JP2016126852A - Positive electrode active material layer for secondary battery, wound device, and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a novel and improved positive electrode active material layer for a secondary battery, which enables the enhancement of flexibility of a positive electrode active material layer while keeping cycle characteristics of a secondary battery; a wound device; and a secondary battery.SOLUTION: To solve the above problem, a positive electrode active material layer for a secondary battery is provided according to an aspect of the present invention, which comprises: a high-elastic modulus layer provided on a positive electrode current collector, and including a first positive electrode active material and a high-elastic modulus binder; and a low-elastic modulus layer provided on the high-elastic modulus layer and including a second positive electrode active material and a low-elastic modulus binder lower than the high-elastic modulus binder in tensile elasticity.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode active material layer for a secondary battery and a secondary battery.

  In recent years, with the downsizing of information processing apparatuses such as mobile phones and notebook personal computers (note PCs), further improvement in characteristics of lithium ion secondary batteries used as power sources for these information processing apparatuses has been demanded. .

  For example, Patent Document 1 discloses a technique for improving the characteristics (capacity and cycle characteristics) of a lithium ion secondary battery by increasing the density of a positive electrode active material layer. In this technique, a plurality of types of active material particles having different average particle sizes are blended at a predetermined blending ratio, and carbon black and expanded graphite are blended at a predetermined blending ratio.

  However, the characteristics of the lithium ion secondary battery cannot be sufficiently improved by simply increasing the density of the positive electrode active material layer. For this reason, it has been proposed to increase the density and thickness of the positive electrode active material layer.

JP 2012-146590 A

  However, when the positive electrode active material layer is densified and further thickened, there is a problem in that the flexibility of the positive electrode active material layer decreases. For this reason, when producing a wound type lithium ion secondary battery, the positive electrode may be damaged. For this reason, the conventional technique has a limit in increasing the thickness of the positive electrode active material layer.

  On the other hand, as a technique for ensuring the flexibility of the positive electrode active material layer, a low elastic modulus binder may be used as the binder for the positive electrode active material layer. However, the low elastic modulus binder can be a factor that deteriorates the characteristics of the lithium ion secondary battery, particularly the cycle characteristics. Therefore, even if the positive electrode active material layer is thickened using a low elastic modulus binder, the characteristics of the lithium ion secondary battery cannot be improved.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to increase the flexibility of the positive electrode active material layer while maintaining the cycle characteristics of the secondary battery. Another object of the present invention is to provide a new and improved positive electrode active material layer for a secondary battery, a wound element, and a secondary battery.

  In order to solve the above problems, according to one aspect of the present invention, a high elastic modulus layer provided on a positive electrode current collector and including a first positive electrode active material and a high elastic modulus binder, and a high elastic modulus A secondary battery comprising: a second positive electrode active material provided on the layer; and a low elastic modulus layer including a low elastic modulus binder having a tensile elastic modulus lower than that of the high elastic modulus binder. A positive electrode active material layer is provided.

  According to this aspect, the positive electrode active material layer includes a high elastic modulus layer and a low elastic modulus layer, and the high elastic modulus layer exists on the positive electrode current collector side, so that the cycle characteristics of the secondary battery are maintained. The flexibility of the positive electrode active material layer can be increased.

  Here, the tensile elastic modulus of the high elastic modulus binder may be 400 to 1200 MPa, and the tensile elastic modulus of the low elastic modulus binder may be 150 to 700 MPa.

  According to this aspect, since the high elastic modulus binder and the low elastic modulus binder have a tensile elastic modulus in the above range, the flexibility of the positive electrode active material layer can be enhanced while maintaining the cycle characteristics of the secondary battery.

  Further, at least one of the high modulus binder and the low modulus binder may contain a copolymer.

  According to this viewpoint, the flexibility of the positive electrode active material layer can be enhanced while maintaining the cycle characteristics of the secondary battery. Furthermore, the tensile elastic modulus of each binder can be arbitrarily adjusted by adjusting the composition ratio of the monomers constituting the copolymer.

