Method for producing positive electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

This method for producing a positive electrode (11) includes: a step of preparing an electroconductive agent dispersion by adding carbon nanotubes having an average fiber length of 150 μm to 300 μm and a dispersant to a dispersion medium; a step of formulating a positive electrode slurry by mixing a positive electrode mixture with the electroconductive agent dispersion; and a step of forming a positive electrode mixture layer on the positive electrode core by applying the positive electrode slurry onto a positive electrode core and drying the coating film. The positive electrode active material includes: a first positive electrode active material having the shape of a secondary particle formed by aggregation of primary particles; and a second positive electrode active material having a single-particle shape. The content of the second positive electrode active material is 10-35 mass% relative to the total mass of the positive electrode active material.

Description

Method for manufacturing a positive electrode for a non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

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

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, high durability, and rapid charging performance, such as in-vehicle applications and power storage applications. The positive electrode, a major component of non-aqueous electrolyte secondary batteries, significantly affects these performances, and therefore much research has been conducted on the positive electrode. For example, Patent Document 1 discloses a positive electrode containing a positive electrode active material with a high Ni content and carbon nanotubes. Using a positive electrode active material with a high Ni content contributes to increasing the capacity of the battery, but in this case, it is not easy to achieve both rapid charging performance and high durability.

[0003] Japanese Patent Application Laid-Open No. 2021-150051

[0004] As disclosed in Patent Document 1, adding carbon nanotubes to a positive electrode is expected to have the effect of reducing the resistance of the positive electrode. However, conventional technologies including the positive electrode of Patent Document 1 still have a lot of room for improvement in terms of reducing the resistance required to achieve rapid charging performance. An object of the present disclosure is to provide a high-capacity nonaqueous electrolyte secondary battery with low initial resistance and excellent durability.

[0005] A method for manufacturing a positive electrode for a non-aqueous electrolyte secondary battery according to the present disclosure includes the steps of: preparing a positive electrode mixture containing a positive electrode active material and a binder; preparing a conductive agent dispersion by adding carbon nanotubes having an average fiber length of 150 μm or more and 300 μm or less and a dispersant to a dispersion medium; mixing the positive electrode mixture and the conductive agent dispersion to prepare a positive electrode slurry; and applying the positive electrode slurry onto a positive electrode core and drying the coating to form a positive electrode mixture layer on the positive electrode core, wherein the positive electrode active material includes a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles and a second positive electrode active material having a single particle shape, and the first positive electrode active material has a composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ(0.9<a≦1.1, 0.87≦b≦0.93, 0<c<0.08, 0≦d≦0.08, 1.9≦e≦2.1), and the second positive electrode active material is a composite oxide represented by the composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.80≦b≦0.87, 0<c<0.10, 0≦d≦0.10, 1.9≦e≦2.1), and the content of the second positive electrode active material is 10% by mass or more and 35% by mass or less with respect to the total mass of the positive electrode active material.

[0006] A non-aqueous electrolyte secondary battery according to the present disclosure is a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, the positive electrode mixture layer including a positive electrode active material, a binder, and carbon nanotubes, the positive electrode active material including a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles, and a second positive electrode active material having a single particle shape, and the first positive electrode active material has a composition formula: Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.87≦b≦0.93, 0<c<0.08, 0≦d≦0.08, 1.9≦e≦2.1), and the second positive electrode active material is a composite oxide represented by the composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.80≦b≦0.87, 0<c<0.10, 0≦d≦0.10, 1.9≦e≦2.1), the content of the second positive electrode active material is 10% by mass or more and 35% by mass or less with respect to the total mass of the positive electrode active material, and the carbon nanotubes have an average fiber length of 2 μm or more and 10 μm or less in the positive electrode mixture layer.

[0007] According to one aspect of the present disclosure, a nonaqueous electrolyte secondary battery having low initial resistance, excellent durability, and high capacity can be provided. The nonaqueous electrolyte secondary battery according to the present disclosure has excellent charge-discharge cycle characteristics, for example, in a high-temperature environment.

[0008] 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention; FIG. 2 is a front view of a positive electrode according to an embodiment of the present invention; FIG. 3 is a front view of a positive electrode according to another embodiment of the present invention;

[0009] As described above, there is a demand for high-capacity nonaqueous electrolyte secondary batteries with low initial resistance and excellent durability. To achieve such batteries, it is important to keep the direct current resistance (DCIR) of the battery low. The present inventors conducted extensive research to solve this problem and found that by mixing a first active material having a secondary particle shape and a high Ni content with a second active material having a single particle shape and a high Ni content in a specific mass ratio, and further manufacturing a positive electrode using carbon nanotubes with an average fiber length of 150 μm or more and 300 μm or less, the DCIR can be effectively reduced while maintaining good durability.

[0010] Hereinafter, with reference to the drawings, an example of an embodiment of a method for manufacturing a positive electrode for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode will be described in detail. Note that the scope of the present disclosure includes embodiments obtained by selectively combining the respective components of the multiple embodiments and modifications described below.

