JPWO2016163114A1 - Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Abstract

Provided are a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery having a protective layer that enhances safety without greatly reducing the charge / discharge capacity. A current collector, an active material layer disposed on the surface of the current collector, and a protective layer disposed so that there is no exposed portion of the active material layer on the surface of the active material layer. The maximum height is 5 μm or less, the active material layer includes a first active material and a second active material, and the average particle diameter D50 of the first active material is larger than the average particle diameter D50 of the second active material, One active material is a lithium cobalt-containing composite metal oxide, the second active material is an olivine-type lithium phosphate composite oxide, and the protective layer includes ceramic powder.

Description

  The present invention relates to a positive electrode used for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, and a nonaqueous electrolyte secondary battery using the positive electrode.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. At present, lithium ion secondary batteries are mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future. A lithium ion secondary battery has an active material capable of inserting and extracting lithium (Li) in a positive electrode and a negative electrode, respectively. The lithium ion secondary battery operates by moving lithium ions in the electrolytic solution provided between both electrodes.

  In order to increase the safety of lithium ion secondary batteries, it has been studied to provide a protective layer on the surface of the active material layer to prevent internal short circuit. For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2008-159385) discloses that an increase in battery temperature during overcharge can be suppressed by providing a ceramic coat layer on the electrode surface. In Patent Document 1, a thermoplastic polymer having a melting point of 110 ° C. to 150 ° C. is used as a binder in the ceramic coat layer, the binder covers 30% to 90% of the ceramic surface, and at a high temperature of 110 ° C. or higher. It is disclosed that the thermoplastic polymer in the ceramic coat layer melts and covers the surface of the active material layer to suppress contact between the active material and the electrolytic solution.

  Basically, the thicker the protective layer, the higher the internal resistance of the battery and the lower the charge / discharge capacity of the battery. In order to maintain the energy density of the battery, the protective layer is preferably thinner. However, in order to ensure the safety of the battery, the protective layer needs to have a certain thickness.

  In addition, when the active material layer has a large surface roughness, if a protective layer is provided on the surface of the active material layer so that there is no exposed portion of the active material layer, the thickness of the protective layer is the surface of the active material layer. It is necessary to be more than the roughness. As a result, the thickness of the protective layer may become such a thickness that the charge / discharge capacity of the battery is extremely reduced.

  Therefore, a nonaqueous electrolyte secondary battery electrode in which a protective layer that enhances safety without greatly reducing the charge / discharge capacity is desired.

JP 2008-159385 A

  The present invention has been made in view of such circumstances, and used a positive electrode for a non-aqueous electrolyte secondary battery in which a protective layer that enhances safety without significantly reducing charge / discharge capacity is disposed, and the positive electrode. An object is to provide a nonaqueous electrolyte secondary battery.

  The inventors of the present invention can reduce the surface roughness of the active material layer by including two types of active materials having different sizes, and the surface roughness of the active material layer is below a certain level. By doing so, it was found that the thickness of the protective layer can be reduced, and that the protective layer ensures safety.

That is, the positive electrode for a non-aqueous electrolyte secondary battery of the present invention has a current collector, an active material layer disposed on the surface of the current collector, and an exposed portion of the active material layer on the surface of the active material layer. A maximum height of the active material layer surface is 5 μm or less, and the maximum height is the current collection in the cross section of the active material layer in the scanning electron microscope image of the cross section of the active material layer Parallel line parallel to the surface of the current collector passing through the uppermost point in the thickness direction of the curve on the surface opposite to the surface on the body side, and to the surface of the current collector passing through the lowest point in the thickness direction of the curve the distance between the parallel parallel lines, the active material layer includes a first active material and a second active material, the average particle diameter D ₅₀ of the first active material average particle diameter D ₅₀ of the second active material The first active material is a lithium cobalt-containing composite metal oxide, and the second active material is It is acid complex oxide, and a protective layer contains ceramic powder.

The first active material has the general formula: at least one _{Li a Co p Ni q Mn r} D s O x (D is the Al, Mg, Ti, Sn, Zn, W, Zr, Mo, is selected from Fe and Na, p + q + r + s = 1, 0 <p <1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0 .2) is a lithium cobalt-containing composite metal oxide having a layered structure represented by the following formula. The second active material is an olivine-type lithium phosphate composite oxide represented by the general formula: LiMPO ₄ (M is Mn, Fe, Co). And at least one of Ni).

It is preferable that the average particle diameter D ₅₀ of the first active material is not more than 15 times 1.5 times or more the average particle diameter D ₅₀ of the second active material.

It is preferable that the average particle diameter D ₅₀ of the ceramic powder is less than 1 × 1/20 or more the average particle diameter D ₅₀ of the second active material.

  When the total of the first active material and the second active material is 100% by mass, the second active material is preferably 10% by mass or more and 40% by mass or less.

The average particle diameter D ₅₀ of the first active material is 3 μm or more and 40 μm or less, the average particle diameter D ₅₀ of the second active material is 0.5 μm or more and 10 μm or less, and the average particle diameter D ₅₀ of the ceramic powder is 0.1 μm. The thickness is preferably 2 μm or less.

  The protective layer preferably contains an aqueous binder.

  The aqueous binder is preferably a water-soluble binder.

  The aqueous binder is preferably polyvinyl alcohol, polyacrylic acid, acrylic resin or methacrylic resin.

  The non-aqueous electrolyte secondary battery of the present invention has the positive electrode for a non-aqueous electrolyte secondary battery.

  In the positive electrode for a non-aqueous electrolyte secondary battery of the present invention, the thickness of the protective layer provided on the active material layer can be reduced so that there is no exposed portion of the active material layer. The non-aqueous electrolyte secondary battery having the positive electrode for the non-aqueous electrolyte secondary battery can improve safety without greatly reducing the charge / discharge capacity.

It is a schematic diagram explaining the positive electrode for nonaqueous electrolyte secondary batteries of this embodiment. 3 is a graph comparing the surface roughness of the cross sections of positive electrode A, positive electrode B, and negative electrode A. FIG.

<Positive electrode for non-aqueous electrolyte secondary battery>
The positive electrode for a non-aqueous electrolyte secondary battery of the present invention includes a current collector, an active material layer disposed on the surface of the current collector, and a protective layer disposed on the surface of the active material layer. The nonaqueous electrolyte secondary battery of the present invention is preferably a lithium ion secondary battery. The present invention will be described below by taking a lithium ion secondary battery as an example.

(Current collector)
The current collector is responsible for taking out electricity in the non-aqueous electrolyte secondary battery, and the current collector is made of a material having high electronic conductivity and electrochemically inactive during charge / discharge. Examples of the current collector material include metal materials such as stainless steel, titanium, nickel, aluminum, and copper, or conductive resins. In particular, from the viewpoint of electrical conductivity, workability, and cost, the material for the current collector is preferably aluminum or copper. The current collector can take the form of a foil, a sheet, a film, a linear shape, a rod shape, a mesh, or the like. As the current collector, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, or a stainless steel foil can be suitably used. When the current collector is a foil, sheet or film, the thickness of the current collector is preferably 10 μm to 50 μm. In view of increasing the battery capacity while maintaining the high strength of the current collector, the thickness of the current collector is particularly preferably 15 μm to 30 μm.

(Active material layer)
The active material layer includes a first active material and a second active material, and includes an active material layer binder and a conductive aid as necessary.

The average particle diameter D ₅₀ of the first active material is larger than the average particle diameter D ₅₀ of the second active material.

The average particle diameter D ₅₀ can be measured by particle size distribution measurement method. The average particle diameter D ₅₀ is that the particle size cumulative value of the volume distribution in the particle size distribution measurement by laser diffraction method is equivalent to 50%. That is, the average particle diameter D ₅₀ means the median size measured by volume.

  By including two kinds of active materials having different particle sizes in the active material layer, a second active material having a smaller particle size is disposed between the first active materials having a larger particle size, The unevenness of the surface of the active material layer is smoothed, and the surface roughness of the active material layer tends to be small.