  Further, at least one of the first positive electrode active material and the second positive electrode active material may contain a lithium-containing transition metal oxide.

  According to this viewpoint, the flexibility of the positive electrode active material layer can be enhanced while maintaining the cycle characteristics of the secondary battery. In addition, higher capacity of secondary batteries can be expected.

  According to another aspect of the present invention, there is provided a winding element comprising the positive electrode active material layer for a secondary battery.

  Since the winding element by this viewpoint contains the said positive electrode active material layer, it can suppress the fracture | rupture of the positive electrode in a winding element, and can improve cycle life.

  According to another aspect of the present invention, there is provided a secondary battery comprising the winding element.

  Since the secondary battery by this viewpoint contains the said winding element, it can suppress the fracture | rupture of the positive electrode in a winding element, and can improve cycle life.

  As described above, according to the present invention, the positive electrode active material layer includes the high elastic modulus layer and the low elastic modulus layer, and the high elastic modulus layer exists on the positive electrode current collector side. The flexibility of the positive electrode active material layer can be increased while maintaining the characteristics.

It is a plane sectional view showing a schematic structure of a lithium ion secondary battery according to an embodiment of the present invention.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

<1. Configuration of lithium ion secondary battery>
First, with reference to FIG. 1, the structure of the lithium ion secondary battery which concerns on embodiment of this invention is demonstrated. FIG. 1 shows a plan sectional view of the winding element 1a and an enlarged view in which a region A of the winding element 1a is enlarged. The lithium ion secondary battery includes a winding element 1a, a nonaqueous electrolyte solution, and an exterior material. The winding element 1a is obtained by winding an electrode laminate in which a belt-like positive electrode 10, a separator 20, a belt-like negative electrode 30, and a separator 20 are laminated in this order in the longitudinal direction and compressing in the arrow B direction.

  The strip-like positive electrode 10 (hereinafter also referred to as “positive electrode 10”) includes a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode current collector 11 is not particularly limited, and is made of, for example, aluminum (Al), stainless steel, nickel plated steel, or the like. A positive electrode terminal is connected to the positive electrode current collector 11.

  Furthermore, the positive electrode active material layer 12 includes a high elastic modulus layer 12a and a low elastic modulus layer 12b. The high elastic modulus layer 12 a is provided on the current collector 11, specifically, on both the front and back surfaces of the current collector 11. The high elastic modulus layer 12a has a higher tensile elastic modulus than the low elastic modulus layer 12b. That is, the high elastic modulus layer 12a is harder than the low elastic modulus layer 12b. Thus, in this embodiment, the positive electrode active material layer 12 has a two-layer structure, and the surface layer side (the side far from the positive electrode current collector 11) of the positive electrode active material layer 12 is softened. The reason for this is as follows. That is, when the positive electrode 10 is bent, distortion is likely to occur on the surface layer side of the positive electrode 10. This is because when the positive electrode 10 is bent, a large stress is applied to the surface layer side of the positive electrode 10. Therefore, in this embodiment, the generation of strain is suppressed by disposing a soft layer on the surface layer side of the positive electrode 10. Hereinafter, each layer constituting the positive electrode active material layer 12 will be described in detail.

  The high elastic modulus layer 12a includes at least a first positive electrode active material and a high elastic modulus binder, and may further include a conductive agent. The first positive electrode active material is not particularly limited as long as it is a material capable of reversibly occluding and releasing lithium ions. For example, lithium-containing transition metal oxide, nickel sulfide, copper sulfide, sulfur, iron oxide And vanadium oxide. Examples of the lithium-containing transition metal oxide include lithium cobalt oxide (LCO), lithium nickelate, lithium nickel cobaltate, lithium nickel cobaltaluminate (hereinafter sometimes referred to as “NCA”), nickel cobalt manganate. Examples thereof include lithium (hereinafter sometimes referred to as “NCM”), lithium manganate, lithium iron phosphate, and the like. These positive electrode active materials may be used independently and 2 or more types may be used together.

