Carbon material, conductive aid, dispersion, electrode mixture layer-forming composition, and secondary battery

Abstract

Provided is a carbon material in which entanglement of carbon fibers is suppressed and electrode resistance can be reduced. [Solution] The carbon material contains carbon fibers having a structure in which cylindrical carbon hexagonal mesh planes are stacked in the fiber thickness direction, and having an average fiber diameter of 140 nm or more and an average fiber length of less than 5.0 μm.

Description

[Technical Field]

[0001] The present disclosure relates to a carbon material, a conductive assistant, a dispersion, a composition for forming an electrode mixture layer, and a secondary battery. [Background technology]

[0002] Taking advantage of their small size, light weight, and high voltage characteristics, secondary batteries are widely used in electronic devices such as notebook PCs, mobile phones, smartphones, tablet PCs, etc. In recent years, against the backdrop of environmental concerns, secondary batteries such as lithium-based secondary batteries have become widespread in electric vehicles (EVs) that run solely on batteries, and hybrid electric vehicles (HEVs) that combine gasoline engines with batteries.

[0003] Composite carbon fibers in which multi-walled carbon nanotubes are uniformly dispersed among graphitized carbon nanofibers and near the surfaces of the graphitized carbon nanofibers have been proposed as a conductivity imparting agent for electrodes of secondary batteries (for example, Patent Document 1). The composite carbon fibers are easily dispersed in matrices such as resins without leaving any aggregates and are excellent in reducing resistance, and when the composite carbon fibers are contained in electrodes of secondary batteries as a conductivity imparting agent, battery characteristics such as capacity retention rate are improved. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5497109 Summary of the Invention [Problem to be solved by the invention]

[0005] When carbon fibers are used as a conductive additive to prepare a slurry for coating electrodes of lithium-ion batteries, it is desirable to obtain a uniform, aggregate-free slurry. However, unlike the composite carbon fibers of Patent Document 1, when ordinary carbon fibers are used, the carbon fibers tend to entangle with each other and form aggregates. When aggregates are easily formed, it becomes difficult to reduce the electrode resistance.

[0006] The present disclosure has been made in view of the above circumstances, and aims to provide a carbon material in which entanglement of carbon fibers is suppressed and electrode resistance can be reduced, as well as a conductive assistant, a dispersion, an electrode mixture layer-forming composition, and a secondary battery, each of which contains the carbon material. [Means for solving the problem]

[0007] Specific means for achieving the above object are as follows. <1> A carbon material comprising carbon fibers having a structure in which cylindrical carbon hexagonal mesh planes are stacked in the fiber thickness direction, with an average fiber diameter of 140 nm or more and an average fiber length of less than 5.0 μm. <2> The ratio of the average fiber length of the carbon fibers to the average fiber diameter of the carbon fibers, that is, average fiber length / average fiber diameter, is 30 or less. <1> The carbon material according to claim 1. <3> The carbon fibers have an average fiber diameter of 300 nm or less. <1> or <2> The carbon material according to claim 1. <4> The compaction resistivity at a compaction density of 0.8 g / cm3 is 0.0180 Ω·cm or less. <1> ~ <3> 1. The carbon material according to any one of the above items. <5> 0.8g / cm 3 The pressure for compressing is less than 1.9 MPa. <1> ~ <4> 1. The carbon material according to any one of the above items. <6> <1> ~ <5> A conductive additive comprising the carbon material according to any one of the above items. <7> <1> ~ <5> A dispersion comprising the carbon material according to any one of the above items. <8> <1> ~ <5> 10. A composition for forming an electrode mixture layer, comprising the carbon material according to any one of the above items. <9> a positive electrode including a positive electrode current collector and a positive electrode mixture layer that is disposed on the positive electrode current collector and that contains a positive electrode active material; and a negative electrode including a negative electrode current collector and a negative electrode mixture layer that is disposed on the negative electrode current collector and that contains a negative electrode active material, At least one of the positive electrode mixture layer and the negative electrode mixture layer <1> ~ <5> A secondary battery comprising the carbon material according to any one of the above items. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to provide a carbon material in which entanglement of carbon fibers is suppressed and electrode resistance can be reduced, as well as a conductive assistant, a dispersion, an electrode mixture layer-forming composition, and a secondary battery each containing the carbon material. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure.

[0010] In the present disclosure, the term "process" includes not only a process that is independent of other processes, but also a process that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved. In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In the present disclosure, each component may contain multiple substances corresponding to the component. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, the composition may contain multiple types of particles corresponding to each component. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component means the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified. In the present disclosure, the terms "layer" and "film" include cases where the layer or film is formed over the entire area when the area in which the layer or film is present is observed, as well as cases where the layer or film is formed over only a portion of the area. In this disclosure, the term "lamination" refers to stacking layers, and two or more layers may be bonded together, or two or more layers may be detachable. In the present disclosure, the term "comprise" of a specific component (for example, a conductive assistant, a dispersion liquid, a conductive layer, an electrode mixture layer) means that other components besides the specific component may be included. In the present disclosure, the term "conductive assistant" refers to an agent that is added to an electrode mixture layer to reduce the resistance of the electrode.

