Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

A non-aqueous electrolyte secondary battery positive electrode (11) that is characterized by comprising a positive electrode collector (40) and a positive electrode mixture layer (42) that is provided on the positive electrode collector (40) and includes a positive electrode active material and a conductive material that includes carbon nanotubes. The non-aqueous electrolyte secondary battery positive electrode (11) is also characterized in that, when the positive electrode mixture layer (42) is bisected in the thickness direction, the mass ratio of carbon nanotubes to positive electrode active material in an upper-half region (42b) that is on a top surface side is smaller than the mass ratio of carbon nanotubes to positive electrode active material in a lower-half region (42a) that is on the positive electrode collector (40) side.

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries have been widely used as high-power, high-energy density secondary batteries. These batteries include a positive electrode, a negative electrode, and a non-aqueous electrolyte, and are charged and discharged by transferring lithium ions and the like between the positive electrode and the negative electrode.

[0003] For example, Patent Documents 1 to 3 disclose non-aqueous electrolyte secondary batteries using a positive electrode having a positive electrode mixture layer containing a positive electrode active material and carbon nanotubes as a conductive material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2012 / 114590 [Patent Document 2] Japanese Patent Application Publication No. 2019-061734 [Patent Document 3] Special Publication No. 2018-501602 Summary of the Invention [Problem to be solved by the invention]

[0005] By adding carbon nanotubes to the positive electrode composite layer, the electronic conductivity of the positive electrode composite layer can be increased, thereby improving the capacity and output of the battery, but the heat generation of the battery in the event of an internal short circuit increases. One possible way to suppress the heat generation of the battery in the event of an internal short circuit is to reduce the amount of carbon nanotubes added, but doing so would reduce the capacity and output of the battery.

[0006] Therefore, an object of the present disclosure is to provide a positive electrode for a nonaqueous electrolyte secondary battery that can suppress heat generation in the battery during an internal short circuit while suppressing a decrease in the capacity and output of the battery, and a nonaqueous electrolyte secondary battery including the positive electrode for the nonaqueous electrolyte secondary battery. [Means for solving the problem]

[0007] A positive electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode current collector and a positive electrode composite layer provided on the positive electrode current collector and containing a positive electrode active material and a conductive material, wherein the conductive material contains carbon nanotubes, and when the positive electrode composite layer is divided into two equal parts in a thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in an upper half region on the surface side is smaller than the mass ratio of the carbon nanotubes to the positive electrode active material contained in a lower half region on the positive electrode current collector side.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the positive electrode is the above-described positive electrode for a non-aqueous electrolyte secondary battery. [Effects of the Invention]

[0009] According to one aspect of the present disclosure, it is possible to suppress heat generation in a battery during an internal short circuit while suppressing a decrease in the capacity and output of the battery. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; [Figure 2] FIG. 2 is a cross-sectional view of a positive electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] An example of an embodiment will be described with reference to the drawings. Note that the nonaqueous electrolyte secondary battery of the present disclosure is not limited to the embodiment described below. Furthermore, the drawings referred to in the description of the embodiment are schematic.

[0012] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. The nonaqueous electrolyte secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing member 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other types of electrode bodies may be used, such as a laminated electrode body formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include cylindrical, prismatic, coin-shaped, or button-shaped metal outer cans, and pouch outer cans formed by laminating a resin sheet and a metal sheet.

[0013] Case body 16 is, for example, a cylindrical metal outer can with a bottom. A gasket 28 is provided between case body 16 and sealing body 17 to ensure airtightness inside the battery. Case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports sealing body 17. Protruding portion 22 is preferably formed in an annular shape along the circumferential direction of case body 16, and supports sealing body 17 on its upper surface.

[0014] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to one another at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the nonaqueous electrolyte secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0015] 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in an insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside an insulating plate 19 and extends toward the bottom of the case body 16. The positive electrode lead 20 is connected to the underside of a filter 23, which is the bottom plate of the sealing body 17, by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

[0016] Each component of the nonaqueous electrolyte secondary battery 10 will be described in detail below.

[0017] [Positive electrode] FIG. 2 is a cross-sectional view of a positive electrode according to one embodiment. The positive electrode 11 includes a positive electrode current collector 40 and a positive electrode composite layer 42 disposed on the positive electrode current collector 40. The positive electrode current collector 40 may be a foil of a metal, such as aluminum, that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface. The positive electrode composite layer 42 includes a positive electrode active material and a conductive material. The conductive material includes carbon nanotubes. The positive electrode composite layer 42 preferably further includes a binder or the like.

