Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery (10) comprises a positive electrode (11), a negative electrode (12), and a non-aqueous electrolyte, the non-aqueous electrolyte secondary battery being characterized in that: the positive electrode (11) has a positive electrode current collector (30) and a positive electrode mixture layer (32) that is disposed on the surface of the positive electrode current collector (30) and contains a positive electrode active material; the positive electrode mixture layer (32) contains a first conductive agent having a hydrogen content of 1.0-2.0 mg / g, and a second conductive agent having a hydrogen content of 0.5 mg / g or less; the positive electrode mixture layer (32) has a first positive electrode mixture layer (34) disposed on the positive electrode current collector (30) side, and a second positive electrode mixture layer (36) disposed on the surface side of the positive electrode mixture layer (32); the first positive electrode mixture layer (34) contains more of the first conductive agent than the second conductive agent in terms of mass; and the second positive electrode mixture layer (36) contains more of the second conductive agent than the first conductive agent in terms of mass.

Description

Nonaqueous electrolyte secondary battery

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

[0002] A positive electrode of a non-aqueous electrolyte secondary battery generally includes a positive electrode current collector that is a metal foil, and a positive electrode mixture layer that includes a positive electrode active material, a conductive material, and a binder and is provided on the positive electrode current collector. Patent Document 1 discloses a positive electrode that uses carbon black, whose BET specific surface area is controlled within a predetermined range, as a conductive material.

[0003] Special Publication No. 2020-524192

[0004] In recent years, with the spread of electric vehicles and other factors, there has been a demand for improving the output characteristics of non-aqueous electrolyte secondary batteries. Furthermore, the resistance at the interface between the positive electrode current collector and the positive electrode mixture layer (interface resistance) significantly affects the output characteristics of the battery. The technology described in Patent Document 1 does not sufficiently consider the interface resistance, and there is still considerable room for improvement.

[0005] A non-aqueous electrolyte secondary battery according to one aspect of 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 current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector and containing a positive electrode active material, the positive electrode mixture layer containing a first conductive agent having a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less and a second conductive agent having a hydrogen content of 0.5 mg / g or less, the positive electrode mixture layer has the first positive electrode mixture layer disposed on the positive electrode current collector side and a second positive electrode mixture layer disposed on the surface side of the positive electrode mixture layer, the first positive electrode mixture layer containing more of the first conductive agent than the second conductive agent, on a mass basis, and the second positive electrode mixture layer containing more of the second conductive agent than the first conductive agent, on a mass basis.

[0006] According to the nonaqueous electrolyte secondary battery of one aspect of the present disclosure, the interface resistance can be reduced.

[0007] 1 is a cross-sectional view of a positive electrode of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention; FIG. 2 is a perspective view of a wound electrode body of the nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0008] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the nonaqueous electrolyte secondary battery. Furthermore, when the following description includes multiple embodiments and modified examples, it is initially assumed that the characteristic portions thereof will be used in appropriate combination.

[0009] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified as the nonaqueous electrolyte secondary battery, but the outer can of the battery is not limited to a cylindrical outer can. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery having a prismatic outer can, a coin battery having a coin-shaped outer can, or a pouch-type battery having an outer can made of a laminate sheet including a metal layer and a resin layer.

[0010] 1 is a cross-sectional view of an example of a nonaqueous electrolyte secondary battery 10. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes an electrode assembly 14, a nonaqueous electrolyte (not shown), and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte.

[0011] As will be described in detail later, the electrode body 14 has 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 electrode body 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.

[0012] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0013] 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

[0014] 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.