[0011] <Carbon materials> The carbon material of the present disclosure includes carbon fibers having a structure in which cylindrical carbon hexagonal mesh planes are laminated in the fiber thickness direction, with an average fiber diameter of 140 nm or more and an average fiber length of less than 5.0 μm. The carbon material of the present disclosure has excellent dispersibility and is easily dispersed in compositions such as a composition for forming an electrode mixture layer. This is because entanglement and aggregation of the carbon fibers are suppressed. As a result, by adding the carbon material to at least one of the positive electrode mixture layer and the negative electrode mixture layer, electrode resistance can be reduced.

[0012] The average fiber diameter of the carbon fibers is preferably 300 nm or less from the viewpoint of producing a secondary battery having excellent cycle characteristics and rate characteristics, and is preferably 150 nm to 250 nm, more preferably 160 nm to 210 nm, and even more preferably greater than 170 nm and equal to or less than 200 nm, from the viewpoint of reducing electrode resistance.

[0013] The average fiber diameter of carbon fibers can be calculated from the arithmetic mean value of the diameters of 300 random fibers observed under the SEM of an electrode. The diameter of a single fiber can be calculated by dividing the fiber shown in the SEM photograph into 11 equal parts along its length, measuring the width at 10 points excluding both ends, and taking the arithmetic mean. Here, the width of the fiber refers to the dimension of the fiber in the direction perpendicular to the longitudinal direction.

[0014] From the viewpoint of dispersibility of the carbon material and producing a secondary battery having excellent cycle characteristics and rate characteristics, the average fiber length of the carbon fibers is preferably 2.0 μm to 4.9 μm, more preferably 3.0 μm to 4.8 μm, even more preferably 3.5 μm to 4.7 μm, and particularly preferably 4.0 μm to 4.6 μm. The average fiber length of the carbon fibers may be less than 4.0 μm, may be 2.0 μm or more and less than 4.0 μm, may be 2.5 μm or more and less than 4.0 μm, or may be 3.0 μm to 3.5 μm.

[0015] The average fiber length of carbon fibers can be measured as follows: First, the electrode is washed with a solvent, and after removing the binder and other materials, the carbon material is dispersed in a dispersion medium, spread on aluminum foil or the like, and after drying, observed with an SEM. The lengths of 300 randomly selected fibers along the fiber axis are measured, and the average fiber length can be calculated by taking the arithmetic mean.

[0016] The ratio of the average fiber length of the carbon fibers to the average fiber diameter of the carbon fibers, that is, average fiber length / average fiber diameter, may be 30 or less, 28 or less, or 20-26.

[0017] The carbon material of the present disclosure has a compressed density of 0.8 g / cm 3 It is preferable that the compacted resistivity is 0.0180 Ω cm or less. When the compacted resistivity is 0.0180 Ω cm or less, the decrease in the electrical conductivity of the carbon material is suppressed, which tends to contribute to improving the battery characteristics. In the carbon material of the present disclosure, the aforementioned compaction resistivity may be 0.0120 Ω·cm to 0.0180 Ω·cm, 0.0130 Ω·cm to 0.0170 Ω·cm, or 0.0140 Ω·cm to 0.0160 Ω·cm. Compressed density of carbon material: 0.8g / cm 3 The measurement of the compaction resistivity can be carried out by the method described in the examples below.

[0018] In the carbon material of the present disclosure, from the viewpoint of being able to reduce the electrode resistance, 0.8 g / cm 3 It is preferable that the pressure for compressing is less than 1.9 MPa.

[0019] Carbon material d 002 is preferably 0.33900 nm or less, and from the viewpoint of battery characteristics, is preferably 0.33780 nm to 0.33880 nm, more preferably 0.33790 nm to 0.33860 nm, and further preferably 0.33795 nm to 0.33840 nm. Carbon material d 002 is the average interplanar spacing d obtained by the X-ray diffraction method of carbon materials, specifically the Gakken method. 002 means.

[0020] The carbon material of the present disclosure may be used as a conductive assistant or the like, and may be used to prepare a dispersion, a composition for forming an electrode mixture layer, or the like.

[0021] The carbon material of the present disclosure may be used in the form of a dispersion in which it is dispersed in a solvent, etc. The dispersion may contain other components such as a positive electrode active material, a negative electrode active material, a binder, and an additive.

[0022] The carbon material of the present disclosure may be used to prepare a composition for forming an electrode mixture layer (electrode mixture layer-forming composition). Examples of the electrode mixture layer-forming composition include a positive electrode mixture layer-forming composition for forming a positive electrode mixture layer, and a negative electrode mixture layer-forming composition for forming a negative electrode mixture layer. The composition for forming a positive electrode mixture layer contains a positive electrode active material and the carbon material of the present disclosure, and may further contain carbon black, a binder, a solvent, and the like as necessary. The composition for forming the negative electrode mixture layer contains a negative electrode active material and the carbon material of the present disclosure, and may further contain a conductive aid, a binder, a solvent, and the like as necessary.

[0023] <Secondary battery> The secondary battery of the present disclosure includes a positive electrode including a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector, and a negative electrode including a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector, wherein at least one of the positive electrode mixture layer and the negative electrode mixture layer contains the carbon material of the present disclosure.

[0024] The secondary battery may have a structure in which a plurality of positive electrodes and negative electrodes housed in an outer casing are stacked in the thickness direction, or may be a laminated secondary battery or a wound secondary battery. The wound secondary battery may be, for example, a cylindrical secondary battery in which an electrode pair obtained by winding a laminate formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are enclosed in a cylindrical outer casing, or a cylindrical secondary battery in which a cell obtained by winding a laminate formed by stacking a positive electrode and a negative electrode with a solid electrolyte interposed therebetween is enclosed in a cylindrical outer casing.

