Positive electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

Provided is a positive electrode for a nonaqueous electrolyte secondary battery, with which a rapid charging characteristic and a storage characteristic (thermal stability) can be improved while suppressing a decrease in battery capacity.　A positive electrode (11) for a nonaqueous electrolyte secondary battery is characterized by having: a positive electrode current collector (40); and a positive electrode active material layer (42) that is provided on one surface or both surfaces of the positive electrode current collector (40), wherein the positive electrode active material layer (42) includes a positive electrode active material, a first binding material, and a second binding material having a lower dielectric constant than the first binding material, the included quantity of the first binding material is greater than that of the second binding material in an upper half region (42b) on the outer surface side when the positive electrode active material layer (42) is divided into two equal parts in the thickness direction, and the included quantity of the second binding material is greater than that of the first binding material in a lower half region (42a) on the positive electrode current collector (40) side when the positive electrode active material layer (42) is divided into two equal parts in the thickness direction.

Description

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

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

[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, the following are known as positive electrodes for non-aqueous electrolyte secondary batteries.

[0004] For example, Patent Document 1 discloses a compound having the general formula Li ａ Me ｂ O ２ (wherein Me is a transition metal containing one or more elements selected from Co, Ni, and Mn, and a and b satisfy the condition 0.9≦a / b≦1.2) A positive electrode for a non-aqueous electrolyte secondary battery is disclosed, which contains a positive electrode active material in which a ferroelectric substance having a relative dielectric constant of 500 or more is sintered onto the surface of a lithium-containing transition metal composite oxide represented by the formula: (wherein Me is a transition metal containing one or more elements selected from Co, Ni, and Mn, and a and b satisfy the condition 0.9≦a / b≦1.2). Patent Document 1 makes it possible to improve output characteristics, but there is room for improvement in terms of battery capacity.

[0005] JP 2011-210694 A

[0006] An object of the present disclosure is to provide a positive electrode for a non-aqueous electrolyte secondary battery that can improve rapid charging characteristics and storage characteristics (thermal stability) while suppressing a decrease in battery capacity, and a non-aqueous electrolyte secondary battery including the positive electrode for a non-aqueous electrolyte secondary battery.

[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 active material layer provided on one or both surfaces of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, a first binder, and a second binder having a lower dielectric constant than the first binder, and is characterized in that, when the positive electrode active material layer is divided into two equal parts in the thickness direction, the content of the first binder is greater than that of the second binder in an upper half region on the outer surface side, and when the positive electrode active material layer is divided into two equal parts in the thickness direction, the content of the second binder is greater than that of the first binder 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 the above-described positive electrode for a non-aqueous electrolyte secondary battery.

[0009] According to one aspect of the present disclosure, it is possible to improve rapid charging characteristics and storage characteristics (thermal stability) while suppressing a decrease in battery capacity.

[0010] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0011] Hereinafter, an example of an embodiment will be described in detail 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 body 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other 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, button-shaped, or other metal exterior cans, and pouch exteriors formed by laminating a resin sheet and a metal sheet.

[0013] The non-aqueous electrolyte has, for example, ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0014] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0015] Furthermore, examples of the solid electrolyte that can be used include solid or gel polymer electrolytes, inorganic solid electrolytes, and the like. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, and the like).

[0016] The case body 16 is, for example, a cylindrical metal outer can with a bottom. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0017] 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 component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other 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.

[0018] 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, 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 by welding or the like to the inner bottom surface of the case body 16, and the case body 16 serves as the negative electrode terminal.

[0019] Each component of the nonaqueous electrolyte secondary battery 10 will be described in detail below.

[0020] [Positive Electrode] FIG. 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 and a positive electrode active material layer 42 disposed on the positive electrode current collector 40. While the positive electrode 11 shown in FIG. 2 is disposed on one side of the positive electrode current collector 40, the present invention is not limited thereto and the positive electrode 11 may be disposed on both sides of 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 the surface layer. The positive electrode active material layer 42 includes a positive electrode active material and a binder. The binder includes a first binder and a second binder having a lower dielectric constant than the first binder. The positive electrode active material layer 42 preferably includes a conductive material.

