Secondary battery

Abstract

This secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode and the negative electrode are wound with the separator interposed therebetween. The positive electrode is provided with a belt-form positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode has a positive electrode edge portion including one end in the short-axis direction of the positive electrode and a positive electrode main portion other than the positive electrode edge portion. The positive electrode edge portion has one or more positive electrode current collector exposure parts that are partially provided along the long-axis direction of the positive electrode current collector. The exposure parts do not have the positive electrode mixture layer from the one end in the short-axis direction to the positive electrode main part. The positive electrode mixture layer includes a positive electrode active material and a binding material. The binding material includes a first resin having a first molecular weight M1 and a second resin having a second molecular weight M2. The relationship M1 > M2 is satisfied.

Description

secondary battery

[0001] The present disclosure relates to secondary batteries.

[0002] Patent Document 1 proposes "a method for manufacturing an electrode plate for a storage device, which comprises discharging a mixture slurry from discharge nozzles corresponding to each of a plurality of discharge regions extending along the length of a strip-shaped current collector to form a mixture layer on the current collector, wherein the positions of the discharge regions are set so that a portion of each of the plurality of discharge regions forms an overlapping portion that overlaps a portion of an adjacent discharge region as viewed in the length direction of the current collector, each of the overlapping portions having a length in the width direction of the current collector of 8 mm or less, and an uncoated portion is provided in at least one of the discharge regions by intermittently discharging the mixture slurry."

[0003] Patent Document 2 proposes "an electrode plate for a power storage device, comprising a substantially rectangular current collector and an active material layer provided on at least one surface of the current collector, the current collector having an uncoated portion at one end in the width direction in a partial region in the longitudinal direction to which an electrode lead is connected, and in the region where the active material layer is provided, the elastic modulus of a first region adjacent to the uncoated portion in the width direction is greater than the elastic modulus of a second region adjacent in the longitudinal direction to a region occupied by the uncoated portion and the first region."

[0004] Patent No. 6965162 Patent No. 6821595

[0005] According to the proposals in Patent Documents 1 and 2, a high-capacity electrode plate can be obtained by reducing the uncoated or uncoated areas of the mixture layer. However, as the energy density of the electrode increases, it becomes more difficult to prevent the electrode from breaking or the mixture layer from peeling off from the current collector.

[0006] One aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator, in which the positive electrode and the negative electrode are wound with the separator interposed therebetween, the positive electrode including a strip-shaped positive electrode current collector and a positive electrode composite layer disposed on the positive electrode current collector, the positive electrode having a positive electrode edge portion including one end in a short side direction of the positive electrode and a positive electrode main portion other than the positive electrode edge portion, the positive electrode edge portion having an exposed portion of the positive electrode current collector provided in one or more locations partially along the long side direction of the positive electrode current collector, the exposed portion not including the positive electrode composite layer from the one end in the short side direction to the positive electrode main portion, the positive electrode composite layer including a positive electrode active material and a binder, the binder including a first resin having a first molecular weight M1 and a second resin having a second molecular weight M2, and satisfying M1 > M2.

[0007] According to the present disclosure, in a secondary battery, it is possible to suppress both fracture of the positive electrode and peeling between the positive electrode current collector and the positive electrode mixture layer.

[0008] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0009] 1 is a schematic cross-sectional view of a secondary battery according to an embodiment of the present invention; 2 is a schematic plan view of a positive electrode according to an embodiment of the present invention;

[0010] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit.

[0011] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0012] Secondary batteries include non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, lithium metal secondary batteries, and solid-state batteries containing gel electrolytes or solid electrolytes. That is, the secondary battery may be a liquid secondary battery containing an electrolytic solution as an electrolyte, or an all-solid-state secondary battery containing a solid electrolyte.

[0013] The secondary battery according to the present disclosure includes a strip-shaped positive electrode, a strip-shaped negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode. The positive electrode and the negative electrode are wound (wound) with the separator interposed therebetween. That is, the secondary battery includes a wound electrode group. The wound electrode group has a columnar outer shape, which may be, for example, cylindrical.

[0014] [Positive Electrode] The positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode composite layer disposed on the positive electrode current collector. The positive electrode composite layer may be in the form of a film. The positive electrode comprises a positive electrode current collector and a positive electrode composite layer formed (or supported) on a partial region of the surface of the positive electrode current collector. Specifically, the positive electrode comprises a positive electrode edge portion including one end of the positive electrode in the short direction, and a positive electrode main portion other than the positive electrode edge portion. The positive electrode edge portion has an exposed portion of the positive electrode current collector provided at one or more locations along the longitudinal direction of the positive electrode current collector. The exposed portion of the positive electrode current collector does not have a positive electrode composite layer from the one end of the short direction to the positive electrode main portion. The exposed portions of the positive electrode current collector may be provided intermittently at multiple locations along the longitudinal direction of the positive electrode current collector.

[0015] The positive electrode mixture layer is composed of a positive electrode mixture. Because the positive electrode mixture contains a positive electrode active material as an essential component, the positive electrode mixture layer may also be referred to as a positive electrode active material layer. The positive electrode mixture layer is supported on one or both surfaces of a positive electrode current collector.

[0016] The positive electrode mixture contains a positive electrode active material and a binder as essential components, and may contain optional components such as a conductive additive, a thickener, etc. The binder contains a first resin having a first molecular weight M1 and a second resin having a second molecular weight M2, and M1 > M2 is satisfied. M1 and M2 may be weight-average molecular weights Mw.