[0018] Positive electrode 11 is produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. onto positive electrode current collector 40, drying the slurry to form positive electrode composite layer 42, and then rolling positive electrode composite layer 42 with a rolling roller or the like. The method for producing positive electrode composite layer 42 will be described in detail below.

[0019] In this embodiment, when the positive electrode mixture layer 42 shown in FIG. 2 is divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in an upper half region 42b on the surface side is smaller than the mass ratio of the carbon nanotubes to the positive electrode active material contained in a lower half region 42a on the positive electrode current collector 40 side. Here, dividing the positive electrode mixture layer 42 into two equal parts in the thickness direction means that when the stacking direction of the positive electrode current collector 40 and the positive electrode mixture layer 42 is the thickness direction of the positive electrode mixture layer 42, the positive electrode mixture layer 42 is divided into two equal parts at a midpoint Z of the thickness of the positive electrode mixture layer 42. Even when the positive electrode mixture layer 42 is formed on both sides of the positive electrode current collector 40, the region on the positive electrode current collector 40 side of the two regions obtained by dividing the positive electrode mixture layer 42 into two equal parts in the thickness direction is referred to as the lower half region 42a, and the region on the surface side of the positive electrode mixture layer 42 that is positioned away from the positive electrode current collector 40 is referred to as the upper half region 42b.

[0020] When carbon nanotubes are uniformly distributed within the positive electrode mixture layer, as in the past, the electronic conductivity of the positive electrode mixture layer is increased, improving the battery's capacity and output. However, this also increases the short-circuit current between the positive electrode and the negative electrode in the event of an internal short circuit, resulting in increased battery heat generation. However, in this embodiment, the mass ratio of carbon nanotubes to the positive electrode active material in the upper half region 42b on the surface side is low, thereby reducing the electronic conductivity of the positive electrode surface and suppressing the short-circuit current between the positive electrode and the negative electrode in the event of an internal short circuit. This reduces battery heat generation in the event of an internal short circuit compared to when carbon nanotubes are uniformly distributed within the positive electrode mixture layer 42. Furthermore, in this embodiment, the mass ratio of carbon nanotubes to the positive electrode active material in the lower half region 42a on the positive electrode current collector 40 side is high, reducing the resistance between the positive electrode current collector 40 and the positive electrode mixture layer 42. This reduces the decrease in battery capacity and output compared to simply reducing the amount of carbon nanotubes added to ensure battery safety.

[0021] The mass ratio of carbon nanotubes to the positive electrode active material contained in the upper half region 42b on the surface side is preferably 0.05 mass% or less, more preferably 0.03 mass% or less, and even more preferably 0 mass%, in order to suppress heat generation in the battery during an internal short circuit.

[0022] The mass ratio of carbon nanotubes to the positive electrode active material contained in the lower half region 42a on the positive electrode current collector 40 side is preferably 0.01 mass% or more and 1 mass% or less, and more preferably 0.04 mass% or more and 0.08 mass% or less, in order to suppress a decrease in battery capacity and output.

[0023] The mass ratio of carbon nanotubes to the positive electrode active material contained in the positive electrode composite layer 42 is preferably 0.005 mass% or more and 0.5 mass% or less, and more preferably 0.02 mass% or more and 0.04 mass% or less, in order to suppress a decrease in battery capacity and output.

[0024] Carbon nanotubes include single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Single-walled carbon nanotubes (SWCNTs) are carbon nanostructures in which one layer of graphene sheets forms a cylindrical shape. Double-walled carbon nanotubes are carbon nanostructures in which two layers of graphene sheets are concentrically stacked to form a cylindrical shape. Multi-walled carbon nanotubes are carbon nanostructures in which three or more layers of graphene sheets are concentrically stacked to form a cylindrical shape. A graphene sheet refers to a layer in which carbon atoms with sp2 hybrid orbitals that form graphite crystals are located at the vertices of a regular hexagon. The shape of carbon nanotubes is not limited. Examples of such shapes include needles, cylindrical tubes, fishbone-shaped (fishbone or cup-stacked), playing card-shaped (platelet), and coil-shaped.

[0025] The fiber length of the carbon nanotubes is, for example, preferably 500 nm or more and 200 μm or less, and more preferably 1 μm or more and 100 μm or less. The fiber length of the carbon nanotubes can be determined by measuring the lengths of 50 random carbon nanotubes using a field emission scanning electron microscope (FE-SEM) and taking the arithmetic average.

