Nonaqueous-electrolyte secondary battery positive electrode and nonaqueous-electrolyte secondary battery

Abstract

A positive electrode for a nonaqueous-electrolyte secondary battery according to one embodiment of the present invention comprises a positive electrode current collector and a positive electrode mixture layer arranged on the positive electrode current collector. The positive electrode mixture layer comprises: a positive electrode active material; an electroconductive agent containing carbon; and a binder. The positive electrode mixture layer has a normal region and a carbon concentration inclination region arranged between the normal region and the positive electrode current collector. The carbon concentration in the carbon concentration inclination region is inclined so as to decrease from the positive electrode current collector side toward the normal region side in the thickness direction of the carbon concentration inclination region, and the average carbon concentration in the carbon concentration inclination region is higher than the average carbon concentration in the normal region.

Description

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

[0001] This disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery, and more particularly to a positive electrode for a non-aqueous electrolyte secondary battery characterized by the distribution of carbon in the positive electrode mixture layer, and to a non-aqueous electrolyte secondary battery containing this positive electrode.

[0002] In recent years, non-aqueous electrolyte secondary batteries, which comprise a positive electrode, a negative electrode, and a non-aqueous electrolyte, and perform charging and discharging by moving Li ions and the like between the positive and negative electrodes, have been widely used as high-power, high-capacity secondary batteries. The positive and negative electrodes of non-aqueous electrolyte secondary batteries generally consist of a current collector and a composite layer disposed on the current collector, and the composite layer contains an active material, a conductive agent, a binder, etc. Patent Document 1 discloses a technique to suppress the increase in electrode resistance by forming a carbon coating layer with irregularities on the surface of the positive electrode current collector to suppress the peeling of the positive electrode composite layer, in order to solve the problem that the positive electrode composite layer containing a positive electrode active material having an olivine structure is easily peeled off from the positive electrode current collector.

[0003] Japanese Patent Publication No. 2013-65482

[0004] In recent years, there has been an increasing demand for higher output power in non-aqueous electrolyte secondary batteries. To increase battery output, it is necessary to reduce the resistance of the electrodes. The inventors of this invention have found that the technology described in Patent Document 1, which involves forming a carbon coating layer on the surface of the positive electrode current collector, is insufficient to achieve further resistance reduction while maintaining high energy density. Instead of providing a carbon coating layer composed solely of carbon, they have found that by providing a carbon concentration gradient region in the positive electrode mixture layer, a positive electrode with high energy density and low resistance can be obtained.

[0005] The purpose of this disclosure is to provide a positive electrode with high energy density and low resistance.

[0006] The positive electrode for a non-aqueous electrolyte secondary battery according to this disclosure comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive agent containing carbon, and a binder, and the positive electrode mixture layer comprises a normal region and a carbon concentration gradient region disposed between the normal region and the positive electrode current collector, wherein the carbon concentration in the carbon concentration gradient region is gradient such that it decreases from the positive electrode current collector side to the normal region side in the thickness direction of the carbon concentration gradient region, and the average carbon concentration in the carbon concentration gradient region is higher than the average carbon concentration in the normal region.

[0007] The non-aqueous electrolyte secondary battery according to this disclosure is characterized by comprising the above-mentioned positive electrode for non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

[0008] The positive electrode for non-aqueous electrolyte secondary batteries according to this disclosure makes it possible to lower the interfacial resistance while maintaining the energy density of the positive electrode, thereby achieving both high capacity and high output of the battery.

[0009] This is a longitudinal cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. This is a schematic diagram showing the results of measuring the cross-section of a positive electrode, which is an example of an embodiment, using an electron beam microanalyzer. This is an example of a diagram showing the change in carbon concentration in the positive electrode mixture layer along the thickness direction.

[0010] In recent years, there has been an increasing demand for higher output power in non-aqueous electrolyte secondary batteries. To increase battery output, it is necessary to reduce the resistance of the electrodes. Through our research, we have found that the method of forming a carbon coating layer composed solely of carbon on the surface of the positive electrode current collector, as described in Patent Document 1, cannot sufficiently reduce the resistance while maintaining the energy density of the positive electrode. After diligent research, we have found that by providing a carbon concentration gradient region in the positive electrode mixture layer, where the carbon concentration decreases from the positive electrode current collector side to the normal region, instead of providing a carbon coating layer, a positive electrode with high energy density and low resistance can be obtained.