[0011] In the following, a nonaqueous electrolyte secondary battery 10 is exemplified, which is a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom, but the outer can of the battery is not limited to a cylindrical outer can. Other embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure include, for example, a prismatic battery having a prismatic outer can, a coin-shaped battery having a coin-shaped outer can, and a pouch-shaped battery having an outer can made of a laminate sheet including a metal layer and a resin layer. In addition, the electrode assembly is not limited to a wound type, and may be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0012] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The nonaqueous electrolyte secondary battery 10 is, for example, a lithium-ion secondary battery. The electrode assembly 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 spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom and an open end in the axial direction. The opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0013] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. 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 the length and width directions. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has 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.

[0014] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0015] The liquid electrolyte (electrolytic solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0016] 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, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, 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 the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, 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. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14, with the positive electrode 11 being particularly described in detail below.

[0021] [Positive Electrode] FIGS. 2 and 3 are front views of the positive electrode 11. As shown in FIGS. 2 and 3, the positive electrode 11 includes a positive electrode core 30 and a positive electrode mixture layer 31 provided on the positive electrode core 30. The positive electrode core 30 can be made of a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface. The positive electrode mixture layer 31 preferably contains a positive electrode active material, a binder, and carbon nanotubes (hereinafter referred to as "CNTs"), and is provided on both sides of the positive electrode core 30. The CNTs function as a conductive agent and form a good conductive path in the positive electrode mixture layer 31.

[0022] The positive electrode 11 has an exposed portion 32 where the surface of the positive electrode core 30 is exposed. The exposed portion 32 is a portion to which the positive electrode lead 20 is connected, and is provided by not forming a positive electrode mixture layer 31 on the positive electrode core 30. The positive electrode 11 can be produced, for example, by applying a positive electrode slurry containing a positive electrode active material, CNTs, etc. to the positive electrode core 30, leaving a portion that will become the exposed portion 32, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0023] In the example shown in FIG. 2 , an exposed portion 32 is formed at one widthwise end of the positive electrode 11. More specifically, the exposed portion 32 is formed in the width direction from one widthwise end of the positive electrode 11, with a length of 50% or less of the overall width of the positive electrode core 30. The length of the exposed portion 32 along the widthwise direction of the positive electrode 11 is preferably 17% to 50%, or 20% to 40%, of the overall width of the positive electrode core 30. In this case, the positive electrode mixture layer 31 includes a first region aligned with the exposed portion 32 in the longitudinal direction of the positive electrode 11, and a second region aligned with the exposed portion 32 and the first region in the widthwise direction of the positive electrode 11. Note that the capacity of each region can be made different by changing the type and compounding ratio of the positive electrode active material contained in the first region and the second region.

[0024] In the example shown in Fig. 3, the exposed portion 32 is formed from one end to the other end in the width direction of the positive electrode 11. The exposed portion 32 is formed, for example, with a substantially constant width across the entire width of the positive electrode 11. When the exposed portion 32 is provided in only a portion of the width direction of the positive electrode 11 as in the example shown in Fig. 2, the formation area of ​​the positive electrode mixture layer 31 is larger than when the exposed portion 32 is provided across the entire width of the positive electrode 11 as in the example shown in Fig. 3, and therefore, the capacity of the battery can be increased.

[0025] Examples of binders contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer 31.

[0026] The positive electrode mixture layer 31 contains at least CNT as a conductive agent. Addition of a small amount of CNT forms a good conductive path in the positive electrode mixture layer 31, reducing the DCIR of the battery and contributing to improving the charge-discharge cycle characteristics. The positive electrode mixture layer 31 may also contain a conductive agent other than CNT, such as carbon black such as acetylene black or ketjen black, graphite, carbon nanofiber, or carbon fiber other than CNT, such as graphene. However, the main component of the conductive agent is preferably CNT, and the positive electrode mixture layer 31 may contain substantially only CNT as the conductive agent.

[0027] The CNT content is preferably 0.01% by mass or more and 5% by mass or less, more preferably 0.1% by mass or more and 2% by mass or less, and particularly preferably 0.3% by mass or more and 1.0% by mass or less, relative to the mass of the positive electrode mixture layer 31. If the CNT content is within this range, it becomes easy to achieve both low resistance (rapid charging performance) and high durability. As will be described in detail later, CNTs having an average fiber length of 150 μm or more and 300 μm or less are used to manufacture the positive electrode 11. In this case, the average fiber length of the CNTs in the positive electrode mixture layer 31 is 2 μm or more and 10 μm or less, and a good conductive path is formed.

[0028] The positive electrode active material includes a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles, and a second positive electrode active material having a single particle shape. The first and second positive electrode active materials are particles of a lithium-nickel composite oxide in which the molar ratio of Ni to the total molar amount of elements excluding Li and O is 80 mol % or more, and the transition metal element contains at least Mn in addition to Ni. The positive electrode active material may also include a third positive electrode active material within the scope of the present disclosure. Examples of the third positive electrode active material include a composite oxide having a Ni molar ratio of less than 80 mol % and a composite oxide that does not contain Mn. The positive electrode active material of this embodiment essentially contains only the first and second positive electrode active materials.

[0029] The first and second positive electrode active materials preferably have a layered rock salt structure. Examples of the layered rock salt structure include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of increasing capacity and stability of the crystal structure.

