Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery is characterized by: comprising a wound electrode body in which a positive electrode (11) and a negative electrode are wound with a separator therebetween; the positive electrode (11) having a positive electrode current collector (40), a first positive electrode mixture layer (41) formed on a first surface (40a) of the positive electrode current collector (40) which faces the outside of the electrode body, and a second positive electrode mixture layer (42) formed on a second surface (40b) of the positive electrode current collector (40) which faces the inside of the electrode body; the first positive electrode mixture layer (41) and the second positive electrode mixture layer (42) each containing a positive electrode active material; and the dibutyl phthalate oil absorption amount of the positive electrode active material contained in the first positive electrode mixture layer (41) being less than the dibutyl phthalate oil absorption amount of the positive electrode active material contained in the second positive electrode mixture layer (42).

Description

[Technical Field]

[0001] The present disclosure relates to 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 Document 1 proposes the use of a positive electrode, in order to provide a nonaqueous electrolyte secondary battery having both excellent output characteristics and favorable charge / discharge cycle characteristics, which includes a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer being composed of two layers divided in the thickness direction, an upper layer relatively close to the surface and a lower layer relatively close to the positive electrode current collector, the dibutyl phthalate oil absorption per unit mass of the lower layer being greater than the dibutyl phthalate oil absorption per unit mass of the upper layer, and the thickness of the lower layer being 40% or less when the thickness of the entire positive electrode active material layer is taken as 100%.

[0004] Furthermore, for example, Patent Document 2 proposes a positive electrode active material made of a lithium-containing composite oxide powder having a dibutyl phthalate oil absorption of 20 mL / 100 g to 40 mL / 100 g. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-100241 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-515465 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that can suppress deterioration in charge-discharge cycle characteristics. [Means for solving the problem]

[0007] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure includes a wound electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, the positive electrode having a positive electrode current collector, a first positive electrode composite layer formed on a first surface of the positive electrode current collector facing outward from the electrode assembly, and a second positive electrode composite layer formed on a second surface of the positive electrode current collector facing inward from the electrode assembly, the first positive electrode composite layer and the second positive electrode composite layer containing positive electrode active materials, and the dibutyl phthalate oil absorption of the positive electrode active material contained in the first positive electrode composite layer is smaller than the dibutyl phthalate oil absorption of the positive electrode active material contained in the second positive electrode composite layer. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, it is possible to suppress deterioration in charge-discharge cycle characteristics. [Brief explanation of the drawings]

[0009] [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

[0010] 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.

[0011] 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 body 17 that closes the opening of the case body 16. Examples of the battery case 15 include a cylindrical or prismatic outer can with a bottom, and a pouch outer body formed by laminating a resin sheet and a metal sheet.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] Each component of the nonaqueous electrolyte secondary battery 10 will be described in detail below.

[0016] [Positive electrode] 2 is a cross-sectional view of a positive electrode according to an embodiment. The positive electrode 11 includes a positive electrode current collector 40, a first positive electrode composite layer 41 formed on a first surface 40a of the positive electrode current collector 40 facing outward from the electrode body 14, and a second positive electrode composite layer 42 formed on a second surface 40b of the positive electrode current collector 40 facing inward from the electrode body 14. 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 first positive electrode composite layer 41 and the second positive electrode composite layer 42 each contain a positive electrode active material, and preferably further contain a binder, a conductive material, or the like.

[0017] Positive electrode 11 is obtained, for example, by applying a first positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. onto first surface 40a of positive electrode current collector 40 and drying it to form first positive electrode composite layer 41, and by applying a second positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. onto second surface 40b of positive electrode current collector 40 and drying it to form second positive electrode composite layer 42. Then, first positive electrode composite layer 41 and second positive electrode composite layer 42 are rolled using a rolling roller or the like to obtain positive electrode 11.

[0018] 2, the positive electrode 11 constituting the wound electrode assembly 14 is curved over the entire length in the longitudinal direction. The positive electrode 11 is manufactured in the form of a flat plate, and becomes curved by being wound together with the negative electrode 12 and separator 13 during the manufacture of the electrode assembly 14. The positive electrode 11 generally has a radius of curvature of about 1 mm to 100 mm, and the radius of curvature differs between the start and end of the winding of the electrode assembly 14.