[0011] The following describes in detail an example of an embodiment of a non-aqueous electrolyte secondary battery according to this disclosure. In the following description, a cylindrical battery in which a wound electrode body is housed in a cylindrical outer casing is given as an example, but the electrode body is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are stacked alternately one by one with a separator in between. Furthermore, the outer casing is not limited to a cylindrical shape and may be, for example, rectangular, coin-shaped, etc., or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0012] Figure 1 is an axial cross-sectional view of a cylindrical secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the secondary battery 10 comprises a wound electrode body 14, an electrolyte, and an outer casing 16 that houses the electrode body 14 and the electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is closed by a sealing body 17. In the following description, for convenience, the side of the battery with the sealing body 17 will be referred to as the top, and the bottom side of the outer casing 16 as the bottom.

[0013] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all rectangular elongated bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape in the longitudinal direction. The separator 13 separates the positive electrode 11 and the negative electrode 12 from each other. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal and transverse directions. The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode body 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the transverse direction of the positive electrode 11 and the negative electrode 12 is the axial direction. In other words, the end faces in the short direction of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode body 14.

[0014] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0015] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0016] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side, and functions as a safety valve. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0017] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the secondary battery 10, with particular emphasis on the positive electrode 11.

[0018] [Positive Electrode] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. Preferably, the positive electrode mixture layer is disposed on both sides of the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal disposed on its surface. The thickness of the positive electrode current collector is, for example, 10 μm or more and 30 μm or less.

[0019] The positive electrode mixture layer comprises a positive electrode active material, a conductive agent, and a binder. The positive electrode active material is the main component of the positive electrode mixture layer. Here, the main component refers to the component with the highest mass ratio among the constituent components of the positive electrode mixture layer. The positive electrode mixture layer preferably contains 80% by mass or more of the positive electrode active material, and more preferably 90% by mass or more, based on the total mass of the positive electrode mixture layer. The thickness of the positive electrode mixture layer is, for example, 10 μm to 150 μm on one side of the positive electrode current collector.

[0020] The positive electrode active material is composed mainly of, for example, a lithium transition metal composite oxide. Elements other than Li contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Si, and P. A suitable example of a lithium transition metal composite oxide is a composite oxide containing at least one of Ni, Co, and Mn. Specific examples include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al. The lithium transition metal composite oxide may have a layered structure. Examples of layered structures of the lithium transition metal composite oxide include a layered structure belonging to space group R-3m and a layered structure belonging to space group C2 / m. From the viewpoint of increasing capacity and ensuring crystal structure stability, it is preferable for the lithium transition metal composite oxide to have a layered structure belonging to space group R-3m.

[0021] The conductive agent contained in the positive electrode mixture layer includes, for example, carbon. The carbon includes, for example, at least one of carbon black and carbon nanotubes. Examples of carbon black include carbon-based particles such as acetylene black (AB), Ketjen black, and furnace black. These may be used individually or in combination of two or more types. The content of the conductive agent in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less, relative to the total mass of the positive electrode mixture layer.

[0022] Examples of binders included in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyimide resins, acrylic resins, polyolefin resins, and polyacrylonitrile (PAN). These may be used individually or in combination of two or more. The binder content in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less, relative to the total mass of the positive electrode mixture layer.

[0023] Figure 2 is a schematic diagram showing the results of measuring a cross-section of a positive electrode 11, which is an example of an embodiment, using an electron beam microanalyzer (EPMA). As described above, the positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 32 disposed on the positive electrode current collector 30. The positive electrode mixture layer 32 has a normal region 36 and a carbon concentration gradient region 34 disposed between the normal region 36 and the positive electrode current collector 30. The carbon concentration in the carbon concentration gradient region 34 is gradient in the thickness direction of the carbon concentration gradient region 34, decreasing from the positive electrode current collector 30 side to the normal region 36 side, and the average carbon concentration in the carbon concentration gradient region 34 is higher than the average carbon concentration in the normal region 36. As a result, the interfacial resistance of the positive electrode 11 can be lowered, and the output of the secondary battery 10 can be increased. In a positive electrode 11 provided with a carbon concentration gradient region 34, it is thought that the interfacial resistance is greatly reduced because the contact area between the positive electrode active material and the conductive agent is greatly increased near the positive electrode current collector 30. Here, the average carbon concentration in the carbon concentration gradient region 34 and the average carbon concentration in the normal region 36 refer to the average value of the carbon concentration in the carbon concentration gradient region 34 and the average value of the carbon concentration in the normal region 36, respectively. Furthermore, since the carbon concentration gradient region 34 contains the positive electrode active material, its energy density is higher than that of a carbon coating layer consisting only of carbon. As a result, even if the thickness of the carbon concentration gradient region 34 is greater than the thickness of the carbon coating layer, a positive electrode having the carbon concentration gradient region 34 can have an energy density comparable to that of a positive electrode having a carbon coating layer. As described above, by using a positive electrode 11 having a carbon concentration gradient region 34, it is possible to achieve both high capacity and high output in the battery.