[0015] The outer can 16 is a cylindrical metal container with a bottom and an opening at one axial end, and the opening of the outer can 16 is closed by a sealing body 17. For ease of explanation, the sealing body 17 side of the battery will be referred to as the "top" and the bottom side of the outer can 16 will be referred to as the "bottom" below.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] Next, the electrode assembly 14 will be described in detail with reference to FIG. 2 . FIG. 2 is a perspective view of the electrode assembly 14. As described above, the electrode assembly 14 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 positive electrode 11, the negative electrode 12, and the separator 13 are all formed in strip shapes and spirally wound around a winding core disposed along a winding axis 29, resulting in an alternating stacked state in the radial direction of the electrode assembly 14. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the width direction of the positive electrode 11 and the negative electrode 12 is the axial direction. The positive electrode lead 20 extends axially from approximately the center in the radial direction between the center and the outermost periphery at the upper end of the electrode assembly 14. The negative electrode lead 21 extends axially from the lower end of the outermost periphery of the electrode assembly 14. The negative electrode lead 21 may extend in the axial direction from the vicinity of the winding axis 29 at the lower end of the electrode body 14 .

[0020] The negative electrode 12 includes a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode current collector can be a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. An example of the negative electrode current collector is a copper or copper alloy foil having a thickness of 5 μm or more and 15 μm or less. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably formed on both sides of the negative electrode current collector. The thickness of the negative electrode mixture layer is, for example, 30 μm or more and 150 μm or less on one side of the negative electrode current collector. The negative electrode 12 can be fabricated by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., onto the negative electrode current collector, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0021] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and earthy graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, the negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a carbon-based active material and a Si-based active material may be used in combination.

[0022] The binder contained in the negative electrode mixture layer can be a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, polyolefin, or styrene-butadiene rubber (SBR). The negative electrode mixture layer may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, or polyvinyl alcohol (PVA). The binder content is, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of the negative electrode active material. A conductive agent such as carbon black, acetylene black, or ketjen black may also be added to the negative electrode mixture layer.

[0023] 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.

[0024] 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 and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. 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.

[0025] Next, the positive electrode 11 constituting the electrode assembly 14 will be described in detail with further reference to Fig. 3. Fig. 3 is a cross-sectional view of the positive electrode 11.

[0026] 3, the positive electrode 11 includes a positive electrode current collector 30 and a positive electrode mixture layer 32 containing a positive electrode active material and disposed on at least one surface of the positive electrode current collector 30. In this embodiment, the positive electrode mixture layer 32 is disposed on both surfaces of the positive electrode current collector 30 except for the portion to which the positive electrode lead 20 is connected.

[0027] The positive electrode current collector 30 can be a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on the surface thereof. An example of the positive electrode current collector 30 is an aluminum or aluminum alloy foil having a thickness of 10 μm or more and 20 μm or less.

[0028] The positive electrode mixture layer 32 has a first positive electrode mixture layer 34 disposed on the positive electrode current collector 30 side, and a second positive electrode mixture layer 36 disposed on the surface side of the positive electrode mixture layer 32. Hereinafter, the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 may be collectively referred to as the positive electrode mixture layer 32. The thickness of the positive electrode mixture layer 32 on one side of the positive electrode current collector 30 is preferably 40 μm or more and 100 μm or less, and more preferably 50 μm or more and 90 μm or less. In this embodiment, the positive electrode mixture layer 32 has a substantially uniform thickness along the longitudinal direction of the electrode plate.

[0029] The positive electrode mixture layer 32 includes a positive electrode active material and a conductive agent. As will be described in detail later, the positive electrode mixture layer 32 includes, as the conductive agents, a first conductive agent having a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less and a second conductive agent having a hydrogen content of 0.5 mg / g or less.

[0030] The positive electrode mixture layer 32 contains a particulate lithium-containing composite oxide as a positive electrode active material. The lithium-containing composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal element constituting the lithium-containing composite oxide is, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, Al, and Mn. Examples of suitable composite oxides include a lithium-containing composite oxide containing Ni, Co, and Mn, and a lithium-containing composite oxide containing Ni, Co, and Al.

[0031] The content of the positive electrode active material is preferably 85 mass % or more and 99.5 mass % or less, and more preferably 90 mass % or more and 99 mass % or less, relative to the mass of the positive electrode mixture layer 32. The content of the positive electrode active material in the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 may be the same or different from each other.

