Compound carbonaceous conductive material and nonaqueous electrolyte secondary battery

Abstract

To provide a carbonaceous conductive material that can sufficiently suppress the deterioration of a carbonaceous conductive material and the side reaction under a high-voltage environment and sufficiently suppress the generation of oxidation current even in a case where a positive electrode of a battery with the specification for the high voltage is in the high-potential state and exposed to a strong oxidizing environment.SOLUTION: A compound carbonaceous conductive material includes carbon nanotube, and a coating layer that coats a surface of the carbon nanotube. The coating layer contains a nitrogen element (N), a boron element (B), and an oxygen element (O). The ratio (B / N ratio) of the content of the boron element (B) to the content of the nitrogen element (N) in the coating layer is 0.7 or more and 1.3 or less. The amount of the boron element (B) in the coating layer is 1.0 mass% or more and 21 mass% or less per 100 mass% of the compound carbonaceous conductive material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a composite carbonaceous conductive material and a non-aqueous electrolyte secondary battery containing the same. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, including lithium-ion secondary batteries, are widely used as power sources for smartphones, notebook computers, and the like, and have recently been used in large-scale batteries for vehicles and the like. To achieve even higher capacity and power output, there is a demand for higher-voltage batteries.

[0003] The positive electrode of a lithium ion secondary battery contains not only a positive electrode active material, which is its main constituent, but also a carbonaceous conductive material such as carbon nanotubes, which has excellent electronic conductivity and complements the electronic conductivity of the positive electrode. In a high-voltage battery as described above, the positive electrode is in a high-potential state and is exposed to a strong oxidizing environment. When the carbonaceous conductive material in the positive electrode is placed in a high-potential environment, the carbonaceous conductive material itself deteriorates or a side reaction occurs between the carbonaceous conductive material and the non-aqueous electrolyte in the lithium-ion secondary battery, generating an oxidation current that does not contribute to the performance of the battery, resulting in a decrease in the performance of the battery as a whole.

[0004] Therefore, as a method for suppressing the deterioration and side reactions of the carbonaceous conductive material, it has been considered to provide a coating layer that covers the surface of the carbonaceous conductive material, as described in Patent Document 1 or Patent Document 2. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-099355 [Patent Document 2] Japanese Patent Application Publication No. 2023-155562 Summary of the Invention [Problem to be solved by the invention]

[0006] However, according to the studies of the present inventors, it has been found that the methods described in Patent Documents 1 and 2 do not sufficiently suppress deterioration and side reactions of the carbonaceous conductive material, particularly in a high-voltage environment higher than conventional ones. The high-voltage environment here refers to a voltage environment of, for example, more than 4.5 V, or even 5.0 V or higher.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a carbonaceous conductive material that can sufficiently suppress the deterioration and side reactions of the carbonaceous conductive material in a high-voltage environment and can sufficiently suppress the generation of oxidation current, even when the positive electrode of a high-voltage battery is in a high-potential state and exposed to a strong oxidizing environment. [Means for solving the problem]

[0008] That is, the present invention is as follows. [1] Carbon nanotubes and a coating layer that coats the surface of the carbon nanotube, the coating layer contains nitrogen (N), boron (B), and oxygen (O), a ratio (B / N ratio) of the content of boron element (B) to the content of nitrogen element (N) in the coating layer is 0.7 or more and 1.3 or less in mass ratio; A composite carbonaceous conductive material, wherein the coating layer contains boron (B) in an amount of 1.0% by mass or more and 21% by mass or less, relative to 100% by mass of the composite carbonaceous conductive material. [2] The composite carbonaceous conductive material according to [1], wherein the total content of the nitrogen element (N) and the nitrogen element (B) contained in the coating layer is 2.0 mass% or more and 50.0 mass% or less relative to 100 mass% of the composite carbonaceous conductive material. [3] The composite carbonaceous conductive material according to [1] or [2], wherein the coating layer contains boron nitride. [4] The composite carbonaceous conductive material according to any one of [1] to [3], wherein the amount of boron (B) contained in the coating layer is 2% by mass or more and 15% by mass or less, relative to 100% by mass of the composite carbonaceous conductive material. [5] The composite carbonaceous conductive material according to any one of [1] to [3], wherein the amount of boron (B) contained in the coating layer is more than 3.6 mass% and not more than 10 mass% relative to 100 mass% of the composite carbonaceous conductive material. [6] The composite carbonaceous conductive material according to any one of [1] to [5], wherein the ratio of the content of boron element (B) and nitrogen element (N) to the content of oxygen element (O) in the composite carbonaceous conductive material ((B+N) / O ratio) is 0.5 or more and 8.0 or less in mass ratio. [7] The composite carbonaceous conductive material according to any one of [1] to [6], wherein the coating layer has a thickness of 0.3 nm or more and 20 nm or less. [8] A positive electrode for a secondary battery containing a positive electrode active material and a conductive material, The conductive material is the composite carbonaceous conductive material according to any one of [1] to [7]. [9] The positive electrode for a secondary battery according to [8], further containing a sulfide solid electrolyte.