[0017] The positive electrode mixture layer can be formed, for example, by applying a positive electrode slurry to the surface of a positive electrode current collector, in which a positive electrode mixture containing particles of a positive electrode active material and a binder, as essential components, and optional components (such as a conductive additive), is dispersed in a dispersion medium, and then drying the slurry. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. Examples of dispersion media for the positive electrode slurry include N-methyl-2-pyrrolidone (NMP), cyclohexanone, alcohol, and ether.

[0018] Here, the binder of the positive electrode composite material contains the first resin and the second resin having different molecular weights, which gives the positive electrode composite material layer the necessary toughness (elongation) and rigidity, making the positive electrode less likely to break and less likely to peel off from the positive electrode current collector.

[0019] Toward higher energy densities in secondary batteries, positive electrode composite layers have become thicker and more densely packed. As the positive electrode composite layer becomes thicker and denser, the elastic modulus of the entire positive electrode increases, necessitating advanced measures to prevent positive electrode fracture and peeling between the positive electrode current collector and the positive electrode composite layer. In particular, when the positive electrode edge has multiple exposed portions of the positive electrode current collector intermittently arranged along the longitudinal direction of the positive electrode current collector, it is important to prevent fracture at the boundary between the exposed portions and the portion containing the positive electrode composite layer. One possible method for preventing fracture at the boundary is to provide first and second regions in the positive electrode composite layer with different elastic moduli. However, as the positive electrode composite layer becomes thicker and denser, achieving a significant difference in elastic modulus becomes increasingly difficult.

[0020] On the other hand, fracture of the positive electrode and peeling of the positive electrode current collector and the positive electrode mixture layer can be controlled by balancing the toughness and rigidity of the positive electrode mixture layer. The balance between the toughness and rigidity of the positive electrode mixture layer varies significantly depending on the molecular weight of the binder. The smaller the molecular weight, the better the toughness, and the higher the molecular weight, the better the rigidity. By using a first resin and a second resin that satisfy M1 > M2 as binders, both the toughness and rigidity can be improved. Furthermore, even in a positive electrode having a thick, high-density positive electrode mixture layer, the toughness and rigidity can be easily controlled by the binder. In this case, there is no need to separate the positive electrode mixture layer into a first region and a second region.

[0021] As described above, the present disclosure is based on the finding that, in a positive electrode in which the positive electrode edge portion has exposed portions of the positive electrode current collector intermittently arranged at a plurality of locations along the longitudinal direction of the positive electrode current collector, a simple method of using a first resin and a second resin that satisfy M1>M2 as binders in combination is effective in suppressing both fracture of the positive electrode and peeling of the positive electrode composite layer.

[0022] If the first molecular weight M1 and the second molecular weight M2 satisfy 1 < M1 / M2, a certain degree of effect can be obtained, but it is preferable that they satisfy 1.5 ≦ M1 / M2, more preferably 2 ≦ M1 / M2, and even more preferably 3 ≦ M1 / M2 or 4 ≦ M1 / M2. The larger the M1 / M2 ratio, the greater the rigidity of the first resin relative to the second resin, or the greater the toughness of the second resin relative to the first resin, and the easier it is to improve the balance between the toughness and rigidity of the electrode composite layer.

[0023] The mass of the positive electrode mixture layer disposed per unit area on the surface of the positive electrode current collector is, for example, 240 g / m ２ or more, 260 g / m ２ or more, 280 g / m ２ The greater the mass of the positive electrode composite layer disposed per unit area on the surface of the positive electrode current collector, the greater the effect of satisfying 1 < M1 / M2. That is, in a positive electrode in which the positive electrode edge portion has multiple exposed portions of the positive electrode current collector intermittently along the longitudinal direction of the positive electrode current collector, both fracture of the positive electrode and peeling of the positive electrode composite layer can be more easily suppressed.

[0024] The mass of the positive electrode mixture layer disposed per unit area on the surface of the positive electrode current collector is, for example, 280 g / m ２ 350g / m or more ２ The mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector may be 350 g / m or less. ２ If the ratio is less than or equal to 1, the effect of suppressing breakage of the positive electrode and peeling of the positive electrode mixture layer, which is achieved by satisfying 1<M1 / M2, is sufficiently exhibited.

[0025] The positive electrode current collector is in a sheet shape and has a first surface on one side and a second surface on the opposite side. The "unit area of ​​the surface of the positive electrode current collector" means the "unit area of ​​the first surface of the positive electrode current collector" or the "unit area of ​​the second surface of the positive electrode current collector." In other words, the mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector means the mass of the positive electrode mixture layer disposed per unit area of ​​one surface of the positive electrode current collector.

[0026] The peel strength of the positive electrode mixture layer from the positive electrode current collector is preferably, for example, 10 N / m or more. The peel strength may be measured by a "peel test in accordance with JIS K 6854-1."

[0027] The positive electrode mixture may contain, for example, 0.1 parts by mass to 10 parts by mass, preferably 0.5 parts by mass to 2 parts by mass of a binder per 100 parts by mass of the positive electrode active material, which can sufficiently suppress peeling of the positive electrode mixture layer from the positive electrode current collector and increase the content of the positive electrode active material in the positive electrode mixture layer, thereby enabling a positive electrode with higher capacity and durability to be obtained.

