut by a cutter. After cutting, the outer peripheral side end portion of the negative electrode 22, which is the outermost peripheral electrode, is fixed by a winding tape 24. Thereby, the electrode body 20 is obtained.

[0064] Note that in the wound state, the negative electrode 22 is attracted to the separator 23A by static electricity. If the separators 23A and 23B are cut in this state, the negative electrode 22 may also be cut together with the separators 23A and 23B, and the negative electrode 22 may become shorter than the specified length. By bending the outer peripheral side end portion of the negative electrode 22 as described above and then cutting the separators 23A and 23B, it is possible to suppress the negative electrode 22 from being cut together with the separators 23A and 23B.

[0065] (Sealing process) The electrode body 20 is sealed by the case 10 as follows. First, the electrode body 20 and the electrolytic solution are housed in the housing portion 11. Subsequently, the positive electrode tab 31 is connected to the positive electrode terminal 13 provided on the case 10, and the negative electrode tab 32 is connected to the inner surface of the case 10. Next, the opening of the housing portion 11 is covered with the lid portion 12, and the peripheral edges of the housing portion 11 and the lid portion 12 are joined by welding or an adhesive or the like. Thereby, a battery is obtained.

[0066] [Effect] In this embodiment, the following effects can be obtained. The ranges of the distances C1 and C2 are set to the ranges described in the embodiment, that is, the ranges that satisfy both of the relational expressions (1) and (2). Thereby, in each of the two flat portions, it becomes possible to oppose the positive electrode active material layer of the positive electrode and the positive electrode active material layer of the negative electrode over a wide range. Therefore, the expansion of the negative electrode during charging occurs uniformly in all directions, and it is possible to suppress the occurrence of local stress concentration in the electrode body. Further, it is possible to suppress the breakage of the negative electrode current collector due to local stress concentration. In addition, since there are a positive electrode tab and a negative electrode tab at the outermost periphery, the distortion of the positive electrode and the negative electrode becomes more prominent by the amount of the step of each lead. However, by making the distance C1 and the distance C2 satisfy the relational expressions (1) and (2), respectively, it becomes difficult to cause breakage. Also, two ends of the positive electrode active material layer are positioned on the flat portion opposite to the flat portion to which the positive electrode tab and the negative electrode tab are connected. As a result, the distorted portions of the positive electrode and the negative electrode caused by the step become symmetric in the plan view of the electrode. Thereby, the distortion can be dispersed and breakage can be further suppressed. Also, by setting the distance C1 = C2, breakage of the negative electrode can be effectively suppressed. Furthermore, as the copper foil of the negative electrode current collector, a copper foil in which impurities (for example, sulfur component) contained in the copper foil are 20 ppm or less and the elongation rate after heat treatment at 200 ° C. is 7% or more is used. By the copper foil stretching during expansion, it is possible to suppress the copper foil from breaking.

Example

[0067] Hereinafter, the present invention will be specifically described by way of examples. However, the present invention is not limited to only these examples.

[0068] [Examples 1 to 4] (Positive electrode manufacturing process) The positive electrode was manufactured as follows. First, 91 parts by mass of lithium cobalt composite oxide (LiCoO2) as a 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 were mixed to obtain a positive electrode mixture, and then dispersed in N-methyl-2-pyrrolidone to obtain a paste-like positive electrode mixture slurry.

[0069] Next, a strip-shaped aluminum foil with a thickness of 19 μm was prepared as the positive electrode current collector. After applying the positive electrode mixture slurry to both sides of this aluminum foil and drying it, it was compression-molded with a roll press to form a positive electrode active material layer, thereby obtaining a positive electrode. At this time, the application position of the positive electrode mixture slurry was adjusted so that positive electrode current collector exposed portions were formed on both sides of one end of the positive electrode. Next, a positive electrode tab made of aluminum was welded and attached to the positive electrode current collector exposed portion that becomes the outer surface of the outer peripheral side end portion after winding among the positive electrode current collector exposed portions formed on both sides of one end of the positive electrode. Next, insulating tapes were respectively attached to the positive electrode current collector exposed portions formed on both sides of one end of the positive electrode (see FIG. 2).

[0070] (Manufacturing process of the negative electrode) The negative electrode was manufactured 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 the binder were mixed to obtain a negative electrode mixture, and then it was dispersed in N-methyl-2-pyrrolidone to obtain a paste-like negative electrode mixture slurry.