Among the examples listed above, the first positive electrode active material is preferably a lithium-containing transition metal oxide, and particularly preferably a lithium salt of a transition metal oxide having a layered rock salt structure. As a lithium salt of a transition metal oxide having such a layered rock salt structure, for example, Li _1-x-yz Ni _x Co _y Al _z O ₂ (NCA) or Li _1-x-yz Ni _x Co _y Mn _z O ₂ (NCM) (0 <x <1, 0 <y <1, 0 <z <1, and x + y + z <1) is represented by a lithium salt of a ternary transition metal oxide. Can be mentioned.

  In addition, the first positive electrode active material may be a material obtained by subjecting each of the above materials to surface treatment in order to suppress side reactions with the electrolytic solution at a high voltage. The average agglomerated particle size of the first positive electrode active material is preferably 10 to 30 μm from the viewpoints of safety and fillability of the first positive electrode active material. The average agglomerated particle size of the first positive electrode active material is a 50% integrated value (D50 value) of the diameter distribution when the secondary particles in which the primary particles of the first positive electrode active material are agglomerated are considered as spheres. And can be measured by a laser diffraction / scattering method.

  In addition, content (for example, volume density) in the high elastic modulus layer 12a of the 1st positive electrode active material is not restrict | limited in particular, If it is content applied to the positive electrode active material layer of the conventional lithium ion secondary battery. Either may be sufficient.

  The high elastic modulus binder bonds the first positive electrode active material and the conductive agent to each other, and bonds the first positive electrode active material and the conductive agent to the positive electrode current collector 11. As the high elastic modulus binder, one having a higher tensile elastic modulus than a low elastic modulus binder described later is selected. Examples of the high elastic modulus binder include polyvinylidene fluoride, a modified product of polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene (hexafluoropropylene). Copolymer, modified vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, ethylene acrylic acid (ethylene acrylate) copolymer Polymer, ethylene methyl acrylate copolymer, vinylidene fluoride (VDF) - acrylate (acrylate) copolymer, acrylonitrile-butadiene rubber (acrylonitrile-butadiene rubber, NBR), hydrogenated acrylonitrile butadiene copolymer (hydrogenated acylonitrile-butadiene) and the like. These binders may be used alone or in combination.

  In addition, when a copolymer is contained in a high elastic modulus binder, the tensile elasticity modulus of a copolymer can be adjusted by adjusting the composition ratio (molar ratio etc.) of the monomer which comprises a copolymer. Specifically, the tensile modulus of the copolymer can be increased by increasing the composition ratio of the monomers constituting the polymer having a high tensile modulus. The high elastic modulus binder preferably contains at least one copolymer.

  The tensile elastic modulus of the high elastic modulus binder is not particularly limited as long as it is higher than the tensile elastic modulus of the low elastic modulus binder described later, but is preferably 400 to 1200 MPa, and more preferably 500 to 900 MPa. When the tensile elastic modulus of the high elastic modulus binder is within this range, a high capacity can be realized while ensuring the flexibility of the positive electrode 10. In addition, the tensile elasticity modulus of this embodiment is a value measured by the tension test based on ASTMD638, for example. The tensile test shown in the Example mentioned later is the value measured by this tensile test.

  The content of the high elastic modulus binder in the high elastic modulus layer 12a is not particularly limited, but is preferably 0.3% to 7%, preferably 0.5% to 3% by mass ratio. This is because if the content of the high elastic modulus binder is low, the adhesion to the current collector is lowered, and if the content is high, the filling property of the electrode may be lowered.

  The conductive agent is, for example, carbon black such as ketjen black or acetylene black, natural graphite, artificial graphite, or the like, but is not particularly limited as long as it is intended to increase the conductivity of the positive electrode.