[0032] The lithium-containing composite oxide is, for example, a secondary particle formed by the aggregation of multiple primary particles. The volume-based median diameter (D50) of the composite oxide is not particularly limited, but is, for example, 3 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less. When the composite oxide is a secondary particle formed by the aggregation of primary particles, the D50 of the composite oxide refers to the D50 of the secondary particles. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the composite oxide (as well as that of the positive electrode active material) can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as a dispersion medium.

[0033] The average particle size of the primary particles constituting the lithium-containing composite oxide is, for example, 0.05 μm or more and 1 μm or less, and is calculated by averaging the diameters of the circumscribed circles of the primary particles extracted by analyzing a scanning electron microscope (SEM) image of the cross section of the secondary particles.

[0034] As described above, the positive electrode mixture layer 32 includes a first conductive agent having a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less, and a second conductive agent having a hydrogen content of 0.5 mg / g or less. The first positive electrode mixture layer 34 includes more of the first conductive agent than the second conductive agent, in terms of mass, and the second positive electrode mixture layer 36 includes more of the second conductive agent than the first conductive agent, in terms of mass. That is, the positive electrode mixture layer 32 includes more of the first conductive agent on the side closer to the positive electrode current collector 30, and more of the second conductive agent on the surface side.

[0035] As a result of investigations by the present inventors, it has been found that when the hydrogen content of the conductive agent present on the positive electrode current collector 30 side is 1.0 mg / g or more and 2.0 mg / g or less, a good conductive path is formed between the conductive agent and the positive electrode current collector 30, and interfacial resistance is reduced. This is presumably because, when the hydrogen content of the conductive agent is 1.0 mg / g or more and 2.0 mg / g or less, surface functional groups that improve the dispersibility of the conductive agent are appropriately formed on the surface of the conductive agent, improving the dispersibility of the conductive agent, and as a result, a good conductive path is formed due to the appropriate dispersion of the conductive agent near the positive electrode current collector 30. Note that examples of surface functional groups that improve the dispersibility of the conductive agent include hydroxyl groups and carboxyl groups.

[0036] On the other hand, a conductive agent having a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less forms a good conductive path with the positive electrode current collector 30, but does not easily form a good conductive path between the conductive agents themselves. Therefore, by having a second conductive agent having a hydrogen content of 0.5 mg or less present on the surface side of the positive electrode mixture layer 32 away from the positive electrode current collector 30, a good conductive path can be formed within the positive electrode mixture layer 32 while maintaining a good conductive path between the positive electrode current collector 30 and the positive electrode mixture layer 32.

[0037] The hydrogen content of the first conductive agent may be 1.0 mg / g or more and 2.0 mg / g or less, but from the viewpoint of further reducing the interfacial resistance, it is preferably 1.1 mg / g or more and 1.9 mg / g or less, and more preferably 1.2 mg / g or more and 1.8 mg / g or less. Note that if the hydrogen content of the conductive agent exceeds 2.0 mg / g, it becomes difficult to form a good conductive path between the conductive agents, and the interfacial resistance may increase instead.

[0038] The carbon monoxide content of the first conductive agent is preferably 10 mg / g or more, more preferably 15 mg / g or more, and even more preferably 20 mg / g or more. It is presumed that the carbon monoxide content of the first conductive agent indicates the amount of carboxyl groups present on the surface of the conductive agent. Therefore, when the carbon monoxide content of the first conductive agent is 10 mg / g or more, a good conductive path is formed between the conductive agent and the positive electrode current collector 30, and the interfacial resistance is reduced. On the other hand, if the amount of carboxyl groups on the surface of the first conductive agent is excessively increased, it becomes difficult to form a good conductive path between the conductive agents, and the interfacial resistance may actually increase. Therefore, the carbon monoxide content of the first conductive agent is preferably 50 mg / g or less, more preferably 45 mg / g or less, and even more preferably 40 mg / g or less. Therefore, the carbon monoxide content of the first conductive agent is preferably 10 mg / g or more and 50 mg / g or less, more preferably 15 mg / g or more and 45 mg / g or less, and even more preferably 20 mg / g or more and 40 mg / g or less.