[10] A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution, A non-aqueous electrolyte secondary battery, wherein the positive electrode is the positive electrode for a non-aqueous electrolyte secondary battery according to [8] or [9].

[11] The nonaqueous electrolyte secondary battery according to

[10] , which can be charged and discharged at a voltage exceeding 4.5 V.

[12] A solid secondary battery comprising a positive electrode, a negative electrode, and a solid electrolyte layer, A solid secondary battery, wherein the positive electrode is the positive electrode for secondary batteries according to [8] or [9].

[13] The solid secondary battery according to

[12] , wherein the solid electrolyte layer contains a sulfide solid electrolyte.

[14] The solid secondary battery according to

[12] or

[13] , which can be charged and discharged at a voltage exceeding 4.5 V.

[15] a precursor layer forming step of forming a precursor layer made of boron oxide so as to cover a part or all of the surface of the carbon nanotubes; a nitriding step of nitriding the boron oxide forming the precursor layer to convert the precursor layer into a coating layer containing elemental boron and elemental nitrogen, a ratio (B / N ratio) of the content of boron element (B) to the content of nitrogen element (N) in the coating layer is 0.7 or more and 1.3 or less in mass ratio; The method for producing a composite carbonaceous conductive material, wherein the amount of boron (B) contained in the coating layer is 1.0% by mass or more and 21% by mass or less relative to 100% by mass of the composite carbonaceous conductive material.

[16] The method for producing a composite carbonaceous conductive material according to

[15] , wherein the precursor layer forming step includes a step of immersing the carbon nanotubes in a solution containing boron element (B) on the surface thereof, or a step of applying a solution containing boron element (B) to the surface of the carbon nanotubes.

[17] The method for producing a composite carbonaceous conductive material according to

[15] or

[16] , wherein the coating layer contains boron nitride.

[18] The method for producing a composite carbonaceous conductive material according to any one of

[15] to

[17] , wherein the coating layer has a thickness of 0.3 nm or more and 20 nm or less. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a carbonaceous conductive material that can sufficiently suppress the deterioration and side reactions of the carbonaceous conductive material in a high-voltage environment and sufficiently suppress the generation of oxidation current, even when the positive electrode of a high-voltage battery is in a high-potential state and exposed to a strong oxidizing environment. DETAILED DESCRIPTION OF THE INVENTION

[0010] A specific configuration of a secondary battery according to one embodiment of the present invention will be described below. <1. Basic structure of non-aqueous electrolyte secondary battery> As shown in FIG. 1, the nonaqueous electrolyte secondary battery according to this embodiment is a lithium ion secondary battery including a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte solution, and a case that accommodates these components. The shape of the lithium ion secondary battery is not particularly limited, but may be, for example, any of a cylindrical shape, a prismatic shape, a laminate shape, a button shape, or the like.

[0011] (1-1. Positive electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode current collector may be any conductive material, for example, a plate or foil, and is preferably made of aluminum, stainless steel, nickel-plated steel, or the like. The positive electrode mixture layer contains a positive electrode active material, and may further contain a conductive material and a positive electrode binder.

[0012] The positive electrode active material is, for example, a lithium-containing transition metal oxide or solid solution oxide, and is not particularly limited as long as it is a material that can electrochemically absorb and release lithium ions. The shape of the positive electrode active material is not particularly limited, but it is preferably in the form of particles. Examples of the lithium-containing transition metal oxide include Li 1.0 Ni 0.88 Co 0.1 Al 0.01 Mg 0.01 O2, but also LiCoO2 and other Li-Co based composite oxides, LiNi x Co y Mn z Examples of solid solution oxides include Li·Ni·Co·Mn-based composite oxides such as LiO2, Li·Ni-based composite oxides such as LiNiO2, and Li·Mn-based composite oxides such as LiMn2O4. a Mn x Co y Ni zO2 (1.150≦a≦1.430, 0.45≦x≦0.6, 0.10≦y≦0.15, 0.20≦z≦0.28), LiMn 1.5 Ni 0.5 Examples include O4, etc. The content (content ratio) of the positive electrode active material is not particularly limited, and may be any content that is applicable to the positive electrode composite layer of a non-aqueous electrolyte secondary battery. These compounds may be used alone or in combination.

[0013] The positive electrode mixture layer according to this embodiment contains at least a composite carbonaceous conductive material, which will be described later, as the conductive material. Specific examples of the conductive material include, in addition to the composite carbonaceous conductive material described below, materials containing one or more selected from carbon black, natural graphite, artificial graphite, fibrous carbon, and sheet-like carbon. Examples of the carbon black include furnace black, channel black, thermal black, ketjen black, and acetylene black. Examples of the fibrous carbon include carbon nanotubes and carbon nanofibers, and examples of the sheet-like carbon include graphene. The content of the conductive material in the positive electrode mixture layer is not particularly limited, but from the viewpoint of achieving both electrical conductivity and battery capacity, it is preferably 0.005% by mass to 5% by mass, more preferably 0.1% by mass to 3% by mass, and even more preferably 0.1% by mass to 2% by mass, based on the total positive electrode mixture layer. When the positive electrode contains multiple types of conductive materials, the content of the conductive material refers to the total content of these materials.