[0030] As described above, the first positive electrode active material is a secondary particle formed by the aggregation of numerous primary particles. While using a secondary particle-type lithium-nickel composite oxide is effective in increasing the capacity of a battery, the composite oxide may undergo particle cracking during charging and discharging, and particle cracking becomes more likely as the Ni content increases. When particle cracking occurs, new active material surfaces without a protective coating are formed, promoting electrolyte decomposition or particles becoming isolated from the conductive path, resulting in significant capacity loss during charging and discharging. Therefore, in this embodiment, these problems are addressed by using a single-particle type second positive electrode active material in combination with CNTs with long fiber lengths.

[0031] The first positive electrode active material is, for example, polycrystalline particles containing 1,000 or more primary particles. The average particle size of the primary particles constituting the first positive electrode active material is preferably 0.5 μm or less, more preferably 0.03 μm or more and 0.3 μm or less. The average particle size of the primary particles is determined by observing the particle surface of the active material using a scanning electron microscope (SEM). From an SEM image of the particle surface, 100 primary particles are selected at random, and the diameters of the circumscribed circles are measured. The average particle size of the primary particles can be calculated by averaging the measured values.

[0032] The volume-based median diameter (hereinafter referred to as "D50") of the first positive electrode active material (secondary particles) is preferably 6 μm or more and 30 μm or less, more preferably 8 μm or more and 25 μm or less, or 10 μm or more and 20 μm or less, or 10 μm or more and 15 μm or less. D50 means the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the positive electrode active material is measured using a laser diffraction particle size distribution measuring device with water as the dispersion medium. The D50 of the positive electrode active material in the examples described below was measured using an MT3000II manufactured by Microtrac-Bell Corporation.

[0033] The first positive electrode active material has the composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.87≦b≦0.93, 0<c<0.08, 0≦d≦0.08, 1.9≦e≦2.1). The Ni content in the first positive electrode active material is 87 mol% or more, more preferably 88 mol% or more, particularly preferably 89 mol% or more, or 90 mol% or more, relative to the total molar amount of metal elements excluding Li, from the viewpoint of increasing capacity, etc. The upper limit of the Ni content is 93 mol% from the viewpoint of improving durability, etc. An example of a suitable range of the Ni content is 88 mol% or more and 93 mol% or less, or 89 mol% or more and 93 mol% or less, or 90 mol% or more and 93 mol% or less.

[0034] The first positive electrode active material contains Mn as an essential component. Mn stabilizes the crystal structure of the composite oxide. The content of Mn in the first positive electrode active material is preferably 1 mol% or more and 8 mol% or less, more preferably 3 mol% or more and 7 mol% or less, relative to the total molar amount of metal elements excluding Li. The first positive electrode active material may also contain at least one selected from Co and Al. When the first positive electrode active material contains Co and Al, the content of each is preferably 8 mol% or less, more preferably 6 mol% or less.

[0035] The first positive electrode active material may further contain small amounts of elements other than Li, O, Ni, Mn, Co, and Al. Examples of other elements include Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. The content of the elements contained in the positive electrode active material can be measured using an ICP optical emission spectrometer (for example, CIROS-120 manufactured by SPECTRO).

[0036] As described above, the second positive electrode active material has a single particle shape. In this specification, a single particle does not refer to a secondary particle formed by agglomeration of a large number of primary particles, but refers to a single crystal particle formed substantially of one or 10 or less primary particles. For example, the particle interface of the primary particles is substantially absent within the particle of the second positive electrode active material. Note that a particle formed by agglomeration of 10 or less primary particles approximates a single particle shape and can be considered to be substantially a single particle.

[0037] The D50 of the second positive electrode active material is, for example, smaller than the D50 of the first positive electrode active material, and is preferably 0.5 μm to 10 μm, or 0.8 μm to 8 μm, or 3 μm to 7 μm. The crystallite size of the second positive electrode active material is 370 Å to 1500 Å, preferably 370 Å to 1000 Å. The crystallite size is calculated from the half-width of the diffraction peak of the (104) plane in the X-ray diffraction pattern by X-ray diffraction using the Scherrer equation shown below. In the formula below, s is the crystallite size, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (104) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this specification, K is 0.9. s = Kλ / B cos θ

[0038] The X-ray diffraction pattern was obtained by powder X-ray diffraction using a powder X-ray diffractometer (RINT-TTR manufactured by Rigaku Corporation, Cu-Kα source) under the following conditions: Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-spline Profile function: divided pseudo-Voigt function Constraint conditions: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the Ni content of each element) ICSD No.: 98-009-4814

[0039] The second positive electrode active material has the composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ(0.9<a≦1.1, 0.80≦b≦0.87, 0<c<0.10, 0≦d≦0.10, 1.9≦e≦2.1). The Ni content in the second positive electrode active material is 80 mol% or more, more preferably 82 mol% or more, particularly preferably 83 mol% or more, or 85 mol% or more, relative to the total molar amount of metal elements excluding Li, from the viewpoint of increasing capacity, etc. The upper limit of the Ni content is 87 mol% from the viewpoint of improving durability, etc. The Ni content in the second positive electrode active material is preferably less than the Ni content of the first positive electrode active material. An example of a suitable range of the Ni content is 82 mol% or more and 87 mol% or less, or 83 mol% or more and 86 mol% or less.