It is preferred that the average particle diameter D ₅₀ of the first active material is not more than 15 times 1.5 times or more the average particle diameter D ₅₀ of the second active material, and more preferably not more than 12 times 1.5 times It is more preferably 1.5 times or more and 5 times or less, particularly preferably 1.5 times or more and 4 times or less, and particularly preferably 2 times or more and 4 times or less. By being in this range, the second active material is likely to be favorably disposed between the first active materials, and the surface roughness of the active material layer tends to be small.

It is preferred that the average particle diameter _{D 50} of the first active material is 3μm or more 40μm or less, more preferably 4μm or more 20μm or less, and more preferably 4μm or more 10μm or less. It is preferable that the second active mean particle diameter _{D 50} of the material is 0.5μm or more 10μm or less, more preferably 0.5μm or more 7μm or less, more preferably 0.5μm or more 4μm or less Further, it is preferably 1 μm or more and 10 μm or less, more preferably 1.5 μm or more and 7 μm or less, and further preferably 1.5 μm or more and 4 μm or less.

  The first active material and the second active material are preferably powders. The shape of the powder is not particularly limited, and examples of the shape of the powder include a spherical shape, a flat shape, and a polygonal shape.

Moreover, when the sum total of a 1st active material and a 2nd active material is 100 mass%, it is preferable that a 2nd active material is 10 to 40 mass%, and is 15 to 35 mass%. More preferably, the content is 20% by mass or more and 30% by mass or less. When small second active material having an average particle diameter D ₅₀ in the active material layer in the above ratio, the surface roughness of the active material layer tends to become smaller.

  The surface roughness of the active material layer can be expressed by the maximum height. The maximum height can be measured as follows.

  First, an electrode having at least a current collector and an active material layer is observed with a scanning electron microscope at a magnification of, for example, 1000 times to obtain an image. This electrode may have a protective layer.

  The acquired image is subjected to image processing, and a cross-sectional curve of the active material layer on the surface opposite to the surface on the current collector side is acquired.

  Since there is no significant difference in the longitudinal direction of the cross-sectional curve, a portion having an arbitrary length direction of 100 μm is specified in the cross-sectional curve, and the maximum height at the specified location is obtained.

  A parallel line (peak line) that passes through the peak of the cross-sectional curve in the thickness direction of the specified location and is parallel to the surface of the current collector, and a bottom line of the cross-sectional curve in the thickness direction of the current collector. And measure the distance between the parallel lines (valley bottom line) and make that distance the maximum height.

  The maximum height is 5 μm or less. The maximum height is more preferably 3 μm or less, still more preferably 1 μm or less.

  Since the protective layer is arrange | positioned, the positive electrode for nonaqueous electrolyte secondary batteries of this invention becomes high safety | security.

  The first active material and the second active material are the following positive electrode active materials.

The first active material has the general formula: at least one _{Li a Co p Ni q Mn r} D s O x (D is the Al, Mg, Ti, Sn, Zn, W, Zr, Mo, is selected from Fe and Na, p + q + r + s = 1, 0 <p <1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0 .2) is a lithium cobalt-containing composite metal oxide having a layered structure represented by the following formula. The second active material is an olivine-type lithium phosphate composite oxide represented by the general formula: LiMPO ₄ (M is Mn, Fe, Co). And at least one of Ni).

It is possible to obtain a high energy density battery by selecting a large _{Li a Co p Ni q Mn r} D s O x of the capacitance as the first active material. By selecting the olivine type lithium phosphate complex oxide as the second active material, the thermal stability of the battery can be improved and the safety of the battery can be enhanced.

Examples of the lithium cobalt-containing composite metal oxide having a layered structure include LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.5 Examples include Co _0.2 Mn _0.3 O ₂ , LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ , and LiCoMnO ₂ .

Examples of the olivine type lithium phosphate complex oxide include LiFePO ₄ , LiMnPO ₄ , LiCoPO _{4 and the} like.

  As the negative electrode active material, a carbon-based material that can occlude and release lithium, an element that can be alloyed with lithium, a compound that includes an element that can be alloyed with lithium, a polymer material, or the like can be used.

  Examples of the carbon-based material include non-graphitizable carbon, artificial graphite, natural graphite, cokes, graphites, glassy carbons, organic polymer compound fired bodies, carbon fibers, activated carbon, and carbon blacks. Here, the organic polymer compound fired body refers to a material obtained by firing and carbonizing a polymer material such as phenols and furans at an appropriate temperature.

  Elements that can be alloyed with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Ge, Sn. , Pb, Sb, Bi. Among them, the element that can be alloyed with lithium is preferably silicon (Si) or tin (Sn), and silicon (Si) is particularly preferable.

Examples of the compound having an element that can be alloyed with lithium include ZnLiAl, AlSb, SiB ₄ , SiB ₆ , Mg ₂ Si, Mg ₂ Sn, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , and CaSi. _{2, CrSi 2, Cu 5 Si} , FeSi 2, MnSi 2, NbSi 2, TaSi 2, VSi 2, WSi 2, ZnSi 2, SiC, Si 3 N 4, Si 2 N 2 O, SiO v (0 <v ≦ 2) SnO _w (0 <w ≦ 2), SnSiO ₃ , LiSiO 2 or LiSnO can be used. As the compound having an element that can be alloyed with lithium, a silicon compound or a tin compound is preferable. As the silicon compound, SiO _x (0.3 ≦ x ≦ 1.6) is preferable. Examples of the tin compound include tin alloys (Cu—Sn alloy, Co—Sn alloy, etc.).

  Examples of the polymer material include polyacetylene and polypyrrole.

  The binder for an active material layer plays a role of connecting the active material and the conductive additive to a current collector.

  As the binder for the active material layer, for example, polyvinylidene fluoride (abbreviation PVDF), polytetrafluoroethylene, tetrafluoroethylene / hexafluoropropylene copolymer (abbreviation FEP), fluorine-containing resins such as fluororubber, polypropylene, polyethylene, etc. Examples include thermoplastic resins, imide resins such as polyimide and polyamideimide, acrylic resins such as poly (meth) acrylic acid, alkoxysilyl group-containing resins, styrene / butadiene rubber, carboxymethylcellulose, polyethylene glycol, and polyacrylonitrile. it can. As the binder for the active material layer, PVDF and polyamideimide are preferable.

  The mixing ratio of the active material layer binder in the active material layer is preferably a mass ratio of active material: active material layer binder = 1: 0.001 to 1: 0.3. Active material: active material layer binder = 1: 0.005 to 1: 0.2 is more preferable, and 1: 0.01 to 1: 0.15 is more preferable. If the binder for the active material layer is too small, the moldability of the electrode may be lowered, and if the binder for the active material layer is too much, the energy density of the electrode may be lowered.

  The conductive auxiliary agent is added to the active material layer as necessary in order to increase the conductivity of the electrode. Carbon black, graphite, acetylene black (abbreviated as AB), ketjen black (registered trademark) (abbreviated as KB), vapor-grown carbon fiber (abbreviated as VGCF), etc., which are carbonaceous fine particles, are used alone or in combination as conductive aids. These can be used in combination. Although there are no particular limitations on the amount of conductive aid used, for example, it can be about 1 to 30 parts by mass with respect to 100 parts by mass of the active material contained in the electrode.

  In order to dispose the active material layer on the surface of the current collector, an active material layer-forming composition containing an active material, a binder for the active material layer, and, if necessary, a conductive additive is prepared. An appropriate solvent may be added to form a paste, which may be applied to the surface of the current collector and then dried. Drying may be performed under normal pressure conditions, or under reduced pressure conditions using a vacuum dryer. What is necessary is just to set drying temperature suitably. In addition, you may compress the electrical power collector in which the active material layer is arrange | positioned so that an electrode density may be raised as needed.

  Conventionally known methods such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, a curtain coating method, a lip coating method, a comma coating method, a die coating method and the like as a coating method of the composition for forming an active material layer May be used.

  Water, N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), γ-butyrolactone, acetone, etc. can be used as solvents for viscosity adjustment It is.

(Protective layer)
The protective layer is disposed so that there is no exposed portion of the active material layer on the surface of the active material layer, and includes ceramic powder. A protective layer contains the binder for protective layers as needed.

  The protective layer contains a large number of ceramic powders.