[0014] Examples of the positive electrode binder include fluorine-containing resins such as polyvinylidene fluoride, ethylene-containing resins such as styrene-butadiene rubber, ethylene-propylene-diene terpolymer, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, polyvinyl alcohol, carboxymethyl cellulose or carboxymethyl cellulose derivatives (such as carboxymethyl cellulose salts), and nitrocellulose. The positive electrode binder is not particularly limited as long as it can bind the positive electrode active material and the conductive material onto the positive electrode current collector.

[0015] (1-2. Negative electrode) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode current collector may be any conductive material, and is preferably, for example, in the form of a plate or foil, and is made of copper, stainless steel, nickel-plated steel, or the like.

[0016] The negative electrode mixture layer contains a negative electrode active material, and may further contain a conductive material and a negative electrode binder. The negative electrode active material is not particularly limited as long as it can electrochemically absorb and release lithium ions. Examples of the negative electrode active material include graphite active materials (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, etc.), Si-based active materials or Sn-based active materials (for example, a mixture of fine particles of silicon (Si) or tin (Sn) or an oxide thereof with a graphite active material, fine particles of silicon or tin, and an alloy based on silicon or tin), metallic lithium, and Li4Ti5O 12 Examples of the negative electrode active material include titanium oxide compounds such as those listed above, and lithium nitrides. One of the above-listed materials may be used alone, or two or more may be used in combination. Silicon oxide is represented by SiOx (0≦x≦2).

[0017] The conductive material is not particularly limited as long as it can increase the conductivity of the negative electrode, and for example, the same materials as those described in the section on the positive electrode can be used. The content of the conductive material in the negative electrode composite layer is not particularly limited, but from the viewpoint of achieving both electrical conductivity and battery capacity, it is preferably 0.005% by mass or more and 5% by mass or less, and more preferably 0.1% by mass or more and 3% by mass or less, of the entire negative electrode composite layer.

[0018] The negative electrode binder is not particularly limited as long as it can bind the negative electrode active material and the conductive material to the negative electrode current collector. The negative electrode binder may be, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene-butadiene copolymer (SBR), or metal salt of carboxymethylcellulose (CMC). One type of binder may be used alone, or two or more types may be used in combination.

[0019] (1-3. Separator) The separator is not particularly limited, and any separator suitable for use in lithium ion secondary batteries may be used. As the separator, a porous membrane or a nonwoven fabric, which exhibits excellent high-rate discharge performance, is preferably used alone or in combination. Resins constituting the separator include, for example, polyolefin resins typified by polyethylene, polypropylene, etc., polyester resins typified by polyethylene terephthalate, polybutylene terephthalate, etc., polyvinylidene difluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene difluoride-tetrafluoroethylene copolymer, vinylidene difluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene di ... copolymer), vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene difluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymerExamples of the porosity of the separator 3 include vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, etc. The porosity of the separator 3 is not particularly limited, and any porosity of the separators of conventional lithium ion secondary batteries can be applied.

[0020] The separator may further include a surface layer that covers the surface of the porous membrane or nonwoven fabric. The surface layer may include an adhesive for bonding to the electrodes to fix the battery element. Examples of the adhesive include vinylidene fluoride-hexafluoropropylene copolymer, acid-modified vinylidene fluoride polymer, and styrene-(meth)acrylic acid ester copolymer.

[0021] (1-4.Non-aqueous electrolyte) The nonaqueous electrolyte may be the same as any nonaqueous electrolyte conventionally used in lithium ion secondary batteries, and has a composition in which an electrolyte salt is contained in a nonaqueous solvent that is a solvent for the electrolyte. Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate; methylformate, methylacetate, methylbutyrate, ethylpropionate, and propylpropionate. propionate, tetrahydrofuran or its derivatives, 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane (1,4-dibutoxyethane, or methyldiglyme, ethers such as ethylene glycol monopropyl ether and propylene glycol monopropyl ether, nitriles such as acetonitrile and benzonitrile, dioxolane or its derivatives, ethylene sulfide, sulfolane, sultone or its derivatives, etc., can be used alone or in combination of two or more. When two or more of the nonaqueous solvents are used in combination, the mixing ratio of the nonaqueous solvents can be the same as that used in conventional lithium ion secondary batteries.

[0022] Examples of the electrolyte salt include LiClO4, LiBF4, LiAsF6, LiPF6, and LIPF6-x (C n F 2n+1 )x[However, 1 <x<6、n=1or2]、LiSCN、LiBr、LiI、Li2SO4、Li2B 10 Cl 10, inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K) such as NaClO4, NaI, NaSCN, NaBr, KClO4, KSCN, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, (CH3)4NBF4, (CH3)4NBr, (C2H5)4NClO4, (C2H5)4NI, (C3H7)4NBr, (n-C4H9)4NClO4, (n-C4H9)4NI, (C2H5)4N-maleate, (C2H5)4N-benzoate, (C2H5)4N-phtalate, lithium stearyl sulfonic acid Examples of suitable ionic salts include organic ionic salts such as lithium, octyl sulfonic acid lithium, and dodecyl benzenesulfonic acid lithium. These ionic compounds can be used alone or in combination of two or more. The concentration of the electrolyte salt can be the same as that of non-aqueous electrolytes used in conventional lithium-ion secondary batteries and is not particularly limited. In this embodiment, it is preferable to use a non-aqueous electrolyte containing the above-mentioned lithium compound (electrolyte salt) at a concentration of approximately 0.8 mol / L to 1.5 mol / L.