[0039] The hydrogen content and carbon monoxide content of the first conductive agent can be measured, for example, by an inert gas fusion-non-dispersive infrared absorption method (measuring device: EMGA-830 manufactured by Horiba, Ltd.) The same applies to the hydrogen content and carbon monoxide content of the second conductive agent described below.

[0040] The BET specific surface area of ​​the first conductive agent is 60 m from the viewpoint of forming a good conductive path between the conductive agent and the positive electrode current collector 30. ２ / g or more, and ２ / g or more is more preferable, and 80m ２ On the other hand, if the BET specific surface area of ​​the first conductive agent is excessively increased, it becomes difficult to form a good conductive path between the conductive agents, and the interface resistance may increase. Therefore, the BET specific surface area of ​​the first conductive agent is preferably 110 m ２ / g or less, and ２ / g or less is more preferable, and 100m ２ Therefore, the BET specific surface area of ​​the first conductive agent is 60 m / g or less.２ / g or more, 110m ２ / g or less, and ２ / g or more, 105m ２ / g or less is more preferable, and ２ / g or more, 100m ２ The BET specific surface area is measured in accordance with the BET method (nitrogen adsorption method) described in JIS R1626.

[0041] The first conductive agent may be, for example, one or more materials selected from the group consisting of carbon black (CB) such as furnace black (FB), acetylene black (AB), and ketjen black (KB), carbon nanotubes (CNT), and graphene. Among these, furnace black is preferably used as the first conductive agent from the viewpoint of achieving a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less. The hydrogen content of the first conductive agent may also be adjusted by subjecting it to a surface treatment. Examples of such surface treatments include surface oxidation using a strong acid. Carboxy groups can be introduced onto the surface of the conductive agent by immersing the conductive agent in a mixed acid of sulfuric acid and nitric acid and then heat-treating it.

[0042] In the first positive electrode mixture layer 34, the content of the first conductive agent relative to the total mass of the positive electrode active material is preferably 0.5 mass% or more, more preferably 0.75 mass% or more, and even more preferably 1.0 mass% or more. In this case, a good conductive path is formed between the conductive agent and the positive electrode current collector 30, reducing interfacial resistance. Furthermore, from the viewpoint of ensuring the amount of positive electrode active material in the positive electrode mixture layer 32 and achieving a high battery capacity, the content of the first conductive agent relative to the total mass of the positive electrode active material in the first positive electrode mixture layer 34 is preferably 2.0 mass% or less, more preferably 1.75 mass% or less, and even more preferably 1.5 mass% or less. Therefore, the content of the first conductive agent relative to the total mass of the positive electrode active material in the first positive electrode mixture layer 34 is preferably 0.5 mass% or more but 2.0 mass% or less, more preferably 0.75 mass% or more but 1.75 mass% or less, and even more preferably 1.0 mass% or more but 1.5 mass% or less.

[0043] The first positive electrode mixture layer 34 may contain a second conductive agent as a conductive agent, but contains the first conductive agent as a main component. More specifically, in the first positive electrode mixture layer 34, the content of the first conductive agent relative to the total mass of the conductive agents is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more.

[0044] As described above, the second conductive agent has a hydrogen content of 0.5 mg / g or less and is mainly contained in the second positive electrode mixture layer 36. The hydrogen content of the second conductive agent is preferably 0.45 mg / g or less, more preferably 0.4 mg / g or less, and even more preferably 0.35 mg / g or less. The smaller the hydrogen content of the second conductive agent, the more easily a good conductive path is formed between the conductive agents, and the lower the internal resistance of the positive electrode mixture layer 32.

[0045] The carbon monoxide content of the second conductive agent is preferably less than 10 mg / g, more preferably 9 mg / g or less, and even more preferably 8 mg / g or less. Here, as described above, the carbon monoxide content of the second conductive agent is presumed to indicate the amount of carboxy groups present on the surface of the conductive agent. Therefore, when the carbon monoxide content of the first conductive agent is less than 10 mg / g, a good conductive path is easily formed between the conductive agents, and the internal resistance of the positive electrode mixture layer 32 is reduced.