[0025] The secondary battery may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween and an electrolytic solution are housed in an exterior packaging material, or may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a solid electrolyte interposed therebetween is housed in an exterior packaging material.

[0026] The type of secondary battery is not particularly limited, and examples include lithium-based secondary batteries, sodium-based secondary batteries, potassium-based secondary batteries, magnesium-based secondary batteries, aluminum-based secondary batteries, etc. Among these, lithium-based secondary batteries that can achieve high energy density at high voltage and sodium-based secondary batteries that can be reduced in cost are preferred. Examples of lithium-based secondary batteries include lithium ion secondary batteries, lithium secondary batteries in which the negative electrode is metallic lithium (including, for example, lithium-sulfur batteries and lithium-air batteries), and liquid electrolyte type batteries and solid electrolyte type batteries containing at least one of an electrolytic solution, a polymer electrolyte, a polymer gel electrolyte, a solid electrolyte, etc. Similarly to the aforementioned lithium-based secondary batteries, secondary batteries other than lithium-based secondary batteries are not limited in terms of the positive electrode active material, the negative electrode active material, the electrolyte, etc., and can take various forms. In the following, an example of a lithium-based secondary battery will be described, but the present invention is not limited to this.

[0027] [Positive electrode] The secondary battery of the present disclosure includes a positive electrode including a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material disposed on the positive electrode current collector.

[0028] The material of the positive electrode current collector is not particularly limited as long as it is an electron-conductive material that does not oxidize and dissolve at high potential, and can be selected from aluminum, nickel, titanium, stainless steel, etc. The state of the positive electrode current collector is not particularly limited, and can be selected from foil, perforated foil, mesh, etc. As an example, an aluminum foil is used as the positive electrode current collector.

[0029] The positive electrode mixture layer may contain the carbon material of the present disclosure. For example, a positive electrode mixture layer-forming composition (a type of electrode mixture layer-forming composition) containing a positive electrode active material and the carbon material of the present disclosure, and further containing carbon black, a binder, a solvent, and the like as necessary, is applied onto a positive electrode current collector, the applied slurry is dried, and then pressed, thereby forming a positive electrode mixture layer on the positive electrode current collector.

[0030] From the viewpoints of energy density and safety, the thickness of the positive electrode mixture layer may be 30 μm or more, may be 50 μm to 70 μm, or may be 70 μm to 100 μm.

[0031] The density of the positive electrode mixture layer is set to 2.0 g / cm from the viewpoint of energy density and safety. 3 It may be 3.0 g / cm or more. 3 It may be 3.0 g / cm or more. 3 ~4.0g / cm 3 may be.

[0032] The weight of the positive electrode mixture layer is 10.0 mg / cm from the viewpoint of energy density and safety. 2 It may be 10.0 mg / cm or more. 2 ~30.0mg / cm 2 may be.

[0033] The average electrode area per sheet (average positive electrode area and average negative electrode area) is 20 cm 2 ~10000cm 2 It may be 300cm 2 ~10000cm 2 may be.

[0034] (Cathode active material) The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material can be appropriately selected depending on the type of secondary battery, and examples thereof include compounds containing at least one of lithium, sodium, potassium, magnesium, and aluminum. Examples of the positive electrode active material include oxides containing nickel, phosphates having an olivine structure, etc. When the secondary battery is a lithium-based secondary battery, the positive electrode active material is LiNi x Mn y Co z Al w O2 (x, y, z, w≧0, x+y+z+w=1), LiMPO4 (M is one or more selected from Fe, Co, Mn and Ni), LiMn a Ni b Examples include O4 (a, b≧0, a+b=2).

[0035] The positive electrode active material is LiNi x Mn y Co z Al w It preferably contains O2 (x, y, z, w≧0, x+y+z+w=1) or LiMPO4 (M is one or more selected from Fe, Co, Mn, and Ni).

[0036] LiNi x Mn y Co z Al w O2 (x, y, z, w≧0, x+y+z+w=1) preferably has a relatively high proportion of nickel, for example, x≧0.5 or more, and Li(Ni x Mn y Co z )O2(x≧0.5, y≦0.3, z≦0.3, x+y+z=1) is more preferable. x Mn y Co z )O2(x≧0.5, y≦0.3, z≦0.3, x+y+z=1) 0.8 Mn 0.1 Co 0.1 )O2, Li(Ni 0.7 Mn 0.2 Co 0.1 )O2, Li(Ni 0.7 Mn 0.1 Co 0.2 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2 and Li(Ni 0.5 Mn 0.2 Co 0.3 )O2 is one example.

[0037] Examples of the positive electrode active material represented by LiMPO4 (M is one or more selected from Fe, Co, Mn, and Ni) include LiFePO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.3 Mn0.7 PO4, LiCoPO4 and LiCo 0.5 Mn 0.5 PO4 is an example.

[0038] In the positive electrode mixture layer, the content of the positive electrode active material is preferably 90.0 mass % or more, more preferably 93.0 mass % or more, and even more preferably 95.0 mass % or more, from the viewpoint of positive electrode capacity.