[0023] Various additives may be added to the non-aqueous electrolyte. Examples of such additives include negative electrode additives, positive electrode additives, ester-based additives, carbonate-based additives, sulfate-based additives, phosphate-based additives, borate-based additives, acid anhydride-based additives, and electrolyte-based additives. Any one of these additives may be added to the non-aqueous electrolyte, or multiple additives may be added to the non-aqueous electrolyte.

[0024] 2. Characteristic Configuration of the Non-Aqueous Electrolyte Secondary Battery According to the Present Embodiment The characteristic configuration of the nonaqueous electrolyte secondary battery according to this embodiment will be described below. The positive electrode mixture layer included in the positive electrode of the nonaqueous electrolyte secondary battery according to this embodiment contains a composite carbon-based conductive material as the conductive material, and more preferably contains only a composite carbonaceous conductive material as the conductive material.

[0025] This composite carbon-based conductive material includes carbon nanotubes and a coating layer that coats the surfaces of the carbon nanotubes. More specifically, the carbon nanotubes and the coating layer that coats the surfaces of the carbon nanotubes are combined and integrated into one.

[0026] The carbon nanotubes are not particularly limited as long as they can be used as a conductive material in secondary batteries, and various types can be used. For example, carbon nanotubes having a single-layer structure or a multi-layer structure having two or more layers may be used.

[0027] The carbon nanotubes contained in the composite carbonaceous conductive material can be any commercially available product. The outer diameter of the carbon nanotubes is preferably 0.4 nm to 50 nm, which can further enhance conductivity. Furthermore, the length of the carbon nanotubes is preferably 100 nm to 5 mm, and more preferably 1 μm to 3 mm, which can further improve the formation of a conductive path in the positive electrode composite layer.

[0028] The coating layer contains nitrogen (N) and boron (B), and the ratio of the boron (B) content to the nitrogen (N) content in the coating layer (B / N mass ratio) is set to 0.7 or more and 1.3 or less. This B / N mass ratio is more preferably 0.7 or more and 0.9 or less, and even more preferably 0.7 or more and 0.85 or less.

[0029] The coating layer also contains oxygen (O), and is preferably adjusted so that the ratio of the content of boron (B) and nitrogen (N) to the content of oxygen (O) in the composite carbonaceous conductive material ((B+N) / O ratio) is 0.5 or more and 8.0 or less in mass ratio. This (B+N) / O mass ratio is more preferably 0.7 or more and 3.0 or less, and even more preferably 1.0 or more and 2.5 or less. This oxygen is thought to be derived from boric acid that is initially added to the carbon nanotubes when forming the coating layer of the composite carbonaceous conductive material according to this embodiment.

[0030] Examples of such coating layers include those containing boron nitride, and more preferably those formed of chemically stable boron nitride. The coating layer may be a single layer or a multilayer. For example, when the coating layer contains boron nitride, the boron nitride may be in the form of hexagonal boron nitride (h-BN), which is energetically very stable. This h-BN is known to have a layered structure similar to graphite and carbon nanotubes. When the coating layer contains a large amount of boron nitride, the layered structure of this boron nitride may result in the coating layer having a multilayer structure.

[0031] The amount of boron (B) contained in the coating layer is adjusted to be 1.0 mass % or more and 21 mass % or less with respect to 100 mass % of the composite carbonaceous conductive material.

[0032] The amount of boron (B) contained in the coating layer is preferably 2.5 mass% or more and 15 mass% or less, more preferably more than 3.6 mass% and 10 mass% or less, and particularly preferably more than 3.6 mass% and 4.5 mass% or less, relative to 100 mass% of the composite carbonaceous conductive material.

[0033] The total content of boron (B) and nitrogen (N) in the coating layer is preferably about 2% by mass or more and 50% by mass or less, based on 100% by mass of the composite carbonaceous conductive material. For example, when the coating layer is formed only from boron nitride as described above, the total content of boron (B) and nitrogen (N) in the coating layer is about 5.8% by mass or more and 48% by mass or less, based on 100% by mass of the composite carbonaceous conductive material.

[0034] The thickness of the coating layer is preferably 0.3 nm to 20 nm, more preferably 0.3 nm to 18 nm, even more preferably 0.3 nm to 17 nm, and particularly preferably 0.3 nm to 10 nm. Even when the coating layer is formed of an insulating material such as boron nitride, a coating layer thickness of 20 nm can sufficiently maintain the conductivity of the positive electrode composite layer. Furthermore, a coating layer thickness of 0.3 nm or more can fully suppress the deterioration of the carbon nanotubes and side reactions between the carbon nanotubes and other battery components such as the electrolyte. Note that a thickness of 0.3 nm is approximately the same as the thickness of one layer of h-BN, which is known to have a hexagonal crystal structure similar to graphite. The thickness of the coating layer can be adjusted, for example, by changing the amount of boric acid added to the carbon nanotubes when forming the coating layer.