[0046] The BET specific surface area of ​​the second conductive agent is 110 m from the viewpoint of forming a good conductive path between the conductive agents. ２ / g or more, 180m ２ / g or less, and ２ / g or more, 170m ２ / g or less is more preferable, and 130m ２ / g or more, 160m ２ It is more preferable that the SiO2 content is 1 / g or less.

[0047] As with the first conductive agent, the second conductive agent may be one or more materials selected from the group consisting of carbon black (CB) such as furnace black (FB), acetylene black (AB), and ketjen black (KB), carbon nanotubes (CNT), and graphene. Among these, from the viewpoint of keeping the hydrogen content at 0.5 mg / g or less, it is preferable to use acetylene black as the second conductive agent.

[0048] In the second positive electrode mixture layer 36, the content of the second conductive agent relative to the total mass of the positive electrode active material is preferably 0.5 mass% or more, more preferably 0.75 mass% or more, and even more preferably 1.0 mass% or more. In this case, good conductive paths are more likely to be formed between the conductive agents, and the internal resistance of the positive electrode mixture layer 32 is reduced. Furthermore, from the viewpoint of ensuring the amount of positive electrode active material in the positive electrode mixture layer 32 and realizing a high battery capacity, the content of the second conductive agent relative to the total mass of the positive electrode active material in the second positive electrode mixture layer 36 is preferably 2.0 mass% or less, more preferably 1.75 mass% or less, and even more preferably 1.5 mass% or less. Therefore, in the second positive electrode mixture layer 36, the content of the second conductive agent relative to the total mass of the positive electrode active material is preferably 0.5 mass% or more and 2.0 mass% or less, more preferably 0.75 mass% or more and 1.75 mass% or less, and even more preferably 1.0 mass% or more and 1.5 mass% or less.

[0049] The second positive electrode mixture layer 36 may contain the first conductive agent as a conductive agent, but the second conductive agent is the main component. More specifically, in the second positive electrode mixture layer 36, the content of the second conductive agent relative to the total mass of the conductive agents is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more.

[0050] As described above, the positive electrode mixture layer 32 includes a first positive electrode mixture layer 34 disposed on the positive electrode current collector 30 side and a second positive electrode mixture layer 36 disposed on the surface side of the positive electrode mixture layer 32. Here, the ratio of the thickness of the first positive electrode mixture layer 34 to the sum of the thicknesses of the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 (the thickness of the positive electrode mixture layer 32) is preferably 0.1 or greater, more preferably 0.15 or greater, and even more preferably 0.2 or greater. In this case, a large amount of the first conductive agent is present on the positive electrode current collector 30 side of the positive electrode mixture layer 32, forming a good conductive path between the conductive agent and the positive electrode current collector 30 and reducing interfacial resistance. On the other hand, as described above, the first conductive agent does not easily form a good conductive path between conductive agents. Therefore, if the thickness ratio of the first positive electrode mixture layer 34, which contains a large amount of the first conductive agent, becomes too large, the internal resistance of the positive electrode mixture layer 32 may increase. Therefore, the ratio of the thickness of the first positive electrode mixture layer 34 to the sum of the thicknesses of the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 is preferably 0.6 or less, more preferably 0.55 or less, and even more preferably 0.5 or less. Therefore, the ratio of the thickness of the first positive electrode mixture layer 34 to the sum of the thicknesses of the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 is preferably 0.1 to 0.6, more preferably 0.15 to 0.55, and even more preferably 0.2 to 0.5.

[0051] The positive electrode mixture layer 32 further contains a binder in addition to the positive electrode active material and conductive agent. Examples of binders contained in the positive electrode mixture layer 32 include fluorine-containing polymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may also be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), or the like. The binder content is, for example, 0.1% by mass or more and 5% by mass or less, preferably 0.1% by mass or more and 2% by mass or less, relative to the total mass of the positive electrode active material. The binder content of the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 may be the same or different from each other.