[0028] The content of the first resin in the total of the first resin and the second resin may be, for example, 5% by mass to 95% by mass, 10% by mass to 90% by mass, or 20% by mass to 80% by mass. In other words, the content of the second resin in the total of the first resin and the second resin may be, for example, 5% by mass to 95% by mass, 10% by mass to 90% by mass, or 20% by mass to 80% by mass.

[0029] Even when the content of the first resin (or the second resin) in the total of the first resin and the second resin is less than 5% by mass, a corresponding effect can be obtained by using the first resin and the second resin in combination so that M1 / M2 is 1 < M1 / M2. However, from the viewpoint of obtaining a positive electrode with higher durability, it is desirable that the content of the first resin (or the second resin) in the total of the first resin and the second resin be 5% by mass or more or 10% by mass or more.

[0030] The first resin is preferably, for example, a fluororesin. Examples of such fluororesins include fluororesins containing vinylidene fluoride units (hereinafter also referred to as "vinylidene fluoride-based resins"). The vinylidene fluoride-based resin may be polyvinylidene fluoride or a copolymer of vinylidene fluoride and other monomers. Examples of other monomers include ethylene, propylene, tetrafluoroethylene (TFE), and hexafluoropropylene (HFP). The proportion of vinylidene fluoride units in all monomer units is preferably in the range of 50 to 100 mol%, and more preferably in the range of 75 to 100 mol%.

[0031] The vinylidene fluoride resin may be polyvinylidene fluoride (PVDF) or a modified product thereof, a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-tetrafluoroethylene copolymer, a vinylidene fluoride-chlorotrifluoroethylene copolymer, a vinylidene fluoride-pentafluoropropylene copolymer, etc. One type of first resin may be used alone, or two or more types may be used in combination.

[0032] When the binder contains three or more resins, the resin with the largest weight-average molecular weight Mw is the first resin, and the resin with the smallest weight-average molecular weight Mw is the second resin. When the weight-average molecular weight Mw of the resin with the largest weight-average molecular weight Mw is Mmax and the weight-average molecular weight Mw of the resin with the smallest weight-average molecular weight Mw is Mmin, the resin with a weight-average molecular weight Mw larger than (Mmax ​​+ Mmin) / 2 is the first resin, and the resin with a weight-average molecular weight Mw smaller than (Mmax ​​+ Mmin) / 2 is the second resin.

[0033] In this specification, the weight average molecular weight of each resin can be measured by an apparatus such as gel permeation chromatography (GPC).

[0034] The weight average molecular weight Mw of the first resin is, for example, 300,000 to 2,000,000, or may be 500,000 to 1,500,000, or may be 1,000,000 to 1,500,000, which increases the rigidity of the positive electrode and makes peeling less likely to occur.

[0035] Examples of the second resin include polyolefin resins such as polyethylene and polypropylene, polyamide resins such as aramid resin, polyimide resins such as polyimide and polyamideimide, acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer, vinyl resins such as polyacrylonitrile and polyvinyl acetate, polyvinylpyrrolidone, polyethersulfone, rubber, etc. Furthermore, vinylidene fluoride resins may also be used as the second resin.

[0036] Among rubbers, nitrile group-containing rubber is preferred because it imparts toughness and elongation to the positive electrode composite layer. The nitrile group-containing rubber may be a copolymer containing acrylonitrile units and diene (e.g., butadiene) units. Specifically, the nitrile group-containing rubber may be nitrile rubber (NBR), hydrogenated nitrile rubber (H-NBR), or modified products thereof.

[0037] The weight-average molecular weight Mw of the second resin is, for example, 1,000 to 500,000, or may be 50,000 to 500,000. This increases the rigidity of the positive electrode and makes peeling less likely to occur. Among these, the weight-average molecular weight Mw of the nitrile group-containing rubber may be 10,000 to 500,000, 20,000 to 300,000, or 100,000 to 300,000.

[0038] The positive electrode active material may be a material that reversibly absorbs and releases lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Representative examples of lithium-containing transition metal oxides include lithium cobalt oxide and lithium nickel oxide, which have a layered, rock-salt crystal structure.

[0039] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn can be used. ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４、 LiMPO ４、 Li ２ MPO ４ F (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B). Here, 0<a≦1.2, 0<b≦0.9, and 2.0≦c2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0040] Among them, Li ａ Ni ｂ Me １－ｂ O ２ (M is at least one selected from the group consisting of Mn, Co, and Al, and 0<a≦1.2, and 0.3≦b<1) is preferred. From the viewpoint of increasing capacity, it is more preferred that 0.85≦b<1 is satisfied. From the viewpoint of stability of the crystal structure, Li-nickel composite oxides containing Co and Al as M are preferred. ａ Ni ｂ Co ｃ Al ｄ O ２ (0<a≦1.2, 0.85≦b<1, 0<c<0.15, 0<d≦0.1, b+c+d=1) is more preferable.

[0041] Examples of the conductive additive include carbon materials such as graphite, carbon blacks such as furnace black and acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), etc. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0042] The positive electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the positive electrode current collector include stainless steel, aluminum, an aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0043] [Negative Electrode] The negative electrode includes a strip-shaped negative electrode current collector. The negative electrode may include the negative electrode current collector and a negative electrode composite layer or a negative electrode active material layer formed (or supported) on the entire or partial surface of the negative electrode current collector. The negative electrode composite layer or the negative electrode active material layer may be in the form of a film. The negative electrode composite layer or the negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector.