[0024] The thickness of the carbon concentration gradient region 34 is, for example, 5 μm to 20 μm. The ratio of the thickness of the carbon concentration gradient region 34 to the thickness of the normal region 36 is 5:95 to 50:50.

[0025] As shown in FIG. 2, a part of the positive electrode active material 40 included in the carbon concentration gradient region 34 may be in contact with the positive electrode current collector 30. Further, the positive electrode active material 40 in contact with the positive electrode current collector 30 may be fitted into the positive electrode current collector 30. Note that the embodiment of the positive electrode 11 is not limited to the embodiment shown in FIG. 2.

[0026] As shown in FIG. 2, EPMA measurement expresses the distribution state of carbon 42 as a conductive agent. Specifically, after a measurement range is determined by scanning electron microscope (SEM) observation, EPMA measurement is performed for C, and the distribution state of carbon 42 can be obtained by superimposing the results of the EPMA measurement and the SEM image. In the carbon concentration gradient region 34, carbon 42 is present in most of the voids between the positive electrode active materials 40, whereas in the normal region 36, the proportion of voids between the positive electrode active materials 40 where carbon 42 is present is reduced compared to that in the carbon concentration gradient region 34.

[0027] FIG. 3 is an example of a diagram showing a change in carbon concentration in the positive electrode mixture layer 32 along the thickness direction. In the carbon concentration gradient region 34, which is a range from the surface of the positive electrode current collector 30 to a predetermined thickness, the carbon concentration decreases as the distance from the surface of the positive electrode current collector 30 increases. In the normal region 36 adjacent to the carbon concentration gradient region 34, the carbon concentration is substantially constant. In the example shown in FIG. 3, the minimum value of the carbon concentration in the carbon concentration gradient region 34 is higher than the carbon concentration in the normal region 36. Note that the mode of change in carbon concentration in the positive electrode mixture layer 32 is not limited to the example shown in FIG. 3. For example, in FIG. 3, the change in carbon concentration in the carbon concentration gradient region 34 changes linearly, but may change in a curved manner. Further, the carbon concentration in the normal region 36 does not have to be constant.

[0028] The change in carbon concentration in the positive electrode mixture layer 32 along the thickness direction can be measured as follows.

[0029] [Method for measuring change in carbon concentration] (1) Mapping measurement is performed on a cross-section of the positive electrode 11 using EPMA. The size of this mapping image is, for example, 150 µm long × 100 µm wide. (2) This mapping image is divided at a pitch of 1 µm in the thickness direction from the surface of the positive electrode current collector 30, and the sum of carbon detection intensities (C value) is calculated for each divided section. (3) By plotting this C value against the distance from the positive electrode current collector, a diagram as shown in FIG. 3 can be obtained. Note that since the carbon concentration in the normal region 36 is substantially uniform, after starting measurement from the current collector side and identifying the boundary between the carbon concentration gradient region 34 and the normal region 36, the measurement for the normal region 36 may be terminated.

[0030] An example of a method for producing the positive electrode 11 will be described. First, carbon 42, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) are mixed to prepare a first mixture slurry. Separately, a positive electrode active material 40, carbon 42, a binder, and a solvent such as NMP are mixed to prepare a second mixture slurry. Then, using a multi-layer die coater, the first mixture slurry and the second mixture slurry are simultaneously applied to both surfaces of the positive electrode current collector 30, and dried. At this time, the first mixture slurry is applied onto the surface of the positive electrode current collector 30, and the second mixture slurry is applied onto the surface of the first mixture slurry. The positive electrode 11 can be produced by rolling this coating film with a rolling roller. By using a multi-layer die coater, the first mixture slurry penetrates between the positive electrode active materials contained in the second mixture slurry, and the carbon concentration gradient region 34 is formed.