[0035] The coating layer may be formed in the form of islands on a portion of the surface of the carbon nanotube, but it is preferable that the coating layer uniformly covers the entire surface of the carbon nanotube to the extent that the carbon nanotube and the nonaqueous electrolyte or solid electrolyte cannot come into direct contact with each other.

[0036] The content of the composite carbonaceous conductive material in the positive electrode composite layer is preferably adjusted so that the content of the conductive material as a whole is within the range of the total content of the conductive material in the positive electrode composite layer described above. From the viewpoint of production costs, however, the content of the composite carbonaceous conductive material in the positive electrode composite layer is preferably 0.005% by mass or more and 3.0% by mass or less, and more preferably 0.005% by mass or more and 2.0% by mass or less.

[0037] 3. Method for manufacturing a non-aqueous electrolyte secondary battery according to this embodiment Next, a method for manufacturing a nonaqueous electrolyte secondary battery according to this embodiment will be described. (3-1. Method for producing composite carbonaceous conductive material) The composite carbonaceous conductive material can be prepared by, for example, a precursor layer forming step of forming a precursor layer made of boron oxide so as to cover a part or all of the surface of the carbon nanotube; The composite carbonaceous conductive material can be produced by a method for producing the composite carbonaceous conductive material, which includes a nitriding step of nitriding the boron oxide forming the precursor layer to convert the precursor layer into a coating layer containing elemental boron and elemental nitrogen.

[0038] The precursor layer forming process may include, for example, a process of immersing the carbon nanotubes in a solution containing boron element (B) on the surface thereof, or a process of applying a solution containing boron element (B) to the surface of the carbon nanotubes. The carbon nanotubes having the solution attached to their surfaces in this manner are dried, for example, by natural drying, to form a precursor layer. The solution containing boron element (B) is not particularly limited, but may be a solution in which a boron-containing compound is dissolved in an appropriate solvent. Examples of boron-containing compounds include boric acid. Examples of solvents for dissolving boric acid include solvents that can be evaporated by natural drying, such as ethanol, but the combinations are not limited to these. The precursor layer can be more stably supported on the surface of the carbon nanotubes, for example, by heating in an inert gas atmosphere such as argon after or instead of the natural drying described above. The heating temperature and heating time can be changed as appropriate, but for example, heating at a temperature of 200°C to 600°C for 5 to 60 minutes is preferred, and heating at a temperature of 300°C to 500°C for 15 to 45 minutes is more preferred.

[0039] The precursor layer formed on the surface of the carbon nanotubes by the method described above contains boron oxide, so a nitriding step is then carried out to nitride this boron oxide to form a coating layer. This nitriding step can be carried out by heating the precursor layer in an atmosphere of a nitrogen-containing gas such as ammonia gas, etc. The heating temperature and heating time can be changed as appropriate, but are preferably, for example, at a temperature of 800°C to 1500°C for 5 to 90 minutes, and more preferably at a temperature of 1000°C to 1200°C for 15 to 60 minutes.

[0040] (3-2. Method for producing positive electrode) The positive electrode is fabricated, for example, as follows. First, the composite carbonaceous conductive material produced as described above, the positive electrode active material, and the positive electrode binder are mixed in a desired ratio, and the mixture is dispersed in a positive electrode slurry solvent to form a positive electrode slurry. Next, this positive electrode slurry is applied to a positive electrode current collector and dried to form a positive electrode composite layer. The application method is not particularly limited. Examples of application methods include a knife coater method, a gravure coater method, a reverse roll coater, and a slit die coater. The following application steps are also performed in the same manner. Next, the positive electrode composite layer is pressed using a press to a desired density. This completes the fabrication of the positive electrode.

[0041] (3-3. Method for producing negative electrode) The negative electrode may be fabricated in the same manner as the positive electrode. First, a mixture of materials constituting the negative electrode composite layer is dispersed in a negative electrode slurry solvent to prepare a negative electrode slurry. Next, the negative electrode slurry is applied to a negative electrode current collector and dried to form a negative electrode composite layer. Next, the negative electrode composite layer is pressed to a desired density using a press. This completes the negative electrode fabrication.

[0042] (3-4. Method for manufacturing non-aqueous electrolyte secondary battery) Next, the separator is sandwiched between the positive electrode and the negative electrode to produce an electrode structure. The electrode structure is then processed into a desired shape (e.g., cylindrical, rectangular, laminated, button-shaped, etc.) and inserted into a container of that shape. Next, a nonaqueous electrolyte is injected into the container, thereby impregnating the pores in the separator and the gaps in the positive electrode and negative electrode with the electrolyte. Through the above steps, a lithium-ion secondary battery can be produced.

[0043] 4. Effects of this embodiment According to the nonaqueous electrolyte secondary battery configured as described above, it is possible to provide a composite carbonaceous conductive material, a positive electrode for a secondary battery, and a nonaqueous electrolyte secondary battery that can sufficiently suppress the deterioration and side reactions of the carbonaceous conductive material in a high-voltage environment and sufficiently suppress the generation of oxidation current, even when the positive electrode in a high-voltage battery is in a high-potential state and exposed to a strong oxidizing environment.