[0044] The negative electrode mixture layer is composed of a negative electrode mixture. The negative electrode active material layer is composed of a negative electrode mixture or a negative electrode active material. Because the negative electrode mixture contains a negative electrode active material as an essential component, the negative electrode mixture layer may also be referred to as a negative electrode active material layer. The negative electrode active material may be a material that reversibly absorbs and releases lithium ions, and may be lithium metal or a lithium alloy. The negative electrode active material layer composed of a material other than the negative electrode mixture is composed of at least one selected from the group consisting of lithium metal and lithium alloy. The negative electrode mixture layer or negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector.

[0045] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive additive, and a thickener. Such a negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry to the surface of a negative electrode current collector, in which a negative electrode mixture containing particles of the negative electrode active material as an essential component and optional components is dispersed in a dispersion medium, and then drying the slurry. The dried coating film may be rolled, if necessary.

[0046] When the negative electrode has a negative electrode composite layer, the negative electrode composite layer may contain an alloy-based material. The alloy-based material contains a phase that reversibly forms an alloy with lithium. The phase that reversibly forms an alloy with lithium may be, for example, silicon (a silicon phase). Such a phase exhibits significant expansion and contraction during charging and discharging. The content of the alloy-based material in the negative electrode composite layer at the edge of the negative electrode may be higher than that at the main portion of the negative electrode. This allows the expansion rate of the negative electrode at the edge of the negative electrode to be easily greater than that at the main portion of the negative electrode.

[0047] The category of alloy-based materials includes Si-containing materials, Sn-containing materials, Si, Sn, Si alloys, Sn alloys, etc. Among them, Si-containing materials have high capacity and are suitable as negative electrode active materials. Si-containing materials contain a silicon phase. Silicon can reversibly form an alloy with lithium. Si-containing materials are materials that can reversibly absorb and release lithium ions.

[0048] Since the silicon-containing material contains silicon (silicon phase), it expands and contracts significantly during charging and discharging. By increasing the content of the silicon-containing material in the negative electrode mixture layer at the edge of the negative electrode compared to the main part of the negative electrode, it is possible to easily increase the expansion rate of the negative electrode at the edge of the negative electrode compared to the main part of the negative electrode.

[0049] The silicon-containing material may be a composite particle containing a silicon phase and a matrix phase in which the silicon phase is dispersed. The matrix phase may be composed of a material having lithium ion conductivity. The matrix phase may include, for example, at least one selected from the group consisting of a silicon oxide phase and a carbon phase.

[0050] The silicon oxide phase contains Si and O, and may further contain a third element other than Si and O. The silicon oxide phase is SiO ２ It may be composed of lithium silicate, or may be composed of both of these.

[0051] The composite particles that are silicon-containing materials (composite particles that include a silicon phase and a matrix phase in which the silicon phase is dispersed) may be, for example, in any one of the following forms (a) to (c):

[0052] (a) A silicon phase and a silicon dioxide (SiO ２ ) phase; and a first composite particle comprising:

[0053] (b) second composite particles comprising a silicon phase and a lithium silicate phase in which the silicon phase is dispersed;

[0054] (c) a third composite particle comprising a silicon phase and a carbon phase in which the silicon phase is dispersed;

[0055] As materials other than the Si-containing material, carbon materials, spinel-type lithium titanium oxides, spinel-type lithium manganese oxides, etc. are preferred. Of these, carbon materials are preferred. The carbon material may be graphite, easily graphitizable carbon (soft carbon), hardly graphitizable carbon (hard carbon), etc. Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity.

[0056] Graphite refers to a carbon material having a (002) plane spacing d002 of, for example, 0.340 nm or less as measured by X-ray diffraction. The crystallite size Lc(002) of the graphite as measured by X-ray diffraction may be, for example, 5 nm or more, 5 nm or more and 300 nm or less, or 10 nm or more and 200 nm or less.

[0057] The average particle size of the graphite is, for example, 1 μm or more and 30 μm or less.

[0058] When graphite and a silicon-containing material are used in combination, the proportion of the silicon-containing material in the negative electrode active material (the total of graphite and silicon-containing material) is, for example, 1% by mass to 20% by mass, or alternatively 3% by mass to 15% by mass, or alternatively 3% by mass to 10% by mass, which makes it easier to achieve a good balance between improved cycle characteristics and a high capacity.

[0059] Examples of binders include resin materials, such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0060] Examples of the conductive additive include carbons such as acetylene black, carbon fibers (carbon nanotubes (CNT), carbon fibers other than CNT), metal fibers, metal powders such as aluminum, etc. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0061] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salts), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. One type of thickener may be used alone, or two or more types may be used in combination.

[0062] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0063] An example of a secondary battery according to an embodiment of the present disclosure will be specifically described below with reference to the drawings. The components described above can be applied to the components of the example secondary battery described below. The components of the example secondary battery described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Of the components of the example secondary battery described below, components that are not essential for the secondary battery according to the present disclosure may be omitted. Note that the diagrams shown below are schematic and do not accurately reflect the shapes and numbers of actual components.

[0064] FIG. 1 is a cross-sectional schematic diagram of a secondary battery 10 according to one example of this embodiment. FIG. 2 is a plan view schematic diagram of a positive electrode according to one example of this embodiment. The actual length of the positive electrode may differ from that shown in the schematic diagram, and the number of positive electrode leads may also differ. The following description is an example, and the structure of the battery is not particularly limited.

