Carbon materials, conductive additives, dispersions, electrode mixture layer forming compositions, and secondary batteries

Abstract

This invention provides a carbon material in which entanglement of carbon fibers is suppressed, thereby reducing electrode resistance. [Solution] A carbon material containing carbon fibers having a structure in which tubular carbon hexagonal mesh surfaces are stacked in the direction of fiber thickness, with 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 carbon materials, conductive aids, dispersions, compositions for forming electrode binder layers, and secondary batteries.

Background Art

[0002] Secondary batteries utilize characteristics such as being small, lightweight, and having a high voltage, and are widely used in electronic devices such as notebook PCs, mobile phones, smartphones, and tablet PCs. In recent years, against the backdrop of environmental issues, 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 a gasoline engine and a battery.

[0003] As a conductive agent for electrodes of secondary batteries, composite carbon fibers in which multi-walled carbon nanotubes are homogeneously dispersed between graphitized carbon nanofibers and near the surface of the graphitized carbon nanofibers have been proposed (for example, Patent Document 1). The composite carbon fibers are easily dispersed in a matrix such as a resin without leaving aggregates and are excellent in the effect of reducing resistance. When the composite carbon fibers are contained as a conductive agent in the electrodes of secondary batteries, battery characteristics such as the capacity retention rate are improved.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

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

[0006] This 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 additive, dispersion, composition for forming an electrode mixture layer, and secondary battery containing the carbon material. [Means for solving the problem]

[0007] The specific means for achieving the aforementioned objectives are as follows: <1> A carbon material containing carbon fibers having a structure in which tubular carbon hexagonal mesh surfaces are stacked in the direction of fiber thickness, 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 to the average fiber diameter of the carbon fiber, which is the average fiber length / average fiber diameter, is 30 or less. <1> The carbon material described above. <3> The average fiber diameter of the carbon fibers is 300 nm or less. <1> or <2> The carbon material described above. <4> The compaction resistivity at a compressive density of 0.8 g / cm³ is 0.0180 Ω·cm or less. <1> ~ <3> A carbon material as described in any one of the following. <5> 0.8 g / cm³ 3 The pressure required to compress it is less than 1.9 MPa. <1> ~ <4> A carbon material as described in any one of the following. <6> <1> ~ <5> A conductive additive containing any one of the carbon materials described in one of the following. <7> <1> ~ <5> A dispersion containing any one of the carbon materials described in one of the following. <8> <1> ~ <5> A composition for forming an electrode mixture layer, comprising any one of the carbon materials described in one of the following. <9> A positive electrode comprising 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 comprising a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector, At least one of the positive electrode mixture layer and the negative electrode mixture layer <1> ~ <5> A secondary battery containing a carbon material as described in any one of the following. [Effects of the Invention]

[0008] According to this 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 additive, dispersion, electrode mixture layer forming composition, and secondary battery containing the carbon material. [Modes for carrying out the invention]

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

[0010] In this disclosure, the term "process" includes not only processes that are independent of other processes, but also processes that cannot be clearly distinguished from other processes, provided that the purpose of such process is achieved. In this disclosure, the numerical range indicated using "~" includes the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described in stages within this disclosure, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in numerical ranges described within this disclosure, the upper or lower limit of that range may be replaced with the values ​​shown in the examples. In this disclosure, each component may contain multiple types of the corresponding substance. If multiple types of the substance 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 types of substances present in the composition, unless otherwise specified. In this disclosure, each component may contain multiple types of particles. If multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value for a mixture of such multiple types of particles present in the composition, unless otherwise specified. In this disclosure, the terms “layer” or “film” include cases where, when the region in which the layer or film exists is observed, it is formed not only over the entire region but also over only a portion of the region. In this disclosure, the term "lamination" refers to stacking layers, and two or more layers may be bonded together or detachable. In this disclosure, the term "contains" in reference to a specific component (e.g., a conductive additive, a dispersion, a conductive layer, an electrode mixture layer) means that it may contain other components in addition to that specific component. In this disclosure, the "conductive additive" is added to the electrode mixture layer to reduce the resistance of the electrode.

[0011] <Carbon materials> The carbon material of this disclosure has a structure in which tubular carbon hexagonal mesh surfaces are stacked in the direction of the fiber thickness, and includes carbon fibers 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 this disclosure has excellent dispersibility and can be easily dispersed in compositions such as compositions for forming electrode mixture layers. This is because entanglement and aggregation of carbon fibers are suppressed. As a result, electrode resistance can be reduced by adding the carbon material to at least one of the positive electrode mixture layer and the negative electrode mixture layer.

