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

Abstract

A positive electrode for a nonaqueous electrolyte secondary battery according to an exemplary embodiment comprises a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector. The positive electrode mixture layer contains a first positive electrode active material, a second positive electrode active material, a conductive agent that contains carbon, and a binder. The positive electrode mixture layer has a normal region and a carbon concentration gradient region located between the normal region and the positive electrode current collector. The carbon concentration in the carbon concentration gradient region is graded so as to decrease from the positive electrode current collector toward the normal region in the thickness direction of the carbon concentration gradient region. The average carbon concentration of the carbon concentration gradient region is higher than the average carbon concentration of the normal region. The carbon concentration gradient region contains the first positive electrode active material and the second positive electrode active material. The normal region contains the first positive electrode active material and does not contain the second positive electrode active material.

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, it is possible to further reduce the resistance of the positive electrode.

[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 surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a first positive electrode active material and a second positive electrode active material, a conductive agent containing carbon, and a binder, wherein 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, the average carbon concentration in the carbon concentration gradient region is higher than the average carbon concentration in the normal region, the carbon concentration gradient region comprises the first positive electrode active material and the second positive electrode active material, and the normal region comprises the first positive electrode active material but does not contain the second positive electrode active material.

[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 electrode 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. Furthermore, we have found that by including a positive electrode active material that has high resistance but also high capacity and high durability in the carbon concentration gradient region, it is possible to achieve high capacity and high durability while maintaining low resistance.

[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 first positive electrode active material and a second positive electrode active material, a conductive agent, and a binder. The positive electrode active material, consisting of the first and second positive electrode active materials, is the main component of the positive electrode mixture layer. Here, the main component means 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 NCM-type lithium transition metal composite oxides containing Ni, Co, and Mn, and NCA-type lithium transition metal composite oxides containing Ni, Co, and Al (but not Mn). That is, a lithium transition metal composite oxide containing Ni, Co, Mn, and Al is defined as an NCM-type lithium transition metal composite oxide. The lithium transition metal composite oxide may have a layered structure. Examples of layered structures for lithium transition metal composite oxides include layered structures belonging to space group R-3m and layered structures belonging to space group C2 / m. From the viewpoint of increasing capacity and crystalline structure stability, lithium transition metal composite oxides are preferably found 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 electrode resistance of the positive electrode 11 can be lowered, and the output power 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 or more and 20 μm or less. Further, the ratio of the thickness of the carbon concentration gradient region 34 to that of the normal region 36 is 5:95 to 50:50.

[0025] The carbon concentration gradient region 34 contains a first positive electrode active material and a second positive electrode active material, and the normal region 36 contains the first positive electrode active material and does not contain the second positive electrode active material. The first positive electrode active material is, for example, the above-mentioned lithium transition metal composite oxide.

[0026] The second positive electrode active material may be a lithium transition metal composite oxide having a higher capacity than the first positive electrode active material. Generally, a high-capacity lithium transition metal composite oxide has higher resistance. Even if the second positive electrode active material has higher resistance than the first positive electrode active material, the carbon concentration gradient region 34 contains more carbon than the normal region 36, so electrode resistance can be suppressed. Therefore, when the positive electrode 11 has the above configuration, both lower resistance and higher capacity of the positive electrode can be achieved at the same time.

[0027] As an example, there may be mentioned a case where the first positive electrode active material is a lithium transition metal composite oxide containing Ni, Co and Al and not containing Mn, and the second positive electrode active material is a lithium transition metal composite oxide containing Ni, Co and Mn. As another example, there may be mentioned a case where the Ni content in the second positive electrode active material is higher than the Ni content in the first positive electrode active material. The Ni content in the second positive electrode active material is, for example, 70% by mass or more and 95% by mass or less, and the Ni content in the first positive electrode active material is, for example, 50% by mass or more and 80% by mass or less. Here, the Ni content refers to the ratio of the molar amount of Ni to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide.