[0026] The outermost diameter of the carbon nanotubes (i.e., fiber diameter) is, for example, preferably 0.5 nm or more and 20 nm or less, and more preferably 1 nm or more and 10 nm or less. The outermost diameter of the carbon nanotubes can be determined by measuring the outer diameters of 50 arbitrary carbon nanotubes using a field emission scanning electron microscope (FE-SEM) or a transmission electron microscope (TEM) and calculating the arithmetic average.

[0027] The conductive material may contain a particulate conductive material in addition to carbon nanotubes. Examples of the particulate conductive material include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. When a particulate conductive material is used, it is preferable that the primary particle diameter is 5 nm or more and 100 nm or less, and that the aspect ratio is less than 10.

[0028] The mass ratio of the particulate conductive material to the positive electrode active material contained in the upper half region 42b on the surface side is preferably 0.05 mass% or more and 1.2 mass% or less in order to suppress a decrease in battery capacity and output, and the mass ratio of the particulate conductive material to the positive electrode active material contained in the lower half region 42a on the positive electrode current collector 40 side is preferably 0.5 mass% or less, and more preferably 0.1 mass% or less.

[0029] The positive electrode active material may be a lithium metal composite oxide containing a transition metal element such as Co, Mn, or Ni. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x Mn2O4, Li x Mn 2-y M yExamples include O4, LiMPO4, Li2MPO4F (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), etc. The positive electrode active material may be used alone or in combination of multiple types.

[0030] Also, in terms of achieving high capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material is Li a Ni x M y O 2-b (where M is at least one element selected from Al, Co, Mn, Fe, Ti, Si, Nb, Mo, W, and Zn, 0 ≤ a < 1.05, 0.7 < x ≤ 0.95, 0 ≤ y ≤ 0.3, 0 ≤ b < 0.05, x + y = 1, and a varies during charge and discharge) preferably contains a lithium nickel composite oxide. By using a positive electrode active material with a high nickel ratio like the above lithium nickel composite oxide, high capacity of the battery can be achieved. However, on the other hand, the heat generation of the battery during internal short circuit tends to increase. However, as in this embodiment, by reducing the content of carbon nanotubes on the surface side of the positive electrode 11, even when using a positive electrode active material with a high nickel ratio, the heat generation of the battery during internal short circuit can be effectively suppressed.

[0031] Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more types.

[0032] An example of a method for producing the positive electrode composite layer 42 will be described. For example, a positive electrode active material, a binder, a conductive material including carbon nanotubes, and the like are mixed together with a solvent to prepare a positive electrode composite slurry for the lower half region 42a. Separately from this slurry, a positive electrode active material, a binder, a conductive material that does not contain carbon nanotubes or that contains a small amount of carbon nanotubes, and the like are mixed together with a solvent to prepare a positive electrode composite slurry for the upper half region 42b. The positive electrode composite slurry for the lower half region 42a is then applied to both sides of the positive electrode current collector 40 and dried. Then, the positive electrode composite slurry for the upper half region 42b is applied to the coating of the positive electrode composite slurry for the lower half region 42a and dried, thereby forming the positive electrode composite layer 42. In the above method, the positive electrode composite slurry for the lower half region 42a is applied and dried, and then the positive electrode composite slurry for the upper half region 42b is applied. However, a method in which the positive electrode composite slurry for the upper half region 42b is applied after the positive electrode composite slurry for the lower half region 42a is applied and before drying may also be used, or the positive electrode composite slurry for the lower half region 42a and the positive electrode composite slurry for the upper half region 42b may be applied simultaneously.

[0033] [Negative electrode] The negative electrode 12 includes a negative electrode current collector and a negative electrode composite layer provided on the negative electrode current collector. The negative electrode current collector may be, for example, a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film having such a metal disposed on its surface.

[0034] The negative electrode mixture layer preferably contains a negative electrode active material, and further contains a binder, a conductive material, etc. Negative electrode 12 can be produced, for example, by preparing a negative electrode mixture slurry containing the negative electrode active material, the binder, etc., applying this negative electrode mixture slurry onto a negative electrode current collector, drying it to form a negative electrode mixture layer, and rolling this negative electrode mixture layer.

[0035] The negative electrode active material can reversibly absorb and release lithium ions, and examples thereof include carbon materials such as natural graphite and artificial graphite, metals that can be alloyed with lithium such as silicon (Si) and tin (Sn), and alloys and composite oxides containing metal elements such as Si and Sn.

[0036] Examples of binders include fluorine-based resins, PAN, polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or salts thereof, polyacrylic acid (PAA) or salts thereof (PAA-Na, PAA-K, etc., or partially neutralized salts), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0037] Examples of conductive materials include carbon-based materials such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), graphite, etc. These may be used alone or in combination of two or more.