[0021] The positive electrode 11 is produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive material, etc. onto the positive electrode current collector 40, drying the slurry to form a positive electrode active material layer 42, and then rolling the positive electrode active material layer 42 with a rolling roller or the like. The method for producing the positive electrode active material layer 42 will be described in detail later.

[0022] In this embodiment, when the positive electrode active material layer 42 shown in FIG. 2 is divided in half in the thickness direction into an upper half region 42b on the outer surface side and a lower half region 42a on the positive electrode current collector 40 side, the content of the first binder is greater than that of the second binder in the upper half region 42b on the outer surface side, and the content of the second binder is greater than that of the first binder in the lower half region 42a on the positive electrode current collector 40 side. In the present disclosure, dividing the positive electrode active material layer 42 in half in the thickness direction means dividing the positive electrode active material layer 42 in half at the midpoint Z of the thickness when the stacking direction of the positive electrode current collector 40 and the positive electrode active material layer 42 is the thickness direction of the positive electrode active material layer 42. In this way, by disposing a large amount of the first binder with a high dielectric constant in the upper half region 42b on the outer surface side, it is possible to attract a large amount of ions (e.g., lithium ions) in the nonaqueous electrolyte from the upper half region 42b during charging, for example. Ions such as lithium ions drawn into the upper half region 42b diffuse into the lower half region 42a due to the concentration gradient. Therefore, if a large number of ions such as lithium ions can be drawn from the upper half region 42b, the ion concentration throughout the positive electrode active material layer 42 can be increased, which is thought to lead to improved rapid charging characteristics. Furthermore, by disposing a large amount of the second binder with a low dielectric constant in the lower half region 42a on the positive electrode current collector 40 side, the high insulating properties of the second binder are thought to lead to improved storage characteristics (thermal stability). Furthermore, because the first binder and the second binder are merely dispersed within the positive electrode active material layer 42, they do not significantly inhibit the charge / discharge reaction of the positive electrode active material, thereby suppressing a decrease in battery capacity.

[0023] The content of the first binder in the upper half region 42b on the outer surface side may be, for example, 1% by mass or more and 10% by mass or less, or 2% by mass or more and 8% by mass or less, relative to the mass of the upper half region 42b. The content of the second binder in the upper half region 42b may be lower than the content of the first binder, but is preferably zero. The content of the second binder in the lower half region 42a on the positive electrode current collector 40 side may be, for example, 1% by mass or more and 10% by mass or less, or 2% by mass or more and 8% by mass or less, relative to the mass of the lower half region 42a. The content of the first binder in the lower half region 42b may be lower than the content of the second binder, but is preferably zero.

[0024] A method for identifying and quantifying the binder contained in the upper half region 42b and the lower half region 42a will be described. Using an oblique cutting device (SAICAS), the positive electrode active material layer 42 in the upper half region 42b is peeled from the positive electrode 11, and the peeled material is dispersed in an organic solvent. Next, the dispersed slurry is separated into a binder-containing slurry and an active material-containing slurry using a centrifuge. The supernatant liquid (binder-containing slurry) after centrifugation is dried and solidified, and the resulting solid content is analyzed as described below to identify and quantify the binder in the upper half region 42b. Examples of organic solvents include N-methyl-2-pyrrolidone (NMP). Examples of methods for identifying the binder include Fourier transform infrared spectroscopy (FT-IR), solid-state NMR, and pyrolysis gas chromatography / mass spectrometry (Py-GC / MS). It is preferable to identify the binder by combining multiple analytical methods. The amount of the binder can be quantified by, for example, thermogravimetry (TG), combustion ion chromatography using a tubular combustion method, or pyrolysis gas chromatography / mass spectrometry (Py-GC / MS). It is preferable to quantify the amount of the binder by combining multiple analytical methods. The binder is also identified and quantified in the lower half region 42a using a similar method. The binder contained in the upper half region 42b and the lower half region 42a can also be identified and quantified by performing time-of-flight secondary ion mass spectrometry (TOF-SIMS) on a cross section of the positive electrode active material layer 42.