A ceramic powder that does not dissolve in a solvent can be used. That is, oxides, nitrides, and carbides are desirable as the ceramic powder. Specific examples of the ceramic powder include Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , MgO, SiC, AlN, BN, talc, mica, kaolinite, CaO, ZnO, and zeolite. As the ceramic powder, Al ₂ O ₃ , SiO ₂ , and TiO ₂ are preferable from the viewpoint of availability, and Al ₂ O ₃ is particularly preferable.

More preferably an average particle diameter _{D 50} of the ceramic powder is preferably 100nm or more 1μm or less is 100nm or more 2μm or less, further preferably 200nm or more 800nm or less, and particularly preferably 300nm or more 600nm or less . When the average particle diameter D ₅₀ of the ceramic powder is too large, the thickness of the protective layer is likely to become larger than the desired thickness. Further, the ceramic powder tends to settle in the slurry during the formation of the protective layer and is difficult to disperse. When the average particle diameter D ₅₀ of the ceramic powder is too small, the ceramic powder is likely to enter into the active material layers when creating protective layer.

It is preferable that the average particle diameter D ₅₀ of the ceramic powder is less than 1 × 1/20 or more the average particle diameter D ₅₀ of the second active material. When the average particle diameter D ₅₀ of the ceramic powder is too small for the average particle diameter D ₅₀ of the second active material, many ceramic powder when creating protective layer is likely to enter into the active material layer.

  Since the ceramic powder has low conductivity and high heat resistance, a protective layer having both insulation and heat resistance is disposed on the surface of the active material layer. Therefore, an internal short circuit in the nonaqueous electrolyte secondary battery can be suppressed even at high temperatures.

  The protective layer is disposed so that there is no exposed portion of the active material layer on the surface of the active material layer. That is, since the active material layer is covered with the protective layer without exposing the active material layer even on the surface of the active material layer, the active material is less likely to come into direct contact with the non-aqueous electrolyte. Therefore, the decomposition reaction of the nonaqueous electrolyte solution by the active material is suppressed, and deterioration of the cycle characteristics of the nonaqueous electrolyte secondary battery can be suppressed. Moreover, since the eluate of the metal component contained in the non-aqueous electrolyte and the decomposition product of the non-aqueous electrolyte can be physically trapped by the protective layer, it is possible to suppress the deposition of the decomposition product on the surface of the active material. As a result, deterioration of the cycle characteristics of the nonaqueous electrolyte secondary battery can be suppressed.

  In the protective layer containing the ceramic powder, pores are formed between the ceramic powder. Since the protective layer has pores, the protective layer has ionic conductivity. Therefore, in the nonaqueous electrolyte secondary battery, the protective layer does not have a large resistance, and the protective layer does not cause a significant decrease in battery capacity of the nonaqueous electrolyte secondary battery.

  If the maximum height of the surface of the active material layer is too large, the protective layer becomes too thick, and the charge / discharge capacity of the nonaqueous electrolyte secondary battery may be reduced.

  The binder for the protective layer binds the active material layer and the protective layer, and binds between the ceramic powders in the protective layer.

  If an organic solvent-based binder is used for the active material layer and an organic solvent-based binder is also used for the protective layer, the organic solvent-based binder of the active material layer is partially dissolved in the organic solvent used when forming the protective layer. This is not preferable because it may be

  When an organic solvent-based binder is used for the active material layer, a water-based binder is preferably used as the protective layer binder. The aqueous binder refers to a resin that dissolves or disperses in an aqueous solvent or a rubber that dissolves or disperses in an aqueous solvent. Examples of the water-based binder include a water-soluble binder and a water-dispersed binder. Here, the aqueous solvent is water or a mixture of water and alcohol. Examples of the alcohol include ethanol, methanol, isopropanol, and butanol. The blending mass ratio of the mixture of water and alcohol is preferably water: alcohol = 50: 50 to 99: 1.

  As the aqueous binder, those having a glass transition point of 60 ° C. or higher are preferable, and those having a glass transition point of 80 ° C. or higher are more preferable. Since the actual use temperature range of the nonaqueous electrolyte secondary battery is about 60 ° C. or less, it is desirable that the aqueous binder does not soften to about 60 ° C. When the glass transition point of the aqueous binder is 60 ° C. or higher, the aqueous binder is not softened even at a high temperature of about 60 ° C. Therefore, the lifetime and safety of the nonaqueous electrolyte secondary battery at high temperatures can be ensured. If the glass transition point of the aqueous binder is 60 ° C. or higher, it can be said that the polymer skeleton of the aqueous binder is hard or the cohesion between molecules is high. If an aqueous binder having a glass transition point of 60 ° C. or higher is used, a protective layer having high strength can be formed. Therefore, even if a negative electrode active material that expands and contracts with the insertion and extraction of Li is used, the protective layer can suppress the expansion and contraction of the negative electrode active material in the active material layer.

  Examples of the water-soluble binder include methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, polystyrene sulfonic acid, polyvinyl sulfonic acid, polyacrylic acid, sodium polyacrylate, polystyrene styrene copolymer, methyl vinyl ether / maleic anhydride copolymer, Polyacrylamide, polyethylene oxide, polyvinyl alcohol, acrylic acid / maleic acid copolymer, acrylic acid / sulfonic acid monomer, hydroxyethyl cellulose, acrylamide-diallyldimethylammonium chloride, diallyldimethylammonium chloride, poly (trimethylaminoethyl methyl methacrylate) Sulfate), isobutyl / maleic anhydride, chitosan, polyvinyl butyral, polyethylene glycol Call, gelatin, polyvinyl ethyl ether, polyvinyl acetate, polyvinyl pyrrolidone, polypropylene oxide, and acrylic resin. A commercially available product can be suitably used as the water-soluble acrylic resin. As a water-soluble acrylic resin, Nippon Shokubai Co., Ltd. PAANA salt (Mw = 5500), Nippon Shokubai Co., Ltd. acrylic acid / maleic acid copolymer salt, Nippon Shokubai Co., Ltd. acrylic acid / sulfonic acid monomer copolymer Salt, styrene-acrylic copolymer (manufactured by BASF Japan Ltd., product number: Joncryl-52J, Mw = 1,700), styrene-acrylic copolymer (manufactured by BASF Japan Ltd., product number: Joncryl-57J, Mw = 4) , 900), styrene-acrylic copolymer (manufactured by BASF Japan Ltd., product number: Joncryl-60J, Mw = 8,500), styrene-acrylic copolymer (manufactured by BASF Japan Ltd., product number: Joncryl-63J, Mw) = 12,500), styrene-acrylic copolymer (BASF Japan Ltd.) Manufactured by Co., Ltd., product number: Joncryl-70J, Mw = 16,500), styrene-acrylic copolymer (manufactured by BASF Japan, product number: Joncryl-JDX-6180, Mw = 14,000), styrene-acrylic copolymer (Manufactured by BASF Japan, product number: Joncryl-HPD-196, Mw = 9,200), styrene-acrylic copolymer (manufactured by BASF Japan, product number: Joncryl-HPD-96J, Mw = 16,500), Styrene-acrylic copolymer (manufactured by BASF Japan, product number: Joncryl-PDX-6137A, Mw = 16,500), styrene-acrylic copolymer (manufactured by BASF Japan, product number: Joncryl-6610, Mw = 8 , 500), acrylic copolymer (BASF Manufactured by Co., Ltd., product number: Joncry-JDX-6500, Mw = 10,000), acrylic copolymer (manufactured by BASF Japan, product number: Joncryl-PDX-6102B, Mw = 60,000), acrylic copolymer (BASF Japan Ltd., product number: Joncryl-PDX-6124, Mw = 60,000). The preferred mass average molecular weight of the water-soluble acrylic resin is 5000 or more and 500,000 or less, more preferably 6000 or more and 200,000 or less, and further preferably 7500 or more and 150,000 or less. The mass average molecular weight can be measured using gel permeation chromatography (GPC) with polystyrene having a known molecular weight as a standard substance.