[0019] Because the positive electrode 11 is manufactured in the form of a flat plate, when it is curved, the convex side is stretched and the concave side is compressed. That is, as shown in FIG. 2 , the first positive electrode composite layer 41 formed on the first surface 40a of the positive electrode current collector 40 facing outward from the electrode body 14 is stretched mainly in the direction of arrow A, and the second positive electrode composite layer 42 formed on the second surface 40b of the positive electrode current collector 40 facing inward from the electrode body 14 is compressed mainly in the direction of arrow B. As a result, voids are generated in the first positive electrode composite layer 41 due to the stretching, which tends to increase the amount of non-aqueous electrolyte penetration. Meanwhile, the second positive electrode composite layer 42 is compressed, which tends to reduce the voids and decrease the amount of non-aqueous electrolyte penetration. If there is a difference in the amount of non-aqueous electrolyte that penetrates the first positive electrode composite layer 41 and the second positive electrode composite layer 42, the reaction between the first positive electrode composite layer 41 and the second positive electrode composite layer 42 will become uneven, resulting in a deterioration in the charge-discharge cycle characteristics.

[0020] Therefore, in this embodiment, the dibutyl phthalate oil absorption of the positive electrode active material contained in the first positive electrode composite layer 41 formed on the first surface 40a of the positive electrode current collector 40 facing outward from the electrode body 14 is set to be smaller than the dibutyl phthalate oil absorption of the positive electrode active material contained in the second positive electrode composite layer 42 formed on the second surface 40b of the positive electrode current collector 40 facing inward from the electrode body 14. As a result, the first positive electrode composite layer 41, which is likely to have a large amount of non-aqueous electrolyte permeation, is made of a positive electrode active material with a small oil absorption, and therefore the amount of non-aqueous electrolyte permeation is suppressed. Meanwhile, the second positive electrode composite layer 42, which is likely to have a small amount of non-aqueous electrolyte permeation, is made of a positive electrode active material with a large oil absorption, and therefore the amount of non-aqueous electrolyte permeation is improved. As a result, the difference in the amount of non-aqueous electrolyte permeated into the first positive electrode composite layer 41 and the second positive electrode composite layer 42 becomes smaller, and the reaction between the first positive electrode composite layer 41 and the second positive electrode composite layer 42 becomes uniform, which is thought to suppress deterioration in the charge-discharge cycle characteristics.

[0021] In this embodiment, in order to suppress a deterioration in charge-discharge cycle characteristics, the dibutyl phthalate oil absorption of the positive electrode active material contained in first positive electrode composite layer 41 is preferably 11 mL / 100 g or more and 19 mL / 100 g or less, more preferably 12 mL / 100 g or more and 18 mL / 100 g or less, and even more preferably 13 mL / 100 g or more and 17 mL / 100 g or less. Furthermore, in this embodiment, in order to suppress a deterioration in charge-discharge cycle characteristics, the dibutyl phthalate oil absorption of the positive electrode active material contained in second positive electrode composite layer 42 is preferably 15 mL / 100 g or more and 23 mL / 100 g or less, more preferably 16 mL / 100 g or more and 22 mL / 100 g or less, and even more preferably 17 mL / 100 g or more and 21 mL / 100 g or less.

[0022] In this embodiment, the dibutyl phthalate oil absorption values ​​of the positive electrode active materials contained in the first positive electrode composite layer 41 and the second positive electrode composite layer 42 are average values. That is, the first positive electrode composite layer 41 and the second positive electrode composite layer 42 may each contain a plurality of positive electrode active materials with different dibutyl phthalate oil absorption values. For example, if the first positive electrode composite layer 41 contains three positive electrode active materials (P1, P2, P3) with different dibutyl phthalate oil absorption values, the dibutyl phthalate oil absorption value of the positive electrode active material contained in the first positive electrode composite layer 41 is the dibutyl phthalate oil absorption value of a mixture of the positive electrode active materials P1, P2, and P3. The same is true for the second positive electrode composite layer 42.