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

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

[0014] From the viewpoint of producing a secondary battery with good carbon material dispersibility and excellent cycle 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, 2.0 μm or more and less than 4.0 μm, 2.5 μm or more and less than 4.0 μm, or 3.0 μm to 3.5 μm.

[0015] The average fiber length of carbon fibers can be measured as follows: First, the electrodes are washed with a solvent to remove binders and other contaminants. The carbon material is then dispersed in a dispersion medium, spread out on aluminum foil or similar material, dried, and observed using a scanning electron microscope (SEM). The length of 300 arbitrary fibers along their fiber axes is measured, and the average fiber length is determined by taking the arithmetic mean of these measurements.

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

[0017] The carbon material disclosed herein has a compressive density of 0.8 g / cm³. 3 It is preferable that the compaction resistivity is 0.0180 Ω·cm or less. Having a compaction resistivity of 0.0180 Ω·cm or less suppresses the decrease in conductivity of the carbon material, which tends to contribute to improved battery characteristics. In the carbon material of this disclosure, the aforementioned consolidation 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. Compression density of carbon material: 0.8 g / cm³ 3 The compaction resistivity can be measured by the method described in the examples below.

[0018] In the carbon material disclosed herein, from the viewpoint of reducing electrode resistance, 0.8 g / cm³ 3 It is preferable that the pressure used for compression is less than 1.9 MPa.

[0019] d of carbon materials 002 The wavelength is preferably 0.33900nm or less, and from the viewpoint of battery characteristics, it is preferably 0.33780nm to 0.33880nm, more preferably 0.33790nm to 0.33860nm, and even more preferably 0.33795nm to 0.33840nm. d of carbon materials 002 This refers to the average interplanar spacing d of carbon materials, specifically determined by X-ray diffraction, or more precisely, by the JSPS method. 002 It means...

[0020] The carbon material of this disclosure may be used as a conductive additive, or in the preparation of dispersions, electrode mixture layer forming compositions, etc.

[0021] The carbon material of this disclosure may be used in the form of a dispersion in a solvent or the like. The dispersion may contain other components such as a positive electrode active material, a negative electrode active material, a binder, and additives.

[0022] The carbon material of this disclosure may be used in the preparation of a composition for forming an electrode mixture layer (composition for forming an electrode mixture layer). Examples of compositions for forming electrode mixture layers include compositions for forming a positive electrode mixture layer and compositions for forming a negative electrode mixture layer. The composition for forming the positive electrode mixture layer comprises a positive electrode active material and the carbon material of this disclosure, and may further optionally contain carbon black, a binder, a solvent, etc. The negative electrode mixture layer forming composition comprises a negative electrode active material and the carbon material of this disclosure, and may further optionally contain a conductive additive, binder, solvent, etc.

[0023] <Secondary battery> The secondary battery of the present disclosure comprises a positive electrode having 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 having 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 take the form of a structure in which multiple positive and negative electrodes are stacked in the thickness direction within an outer casing, a laminated secondary battery, or a wound secondary battery. As an example of a wound secondary battery, it may be a cylindrical secondary battery in which an electrode pair and electrolyte obtained by winding a laminate in which positive and negative electrodes are stacked with a separator in between are sealed inside a cylindrical outer casing, or a cylindrical secondary battery in which a cell obtained by winding a laminate in which positive and negative electrodes are stacked with a solid electrolyte in between is sealed inside a cylindrical outer casing.

[0025] A secondary battery may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a separator in between, and an electrolyte are housed in an outer casing, or it may be a battery in which a laminate formed by stacking a positive electrode and a negative electrode with a solid electrolyte in between is housed in an outer casing.

[0026] The types of secondary batteries are not particularly limited, and include lithium-based secondary batteries, sodium-based secondary batteries, potassium-based secondary batteries, magnesium-based secondary batteries, and aluminum-based secondary batteries. Among these, lithium-based secondary batteries, which can achieve high voltage and high energy density, and sodium-based secondary batteries, which can be cost-effective, are preferred. Examples of lithium-based secondary batteries include lithium-ion secondary batteries and lithium-based secondary batteries in which the negative electrode is metallic lithium (including, for example, lithium-sulfur batteries and lithium-air batteries). These include liquid electrolyte batteries and solid electrolyte batteries that contain at least one of the following: electrolyte, polymer electrolyte, polymer gel electrolyte, or solid electrolyte. Furthermore, for secondary batteries other than lithium-based secondary batteries, the positive electrode active material, negative electrode active material, electrolyte, etc., are not limited and can take various forms, similar to the lithium-based secondary batteries mentioned above. The following describes an example of a lithium-ion secondary battery, but the present invention is not limited to this example.