[0040] The second positive electrode active material contains Mn as an essential component. The content of Mn in the second positive electrode active material is preferably 1 mol% or more and 8 mol% or less, more preferably 3 mol% or more and 7 mol% or less, relative to the total molar amount of metal elements excluding Li. The second positive electrode active material may also contain at least one selected from Co and Al. When the second positive electrode active material contains Co and Al, the respective contents are preferably 8 mol% or less, more preferably 6 mol% or less. The second positive electrode active material may further contain small amounts of elements other than Li, O, Ni, Mn, Co, and Al.

[0041] The content of the second positive electrode active material is 10% by mass or more and 35% by mass or less, more preferably 15% by mass or more and 30% by mass or less, or 15% by mass or more and 25% by mass or less, or 20% by mass or more and 30% by mass or less, based on the total mass of the positive electrode active material. If the content of the second positive electrode active material is within this range, high capacity and high durability can be more effectively achieved. The content of the first positive electrode active material is 65% by mass or more and 90% by mass or less, more preferably 70% by mass or more and 85% by mass or less, based on the total mass of the positive electrode active material.

[0042] The positive electrode mixture layer 31 contains, for example, 0.01% by mass or more and 5% by mass or less of CNTs, and has a conductive path formed by the CNTs. CNTs are conductive carbon fibers with a diameter (outer diameter) of 100 nm or less and have an extremely large aspect ratio (ratio of fiber length to fiber diameter). The aspect ratio of the CNTs is, for example, 20 times or more, preferably 50 times or more. CNTs with a high aspect ratio form linear contact with the active material and the core, rather than point contact. Therefore, a good conductive path can be formed with a small amount of addition.

[0043] The CNTs preferably have an average fiber length of 2 μm or more and 10 μm or less in the positive electrode mixture layer 31. If the average fiber length of the CNTs is within this range, a good conductive path is formed in the positive electrode mixture layer 31, and low resistance and high durability can be more highly compatible. The average fiber length of the CNTs is determined by analyzing a cross-sectional image of the positive electrode mixture layer 31 using an SEM or a transmission electron microscope (TEM). The average fiber length of the CNTs is determined by measuring the lengths of 100 randomly selected CNTs and arithmetically averaging the measured values. The fiber length means the length of the CNTs when stretched linearly.

[0044] The average fiber diameter (outer diameter) of the CNTs is, for example, 20 nm or less, and may be 15 nm or less. If the average fiber diameter is 20 nm or less, DCIR is more effectively reduced. The lower limit of the average fiber diameter of the CNTs is not particularly limited, but an example is 1 nm. The average fiber diameter of the CNTs is, for example, 10 nm or more and 20 nm or less. The average fiber diameter of the CNTs can be determined, similar to the average fiber diameter, by analyzing a cross-sectional image of the positive electrode mixture layer 31 using a SEM or a TEM.

[0045] The CNT may be either a single-walled CNT (SWCNT) or a multi-walled CNT (MWCNT). The positive electrode mixture layer 31 may contain SWCNT and MWCNT. SWCNT has a structure in which one layer of graphite sheet is formed into a tubular shape, and MWCNT has a structure in which multiple layers of graphite sheet are formed into a tubular shape. An example of MWCNT is a double-walled CNT having a two-wall structure.

[0046] The preferred BET specific surface area of ​​CNT varies slightly depending on the type of CNT. For example, ２ / g or more, more preferably 500m ２ The upper limit of the BET specific surface area is not particularly limited, but is, for example, 1500 m ２ The BET specific surface area is measured in accordance with the BET method (nitrogen adsorption method) described in JIS R1626.

[0047] The manufacturing process of the positive electrode 11 includes the following steps: (a) a step of preparing a positive electrode mixture containing a positive electrode active material and a binder, (b) a step of preparing a conductive agent dispersion by adding CNTs having an average fiber length of 150 μm or more and 300 μm or less and a dispersant to a dispersion medium, (c) a step of mixing the positive electrode mixture and the conductive agent dispersion to prepare a positive electrode slurry, and (d) a step of applying the positive electrode slurry onto the positive electrode core 30 and drying the coating to form a positive electrode mixture layer 31 on the positive electrode core 30. As will be described in detail later, the conductive agent dispersion preferably contains a hydrogenated nitrile rubber as a dispersant.

[0048] The conductive agent dispersion and the positive electrode slurry are described in detail below. The conductive agent dispersion contains at least a conductive agent containing CNTs, a dispersant, and an aprotic polar solvent. The conductive agent dispersion is added to the positive electrode slurry, and the conductive agent contained in the conductive agent dispersion forms a conductive path in the positive electrode mixture layer 31. In the conductive agent dispersion, the dispersant is dissolved in the polar solvent, and the CNTs are dispersed in the polar solvent by the action of the dispersant. The solid content (conductive agent and dispersant) concentration of the conductive agent dispersion is, for example, 0.1% by mass or more and 20% by mass or less. From the viewpoint of achieving both CNT dispersibility and productivity, it is preferably 0.2% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less.