[0044] 5. Other embodiments of the present invention The present invention is not limited to the above-described embodiment. In the above-described embodiment, the composite carbonaceous conductive material is contained in the positive electrode composite layer. However, the composite carbonaceous conductive material according to the present invention may be contained in other layers constituting the secondary battery, such as a protective layer that protects the negative electrode composite layer or the positive electrode composite layer.

[0045] Furthermore, in the above-described embodiment, a nonaqueous electrolyte solid secondary battery including a separator and an electrolytic solution has been described as an example of a secondary battery. However, the secondary battery is not limited to this, and the composite carbonaceous conductive material according to the present invention can be widely used as a conductive material in semi-solid batteries including a gel-like electrolytic solution, solid batteries including a solid electrolyte layer made of a sulfide solid electrolyte or the like, and all-solid batteries not including any electrolytic solution at all. Furthermore, the present invention is not limited to these embodiments, and it goes without saying that various modifications are possible without departing from the spirit of the present invention. [Example]

[0046] The present invention will be described in more detail below based on specific examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples. In the following examples, composite carbonaceous conductive materials having the properties shown in Table 1 were first produced, and nonaqueous electrolyte secondary batteries and solid secondary batteries were fabricated using these materials, and the generation of oxidation current was examined for each of the fabricated batteries.

[0047] <Preparation of composite carbonaceous conductive material> Example 1 First, a boric acid-ethanol solution was prepared by dispersing 0.48 g of boric acid in 10 mL of ethanol. 450 mg of Sigma-Aldrich multiwall carbon nanotubes (MWCNT) were added to the boric acid-ethanol solution and allowed to dry overnight at room temperature while stirring to produce MWCNTs loaded with boric acid. The mixture was then heated to 400 °C in a tubular electric furnace under an argon atmosphere and maintained for 30 minutes to prepare MWCNTs loaded with boron oxide. This was followed by nitriding at 1100 °C for 40 minutes in an ammonia / argon mixture (1:4) to produce MWCNTs coated with boron nitride, yielding composite carbonaceous conductive material (A).

[0048] Example 2 10 mg of the MWCNTs of Example 1 were dispersed in 140 mL of 2-propanol, filtered through a membrane filter by suction, dried under vacuum at 120°C, and then punched out into an 11 mm diameter circle to produce an MWCNT film. The MWCNT film was then immersed for 85 minutes in a boric acid solution prepared by dispersing 0.48 g of boric acid in 10 mL of water to prepare an MWCNT film carrying boric acid. A self-supporting film-like composite carbonaceous conductive material (B) was obtained by the same procedure as in Example 1.

[0049] Example 3 A composite carbonaceous conductive material (C) was obtained in the same manner as in Example 1, except that 230 mg of MWCNT was used.

[0050] Example 4 A composite carbonaceous conductive material (D) was obtained in the same manner as in Example 1, except that 90.5 mg of MWCNT was used.

[0051] (Comparative Example 2) A composite carbonaceous conductive material (E) was obtained in the same manner as in Example 1, except that 22.7 mg of MWCNT was used.

[0052] <Evaluation of composite carbonaceous conductive materials> The composite carbonaceous conductive materials of Examples 1 and 2 and Comparative Examples 1 and 2 prepared by the above-described method were evaluated as follows. (Elemental analysis) The boron and nitrogen contained in each composite carbonaceous conductive material were quantitatively analyzed using an X-ray photoelectron spectroscopy system (XPS, Thermo Fisher Scientific, ESCALAB 250Xi model) according to JIS K0152:2014. The quantitative analysis revealed that the coating layer of each composite carbonaceous conductive material was composed of boron, nitrogen, and oxygen, and the boron and nitrogen content in the composite carbonaceous conductive material is shown in Table 1. Note that oxygen is contained not only in the coating layer but also in the carbon nanotubes, whereas nitrogen and boron are contained only in the coating layer. Furthermore, the XPS analysis revealed that some of the constituent elements of the coating layer are supported by chemical bonding between the carbon nanotubes and the coating layer. As an example of such chemical bonds, a covalent bond (NC) between the nitrogen element constituting the coating layer and the carbon element constituting the CNT has been observed, and the existence of a bond (BOC) between boron and carbon elements via an oxygen element has also been inferred based on information such as the abundance ratio of the covalent bond between boron and oxygen (BO) and the covalent bond between oxygen and carbon (OC).

[0053] (Measurement of coating layer thickness) The thickness of the coating layer of each composite carbonaceous conductive material was measured as follows. First, images of the composite carbonaceous conductive material were obtained by a transmission electron microscope (TEM) and an electron energy loss spectroscopy (EELS), as well as elemental mapping images. A JEOL field emission transmission electron microscope (JEM-ARM200F) and a Gatan electron energy loss spectroscopy (GIF QUANTUM ER) were used. The thickness of the coating layer of the composite carbonaceous conductive material contained in the TEM image and identified from the transmission observation image and elemental mapping image was measured at 10 locations, and the average value of these measurements was taken as the coating layer thickness.