[0027] [Positive electrode] The secondary battery of this disclosure comprises a positive electrode having 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 does not oxidize and dissolve at high potential and is electrically conductive; it 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, aluminum foil is used as the positive electrode current collector.

[0029] The positive electrode mixture layer may contain the carbon material of this disclosure. For example, a positive electrode mixture layer is formed on a positive electrode current collector by coating it with a positive electrode active material and the carbon material of this disclosure, and optionally further containing carbon black, a binder, a solvent, etc., using a composition for forming a positive electrode mixture layer (a type of composition for forming an electrode mixture layer), drying the coated slurry, and then pressing it.

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

[0031] The density of the positive electrode active material layer may be 2.0 g / cm 3 or more, may be 3.0 g / cm 3 or more, and may be 3.0 g / cm 3 to 4.0 g / cm 3 from the viewpoints of energy density and safety.

[0032] The basis weight of the positive electrode active material layer may be 10.0 mg / cm 2 or more, and may be 10.0 mg / cm 2 to 30.0 mg / cm 2 from the viewpoints of energy density and safety.

[0033] The average electrode area (average positive electrode area and average negative electrode area) per sheet may be 20 cm 2 to 10000 cm 2 or may be 300 cm 2 to 10000 cm 2 from the viewpoints of energy density and safety.

[0034] (Positive electrode active material) The positive electrode active material layer contains a positive electrode active material. The positive electrode active material can be appropriately selected according to the type of the secondary battery. For example, compounds containing at least one of lithium, sodium, potassium, magnesium, and aluminum can be mentioned. Examples of the positive electrode active material include oxides containing nickel, phosphates having an olivine-type structure, etc. When the secondary battery is a lithium-based secondary battery, examples of the positive electrode active material include 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 O4 (a, b ≧ 0, a + b = 2), etc.

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

[0036] LiRing x Mn y Co z Al w For O2(x, y, z, w≧0, x+y+z+w=1), it is preferable that the nickel content is relatively high, for example, x≧0.5 or higher, and Li(Ni x Mn y Co z It is more preferable that O2(x≧0.5, y≦0.3, z≦0.3, x+y+z=1). Li(Ni x Mn y Co z As a positive electrode active material represented by O2 (x≧0.5, y≦0.3, z≦0.3, x+y+z=1), for example, Li(Ni 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 positive electrode active materials represented as LiMPO4 (where 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 one example.

[0038] In the positive electrode mixture layer, the content of the positive 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, 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% by mass or less, more preferably 97.0% by mass or less, and even more preferably 96.5% by mass or less, from the viewpoint of ensuring the amount of other components.

[0040] The positive electrode mixture layer may contain the carbon material of this disclosure. When the positive electrode mixture layer contains the above-mentioned carbon material, the content of the above-mentioned carbon material in the positive electrode mixture layer is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by 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 cathode mixture layer may contain carbon materials other than the carbon material of this disclosure (other carbon materials). Examples of other carbon materials include carbon fibers that do not meet the average fiber diameter or average fiber length requirements of the carbon fibers contained in the carbon material of this disclosure, composite carbon fibers, carbon nanotubes such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), and carbon nanofibers. The cathode mixture layer may, for example, contain carbon nanotubes and carbon black as described below, together with the carbon material of this disclosure.

[0043] (Carbon Black) The positive electrode mixture layer may further contain carbon black. Carbon black is used as a conductive additive in 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 size of carbon black may be between 10 nm and 100 nm. Here, "primary particle" refers to the portion corresponding to one particle contained within a group of particles called an aggregate. When the primary particle size of carbon black is within this range, it tends to disperse uniformly on the surface of the active material. From the viewpoint of improving dispersibility, the primary particle size of carbon black is preferably between 20 nm and 80 nm, and more preferably between 30 nm and 70 nm.

[0045] The primary particle size of carbon black can be determined by selecting 100 arbitrary primary carbon black particles from SEM images of the electrode and cross-sectional SEM images, and then taking the arithmetic mean of the maximum particle length measurements using image recognition software.