[0031] [Negative Electrode] The negative electrode 12 may, for example, have a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, or a metallic Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode current collector, and lithium metal may be deposited on the surface of the negative electrode current collector by charging. When the negative electrode 12 has a negative electrode mixture layer, it is preferable that the negative electrode mixture layer is disposed on both sides of the negative electrode current collector. For the negative electrode current collector, a foil of a metal that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal disposed on its surface, can be used. The thickness of the negative electrode current collector is, for example, 5 μm to 30 μm. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer is, for example, 10 μm to 150 μm on one side of the negative electrode current collector. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., to the surface of a negative electrode current collector, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0032] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si and Sn, or lithium titanium composite oxides may be used as the negative electrode active material. Furthermore, materials with a carbon coating may also be used. For example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0033] Examples of binders included in the negative electrode mixture layer include styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0034] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0035] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0036] [Non-aqueous electrolytes] Non-aqueous electrolytes, for example, have lithium ion conductivity. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0037] A liquid electrolyte (electrolyte solution) includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0038] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters 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 (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0039] Examples of the above ethers include: cyclic ethers such as 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, and crown ether; and chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl 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.

[0040] The electrolyte salt is preferably a lithium salt. As the lithium salt, LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic carboxylic acid lithium, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of the phosphate include lithium difluorophosphate (LiPO ２ F ２Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. Of these, LiPF is used from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ It is preferable to use the following. The concentration of the lithium salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 1.8 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0041] Non-aqueous electrolytes may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0042] Examples of unsaturated cyclic carbonate esters include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. Unsaturated cyclic carbonate esters may be used individually or in combination of two or more. Some hydrogen atoms in the unsaturated cyclic carbonate esters may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by the intermolecular condensation of multiple carboxylic acid molecules, but it is preferable that it be an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0043] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0044] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanate methylcyclohexane (BIMCH). Examples of sultone compounds include propanesultone and propensultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethylborate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethylphosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethylphosphite and tris(trimethylsilyl)phosphite.

[0045] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. 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. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0046] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0047] <Example 1> [Fabrication of the positive electrode] As the positive electrode active material, aluminum-containing lithium nickel cobalt oxide (LiNi ０．８０ Co ０．１５ Al ０．０５ O ２The following was used: Acetylene black (AB) and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 4:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare the first combination slurry. The positive electrode active material, AB, and PVDF were mixed in a mass ratio of 98:1:1, and an appropriate amount of NMP was added to prepare the second combination slurry.

[0048] Using a multilayer die coater, the first mixture slurry was applied to the surface of the aluminum foil positive electrode current collector, and the second mixture slurry was applied to the surface of the first mixture slurry, thereby simultaneously coating both sides of the positive electrode current collector with the first and second mixture slurry, and allowing them to dry. Subsequently, the coating film was rolled with a rolling roller to produce the positive electrode. The thickness of the positive electrode mixture layer (i.e., the sum of the normal region and the carbon concentration gradient region) on one side of the positive electrode current collector was 70 μm, of which the thickness of the carbon concentration gradient region was 10 μm.

[0049] [Measurement of interfacial resistance] The interfacial resistance of the positive electrode prepared as described above was measured using an electrode resistance meter (device name: RM2610) manufactured by HIOKI E.E. CORPORATION.

[0050] [Measurement of Peel Strength] The positive electrode prepared above was cut to a predetermined size to form a test specimen. Using Nitto Denko double-sided tape #515, the positive electrode mixture layer on one side of the test specimen was attached to a stainless steel substrate with a smooth surface, so that the stainless steel substrate to which the test specimen was fixed was horizontal. One end of the positive electrode current collector in the longitudinal direction of the test specimen was fixed to a movable jig of a tensile testing machine (Tensilon universal testing machine STB-1225L manufactured by A&D Co., Ltd.). The setting was configured to peel the positive electrode current collector in a direction of 90° to the substrate surface of the stainless steel substrate, and then the movable jig was moved to peel the positive electrode mixture layer and the positive electrode current collector of the test specimen at a speed of 200 mm / min. At that time, the tensile direction was always maintained at 90° to the substrate surface of the stainless steel substrate to which the test specimen was fixed. The stable peel strength value was read when 30 mm or more of the test specimen had been peeled off. The above measurements were performed on five test specimens, and the average of the measured values ​​was defined as the peel strength (N / m).

[0051] <Example 2> In preparing the positive electrode, the amount of the second mixture slurry applied was adjusted so that the thickness of the positive electrode mixture layer was the same as in Example 1, while tripling the amount of the first mixture slurry applied. The positive electrode was then prepared and evaluated in the same manner as in Example 1. The thickness of the positive electrode mixture layer (i.e., the sum of the normal region and the carbon concentration gradient region) arranged on one side of the positive electrode current collector was 70 μm, of which the thickness of the carbon concentration gradient region was 30 μm.