[0049] As described above, the average fiber length of the CNTs used in preparing the conductive agent dispersion is 150 μm or more and 300 μm or less, more preferably 180 μm or more and 250 μm or less. In this case, the average fiber length of the CNTs in the positive electrode mixture layer 31 is, for example, 2 μm or more and 10 μm or less, and a good conductive path is formed in the positive electrode mixture layer 31. This allows the battery to achieve both low resistance and high durability to a higher degree. The average fiber length of the CNTs is determined by observing the CNT powder using an SEM or TEM, analyzing the acquired image, measuring the lengths of 100 CNTs arbitrarily selected from the image, and arithmetically averaging the measured values.

[0050] The CNTs are cut and shortened by shear forces acting in the steps of preparing the conductive agent dispersion and the positive electrode slurry. In other words, the positive electrode 11 is manufactured using CNTs with an average fiber length of 150 μm or more and 300 μm or less, so that the average fiber length of the CNTs in the positive electrode mixture layer 31 is 2 μm or more and 10 μm or less.

[0051] The content of CNT in the conductive agent dispersion is preferably 0.05% by mass or more and 20% by mass or less, more preferably 0.1% by mass or more and 10% by mass or less. Even when a conductive agent other than CNT is added, the content of the conductive agent is preferably within this range. If the CNT content is within this range, the dispersibility of the CNT is improved while ensuring good productivity of the conductive agent dispersion, and the DCIR reduction effect is more pronounced. The preferred content varies somewhat depending on the type of CNT; for example, for SWCNT, 0.1% by mass or more and 5% by mass or less is particularly preferred, and for MWCNT, 2% by mass or more and 7% by mass or less is particularly preferred.

[0052] The conductive agent dispersion is prepared by mixing a conductive agent, a dispersant, and an aprotic polar solvent. A conventionally known disperser or mixer can be used to mix (knead) these raw materials. Examples of mixers include a planetary mixer, a homomixer, a pin mixer, a high-speed mixer, a disperser, a roll mill, a ball mill, a jet mill, and a kneader. If the shear force acting on the CNTs is too strong, the fibers will be cut, so it is preferable to apply an appropriate shear force to the conductive agent dispersion.

[0053] As described above, the dispersant preferably contains at least a nitrile-based rubber. The presence of a nitrile-based rubber in the conductive agent dispersion liquid ensures good dispersion of long-fiber CNTs throughout the manufacturing process of the positive electrode 11. The nitrile-based rubber is a copolymer of a monomer containing unsaturated nitrile and a conjugated diene as raw materials, and may be a copolymer essentially consisting of only unsaturated nitrile and a conjugated diene. The molar ratio of unsaturated nitrile to conjugated diene is, for example, 10:90 to 50:50. The weight-average molecular weight of the nitrile-based rubber is not particularly limited, but an example is 5,000 to 2,000,000. At least a portion of the dispersant may function as a binder for the positive electrode mixture layer 31.

[0054] The nitrile rubber may be a hydrogenated nitrile rubber. The hydrogenated nitrile rubber contains, for example, structural units derived from an unsaturated nitrile, structural units derived from a conjugated diene, and structural units derived from a hydrogenated conjugated diene. An example of a suitable hydrogenated nitrile rubber is a partially hydrogenated nitrile rubber in which 80 mol% or more of the structural units derived from a conjugated diene are hydrogenated. An example of an unsaturated nitrile is acrylonitrile or methacrylonitrile, preferably acrylonitrile. An example of a conjugated diene is a conjugated diene having 3 to 6 carbon atoms, preferably butadiene.

[0055] The nitrile rubber is preferably contained in an amount of 10 parts by mass or more and 100 parts by mass or less per 100 parts by mass of CNT. More preferably, it is 10 parts by mass or more and 80 parts by mass or less, and particularly preferably, it is 20 parts by mass or more and 50 parts by mass or less. In this case, it becomes easier to maintain a good dispersion state of the CNT for a long period of time. The conductive agent dispersion may contain only the nitrile rubber as the dispersant, or other dispersants may be used in combination. The mass ratio of the nitrile rubber in the dispersant is preferably 30% by mass or more, more preferably 40% by mass or more.

[0056] Other dispersants that can be used in combination with nitrile rubber include polyvinyl alcohol, polyvinylpyrrolidone (PVP), polyalkylene oxide, polyvinyl acetal, polyvinyl ether, cellulose, chitins, chitosans, starch, and derivatives thereof. Among these, PVP or its derivatives (PVPs) are preferred. When PVPs are used in combination, the mass ratio of nitrile rubber to PVPs is, for example, 40:60 to 60:40. The weight-average molecular weight of the PVPs is not particularly limited, but an example is 5,000 to 1,000,000. The conductive agent dispersion may also contain PVdF, which is added to the positive electrode slurry as a binder.

[0057] The conductive agent dispersion contains an aprotic polar solvent as a dispersion medium. The aprotic polar solvent may be any solvent that can dissolve the dispersant and disperse the CNTs. Because the conductive agent dispersion is added to the positive electrode slurry, the polar solvent is preferably miscible with the solvent of the positive electrode slurry, and may be the same type of solvent as the solvent of the positive electrode slurry. Examples of aprotic polar solvents include N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, tetrahydrofuran, dimethylformamide, acetone, ethyl acetate, and dimethyl sulfoxide. Of these, it is preferable to use NMP.