[0023] When the oil absorption of the mixture of multiple positive electrode active materials in first positive electrode composite layer 41 is 11 mL / 100 g or more and 19 mL / 100 g or less, it is desirable that the dibutyl phthalate oil absorption of all of the positive electrode active materials be 11 mL / 100 g or more and 19 mL / 100 g or less. However, as long as the dibutyl phthalate oil absorption of the mixture of multiple positive electrode active materials contained in first positive electrode composite layer 41 is 11 mL / 100 g or more and 19 mL / 100 g or less, the dibutyl phthalate oil absorption of each positive electrode active material does not have to satisfy the above range. For example, when first positive electrode composite layer 41 contains two types of positive electrode active materials (P1, P2) with different dibutyl phthalate oil absorptions, the dibutyl phthalate oil absorption of positive electrode active material P1 may be less than 11 mL / 100 g, for example, and the dibutyl phthalate oil absorption of positive electrode active material P2 may be greater than 19 mL / 100 g, for example, as long as the dibutyl phthalate oil absorption of the mixture consisting of positive electrode active materials P1 and P2 is 11 mL / 100 g or more and 19 mL / 100 g or less. In this case, it is necessary to adjust the contents of positive electrode active materials P1 and P2 so that the dibutyl phthalate oil absorption of the mixture consisting of positive electrode active materials P1 and P2 is 11 mL / 100 g or more and 19 mL / 100 g or less.

[0024] Similarly, in second positive electrode composite layer 42, when the oil absorption of the mixture of multiple positive electrode active materials is 15 mL / 100 g or more and 23 mL / 100 g or less, it is desirable that the dibutyl phthalate oil absorption of all of the positive electrode active materials be 15 mL / 100 g or more and 23 mL / 100 g or less. However, as long as the dibutyl phthalate oil absorption of the mixture of multiple positive electrode active materials contained in second positive electrode composite layer 42 is 15 mL / 100 g or more and 23 mL / 100 g or less, the dibutyl phthalate oil absorption of each positive electrode active material does not have to satisfy the above range. For example, when second positive electrode composite layer 42 contains two types of positive electrode active materials (P1, P2) with different dibutyl phthalate oil absorptions, the dibutyl phthalate oil absorption of positive electrode active material P1 may be less than 15 mL / 100 g, for example, and the dibutyl phthalate oil absorption of positive electrode active material P2 may be greater than 23 mL / 100 g, for example, as long as the dibutyl phthalate oil absorption of the mixture consisting of positive electrode active materials P1 and P2 is 15 mL / 100 g or more and 23 mL / 100 g or less. In this case, it is necessary to adjust the contents of positive electrode active materials P1 and P2 so that the dibutyl phthalate oil absorption of the mixture consisting of positive electrode active materials P1 and P2 is 15 mL / 100 g or more and 23 mL / 100 g or less.

[0025] The dibutyl phthalate oil absorption of the positive electrode active material is measured according to DBP (dibutyl phthalate) absorption method A (mechanical method) specified in JIS K-6217-4, "Carbon black for rubber - Fundamental properties - Part 4: Determination of DBP absorption." Specifically, using an absorption tester (manufactured by Asahi Research Institute Co., Ltd., model name "S-500"), DBP is added at a constant speed to a sample (positive electrode active material) being stirred with two blades, and the change in viscosity characteristics during this process is detected by a torque detector. The output is converted into torque by a microcomputer, and the DBP corresponding to the torque at 100% of the maximum torque generated is converted per 100g of sample (positive electrode active material) to determine the dibutyl phthalate oil absorption.

[0026] In this embodiment, when the first positive electrode mixture layer 41 is divided into two equal parts in the thickness direction, the dibutyl phthalate oil absorption of the positive electrode active material contained in the upper half region 41b is preferably smaller than the dibutyl phthalate oil absorption of the positive electrode active material contained in the lower half region 41a. This more effectively suppresses the amount of non-aqueous electrolyte permeation into the first positive electrode mixture layer 41. Here, dividing the first positive electrode mixture layer 41 into two equal parts in the thickness direction means that when the stacking direction of the positive electrode current collector 40 and the first positive electrode mixture layer 41 is the thickness direction of the first positive electrode mixture layer 41, the first positive electrode mixture layer 41 is divided into half at the middle Z1 of the thickness. The first positive electrode composite layer 41 is divided into two equal parts in the thickness direction, and the first positive electrode composite layer 41 located closer to the positive electrode current collector 40 is designated as the lower half region 41a, and the first positive electrode composite layer 41 located farther from the positive electrode current collector 40 is designated as the upper half region 41b.