  The low elastic modulus layer 12b is provided on the surface of each high elastic modulus layer 12a. The low elastic modulus layer 12b includes at least a second positive electrode active material and a low elastic modulus binder, and may further include a conductive agent. The second positive electrode active material is selected from the same materials as the first positive electrode active material. The first positive electrode active material and the second positive electrode active material may be made of the same material or different materials. The content of the second positive electrode active material in the low elastic modulus layer 12b may be about the same as that of the first positive electrode active material. The same kind of conductive agent as that contained in the high elastic modulus layer 12a can be used.

  The low elastic modulus binder bonds the second positive electrode active material and the conductive agent to each other, and bonds the second positive electrode active material and the conductive agent to the high elastic modulus layer 12a. The low elastic modulus binder is selected to have a lower tensile elastic modulus than the high elastic modulus binder. Examples of the low elastic modulus binder include polyvinylidene fluoride, a modified product of polyvinylidene fluoride, a vinylidene fluoride-tetrafluoroethylene copolymer, and a vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene (hexafluoropropylene). Copolymer, modified vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, ethylene acrylic acid (ethylene acrylate) copolymer Polymer, ethylene methyl acrylate copolymer, vinylidene fluoride (VDF) - acrylate (acrylate) copolymer, acrylonitrile-butadiene rubber (acrylonitrile-butadiene rubber, NBR), hydrogenated acrylonitrile butadiene copolymer (hydrogenated acylonitrile-butadiene) and the like. These binders may be used alone or in combination.

  In addition, when a copolymer is contained in a low elastic modulus binder, the tensile elasticity modulus of a copolymer can be adjusted by adjusting the composition ratio (molar ratio etc.) of the monomer which comprises a copolymer. Specifically, the tensile modulus of the copolymer can be lowered by increasing the composition ratio of the monomers constituting the polymer having a low tensile modulus. The low elastic modulus binder preferably contains at least one copolymer.

  The tensile modulus of the low modulus binder is not particularly limited as long as it is lower than the tensile modulus of the high modulus binder, but is preferably 150 to 700 MPa, and preferably 200 to 500 MPa. When the tensile elastic modulus of the low elastic modulus binder is a value within such a range, a high capacity can be realized while ensuring the flexibility of the positive electrode 10. The tensile elastic modulus is a value measured by a tensile test based on ASTM D638, for example. The tensile test shown in the Example mentioned later is the value measured by this tensile test.

  The content of the low elastic modulus binder in the low elastic modulus layer 12b is not particularly limited, but is preferably 0.3% to 7%, preferably 0.5% to 3% in terms of mass ratio. This is because when the content of the low elastic modulus binder is low, the adhesion to the current collector is lowered, and when the content is large, the filling property of the electrode may be lowered.

  The low elastic modulus binder can be a factor that reduces the capacity density of the lithium ion secondary battery 1. Therefore, from the viewpoint of increasing the capacity density of the lithium ion secondary battery 1, the low elastic modulus layer 12b is preferably as thin as possible. On the other hand, if the low elastic modulus layer 12b is too thin, the positive electrode active material layer 12 may not be sufficiently flexible. For this reason, it is preferable that the ratio of the thickness of the high modulus layer 12a and the low modulus layer 12b (low modulus layer / high modulus layer) is 0.2-2.

  The thickness of the positive electrode active material layer 12 is not particularly limited, and can be at least as thick as a conventional lithium ion secondary battery. Furthermore, in this embodiment, since the positive electrode active material layer 12 has excellent flexibility, it is possible to increase the thickness of the positive electrode active material layer 12 as compared with the conventional case.

  The separator 20 (hereinafter also referred to as “separator 20”), the strip-shaped negative electrode 30 (hereinafter also referred to as “negative electrode 30”), the electrolytic solution, and the exterior material are those that can be used in a general lithium ion secondary battery. Can be used arbitrarily. These will be briefly described as follows.

