Secondary battery

Abstract

Disclosed is a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode and the negative electrode are wound together with the separator therebetween. The positive electrode includes a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode has a first region including one end in the transverse direction of the positive electrode and a second region that is not the first region. The first region has at least one exposed portion of the positive electrode current collector, the exposed portion being partially provided along the longitudinal direction of the positive electrode current collector. The exposed portion does not have the positive electrode mixture layer from the one end in the transverse direction to the second region. The positive electrode mixture layer contains a positive electrode active material and a binder. The mass content C1 of the binder contained in the positive electrode mixture layer in the first region is greater than the mass content C2 of the binder contained in the positive electrode mixture layer in the second region.

Description

Secondary battery Cross-reference to related applications

[0001] This disclosure claims the benefit of priority with respect to Japanese Patent Application No. 2024-169316, filed on September 27, 2024, with the Japan Patent Office, and the entire contents of the said patent application are incorporated herein by reference.

[0002] This disclosure relates to a secondary battery.

[0003] Patent Document 1 proposes "a method for manufacturing an electrode plate for a power storage device, in which a composition layer is formed on a strip-shaped current collector by discharging a composition slurry from discharge nozzles corresponding to respective discharge regions extending along the length direction thereof, wherein the positions of the discharge regions are set so that a part of each of the plurality of discharge regions forms an overlapping portion that overlaps a part of an adjacent discharge region as viewed in the length direction of the current collector, the overlapping portion has 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 composition slurry."

[0004] Patent Document 2 proposes "a power storage device electrode plate including a substantially rectangular current collector and an active material layer provided on at least one surface of the current collector, wherein the current collector has a plain portion to which an electrode lead is connected at one end in the width direction in a partial region in the longitudinal direction, and in a region where the active material layer is provided, the elastic modulus of a first region adjacent to the plain portion in the width direction is larger than the elastic modulus of a second region adjacent to the plain portion and the region occupied by the first region in the longitudinal direction."

[0005] Japanese Patent No. 6965162, Japanese Patent No. 6821595

[0006] An electrode lead is connected to the uncoated portion (or plain portion) of the composition layer of Patent Documents 1 and 2. When the electrode is wound, the curvature of the uncoated portion to which the electrode lead is connected becomes small, and the curvature in the vicinity of the uncoated portion of the composition layer becomes large. Therefore, breakage is likely to occur during winding in the vicinity of the uncoated portion of the composition layer of the electrode.

[0007] One aspect of the present disclosure relates to a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided partially along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from the one end in the short direction to the second region, the positive electrode mixture layer comprises a positive electrode active material and a binder, and the mass content C1 of the binder contained in the positive electrode mixture layer in the first region is greater than the mass content C2 of the binder contained in the positive electrode mixture layer in the second region.

[0008] According to this disclosure, it is possible to suppress the breakage of the positive electrode in a secondary battery.

[0009] This is a schematic cross-sectional view of a secondary battery according to one embodiment. This is a schematic plan view of the positive electrode according to one embodiment. This is a schematic plan view of the negative electrode according to one embodiment.

[0010] Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0011] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit is not greater than or equal to the upper limit.

[0012] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0013] Secondary batteries include lithium-ion secondary batteries, lithium metal secondary batteries, and non-aqueous electrolyte secondary batteries such as solid batteries containing gel electrolytes or solid electrolytes. In other words, secondary batteries may be liquid-type secondary batteries containing an electrolyte solution, or all-solid-state secondary batteries containing a solid electrolyte.

[0014] The secondary battery according to this disclosure comprises a strip-shaped positive electrode, a strip-shaped negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes. The positive and negative electrodes are wound around each other via the separator. In other words, the secondary battery comprises a wound-type electrode group. The cross-sectional shape of the wound-type electrode group perpendicular to the winding axis may be, for example, circular or elliptical, and the outer shape may be, for example, cylindrical, but is not limited to these.

[0015] [Positive Electrode] The positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer may be in the form of a film. The positive electrode has a positive electrode current collector and a positive electrode mixture layer formed (or supported) on a portion of the surface of the positive electrode current collector. Specifically, the positive electrode has a first region (positive electrode edge) including one end in the short direction of the positive electrode and a second region (positive electrode main part) other than the first region. The first region has one or more exposed portions of the positive electrode current collector (hereinafter also referred to as "uncoated portions") provided along the longitudinal direction of the positive electrode current collector. The exposed portions of the positive electrode current collector do not have a positive electrode mixture layer from one end in the short direction to the second region. Multiple exposed portions of the positive electrode current collector may be provided intermittently along the longitudinal direction of the positive electrode current collector. The exposed portion of the positive electrode current collector may be a partially exposed portion that is roughly rectangular in shape and has a predetermined width in the longitudinal direction of the positive electrode current collector.

[0016] The positive electrode mixture layer is composed of a positive electrode mixture. Since the positive electrode mixture contains a positive electrode active material as an essential component, the positive electrode mixture layer may also be called the positive electrode active material layer. The positive electrode mixture layer is supported on one or both surfaces of the positive electrode current collector.