[0046] When the positive electrode mixture layer contains carbon black, the carbon black content is preferably 0.2% by mass or more, more preferably 0.6% by mass or more, and even more preferably 1.0% by 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% by mass or less, more preferably 4.0% by mass or less, and even more preferably 2.0% by 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. Suitable binders include those commonly used in positive electrode mixture layers for lithium-ion secondary batteries. Examples of binders include polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE).

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

[0050] In the positive electrode mixture layer, the binder content is preferably 5.0% by mass or less, more preferably 4.5% by mass or less, and even more preferably 4.0% by 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 dispersants and additives. For example, it may contain various dispersants for dispersing the positive electrode active material, and agents for surface modification of the positive electrode active material.

[0052] [Negative electrode] The secondary battery comprises a negative electrode which includes 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 electronically 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 this disclosure. For example, a composition for a negative electrode binder layer (a kind of composition for forming an electrode binder layer) containing a negative electrode active material and the carbon material of the present disclosure, and further containing a conductive assistant, a binder, a solvent, etc. as required, is applied onto the negative electrode current collector, the applied 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 x (0 < x ≤ 1.5), BiO x (0 < x ≤ 1.5), ZnO x (0 < x ≤ 1), and other oxides. Among them, it is preferable that the negative electrode active material contains graphite. Also, at least a part of the surface of the negative electrode active material may be coated with amorphous carbon.

[0059] 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% by mass or less, more preferably 97.0% by mass or less, and even more preferably 96.5% by mass or less, from the viewpoint of ensuring the amount of other components.

[0061] The negative electrode mixture layer may contain the carbon material of this disclosure. When the negative electrode mixture layer contains the above-mentioned carbon material, the content of the above-mentioned carbon material in the negative electrode mixture layer is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by 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. Examples of conductive additives include carbon-containing materials such as carbon black and graphene.

[0064] (binder) The negative electrode mixture layer may contain a binder. Examples of binders include PVdF, PTFE, etc., as with the positive electrode mixture layer, as well as styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), etc.

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

[0066] In the negative electrode mixture layer, the binder content is preferably 5.0% by mass or less, more preferably 4.5% by mass or less, and even more preferably 4.0% by 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 dispersants and additives. For example, it may contain various dispersants for dispersing the negative electrode active material, and agents for surface modification of the negative electrode active material.

[0068] (Exterior materials) The outer casing material for housing the positive and negative electrodes is not limited as long as it can accommodate the positive and negative electrodes, and optionally a separator and electrolyte, or a solid electrolyte. Examples of outer casing materials include commercially available battery packs, 18650 type cylindrical cells, and those packaged in aluminum foil, and the outer casing material can be freely designed and used.

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

[0070] (electrolyte) The secondary battery may contain an electrolyte. There are no particular restrictions on the electrolyte, and any electrolyte that can be used in a typical secondary battery can be suitably used. For example, an organic solvent in which a lithium salt is dissolved in a concentration of 0.5 mol / L to 2.0 mol / L 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). Organic solvents listed here and others may be appropriately selected and mixed. Examples of electrolyte additives include vinylene carbonate (VC), propane sultone (PS), and fluoroethylene carbonate (FEC). When additives are used, the additive content 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, based on 100% by mass of the organic solvent.

[0073] (Ionic liquid) Ionic liquids may be used as electrolytes, or ionic liquids may be used in combination with the aforementioned organic solvents. The ionic liquid is not particularly limited, and examples include combinations of cations such as imidazolium cations, pyrrolidinium cations, piperinidium cations, and ammonium cations with anions such as bis(trifluoromethane)sulfonamide anions.

[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 the positive electrode and negative electrode are sandwiched between solid electrolytes.

[0075] Examples of solid electrolytes include polymer electrolytes and inorganic solid electrolytes. Polymer electrolytes are not particularly limited, and examples include polymers such as polyethylene oxide impregnated with the above-mentioned lithium salt. Inorganic solid electrolytes are not particularly limited, and examples include Li 13 Ti 1.7 Al 0.3 Examples include (PO4)3 and Li2S-P2S5.

[0076] The secondary battery of this 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 such as power tools, vacuum cleaners, electric bicycles, drones, and electric vehicles; and for storing electricity obtained from fuel cells, solar power generation, wind power generation, etc.