The separator 20 is not particularly limited, and any separator can be used as long as it is used as a separator for a general lithium ion secondary battery. As the separator, it is preferable to use a porous film or a non-woven fabric exhibiting excellent high rate discharge performance alone or in combination. The separator may be coated with an inorganic material such as Al ₂ O ₃ , Mg (OH) ₂ , SiO ₂ . Examples of the material constituting the separator include, for example, polyolefin resins represented by polyethylene, polypropylene, and the like, polyethylene terephthalate, and polybutylene terephthalate. Polyester resin, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinylether (perfluorovinylether) Polymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone (Hexafluoroacetone) copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetra Fluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetra It can be used Ruoroechiren copolymer. The porosity of the separator is not particularly limited, and the porosity of the separator of the lithium ion secondary battery can be arbitrarily applied.

The negative electrode 30 includes a current collector 31 and a negative electrode active material layer 32. The current collector 31 is made of, for example, copper (Cu), nickel (Ni), or the like. Here, as long as the negative electrode active material layer 32 is used as a negative electrode active material layer of a lithium ion secondary battery, what kind of thing may be sufficient as it. For example, the negative electrode active material layer 32 includes a negative electrode active material, and may further include a negative electrode binder. Examples of the negative electrode active material include graphite active materials (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, etc.), silicon (Si), tin (Sn), or oxides thereof. A mixture of fine particles of graphite and a graphite active material, fine particles of silicon or tin, alloys based on silicon or tin, and titanium oxide (TiO _x ) compounds such as Li ₄ Ti ₅ O ₁₂ can be used. . Note that the oxide of silicon is represented by SiO _x (0 ≦ x ≦ 2). In addition to these, for example, metallic lithium can be used as the negative electrode active material.

  Examples of the binder for the negative electrode include polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, SBR butadiene, and acrylonitrile rubber. Examples thereof include butadiene rubber, fluoroelastomer, polyvinyl acetate, polymethyl methacrylate, polyethylene, and nitrocellulose. The negative electrode binder is not particularly limited as long as it can bind the negative electrode active material and the conductive agent onto the current collector 21. In addition, the content of the negative electrode binder is not particularly limited, and may be any content as long as it is applied to the negative electrode active material layer of the lithium ion secondary battery.

  As the electrolytic solution, the same non-aqueous electrolytic solution conventionally used for lithium secondary batteries can be used without any particular limitation. Here, the electrolytic solution has a composition in which an electrolyte salt is contained in a non-aqueous solvent. Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, vinylene carbonate and the like. ); Cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate linear carbonates such as ethyl carbonate; chain esters such as methyl formate, methyl acetate, methyl butyrate; tetrahydrofuran or its derivatives; 3-dioxane (1,3-dioxane), 1,4-dioxane (1,4-dioxane), 1,2-dimethoxyethane (1,2-dioxyethane), 1,4-dibutoxyethane (1,4- ethers such as dibutoxyether and methyldiglyme; acetonitrile, benzonitrile (ben) nitriles such as zontrile; dioxolane or a derivative thereof; ethylene sulfide, sulfolane, sultone, a derivative thereof, or the like alone, or a mixture of two or more thereof However, the present invention is not limited to these.

Examples of the electrolyte salt include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO _4. , Inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K), such as KSCN, LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , (CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (n-C ₄ H ₉ ) ₄ NC _{lO 4, (n-C 4} H 9) 4 NI, (C 2 H 5) 4 N-maleate, (C 2 H 5) 4 N-benzoate, (C 2 H 5) 4 N-phtalate, stearyl acid Organic ion salts such as lithium (lithium stearyl sulfate), lithium octyl sulfonate (lithium octyl sulfate), lithium dodecylbenzene sulfonate (lithium dodecylbenzene sulfate), and the like can be used. These ionic compounds can be used alone or in admixture of two or more. Further, the concentration of the electrolyte salt may be the same as that of the nonaqueous electrolytic solution used in the conventional lithium secondary battery, and is not particularly limited. In the present invention, an electrolytic solution containing an appropriate lithium compound (electrolyte salt) at a concentration of about 0.5 to 2.0 mol / L can be used. The exterior material is, for example, an aluminum laminate.