[0027] In this embodiment, when the second positive electrode mixture layer 42 is divided into two equal parts in the thickness direction, the dibutyl phthalate oil absorption of the positive electrode active material contained in the upper half region 42b is preferably greater than the dibutyl phthalate oil absorption of the positive electrode active material contained in the lower half region 42a. This makes it possible to further improve the amount of non-aqueous electrolyte permeation into the second positive electrode mixture layer 42. Here, dividing the second 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 second positive electrode mixture layer 42 is the thickness direction of the second positive electrode mixture layer 42, the second positive electrode mixture layer 42 is divided into half at the middle Z2 of the thickness. The second positive electrode composite layer 42 is divided into two equal parts in the thickness direction, and the second positive electrode composite layer 42 located closer to the positive electrode current collector 40 is designated as the lower half region 42a, and the second positive electrode composite layer 42 located farther from the positive electrode current collector 40 is designated as the upper half region 42b.

[0028] 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-yO2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x , LiMn2O4, Li x Mn 2-y M y 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). These may be used alone or in combination of multiple kinds. In terms of achieving high capacity of the non-aqueous electrolyte secondary battery, the cathode active material is Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (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. lithium nickel composite oxides are preferably included.

[0029] The cathode active material can be obtained, for example, by mixing a precursor and a lithium compound and firing the mixture. The precursor can be obtained, for example, by dropping an alkaline solution such as sodium hydroxide while stirring a solution containing one or more metal salts such as transition metals to precipitate (co-precipitate) metal hydroxides, and then heat-treating the precipitated metal hydroxides by adjusting the heat treatment temperature, heat treatment time, etc. during this heat treatment. By adjusting the heat treatment temperature, heat treatment time, etc. during this heat treatment, precursors with different dibutyl phthalate oil absorption amounts can be obtained, and thus cathode active materials with different dibutyl phthalate oil absorption amounts can be obtained.

[0030] Examples of conductive materials include carbon particles 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.

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

[0032] [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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] Examples of conductive materials include carbon particles 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.

[0037] [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.

[0038] [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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt 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 plural 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

[0043] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0044] (Production of lithium metal composite oxide A) After obtaining a nickel-cobalt-aluminum composite hydroxide by coprecipitation and subjecting it to a heat treatment to obtain a precursor, the precursor and lithium hydroxide monohydrate (LiOH·H2O) were mixed so that the atomic ratio of lithium, nickel, cobalt, and aluminum became Li:Ni:Co:Al = 1.00:0.82:0.15:0.03. By firing this mixed powder in an electric furnace under an oxygen atmosphere at 750 °C for 15 hours, lithium metal composite oxide A was obtained.

[0045] (Production of lithium metal composite oxides B to F) Lithium metal composite oxides B to F were produced under the same conditions as lithium metal composite oxide A, except that the heating temperature and heating time when the nickel-cobalt-aluminum composite hydroxide was heat-treated were changed.

[0046] Table 1 shows the dibutyl phthalate oil absorption of lithium metal composite oxides A to F. The method for measuring the dibutyl phthalate oil absorption is as described above.

[0047] [Table 1]

[0048] Example 1 [Preparation of positive electrode] A lithium metal composite oxide B as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) with an average molecular weight of 1.1 million as a binder were mixed in a mass ratio of 98:1:1 in an N-methylpyrrolidone (NMP) solvent to prepare a slurry with a solid content of 70 mass %. This was designated as a first positive electrode composite slurry.

[0049] In addition, lithium metal composite oxide E as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) with an average molecular weight of 1.1 million as a binder were mixed in a mass ratio of 98:1:1 in an N-methylpyrrolidone (NMP) solvent to prepare a slurry with a solid content of 70 mass %. This was designated as a second positive electrode composite slurry.

[0050] The first positive electrode composite slurry was applied to one side of a 15 μm-thick aluminum foil, and the second positive electrode composite slurry was applied to the other side of the aluminum foil. The resulting mixture was then dried and rolled with a rolling roller to produce a positive electrode having a first positive electrode composite layer on one side of a positive electrode current collector and a second positive electrode composite layer on the other side of the positive electrode current collector.

[0051] [Preparation of negative electrode] 95 parts by mass of graphite powder, 5 parts by mass of silicon oxide, and 1 part by mass of carboxymethyl cellulose (CMC) were mixed with an appropriate amount of water. 1.2 parts by mass of styrene-butadiene rubber (SBR) and an appropriate amount of water were added to this mixture 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.

[0052] [Preparation of non-aqueous electrolyte] Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent consisting of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (volume ratio of EC:DMC = 1:3), and LiPF6 was dissolved therein at a concentration of 1 mol / L to form a non-aqueous electrolyte.