[0058] The aprotic polar solvent may be a single solvent or a combination of two or more solvents. For example, the mass ratio of NMP in the polar solvent may be 50 mass% or more, or even 100 mass%. The polar solvent may be substantially composed of NMP alone. The content of the polar solvent is, for example, 80 mass% or more, preferably 85 mass% or more and 99.5 mass% or less, more preferably 90 mass% or more and 99 mass% or less, of the mass of the conductive agent dispersion liquid.

[0059] The positive electrode slurry contains a conductive agent dispersion, two types of positive electrode active material, a binder, and a dispersion medium. The positive electrode slurry may be prepared by mixing these raw materials at once, but is preferably prepared, for example, by dry-blending powders of the positive electrode active material and the binder, adding the mixture to a dispersion medium, and then mixing the conductive agent dispersion. That is, as described above, it is preferable to have a step of preparing a positive electrode mixture containing the positive electrode active material and the binder, separate from the step of preparing the conductive agent dispersion. The solid content concentration of the positive electrode slurry is, for example, 70% by mass or more and 90% by mass or less.

[0060] The proportion of the positive electrode active material in the solid content of the positive electrode slurry is, for example, 90% by mass or more, preferably 90% by mass or more and 99.5% by mass or less, and more preferably 95% by mass or more and 99% by mass or less. The proportion of CNT in the solid content of the positive electrode slurry is, for example, 0.05% by mass or more and 10% by mass or less, preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. The amount of conductive agent dispersion liquid added is adjusted so that the CNT content falls within this range. The proportion of the binder in the solid content of the positive electrode slurry is, for example, 0.1% by mass or more and 5% by mass or less.

[0061] The dispersion medium disperses the positive electrode active material and the conductive agent and dissolves the binder, for example. Depending on the type of binder, water, lower alcohols such as ethanol, etc. may be used as the dispersion medium. However, an aprotic polar solvent is generally used, as in the case of the conductive agent dispersion liquid. Examples of the dispersion medium include NMP, methyl ethyl ketone, and dimethylformamide. Among these, NMP is preferably used. In this embodiment, the same type of dispersion medium (NMP) is used in the conductive agent dispersion liquid and the positive electrode slurry.

[0062] A conventionally known mixer or the like can be used to mix (knead) the positive electrode active material, conductive agent dispersion, binder, and dispersion medium. Examples of mixers or the like include a planetary mixer, homomixer, pin mixer, high-speed mixer, disperser, roll mill, ball mill, jet mill, kneader, etc. Among these, it is preferable to use a planetary mixer. A planetary mixer is a rotation-revolution type agitation mixer that can impart strong shear force to the slurry by planetary motion of blades.

[0063] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with such a metal disposed on its surface. The negative electrode mixture layer contains a negative electrode active material and a binder and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be fabricated, for example, by applying a negative electrode slurry containing a negative electrode active material and a binder to the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core. Metallic lithium foil can also be used as the negative electrode 12. Alternatively, the negative electrode 12 may be composed only of a negative electrode core, with metallic lithium being deposited on the core surface during battery charging.

[0064] The negative electrode active material is not particularly limited as long as it reversibly absorbs and releases lithium ions, and typically, carbon materials such as graphite are used. Furthermore, elements that alloy with Li, such as Si and Sn, or materials containing such elements, may also be used as the negative electrode active material. Among these, silicon-containing materials containing Si are preferred. Furthermore, lithium titanate, which has a higher charge / discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. One type of negative electrode active material may be used alone, or multiple types may be used in combination.

[0065] The carbon material functioning as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among these, it is preferable to use artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, or a mixture thereof. Examples of silicon-containing materials functioning as the negative electrode active material include silicon alloys, silicon compounds, and composite materials containing Si. A suitable silicon-containing material is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

[0066] As with the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, SBR is preferably used. A single binder may be used, or multiple binders may be used in combination. The negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. These function as thickeners in the negative electrode slurry. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the negative electrode mixture layer. The negative electrode mixture layer may also contain a conductive agent such as CNT.

[0067] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0068] 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 phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0069] The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0070] Example 1 Preparation of Conductive Agent Dispersion MWCNT (average fiber diameter: 15 nm, average fiber length: 200 μm) was used as the conductive agent, and a mixture of hydrogenated nitrile rubber (H-NBR) obtained by hydrogen-substitution of a copolymer of acrylonitrile and butadiene and polyvinylpyrrolidone (PVP) in a mass ratio of 1:1 was used as the dispersant. MWCNT and dispersant were dry-blended in a mass ratio of 2:1 to form a powder, which was then added to N-methyl-2-pyrrolidone (NMP) and kneaded using a ball mill, thereby obtaining a conductive agent dispersion containing approximately 5 mass% MWCNT.