[0039] In the positive electrode mixture layer, the content of the positive electrode active material is preferably 98.0 mass % or less, more preferably 97.0 mass % or less, and even more preferably 96.5 mass % or less, from the viewpoint of ensuring the amounts of other components.

[0040] The positive electrode mixture layer may contain the carbon material of the present disclosure. When the positive electrode mixture layer contains the above carbon material, the content of the above carbon material in the positive electrode mixture layer is preferably 0.1 mass % or more, more preferably 0.3 mass % or more, and even more preferably 0.5 mass % or more.

[0041] In the positive electrode mixture layer, the content of the carbon material is preferably 3.0% by mass or less, more preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less.

[0042] The positive electrode mixture layer may contain a carbon material (other carbon material) other than the carbon material of the present disclosure. Examples of other carbon materials include carbon fibers that do not satisfy the average fiber diameter or average fiber length requirements of the carbon fibers contained in the carbon material of the present disclosure, composite carbon fibers, carbon nanotubes such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), and carbon nanofibers. The positive electrode mixture layer may contain, for example, carbon nanotubes and carbon black, which will be described later, in addition to the carbon material of the present disclosure.

[0043] (carbon black) The positive electrode mixture layer may further contain carbon black. Carbon black is used as a conductive additive for secondary batteries. Examples of carbon black include Denka Black (registered trademark, manufactured by Denka Co., Ltd.), C-NERGY (registered trademark) Super C45, C65 (manufactured by Imerys Graphite & Carbon), and Ketjen Black (manufactured by Ketjen Black International).

[0044] The primary particle diameter of the carbon black may be 10 nm to 100 nm. The term "primary particle" here refers to a portion corresponding to a single particle contained in multiple particles called an aggregate. If the primary particle diameter of the carbon black falls within this range, it tends to be uniformly dispersed on the surface of the active material. From the viewpoint of improving dispersibility, the primary particle diameter of the carbon black is preferably 20 nm to 80 nm, and more preferably 30 nm to 70 nm.

[0045] The primary particle diameter of carbon black can be determined by randomly selecting 100 primary carbon black particles from the SEM photograph and cross-sectional SEM photograph of the electrode and then arithmetically averaging the results of measuring the maximum particle length using image recognition software.

[0046] When the positive electrode mixture layer contains carbon black, the carbon black content is preferably 0.2 mass% or more, more preferably 0.6 mass% or more, and even more preferably 1.0 mass% or more, from the viewpoint of obtaining excellent cycle characteristics and rate characteristics.

[0047] In the positive electrode mixture layer, the carbon black content is preferably 6.0 mass % or less, more preferably 4.0 mass % or less, and even more preferably 2.0 mass % or less, from the viewpoint of ensuring the capacity of the positive electrode.

[0048] (binder) The positive electrode mixture layer may contain a binder. Examples of binders that can be used in the positive electrode mixture layer of a typical lithium-ion secondary battery include polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE).

[0049] When the positive electrode mixture layer contains a binder, the content of the binder is preferably 1.0 mass % or more, more preferably 1.5 mass % or more, and even more preferably 2.0 mass % or more, from the viewpoint of obtaining the binder function.

[0050] In the positive electrode mixture layer, the binder content is preferably 5.0 mass % or less, more preferably 4.5 mass % or less, and even more preferably 4.0 mass % or less, from the viewpoint of suppressing an increase in the resistance of the positive electrode.

[0051] (Other ingredients) In addition to the above, the positive electrode mixture layer may contain other components such as a dispersant, an additive, etc. For example, it may contain various dispersants for dispersing the positive electrode active material, agents for surface modification of the positive electrode active material, etc.

[0052] [Negative electrode] The secondary battery includes a negative electrode including a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector.

[0053] The material of the negative electrode current collector is not particularly limited as long as it is an electron-conductive material, and can be selected from copper, nickel, titanium, stainless steel, etc. The state of the negative electrode current collector is not particularly limited, and can be selected from foil, perforated foil, mesh, etc. As an example, copper foil is used as the negative electrode current collector.

[0054] The negative electrode mixture layer may contain the carbon material of the present disclosure. For example, a composition for a negative electrode binder layer (a kind of composition for forming an electrode binder layer), which contains a negative electrode active material and the carbon material of the present disclosure on a negative electrode current collector, and further contains a conductive assistant, a binder, a solvent, etc. as required, is coated, the coated slurry is dried, and then pressed to form a negative electrode binder layer on the negative electrode current collector.

[0055] From the viewpoints of energy density and safety, the thickness of the negative electrode binder layer may be 30 μm or more, may be 50 μm to 100 μm, or may be 100 μm to 150 μm.

[0056] From the viewpoints of energy density and safety, the density of the negative electrode binder layer may be 1.3 g / cm 3 or more, and may be 1.5 g / cm 3 to 2.0 g / cm 3 or more.

[0057] From the viewpoints of energy density and safety, the basis weight of the negative electrode binder layer may be 5.0 mg / cm 2 or more, and may be 10 mg / cm 2 to 20 mg / cm 2 or more.