[0017] 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 and a thickener. The mass content C1 of the binder contained in the positive electrode mixture layer in the first region (hereinafter also referred to as the "first positive electrode mixture section") is greater than the mass content C2 of the binder contained in the positive electrode mixture layer in the second region (hereinafter also referred to as the "second positive electrode mixture section") (C1 > C2). In other words, the binder is unevenly distributed in the first region. It is sufficient to satisfy C1 > C2, but it is preferable that there is a difference of at least 5% by mass between the mass content C1 of the binder contained in the first positive electrode mixture section and the mass content C2 of the binder contained in the second positive electrode mixture section.

[0018] When the mass content C1 of the binder in the first positive electrode mixture and the mass content C2 of the binder in the second positive electrode mixture satisfy C1 > C2, the strain applied per unit mass of binder in the first positive electrode mixture becomes relatively smaller than the strain applied per unit mass of binder in the second positive electrode mixture. In other words, the fracture strain of the positive electrode in the first region becomes relatively larger than that of the positive electrode in the second region. As a result, fracture of the positive electrode (hereinafter also referred to as the "first positive electrode portion") near the uncoated portion of the first region (exposed portion of the positive electrode current collector) is suppressed. The fracture strain of the positive electrode is the magnitude of the strain that occurs in the positive electrode before fracture occurs, and the larger the fracture strain, the less likely the positive electrode is to fracture.

[0019] In other words, by unevenly distributing the binder in the first region, the fracture strain of the positive electrode (first positive electrode portion) in the first region can be made relatively larger than the fracture strain of the positive electrode (second positive electrode portion) in the second region. This improves the winding ability of the electrode when the positive electrode lead is connected to the exposed portion of the positive electrode current collector. In addition, because the second region does not contain excessive binder, the increase in resistance in the second region is suppressed.

[0020] The ratio of the mass content C1 of the binder in the first positive electrode mixture to the mass content C2 of the binder in the second positive electrode mixture, C1 / C2, is preferably 2 or less, but may also be 1.8 or less, 1.6 or less, or 1.5 or less. Because C1 / C2 is within the above range, the second region does not contain an excess of binder, and therefore the increase in resistance in the second region is easily suppressed.

[0021] The ratio of the mass content C1 of the binder in the first positive electrode mixture to the mass content C2 of the binder in the second positive electrode mixture, C1 / C2, is preferably 1.05 or more, may be 1.1 or more, 1.2 or more, or 1.3 or more. When C1 / C2 is within the above range, the winding properties of the electrode when the positive electrode lead is connected to the exposed portion of the positive electrode current collector tend to improve.

[0022] A preferred range for the C1 / C2 ratio is, for example, 1.05 to 2.0, but may also be 1.1 to 1.8, 1.2 to 1.6, 1.2 to 1.5, or 1.3 to 1.5.

[0023] The binder may include, for example, a fluorinated polymer. Fluorinated polymers can exhibit high binding strength. A fluorinated polymer is a general term for polymers that have fluorine atoms (F) bonded to carbon atoms that constitute the main chain. Because fluorine atoms have a small atomic radius and polarizability, the carbon-fluorine bond is stable, and they have crystalline properties, which can also achieve excellent heat resistance, weather resistance, and chemical resistance.

[0024] The fluorine-based polymer may be, but is not limited to, polyvinylidene fluoride polymers or polytetrafluoroethylene polymers. In particular, the fluorine-based polymer preferably contains polyvinylidene fluoride polymers, and the polyvinylidene fluoride polymer may account for 50% or more by mass, and even more than 80% by mass, of the fluorine-based polymer, or 100% of the fluorine-based polymer may be polyvinylidene fluoride polymers.

[0025] Polyvinylidene fluoride polymers are fluorinated polymers containing vinylidene fluoride units. Polyvinylidene fluoride polymers can exhibit high binding strength. Because polyvinylidene fluoride polymers maintain polarity at the vinylidene fluoride units, they have a higher affinity for polar solvents such as N-methyl-2-pyrrolidone (NMP) compared to other fluorinated polymers.

[0026] The polyvinylidene fluoride polymer may be polyvinylidene fluoride itself, 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 to the total monomer units is preferably in the range of 50 to 100 mol%, and more preferably in the range of 75 to 100 mol%.

[0027] The polyvinylidene fluoride polymer may be polyvinylidene fluoride (PVDF) and its modified forms, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-tetrafluoroethylene copolymer, polyvinylidene fluoride-chlorotrifluoroethylene copolymer, polyvinylidene fluoride-pentafluoropropylene copolymer, etc. The polyvinylidene fluoride polymer may be used alone or in combination of two or more types.

[0028] Polytetrafluoroethylene polymers are fluorine-based polymers containing tetrafluoroethylene units. Polytetrafluoroethylene polymers may be polytetrafluoroethylene (PTFE) or copolymers of tetrafluoroethylene and other monomers. Examples of other monomers include ethylene, propylene, and hexafluoropropylene (HFP). The proportion of tetrafluoroethylene units to total monomer units is preferably in the range of 50 to 100 mol%, and more preferably in the range of 75 to 100 mol%.