[0038] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator whose surface is coated with a material such as an aramid-based resin or ceramic may be used.

[0039] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0040] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0041] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0042] As the halogen-substituted compound, it is preferable to use a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, a fluorinated chain carboxylate such as methyl fluoropropionate (FMP), or the like.

[0043] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 1 or more}, and the like. The lithium salt may be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of the solvent.

Examples

[0044] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0045] (Example 1) Nickel sulfate, cobalt sulfate, and aluminum sulfate were mixed in an aqueous solution so that the molar ratio was 87:9:4, and co-precipitated to obtain the precursor substance (Ni, Co, Al)(OH)2. Then, this precursor substance and lithium hydroxide monohydrate (LiOH·H2O) were mixed so that the molar ratio was 1:1.03. This mixed powder was fired in an electric furnace under an oxygen atmosphere at 750 °C for 12 hours to obtain a positive electrode active material.

[0046] The positive electrode active material, carbon nanotubes, and polyvinylidene fluoride were mixed so that the mass ratio of the carbon nanotubes to the positive electrode active material was 0.05 mass%, and an appropriate amount of N-methylpyrrolidone (NMP) was added to the mixture to prepare a slurry. This was designated as positive electrode composite slurry A.

[0047] The positive electrode active material, carbon nanotubes, and polyvinylidene fluoride were mixed so that the mass ratio of carbon black to the positive electrode active material was 1.0 mass%, and an appropriate amount of N-methylpyrrolidone (NMP) was added to the mixture to prepare a slurry, which was designated as positive electrode composite slurry B.

[0048] The positive electrode composite slurry A was applied to both sides of a 15 μm-thick aluminum foil and dried to form a first coating, and then the positive electrode composite slurry B was applied to the first coating and dried to form a second coating. The coating was then rolled using a rolling roller to produce a positive electrode in which a positive electrode composite layer was formed on both sides of the positive electrode current collector. The thickness ratio of the first coating to the second coating was 75:25.

[0049] Calculations based on the slurry composition, thickness ratio, etc. revealed that when the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side was 0.05 mass%, and the mass ratio of the carbon black was 0 mass%. Furthermore, when the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side was 0.025 mass%, and the mass ratio of the carbon black was 0.5 mass%.

[0050] [Preparation of negative electrode] Graphite, sodium carboxymethylcellulose, and a styrene-butadiene copolymer were mixed in a mass ratio of 98:1:1, and the mixture was kneaded with water to prepare a negative electrode composite slurry. This negative electrode composite slurry was applied to both sides of an 8 μm-thick copper foil, the coating was dried, and then rolled with a rolling roller to produce a negative electrode in which a negative electrode composite layer was formed on both sides of the negative electrode current collector.

[0051] [Preparation of non-aqueous electrolyte] LiPF6 was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (volume ratio of EC:MEC = 1:3), which was used as a non-aqueous electrolyte.

[0052] [Secondary battery production] (1) A 20 μm thick separator (a composite film of polyethylene and polypropylene) was placed between the positive and negative electrodes and wound around them to prepare a wound electrode assembly. Leads were attached to both the positive and negative electrodes. (2) The electrode body was inserted into the case body, the negative electrode lead was welded to the bottom of the case body, and the positive electrode lead was welded to the sealing body. (3) After the non-aqueous electrolyte was poured into the case body, the open end of the case body was crimped to a sealing member via a gasket. Non-aqueous electrolysis was performed to produce a secondary battery.

[0053] <Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the thickness ratio of the first coating film to the second coating film was 95:5. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side was 0.05 mass%, and the mass ratio of the carbon black was 0 mass%. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side was 0.045 mass%, and the mass ratio of the carbon black was 0.1 mass%.

[0054] Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the thickness ratio of the first coating film to the second coating film was 50:50. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side was 0.05 mass%, and the mass ratio of the carbon black was 0 mass%. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side was 0 mass%, and the mass ratio of the carbon black was 1 mass%.

[0055] Example 4 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the thickness ratio of the first coating film to the second coating film was 25:75. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side was 0.025 mass%, and the mass ratio of the carbon black was 0.5 mass%. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side was 0 mass%, and the mass ratio of the carbon black was 1 mass%.

[0056] <Comparative Example 1> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the thickness ratio of the first coating film to the second coating film was 100:0. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side was 0.05 mass%, and the mass ratio of the carbon black was 0 mass%. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side was 0.05 mass%, and the mass ratio of the carbon black was 0 mass%.