[0058] (Negative electrode active material) The negative electrode binder layer contains a negative electrode active material. Examples of the negative electrode active material include Si, SiO x (0 < x ≤ 2), soft carbon, hard carbon, graphite, a composite of silicon and carbon, Li4Ti5O 12 , metallic Li, InO x (0 < x ≤ 1.5), AlO x (0 < x ≤ 1.5), AgO x (0 < x ≤ 0.5), CdO x (0 < x ≤ 1), SbO[[ID=​​​​​​​ In the negative electrode mixture layer, the content of the negative electrode active material is preferably 90.0% by mass or more, more preferably 93.0% by mass or more, and even more preferably 95.0% by mass or more.

[0060] In the negative electrode mixture layer, the content of the negative electrode active material is preferably 98.0 mass % or less, more preferably 97.0 mass % or less, and even more preferably 96.5 mass % or less, from the viewpoint of ensuring the amounts of other components.

[0061] The negative electrode mixture layer may contain the carbon material of the present disclosure. When the negative electrode mixture layer contains the above carbon material, the content of the above carbon material in the negative electrode mixture layer is preferably 0.1 mass % or more, more preferably 0.3 mass % or more, and even more preferably 0.5 mass % or more.

[0062] In the negative electrode mixture layer, the content of the carbon material is preferably 3.0% by mass or less, more preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less.

[0063] (Conductive additive) The negative electrode mixture layer may contain a conductive additive, such as a carbon-containing material such as carbon black or graphene.

[0064] (binder) The negative electrode mixture layer may contain a binder, such as PVdF, PTFE, etc., as in the positive electrode mixture layer, or styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), etc.

[0065] When the negative electrode mixture layer contains a binder, the content of the binder is preferably 1.0 mass % or more, more preferably 1.5 mass % or more, and even more preferably 2.0 mass % or more, from the viewpoint of obtaining the binder function.

[0066] In the negative electrode mixture layer, the binder content is preferably 5.0 mass % or less, more preferably 4.5 mass % or less, and even more preferably 4.0 mass % or less, from the viewpoint of suppressing an increase in the resistance of the negative electrode.

[0067] (Other ingredients) In addition to the above, the negative electrode mixture layer may contain other components such as a dispersant, an additive, etc. For example, it may contain various dispersants for dispersing the negative electrode active material, agents for surface modification of the negative electrode active material, etc.

[0068] (exterior materials) The exterior material for housing the positive electrode and the negative electrode is not limited as long as it can house the positive electrode, the negative electrode, and, if necessary, a separator and an electrolytic solution or a solid electrolyte, etc. Examples of the exterior material include commonly available battery packs, 18650-type cylindrical cells, and those packed in aluminum packaging, and the exterior material can be freely designed and used.

[0069] (separator) The secondary battery may include a separator between the positive electrode and the negative electrode. The separator can be freely selected from those usable in general secondary batteries, such as microporous films made of polyethylene or polypropylene. Separators containing particles such as SiO2 or Al2O3 as a filler, or separators with these particles attached to the surface, can also be used.

[0070] (electrolyte) The secondary battery may contain an electrolytic solution. There are no particular limitations on the electrolytic solution, and any electrolytic solution that can be used in a typical secondary battery can be suitably used. For example, an organic solvent in which 0.5 mol / L to 2.0 mol / L of lithium salt is dissolved can be used.

[0071] Examples of lithium salts include LiPF6, LiBF4, LiClO4, LiAsF6, and LiFSI.

[0072] Examples of organic solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and propylene carbonate (PC). The organic solvent may be a mixture of the above-listed organic solvents and other organic solvents. Examples of additives for the electrolyte solution include vinylene carbonate (VC), propane sultone (PS), and fluoroethylene carbonate (FEC). When an additive is used, the content of the additive is preferably 0.01% to 20% by mass, more preferably 0.1% to 10% by mass, and even more preferably 0.5% to 5% by mass, relative to 100% by mass of the organic solvent.

[0073] (ionic liquid) An ionic liquid may be used as the electrolyte, or an ionic liquid may be used in combination with the organic solvent described above. The ionic liquid is not particularly limited, and examples thereof include a combination of a cation such as an imidazolium cation, a pyrrolidinium cation, a piperidinium cation, or an ammonium cation with an anion such as a bis(trifluoromethane)sulfonamide anion.

[0074] (solid electrolyte) A solid electrolyte may be used as the electrolyte. When a solid electrolyte is used, a separator is not required, and a battery (for example, an all-solid-state lithium ion secondary battery) can be formed in which a positive electrode and a negative electrode are sandwiched between the solid electrolyte.

[0075] Examples of the solid electrolyte include polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte is not particularly limited, and examples thereof include polymers such as polyethylene oxide impregnated with the lithium salts. The inorganic solid electrolyte is not particularly limited, and examples thereof include Li 13 Ti 1.7 Al 0.3 (PO4)3, Li2S-P2S5, etc.

[0076] The secondary battery of the present disclosure can be used as a power source for electronic devices such as smartphones, tablet PCs, and personal digital assistants; as a power source for electric motors for power tools, vacuum cleaners, electric bicycles, drones, and electric vehicles; and for storing electricity obtained from fuel cells, solar power generation, wind power generation, and the like.

[0077] <Method of manufacturing carbon materials> The method for producing a carbon material according to the present disclosure includes introducing a raw material liquid containing a carbon source and a catalyst source into a reaction tube having an inner diameter of 400 mm or less using a carrier gas, and heating the components introduced into the reaction tube. The method for producing a carbon material according to the present disclosure is a production method using a floating catalyst method. In the floating catalyst method, a hollow tube is generally formed starting from the catalyst source at the beginning of the reaction, and carbon fibers grow in the longitudinal direction. The longitudinal growth continues until the catalytic activity is lost, for example, due to the catalyst particle surface being covered with carbon. Once the longitudinal growth is completed, pyrolytic carbon is deposited on the surface of the hollow tube, and the fiber grows in the diameter direction, forming a structure in which cylindrical carbon hexagonal mesh planes are stacked in the diameter direction.