[0029] The weight-average molecular weight Mw of the polyvinylidene fluoride polymer and the polytetrafluoroethylene polymer is, for example, 300,000 to 2,000,000, and may also be 500,000 to 1,500,000, or 1,000,000 to 1,500,000.

[0030] In this specification, the weight-average molecular weight can be measured using an instrument such as gel permeation chromatography (GPC) for each resin.

[0031] The positive electrode mixture layer can be formed, for example, by dispersing a positive electrode mixture containing particles of the essential component, the positive electrode active material, a binder, and optional components (such as conductive additives), in a dispersion medium, applying the slurry to the surface of the positive electrode current collector, and drying it. The dried coating may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces. As the dispersion medium for the positive electrode slurry, N-methyl-2-pyrrolidone (NMP), cyclohexanone, alcohols, ethers, etc., can be used.

[0032] To increase the energy density of secondary batteries, the positive electrode mixture layer is increasingly being filled to be thicker and denser. As the positive electrode mixture layer becomes thicker and denser, more sophisticated measures are needed to prevent positive electrode fracture. Specifically, if the first region has one or more or intermittently multiple exposed portions of the positive electrode current collector along the longitudinal direction of the positive electrode current collector, it is important to suppress fracture near these exposed portions. In such circumstances, forming a positive electrode mixture layer with the above configuration where the mass content C1 and mass content C2 satisfy C1 > C2 is an effective measure to suppress positive electrode fracture.

[0033] The mass of the positive electrode mixture layer per unit area on the surface of the positive electrode current collector is, for example, 240 g / m². ２ The above is also acceptable, 260 g / m ２ The above is also acceptable, 280 g / m ２The above is also acceptable. The larger the mass of the positive electrode mixture layer distributed per unit area on the surface of the positive electrode current collector, the greater the effect of unevenly distributing the binder in the first region. In other words, the effect of suppressing fracture of the first positive electrode becomes more pronounced. The mass of the positive electrode mixture layer distributed per unit area on the surface of the positive electrode current collector is, for example, 280 g / m². ２ 350g / m or more ２ The following is also acceptable.

[0034] The positive electrode current collector is sheet-like and has a first surface on one side and a second surface on the opposite side. "Unit area of ​​the surface of the positive electrode current collector" means either "unit area of ​​the first surface of the positive electrode current collector" or "unit area of ​​the second surface of the positive electrode current collector." In other words, the mass of the positive electrode mixture layer per unit area of ​​the surface of the positive electrode current collector means the mass of the positive electrode mixture layer per unit area of ​​one of the surfaces of the positive electrode current collector.

[0035] The first positive electrode mixture in the first region and the second positive electrode mixture in the second region may each contain, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, and more preferably 0.5 to 2 parts by mass of binder per 100 parts by mass of positive electrode active material. This provides a sufficient effect to suppress fracture of the first positive electrode and increases the content of positive electrode active material in the positive electrode mixture layer, making it possible to obtain a higher-capacity positive electrode.

[0036] The positive electrode mixture may contain binders other than fluorine-based polymers, but it is preferable that the fluorine-based polymer accounts for 80% by mass or more, and more preferably 90% by mass or more, of the binder.

[0037] The binder can be separated, for example, by separating the positive electrode mixture layer from the positive electrode and reacting it with an acidic aqueous solution. The acid dissolves components such as the positive electrode active material in the acidic aqueous solution. Subsequently, the acidic aqueous solution is filtered to obtain a residue containing the binder. The binder contained in such a residue can be quantitatively separated using separation devices such as centrifugation, solvent extraction, or column chromatography. The separated components can be analyzed qualitatively and quantitatively using various analytical instruments.

[0038] The positive electrode active material can be, for example, a material that reversibly occludes and releases lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Representative examples of the lithium-containing transition metal oxide include lithium cobaltate and lithium nickelate having a layered crystal structure and a rock salt type.

[0039] As the positive electrode active material, for example, a composite oxide containing lithium and transition metals such as Ni, Co, and Mn can be used. For example, Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ [[ID=**28**]]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, K, Ca, Rb, Sr, Sc, Y, Ti, Zr, V, Nb, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B. Here, 0 < a ≤ 1.2, 0 < b ≤ 0.9, 2.0 ≤ c ≤ 2.3. The value of a indicating the molar ratio of lithium increases and decreases by charge and discharge.) can be mentioned.

[0040] Among them, Li <00QQ034>Ni １－ｂ M ｂ O ２ **Note**: There seems to be a formatting issue with the tag `**28**` which might be a mistake. It's translated as `M` as per the context, but the tag might need to be corrected in the original source.Lithium nickel composite oxides represented by (M being at least one selected from the group consisting of Mn, Co, and Al, with 0 < a ≤ 1.2 and 0 < b < 0.7) are preferred. From the viewpoint of increasing capacity, it is more preferable that 0 < b < 0.2 is satisfied. From the viewpoint of crystal structure stability, Li containing Co and Al as M is preferred. ａ Ni １－ｂ Co ｄ Al ｅ O ｃ Or, Li including Co and Mn as M ａ Ni １－ｂ Co ｄ Mn ｅ O ｃ (0 < a ≤ 1.2, 0 < b < 0.2, 0 < d < 0.15, 0 < e ≤ 0.1, b = d + e) ​​is even more preferable.