[0071] [Preparation of Positive Electrode Slurry] A positive electrode active material having the composition formula LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ A first lithium transition metal composite oxide A1 (D50: 13 μm) having a secondary particle shape (polycrystalline) formed by aggregation of primary particles, and a composition formula LiNi ０．８５ Co ０．０５ Mn ０．１０ O ２ A second lithium transition metal composite oxide B1 (D50:5 μm) having a single particle shape (single crystal) was mixed at a mass ratio of 80:20 with polyvinylidene fluoride (PVdF) as a binder. The positive electrode active material and binder were dry-blended at a mass ratio of 100:1 to form a powder, which was then added to NMP. Subsequently, the conductive agent dispersion was added and kneaded so that the mass ratio of the positive electrode active material to MWCNT was 100:0.5, to obtain a positive electrode slurry containing the positive electrode active material, binder, and MWCNT.

[0072] [Fabrication of Positive Electrode] The above-mentioned positive electrode slurry was applied to both sides of a positive electrode core (total width: 750 mm) made of aluminum foil, and the coating was dried and compressed. The positive electrode core was then cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core. At this time, an exposed portion was provided at the longitudinal center of the positive electrode, extending 20 mm in the width direction (33% of the electrode plate width) from one end of the positive electrode core in the width direction, exposing the core surface. The width (electrode plate width) of the positive electrode core (positive electrode) after cutting was 60 mm. Hereinafter, a structure in which an exposed portion is provided in a portion of the width direction of the positive electrode core is referred to as a "partially exposed structure." The average fiber length of the CNTs in the positive electrode mixture layer was 6 μm (the same applies to Examples 2 to 4 and Comparative Examples 2 and 4 described below).

[0073] [Fabrication of Negative Electrode] Graphite was used as the negative electrode active material. The negative electrode active material, a dispersion of styrene butadiene rubber (SBR), and a sodium salt of carboxymethyl cellulose (CMC-Na) were mixed in a solids mass ratio of 98:1:1, and water was used as the dispersion medium to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to both sides of a negative electrode core made of copper foil, leaving a predetermined exposed portion. The coating was then dried and compressed, and the negative electrode core was cut to a predetermined electrode size to obtain a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode core.

[0074] [Preparation of non-aqueous electrolyte] LiPF 6 was dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate at a volume ratio of 3:7 (25°C) at a concentration of 1.2 mol / L. ６ was dissolved to obtain a non-aqueous electrolyte solution.

[0075] [Preparation of Test Cell] The positive electrode with an aluminum lead welded to its exposed portion and the negative electrode with a nickel lead welded to its exposed portion were spirally wound with a separator interposed therebetween to prepare a wound electrode assembly. This electrode assembly was housed in a cylindrical outer can with a bottom, the nonaqueous electrolyte solution was poured into it, and the opening of the outer can was sealed with a sealing member via a gasket to obtain a test cell (nonaqueous electrolyte secondary battery) X1.

[0076] Example 2 A positive electrode and a test cell X2 were obtained in the same manner as in Example 1, except that in the production of the positive electrode, an exposed portion was formed across the entire width of the positive electrode substrate (no partially exposed structure).

[0077] Example 3 A positive electrode and a test cell X3 were obtained in the same manner as in Example 2, except that in the preparation of the first positive electrode slurry, the mass ratio of the lithium transition metal composite oxides A1 and B1 was changed to 90:10.

[0078] Example 4 A positive electrode and a test cell X4 were obtained in the same manner as in Example 2, except that in the preparation of the first positive electrode slurry, the mass ratio of the lithium transition metal composite oxides A1 and B1 was changed to 70:30.

[0079] Comparative Example 1 In the preparation of the conductive agent dispersion, MWCNT (average fiber diameter: 15 nm, average fiber length: 20 μm) was used, and hydrogenated nitrile rubber (H-NBR) was not used. In the preparation of the first positive electrode slurry, instead of the lithium transition metal composite oxide B1 having a single particle shape, a compound having the composition formula LiNi ０．８５ Co ０．０５ Mn ０．１０ O ２ A positive electrode and a test cell Y1 were obtained in the same manner as in Example 2, except that lithium transition metal composite oxide B2 having a secondary particle shape (polycrystalline) formed by aggregation of primary particles was used. The average fiber length of the CNTs in the positive electrode mixture layer was 6 μm (the same applies to Comparative Examples 3 and 5 described below).

[0080] Comparative Example 2 In the preparation of the first positive electrode slurry, a lithium transition metal composite oxide B1 having the composition formula LiNi ０．８５ Co ０．０５ Mn ０．１０ O ２ A positive electrode and a test cell Y2 were obtained in the same manner as in Example 2, except that lithium transition metal composite oxide B2 having the shape of secondary particles formed by aggregation of primary particles was used.

[0081] Comparative Example 3 A positive electrode and a test cell Y3 were obtained in the same manner as in Example 2, except that MWCNT (average fiber diameter: 15 nm, average fiber length: 20 μm) were used and hydrogenated nitrile rubber (H-NBR) was not used in preparing the conductive agent dispersion liquid.

[0082] Comparative Example 4 A positive electrode and test cell Y4 were obtained in the same manner as in Example 2, except that in the preparation of the first positive electrode slurry, the mass ratio of the lithium transition metal composite oxides A1 and B1 was changed to 60:40.