[0077] <Method for manufacturing carbon materials> The present disclosure's method for producing a carbon material includes introducing a raw material liquid containing a carbon source and a catalyst source into a reaction tube with an inner diameter of 400 mm or less using a carrier gas, and heating the components introduced into the reaction tube. The present disclosure's method for producing a carbon material is a floating catalyst method. In the floating catalyst method, a hollow tube is generally formed starting from the catalyst source in the initial stages of the reaction, and carbon fibers grow in the longitudinal direction. Growth in the longitudinal direction continues until catalytic activity is lost, such as when the surface of the catalyst particles is covered with carbon. Once growth in the longitudinal direction is complete, pyrolysis carbon is deposited on the surface of the hollow tube, and growth in the thickness direction of the fibers progresses, forming a structure in which cylindrical carbon hexagonal mesh surfaces are stacked in the thickness direction.

[0078] In this disclosure, the structure in which tubular carbon hexagonal mesh surfaces are stacked in the thickness direction refers to a structure in which multiple tubular structures formed by winding carbon hexagonal mesh surfaces are stacked along the thickness direction (also referred to as a specific structure). A specific structure can be confirmed, for example, by observing a carbon material with a transmission electron microscope (TEM) as follows: Observe 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 the cross-section can be confirmed when the carbon fiber is cut in a direction intersecting the longitudinal direction (hereinafter also referred to as a "TEM cross-sectional image"). If, in the TEM longitudinal image, multiple lines along the longitudinal direction exist inside the carbon fiber, and in the TEM cross-sectional image, multiple closed curves with different maximum diameters exist, and the closed curves are arranged sequentially towards the inside as the maximum diameter decreases, then it can be confirmed that the carbon fiber has a specific structure. Furthermore, by observing diffraction lines similar to those of graphite particles for the (002), (100), (101), (110), or (112) planes using X-ray diffraction (XRD), the structure in which carbon hexagonal mesh surfaces are stacked can be confirmed. The structure in which cylindrical carbon hexagonal mesh surfaces are stacked in the thickness direction may be a structure in which multiple cylindrical carbon hexagonal mesh surfaces of different diameters are arranged so as to have concentric cross-sections (for example, like a concentric multi-tube), and the central axes (lines connecting the centers of each cross-section of a given tube) of multiple cylindrical carbon hexagonal mesh surfaces of different diameters do not all have to be aligned, and only some of the central axes may be aligned. The shape of the cross-section of the tube is not limited to a perfect circle, but may be an ellipse, a polygon, etc., and a part of the outer circumference may be a perfect circle, ellipse, other curve, polygon, or a combination thereof (the above "closed curve" refers to such a shape. In these cases, the "central axis" is the line connecting the centroids of each cross-section). The structure in which cylindrical carbon hexagonal mesh surfaces are stacked in the thickness direction may be a structure in which the central axes of multiple cylindrical carbon hexagonal mesh surfaces with different maximum cross-sectional widths (for example, elliptical or polygonal cross-sections) are all arranged to coincide, or the central axes may not all be aligned, or only some of the central axes may be aligned.

[0079] In the manufacturing method of the present disclosure, the raw material liquid is introduced into a reaction tube with an inner diameter of 400 mm or less, and the carbon material of the present disclosure is obtained by heating it inside the reaction tube. This is presumed to be because reducing the inner diameter of the reaction tube makes it easier to obtain carbon fibers with a relatively large fiber diameter and short fiber length.

[0080] The carbon source can be any raw material used in the suspended catalyst method, such as decahydronaphthalene (decalin), toluene, benzene, hexane, cyclohexane, xylene, ethylbenzene, cyclohexane, and ethylcyclohexane.

[0081] The catalyst source can be any catalyst used in the suspension catalyst method, such as ferrocene or sulfur compounds. A combination of ferrocene and sulfur compounds may also be used as the catalyst source. The amount of catalyst source in the raw material liquid may be 1% to 10% by mass, or 2% to 5% by mass.

[0082] Any gas that can be introduced into the reaction tube from the raw material liquid can be used as the carrier gas, such as hydrogen. Alternatively, the raw material liquid may be gasified and then introduced into the reaction tube as 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 making it easier to obtain carbon fibers having an average fiber diameter and average fiber length within a desired range. Increasing the raw material liquid flow rate and thus increasing the ratio makes it easier for carbon coating to occur on the catalyst source surface, suppressing length growth, and thus making it easier to obtain carbon fibers with a large average fiber diameter and a small average fiber length. Increasing the hydrogen flow rate and thus decreasing the ratio makes it harder for carbon coating to occur on the catalyst source surface, promoting length growth, and thus making it easier to obtain carbon fibers with a small average fiber diameter and a large average fiber length.