<2. Manufacturing method of non-aqueous electrolyte lithium ion secondary battery>
Next, the manufacturing method of a nonaqueous electrolyte lithium ion secondary battery is demonstrated.
(Method for producing strip-shaped positive electrode)
The positive electrode 10 is produced by the following method, for example. First, the high elastic modulus layer 12 a is formed on the positive electrode current collector 11. That is, the material for the high modulus layer 12 a is dispersed in an organic solvent or water to form a mixture slurry for the high modulus layer, and this mixture slurry is applied onto the positive electrode current collector 11. Thereby, a coating layer is formed. Next, the low elastic modulus layer 12b is formed on the high elastic modulus layer 12a before the coating layer is dried. That is, the low elastic modulus layer 12b is dispersed in an organic solvent or water to form a low elastic modulus layer mixture slurry, and this mixture slurry is applied onto the high elastic modulus layer 12a. Thereby, two coating layers are formed. Next, the coating layer is dried. Thereby, the low elastic modulus layer 12b is formed on the high elastic modulus layer 12a.

  The coating method is not particularly limited. For example, a doctor blade method, a slot die method, a knife coater method, a gravure coater method, or the like is used. Also good. Moreover, each coating process is not specifically limited, You may apply a high elastic modulus layer and a low elastic modulus layer simultaneously.

(Method for producing strip-shaped negative electrode)
The negative electrode 30 is produced by the following method, for example. That is, a negative electrode mixture slurry is formed by dispersing the material of the negative electrode active material layer in a solvent (for example, water), and this negative electrode mixture slurry is applied onto the current collector. Thereby, a coating layer is formed. Next, the coating layer is dried. In the negative electrode mixture slurry, fluororesin fine particles and elastomeric polymer fine particles are dispersed in the negative electrode active material layer 10a. Next, the dried coating layer is rolled together with the negative electrode current collector 10b. Thereby, the negative electrode 30 is produced.

(Wound element and battery manufacturing method)
Next, the positive electrode 10, the separator 20, the negative electrode 30, and the separator 20 are laminated in this order to produce an electrode laminate. Next, the electrode laminate is wound. Thereby, the winding element 1a is produced. Subsequently, the flat winding element 1a is produced by crushing the winding element 1a. Next, the flat winding element 1a is inserted into the exterior body (for example, a laminate film) together with the non-aqueous electrolyte, and the exterior body is sealed, whereby the lithium ion secondary battery 1 is manufactured. Note that, when sealing the exterior body, the terminals that are electrically connected to the current collectors are projected outside the exterior body.

Example 1
Next, examples of the present invention will be described. In Example 1, the lithium ion secondary battery 1 according to Example 1 was manufactured through the following steps.

(Preparation of positive electrode)
(Preparation of high modulus layer)
A mixture slurry for a high elastic modulus layer is prepared by dissolving and dispersing lithium cobaltate, carbon black, and a high elastic modulus binder in N-methylpyrrolidone at a mass ratio of solids of 97.6: 1.2: 1.2. Produced. Here, the high elastic modulus binder was a VdF / TFE copolymer having a tensile elastic modulus of 800 MPa. Next, this mixture slurry was applied to both surfaces of a 12 μm thick aluminum foil current collector (positive electrode current collector 11) to prepare a coating layer. The thickness of the coating layer was adjusted so that the thickness after drying (that is, the thickness of the high elastic modulus layer 12a) was 60 μm. Then, the high elastic modulus layer 12a was formed on the positive electrode current collector 11 by drying the coating layer.