[0065] The secondary battery 10 may be, for example, a lithium ion secondary battery or a lithium secondary battery (lithium metal secondary battery). As shown in Fig. 1 , the secondary battery 10 includes a non-polar case 11, a wound electrode group 14, a plurality of positive electrode leads 112 made of a conductor, a positive electrode terminal 16 made of a conductor, an end surface current collector plate 19 made of a conductor, a negative electrode current collector plate 22 made of a conductor, and a sealing plate 23.

[0066] The case 11 is formed in a cylindrical shape with a bottom and an opening at one end (the lower end in FIG. 1 ). The case 11 is made of metal. A through-hole 12 through which a positive electrode terminal 16 is inserted is formed in the center of the bottom (the upper end in FIG. 1 ) of the case 11. The case 11 accommodates an electrolyte (not shown) together with an electrode group 14. Near the opening in the case 11, a recess 13 is formed that is recessed radially inward of the case 11.

[0067] The electrode group 14 has a positive electrode 110 and a negative electrode 120. The electrode group 14 is a wound electrode group formed by winding the positive electrode 110 and the negative electrode 120 with a separator (not shown) interposed therebetween. The electrode group 14 has a generally cylindrical shape as a whole.

[0068] One end of each of the positive electrode leads 112 is connected to an exposed portion 113b of the positive electrode current collector of the positive electrode edge portion 113 of the positive electrode 110. The other end of each of the positive electrode leads 112 is provided so as to stand upright from one end face of the electrode group 14.

[0069] The positive electrode leads 112 are stacked on top of each other and connected to the positive electrode terminal 16 by welding. In this embodiment, the number of positive electrode leads 112 is eight, but is not limited to this. Also, only four of the eight positive electrode leads 112 are shown in FIG. 1 .

[0070] The material of each positive electrode lead 112 is, for example, stainless steel, aluminum, an aluminum alloy, nickel, a nickel alloy, or the like.

[0071] An insulating member 24 for electrically insulating the electrode group 14 from the bottom of the case 11 is disposed between the electrode group 14 and the bottom of the case 11. The insulating member 24 is made of, for example, an insulating resin. The insulating member 24 may be attached to the bottom of the case 11.

[0072] The positive electrode terminal 16 is provided on the opposite side of the electrode group 14, with multiple positive electrode leads 112 sandwiched between them. The positive electrode terminal 16 is inserted into the through-hole 12 in the bottom of the case 11 and penetrates the bottom of the case 11. The positive electrode terminal 16 is made of metal, and a rivet or the like is used. The positive electrode terminal 16 is insulated from the case 11 by a positive electrode gasket 26 made of an insulating material. An insulating plate 25 is arranged between the positive electrode terminal 16 and the electrode group 14 to electrically insulate them from each other.

[0073] The positive electrode terminal 16 includes a first terminal member 17 extending from the inside to the outside of the case 11 and a disk-shaped second terminal member 18 joined to the first terminal member 17 and exposed to the outside of the case 11. The first terminal member 17 includes a disk-shaped first portion 17a, a hollow cylindrical second portion 17b formed continuously with the first portion 17a and inserted into the through-hole 12, and a third portion 17c extending radially outward from the end of the second portion 17b and joined to the second terminal member 18. The first terminal member 17 is welded to multiple positive electrode leads 112 at the first portion 17a by a laser irradiated in a direction from the first terminal member 17 toward the electrode group 14. Thus, the positive electrode terminal 16 is electrically connected to the positive electrode 110 via the multiple positive electrode leads 112 and functions as an external positive electrode terminal for the secondary battery 10. The first terminal member 17 is an example of a terminal member.

[0074] Of the multiple positive electrode leads 112, at least the positive electrode lead 112 closest to the electrode group 14 (the lowermost positive electrode lead 112 in FIG. 1 ) has a folded portion 112a formed by folding over a part (specifically, a part of the tip side) of the positive electrode lead 112 and on which a part of the laser mark LM by the laser is formed. The folded portion 112a is disposed on the opposite side of the electrode group 14 with the insulating plate 25 interposed therebetween.

[0075] The end surface current collector 19 is made of metal. The shape of the end surface current collector 19 is not particularly limited, and may be, for example, generally cross-shaped as a whole. The end surface current collector 19 is electrically connected to the negative electrode 120 of the electrode group 14.

[0076] The negative current collector 22 is electrically connected to the end current collector 19 via a metal connecting plate 21 (which may be formed, for example, in a ring shape). Thus, the negative current collector 22 is electrically connected to the negative electrode 120. The negative current collector 22 and the connecting plate 21 may be welded to each other (for example, by laser welding). The connecting plate 21 and the end current collector 19 may be welded to each other (for example, by laser welding). The negative current collector 22 may be directly connected to the end current collector 19. In this case, the connecting plate 21 is not necessary. The negative current collector 22 has one or more injection holes 22a for injecting electrolyte into the case 11. The outer edge of the negative current collector 22 is welded (for example, by laser welding) to the recess 13 of the case 11. Thus, the case 11 is electrically connected to the negative electrode 120 via the negative current collector 22 and the like.

[0077] The sealing plate 23 seals the opening of the case 11. The sealing plate 23 is made of metal and has a generally circular plate shape. The sealing plate 23 is insulated from the case 11 by a negative electrode gasket 27. In this embodiment, the sealing plate 23 is not electrically connected to either the positive electrode 110 or the negative electrode 120 of the electrode group 14, but this is not limited to this. The sealing plate 23 has an explosion-proof mechanism (not shown) that activates when the internal pressure of the case 11 exceeds a predetermined value.