[0078] In the present disclosure, a structure in which tubular carbon hexagonal mesh planes are stacked in the thickness direction refers to a structure (also referred to as a specific structure) in which multiple structures in which carbon hexagonal mesh planes are rolled into a cylindrical shape are stacked along the thickness direction. The specific structure can be confirmed by observing a carbon material with, for example, a transmission electron microscope (TEM) as follows: An image in which the longitudinal direction of the carbon fiber can be confirmed (hereinafter also referred to as a "TEM longitudinal image") and an image in which a cross section of the carbon fiber cut in a direction intersecting the longitudinal direction can be confirmed (hereinafter also referred to as a "TEM cross-sectional image") are observed. If the TEM longitudinal image shows multiple lines running along the longitudinal direction within the carbon fiber, and the TEM cross-sectional image shows multiple closed curves with different maximum diameters, the closed curves being arranged in order from the largest diameter to the smallest diameter, the carbon fiber can be confirmed to have a specific structure. Furthermore, in X-ray diffraction (XRD), a structure in which carbon hexagonal mesh planes are stacked can be confirmed by observing diffraction lines similar to those of graphite particles for the (002), (100), (101), (110), or (112) planes. The structure in which cylindrical carbon hexagonal mesh planes are stacked in the thickness direction may be a structure in which multiple cylindrical carbon hexagonal mesh planes of different diameters are arranged to have concentric cross sections (e.g., like a concentric multi-tube). The central axes (lines connecting the centers of the cross sections of a cylinder) of multiple cylindrical carbon hexagonal mesh planes of different diameters do not all need to be aligned, and only some of the central axes may be aligned. The cross-sectional shape of the cylinder is not limited to a perfect circle but may be an ellipse, a polygon, or the like. Part of the periphery may be a perfect circle, an ellipse, another curve, a polygon, or a combination thereof (the above-mentioned "closed curve" refers to such a shape. In these cases, the "central axis" is defined as the line connecting the centers of gravity of the cross sections). The structure in which cylindrical carbon hexagonal mesh planes are stacked in the thickness direction may be a structure in which multiple cylindrical carbon hexagonal mesh planes (for example, the cross section is elliptical or polygonal) with different maximum cross-sectional widths are arranged so that the central axes all coincide, or the central axes do not all have to be aligned, or only some of the central axes may be aligned.

[0079] In the production method of the present disclosure, the raw material liquid is introduced into a reaction tube having an inner diameter of 400 mm or less, and heated in the reaction tube, thereby obtaining the carbon material of the present disclosure. The reason for this is presumably that by reducing the inner diameter of the reaction tube, it becomes easier to obtain carbon fibers having a relatively large fiber diameter and a short fiber length.

[0080] The carbon source may be any raw material that is used in the floating catalyst method, and examples thereof include decahydronaphthalene (decalin), toluene, benzene, hexane, cyclohexane, xylene, ethylbenzene, cyclohexane, and ethylcyclohexane.

[0081] The catalyst source may be any catalyst used in the floating catalyst method, such as ferrocene or a sulfur compound. Ferrocene and a sulfur compound may be used in combination as the catalyst source. The amount of the catalyst source in the raw material liquid may be 1% by mass to 10% by mass, or may be 2% by mass to 5% by mass.

[0082] The carrier gas may be any gas that can introduce the raw material liquid into the reaction tube, such as hydrogen, etc. Alternatively, the raw material liquid may be gasified and introduced into the reaction tube by the carrier gas.

[0083] The ratio of the raw material liquid flow rate (g / min) to the hydrogen flow rate (NL / min) may be 0.30 to 1.00 g / NL, or 0.40 to 0.80 g / NL, from the viewpoint of facilitating the production of carbon fibers having an average fiber diameter and an average fiber length within the desired range. Increasing the raw material liquid flow rate to increase the ratio makes it easier for carbon coating to occur on the catalyst source surface and suppresses length growth, making it easier to produce carbon fibers with a large average fiber diameter and a small average fiber length. Increasing the hydrogen flow rate to decrease the ratio makes it harder for carbon coating to occur on the catalyst source surface and promotes length growth, making it easier to produce carbon fibers with a small average fiber diameter and a large average fiber length.

[0084] The heating temperature when the raw material liquid introduced into the reaction tube is heated is not particularly limited, and may be, for example, 1000° C. or higher. In particular, from the viewpoint of facilitating the production of carbon fibers having an average fiber diameter and an average fiber length within a desired range, the wall surface temperature at the center in the flow direction of the raw material liquid in the reaction tube is preferably 1250° C. or higher, and more preferably 1250° C. to 1350° C.