[0025] The dielectric constant of the binder can be measured by preparing a binder sheet measuring 100 mm x 100 mm x 1 mm and using an LCR meter (Agilent Technologies, Precision LCR Meter HP4284A) at a measurement temperature of 20°C and a frequency of 1000 Hz using the parallel plate capacitor method. Since it is usually difficult to measure the dielectric constant using binder extracted from the upper half region 42b or the lower half region 42a, the same binder as the binder identified by the above-mentioned method can be prepared and the dielectric constant measured by the above-mentioned method. Furthermore, considering the amount of binder quantified by the above-mentioned method, it is possible to determine whether the upper half region 42b and the lower half region 42a contain a larger amount of the first binder (high dielectric constant) or the second binder (low dielectric constant).

[0026] The dielectric constant (εA) of the first binder is preferably 7.5 F / m or more, more preferably 9 F / m or more. The ratio (εB / εA) of the dielectric constant (εA) of the first binder to the dielectric constant (εB) of the second binder is preferably 1.2 or more, more preferably 1.2 to 4.5. By satisfying the above ranges for εA and εB / εA, for example, rapid charging characteristics may be further improved.

[0027] The first binder is preferably a rubber-based binder because it has a high dielectric constant and high dispersibility, and the second binder is preferably a fluorine-based binder because it has high insulating properties and high binding properties.

[0028] Examples of rubber-based binders include nitrile butadiene rubber (NBR), hydrogenated nitrile butadiene rubber (HNBR), styrene butadiene rubber (SBR), high styrene rubber, ethylene propylene rubber, butyl rubber, chloroprene rubber, isoprene rubber, acrylonitrile butadiene rubber, acrylonitrile rubber, acrylic rubber, silicone rubber, etc. Among these, hydrogenated nitrile butadiene rubber (HNBR) is preferred because of its high dielectric constant.

[0029] Examples of fluorine-based binders include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, and other polyvinylidene fluoride-based polymers, polytetrafluoroethylene (PTFE), polytrifluoroethylene (PTrFE), etc. Among these, polyvinylidene fluoride is preferred because of its high insulating properties and high binding properties.

[0030] The first binder and the second binder are not limited to the above, and may be, for example, polyacrylonitrile (PAN), polyimide resin, acrylic resin, polyolefin resin, carboxymethyl cellulose, polyacrylic acid (PAA), polyvinyl alcohol (PVA), etc.

[0031] In addition to the first binder and the second binder, the upper half region 42 b and the lower half region 42 a may contain a third binder, as long as the effects of the present disclosure are not impaired. In the present disclosure, when three or more types of binders are contained in the positive electrode active material layer 42, the binder having the highest dielectric constant among them corresponds to the first binder, and the binder having the lowest dielectric constant corresponds to the second binder.

[0032] Examples of the conductive material include carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes, etc. These may be used alone or in combination.

[0033] The conductive material is preferably contained in a larger amount in the upper half region 42 b than in the lower half region 42 a, in order to improve rapid charging characteristics. The content of the conductive material may be, for example, in the range of 0.01 mass % to 10 mass % and preferably 1 mass % to 5 mass % relative to the mass of the positive electrode active material layer 42.