  Among the water-soluble binders, those having a glass transition point of 60 ° C. or higher are more preferable. As a water-soluble binder having a glass transition point of 60 ° C. or more, for example, methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, polystyrene sulfonic acid, polyvinyl sulfonic acid, polyacrylic acid may vary depending on the degree of polymerization and the composition ratio of the copolymer. , Sodium polyacrylate, polystyrene styrene copolymer, methyl vinyl ether / maleic anhydride copolymer, polyacrylamide, polyvinyl alcohol, acrylic acid / maleic acid copolymer, acrylic acid / sulfonic acid monomer, hydroxyethyl cellulose, acrylamide Examples include diallyldimethylammonium chloride, isobutyl / maleic anhydride, chitosan, polyvinyl butyral, gelatin, and polyvinylpyrrolidone.

  Examples of the water-dispersed binder include acrylic resin, methacrylic resin, polyurethane, epoxy resin, styrene resin, and vinyl resin. These water-dispersed binders may be used alone, but may be used as a combination of two or more types or as a copolymer. Examples of water-dispersed (meth) acrylic resins include (meth) acrylic emulsions. “(Meth) acryl” means at least one selected from “acryl” and “methacryl”. In the present invention, the “(meth) acrylic emulsion” is a dispersion in which a (meth) acrylic resin that is sparingly soluble in water is dispersed in water as a dispersion medium with a surfactant or the like as a dispersoid, so-called resin. It is an emulsion. The term “water-soluble” refers to dissolution in water at the molecular level when mixed with water. A commercially available product can be suitably used as the (meth) acrylic emulsion. The average particle diameter of the (meth) acrylic resin dispersed in the (meth) acrylic emulsion is preferably 0.5 μm or less, and more preferably 0.2 μm or less. If the average particle diameter of the (meth) acrylic resin dispersed in the (meth) acrylic emulsion is too large, the protective layer may be too thick. Here, the average particle diameter is calculated by measuring the particle size distribution of the dispersoid using a dynamic light scattering method in a dispersion containing 0.01 mg to 100 g of the dispersoid in 1 L of the dispersion medium. As the (meth) acrylic emulsion, (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncry-711), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-7182), (Meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-7696), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-7511), (meth) acrylic emulsion (Product number manufactured by BASF Japan, product number: Joncryl-PDX-7326), (meth) acrylic emulsion (product manufactured by BASF Japan, product number: Joncryl-PDX-7370), (meth) acrylic emulsion (BASF Japan, product number: Joncry-PDX-7341), (meth) acrylic emulsion (BASF Japan, product number: Joncryl-PDX-7687), (meth) acrylic emulsion (BASF Japan Co., Ltd.) Manufactured, product number: Joncryl-74J), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-7639), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-) 7323), (meth) acrylic emulsion (manufactured by BASF Japan Ltd., product number: Joncryl-PDX-7777), (meth) acrylic emulsion (manufactured by BASF Japan Ltd., product number: Joncryl-7600) (Meth) acrylic emulsion (BASF Japan, product number: Joncry-775), (Meth) acrylic emulsion (BASF Japan, product number: Joncry-537J), (meth) acrylic emulsion (BASF Japan stock) Made by company, product number: Joncryl-PDX-7692), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-7611), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl) -PDX-7630A), (meth) acrylic emulsion (BASF Japan KK, part number: Joncry-352J), (meth) acrylic emulsion (BASF Japan KK, part number: Joncry) -352D), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-7145), (meth) acrylic emulsion (manufactured by BASF Japan, product number: Joncryl-PDX-7164), (meta ) Acrylic emulsion (BASF Japan, product number: Joncry-PDX-7430), (meth) acrylic emulsion (BASF Japan, product number: Joncryl-PDX-7440), (meth) acrylic emulsion (BASF) Japan Co., Ltd., product number: Joncry-8380), (meth) acrylic emulsion (BASF Japan Co., Ltd. product number: Joncry-8383).

  The aqueous binder is preferably polyvinyl alcohol, polyacrylic acid, acrylic resin or methacrylic resin.

  When an organic solvent binder is used as the protective layer binder, examples of the organic solvent binder include polyvinylidene fluoride (abbreviation PVDF), polytetrafluoroethylene, tetrafluoroethylene / hexafluoropropylene copolymer (abbreviation FEP), Examples thereof include fluorine-containing resins such as fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, acrylic resins such as poly (meth) acrylic acid, and alkoxysilyl group-containing resins.

  The protective layer may further contain a dispersant as necessary. As the dispersant, commercially available products can be used as appropriate.

  A preferable mass ratio of the ceramic powder and the binder for the protective layer in the protective layer is 86:14 to 99: 1, more preferably 87:13 to 98: 2, and still more preferably 88:12 to 97: 3. is there. If the protective layer binder content is too low in the protective layer, the protective layer will be reduced in binding strength to the active material layer, or the protective layer will collapse due to reduced binding force between the ceramic powders in the protective layer. There is a risk. In addition, the flexibility of the entire protective layer is lost, and the protective layer may be broken by the pressure applied to the electrode. When there is too much content of the binder for protective layers in a protective layer, there exists a possibility that the heat resistance of a protective layer may fall.

  Further, the amount of the ceramic powder in the protective layer is preferably such that pores remain between the ceramic powders. If the pores between the ceramic powders are reduced, the ion conductivity of the protective layer is lowered, and it may be difficult to secure the battery capacity.

  The thickness of the protective layer is preferably 1 μm or more and 12 μm or less, more preferably 2 μm or more and 9 μm or less, and particularly preferably 3 μm or more and less than 6 μm. If the thickness of the protective layer is too small, the effect of preventing a short circuit of the nonaqueous electrolyte secondary battery may not be exhibited. If the thickness of the protective layer is too large, the charge / discharge capacity of the nonaqueous electrolyte secondary battery may be reduced. Here, the thickness of the protective layer is determined by subtracting the thickness of the electrode before forming the protective layer from the thickness of the electrode having the protective layer. The thickness of each electrode is an average value of the results of measuring each electrode at 10 points at intervals of 5 mm.

  A method for disposing the protective layer on the active material layer is not particularly limited. For example, the protective layer can be disposed on the active material layer by the following method. A protective layer slurry can be prepared by dispersing a protective layer material in a solvent, applying the protective layer slurry onto the active material layer, and drying after application. . As a coating method, a conventionally known method such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method may be used.

  20 mass% or more and 70 mass% or less are preferable, and, as for the solid content concentration in the slurry for protective layers, 30 mass% or more and 60 mass% or less are more preferable. If solid content concentration exists in the said range, solid content will be easy to disperse | distribute in the slurry for protective layers.

  FIG. 1 is a schematic diagram illustrating the electrode for a nonaqueous electrolyte secondary battery according to this embodiment. In FIG. 1, an active material layer 5 is disposed on the current collector 1, and a protective layer 6 is disposed on the active material layer 5. The active material layer 5 includes a first active material 2, a second active material 3, and an active material layer binder 4. The protective layer 6 includes a ceramic powder 61 and a protective layer binder 62.

  In FIG. 1, in the active material layer 5, the second active material 3 having a small particle size is well disposed between the first active materials 2 having a large particle size, and the unevenness of the surface of the active material layer 5 is small.

  In the protective layer 6 of FIG. 1, a plurality of ceramic powders 61 are arranged along the unevenness of the surface of the active material layer 5, and the protective layer binder 62 includes the ceramic powders 61 and the ceramic powder 61 and the active material layer 5. It is bound.

  Since the unevenness of the surface of the active material layer 5 is small, the protective layer 6 covers the surface of the active material layer 5 so that there is no exposed portion of the active material layer 5 even if the thickness of the protective layer 6 is not extremely increased. be able to.

  The plurality of ceramic powders 61 or the ceramic powder 61 and the active material layer 5 are bound by the protective layer binder 62. In the protective layer 6, pores 7 are formed between the ceramic powders 61 and between the ceramic powder 61 and the protective layer binder 62.

(Non-aqueous electrolyte secondary battery)
The non-aqueous electrolyte secondary battery of the present invention includes the positive electrode for a non-aqueous electrolyte secondary battery.

  The non-aqueous electrolyte secondary battery of the present invention has a positive electrode, a negative electrode, and a non-aqueous electrolyte.

(Negative electrode)
The negative electrode has a current collector and a negative electrode active material layer.