[0041] Examples of conductive additives include carbon materials such as graphite, carbon black such as furnace black and acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), and graphene. Conductive additives may be used individually or in combination of two or more types.

[0042] As the positive electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet) can be used. Examples of materials for the positive electrode current collector include stainless steel, aluminum, 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 comprises a strip-shaped negative electrode current collector. The negative electrode may have a negative electrode current collector and a negative electrode mixture layer or negative electrode active material layer formed (or supported) on all or part of the surface of the negative electrode current collector. The negative electrode mixture layer or negative electrode active material layer may be in the form of a film. The negative electrode mixture layer or 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 the negative electrode mixture. The negative electrode active material layer is composed of the negative electrode mixture or the negative electrode active material. Since the negative electrode mixture contains the negative electrode active material as an essential component, the negative electrode mixture layer may also be called the negative electrode active material layer. The negative electrode active material may be a material that reversibly intercepts and releases lithium ions, a lithium metal, or a lithium alloy. The negative electrode active material layer, which is composed of materials 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 the 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 binders, conductive additives, thickeners, etc., as optional components. Such a negative electrode mixture layer can be formed, for example, by dispersing a negative electrode slurry containing particles of the essential negative electrode active material and optional components in a dispersion medium, applying it to the surface of a negative electrode current collector, and drying it. The dried coating may be rolled if necessary.

[0046] If the negative electrode comprises a negative electrode mixture layer, the negative electrode mixture layer may contain an alloying material. The alloying 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 (silicon phase). Such a phase exhibits very large expansion and contraction due to charging and discharging. The content of the alloying material in the negative electrode mixture layer may be higher in the negative electrode edge than in the negative electrode core. This makes it easy to increase the expansion rate of the negative electrode at the negative electrode edge compared to the negative electrode core.

[0047] The category of alloying materials includes Si-containing materials, Sn-containing materials, Si-Sn-Si alloys, and Sn alloys. Among these, Si-containing materials are suitable as anode active materials due to their high capacity. Si-containing materials contain a silicon phase. Silicon can reversibly form alloys with lithium. Si-containing materials are materials that can reversibly intercept and release lithium ions.

[0048] The silicon-containing material may be a composite particle comprising 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. For example, the matrix phase may include at least one selected from the group consisting of a silicon oxide phase and a carbon phase.

[0049] The silicon oxide phase contains Si and O, and may also contain a third element other than Si and O. ２ It may be composed of [a certain material], or it may be composed of lithium silicate, or it may be composed of both of these.

[0050] The silicon-containing composite particles (composite particles comprising a silicon phase and a matrix phase in which the silicon phase is dispersed) may, for example, take any of the following forms (a) to (c).

[0051] (a) A silicon phase and silicon dioxide (SiO₂) in which the silicon phase is dispersed. ２ A first composite particle containing the ) phase.

[0052] (b) A second composite particle comprising a silicon phase and a lithium silicate phase in which the silicon phase is dispersed.

[0053] (c) A third composite particle comprising a silicon phase and a carbon phase in which the silicon phase is dispersed.

[0054] Materials other than Si-containing materials include carbon materials, spinel-type lithium titanium oxide, and spinel-type lithium manganese oxide. Among these, carbon materials are preferred. Carbon materials may include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Among these, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity.

[0055] Graphite refers to a carbon material in which the interplanar spacing d002 of (002) planes, as measured by X-ray diffraction, is, for example, 0.340 nm or less. The crystallite size Lc(002) of 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. The average grain size of graphite is, for example, 1 μm or more and 30 μm or less.

[0056] When graphite and silicon-containing materials are used in combination, the proportion of silicon-containing material in the negative electrode active material (total of graphite and silicon-containing material) is, for example, 1% to 20% by mass, but may also be 3% to 15% by mass, or 3% to 10% by mass. In this case, a good balance between improved cycle characteristics and increased capacity can be easily obtained.

[0057] Examples of binders include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins like polyethylene and polypropylene; polyamide resins like aramid resin; polyimide resins like polyimide and polyamideimide; acrylic resins like polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymer; vinyl resins like polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials like styrene-butadiene copolymer rubber (SBR). A single binder may be used alone, or two or more may be used in combination.

[0058] Examples of conductive additives include carbon compounds such as acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), graphene, metal fibers, and metal powders such as aluminum. Conductive additives may be used individually or in combination of two or more types.

[0059] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as Na salts), cellulose derivatives such as methylcellulose (cellulose ethers, etc.), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. A single thickening agent may be used alone, or two or more may be used in combination.

[0060] As the negative electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet) can be used. Examples of materials 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.

[0061] Hereinafter, an example of a secondary battery according to the embodiment of this disclosure will be specifically described with reference to the drawings. The components of the secondary battery example described below can be the components described above. The components of the secondary battery example described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Among the components of the secondary battery example described below, components that are not essential to the secondary battery according to this disclosure may be omitted. Note that the figures shown below are schematic and do not accurately reflect the actual shape and number of components.