[0052] Next, a method for producing a positive electrode 11 including a positive electrode mixture layer 32 having a first positive electrode mixture layer 34 and a second positive electrode mixture layer 36 will be described. The positive electrode 11 can be produced, for example, through an active material production step of producing a positive electrode active material, a preparation step of preparing a positive electrode mixture slurry containing the positive electrode active material, a conductive agent, and a binder, a coating step of applying the positive electrode mixture slurry to the surface of a positive electrode current collector 30 and drying it, and a rolling step of rolling the coating film produced in the coating step. Here, in the preparation step, a first positive electrode mixture slurry containing a first conductive agent as a conductive agent and a second positive electrode mixture slurry containing a second conductive agent as a conductive agent are produced.

[0053] In the coating process, for example, the first positive electrode mixture slurry is coated on the positive electrode current collector 30, and then the second positive electrode mixture slurry is coated on the coating film. This allows the formation of a two-layer positive electrode mixture layer 32 including a first positive electrode mixture layer formed from the first positive electrode mixture slurry and a second positive electrode mixture layer formed from the second positive electrode mixture slurry. In this case, the thicknesses of the first positive electrode mixture layer 34 and the second positive electrode mixture layer 36 can be adjusted by the amounts of the first positive electrode mixture slurry and the second positive electrode mixture slurry coated. Note that the method for forming the positive electrode mixture layer 32 is not limited to this. For example, the first positive electrode mixture slurry and the second positive electrode mixture slurry may be simultaneously coated on the surface of the positive electrode current collector 30 using a coating device having two discharge ports, so that the first positive electrode mixture slurry and the second positive electrode mixture slurry are coated in this order from the surface side of the positive electrode current collector 30.

[0054] Hereinafter, the present disclosure will be further described with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0055] Example 1 [Fabrication of Positive Electrode] As a positive electrode active material, LiNi ０．８７ Co ０．０９ Al ０．０４ O ２A lithium composite oxide represented by the formula (I) was used. The above positive electrode active material, furnace black (FB) as a conductive agent, and polyvinylidene fluoride powder as a binder were mixed in a mass ratio of 100:1:0.9, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a first positive electrode mixture slurry. The above positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride powder as a binder were mixed in a mass ratio of 100:1:0.9, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a second positive electrode mixture slurry. The hydrogen contents of the furnace black and acetylene black were measured using the above-mentioned method and were found to be 1.2 mg / g and 0.3 mg / g, respectively. The carbon monoxide contents of the furnace black and acetylene black were measured using the above-mentioned method and were found to be 20 mg / g and 8 mg / g, respectively. The BET specific surface area of ​​furnace black and acetylene black is 100 m ２ / g, 133m ２ / g.

[0056] The first positive electrode mixture slurry was then applied to both sides of a positive electrode current collector made of aluminum foil. The second positive electrode mixture slurry was then applied to the coating of the first positive electrode mixture slurry, and the coating was dried and compressed. The electrode was then cut to a predetermined electrode size, and a positive electrode was fabricated in which a two-layer structure positive electrode mixture layer including a lower layer (first positive electrode mixture layer) and an upper layer (second positive electrode mixture layer) was formed on both sides of the positive electrode current collector. An exposed portion was also provided on a portion of the positive electrode, exposing the surface of the positive electrode current collector. The first positive electrode mixture slurry and the second positive electrode mixture slurry were applied in a mass ratio of 40:60. That is, the ratio of the thickness of the first positive electrode mixture layer to the sum of the thicknesses of the first and second positive electrode mixture layers (thickness of the positive electrode mixture layer) was 0.4, and the ratio of the thickness of the second positive electrode mixture layer to the sum of the thicknesses of the first and second positive electrode mixture layers (thickness of the positive electrode mixture layer) was 0.6. The thickness of the positive electrode mixture layer was 80 μm on one side of the positive electrode current collector.

[0057] [Preparation of Negative Electrode] A mixture of graphite and SiO2 mixed in a mass ratio of 95:5 was used as the negative electrode active material. The above negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene copolymer (SBR) as a binder were mixed in a mass ratio of 98:1:1, and water was added to prepare a first negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and compressed, and then cut to a predetermined electrode size to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector. Furthermore, an exposed portion in which the surface of the negative electrode current collector was exposed was provided in a portion of the negative electrode.