[0034] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. ｘ CoO ２ , Li ｘ NiO ２ , Li ｘ MnO２ , Li ｘ Co ｙ Ni １－ｙ O ２ , Li ｘ Co ｙ M １－ｙ O ｚ , Li ｘ Ni １－ｙ M ｙ O ｚ , Li ｘ Mn ２ O ４ , Li ｘ Mn ２－ｙ M ｙ O ４ , LiMPO ４ , Li ２ MPO ４ F (M: at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3). These may be used alone or in combination. In terms of increasing the capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material is preferably Li ｘ NiO ２ , Li ｘ Co ｙ Ni １－ｙ O ２ , Li ｘ Ni １－ｙ M ｙ O ｚ It is preferable that the lithium-nickel composite oxide contains a lithium-nickel composite oxide such as (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B; 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3).

[0035] The mass per unit area of ​​the positive electrode active material layer 42 is 240 m per side. ２ / g or more, and 280 g / m ２ It is more preferable that the mass per unit area of ​​the positive electrode active material layer 42 is in the above range, thereby increasing the battery capacity. The upper limit of the mass per unit area of ​​the positive electrode active material layer 42 is not particularly limited, but is, for example, 400 g / m per side. ２ The following is fine.

[0036] An example of a method for manufacturing the positive electrode active material layer 42 according to this embodiment will be described. First, a first positive electrode composite slurry containing a positive electrode active material, a second binder with a low dielectric constant, and the like is prepared. Separately, a second positive electrode composite slurry containing a positive electrode active material, a first binder with a high dielectric constant, and the like is prepared. The first positive electrode composite slurry is then applied to the positive electrode current collector 40 and dried. After that, the second positive electrode composite slurry is applied to the coating film derived from the first positive electrode composite slurry and dried to form a coating film derived from the second positive electrode composite slurry, thereby forming the positive electrode active material layer 42. Note that in the above method, the first positive electrode composite slurry is applied and dried, and then the second positive electrode composite slurry is applied. However, a method in which the second positive electrode composite slurry is applied after the first positive electrode composite slurry is applied and before drying may also be used, or a method in which the first positive electrode composite slurry and the second positive electrode composite slurry are applied simultaneously may also be used.

[0037] [Negative Electrode] The negative electrode 12 has a negative electrode current collector and a negative electrode active material layer provided on one or both sides of 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 with such a metal disposed on the surface layer. The negative electrode active material layer contains a negative electrode active material and may further contain a binder, a conductive material, etc. The 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 to the negative electrode current collector, drying it to form a negative electrode active material layer, and rolling this negative electrode active material layer.

[0038] The negative electrode active material can reversibly absorb and release lithium ions, and examples thereof include carbon materials such as graphite particles, non-graphitizable carbon particles, and easily graphitizable carbon particles, Si-based materials, and Sn-based materials. Examples of the Si-based materials include Si, alloys containing Si, and SiO Ｘ Examples of silicon oxides include SiO Ｘ The volume change caused by charging and discharging is smaller than that of Si, so Si-based materials are considered to have SiO Ｘ It is particularly preferable to use SiO Ｘ is, for example, amorphous SiO ２ It has a structure in which fine Si particles are dispersed in the matrix.

[0039] Examples of the binder include polyacrylonitrile (PAN), polyimide resin, acrylic resin, polyolefin resin, carboxymethyl cellulose, polyacrylic acid (PAA), polyvinyl alcohol (PVA), the above-mentioned rubber-based binder, the above-mentioned fluorine-based binder, etc. Examples of the conductive material include the same materials as those on the positive electrode 11 side.

[0040] [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.

[0041] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0042] Example 1 [Fabrication of Positive Electrode] Aluminum-containing lithium nickel cobalt oxide (LiNi) was used as the positive electrode active material. ０．８８ Co ０．０９ Al ０．０３ O ２ 100 parts by mass of the above positive electrode active material, 1 part by mass of graphite as a conductive material, and 1 part by mass of polyvinylidene fluoride (PVdF, dielectric constant (εB)=4 F / m) as a second binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a first positive electrode composite slurry.

[0043] Furthermore, 100 parts by mass of the above positive electrode active material, 1 part by mass of graphite as a conductive material, and 1 part by mass of hydrogenated nitrile butadiene rubber (HNBR, dielectric constant (εA)=9 F / m) as a first binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a second positive electrode composite slurry.