  The negative electrode active material layer includes a negative electrode active material and a binder for the active material layer, and includes a conductive additive as necessary. As the current collector, the negative electrode active material, the binder for the active material layer, and the conductive auxiliary agent, the same ones as described above can be used.

(Positive electrode)
The positive electrode includes a current collector and a positive electrode active material layer.

  The positive electrode active material layer includes a positive electrode active material and a binder for the active material layer, and includes a conductive additive as necessary. As the current collector, the positive electrode active material, the binder for the active material layer, and the conductive auxiliary agent, the same ones as described above can be used.

(Nonaqueous electrolyte)
The nonaqueous electrolytic solution includes a solvent and an electrolyte dissolved in the solvent. An additive may be further added to the non-aqueous electrolyte.

  As the solvent, for example, cyclic esters, chain esters, and ethers can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of the chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers that can be used include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane.

Moreover, as an electrolyte dissolved in the non-aqueous electrolyte, for example, a lithium salt such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ can be used.

As the non-aqueous electrolyte, for example, a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 is} added in a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and dimethyl carbonate to 0.5 mol / l to 1.7 mol. A solution dissolved at a concentration of about 1 / l can be used.

  The nonaqueous electrolyte secondary battery of the present invention may further have a separator.

  The separator separates the positive electrode and the negative electrode, and allows ions to pass while preventing a short circuit of current due to contact between the two electrodes. As the separator, for example, a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramics can be used. The separator may be composed of a plurality of laminated porous membranes.

  The non-aqueous electrolyte secondary battery can be mounted on a vehicle. The vehicle may be a vehicle that uses electric energy from a battery as a whole or a part of a power source. For example, an electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, a hybrid railway vehicle, an electric forklift, an electric wheelchair, and an electric assist. Bicycles and electric motorcycles are examples.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described in more detail with reference to examples.

<Measurement of surface roughness of active material layer>
(Preparation of positive electrode)
As the positive electrode active material, LiNi _0.5 Co _0.2 Mn _0.3 O ₂ having an average particle diameter D ₅₀ of 6 μm and an average particle diameter D ₅₀ of 0.5 μm, 1.5 μm, 2 μm, 3 μm, 4 μm, 8 μm, 12 μm LiFePO ₄ was prepared. Acetylene black (hereinafter referred to as “AB”) was prepared as a conductive additive, and polyvinylidene fluoride (hereinafter referred to as “PVDF”) was prepared as a binder.

(Preparation of positive electrode A)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB having an average particle diameter D ₅₀ of 3 μm, and PVDF were respectively 67 parts by mass, 27 parts by mass, 3 parts by mass, and 3 parts by mass. The mixture was mixed at a ratio, and the mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (hereinafter referred to as NMP) to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 29 mass%.

An aluminum foil having a thickness of 20 μm was prepared as a current collector. The slurry was placed on the current collector, and applied using a doctor blade so that the slurry became a film. The obtained sheet was dried at 80 ° C. for 20 minutes to volatilize and remove NMP. Thereafter, the current collector and the coated material on the current collector were firmly and closely joined by a roll press. The density of the positive electrode active material layer was set to 2.9 g / cm ³ . The density of the positive electrode active material layer here is based on the formula of mass (g) of positive electrode active material layer / ((thickness of positive electrode active material layer (cm)) × (area of positive electrode active material layer (cm ² ))). Calculated. The joined product was heated with a vacuum dryer at 120 ° C. for 6 hours to obtain a current collector on which a positive electrode active material layer was formed. This was designated as positive electrode A. The thickness of the positive electrode active material layer of the positive electrode A was 93 μm.

(Preparation of positive electrode B)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , AB, and PVDF were mixed at a ratio of 94 parts by mass, 3 parts by mass, and 3 parts by mass, respectively, and this mixture was dispersed in an appropriate amount of NMP. A positive electrode B was prepared in the same manner as the positive electrode A except that the positive electrode active material layer slurry was prepared. The thickness of the positive electrode active material layer of the positive electrode B was 78 μm.

(Preparation of positive electrode C)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB with an average particle diameter D ₅₀ of 3 μm, and PVDF were respectively 70 parts by weight, 24 parts by weight, 3 parts by weight, and 3 parts by weight. A positive electrode C was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio and the mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 26 mass%. The thickness of the positive electrode active material layer of the positive electrode C was 91 μm.

(Preparation of positive electrode D)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB having an average particle diameter D ₅₀ of 3 μm, and PVDF were mixed in 84 parts by mass, 10 parts by mass, 3 parts by mass, and 3 parts by mass, respectively. A positive electrode D was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio and the mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 12 mass%. The thickness of the positive electrode active material layer of the positive electrode D was 84 μm.

(Preparation of positive electrode E)
And the _{AB LiNi 0.5 Co 0.2 Mn 0.3 O} 2 with an average particle diameter _{D 50} of 0.5 [mu] m LiFePO _4, and PVDF, respectively 67 parts by weight, 27 parts by weight, 3 parts by weight, 3 parts by mass A positive electrode E was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio of parts and the mixture was dispersed in an appropriate amount of NMP to prepare a slurry for the positive electrode active material layer. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 29 mass%. The thickness of the positive electrode active material layer of the positive electrode E was 93 μm.

(Preparation of positive electrode F)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB having an average particle diameter D ₅₀ of 0.5 μm, and PVDF were respectively 70 parts by mass, 24 parts by mass, 3 parts by mass, and 3 parts by mass. A positive electrode F was prepared in the same manner as the positive electrode A, except that the mixture was mixed in a proportion of parts, and this mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 26 mass%. The thickness of the positive electrode active material layer of the positive electrode F was 91 μm.

(Preparation of positive electrode G)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB having an average particle diameter D ₅₀ of 0.5 μm, and PVDF were respectively 77 parts by mass, 17 parts by mass, 3 parts by mass, 3 parts by mass. A positive electrode G was prepared in the same manner as the positive electrode A, except that the mixture was dispersed in a proportion of parts, and this mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the proportion of LiFePO ₄ was 18 mass%. The thickness of the positive electrode active material layer of the positive electrode G was 89 μm.

(Preparation of positive electrode H)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB having an average particle diameter D ₅₀ of 1.5 μm, and PVDF were respectively 70 parts by mass, 24 parts by mass, 3 parts by mass, and 3 parts by mass. A positive electrode H was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio of parts and the mixture was dispersed in an appropriate amount of NMP to prepare a slurry for the positive electrode active material layer. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 26 mass%. The thickness of the positive electrode active material layer of the positive electrode H was 91 μm.

(Preparation of positive electrode I)
And the _{AB LiNi 0.5 Co 0.2 Mn 0.3 O} 2 with an average particle diameter _{D 50} of 2 [mu] m LiFePO _4, and PVDF, 70 parts by weight, respectively, 24 parts by weight, 3 parts by weight, 3 parts by weight of A positive electrode I was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio and the mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 26 mass%. The thickness of the positive electrode active material layer of the positive electrode I was 91 μm.

(Preparation of positive electrode J)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB with an average particle diameter D ₅₀ of 4 μm, and PVDF were 70 parts by mass, 24 parts by mass, 3 parts by mass, and 3 parts by mass, respectively. A positive electrode J was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio and the mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 26 mass%. The thickness of the positive electrode active material layer of the positive electrode J was 91 μm.

(Preparation of positive electrode K)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB with an average particle diameter D ₅₀ of 8 μm, and PVDF were respectively 70 parts by weight, 24 parts by weight, 3 parts by weight, and 3 parts by weight. A positive electrode K was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio and the mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 26 mass%. The thickness of the positive electrode active material layer of the positive electrode K was 91 μm.

(Preparation of positive electrode L)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiFePO ₄ and AB with an average particle diameter D ₅₀ of 12 μm, and PVDF were 70 parts by mass, 24 parts by mass, 3 parts by mass, and 3 parts by mass, respectively. A positive electrode L was prepared in the same manner as the positive electrode A, except that the mixture was mixed at a ratio and the mixture was dispersed in an appropriate amount of NMP to prepare a positive electrode active material layer slurry. When the sum of the mass of LiNi _0.5 Co _0.2 Mn _0.3 O _{2 and} the mass of LiFePO ₄ was 100 mass%, the ratio of LiFePO ₄ was 26 mass%. The thickness of the positive electrode active material layer of the positive electrode L was 91 μm.