[0053] [Secondary battery production] (1) After attaching leads to each of the positive and negative electrodes, a 20 μm thick polyethylene separator was placed between the positive and negative electrodes, and these were wound so that the first positive electrode composite layer of the positive electrode was on the outer periphery and the second positive electrode composite layer was on the inner periphery, thereby producing a wound electrode body. (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 member. (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.

[0054] <Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that lithium metal composite oxide D was used as the positive electrode active material used in the first positive electrode mixture slurry.

[0055] Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that lithium metal composite oxide C was used as the positive electrode active material used in the second positive electrode mixture slurry.

[0056] Example 4 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that lithium metal composite oxide A was used as the positive electrode active material used in the first positive electrode mixture slurry.

[0057] <Example 5> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that lithium metal composite oxide B was used as the positive electrode active material used in the first positive electrode composite slurry, and lithium metal composite oxide F was used as the positive electrode active material used in the second positive electrode composite slurry.

[0058] <Comparative Example 1> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that lithium metal composite oxide E was used as the positive electrode active material used in the first positive electrode composite slurry, and lithium metal composite oxide B was used as the positive electrode active material used in the second positive electrode composite slurry.

[0059] <Comparative Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the same lithium metal composite oxide B as in the first positive electrode mixture slurry was used as the positive electrode active material for the second positive electrode mixture slurry.

[0060] <Comparative Example 3> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the same lithium metal composite oxide E as in the second positive electrode mixture slurry was used as the positive electrode active material for the first positive electrode mixture slurry.

[0061] [Evaluation of charge / discharge cycle characteristics] The nonaqueous electrolyte secondary batteries of each Example and Comparative Example were subjected to constant current charging at a current of 0.7 It in a temperature environment of 25°C until the voltage reached 4.2 V, followed by constant voltage charging at a voltage of 4.2 V until the current reached 0.05 It. They were then subjected to constant current discharging at a current of 0.7 It until the voltage reached 2.5 V. This charge / discharge cycle constitutes one cycle, and 300 cycles were repeated, and the capacity retention rate was calculated using the following formula. Capacity retention rate (%) = (300th cycle discharge capacity / 1st cycle discharge capacity) x 100

[0062] Table 2 summarizes the results of the charge-discharge cycle characteristics of each example and each comparative example.

[0063] [Table 2]

[0064] All of Examples 1 to 5 exhibited higher capacity retention rates than Comparative Examples 1 to 3. For these reasons, by using a positive electrode in which the dibutyl phthalate oil absorption of the positive electrode active material contained in the first positive electrode composite layer formed on the first surface of the positive electrode current collector facing the outside of the electrode assembly is smaller than the oil absorption of the positive electrode active material contained in the second positive electrode composite layer formed on the second surface of the positive electrode current collector facing the inside of the electrode assembly, as in Examples 1 to 5, deterioration of charge-discharge cycle characteristics can be suppressed. Furthermore, since Examples 1 to 3 among Examples 1 to 5 exhibited capacity retention rates of 85% or higher, in terms of further suppressing charge-discharge cycle characteristics, it is preferable that the dibutyl phthalate oil absorption of the positive electrode active material contained in the first positive electrode composite layer be 11 mL / 100 or more and 19 mL / 100 g or less, and that the dibutyl phthalate oil absorption of the positive electrode active material contained in the second positive electrode composite layer be 15 mL / 100 g or more and 23 mL / 100 g or less. [Explanation of symbols]

[0065] 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, 41 first positive electrode composite layer, 41a, 42a lower half region, 41b, 42b upper half region, 42 second positive electrode composite layer.

Claims

[Claim 1] The battery includes a wound electrode body in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, the positive electrode includes a positive electrode current collector, a first positive electrode composite layer formed on a first surface of the positive electrode current collector facing outward from the electrode body, and a second positive electrode composite layer formed on a second surface of the positive electrode current collector facing inward from the electrode body, the first positive electrode mixture layer and the second positive electrode mixture layer contain a lithium metal composite oxide as a positive electrode active material, the dibutyl phthalate oil absorption of the lithium metal composite oxide contained in the first positive electrode mixture layer is smaller than the dibutyl phthalate oil absorption of the lithium metal composite oxide contained in the second positive electrode mixture layer, a dibutyl phthalate oil absorption of the lithium metal composite oxide contained in the first positive electrode mixture layer is 11 mL / 100 or more and 19 mL / 100 g or less, and a dibutyl phthalate oil absorption of the lithium metal composite oxide contained in the second positive electrode mixture layer is 15 mL / 100 g or more and 23 mL / 100 g or less.

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