[0078] The positive electrode 110 shown in Fig. 2 is in a state before being wound into the electrode group 14. In Fig. 2, arrow Y1 indicates the winding direction of the positive electrode 110 when producing the electrode group 14, and is the longitudinal direction of the positive electrode 110. Furthermore, in Fig. 2, arrow Y2, which is perpendicular to arrow Y1, indicates the direction of the winding axis of the positive electrode 110 (i.e., the direction of the winding axis of the electrode group 14) and the widthwise direction of the positive electrode 110.

[0079] 2, the positive electrode 110 has a positive electrode edge portion 113 including one end 110a in the lateral direction of the positive electrode 110, and a positive electrode main portion 114 other than the positive electrode edge portion 113. The positive electrode main portion 114 is a region from the positive electrode center end portion 113a of the positive electrode edge portion 113 to the other end 110b in the lateral direction of the positive electrode 110. The ratio of the width (length in the lateral direction) of the positive electrode edge portion 113 to the width (length in the lateral direction) of the positive electrode main portion 114 is, for example, in the range of 1:15 to 3:4 or 1:12 to 1:6.

[0080] The positive electrode edge portion 113 of the positive electrode 110 has an exposed portion 113b of the positive electrode current collector where a positive electrode composite material layer is not disposed on the positive electrode current collector, and a first positive electrode composite material portion 113c where a positive electrode composite material layer is disposed on the positive electrode current collector. The positive electrode main portion 114 has a second positive electrode composite material portion 114c where a positive electrode composite material layer is disposed on the positive electrode current collector.

[0081] The exposed portions 113b of the positive electrode current collector are provided at multiple (e.g., eight) intervals along the longitudinal direction of the positive electrode current collector. The exposed portions 113b do not have a positive electrode composite layer from one end 110a of the positive electrode 110 in the lateral direction to the positive electrode main portion 114.

[0082] The length of each exposed portion 113b of the positive electrode current collector in the longitudinal direction of the positive electrode current collector may be 1% to 10% of the length of the positive electrode current collector in the longitudinal direction, and the total length of all exposed portions 113b of the positive electrode current collector in the longitudinal direction of the positive electrode current collector may be 1% to 20%, 5% to 20%, or 8% to 20% of the length of the positive electrode current collector in the longitudinal direction.

[0083] It is desirable that the intervals between the exposed portions 113b of adjacent positive electrode current collectors be as uniform as possible. For example, if the length of the positive electrode current collector is L100 and the number of exposed portions 113b of the positive electrode current collector is n, the intervals between the exposed portions 113b of adjacent positive electrode current collectors may be 0.8×L100 / n to 1.2×L100 / n.

[0084] Each of the exposed portions 113b of the positive electrode current collector is connected to a tab-shaped positive electrode lead 112. The plurality of positive electrode leads 112 are bundled together and connected to the first portion 17a of the first terminal member 17.

[0085] The mass per unit area W1 of the positive electrode composite layer arranged on the surface of the positive electrode current collector in the first positive electrode composite portion 113c may be the same as the mass per unit area W2 of the positive electrode composite layer in the second positive electrode composite portion 114c. When W1 and W2 are different, the ratio of the difference (ΔW) between W1 and W2 to W1 may be, for example, 4% or less, 3% or less, 2% or less, or 1% or less.

[0086] The negative electrode 120 has a negative electrode composite layer in a region from one end of the negative electrode 120 facing the positive electrode edge portion 113 to just before the other end in the short direction, and may have an exposed portion of the negative electrode current collector on the other end 120b in the short direction of the negative electrode 120 where the negative electrode composite layer is not disposed on the negative electrode current collector. The exposed portion of the negative electrode current collector is formed along the longitudinal direction of the negative electrode current collector, is exposed at the end face of the electrode group 14, and may be connected to the end face current collector 19 by, for example, laser welding. Note that the configuration of the negative electrode is not particularly limited, and the structure of the battery is not limited to the structure shown in FIG. 1 . The negative electrode may not be connected to the end face current collector 19, but may be electrically connected to the case 11 via a predetermined negative electrode lead.

[0087] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives.

[0088] The gel electrolyte contains a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0089] As the solid electrolyte, for example, a material known in all-solid-state lithium ion secondary batteries (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0090] For example, a liquid non-aqueous electrolyte is prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that ionizes in the electrolyte, and may include, for example, a lithium salt. The electrolyte may contain various additives. The electrolyte is usually used in its liquid state, but its fluidity may be limited by a gelling agent or the like.