(Preparation of negative electrode A)
As the negative electrode active material, SiO having an average particle diameter D ₅₀ of 4 μm and natural graphite having an average particle diameter D ₅₀ of 20 μm were prepared. A polyamideimide resin (hereinafter referred to as PAI) was prepared as a binder resin. AB was prepared as a conductive aid.

  A mixture was prepared by mixing at a ratio of SiO / natural graphite / AB / PAI = 32/50/8/10 (mass ratio). This mixture was dispersed in an appropriate amount of NMP to prepare a slurry.

This slurry was applied to a copper foil having a thickness of 20 μm, which is a negative electrode current collector, in a film shape using a doctor blade. The current collector coated with the slurry was dried and pressed to obtain a bonded product. The density of the negative electrode active material layer was set to 1.6 g / cm ³ . The density of the negative electrode active material layer referred to here is based on the formula of mass (g) of negative electrode active material layer / ((thickness of negative electrode active material layer (cm)) × (area of negative electrode active material layer (cm ² ))). Calculated. The joined product was heated and dried with a vacuum dryer at 200 ° C. for 2 hours to obtain a copper foil on which a negative electrode active material layer was formed. This was designated as negative electrode A. The thickness of the negative electrode active material layer of the negative electrode A was 48 μm.

(Scanning electron microscope (hereinafter referred to as SEM) observation)
Cross sections of the positive electrode A, the positive electrode B, and the negative electrode A were observed with a SEM at a magnification of 1000 times, and images were acquired.

  The acquired image was subjected to image processing, and a cross-sectional curve of the active material layer on the surface opposite to the current collector-side surface was acquired.

  Since there was no significant difference in the longitudinal direction of the cross-sectional curve, an arbitrary location of 100 μm in the longitudinal direction was specified, and the maximum height in this region was obtained.

  A parallel line (peak line) that passes through the peak of the cross-sectional curve in the thickness direction of the specified location and is parallel to the surface of the current collector, and a bottom line of the cross-sectional curve in the thickness direction of the current collector. The distance between the two parallel lines (valley line) was measured, and the distance was taken as the maximum height.

  A graph comparing the cross-sectional curves of the positive electrode A, the positive electrode B, and the negative electrode A is shown in FIG. As shown in FIG. 2, the maximum height of the positive electrode A was 1.5 μm, the maximum height of the positive electrode B was 5.1 μm, and the maximum height of the negative electrode A was 7.6 μm.

  The positive electrode A uses two types of active materials having different particle diameters, and the positive electrode B uses only one type of active material. The maximum height of the positive electrode A was significantly smaller than the maximum height of the positive electrode B. From this, it was found that the surface roughness of the active material layer can be remarkably reduced by using two kinds of active materials having different particle diameters for the active material layer.

  In the negative electrode A, two active materials having different particle diameters were used, but the maximum height of the active material layer was large. From this, it was found that there are preferable combinations of active materials having different particle diameters in order to reduce the maximum height of the active material layer. It was estimated that the maximum height of the surface of the active material layer can be reduced by using a preferable combination of active materials in the negative electrode.

In addition, the maximum height of the positive electrode C to the positive electrode L was obtained by the same method. The maximum height of the positive electrode C is 1.8 μm, the maximum height of the positive electrode D is 2 μm, the maximum height of the positive electrode E is 0.7 μm, the maximum height of the positive electrode F is 0.7 μm, and the maximum height of the positive electrode G is 0 .9 μm, the maximum height of the positive electrode H is 1 μm, the maximum height of the positive electrode I is 1.5 μm, the maximum height of the positive electrode J is 3 μm, the maximum height of the positive electrode K is 4 μm, and the maximum height of the positive electrode L is 4. It was 5 μm.

<Preparation of slurry for protective layer>
An Al ₂ O ₃ powder (manufactured by Sumitomo Chemical Co., Ltd.) having an average particle diameter D ₅₀ = 540 nm is prepared as a ceramic powder for the protective layer, and polyvinyl alcohol (hereinafter referred to as PVA) as a protective layer binder. ) = 2200, glass transition point (Tg) 85 ° C.), polyacrylic acid (hereinafter referred to as PAA, glass transition point (Tg) 106 ° C.), two types of acrylic emulsions (BASF Japan Ltd.) Manufactured, product number: Joncryl-PDX7430, solid content acid value: 30 mg KOH / g, Tg: 34 ° C., average particle size: 0.12 μm, Mw: 200,000 or more, manufactured by BASF Japan Ltd., product number: Joncryl-537J, solid content Acid value: 40 mg KOH / g, Tg: 49 ° C., average particle size: 0.07 μm, Mw: 200,000 or more) and one kind Water-soluble acrylic resin (BASF Japan Ltd., product number: Joncryl-63J, acid value: 213mgKOH / g, Tg: 73 ℃, Mw: 12500) was prepared.

PVA was dissolved in water, Al ₂ O ₃ powder was added, and mixed for 30 minutes with a disper to obtain a slurry for the first protective layer. The mass ratio of water, PVA, and Al ₂ O ₃ was set to water / PVA / Al ₂ O ₃ = 70 / 3.3 / 26.7. The solid content concentration of the slurry for the first protective layer was 30% by mass. A second protective layer slurry was produced in the same manner as the first protective layer slurry except that PVA was changed to PAA. The solid content concentration of the second protective layer slurry was 30% by mass. Prepare a mixed solution of PVA and water-soluble acrylic resin (Joncry-63J), add Al ₂ O ₃ powder, mix for 1 hour with a disper, add an acrylic emulsion (Joncry-PDX7430), Was mixed for 1 hour to obtain a slurry for the third protective layer. The mass ratio of water, PVA, acrylic emulsion, water-soluble acrylic resin, and Al ₂ O ₃ is as follows: water / PVA / acrylic emulsion / water-soluble acrylic resin / Al ₂ O ₃ = 70 / 1.5. /1.5/0.3/26.7. The solid content concentration of the third protective layer slurry was 30% by mass. An aqueous PVA solution is prepared, Al ₂ O ₃ powder is added, mixed with a disper for 1 hour, an acrylic emulsion (Joncry-537J) is added, and further mixed with a disper for 1 hour, and a slurry for the fourth protective layer is prepared. Obtained. The mass ratio of water, PVA, acrylic emulsion, and Al ₂ O ₃ was set to water / PVA / acrylic emulsion / Al ₂ O ₃ = 70 / 1.65 / 1.65 / 26.7. The solid content concentration of the fourth protective layer slurry was 30% by mass. An aqueous solution of water-soluble acrylic resin (Joncry-63J) is prepared, Al ₂ O ₃ powder is added, mixed with a disper for 1 hour, then an acrylic emulsion (Joncry-PDX7430) is added, and further with a disper for 1 hour. By mixing, a slurry for the fifth protective layer was obtained. The mass ratio of water, acrylic emulsion, water-soluble acrylic resin and solid content of Al ₂ O ₃ is water / acrylic emulsion / water-soluble acrylic resin / Al ₂ O ₃ = 70 / 3.0 / 0.3. /26.7. The solid content concentration of the fifth protective layer slurry was 30% by mass.

(Positive electrode of Example 1)
The slurry for 1st protective layers was apply | coated to the positive electrode A using the applicator. The positive electrode A coated with the first protective layer slurry was dried by heating in a vacuum dryer at 120 ° C. for 6 hours to obtain a positive electrode of Example 1. The thickness of the protective layer of the positive electrode of Example 1 was 3 μm. The thickness of the protective layer was determined by subtracting the thickness of the positive electrode A from the thickness of the positive electrode in Example 1. The thickness of each positive electrode was the average value of the results of measuring each positive electrode at 10 points at intervals of 5 mm.

(Positive electrode of Example 2)
A positive electrode of Example 2 was produced in the same manner as the positive electrode of Example 1 except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the positive electrode of Example 2 was 6 μm.

(Positive electrode of Example 3)
A positive electrode of Example 3 was produced in the same manner as the positive electrode of Example 1 except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the positive electrode of Example 3 was 9 μm.