[0058] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:3 (25°C). Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of the mixed solvent to prepare a non-aqueous electrolyte solution containing LiPF ６ was dissolved to a concentration of 1 mol / L to prepare a non-aqueous electrolyte solution.

[0059] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] A positive electrode lead was attached to the exposed portion of the positive electrode, and a negative electrode lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator between them to fabricate a wound electrode assembly. Then, insulating plates were placed on the top and bottom of the electrode assembly, respectively. The negative electrode lead was welded to the case body, and the positive electrode lead was welded to a sealing member. The electrode assembly was then housed in the case body. After injecting non-aqueous electrolyte into the case body under reduced pressure, the opening of the case body was sealed with a sealing member via a gasket to fabricate a test cell.

[0060] [Measurement of Interface Resistance] The interface resistance of the positive electrode was measured using an electrode resistance measuring device (device name: RM2610) manufactured by Hioki E.E. Corp. The measurement current was set to 100 μA and the voltage range was set to 0.5 V.

[0061] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that in fabricating the positive electrode, only the second positive electrode mixture slurry was applied to the surface of the positive electrode current collector. That is, the positive electrode of Comparative Example 1 had a single-layer structure. The thickness of the positive electrode mixture layer was 80 μm on one side of the positive electrode current collector, the same as in Example 1.

[0062] Comparative Example 2 A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that in fabricating the positive electrode, only the first positive electrode mixture slurry was applied to the surface of the positive electrode current collector. That is, the positive electrode of Comparative Example 2 had a single-layer structure. The thickness of the positive electrode mixture layer was 80 μm on one side of the positive electrode current collector, the same as in Example 1.

[0063] Comparative Example 3 A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the second positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and then the first positive electrode mixture slurry was applied onto the coating of the second positive electrode mixture slurry. That is, the positive electrode mixture layer of Comparative Example 3 had a two-layer structure in which a first positive electrode mixture layer made of the second positive electrode mixture slurry was disposed on the positive electrode current collector side, and a second positive electrode mixture layer made of the first positive electrode mixture slurry was disposed on the surface side of the positive electrode mixture layer. The first positive electrode mixture slurry and the second positive electrode mixture slurry were applied in a mass ratio of 60:40. That is, the ratio of the thickness of the first positive electrode mixture layer to the sum of the thicknesses of the first and second positive electrode mixture layers (the thickness of the positive electrode mixture layer) was 0.4, and the ratio of the thickness of the second positive electrode mixture layer to the sum of the thicknesses of the first and second positive electrode mixture layers (the thickness of the positive electrode mixture layer) was 0.6. The thickness of the positive electrode mixture layer was 80 μm on one side of the positive electrode current collector, similar to Example 1.

[0064] The interfacial resistance of each battery of the examples and comparative examples is shown in Table 1. Table 1 also shows the layer structure of the positive electrode mixture layer, the type of conductive agent contained in the positive electrode mixture layer, and the thickness ratio. The interfacial resistances of the examples and comparative examples shown in Table 1 are expressed relative to the interfacial resistance of Comparative Example 1, which is set to 100. A smaller value of the interfacial resistance means a lower resistance.

[0065]

[0066] As shown in Table 1, the interface resistance in Example 1 is reduced compared to the comparative example. Therefore, by providing the positive electrode with a two-layer structure consisting of a first positive electrode mixture layer and a second positive electrode mixture layer, and by providing a large amount of the first conductive agent having a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less in the first positive electrode mixture layer on the positive electrode current collector side, and providing a large amount of the second conductive agent having a hydrogen content of 0.5 mg / g or less in the second positive electrode mixture layer on the surface side, the interface resistance of the positive electrode can be reduced.