[0083] Comparative Example 5 A positive electrode and a test cell Y5 were obtained in the same manner as in Example 2, except that the lithium transition metal composite oxide B1 was not used in preparing the first positive electrode slurry.

[0084] For each test cell of the Examples and Comparative Examples, the initial charge / discharge capacity, DCIR, and capacity retention rate and DCIR after charge / discharge cycling were measured by the following methods, and the evaluation results are shown together with the physical properties of the positive electrode in Table 1. The evaluation results of each test cell shown in Table 1 are relative values, with the evaluation result of test cell Y1 of Comparative Example 1 set to 100.

[0085] [Room Temperature Cycle Test] In a temperature environment of 25°C, the test cell was charged at a constant current of 0.5 C until the cell voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V down to 0.1 C. The test cell was then discharged at a constant current of 0.5 C until the cell voltage reached 2.5 V. The charge capacity and discharge capacity at this time were measured and defined as the initial charge capacity and initial discharge capacity. This charge / discharge cycle was repeated 300 times, and the discharge capacity at the 300th cycle was divided by the initial discharge capacity to calculate the capacity retention rate.

[0086] [High-Temperature Cycle Test] The above charge-discharge cycle was repeated 300 times in a temperature environment of 45° C., and the capacity retention rate was calculated by dividing the discharge capacity at the 300th cycle by the initial discharge capacity.

[0087] [Evaluation of DCIR] In a temperature environment of 25° C., the test cells in the initial state and after the above charge-discharge cycles were CC charged at a constant current of 0.5 C until the depth of charge (SOC) reached 10%, and then CC discharged for 10 seconds at a constant current of 0.5 C. The DCIR was calculated by dividing the difference between the open circuit voltage (OCV) and the closed circuit voltage (CCV) 10 seconds after discharge by the discharge current 10 seconds after discharge.

[0088]

[0089] As shown in Table 1, all of the test cells X1 to X4 of the example had a lower initial DCIR before the charge-discharge cycles compared to the test cell Y1 of Comparative Example 1. Furthermore, regarding the capacity retention rate and DCIR after the charge-discharge cycles, no significant difference was confirmed in the values ​​in a room temperature environment, but the values ​​in a high temperature environment were significantly improved.

[0090] Although the test cell Y2 of Comparative Example 2 has a low initial resistance, it experiences a larger capacity decrease and resistance increase after the high-temperature cycle test than the test cells X1 to X4 of the Examples. Furthermore, the test cells Y3 to Y5 of Comparative Examples 3 to 5 have a higher initial resistance than the test cell Y1 of Comparative Example 1.

[0091] From the above results, it can be seen that the test cells X1 to X4 of the examples can achieve both low initial resistance and excellent charge-discharge cycle characteristics at a high level.

[0092] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core, 31 Positive electrode mixture layer, 32 Exposed portion

Claims

1. A method for manufacturing a cathode composite material, comprising: preparing a cathode mixture containing a cathode active material and a binder; preparing a conductive agent dispersion by adding carbon nanotubes having an average fiber length of 150 μm or more and 300 μm or less and a dispersant to a dispersion medium; mixing the cathode mixture and the conductive agent dispersion to prepare a cathode slurry; applying the cathode slurry onto a cathode core and drying the coating to form a cathode mixture layer on the cathode core; wherein the cathode active material comprises a first cathode active material having a secondary particle shape formed by aggregation of primary particles and a second cathode active material having a single particle shape; and the first cathode active material has a composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.87≦b≦0.93, 0<c<0.08, 0≦d≦0.08, 1.9≦e≦2.1), and the second positive electrode active material is a composite oxide represented by the composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.80≦b≦0.87, 0<c<0.10, 0≦d≦0.10, 1.9≦e≦2.1), and the content of the second positive electrode active material is 10% by mass or more and 35% by mass or less with respect to the total mass of the positive electrode active material.

2. The method for producing a positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the conductive agent dispersion contains hydrogenated nitrile rubber as the dispersant.

3. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core, the positive electrode mixture layer includes a positive electrode active material, a binder, and carbon nanotubes, the positive electrode active material includes a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles, and a second positive electrode active material having a single particle shape, and the first positive electrode active material has a composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.87≦b≦0.93, 0<c<0.08, 0≦d≦0.08, 1.9≦e≦2.1), and the second positive electrode active material is a composite oxide represented by the composition formula Li ａ Ni ｂ Co （１－ｂ－ｃ） Mn ｃ Al ｄ O ｅ (0.9<a≦1.1, 0.80≦b≦0.87, 0<c<0.10, 0≦d≦0.10, 1.9≦e≦2.1), a content of the second positive electrode active material is 10% by mass or more and 35% by mass or less with respect to a total mass of the positive electrode active material, and the carbon nanotubes have an average fiber length of 2 μm or more and 10 μm or less in the positive electrode mixture layer.

4. The nonaqueous electrolyte secondary battery according to claim 3, wherein an exposed portion is formed at one widthwise end of the positive electrode, where the surface of the positive electrode core is exposed and a positive electrode lead is connected, and the positive electrode mixture layer includes a first region aligned with the exposed portion in the longitudinal direction of the positive electrode, and a second region aligned with the exposed portion and the first region in the widthwise direction of the positive electrode.

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