(Positive electrode of Example 4)
A positive electrode of Example 4 was produced in the same manner as the positive electrode of Example 1 except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the positive electrode of Example 4 was 12 μm.

(Positive electrode of Example 5) A positive electrode of Example 5 was produced in the same manner as the positive electrode of Example 1, except that the slurry for the third protective layer was applied to the positive electrode C. The thickness of the protective layer of the positive electrode of Example 5 was 3 μm.

(Positive electrode of Comparative Example 1)
The positive electrode A was used as the positive electrode of Comparative Example 1. The positive electrode of Comparative Example 1 is a positive electrode on which no protective layer is formed.

(Positive electrode of Comparative Example 2)
A positive electrode of Comparative Example 2 was produced in the same manner as the positive electrode of Example 1 except that the first protective layer slurry was thinly applied. The thickness of the protective layer of the positive electrode of Comparative Example 2 was 0.5 μm.

(Positive electrode of Comparative Example 3)
A positive electrode of Comparative Example 3 was produced in the same manner as the positive electrode of Comparative Example 2, except that the first protective layer slurry was applied to the positive electrode B. The thickness of the protective layer of the positive electrode of Comparative Example 3 was 0.5 μm.

(Positive electrode of Comparative Example 4)
A positive electrode of Comparative Example 4 was produced in the same manner as the positive electrode of Comparative Example 3, except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the positive electrode of Comparative Example 4 was 3 μm.

(Positive electrode of Comparative Example 5)
A positive electrode of Comparative Example 5 was produced in the same manner as the positive electrode of Comparative Example 3, except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the positive electrode of Comparative Example 5 was 6 μm.

(Positive electrode of Comparative Example 6)
A positive electrode of Comparative Example 6 was produced in the same manner as the positive electrode of Comparative Example 3, except that the first protective layer slurry was applied to a thickness of 9 μm. The thickness of the protective layer of the positive electrode of Comparative Example 6 was 9 μm.

(Positive electrode of Comparative Example 7)
A positive electrode of Comparative Example 7 was produced in the same manner as the positive electrode of Example 1 except that the first protective layer slurry was applied to the positive electrode K. The thickness of the protective layer of the positive electrode of Comparative Example 7 was 3 μm.

(Negative electrode of Test Example 1)
The slurry for 1st protective layers was apply | coated to the negative electrode A using the applicator. The negative electrode A to which the first protective layer slurry was applied was heat-dried at 200 ° C. for 2 hours to obtain a negative electrode of Test Example 1. The thickness of the protective layer of the negative electrode of Test Example 1 was 0.5 μm.

(Negative electrode of Test Example 2)
A negative electrode of Test Example 2 was produced in the same manner as the negative electrode of Test Example 1 except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the negative electrode of Test Example 2 was 3 μm.

(Negative electrode of Test Example 3)
A negative electrode of Test Example 3 was produced in the same manner as the negative electrode of Test Example 1 except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the negative electrode of Test Example 3 was 6 μm.

(Negative electrode of Test Example 4)
A negative electrode of Test Example 4 was produced in the same manner as the negative electrode of Test Example 1 except that the first protective layer slurry was applied thickly. The thickness of the protective layer of the negative electrode of Test Example 4 was 9 μm.

<Electrical insulation evaluation 1>
The positive electrode of Examples 1-3 mentioned above, the positive electrode of Comparative Examples 2-6, and the negative electrode of Test Examples 1-4 were extracted with the circular punch of (phi) 16mm size, and it was set as the evaluation pole.

  Each evaluation electrode was sandwiched between the upper and lower pairs of measurement electrodes, and the resistance value in the cross-sectional direction when a 5 kg load was evenly applied between the upper and lower pairs of measurement electrodes was measured as the resistance value of the 1000 kHz AC resistance. This measured value indicates a resistance component against current flowing through the protective layer, and the higher the resistance value, the higher the insulation. The results are shown in Table 1.

  As shown in Table 1, in the positive electrode A using active material layers containing active materials having different particle sizes, the maximum height of the active material layer could be easily reduced. If the resistance value in Table 1 exceeds 3 kΩ, it can be determined that the insulation is high. This high insulating property is considered to be an index indicating that the protective layer is formed so that there is no exposed portion of the active material layer on the surface of the active material layer. It was found that the positive electrode A having a small maximum height can ensure high insulation even when the protective layer is thin, as compared with the positive electrode B and the negative electrode A having a large maximum height. Further, from the results of the positive electrode B and the negative electrode A, it was found that it is difficult to form a protective layer so that there is no exposed portion of the active material layer unless a protective layer having a thickness larger than the maximum height of the cross section of the active material layer is formed. .

<Electrical insulation evaluation 2>
For each of the positive electrodes A to L described above, protective layers having different thicknesses were formed and an electrical insulation test was performed. The protective layer is one using PVA as a protective layer binder, one using PAA, one using PVA, an acrylic emulsion (J-PDX7430), and a water-soluble acrylic resin (J-63J). Created. The method for producing the protective layer is the same as the method for producing the positive electrode of Example 1, and the slurry for the protective layer used in the production of the protective layer is appropriately changed from the slurry for the first protective layer to the slurry for the second protective layer, the third protective layer. The slurry was changed. The electrical insulation test method was the same as the electrical insulation evaluation 1 described above. The results are shown in Table 2.

From Table 2, when comparing the positive electrode A to L, as the average particle diameter _{D 50} of the LiFePO ₄ is increased, it was found that there is a tendency that the electrode surface maximum height increases. When the positive electrode A, compared C～L, who average particle diameter _{D 50} of the LiFePO ₄ is smaller than the average particle diameter _{D 50} of the _LiNi 0.5 _Co 0.2 _Mn 0.3 _{O 2} is the electrode surface maximum height Was found to be small. Further, in the positive electrode C, it was found that the electrode resistances of those using PVA as a protective layer binder, those using PAA, and those using PVA + acrylic resin were the same. Positive electrode A that proportion of LiFePO ₄ are different, a positive electrode C, and comparing the positive electrode _D, when the sum of the LiNi _0.5 Co _{0.2 Mn} 0.3 _{O 2} and LiFePO ₄ is 100 mass%, LiFePO ₄ is 12 It has been found that the electrode surface maximum height decreases as the mass% of LiFePO ₄ increases in the mass% to 29 mass%.

<Measurement of resistance of lithium ion secondary battery>
(Production of laminated lithium ion secondary battery)
(Laminated lithium ion secondary battery of Example 1)
The positive electrode of Example 1 was cut into a predetermined shape (rectangular shape with a positive electrode active material layer area of 25 mm × 30 mm) and used as a positive electrode, and the negative electrode A was cut into a predetermined shape (rectangular shape with a negative electrode active material layer area of 26 mm × 31 mm). The laminate type lithium ion secondary battery of Example 1 used as was manufactured as follows.

A rectangular sheet (27 × 32 mm, thickness 25 μm) made of polypropylene resin as a separator was sandwiched between the positive electrode and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed, and then an electrolyte solution was injected into the bag-like laminated film. As electrolytes, fluoroethylene carbonate (hereinafter referred to as FEC), ethylene carbonate (hereinafter referred to as EC), ethyl methyl carbonate (hereinafter referred to as EMC), and dimethyl carbonate (hereinafter referred to as DMC). ) In a solvent mixed with FEC: EC: EMC: DMC = 4: 26: 30: 40 (volume ratio) was used so that LiPF ₆ was dissolved at 1 mol / l. Thereafter, the remaining one side was sealed to obtain a laminate type lithium ion secondary battery in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed. Note that the positive electrode and the negative electrode have a tab that can be electrically connected to the outside, and a part of the tab extends to the outside of the laminated lithium ion secondary battery. The laminated lithium ion secondary battery of Example 1 was produced through the above steps.

(Laminated lithium ion secondary battery of Example 2)
A laminated lithium ion secondary battery of Example 2 was produced in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Example 2 was used as the positive electrode.

(Laminated lithium ion secondary battery of Example 3)
A laminated lithium ion secondary battery of Example 3 was produced in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Example 3 was used as the positive electrode.

(Laminated lithium ion secondary battery of Example 4)
A laminated lithium ion secondary battery of Example 4 was produced in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Example 4 was used as the positive electrode.