[0067] The present disclosure will be further described by the following embodiments. Configuration 1: 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 current collector and a positive electrode mixture layer disposed on a surface of the positive electrode current collector and containing a positive electrode active material, the positive electrode mixture layer contains a first conductive agent having a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less and a second conductive agent having a hydrogen content of 0.5 mg / g or less, the positive electrode mixture layer has a first positive electrode mixture layer disposed on the positive electrode current collector side and a second positive electrode mixture layer disposed on a surface side of the positive electrode mixture layer, the first positive electrode mixture layer contains more of the first conductive agent than the second conductive agent, on a mass basis, and the second positive electrode mixture layer contains more of the second conductive agent than the first conductive agent, on a mass basis. Aspect 2: The nonaqueous electrolyte secondary battery according to Aspect 1, wherein the carbon monoxide content of the first conductive agent is 10 mg / g or more and 50 mg / g or less, and the carbon monoxide content of the second conductive agent is less than 10 mg / g. Aspect 3: The BET specific surface area of ​​the first conductive agent is 60 m ２ / g or more, 110m ２ / g or less, and the BET surface area of ​​the second conductive agent is 110 m ２ / g, 180m ２ / g or less. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the first conductive agent is furnace black and the second conductive agent is acetylene black. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein a ratio of the thickness of the first positive electrode mixture layer to the sum of the thickness of the first positive electrode mixture layer and the thickness of the second positive electrode mixture layer is 0.1 or more and 0.6 or less. Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein, in the first positive electrode mixture layer, the content of the first conductive agent is 0.5% by mass or more and 2.0% by mass or less with respect to the total mass of the positive electrode active material. Configuration 7: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein, in the first positive electrode mixture layer, the content of the first conductive agent is 90% by mass or more with respect to the total mass of the conductive agents. Configuration 8: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein in the second positive electrode mixture layer, the content of the second conductive agent relative to the total mass of the positive electrode active material is 0.5 mass% or more and 2.0 mass% or less. Configuration 9: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein in the second positive electrode mixture layer, the content of the second conductive agent relative to the total mass of the conductive agents is 90 mass% or more.

[0068] REFERENCE SIGNS LIST 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 exterior body, 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, 29 winding shaft, 30 positive electrode current collector, 32 positive electrode mixture layer, 34 first positive electrode mixture layer, 36 second positive electrode mixture layer.

Claims

1. 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 current collector and a positive electrode mixture layer disposed on a surface of the positive electrode current collector and containing a positive electrode active material, the positive electrode mixture layer contains a first conductive agent having a hydrogen content of 1.0 mg / g or more and 2.0 mg / g or less, and a second conductive agent having a hydrogen content of 0.5 mg / g or less, the positive electrode mixture layer has a first positive electrode mixture layer disposed on the positive electrode current collector side, and a second positive electrode mixture layer disposed on a surface side of the positive electrode mixture layer, the first positive electrode mixture layer contains more of the first conductive agent than the second conductive agent, on a mass basis, and the second positive electrode mixture layer contains more of the second conductive agent than the first conductive agent, on a mass basis.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon monoxide content of the first conductive agent is 10 mg / g or more and 50 mg / g or less, and the carbon monoxide content of the second conductive agent is less than 10 mg / g.

3. The BET specific surface area of ​​the first conductive material is 60 m ２ / g or more, 110m ２ / g or less, and the BET surface area of ​​the second conductive agent is 110 m ２ / g, 180m ２ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the capacitance is 0.1 to 0.5 μm.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first conductive agent is furnace black and the second conductive agent is acetylene black.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of a thickness of the first positive electrode mixture layer to a sum of a thickness of the first positive electrode mixture layer and a thickness of the second positive electrode mixture layer is 0.1 or more and 0.6 or less.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the first positive electrode mixture layer, the content of the first conductive agent relative to the total mass of the positive electrode active material is 0.5 mass % or more and 2.0 mass % or less.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the first conductive agent in the first positive electrode mixture layer is 90 mass % or more with respect to the total mass of the conductive agents.

8. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the second positive electrode mixture layer, the content of the second conductive agent relative to the total mass of the positive electrode active material is 0.5 mass % or more and 2.0 mass % or less.

9. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the second conductive agent in the second positive electrode mixture layer is 90 mass % or more with respect to the total mass of the conductive agents.

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