[0044] The first positive electrode composite slurry was applied to both sides of a positive electrode current collector made of aluminum foil (thickness 15 μm), and the coating film was dried. Next, the second positive electrode composite slurry was applied to the coating film derived from the first positive electrode composite slurry and dried. The coating thickness ratio of the second positive electrode composite slurry / the first positive electrode composite slurry was 50 / 50, and the mass per unit area of ​​the coating film (positive electrode active material layer) was 200 g / m per side. ２ The coating film was then rolled using a rolling roller to prepare a positive electrode. In the prepared positive electrode, the upper half of the outer surface of the positive electrode active material layer contained hydrogenated nitrile butadiene rubber (HNBR, dielectric constant (εA) = 9 F / m), and the lower half of the positive electrode active material layer on the positive electrode current collector side contained polyvinylidene fluoride (PVdF, dielectric constant (εB) = 4 F / m).

[0045] [Fabrication of Negative Electrode] Graphite, sodium carboxymethyl cellulose, 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 a copper foil with a thickness of 8 μm, and after drying the coating, the foil was rolled with a rolling roller to prepare a negative electrode.

[0046] [Preparation of non-aqueous electrolyte] Five parts by mass of vinylene carbonate (VC) was added to a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate were mixed in a volume ratio of 1:3, and LiPF ６ was dissolved in the solution at a concentration of 1.5 mol / L. This was used as a non-aqueous electrolyte.

[0047] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] (1) A positive electrode lead was attached to the positive electrode, and a negative electrode lead was attached to the negative electrode. A separator made of a polyethylene microporous film was then interposed between the positive and negative electrodes, and the resulting structure was wound to produce a wound electrode assembly. (2) Insulating plates were placed above and below the electrode assembly, and the negative electrode lead was welded to the case body and the positive electrode lead was welded to a sealing member, and the electrode assembly was housed within the case body. (3) A non-aqueous electrolyte was injected into the case body under reduced pressure, and the opening of the case body was then sealed with a sealing member via a gasket. This resulted in a non-aqueous electrolyte secondary battery.

[0048] <Example 2> In the production of the positive electrode, the mass per unit area of ​​the coating film (positive electrode active material layer) was set to 240 g / m per side. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the above-mentioned

[0049] <Example 3> In the production of the positive electrode, the mass per unit area of ​​the coating film (positive electrode active material layer) was set to 280 g / m per side. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the above-mentioned

[0050] Example 4 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that polytetrafluoroethylene (PTFE, dielectric constant (εB)=2.2 F / m) was used as the second binder contained in the first positive electrode composite slurry in fabricating the positive electrode.

[0051] Example 5 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that polytrifluoroethylene (PTrFE, dielectric constant (εB)=7.5 F / m) was used as the second binder contained in the first positive electrode composite slurry in fabricating the positive electrode.

[0052] Example 6 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the amount of the conductive material contained in the first positive electrode composite slurry was set to 0.75 parts by mass, and the amount of the conductive material contained in the second positive electrode composite slurry was set to 1.25 parts by mass.

[0053] Example 7 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that polytrifluoroethylene (PTrFE, dielectric constant (εB)=7.5 F / m) was used as the first binder contained in the second positive electrode composite slurry in fabricating the positive electrode.

[0054] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the second positive electrode composite slurry was applied to both sides of a positive electrode current collector made of aluminum foil, the coating film was dried, and then rolled with a rolling roller. The positive electrode of Comparative Example 1 contained hydrogenated nitrile butadiene rubber (HNBR, dielectric constant (εA) = 9 F / m) in the upper half region on the outer surface side of the positive electrode active material layer and the lower half region on the positive electrode current collector side.