(Laminated lithium ion secondary battery of Example 5)
A laminated lithium ion secondary battery of Example 5 was produced in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Example 5 was used as the positive electrode.

(Laminated lithium ion secondary battery of Comparative Example 1)
A laminated lithium ion secondary battery of Comparative Example 1 was produced in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Comparative Example 1 was used as the positive electrode.

(Laminated lithium ion secondary battery of Comparative Example 2)
A laminated lithium ion secondary battery of Comparative Example 2 was produced in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Comparative Example 2 was used as the positive electrode.

(Laminated lithium ion secondary battery of Comparative Example 3)
A laminated lithium ion secondary battery of Comparative Example 3 was prepared in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Comparative Example 1 was used as the positive electrode and the negative electrode of Test Example 2 was used as the negative electrode. Produced.

(Laminated lithium ion secondary battery of Comparative Example 4)
A laminated lithium ion secondary battery of Comparative Example 4 was produced in the same manner as the laminated lithium ion secondary battery of Example 1 except that the positive electrode of Comparative Example 4 was used as the positive electrode.

(Laminated lithium ion secondary battery of Comparative Example 5)
A laminated lithium ion secondary battery of Comparative Example 5 was produced in the same manner as the laminated lithium ion secondary battery of Example 1, except that the positive electrode of Comparative Example 7 was used as the positive electrode.

<Cell resistance evaluation>
The cell resistances of the laminated lithium ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 5 were measured. The cell resistance (Ω) was measured at a voltage of 3.6 V and a 3 C rate of 60 mA for 10 seconds.

  A smaller cell resistance value indicates a lower electrode resistance. The low resistance of the electrode indicates that the output inherent to the electrode can be exhibited even with the protective layer. Further, since the cell resistance is measured at a 3C rate, the measured value of the cell resistance is also an index indicating a high rate characteristic. In each example and each comparative example, three batteries each having the same configuration were prepared, the resistance of each battery was measured, and the average value was calculated.

  Table 3 shows the average cell resistance results of the laminated lithium ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 5.

<Electrical insulation evaluation 3>
The positive electrodes of Examples 1 to 5, the positive electrodes of Comparative Examples 1 to 2, 4, and 7, and the negative electrode of Test Example 2 were cut into a predetermined shape (circular shape of φ16 mm size) and used as evaluation electrodes.

  Each evaluation electrode was sandwiched between the upper and lower pairs of measurement electrodes, and the resistance value in the cross-sectional direction when a 5 kg load was evenly applied between the upper and lower pairs of measurement electrodes was measured as the resistance value of the 1000 kHz AC resistance. The resistance value in the cross-sectional direction indicates the resistance against the current flowing through the protective layer, and the higher the resistance value, the higher the insulation. The case where the resistance value was more than 3 kΩ was evaluated as ◯, and the case where the resistance value was 3 kΩ or less was evaluated as x. The evaluation results of resistance values are shown in Table 3 as ◯ × as electrical insulation evaluation. In addition, the electrical insulation test result of the negative electrode of Test Example 2 is described in the column of the laminate type lithium ion secondary battery of Comparative Example 3. The results of the electrical insulation test of the positive electrode of Comparative Example 7 are listed in the column of the laminate type lithium ion secondary battery of Comparative Example 5.

  From the results of Table 3, it was found that the positive electrodes of Examples 1 to 5 were more excellent in electrical insulation than the positive electrodes of Comparative Examples 1, 2, 4, and 7 and the negative electrode of Test Example 2. Moreover, the value of the cell resistance of the laminate type lithium ion secondary batteries of Examples 1 to 5 is almost the same as the cell resistance of the laminate type lithium ion secondary battery of Comparative Example 1 having a positive electrode on which no protective layer is disposed. In the laminated lithium ion secondary batteries of Examples 1 to 5, no significant increase in cell resistance due to the protective layer was confirmed.

In the laminate type lithium ion secondary battery of Comparative Example 2, it cannot be said that the protective layer is disposed on the active material layer so that there is no exposed portion of the active material layer in the positive electrode of Comparative Example 2. In the laminated lithium ion secondary battery, the cell resistance was not so high, but insulation could not be secured. In the laminate type lithium ion secondary battery of Comparative Example 4, the positive electrode of Comparative Example 4 did not contain the second active material. Since the maximum height of the surface of the active material layer in the positive electrode of Comparative Example 4 is as high as 5.1 μm and the thickness of the protective layer is 3 μm, the active material is formed so that there is no exposed portion of the active material layer. It could not be said that they were arranged in layers. Therefore, although the cell resistance was not so high in the laminated lithium ion secondary battery of Comparative Example 4, insulation could not be ensured. In laminated lithium ion secondary battery of Comparative Example 5 had an average particle diameter D ₅₀ of the first active material is smaller than the average particle diameter D ₅₀ of the second active material in the positive electrode of Comparative Example 7. In the positive electrode of Comparative Example 7, it cannot be said that the protective layer is arranged on the active material layer so that there is no exposed portion of the active material layer, and the cell resistance is too high in the laminated lithium ion secondary battery of Comparative Example 5. Although there was no insulation, insulation could not be secured.

  1: current collector, 2: first active material, 3: second active material, 4: active material layer binder, 5: active material layer, 6: protective layer, 61: ceramic powder, 62: protective layer binder , 7: pores.

Claims (9)

The active material layer comprising: a current collector; an active material layer disposed on the surface of the current collector; and a protective layer disposed such that there is no exposed portion of the active material layer on the surface of the active material layer The maximum height of the surface is 5 μm or less,
In the scanning electron microscope image of the cross section of the active material layer, the maximum height passes through the peak in the thickness direction of the curve on the surface opposite to the current collector side surface of the cross section of the active material layer. A distance between a parallel line (peak line) parallel to the surface of the body and a parallel line (valley line) passing through the valley point in the thickness direction of the curve and parallel to the surface of the current collector ,
The active material layer includes a first active material and a second active material, the average particle diameter D ₅₀ of the first Ichikatsu material is greater than the average particle diameter D ₅₀ of the second active material,
Wherein the Ichikatsu material has the general formula: at least one _{Li a Co p Ni q Mn r} D s O x (D is the Al, Mg, Ti, Sn, Zn, W, Zr, Mo, is selected from Fe and Na P + q + r + s = 1, 0 <p <1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0.2) is a lithium cobalt-containing composite metal oxide having a layered structure represented by
The second active material is an olivine-type lithium phosphate composite oxide represented by the general formula: LiMPO ₄ (M is at least one of Mn, Fe, Co, and Ni),
The protective layer includes a ceramic powder, and a positive electrode for a non-aqueous electrolyte secondary battery. Wherein the Ichikatsu average particle diameter D ₅₀ is the second active mean particle size for a non-aqueous electrolyte secondary battery positive electrode according to claim 1 D or less 15 times 1.5 times the ₅₀ substances substances. The average particle diameter D ₅₀ is non-aqueous electrolyte secondary battery positive electrode according to claim 1 or 2 less than 1 × 1/20 or more the average particle diameter D ₅₀ of the second active material of the ceramic powder.   The second active material is 10 mass% or more and 40 mass% or less when the total of the first active material and the second active material is 100 mass%. Positive electrode for non-aqueous electrolyte secondary battery. The average particle diameter D ₅₀ of the first active material is 3 μm or more and 40 μm or less, the average particle diameter D ₅₀ of the second active material is 0.5 μm or more and 10 μm or less, and the average particle diameter D ₅₀ of the ceramic powder is It is 0.1 micrometer or more and 2 micrometers or less, The positive electrode for nonaqueous electrolyte secondary batteries as described in any one of Claims 1-4.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the maximum height of the surface of the active material layer is 3 μm or less.   The said protective layer is a positive electrode for nonaqueous electrolyte secondary batteries as described in any one of Claims 1-6 containing an aqueous binder.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 7, wherein the aqueous binder is polyvinyl alcohol, polyacrylic acid, an acrylic resin, or a methacrylic resin.   It has a positive electrode for nonaqueous electrolyte secondary batteries as described in any one of Claims 1-8, The nonaqueous electrolyte secondary battery characterized by the above-mentioned.

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