[0052] <Comparative Example 1> In the preparation of the positive electrode, the positive electrode was prepared and evaluated in the same manner as in Example 1, except that the first mixture slurry was not used, and only the second mixture slurry was applied so that the thickness of the positive electrode mixture layer was the same as in Example 1. The thickness of the positive electrode mixture layer placed on one side of the positive electrode current collector was 70 μm, and there was no carbon concentration gradient region.

[0053] <Comparative Example 2> In the preparation of the positive electrode, the positive electrode was prepared in the same manner as in Example 1, except that the first mixture slurry and the second mixture slurry were not applied simultaneously, but the first mixture slurry was applied to both sides of the aluminum foil, dried, then the second mixture slurry was applied and dried, and then the coating film was rolled with a rolling roller to prepare the positive electrode. The application amounts of the first mixture slurry and the second mixture slurry were the same as in Example 1. The thickness of the carbon coat layer placed on one side of the positive electrode current collector was 3 μm, and the thickness of the positive electrode mixture layer placed on the carbon coat layer was 67 μm. The positive electrode mixture layer did not have a carbon concentration gradient region and consisted only of a normal region. That is, the carbon coat layer was formed from the first mixture slurry, and the positive electrode mixture layer did not have a carbon concentration gradient region and was formed from the second mixture slurry.

[0054] Table 1 shows the evaluation results of the positive electrodes of the Examples and Comparative Examples. In Table 1, the interfacial resistance, peel strength, and unipolar capacity of the positive electrodes of Examples and Comparative Example 2 are expressed relatively, with the interfacial resistance, peel strength, and unipolar capacity of the positive electrode of Comparative Example 1 each set to 100. Here, the total capacity was calculated from the amount of positive electrode active material contained in the positive electrode mixture layer, and this total capacity was normalized by the weight per unit area of ​​the positive electrode to obtain the unipolar capacity.

[0055]

[0056] As shown in Table 1, the positive electrodes of the examples all suppress interfacial resistance while maintaining a similar unipolar capacitance compared to the positive electrodes of the comparative examples. The positive electrode of Comparative Example 2 has lower interfacial resistance than the positive electrode of Comparative Example 1, but the effect of suppressing interfacial resistance is smaller compared to the positive electrodes of the examples. The application amounts of the first and second mixture slurries were the same in Example 1 and Comparative Example 2, and the peel strength of the positive electrode of the examples was improved to the same level as that of Comparative Example 2, which has a carbon coating layer on the surface of the positive electrode current collector.

[0057] This disclosure is further illustrated by the following embodiments. Configuration 1: A positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive agent containing carbon, and a binder, the positive electrode mixture layer having a normal region and a carbon concentration gradient region disposed between the normal region and the positive electrode current collector, the carbon concentration in the carbon concentration gradient region being gradient such that it decreases from the positive electrode current collector side to the normal region side in the thickness direction of the carbon concentration gradient region, and the average carbon concentration in the carbon concentration gradient region being higher than the average carbon concentration in the normal region. Configuration 2: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the thickness of the carbon concentration gradient region is 5 μm or more and 20 μm or less. Configuration 3: A positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the ratio of the thickness of the carbon concentration gradient region to the normal region is 5:95 to 50:50. Configuration 4: A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the carbon comprises at least one of carbon black and carbon nanotubes. Configuration 5: A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein a portion of the positive electrode active material contained in the carbon concentration gradient region is in contact with the positive electrode current collector. Configuration 6: A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 5, a negative electrode, and a non-aqueous electrolyte.

[0058] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive agent containing carbon, and a binder, the positive electrode mixture layer has a normal region and a carbon concentration gradient region disposed between the normal region and the positive electrode current collector, the carbon concentration in the carbon concentration gradient region is gradient such that it decreases from the positive electrode current collector side to the normal region side in the thickness direction of the carbon concentration gradient region, and the average carbon concentration in the carbon concentration gradient region is higher than the average carbon concentration in the normal region.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the thickness of the carbon concentration gradient region is 5 μm or more and 20 μm or less.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of the thickness of the carbon concentration gradient region to the thickness of the normal region is 5:95 to 50:

50.

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon comprises at least one of carbon black and carbon nanotubes.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a portion of the positive electrode active material contained in the carbon concentration gradient region is in contact with the positive electrode current collector.

6. A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, a negative electrode, and a non-aqueous electrolyte.

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