(Production of low elastic modulus layer)
A mixture slurry for a low elastic modulus layer is prepared by dissolving and dispersing lithium cobaltate, carbon black, and a low elastic modulus binder in N-methylpyrrolidone at a mass ratio of solids of 97.6: 1.2: 1.2. Produced. Here, the low elastic modulus binder was a mixture of a VdF / TFE copolymer and hydrogenated NBR. The tensile elastic modulus of this mixture was 300 MPa. Subsequently, this mixture slurry was applied to the surface of each high elastic modulus layer 12a to prepare a coating layer. The thickness of the coating layer was adjusted so that the thickness after drying (that is, the thickness of the low elastic modulus layer 12b) was 60 μm. Then, the low elastic modulus layer 12b was formed on the high elastic modulus layer 12a by drying the coating layer. Accordingly, the thickness ratio between the high elastic modulus layer 12a and the low elastic modulus layer 12b is 1. Thereafter, the positive electrode current collector 11 and the positive electrode active material layer 12 were rolled to produce the positive electrode 10. The thickness of the positive electrode active material layer 12 after rolling was 72 μm. The total thickness of the positive electrode 10 was 156 μm, and the electrode density was 4.15 g / cm ³ . Next, an aluminum lead wire was welded to the end of the positive electrode 10.

(Preparation of negative electrode)
A negative electrode mixture slurry was prepared by dissolving and dispersing graphite, styrene butadiene rubber (SBR), and sodium salt of carboxymethyl cellulose in an aqueous solvent at a mass ratio of solid content of 98: 1: 1. Subsequently, this negative electrode mixture slurry was applied to both surfaces of a 6 μm thick copper foil current collector (negative electrode current collector 31) and then dried. The negative electrode active material layer 32 was obtained by rolling the coating layer after drying. The negative electrode 30 was obtained through the above steps. The total thickness of the negative electrode 30 was 186 μm, and the electrode density was 1.7 g / cm ³ . Thereafter, a nickel lead wire was welded to the end of the negative electrode 30.

(Production of wound element)
The positive electrode 10, the separator 20 (ND314 manufactured by Asahi Kasei E-Materials Co., Ltd.), the negative electrode 30, and the separator 20 were laminated in this order, and this laminate was wound in the longitudinal direction using a winding core having a diameter of 3 cm. After fixing the end with tape, the winding core is removed, the cylindrical electrode winding element is sandwiched between two metal plates having a thickness of 3 cm, and held for 3 seconds, so that the flat winding element 1a Got.

(Production of battery)
The electrode winding element was sealed under reduced pressure with an electrolyte so that two lead wires were exposed to a laminate film composed of three layers of polypropylene / aluminum / nylon, thereby producing a battery. As the electrolytic solution, a solution obtained by dissolving 10% by volume of FEC (fluoroethylene carbonate) and 1.3M LiPF ₆ in a solvent in which ethylene carbonate / dimethyl carbonate was mixed at a ratio of 3 to 7 (volume ratio) was used. The battery was sandwiched between two 3 cm thick metal plates heated to 90 ° C. and held for 5 minutes. The lithium ion secondary battery 1 was produced through the above steps.

(Bending test)
The positive electrode 10 was bent by an MIT type folding resistance tester manufactured by Yasuda Seiki Seisakusho. And the load (N) when the positive electrode 10 fractured was measured. The larger the load, the higher the flexibility of the positive electrode 10.

(Cycle test)
First, in the first cycle, CC-CV charge (constant current constant voltage charge) is performed at 0.1 C until the voltage reaches 4.4 V, and CC discharge (at 0.1 C until the voltage reaches 2.75 V ( Constant current discharge). Next, in the second cycle, CC-CV charge was performed at 0.2 C until the voltage reached 4.4 V, and CC discharge was performed at 0.2 C until the voltage reached 2.75 V. Further, after the third cycle, CC-CV charging was performed at 1.0 C until the voltage reached 4.4 V, and CC discharging was performed at 1.0 C until the voltage reached 3.00 V.

  A numerical value obtained by dividing the discharge capacity at the 300th cycle by the discharge capacity at the third cycle was defined as the capacity retention rate. Table 1 summarizes the binder composition and Table 2 summarizes the evaluation results.

(Examples 2-9, Comparative Examples 1-5)
The same treatment as in Example 1 was performed except that the high elastic modulus binder and the low elastic modulus binder were changed to those shown in Table 1. The evaluation results are summarized in Table 2. In addition, all the electrode densities of the positive electrode were 4.15 g / cm ³ .