[0062] Figure 1 is a schematic cross-sectional view of a secondary battery 10 according to an example of this embodiment. Figure 2 is a schematic plan view of the positive electrode according to an example of this embodiment. Figure 3 is a schematic plan view of the negative electrode according to an example of this embodiment. The actual lengths of the positive and negative electrodes may differ from those shown in the schematic diagrams, and the number of positive electrode leads may also differ.

[0063] 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 Figure 1, the secondary battery 10 comprises a non-polar case 11, a wound electrode group 14, a plurality of positive electrode leads 112 made of conductors, a positive electrode terminal 16 made of conductors, an end face current collector plate 19 made of conductors, a negative electrode current collector plate 22 made of conductors, and a sealing plate 23.

[0064] The case 11 is formed in a bottomed cylindrical shape with an opening at one end (the lower end in Figure 1). The case 11 is made of metal. A through hole 12 is formed in the center of the bottom of the case 11 (the upper end in Figure 1), through which the positive electrode terminal 16 is inserted. The case 11 houses an electrolyte (not shown) together with the electrode group 14. A recess 13 is formed near the opening in the case 11, indented radially inward.

[0065] The electrode group 14 has a positive electrode 110 and a negative electrode 120. The electrode group 14 is a wound-type electrode group formed by winding the positive electrode 110 and the negative electrode 120 with a separator (not shown) in between. The electrode group 14 is generally cylindrical in shape as a whole.

[0066] Each of the multiple positive electrode leads 112 has one end connected to the exposed portion 113b of the positive electrode current collector in the first region (positive electrode edge) 113 of the positive electrode 110. The other ends of the multiple positive electrode leads 112 are provided so as to be planted from one end face of the electrode group 14.

[0067] Multiple positive leads 112 are stacked on top of each other and connected to the positive terminal 16 by welding. In this embodiment, there are eight positive leads 112, but the number is not limited to this. Also, in Figure 1, only four of the eight positive leads 112 are shown.

[0068] The material of each positive electrode lead 112 is, for example, stainless steel, aluminum, aluminum alloy, nickel, nickel alloy, etc.

[0069] An insulating member 24 is placed between the electrode group 14 and the bottom of the case 11 to electrically insulate them. 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.

[0070] The positive electrode terminal 16 is located on the opposite side of the electrode group 14, sandwiching the multiple positive electrode leads 112. The positive electrode terminal 16 is inserted through a through hole 12 at the bottom of the case 11 and penetrates the bottom of the case 11. The positive electrode terminal 16 is made of metal, and rivets or the like are used. The positive electrode terminal 16 is insulated from the case 11 by a positive electrode gasket 26 made of insulating material. An insulating plate 25 is placed between the positive electrode terminal 16 and the electrode group 14 to electrically insulate them from each other.

[0071] The positive electrode terminal 16 has a first terminal member 17 that extends both inside and outside the case 11, and a disc-shaped second terminal member 18 that is joined to the first terminal member 17 and exposed to the outside of the case 11. The first terminal member 17 comprises a disc-shaped first portion 17a, a hollow cylindrical second portion 17b that is continuously formed with the first portion 17a and inserted through the through hole 12, and a third portion 17c that extends radially outward from the end of the second portion 17b and to which the second terminal member 18 is joined. The first terminal member 17 is welded to a plurality of positive electrode leads 112 at the first portion 17a by a laser irradiated in the 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 plurality of positive electrode leads 112 and functions as an external positive electrode terminal of the secondary battery 10. The first terminal member 17 is an example of a terminal member.

[0072] 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 Figure 1) has a folded portion 112a formed by folding a part of the positive electrode lead 112 (specifically, a part of the tip side), and a part of the laser mark LM formed by the laser is formed thereon. The folded portion 112a is positioned on the opposite side of the electrode group 14, with the insulating plate 25 in between.

[0073] The end face current collector plate 19 is made of metal. The shape of the end face current collector plate 19 is not particularly limited; for example, it may be roughly cross-shaped overall. The end face current collector plate 19 is electrically connected to the negative electrode 120 of the electrode group 14.

[0074] The negative electrode current collector plate 22 is electrically connected to the end face current collector plate 19 via a metal connecting plate 21 (which may be formed in the shape of a ring, for example). Thus, the negative electrode current collector plate 22 is electrically connected to the negative electrode 120. The negative electrode current collector plate 22 and the connecting plate 21 may be welded to each other (for example, by laser welding). The connecting plate 21 and the end face current collector plate 19 may also be welded to each other (for example, by laser welding). The negative electrode current collector plate 22 may be directly connected to the end face current collector plate 19. In this case, the connecting plate 21 is not necessary. The negative electrode current collector plate 22 has one or more injection holes 22a for injecting electrolyte into the case 11. The negative electrode current collector plate 22 is welded (for example, by laser welding) to the recess 13 of the case 11 at its outer edge. Thus, the case 11 is electrically connected to the negative electrode 120 via the negative electrode current collector plate 22, etc.