[0085] To improve the crystallinity of the carbon material obtained by the floating catalyst method, it is possible to perform a heat treatment at 800°C to 1500°C in an inert gas atmosphere such as argon, followed by a graphitization treatment at 2000°C to 3000°C. The graphitization treatment can simultaneously evaporate and remove the catalyst metal, enabling the carbon material to be highly purified. After heat treatment (first firing step) at 800°C to 1500°C, preferably 900°C to 1300°C, in an inert gas atmosphere such as argon, the temperature may be increased to 200°C to 500°C, preferably 1200°C to 1700°C, and maintained for approximately 30 minutes to 3 hours (second firing step). The temperature during graphitization treatment is preferably 2500°C to 3000°C, more preferably 2600°C to 3000°C. [Example]

[0086] The present disclosure will be specifically described below using examples, but the scope of the present disclosure is not limited to these examples. The physical properties of the carbon materials obtained in the examples and comparative examples were measured by the methods described below.

[0087] (Examples 1 to 5, 8, and 10) A reactor was prepared, consisting of a cylindrical reaction tube with an inner diameter of 370 mm and a length of 2000 mm, and a heater. A two-fluid mixing nozzle for supplying raw materials was placed at the top of the reaction tube. Ferrocene and sulfur were dissolved in benzene to prepare a raw material solution (3.5 mass % ferrocene, 0.08 mass % sulfur in the raw material solution). The prepared raw material solution and hydrogen were supplied at a flow rate according to the inner diameter of the reaction tube and sprayed into the reaction tube from a two-fluid mixing nozzle. The ratio of the raw material solution flow rate to the hydrogen flow rate was 0.54 to 0.71 g / NL, and the raw material solution flow rate and hydrogen flow rate were adjusted appropriately for each example. The temperature of the reaction tube (temperature of the wall surface inside the reaction tube) was adjusted to 1230°C to 1290°C, and the sprayed raw material liquid was passed through the reaction tube to obtain a carbon material containing carbon fibers. At this time, the temperature of the furnace wall in the middle of the reaction tube was 1230°C.

[0088] (Examples 6, 7, and 9) A reactor was prepared, consisting of a cylindrical reaction tube with an inner diameter of 500 mm and a length of 2000 mm, and a heater. A two-fluid mixing nozzle for supplying raw materials was placed at the top of the reaction tube. Ferrocene and sulfur were dissolved in benzene to prepare a raw material solution (3.5 mass% ferrocene and 0.08 mass% sulfur in the raw material solution). The prepared raw material solution and hydrogen were supplied at a flow rate according to the inner diameter of the reaction tube and sprayed into the reaction tube from a two-fluid mixing nozzle. The ratio of the raw material solution flow rate to the hydrogen flow rate was 0.54 to 0.71 g / NL. The temperature of the reaction tube (temperature of the wall inside the reaction tube) was adjusted to 1230°C to 1290°C, and the sprayed raw material liquid was passed through the reaction tube to obtain a carbon material containing carbon fibers. At this time, the temperature of the furnace wall in the middle of the reaction tube was 1270°C.

[0089] (Comparative Examples 1 to 3) A reactor was prepared, consisting of a cylindrical reaction tube with an inner diameter of 590 mm and a length of 2000 mm, and a heater. A two-fluid mixing nozzle for supplying raw materials was placed at the top of the reaction tube. The raw material liquid and hydrogen prepared in the same manner as in Example 1 were supplied at a flow rate according to the inner diameter of the reaction tube and sprayed into the reaction tube from a two-fluid mixing nozzle. The ratio of the raw material liquid flow rate to the hydrogen flow rate at this time was 0.54 g / NL to 0.71 g / NL, and the raw material liquid flow rate and hydrogen flow rate were adjusted appropriately for each comparative example. The temperature of the reaction tube (temperature of the wall surface inside the reaction tube) was adjusted to 1230°C to 1290°C, and the sprayed raw material liquid was passed through the reaction tube to obtain a carbon material containing carbon fibers. At this time, the temperature of the furnace wall in the middle of the reaction tube was 1290°C.

[0090] (Transmission electron microscope (TEM) observation) The carbon fibers contained in each of the carbon materials obtained in Examples 1 to 10 and Comparative Examples 1 to 3 were dispersed in ethanol, scooped up with a microgrid, and dried to prepare samples. The samples were observed by TEM. It was confirmed that the carbon fibers contained in each of the carbon materials obtained in Examples 1 to 10 and Comparative Examples 1 to 3 all had a structure in which tubular hexagonal carbon mesh planes were laminated in the fiber thickness direction as follows. First, the carbon materials were observed using TEM-EDX to obtain an image in which the longitudinal direction of the carbon fiber could be confirmed (hereinafter also referred to as a "TEM longitudinal image") and an image in which a cross section of the carbon fiber cut in a direction intersecting the longitudinal direction could be confirmed (hereinafter also referred to as a "TEM cross-sectional image"). It was then confirmed that in the TEM longitudinal image, multiple lines were present inside the carbon fiber along the longitudinal direction, and in the TEM cross-sectional image, multiple closed curves with different maximum diameters were present in a concentric pattern. From the above, it was confirmed that the carbon fibers had a structure in which tubular hexagonal carbon mesh planes were laminated in the fiber thickness direction. Device name: JEM-ARM200F (manufactured by JEOL Ltd.)