[0054] (Evaluation of sheet resistance) Five milligrams of any one of the carbonaceous conductive materials prepared as described above in Examples 1, 3, and 4, and Comparative Examples 1 and 2, was dispersed in 30 mL of a 0.5% by mass aqueous solution of sodium dodecylbenzenesulfonate (SDBS), and the dispersion was suction-filtered onto a membrane filter. For Example 1 and Comparative Example 1, the dispersion was peeled from the membrane filter, vacuum-dried at 120°C, and punched into an 11 mm diameter circle to produce a free-standing membrane. For Examples 3, 4, and Comparative Example 2, the dispersion was left to air-dry at room temperature without being peeled from the membrane filter, and then used as a sample for sheet resistance measurement. The sheet resistance of each of these films and that of Example 2 were measured using a voltage-current generating measurement device (Keithley Instruments, 2400 Series Source Meter) using the four-probe method, and compared. The results are shown in Table 1.

[0055] <Evaluation of voltage resistance: Cyclic voltammetry (CV) measurements (1 and 2): Non-aqueous electrolyte> (Preparation of a free-standing positive electrode film for evaluation) 5 mg of any one of the carbonaceous conductive materials of Example 1 and Comparative Example 1 (carbon nanotubes (MWCNTs) not forming a coating layer) prepared as described above was dispersed in 30 mL of a 0.5 mass % SDBS aqueous solution, filtered through a membrane filter by suction, and then vacuum dried at 120°C. After that, a free-standing film with an 11 mm diameter was punched out, and this was used as a positive electrode for CV(1) evaluation.

[0056] (Counter electrode and electrolyte, preparation of coin cell for CV(1)) A porous polypropylene separator and a counter electrode made of metallic lithium were punched out to a size that fit the coin cell. The electrolyte was a 1.0 M LiPF6 solution in a 50 / 50 (volume ratio) mixture of ethylene carbonate and diethyl carbonate. Coin cells (2032) were fabricated using the CV evaluation electrode fabricated as described above, the separator, and the counter electrode, and the CV(1) evaluation cells for Examples 1 and 2 and Comparative Example 1 listed in Table 1 were fabricated.

[0057] (Electrolyte, preparation of coin cells for CV(2)) The electrolyte used was a 10 / 15 / 75 (volume ratio) mixture of ethylene carbonate, propylene carbonate, and ethyl propionate in which 1.3 M of LiPF6 was dissolved. The positive electrode used was a free-standing film of the composite carbonaceous conductive material of Example 2. Except for this, coin cells (2032) were prepared in the same manner as in the preparation of the CV(1) evaluation cell, and CV(2) evaluation cells for Example 2 and Comparative Example 1 listed in Table 1 were prepared.

[0058] (CV measurements 1 and 2) Using a VMP-3 Potentiostat manufactured by BioLogic as a CV measurement device, the current values ​​were measured by scanning the oxidation-reduction potential in the range of 3.0 V to 5.0 V (vs. Li) at a scanning rate of 5 mV / min for each of the CV (1 and 2) evaluation cells of Examples 1 and 2 and Comparative Example 1, and the current values ​​at 5 V were compared. The results are shown in Table 1.

[0059] <Evaluation of Voltage Withstand: Cyclic Voltammetry (CV) Measurement (3): Solid Electrolyte> (Preparation of positive electrode pellets for evaluation) Any one of the carbonaceous conductive materials of Example 3 and Comparative Example 1 and a sulfide solid electrolyte (Li6PS5Cl) were dispersed in a solid content ratio of 5:95 by mass using a dry mixer (FM3 Mixer, manufactured by Nippon Coke and Engineering Co., Ltd.) at 2000 rpm for 3 minutes, and then pellets of 10 mm diameter x 0.1 mm thickness were produced using a powder molding press (Lab Press, manufactured by LabNect) at a pressure of 15 MPa, to prepare positive electrode pellets for CV(2) evaluation.

[0060] (Preparation of separator pellets for evaluation) The sulfide solid electrolyte (Li6PS5Cl) was formed into a pellet of 10 mm diameter x 0.5 mm thickness at a pressure of 15 MPa using the above-mentioned powder molding press, and this was used as a solid electrolyte layer pellet for CV evaluation.

[0061] (CV(3) Preparation of evaluation cell) The positive electrode pellet and solid electrolyte layer pellet for CV evaluation described above were used, and metallic lithium was punched out to a size of 10 mm diameter for the counter electrode. The positive electrode pellet for CV(2) evaluation, the solid electrolyte layer pellet, and the counter electrode were then placed in a solid battery cell (KP-SolidCell, manufactured by Hosen Co., Ltd.) in this order, and pressurized at a pressure of 20 MPa to produce CV(3) evaluation cells for Example 3 and Comparative Example 1 listed in Table 1.

[0062] (CV measurement 3) The CV measurement device used was a VMP-3 Potentiostat manufactured by BioLogic, and the current values ​​were measured by scanning the oxidation-reduction potential in the range of 1.1 V to 6.0 V (vs. Li) at a scanning rate of 5 mV / min for each of the CV(3) evaluation cells of Example 3 and Comparative Example 1, and the current values ​​at 6.0 V were compared. The results are shown in Table 1.