[0075] The sealing plate 23 seals the opening of the case 11. The sealing plate 23 is made of metal and is generally disc-shaped. The sealing plate 23 is insulated from the case 11 by the 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 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.

[0076] The positive electrode 110 shown in Figure 2 is in its state before being wound as part of the electrode group 14. In Figure 2, arrow Y1 is the winding direction of the positive electrode 110 when manufacturing the electrode group 14, and is the longitudinal direction of the positive electrode 110. Also in Figure 2, arrow Y2, perpendicular to arrow Y1, is the winding axis direction of the positive electrode 110 (i.e., the winding axis direction of the electrode group 14), and is the short-axis direction of the positive electrode 110.

[0077] As shown in Figure 2, the positive electrode 110 has a first region (positive electrode edge) 113 that includes one end 110a in the short direction of the positive electrode 110, and a second region (positive electrode main part) 114 other than the first region 113. The second region 114 is the region from the positive electrode central end 113a of the first region 113 to the other end 110b in the short direction of the positive electrode 110. The ratio of the width (length in the short direction) of the first region 113 to the width (length in the short direction) of the second region 114 is, for example, in the range of 1:15 to 3:4 or 1:12 to 1:6.

[0078] The first region 113 of the positive electrode 110 has an exposed portion 113b of the positive electrode current collector where the positive electrode mixture layer is not placed on the positive electrode current collector, and a first positive electrode mixture portion 113c where the positive electrode mixture layer is placed on the positive electrode current collector. The second region 114 has a second positive electrode mixture portion 114c where the positive electrode mixture layer is placed on the positive electrode current collector.

[0079] The exposed portion 113b of the positive electrode current collector is provided intermittently at one or more locations along the longitudinal direction of the positive electrode current collector. The exposed portion 113b does not have a positive electrode mixture layer from one end 110a in the short direction of the positive electrode 110 to the second region 114.

[0080] The length of each exposed portion 113b of the positive electrode current collector in the longitudinal direction may be 1% to 10% of the longitudinal length of the positive electrode current collector, and the sum of the lengths of all exposed portions 113b of the positive electrode current collector in the longitudinal direction may be 1% to 20%, 5% to 20%, or 8% to 20% of the longitudinal length of the positive electrode current collector.

[0081] When a positive electrode current collector has multiple exposed portions 113b, it is desirable that the spacing between adjacent exposed portions 113b of the positive electrode current collectors be as uniform as possible. For example, if the longitudinal 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 spacing between adjacent exposed portions 113b of the positive electrode current collectors may be between 0.8 × L100 / n and 1.2 × L100 / n.

[0082] Each of the exposed portions 113b of the positive electrode current collector is connected to a tab-shaped positive electrode lead 112. Multiple positive electrode leads 112 are bundled together and connected to the first portion 17a of the first terminal member 17.

[0083] The mass W1 per unit area of ​​the positive electrode mixture layer arranged on the surface of the positive electrode current collector in the first positive electrode mixture section 113c may be the same as the mass W2 per unit area of ​​the positive electrode mixture layer in the second positive electrode mixture section 114c. If W1 and W2 are different, the ratio of the difference between W1 and W2 (ΔW) to W1 may be, for example, 4% or less, 3% or less, 2% or less, or 1% or less.

[0084] As shown in Figure 3, the negative electrode 120 has a negative electrode edge portion 123 that faces at least a portion (preferably 70% or more) of the first region 113, and a negative electrode main portion 124 other than the negative electrode edge portion 123. The negative electrode main portion 124 faces at least a portion (preferably 70% or more) of the second region 114. That is, the negative electrode 120 has a negative electrode edge portion 123 that includes one end 120a in the short direction of the negative electrode 120, and a negative electrode main portion 124 other than the negative electrode edge portion 123. The negative electrode main portion 124 is the region from the negative electrode central end 123a of the negative electrode edge portion 123 to the other end 120b in the short direction of the negative electrode 120. The ratio of the width (length in the short direction) of the negative electrode edge portion 123 to the width (length in the short direction) of the negative electrode main portion 124 is, like the positive electrode 110, in the range of, for example, 1:15 to 3:4 or 1:12 to 1:6.

[0085] The other end 120b of the negative electrode 120 in the short direction has an exposed portion 123b of the negative electrode current collector where the negative electrode mixture layer is not disposed on the negative electrode current collector. The exposed portion 123b of the negative electrode current collector is formed along the longitudinal direction of the negative electrode current collector. Therefore, the exposed portion 123b of the negative electrode current collector is exposed at the other end face of the electrode group 14. The exposed portion 123b of the negative electrode current collector is connected to the end face current collector plate 19, for example, by laser welding.

[0086] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolyte solution), a gel electrolyte, or a solid electrolyte. A liquid electrolyte is, for example, an electrolyte solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolyte solution may contain known additives.

[0087] The gel-like electrolyte comprises a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0088] As the solid electrolyte, for example, materials known for use in all-solid-state lithium-ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) are used.

[0089] For example, liquid non-aqueous electrolytes are prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that undergoes ion dissociation in the electrolyte, and may include, for example, lithium salts. Various additives may be included in the electrolyte. Electrolytes are usually used in liquid form, but they may also be in a state where their fluidity is restricted by gelling agents or other means.