[0055] Comparative Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the first positive electrode composite slurry was applied to both sides of a positive electrode current collector made of aluminum foil, the coating was dried, and then rolled with a rolling roller. The positive electrode of Comparative Example 1 contained polyvinylidene fluoride (PVdF, dielectric constant (εB) = 4 F / m) in the upper half region on the outer surface side of the positive electrode active material layer and in the lower half region on the positive electrode current collector side.

[0056] <Comparative Example 3> In the production of the positive electrode, the mass per unit area of ​​the coating film (positive electrode active material layer) was 240 g / m per side. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 2, except that the above-mentioned

[0057] <Comparative Example 4> In the production of the positive electrode, the mass per unit area of ​​the coating film (positive electrode active material layer) was 280 g / m per side. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 2, except that the above-mentioned

[0058] Comparative Example 5: Aluminum-containing lithium nickel cobalt oxide particles (LiNi ０．８８ Co ０．０９ Al ０．０３ O ２ ) 1 mol of BaTiO, a ferroelectric with a dielectric constant of 1500 ３ These were mixed using a Mechanofusion (trade name) manufactured by Hosokawa Micron Corporation to obtain the above BaTiO ３ The particles were uniformly dispersed on the surface of the aluminum-containing lithium nickel cobalt oxide particles, and then the resultant was fired at 700°C in air to form BaTiO ３ A positive electrode active material in which the particles were sintered was obtained.

[0059] 100 parts by mass of the above positive electrode active material, 1 part by mass of graphite as a conductive material, and 1 part by mass of polyvinylidene fluoride (PVdF, dielectric constant (εB)=4 F / m) as a binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode composite slurry.

[0060] In Comparative Example 5, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the positive electrode was fabricated using the above positive electrode composite slurry.

[0061] [Measurement of Battery Capacity] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.3 C to 4.2 V, and then charged at a constant voltage of 1 / 50 C at 4.2 V. They were then discharged at a constant current of 0.3 C to 2.5 V. The discharge capacity at this time was measured as the battery capacity, and the results are summarized in Table 1. In Table 1, the battery capacity of Example 1 was set as the reference (100), and the battery capacities of the other Examples and Comparative Examples were shown as relative values.

[0062] [Measurement of Capacity Retention Rate in Rapid Charge Cycles] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 2.0 C to 4.2 V, and then charged at a constant voltage of 1 / 50 C at 4.2 V. They were then discharged at a constant current of 0.5 C to 2.5 V. This cycle of charge and discharge constitutes one cycle, and 150 cycles were performed. The capacity retention rate in rapid charge cycles of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example was calculated using the following formula, and the results are summarized in Table 1. Capacity retention rate = (discharge capacity at 150th cycle / discharge capacity at 1st cycle) × 100

[0063] [Storage Test] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.5C to 4.2V, and then charged at a constant voltage of 1 / 50C at 4.2V. The batteries charged in this manner were discharged at a constant current of 0.5C at 25°C until the battery voltage reached 2.5V, and the capacity was measured and calculated as the discharge capacity before the storage test. Then, at an ambient temperature of 25°C, the batteries were again charged at a constant current of 0.5C to 4.2V, and then charged at a constant voltage of 1 / 50C at 4.2V, and then stored in an atmosphere at 60°C for 30 days. After storage, the batteries were discharged at a constant current of 0.5C until the battery voltage reached 2.5V, and the capacity was measured and calculated as the discharge capacity after the storage test. The remaining capacity was then calculated using the following formula: remaining capacity (%) = (discharge capacity after storage test / discharge capacity after storage test) x 100

[0064]