[0028] The second positive electrode active material may be a lithium transition metal composite oxide having higher durability than the first positive electrode active material. Generally, a highly durable lithium transition metal composite oxide has higher resistance. Even if the second positive electrode active material has higher resistance than the first positive electrode active material, the carbon concentration gradient region 34 contains more carbon than the normal region 36, so that electrode resistance can be suppressed. Therefore, when the positive electrode 11 has the above configuration, both low resistance and high durability of the positive electrode can be achieved at the same time.

[0029] For example, the first positive electrode active material is polycrystalline, and the second positive electrode active material is single crystal. Single crystal refers to particles constituted by a single crystal structure, while polycrystal refers to secondary particles formed by aggregation of single crystals (primary particles) having a small particle diameter. In polycrystals, repeated charge and discharge may cause cracks between primary particles, leading to a decrease in battery capacity. In contrast, single crystals are composed only of primary particles and do not cause the above phenomenon, so high durability can be achieved. The particle diameter of the primary particles constituting the secondary particles is, for example, 0.05 μm to 1 μm. The particle diameter of a primary particle is measured as the diameter of a circumscribed circle in a particle image observed with a scanning electron microscope (SEM).

[0030] As shown in FIG. 2, a part of the positive electrode active material 40 contained in the carbon concentration gradient region 34 may be in contact with the positive electrode current collector 30. The positive electrode active material 40 contained in the carbon concentration gradient region 34 may be either the first positive electrode active material 40a or the second positive electrode active material 40b, and the positive electrode active material 40 contained in the normal region 36 is the first positive electrode active material 40a. In FIG. 2, the first positive electrode active material 40a is shown as unhatched particles, and the second positive electrode active material 40b is shown as hatched particles. As shown in FIG. 2, a part of the first positive electrode active material 40a 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. The embodiment of the positive electrode 11 is not limited to the embodiment shown in FIG. 2.

[0031] As shown in Figure 2, the distribution of carbon 42 as a conductive agent is revealed by EPMA measurement. Specifically, after determining the measurement range by scanning electron microscopy (SEM) observation, EPMA measurement is performed on C, and the distribution of carbon 42 can be determined by overlaying the EPMA measurement results with the SEM image. In the carbon concentration gradient region 34, carbon 42 is present in almost all of the voids between the positive electrode active materials 40, but in the normal region 36, the proportion of carbon 42 present in the voids between the positive electrode active materials 40 is reduced compared to the carbon concentration gradient region 34.

[0032] Figure 3 is an example of a diagram showing the change in carbon concentration in the positive electrode mixture layer 32 along the thickness direction. In the carbon concentration gradient region 34, which is the 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 approximately constant. In the example shown in Figure 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 manner of change in the carbon concentration in the positive electrode mixture layer 32 is not limited to the example shown in Figure 3. For example, in Figure 3, the change in carbon concentration in the carbon concentration gradient region 34 changes linearly, but it may also change curvilinearly. Also, the carbon concentration in the normal region 36 does not have to be constant.

[0033] The change in carbon concentration in the positive electrode mixture layer 32 along the thickness direction can be measured as follows.

[0034] [Method for measuring changes in carbon concentration] (1) Using EPMA, perform mapping measurements of the cross-section of the positive electrode 11. The size of this mapping image is, for example, 150 μm vertically × 100 μm horizontally. (2) Divide this mapping image into sections at 1 μm intervals in the thickness direction from the surface of the positive electrode current collector 30, and calculate the sum of the carbon detection intensity (C value) for each section. (3) By plotting this C value against the distance from the positive electrode current collector, a figure like the one shown in Figure 3 can be obtained. Note that since the carbon concentration in the normal region 36 is approximately uniform, measurements can be started from the current collector side, and once the boundary between the carbon concentration gradient region 34 and the normal region 36 is determined, measurements for the normal region 36 may be terminated.