[0090] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonate esters having unsaturated bonds, such as vinylene carbonate (VC), may also be used. Cyclic carbonate esters having fluorine atoms, such as fluoroethylene carbonate (FEC), may also be used. Examples of linear carbonate esters 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 linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0091] 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 １０Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenedioleate(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 ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ Examples include the following. A single lithium salt may be used alone, or two or more may be used in combination. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0092] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0093] (Note) The following technologies are disclosed as described above. (Technical 1) A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided partially along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from the end in the short direction to the second region, the positive electrode mixture layer comprises a positive electrode active material and a binder, and the mass content C1 of the binder contained in the positive electrode mixture layer in the first region is greater than the mass content C2 of the binder contained in the positive electrode mixture layer in the second region. (Technology 2) The secondary battery according to Technology 1, wherein the binder comprises at least a fluorine-based polymer. (Technology 3) The secondary battery according to Technology 2, wherein the fluorine-based polymer comprises at least a polyvinylidene fluoride-based polymer. (Technology 4) The secondary battery according to any one of Technology 1 to 3, wherein the ratio C1 / C2 of the mass content C1 of the binder contained in the positive electrode mixture layer in the first region to the mass content C2 of the binder contained in the positive electrode mixture layer in the second region is 2 or less. (Technology 5) The secondary battery according to any one of Technology 1 to 4, wherein the ratio C1 / C2 of the mass content C1 of the binder contained in the positive electrode mixture layer in the first region to the mass content C2 of the binder contained in the positive electrode mixture layer in the second region is 1.05 or more.

[0094] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0095] <Example 1> [Preparation of the positive electrode] <First positive electrode slurry> An appropriate amount of NMP was added to the first positive electrode mixture to obtain the first positive electrode slurry. The first positive electrode mixture was a mixture of a lithium-containing composite oxide, which is the positive electrode active material, carbon black, which is a conductive additive, and a binder. The lithium-containing composite oxide was LiNi ０．８ Co０．１ Mn ０．１ O ２ The following was used. In the positive electrode mixture, the mass ratio of lithium-containing composite oxide to carbon black was set to 98:1. In the positive electrode mixture, the mass content C1 of the binder was set to 1.1% by mass.

[0096] Polyvinylidene fluoride (weight-average molecular weight Mw = approximately 1,000,000) was used as the binder.

[0097] <Second Cathode Slurry> The second cathode slurry was obtained in the same manner as the first cathode slurry, except that the mass content C2 of the binder in the cathode mixture was set to 1.0% by mass.

[0098] <Formation of the First and Second Positive Electrode Mixture Sections> The first positive electrode slurry and the second positive electrode slurry were applied to both sides of an aluminum foil positive electrode current collector to a predetermined thickness, the coating was dried, and the material was rolled to form a positive electrode mixture layer comprising the first and second positive electrode mixture sections, thereby obtaining a positive electrode as shown in Figure 2. Specifically, the first 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 to a predetermined thickness, and the second positive electrode slurry was applied to the remaining part of the positive electrode current collector to the same thickness, the coating was dried, and the material was rolled to form a positive electrode comprising a first region having the first positive electrode mixture section and a second region having the second positive electrode mixture section. The mass of the positive electrode mixture layer per unit area on the surface of the positive electrode current collector was 300 g / m² per side of the positive electrode current collector. ２ In the first region, eight exposed portions of the positive electrode current collector were provided, and a positive electrode lead was attached to each exposed portion.

[0099] The width of the first region (length in the shorter direction) was set to 12 mm, and the width of the second region (length in the shorter direction) was set to 62 mm (the ratio of the length in the shorter direction of the first region to the length in the shorter direction of the second region is 1:5.2).

[0100] The sum of the lengths of the exposed portions of the eight positive electrode current collectors in the longitudinal direction is 10% of the longitudinal length of the positive electrode current collector.

[0101] If the length of the positive electrode current collector is L100 = 720 mm, and the number of exposed parts of the positive electrode current collector is n = 8, then the distance between adjacent exposed parts is L100 / 8 = 90.

[0102] [Fabrication of the negative electrode] SiO₂, the active material of the negative electrode ｘ (x = 1.0) A suitable amount of water was added to a negative electrode mixture containing graphite as the negative electrode active material, styrene-butadiene copolymer rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener to obtain a negative electrode slurry. In the negative electrode mixture, SiO ｘ The mass ratio of graphite, SBR, and CMC was set to 5:93:1:1.

[0103] A negative electrode slurry was applied to both sides of a copper foil, which served as the negative electrode current collector, to a predetermined thickness, dried, and rolled to form a negative electrode mixture layer, resulting in the negative electrode shown in Figure 3. Specifically, the negative electrode slurry was uniformly applied to the surface of the copper foil to a predetermined thickness, dried, and rolled to simultaneously form the first negative electrode mixture portion and the second negative electrode mixture portion. However, a portion of one end of the negative electrode current collector was left exposed. The thickness of the negative electrode mixture portion was appropriately varied according to the thickness of the positive electrode mixture layer.