[0057] <Comparative Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the thickness ratio of the first coating film to the second coating film was 0:100. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side was 0 mass%, and the mass ratio of the carbon black was 1 mass%. When the positive electrode mixture layer was divided into two equal parts in the thickness direction, the mass ratio of the carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side was 0 mass%, and the mass ratio of the carbon black was 1 mass%.

[0058] [Battery capacity measurement] The nonaqueous electrolyte secondary batteries of each Example and Comparative Example were subjected to constant current charging at a maximum current value of 0.3 It in a temperature environment of 25°C until the voltage reached 4.2 V, and then constant voltage charging at 4.2 V until the current value dropped to 0.05 It. The discharge cut-off voltage was then set to 2.5 V, and constant current discharging was performed at a current of 0.2 It. The discharge capacity at this time was measured. Table 1 shows the discharge capacities of the other Examples and Comparative Examples as relative values, with the discharge capacity of Comparative Example 1 set as the reference (100).

[0059] [Output Measurement] The nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.3 It at a temperature of 25°C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current dropped to 0.05 It. The batteries were then discharged at 1.0 It for 30 seconds, and the resulting power (output) was measured. Table 1 shows the outputs of the other Examples and Comparative Examples as relative values, with the output of Comparative Example 1 set as the reference (100).

[0060] [Measurement of battery temperature during internal short circuit] The nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.3 It at a temperature of 25°C until the voltage reached 4.2 V, and then at a constant voltage of 4.2 V until the current reached 0.05 It. Each battery was then placed on a concrete block, and a 15.8 mm diameter rod was placed on top of the battery so as to cross the axial center of the battery. A 9.1 kg weight was dropped onto the rod from a height of 61 cm directly above the battery, causing an internal short circuit. The maximum temperature reached on the battery surface at this time was measured. Table 1 shows the maximum temperatures reached in the other Examples and Comparative Examples as relative values, with the maximum temperature reached in Comparative Example 1 set as the reference (100).

[0061] [Table 1]

[0062] In Comparative Example 2, which did not contain carbon nanotubes, heat generation during an internal short circuit was suppressed compared to Comparative Example 1, in which carbon nanotubes were uniformly contained in the positive electrode composite layer, but the battery capacity and output were significantly lower than in Comparative Example 1. On the other hand, in Examples 1 to 4, in which the mass ratio of carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side when the positive electrode composite layer was divided into two equal parts in the thickness direction was smaller than the mass ratio of carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side, heat generation during an internal short circuit was suppressed compared to Comparative Example 1, and while the battery capacity and output were lower compared to Comparative Example 1, the degree of the decrease was suppressed compared to Comparative Example 2. [Explanation of symbols]

[0063] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 battery case, 16 case body, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 protruding portion, 23 filter, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 40 positive electrode current collector, 42 positive electrode composite layer, 42a lower half region, 42b upper half region.

Claims

1. a positive electrode current collector; and a positive electrode mixture layer provided on the positive electrode current collector and containing a positive electrode active material and a conductive material, the conductive material includes carbon nanotubes; a positive electrode for a non-aqueous electrolyte secondary battery, wherein, when the positive electrode mixture layer is divided into two equal parts in a thickness direction, a mass ratio of the carbon nanotubes to the positive electrode active material contained in an upper half region on a surface side is smaller than a mass ratio of the carbon nanotubes to the positive electrode active material contained in a lower half region on the positive electrode current collector side.

2. The positive electrode active material has the general formula: Li ａ Ni ｘ M ｙ O ２－ｂ 2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, comprising a lithium nickel composite oxide represented by the formula: (wherein M is at least one element selected from Al, Co, Mn, Fe, Ti, Si, Nb, Mo, W, and Zn, and 0≦a<1.05, 0.7<x≦0.95, 0≦y≦0.3, 0≦b<0.05, and x+y=1).

3. 3 . The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 , wherein a mass ratio of the carbon nanotubes to the positive electrode active material contained in the upper half region on the surface side is 0.05 mass % or less.

4. 4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a mass ratio of the carbon nanotubes to the positive electrode active material contained in the lower half region on the positive electrode current collector side is 0.01 mass% or more and 1 mass% or less.

5. 5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a mass ratio of the carbon nanotubes to the positive electrode active material contained in the positive electrode mixture layer is 0.005 mass% or more and 0.5 mass% or less.

6. the conductive material includes a particulate conductive material; 6. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a mass ratio of the particulate conductive material to the positive electrode active material contained in the upper half region on the surface side is 0.05 mass% or more and 1.2 mass% or less, and a mass ratio of the particulate conductive material to the positive electrode active material contained in the lower half region on the positive electrode current collector side is 0.5 mass% or less.

7. A positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode is the positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6.

Images