[0035] An example of a method for manufacturing the positive electrode 11 is described below. First, the second positive electrode active material, carbon, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) are mixed to prepare the first mixture slurry. Then, the first positive electrode active material, carbon, a binder, and a solvent such as NMP are mixed to prepare the second mixture slurry. Next, the first mixture slurry and the second mixture slurry are simultaneously applied to both sides of the positive electrode current collector 30 using a multilayer die coater and dried. At this time, the first mixture slurry is applied to the surface of the positive electrode current collector 30, and the second mixture slurry is applied to the surface of the first mixture slurry. The positive electrode 11 can be manufactured by rolling this coating with a rolling roller. By using a multilayer die coater, the first mixture slurry penetrates between the first positive electrode active material particles contained in the second mixture slurry, forming a carbon concentration gradient region 34. As a result, the carbon concentration gradient region 34 contains both the first positive electrode active material and the second positive electrode active material.

[0036] [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.

[0037] 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.

[0038] 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.

[0039] [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.

[0040] 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.

[0041] [Non-aqueous electrolytes] Non-aqueous electrolytes, for example, have lithium ion conductivity. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0042] 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).

[0043] 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).

[0044] 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.

[0045] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include 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 , lithium lower aliphatic carboxylate, LiCl, LiBr, LiI, phosphate, borate, and imide salt. As the phosphate, 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 imide lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) and others 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.

[0046] 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.

[0047] 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.

[0048] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0049] 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.

[0050] 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.

[0051] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0052] <Examples> [Fabrication of the positive electrode] As the first positive electrode active material, aluminum-containing lithium nickel cobalt oxide (LiNi ０．８０ Co ０．１５ Al ０．０５ O ２) is used, and as the second positive electrode active material, manganese-containing lithium nickel cobalt oxide (LiNi ０．８０ Co ０．１５ Mn ０．０５ O ２ The following was used: The second positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 10:72:18, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare the first combination slurry. The first 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.

[0053] 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.

[0054] [Measurement of Interfacial Resistance and Compound Resistance] The positive electrode prepared as described above was measured for interfacial resistance and compound resistance using an electrode resistance meter (device name: RM2610) manufactured by HIOKI E.E. CORPORATION. Interfacial resistance is the resistance value at the interface between the current collector and the compound layer, and compound resistance is the resistance value of the compound layer.

[0055] [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).

[0056] <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 (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, and there was no carbon concentration gradient region.

[0057] <Comparative Example 2> In the preparation of the positive electrode, the composition of the first mixture slurry was changed, and the application method of the first and second mixture slurry was changed, but the positive electrode was prepared and evaluated in the same manner as in Example 1. The first mixture slurry was prepared by mixing AB and PVDF in a mass ratio of 4:1 without using the second positive electrode active material. The first and second mixture slurry were not applied simultaneously. The first mixture slurry was applied to both sides of the aluminum foil and 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 and 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. Specifically, the carbon coating layer was formed from the first mixture slurry, and the positive electrode mixture layer, which did not have a carbon concentration gradient region, was formed from the second mixture slurry.

[0058] <Comparative Example 3> In the preparation of the positive electrode, the first mixture slurry was prepared by mixing AB and PVDF in a mass ratio of 4:1 without using the second positive electrode active material, except that the positive electrode was prepared in the same manner as in Example 1 and evaluated. 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, and the thickness of the carbon concentration gradient region was 10 μm.

[0059] <Comparative Example 4> In the preparation of the positive electrode, the composition of the first and second mixture slurries was changed, but the positive electrode was prepared and evaluated in the same manner as in Example 1. The first mixture slurry was prepared by mixing AB and PVDF in a mass ratio of 4:1 without using the second positive electrode active material. The second mixture slurry was prepared by mixing the first and second positive electrode active materials in a mass ratio of 1:9 to create a mixed positive electrode active material, and then mixing this mixed positive electrode active material with AB and PVDF in a mass ratio of 98:1:1. 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, and the thickness of the carbon concentration gradient region was 10 μm.