[0063] [Table 1]

[0064] <Considerations on Examples and Comparative Examples> The results in Table 1 show that in Examples 1 to 3, the oxidation current was suppressed in all evaluations, including the CV evaluations using a liquid nonaqueous electrolyte (1 and 2) and the CV evaluation using a solid electrolyte (3), compared to Comparative Example 1, which did not contain boron nitride. This effect is believed to be due to the formation of a coating layer containing boron nitride, which is electrochemically very stable, on the surface of the carbon nanotubes. On the other hand, in Comparative Example 2, in which the content of boron element in the composite carbonaceous conductive material exceeded 21 mass %, the sheet resistance was more than 100 times higher than that of Comparative Example 1, resulting in the material being unusable as a conductive material. From the above experimental results, it was found that, according to the present invention, by forming a coating layer containing predetermined amounts of boron and nitrogen elements on a carbonaceous conductive material, it is possible to suppress side reactions between the carbonaceous conductive material and the electrolyte while maintaining the conductivity of the conductive material.

Claims

1. Carbon nanotubes and a coating layer that coats the surface of the carbon nanotube, the coating layer contains nitrogen (N), boron (B), and oxygen (O), a ratio (B / N ratio) of the content of boron element (B) to the content of nitrogen element (N) in the coating layer is 0.7 or more and 1.3 or less in mass ratio; A composite carbonaceous conductive material, wherein the coating layer contains boron (B) in an amount of 1.0% by mass or more and 21% by mass or less, relative to 100% by mass of the composite carbonaceous conductive material.

2. 2. The composite carbonaceous conductive material according to claim 1, wherein a total content of nitrogen element (N) and nitrogen element (B) contained in the coating layer is 2.0 mass% or more and 50.0 mass% or less, relative to 100 mass% of the composite carbonaceous conductive material.

3. The composite carbonaceous conductive material of claim 1 , wherein the coating layer comprises boron nitride.

4. 2. The composite carbonaceous conductive material according to claim 1, wherein the coating layer contains boron (B) in an amount of 2% by mass or more and 15% by mass or less relative to 100% by mass of the composite carbonaceous conductive material.

5. 2. The composite carbonaceous conductive material according to claim 1, wherein the coating layer contains boron (B) in an amount of more than 3.6 mass % and not more than 10 mass % relative to 100 mass % of the composite carbonaceous conductive material.

6. 2. The composite carbonaceous conductive material according to claim 1, wherein a ratio of the content of boron element (B) and nitrogen element (N) to the content of oxygen element (O) in the composite carbonaceous conductive material ((B+N) / O ratio) is 0.5 or more and 8.0 or less in mass ratio.

7. 2. The composite carbonaceous conductive material according to claim 1, wherein the coating layer has a thickness of 0.3 nm or more and 20 nm or less.

8. A positive electrode for a secondary battery containing a positive electrode active material and a conductive material, A positive electrode for a secondary battery, wherein the conductive material is the composite carbonaceous conductive material according to any one of claims 1 to 7.

9. The positive electrode for a secondary battery according to claim 8 , further comprising a sulfide solid electrolyte.

10. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution, 9. A non-aqueous electrolyte secondary battery, wherein the positive electrode is the positive electrode for a non-aqueous electrolyte secondary battery according to claim 8.

11. The nonaqueous electrolyte secondary battery according to claim 10, which can be charged and discharged at a voltage exceeding 4.5 V.

12. A solid secondary battery including a positive electrode, a negative electrode, and a solid electrolyte layer, A solid secondary battery, wherein the positive electrode is the positive electrode for a secondary battery according to claim 8.

13. The solid secondary battery according to claim 12 , wherein the solid electrolyte layer contains a sulfide solid electrolyte.

14. The solid secondary battery according to claim 12, which can be charged and discharged at a voltage exceeding 4.5 V.

15. a precursor layer forming step of forming a precursor layer made of boron oxide so as to cover a part or all of the surface of the carbon nanotubes; a nitriding step of nitriding the boron oxide forming the precursor layer to convert the precursor layer into a coating layer containing elemental boron and elemental nitrogen, a ratio (B / N ratio) of the content of boron element (B) to the content of nitrogen element (N) in the coating layer is 0.7 or more and 1.3 or less in mass ratio; The method for producing a composite carbonaceous conductive material, wherein the amount of boron (B) contained in the coating layer is 1.0 mass % or more and 21 mass % or less relative to 100 mass % of the composite carbonaceous conductive material.

16. 16. The method for producing a composite carbonaceous conductive material according to claim 15, wherein the precursor layer forming step includes a step of immersing the carbon nanotubes in a solution containing boron element (B) on the surface thereof, or a step of applying a solution containing boron element (B) to the surface of the carbon nanotubes.

17. 16. The method for producing a composite carbonaceous conductive material according to claim 15, wherein the coating layer comprises boron nitride.

18. 16. The method for producing a composite carbonaceous conductive material according to claim 15, wherein the coating layer has a thickness of 0.3 nm or more and 20 nm or less.

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