[0091] (Average fiber length and average fiber diameter) The average fiber length (μm) and average fiber diameter (nm) of the carbon fibers contained in each of the obtained carbon materials were measured. The carbon fibers were photographed panoramic-wise using a scanning electron microscope at 4000x magnification or more, and a field of view was prepared in which both ends of the carbon fibers could be measured. The lengths of 200 randomly selected carbon fibers were measured, and the average of these measurements was taken as the average fiber length. The diameters of 200 randomly selected carbon fibers were measured using a scanning electron microscope at 20,000x magnification, and the average of these measurements was used as the average fiber diameter. The fiber diameter was measured at one point on the carbon fiber, excluding both ends. The measurement results are shown in Table 1.

[0092] (d 002 ) Using a horizontal sample type multipurpose X-ray diffractometer (Ultima IV, manufactured by Rigaku Corporation), According to the Carbon Materials Experimental Techniques (Analysis and Analysis Edition), edited by the Carbon Materials Society, the d of the carbon material was measured using silicon powder as an internal standard. 002 Measurements were carried out.

[0093] (Consolidation resistivity measurement) One gram of carbon material was placed in a special jig. The jig had a space measuring 1 cm long, 4 cm wide, and 9 cm high. Two of the 1 cm long and 4 cm wide surfaces were made of metal and electrically conductive. The bottom surface, also measuring 1 cm long and 4 cm wide, had two metal points (φ=3 mm) for measuring voltage, ensuring electrical continuity. The distance between the metal points was 1 cm. The non-metallic parts were made of plastic and were insulating. The push rod attached to the special jig measured 0.999 cm long, 3.999 cm wide, and 9 cm high, allowing the jig's space to be compressed in the vertical direction. A 100 mA current was applied to the metal on the side, and the height of the carbon material and the voltage between the voltage terminals were measured when a specified load was applied from above the push rod. From these measurements, the density and volume resistivity of the carbon material were recorded. The volume resistivity measured in this way is called the "compacted resistivity." In particular, when the carbon material was compressed to a density of 0.8 g / cm 3 For comparison, the value of the consolidation resistivity when compressed to 0.8 g / cm was used. 3 In Examples 6, 7, and 9 and Comparative Examples 1 to 3, the pressure when compressing the samples to the desired size was less than 1.9 MPa. 3 The pressure when compressing was 1.9 MPa or more in all cases.

[0094] (Preparation of samples for resistance measurement) As the positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.1A slurry was prepared by adding 96.5 parts by mass of O2 (50% diameter in the cumulative particle size distribution on a volume basis, D50 = 38 μm), 0.5 parts by mass of a conductive additive such as a carbon material obtained in the Examples or Comparative Examples, carbon black (C-NERGY® Super C65, manufactured by Imerys Graphite & Carbon, primary particle diameter: 33 nm) as another conductive additive, 1 part by mass of PVDF (#5130 manufactured by Kureha) as a binder, and N-methyl-2-pyrrolidone (NMP, manufactured by Kishida Chemical Co., Ltd.) as a solvent for slurry preparation. The mixture was kneaded using a WAWATORI MIXER (ARE-100 manufactured by Thinky Corporation). The resulting slurry was applied to a 20 μm-thick aluminum foil using a doctor blade with a 70 μm gap, dried on a hot plate at 90 °C, and then dried in a vacuum oven at 90 °C for 1 hour. A 2 cm long and 2 cm wide sample was cut from the mixture to serve as a measurement sample.

[0095] (resistance measurement) As a pretreatment for measuring the electrode resistance value, the electrode was vacuum dried at 100°C for 12 hours in a vacuum dryer in a dry room with a dew point of -60°C or less. The volume resistivity of the measurement sample was measured in a dry room using an electrode resistance measurement system RM2610 (manufactured by Hioki E.E. Corporation).

[0096] The average fiber diameter, average fiber length, compaction resistivity, d 002 The results of the electrode resistance are shown in Table 1.

[0097] [Table 1]

[0098] As shown in Table 1, by using the carbon materials of Examples 1 to 10, lower resistance was obtained than when the carbon materials of Comparative Examples 1 to 3 were used.

Claims

1. The carbon fiber has a structure in which cylindrical carbon hexagonal mesh planes are laminated in the thickness direction of the fiber, and has an average fiber diameter of 140 nm or more and an average fiber length of 4.0 μm or more and less than 5.0 μm, Compressed density 0.8 g / cm ３ A carbon material having a compaction resistivity of 0.0180 Ω cm or less.

2. 2. The carbon material according to claim 1, wherein the ratio of the average fiber length of the carbon fibers to the average fiber diameter of the carbon fibers, that is, average fiber length / average fiber diameter, is 30 or less.

3. 2. The carbon material according to claim 1, wherein the carbon fibers have an average fiber diameter of 300 nm or less.

4. 0.8 g / cm ３ The carbon material according to claim 1, wherein the pressure for compressing the carbon material to a pressure of less than 1.9 MPa.

5. A conductive additive comprising the carbon material according to any one of claims 1 to 4.

6. A dispersion comprising the carbon material according to any one of claims 1 to 4.

7. A composition for forming an electrode mixture layer, comprising the carbon material according to any one of claims 1 to 4.

8. a positive electrode including a positive electrode current collector and a positive electrode mixture layer that is disposed on the positive electrode current collector and that contains a positive electrode active material; and a negative electrode including a negative electrode current collector and a negative electrode mixture layer that is disposed on the negative electrode current collector and that contains a negative electrode active material, A secondary battery in which at least one of the positive electrode mixture layer and the negative electrode mixture layer contains the carbon material according to any one of claims 1 to 4.

Images