※二層比は、高弾性率層１２ａと低弾性率層１２ｂとの厚さの比（低弾性率層／高弾性率層）を示す。

* The two-layer ratio indicates the ratio of the thicknesses of the high elastic modulus layer 12a and the low elastic modulus layer 12b (low elastic modulus layer / high elastic modulus layer).

※比較例１、２、４、５では、巻回素子作製時に正極１０が破断したため、容量維持率を評価できなかった。

* In Comparative Examples 1, 2, 4, and 5, the capacity retention rate could not be evaluated because the positive electrode 10 was broken during the production of the winding element.

  According to Tables 1 and 2, since Examples 1-7 have a higher load at break than Comparative Examples 1, 2, 4, and 5, the flexibility is higher than Comparative Examples 1, 2, 4, and 5. I can say that. In addition, Examples 1 to 7 showed high cycle characteristics. On the other hand, Comparative Example 3 showed higher flexibility than Examples 1-7, but the cycle characteristics were very low.

  Therefore, in Examples 1-7, it can be said that cycling characteristics are high and the flexibility of the positive electrode active material layer 12 is improved. Further, according to Comparative Examples 1 to 3, even if the positive electrode active material layer 12 is formed only by the high elastic modulus layer 12a or the low elastic modulus layer 12b, the wound element cannot be produced, or the cycle characteristics are remarkably deteriorated. I found out that Further, according to Comparative Examples 4 to 5, it was found that the effect cannot be obtained unless the low elastic modulus layer 12b is formed on the high elastic modulus layer 12a.

  Furthermore, the results of Examples 1-4 are better than those of Examples 5-7. Therefore, it can be said that the tensile elastic modulus of the high elastic modulus binder is preferably 500 to 900 MPa, and the tensile elastic modulus of the low elastic modulus binder is preferably 200 to 500 MPa.

  As described above, according to the positive electrode active material 12 according to the present embodiment, the flexibility of the positive electrode active material layer 12 can be improved while maintaining the cycle characteristics of the lithium ion secondary battery 1. As a result, it is possible to further increase the thickness of the positive electrode active material layer 12.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

  For example, although the wound element type lithium ion secondary battery is shown in the above embodiment, the present invention is not limited to such an example. For example, the present invention may be applied to a stacked lithium ion secondary battery. For example, the lithium ion secondary battery may be any one of a cylindrical shape, a square shape, a laminate shape, a button shape, and the like. Further, the negative electrode 30 may have the same structure as the positive electrode 10.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 1a Winding element 10 Positive electrode 11 Positive electrode collector 12 Positive electrode active material layer 12a High elastic modulus layer 12b Low elastic modulus layer 20 Separator 30 Negative electrode active material layer 31 Negative electrode current collector 32 Negative electrode active material layer

Claims (6)

A high elastic modulus layer provided on the positive electrode current collector and including a first positive electrode active material and a high elastic modulus binder;
A low elastic modulus layer provided on the high elastic modulus layer and including a second positive electrode active material and a low elastic modulus binder having a tensile elastic modulus lower than that of the high elastic modulus binder. A positive electrode active material layer for a secondary battery. The tensile elastic modulus of the high elastic modulus binder is 400 to 1200 MPa,
The positive electrode active material layer for a secondary battery according to claim 1, wherein the low elastic modulus binder has a tensile elastic modulus of 150 to 700 MPa.   The positive electrode active material layer for a secondary battery according to claim 1, wherein at least one of the high elastic modulus binder and the low elastic modulus binder includes a copolymer.   4. The secondary according to claim 1, wherein at least one of the first positive electrode active material and the second positive electrode active material includes a lithium-containing transition metal oxide. 5. A positive electrode active material layer for a battery.   A winding element comprising the positive electrode active material layer for a secondary battery according to any one of claims 1 to 4. A secondary battery comprising the winding element according to claim 5.

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