[0091] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonates having an unsaturated bond, such as vinylene carbonate (VC), may also be used. Cyclic carbonates having a fluorine atom, such as fluoroethylene carbonate (FEC), may also be used. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0092] Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0093] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0094] (Additional Note) The above description discloses the following techniques. a positive electrode current collector having a width of 100 mm and a thickness of 100 mm, and a positive electrode composite layer disposed on the positive electrode current collector; a positive electrode edge portion including one end of the positive electrode in a width direction, and a positive electrode main portion other than the positive electrode edge portion; the positive electrode edge portion having an exposed portion of the positive electrode current collector provided in one or more locations partially along the length direction of the positive electrode current collector, the exposed portion not including the positive electrode composite layer from the one end of the width direction to the positive electrode main portion; the positive electrode composite layer including a positive electrode active material and a binder; and the binder including a first resin having a first molecular weight M1 and a second resin having a second molecular weight M2, satisfying M1 > M2. (Technology 2) The secondary battery according to Technology 1, wherein the positive electrode edge portion has exposed portions of the positive electrode current collector provided at a plurality of intervals along the longitudinal direction of the positive electrode current collector. (Technology 3) The mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 240 g / m ２ The secondary battery according to Technology 1 or Technology 2, wherein the mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 260 g / m ２ The secondary battery according to any one of the above techniques 1 to 3, wherein the mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 280 g / m ２ The secondary battery according to any one of Techniques 1 to 4, wherein the mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 280 g / m ２ 350g / m or more ２The secondary battery according to any one of Techniques 1 to 5, wherein the ratio of the length of the positive electrode edge portion in the short-side direction to the length of the positive electrode main portion in the short-side direction is in the range of 1:12 to 1:6. (Technology 9) The secondary battery according to any one of Techniques 1 to 8, wherein the total length of all the exposed portions of the positive electrode current collector in the longitudinal direction of the positive electrode current collector is 1% to 20% of the length of the positive electrode current collector in the longitudinal direction. (Technology 10) The secondary battery according to any one of Techniques 1 to 9, wherein the distance between adjacent exposed portions is 0.8×L / n to 1.2×L / n, where L100 is the length of the positive electrode current collector and n is the number of exposed portions of the positive electrode current collector. (Technology 11) The secondary battery according to any one of Technologies 1 to 10, wherein a difference ΔW between a mass W1 of the positive electrode mixture layer of the positive electrode peripheral portion disposed per unit area of ​​the surface of the positive electrode current collector and a mass W2 of the positive electrode mixture layer of the positive electrode main portion disposed per unit area of ​​the surface of the positive electrode current collector is 4% or less of W1.

[0095] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0096] Examples 1 to 8 and Comparative Examples 1 to 11 [Preparation of Positive Electrode] An appropriate amount of NMP was added to the positive electrode mixture to obtain a positive electrode slurry. The positive electrode mixture was a mixture of a lithium-containing composite oxide as a positive electrode active material, carbon black as a conductive agent, and a binder. The lithium-containing composite oxide was LiNi ０．８ Co ０．１ Mn ０．１ O ２ In the positive electrode mixture, the mass ratio of the lithium-containing composite oxide, the carbon black, and the binder was 98:1:1.

[0097] The following four types of binder were used alone or in the combinations shown in Table 1. First resin A: polyvinylidene fluoride (PVdF) with a weight-average molecular weight M1 of approximately 1,000,000. Second resin B: nitrile group-containing rubber with a weight-average molecular weight M2 that satisfies M1 / M2 = 2. Second resin C: nitrile group-containing rubber with a weight-average molecular weight M2 that satisfies M1 / M2 = 5. Second resin D: nitrile group-containing rubber with a weight-average molecular weight M2 that satisfies M1 / M2 = 10. When the first resin and the second resin were used in combination, the content of the first resin (or the second resin) in the total of the first resin and the second resin was 50 mass % (mass ratio 1:1).

[0098] The positive electrode slurry was applied to both sides of an aluminum foil positive electrode current collector at a predetermined thickness, and the coating was dried and rolled to form a positive electrode composite layer, resulting in a positive electrode as shown in FIG. 2 . Specifically, the positive electrode slurry was intermittently applied to one end of the aluminum foil in the short direction along the longitudinal direction of the positive electrode current collector at a predetermined thickness. The positive electrode slurry was then applied to the remaining portion of the positive electrode current collector at the same thickness. The coating was then dried and rolled to form a positive electrode having a positive electrode edge portion with a first positive electrode composite portion and a positive electrode main portion with a second positive electrode composite portion. The mass of the positive electrode composite layer disposed per unit area on the surface of the positive electrode current collector was controlled to the value shown in Table 1. Eight exposed portions of the positive electrode current collector were provided at the positive electrode edge portion, and a positive electrode lead was attached to each exposed portion. The width of the positive electrode edge portion was set to 12 mm, and the width of the positive electrode main portion was set to 62 mm.

[0099] <Peel Strength> The peel strength of the positive electrode composite layer from the positive electrode current collector was measured by a peel test in accordance with JIS K 6854-1. A positive electrode measuring 1.5 cm x 7 cm was cut out as a test piece. The average peel strength was measured at a gripping movement speed of 24 mm / min. The average peel strength of each battery is shown in Table 1 as a relative value of the peel strength of each battery when the peel strength of Battery B1 is set to 1.0.

[0100] [Fabrication of Negative Electrode] SiO as the negative electrode active material ｘ(x=1.0), graphite as a negative electrode active material, styrene-butadiene copolymer rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were added to a negative electrode mixture in an appropriate amount of water to obtain a negative electrode slurry. ｘ The mass ratio of graphite:SBR:CMC was 5:93:1:1.

[0101] The negative electrode slurry was applied to both sides of a copper foil serving as a negative electrode current collector in a predetermined thickness, dried, and rolled to form a negative electrode composite layer, thereby obtaining a negative electrode as shown in FIG. 3. Specifically, the negative electrode slurry was applied uniformly to a predetermined thickness on the surface of the copper foil, dried, and rolled to simultaneously form a first negative electrode composite portion and a second negative electrode composite portion. However, a portion of one end of the negative electrode current collector was left as an exposed portion. The thickness of the negative electrode composite portion was appropriately changed depending on the thickness of the positive electrode composite layer.

[0102] [Preparation of Electrolyte] Vinylene carbonate (VC) was added to a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:3, and LiPF ６ The VC content in the entire electrolyte was 5 mass %. ６ The concentration was set to 1.5 mol / L.