[0060] Table 1 shows the evaluation results of the positive electrodes of the examples and comparative examples. In Table 1, the interfacial resistance, composite resistance, peel strength, and unipolar capacity of the positive electrodes of the examples and comparative examples 2 to 4 are expressed relatively, with the interfacial resistance, composite 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 composite layer, and this total capacity was normalized by the weight per unit area of ​​the positive electrode to obtain the unipolar capacity.

[0061]

[0062] As shown in Table 1, the positive electrode of the example suppresses interfacial resistance and composite resistance while maintaining a similar unipolar capacitance to the positive electrode of the comparative example, thereby suppressing electrode resistance. Furthermore, the positive electrode of the example also exhibits improved peel strength to a level comparable to that of Comparative Example 2, which has a carbon coating layer on the surface of the positive electrode current collector.

[0063] 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 surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a first positive electrode active material and a second positive electrode active material, a conductive agent containing carbon, and a binder, 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, 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, the average carbon concentration in the carbon concentration gradient region is higher than the average carbon concentration in the normal region, the carbon concentration gradient region comprises the first positive electrode active material and the second positive electrode active material, and the normal region comprises the first positive electrode active material but does not contain the second positive electrode active material. 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: The 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 thickness of the normal region is 5:95 to 50:50. Configuration 4: The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the carbon includes at least one of carbon black and carbon nanotubes. Configuration 5: The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein a portion of the first positive electrode active material or the second positive electrode active material contained in the carbon concentration gradient region is in contact with the positive electrode current collector. Configuration 6: The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the first positive electrode active material is a lithium transition metal composite oxide. Configuration 7: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 6, wherein the second positive electrode active material is a lithium transition metal composite oxide having a higher capacity than the first positive electrode active material. Configuration 8: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 7, wherein the first positive electrode active material is a lithium transition metal composite oxide containing Ni, Co, and Al, but not Mn, and the second positive electrode active material is a lithium transition metal composite oxide containing Ni, Co, and Mn.Configuration 9: A positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 7 or 8, wherein the Ni content in the second positive electrode active material is higher than the Ni content in the first positive electrode active material. Configuration 10: A positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 6, wherein the second positive electrode active material is a lithium transition metal composite oxide having higher durability than the first positive electrode active material. Configuration 11: A positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 10, wherein the first positive electrode active material is multi-crystal and the second positive electrode active material is single-crystal. Configuration 12: 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 11, a negative electrode, and a non-aqueous electrolyte.

[0064] 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 surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a first positive electrode active material and a second positive electrode active material, a conductive agent containing carbon, and a binder, 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, 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, the average carbon concentration in the carbon concentration gradient region is higher than the average carbon concentration in the normal region, the carbon concentration gradient region comprises the first positive electrode active material and the second positive electrode active material, and the normal region comprises the first positive electrode active material and does not contain the second positive electrode active material.

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

6. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the first positive electrode active material is a lithium transition metal composite oxide.

7. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 6, wherein the second positive electrode active material is a lithium transition metal composite oxide having a higher capacity than the first positive electrode active material.

8. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 7, wherein the first positive electrode active material is a lithium transition metal composite oxide containing Ni, Co, and Al, but not Mn, and the second positive electrode active material is a lithium transition metal composite oxide containing Ni, Co, and Mn.

9. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 7, wherein the Ni content in the second positive electrode active material is higher than the Ni content in the first positive electrode active material.

10. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 6, wherein the second positive electrode active material is a lithium transition metal composite oxide having higher durability than the first positive electrode active material.

11. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 10, wherein the first positive electrode active material is a multi-crystal and the second positive electrode active material is a single crystal.

12. 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 11, a negative electrode, and a non-aqueous electrolyte.

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