[0065] As can be seen from Table 1, in all of Examples 1 to 7, the capacity retention rate in the rapid charge cycle was 80% or more, and the remaining capacity in the storage test was 85%, and no decrease in battery capacity was observed during normal charge and discharge. On the other hand, in Comparative Example 1, the capacity retention rate in the rapid charge cycle was 80% or more, but the remaining capacity in the storage test was 65%, lower than that of the Examples. Furthermore, in Comparative Examples 2 to 4, the remaining capacity in the storage test was equivalent to that of the Examples, but the capacity retention rate in the rapid charge cycle was 70% or less, significantly lower than that of the Examples. Comparative Example 5 showed good results for both the capacity retention rate in the rapid charge cycle and the remaining capacity in the storage test, but the battery capacity during normal charge and discharge was lower than that of the Examples and other Comparative Examples. From these results, as in Examples 1 to 7, when the positive electrode active material layer is divided into two equal parts in the thickness direction, the upper half of the region on the outer surface side has a greater content of the first binder with a higher dielectric constant than the second binder with a lower dielectric constant, and when the positive electrode active material layer is divided into two equal parts in the thickness direction, the lower half of the region on the positive electrode current collector side has a greater content of the second binder with a lower dielectric constant than the first binder with a higher dielectric constant. By using a positive electrode, it can be said that it is possible to improve the rapid charging characteristics and storage characteristics (thermal stability) while suppressing a decrease in battery capacity.

[0066] [Notes] (1) A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer provided on one or both surfaces of the positive electrode current collector, wherein the positive electrode active material layer includes a positive electrode active material, a first binder, and a second binder having a lower dielectric constant than the first binder, and wherein, when the positive electrode active material layer is divided into two equal parts in the thickness direction, an upper half region on the outer surface side contains a larger amount of the first binder than the second binder, and when the positive electrode active material layer is divided into two equal parts in the thickness direction, an upper half region on the positive electrode current collector side contains a larger amount of the second binder than the first binder. (2) The positive electrode for a non-aqueous electrolyte secondary battery according to (1) above, wherein the dielectric constant (εA) of the first binder is 7.5 F / m or more, and the ratio (εA / εB) of the dielectric constant (εB) of the first binder to the dielectric constant (εB) of the second binder is 1.2 or more. (3) The positive electrode for a non-aqueous electrolyte secondary battery according to (1) or (2) above, wherein the positive electrode active material layer contains a conductive material, and the conductive material is contained in a larger amount in an upper half region on the outer surface side than in a lower half region on the positive electrode current collector side. (4) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of (1) to (3) above, wherein the first binder is a rubber-based binder, and the second binder is a fluorine-based binder. (5) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of (1) to (4), wherein the first binder is hydrogenated nitrile butadiene rubber (HNBR) and the second binder is polyvinylidene fluoride (PVdF). (6) The mass per unit area of ​​the positive electrode active material layer is 240 g / m per side. ２ (7) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of (1) to (5) above, wherein the mass per unit area of ​​the positive electrode active material layer is 280 g / m2 or more per side. (8) A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to any one of (1) to (7) above.

[0067] REFERENCE SIGNS LIST 10 nonaqueous 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 active material layer, 42a lower half region, 42b upper half region.

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer provided on one or both sides of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, a first binder, and a second binder having a lower dielectric constant than the first binder, wherein when the positive electrode active material layer is divided into two equal parts in the thickness direction, an upper half region on the outer surface side contains a greater amount of the first binder than the second binder, and when the positive electrode active material layer is divided into two equal parts in the thickness direction, the lower half region on the positive electrode current collector side contains a greater amount of the second binder than the first binder.

2. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the dielectric constant (εA) of the first binder is 7.5 F / m or more, and the ratio (εA / εB) of the dielectric constant (εB) of the first binder to the dielectric constant (εB) of the second binder is 1.2 or more.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode active material layer contains a conductive material, and the conductive material is contained in a larger amount in an upper half region on the outer surface side than in a lower half region on the positive electrode current collector side.

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the first binder is a rubber-based binder, and the second binder is a fluorine-based binder.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the first binder is hydrogenated nitrile butadiene rubber (HNBR) and the second binder is polyvinylidene fluoride (PVdF).

6. The mass per unit area of ​​the positive electrode active material layer is 240 g / m per side. ２ The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2.

7. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 6, wherein the mass per unit area of ​​the positive electrode active material layer is 280 g / m2 or more per side.

8. A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2.

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