[0071] Next, a strip-shaped copper foil with a thickness of 6 μm was prepared as the negative electrode current collector. After applying the negative electrode mixture slurry to both sides of the copper foil and drying it, it was compression-molded with a roll press to form a negative electrode active material layer, thereby obtaining a negative electrode. At this time, the application position of the negative electrode mixture slurry was adjusted so that negative electrode current collector exposed portions were formed on both sides of one end of the negative electrode. Next, a negative electrode tab made of nickel was welded and attached to the negative electrode current collector exposed portion that becomes the inner surface of the outer peripheral side end portion after winding among the negative electrode current collector exposed portions formed on both sides of one end of the negative electrode. Next, insulating tapes were respectively attached to the negative electrode current collector exposed portions formed on both sides of one end of the negative electrode (see FIG. 2).

[0072] (Manufacturing process of the electrolytic solution) The electrolytic solution was prepared as follows. First, ethylene carbonate (EC) and propylene carbonate (PC) were mixed so that the mass ratio was EC:PC = 1:1 to prepare a mixed solvent. Next, lithium hexafluorophosphate (LiPF6) as an electrolyte salt was dissolved in this mixed solvent so that the concentration was 1.0 mol / kg to prepare an electrolytic solution.

[0073] (Manufacturing Process of Battery) The battery was manufactured as follows. First, a positive electrode, a negative electrode, and two separators were wound around a winding core to obtain a wound electrode body having a flat shape. As the separator, a microporous polyethylene film with a thickness of 25 μm was used. Subsequently, the outer peripheral side end portion of the negative electrode was bent by a jig. Next, after supporting the separator above the electrode body with a support member, the separator was cut by a cutter. Then, the outer peripheral side end portion of the negative electrode, which is the outermost peripheral electrode, was fixed with a winding tape. Thus, an electrode body was obtained. Next, the electrode body and the electrolytic solution were housed in the housing portion of a metal can, the opening of the housing portion was covered with a lid portion, and the peripheral edge portions of the housing portion and the lid portion were joined to seal the metal can. Thus, the target battery was obtained.

[0074] Note that the length in the longitudinal direction of the electrode body was set to 25 mm. And in the manufacturing process of the positive electrode, the start winding position and the end winding position of the positive electrode current collector were appropriately adjusted. Also, by adjusting the coating position of the positive electrode mixture slurry, the positions of the end portions of the positive electrode active material layer on the start winding end portion side of the positive electrode and the end portions of the positive electrode active material layer on the end winding end portion side of the positive electrode were appropriately adjusted. The above adjustments were made so as to satisfy the relational expressions (1) and (2).

[0075] [Comparative Examples 1 to 4] A battery was obtained in the same manner as in Example 1 except that the adjustment was made so as not to satisfy the relational expressions (1) and (2).

[0076] (Breakage Incidence Rate) The breakage incidence rate was evaluated as follows. The battery was overcharged until the SOC (State of Charge) of the battery reached 150%, and the overcharged battery was disassembled. At that time, the breakage of the copper foil of the negative electrode current collector was visually confirmed, and the ratio of the total number of batteries in which breakage occurred to the number of manufactured batteries (evaluation number) was defined as the breakage incidence rate. Note that the number of manufactured batteries was 100.

[0077] (Breakage Incidence Rate after Cycle Charge and Discharge) The breakage occurrence rate after cycle charge and discharge was evaluated as follows. In an environment of 40°C, the battery was charged and discharged at 1C (Capacity) / 1C, and one cycle of charge and discharge was defined as one cycle. The battery was charged and discharged 10,000 times. After the cycle charge and discharge, the battery was disassembled. At that time, the breakage of the copper foil of the negative electrode current collector was visually confirmed, and the ratio of the total number of batteries with breakage to the total number of manufactured batteries was defined as the breakage occurrence rate after cycle charge and discharge. The number of manufactured batteries was 100.

[0078] Table 1 shows the configurations and evaluation results of the batteries of Examples 1 to 4 and Comparative Examples 1 to 4.

[0079]

Table 1

[0080] It can be seen from Table 1 that In the batteries of Examples 1 to 4 where C1 / W and C2 / W satisfy the relational expressions (1) and (2), the breakage occurrence rate could be made 0%. On the other hand, in the batteries of Comparative Examples 1 to 4 where C1 / W and C2 / W do not satisfy the relational expressions (1) and (2), the breakage occurrence rate became 20% or more. Also, in the batteries of Examples 1 to 4, the breakage occurrence rate after cycle charge and discharge could be made 10% or less. On the other hand, in the batteries of Comparative Examples 1 to 4, the breakage occurrence rate after cycle charge and discharge became 60% or more. Also, as in Example 4, when C1 = C2, the breakage occurrence rate was 0%, and the breakage occurrence rate after cycle charge and discharge was also as low as 8%. Also, in the batteries that satisfy only one of the relational expressions (1) and (2) as in Comparative Examples 1 to 3, the breakage occurrence rate was as high as 21% to 32%, and the breakage occurrence rate after cycle charge and discharge was also as high as 69% to 90%.