[0103] [Preparation of Secondary Batteries] In an inert gas atmosphere, a positive electrode and a negative electrode were wound with a separator (a polyethylene microporous film) interposed therebetween to prepare an electrode assembly. The electrodes were stacked so that the edge of the positive electrode was positioned on one end face of the electrode assembly and the exposed portion of the negative electrode current collector was positioned on the other end face of the electrode assembly. Using the electrode assembly and electrolyte, cylindrical lithium-ion secondary batteries (Batteries A1 to A8 of the Examples and Batteries B1 to B11 of the Comparative Examples) as shown in FIG. 1 were completed.

[0104] The positive electrode leads were bundled and electrically connected to the first portion of the first terminal member. The exposed portion of the negative electrode current collector was connected to an end surface current collector by laser welding, and the end surface current collector was electrically connected to the negative electrode current collector via a connecting plate.

[0105] [Evaluation] (Rolling property) Ten secondary batteries each of the examples and comparative examples were produced, and then disassembled to check for fractures in the positive electrode. Table 1 shows the results: ◯ when no fractures were observed, △ when no fractures were observed but minute cracks (1 mm or less) occurred, and × when at least one fracture was observed.

[0106] (Capacity Ratio) In a temperature environment of 25°C, constant current charging was performed at 0.2 C, and when the battery voltage reached 4.2 V, constant voltage charging was performed until the charging current reached 0.02 C. After a 10-minute break, constant current discharging was performed at 0.2 C until the battery voltage reached 2.5 V. The discharge capacity at this time was determined, and the volumetric energy density of the electrode group was calculated. Table 1 shows the volumetric energy densities of the other batteries as capacity ratios (%) when the volumetric energy densities of batteries B1 to B3 of Comparative Examples 1 to 3 were taken as 100%.

[0107]

[0108] It can be seen from Table 1 that when the first resin and the second resin are used in combination as binders, higher peel strength and rollability (breakage resistance) can be obtained compared to when only the first resin or only the second resin is used. It can also be seen that the greater the mass of the positive electrode mixture layer arranged per unit area on the surface of the positive electrode current collector, the more difficult it is to achieve both peel strength and rollability.

[0109] The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0110] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0111] 10: Secondary battery 11: Case 12: Through hole 13: Recess 14: Electrode group 16: Positive electrode terminal 17: First terminal member (terminal member) 17a: First portion 17b: Second portion 17c: Third portion 18: Second terminal member 19: End surface current collector plate 21: Connecting plate 22: Negative electrode current collector plate 22a: Injection hole 23: Sealing plate 24: Insulating member 25: Insulating plate 26: Positive electrode gasket 27: Negative electrode gasket

[0112] 110: Positive electrode 110a: One end 110b: Other end 112: Positive electrode lead 112a: Folded portion 113: Positive electrode edge portion 113a: Positive electrode central end portion 113b: Positive electrode current collector exposed portion 113c: First positive electrode composite portion 114: Positive electrode main portion 114c: Second positive electrode composite portion

[0113] 120: negative electrode

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, the positive electrode and the negative electrode being wound with the separator interposed therebetween, the positive electrode comprising a band-shaped positive electrode collector and a positive electrode mixture layer disposed on the positive electrode collector, the positive electrode having a positive electrode edge portion including one end of the positive electrode in a short side direction and a positive electrode main portion other than the positive electrode edge portion, the positive electrode edge portion having an exposed portion of the positive electrode collector provided in one or more locations partially along the long side direction of the positive electrode collector, the exposed portion not including the positive electrode mixture layer from the one end in the short side direction to the positive electrode main portion, the positive electrode mixture layer including a positive electrode active material and a binder, the binder including a first resin having a first molecular weight M1 and a second resin having a second molecular weight M2, and satisfying M1>M2.

2. The secondary battery according to claim 1, wherein the positive electrode edge portion has exposed portions of the positive electrode current collector provided at multiple intermittent locations along the longitudinal direction of the positive electrode current collector.

3. The mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 240 g / m ２ The secondary battery according to claim 1 .

4. The mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 260 g / m ２ The secondary battery according to claim 1 .

5. The mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 280 g / m ２ The secondary battery according to claim 1 .

6. The mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector is 280 g / m ２ 350g / m or more ２ The secondary battery according to claim 1 , wherein:

7. The secondary battery according to any one of claims 1 to 6, wherein the first molecular weight M1 and the second molecular weight M2 satisfy 2≦M1 / M2.

8. A secondary battery according to any one of claims 1 to 6, wherein the ratio of the length of the positive electrode edge portion in the short side direction to the length of the positive electrode main portion in the short side direction is in the range of 1:12 to 1:

6.

9. The secondary battery according to any one of claims 1 to 6, wherein a total length of all of the exposed portions of the positive electrode current collector in the longitudinal direction of the positive electrode current collector is 1% to 20% of the length of the positive electrode current collector in the longitudinal direction.

10. The secondary battery according to any one of claims 1 to 6, wherein the length of the positive electrode current collector is L100 and the number of the exposed portions of the positive electrode current collector is n, and the distance between adjacent exposed portions is 0.8 x L100 / n to 1.2 x L100 / n.

11. The secondary battery according to any one of claims 1 to 6, wherein a difference ΔW between a mass W1 of the positive electrode mixture layer of the positive electrode edge portion disposed per unit area on the surface of the positive electrode current collector and a mass W2 of the positive electrode mixture layer of the positive electrode main portion disposed per unit area on the surface of the positive electrode current collector is 4% or less of W1.

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