[0104] [Preparation of electrolyte] Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:3 to form a non-aqueous solvent, to which vinylene carbonate (VC) is added, and LiPF is prepared. ６ The electrolyte was prepared by dissolving the following. The VC content relative to the total electrolyte was 5% by mass. LiPF in the electrolyte ６ The concentration was set to 1.5 mol / L.

[0105] [Fabrication of Secondary Battery] An electrode group was fabricated by winding the positive and negative electrodes with a separator (microporous polyethylene membrane) in between in an inert gas atmosphere. The electrodes were stacked so that the edge of the positive electrode was positioned on one end face of the electrode group, and the exposed portion of the negative electrode current collector was positioned on the other end face of the electrode group. Using the electrode group and electrolyte, a cylindrical lithium-ion secondary battery (battery A1) as shown in Figure 1 was completed.

[0106] Multiple positive leads were bundled together and electrically connected to the first portion of the first terminal member. The exposed portion of the negative current collector was connected to the end face current collector plate by laser welding, and the end face current collector plate was electrically connected to the negative current collector plate via a connecting plate.

[0107] <Example 2> Battery A2 was prepared in the same manner as in Example 1, except that the mass content C1 of the binder in the positive electrode mixture was set to 1.50% by mass in the preparation of the first positive electrode slurry.

[0108] <Example 3> Battery A3 was prepared in the same manner as in Example 1, except that the mass content C1 of the binder in the positive electrode mixture was set to 2.00% by mass in the preparation of the first positive electrode slurry.

[0109] <Comparative Example 1> Battery B1 was prepared in the same manner as in Example 1, except that the second positive electrode slurry was used instead of the first positive electrode slurry in the first positive electrode mixture section.

[0110] [Evaluation 1] (DCIR) Under conditions of 25°C, constant current charging at 0.2C was performed to charge the battery to 10% of its full capacity. After pausing until the voltage stabilized, constant current discharge at 0.5C was performed for 30 seconds. Note that 1C of current refers to the current value required to fully charge the battery in one hour. The DCIR was calculated by determining the resistance value from the voltage change and current value at this time. When the DCIR of battery B1 was set to 100, a value of ○ was used if the DCIR of each battery was less than 102, and a △ was used if the DCIR was 102 or more and less than 105.

[0111] Here, a fully charged state is defined as the state achieved when, under conditions of 25°C, constant current charging at 0.2C is performed, and once the battery voltage reaches 4.2V, constant voltage charging is performed until the charging current reaches 0.02C.

[0112] [Evaluation 2] (Winding Ability) Ten secondary batteries each of the example and comparative example were prepared, then disassembled to check for fractures in the first positive electrode section. Table 1 shows ○ if no fracture was found, △ if no fracture was found but only minute cracks of 1 mm or less in length were found, and × if one or more fractures were found.

[0113]

[0114] Table 1 shows that when the binder is concentrated in the first region, winding performance improves and breakage in the first positive electrode becomes less likely.

[0115] The secondary battery described herein is useful as a primary power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0116] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0117] 10: Secondary battery 11: Case 12: Through hole 13: Recess 14: Electrode group 16: Positive terminal 17: First terminal member (terminal member) 17a: First part 17b: Second part 17c: Third part 18: Second terminal member 19: End face current collector plate 21: Connecting plate 22: Negative current collector plate 22a: Injection hole 23: Sealing plate 24: Insulating member 25: Insulating plate 26: Positive gasket 27: Negative gasket

[0118] 110: Positive electrode 110a One end 110b Other end 112 Positive electrode lead 112a: Folded portion 113 First region (positive electrode edge) 113a Central end of positive electrode 113b Exposed portion of positive electrode current collector 113c First positive electrode mixture portion 114 Second region (positive electrode main portion) 114c Second positive electrode mixture portion

[0119] 120: Negative electrode 120a One end 120b Other end 123 Negative electrode edge 123a Negative electrode central end 123b Negative electrode current collector exposed part 123c First negative electrode mixture part 124 Negative electrode main part 124c Second negative electrode mixture part

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided partially along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from the one end in the short direction to the second region, the positive electrode mixture layer comprises a positive electrode active material and a binder, and the mass content C1 of the binder contained in the positive electrode mixture layer in the first region is greater than the mass content C2 of the binder contained in the positive electrode mixture layer in the second region.

2. The secondary battery according to claim 1, wherein the binder comprises at least a fluorine-based polymer.

3. The secondary battery according to claim 2, wherein the fluorine-based polymer comprises at least a polyvinylidene fluoride-based polymer.

4. The secondary battery according to claim 1, wherein the ratio C1 / C2 of the mass content C1 of the binder contained in the positive electrode mixture layer in the first region to the mass content C2 of the binder contained in the positive electrode mixture layer in the second region is 2 or less.

5. The secondary battery according to claim 1, wherein the ratio C1 / C2 of the mass content C1 of the binder contained in the positive electrode mixture layer in the first region to the mass content C2 of the binder contained in the positive electrode mixture layer in the second region is 1.05 or more.

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