[0081] [Examples 5 to 11] Next, with C1 / W = 0.10 and C2 / W = 0.10, a battery that satisfies the relational expressions (1) and (2) was fabricated. The method for fabricating the battery was the same as in Example 1. While varying the sulfur content and copper foil elongation rate included in the copper foil of the negative electrode current collector, the same evaluations as in Example 1 and the like were conducted.

[0082] Table 2 shows the configurations and evaluation results of the batteries of Examples 5 to 11.

[0083] [Table 2]

[0084] The following can be understood from Table 2. In the batteries of Examples 5 to 11 where C1 / W and C2 / W satisfy the relational expressions (1) and (2), the breakage occurrence rate could be made 0%. Also, the breakage occurrence rate after cycle charge and discharge could be made 12% or less. Furthermore, in Examples 7 to 10 where the sulfur content of the copper foil included in the negative electrode current collector was 20 ppm or less and the copper foil elongation rate was 7% or more, the breakage occurrence rate could be made to a single digit (8% or less).

[0085] [Comparative Examples 5 to 11] Next, with C1 / W = 0.40 and C2 / W = 0.48, a battery that does not satisfy the relational expressions (1) and (2) was fabricated. The method for fabricating the battery was the same as in Example 1. While varying the sulfur content and copper foil elongation rate included in the copper foil of the negative electrode current collector, the same evaluations as in Example 1 and the like were conducted.

[0086] Table 3 shows the configurations and evaluation results of the batteries of Comparative Examples 5 to 11.

[0087] [Table 3]

[0088] The following can be understood from Table 3. In the batteries of Comparative Examples 5 to 11 where C1 / W and C2 / W do not satisfy the relational expressions (1) and (2), the breakage occurrence rate was as high as 69% or more. In addition, the breakage occurrence rate after cycle charge and discharge all became 100%. Thus, in the batteries where C1 / W and C2 / W do not satisfy the relational expressions (1) and (2), even when the sulfur content of the copper foil and the elongation rate of the copper foil were changed, both the breakage occurrence rate and the breakage occurrence rate after cycle charge and discharge became high values.

[0089] <Modification example> As described above, the embodiments and examples of the present invention have been specifically described. However, the present invention is not limited to the above-described embodiments and examples, and various modifications based on the technical idea of the present invention are possible.

[0090] For example, the configurations, methods, steps, shapes, materials, and numerical values, etc. exemplified in the above-described embodiments and examples are merely examples, and different configurations, methods, steps, shapes, materials, and numerical values, etc. may be used as necessary. In addition, the configurations, methods, steps, shapes, materials, and numerical values, etc. of the above-described embodiments and examples can be combined with each other as long as they do not deviate from the gist of the present invention.

[0091] In addition, the chemical formulas of compounds and the like exemplified in the above-described embodiments are representative, and as long as they are the general names of the same compounds, they are not limited to the valences and the like described. Also, in the numerical ranges described step by step in the above-described embodiments, the upper limit value or the lower limit value of a certain step's numerical range may be replaced with the upper limit value or the lower limit value of another step's numerical range. In addition, the materials exemplified in the above-described embodiments can be used alone or in combination of two or more without particular notice.

Explanation of symbols

[0092] 10 ··· Case 20 ··· Electrode body 20A ··· Flat part 21 ··· Positive electrode 21A ··· Positive electrode current collector 21B1, 21B2 ··· Positive electrode active material layer 22 ··· Negative electrode 22A ··· Negative electrode current collector 22B1, 22B2 ··· Negative electrode active material layer 23A, 23B ··· Separator 31 ··· Positive electrode tab 32 ··· Negative electrode tab 41A, 41B ··· End portion P51, P52 ··· Folding position

Claims

[Claim 1] a storage element having an elongated cylindrical shape in which a positive electrode having a positive electrode active material layer formed on a positive electrode current collector and a negative electrode having a negative electrode active material layer formed on a negative electrode current collector are wound together; An exterior body; having At least two folding positions are present in either the positive electrode or the negative electrode located at the innermost periphery of the energy storage element, when viewed from the axial direction of the winding, a straight line is drawn passing through at least two points on either the positive electrode current collector or the negative electrode current collector located at the innermost periphery of the energy storage element, and a position where the longest straight line intersects with one of the positive electrode current collector or the negative electrode current collector at the at least two points is a folding position; A secondary battery that satisfies the following relational expressions (1) and (2), where C1 is the distance between an end of the positive electrode active material layer at a winding start end side of the positive electrode and the folded-back position close to the end of the positive electrode active material layer, C2 is the distance between the end of the positive electrode active material layer at a winding end end side of the positive electrode and the folded-back position close to the end of the positive electrode active material layer, and W is the length in the longitudinal direction of the storage element. 0.02≦C1 / W≦0.12...Formula (1) 0.02≦C2 / W≦0.12...Formula (2)

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