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

[0085] To improve the crystallinity of carbon materials obtained by the floating catalyst method, heat treatment may be performed at 800°C to 1500°C under an inert gas atmosphere such as argon, followed by graphitization treatment at 2000°C to 3000°C. Graphitization treatment simultaneously removes the catalyst metal by evaporation, enabling high-purity carbon materials. After performing a heat treatment (first firing step) at 800°C to 1500°C, preferably 900°C to 1300°C, under an inert gas atmosphere such as argon, the temperature may be raised to 200°C to 500°C, preferably 1200°C to 1700°C, and held for 30 minutes to 3 hours (second firing step). Furthermore, the temperature for the graphitization treatment is preferably 2500°C to 3000°C, and more preferably 2600°C to 3000°C. [Examples]

[0086] The present disclosure will be described in detail below with reference to 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 following method.

[0087] (Examples 1-5, 8, 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 starting material solution (3.5% by mass of ferrocene and 0.08% by mass of sulfur in the starting material solution). The prepared starting material solution and hydrogen were supplied at a flow rate corresponding to the inner diameter of the reaction tube and sprayed into the reaction tube from a two-fluid mixing nozzle. The ratio of the starting material solution flow rate to the hydrogen flow rate at this time was 0.54 to 0.71 g / NL, and the starting material solution flow rate and hydrogen flow rate were adjusted as appropriate for each example. A carbon material containing carbon fibers was obtained by passing a sprayed raw material liquid through a reaction tube whose temperature (temperature of the wall inside the reaction tube) was adjusted to 1230°C to 1290°C. At this time, the temperature of the furnace wall in the middle section of the reaction tube was 1230°C.

[0088] (Examples 6, 7, 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 starting material solution (3.5% by mass of ferrocene and 0.08% by mass of sulfur in the starting material solution). The prepared starting material solution and hydrogen were supplied at a flow rate corresponding to the inner diameter of the reaction tube and sprayed into the reaction tube from a two-fluid mixing nozzle. The ratio of the starting material solution flow rate to the hydrogen flow rate at this time was 0.54 to 0.71 g / NL. A carbon material containing carbon fibers was obtained by passing a sprayed raw material liquid through a reaction tube whose temperature (temperature of the wall inside the reaction tube) was adjusted to 1230°C to 1290°C. At this time, the temperature of the furnace wall in the middle section of the reaction tube was 1270°C.

[0089] (Comparative Examples 1-3) 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 solution and hydrogen, prepared in the same manner as in Example 1, were supplied at a flow rate corresponding 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 at this time was 0.54 g / NL to 0.71 g / NL, and the raw material solution flow rate and hydrogen flow rate were adjusted as appropriate for each comparative example. A carbon material containing carbon fibers was obtained by passing a sprayed raw material liquid through a reaction tube whose temperature (temperature of the wall inside the reaction tube) was adjusted to 1230°C to 1290°C. At this time, the temperature of the furnace wall in the middle section of the reaction tube was 1290°C.

[0090] (Transmission electron microscope (TEM) observation) The carbon fibers contained in each carbon material obtained in Examples 1-10 and Comparative Examples 1-3 were dispersed in ethanol, scooped up with a microgrid, and dried to obtain the samples. TEM observation of the samples was performed. In Examples 1-10 and Comparative Examples 1-3, it was confirmed that the carbon fibers contained in each carbon material had a structure in which tubular carbon hexagonal mesh surfaces were stacked in the direction of the fiber thickness, as described below. First, the carbon material was observed using TEM-EDX to obtain an image showing the longitudinal direction of the carbon fiber (hereinafter also referred to as the "TEM longitudinal image") and an image showing the cross-section when the carbon fiber is cut in a direction intersecting the longitudinal direction (hereinafter also referred to as the "TEM cross-sectional image"). In the TEM longitudinal image, it was confirmed that there were multiple lines along the longitudinal direction inside the carbon fiber, and in the TEM cross-sectional image, it was confirmed that there were multiple closed curves with different maximum diameters arranged concentrically. Thus, the structure in which tubular carbon hexagonal mesh surfaces are stacked in the direction of the fiber thickness in the carbon fiber was confirmed. 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) were measured for the carbon fibers contained in each of the obtained carbon materials. Using a scanning electron microscope, we created a panoramic image of carbon fibers at over 4000x magnification, preparing a field of view that allowed for measurement of both ends of the carbon fibers. We measured the lengths of 200 randomly selected carbon fibers and averaged them to determine 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 defined as the average fiber diameter. The fiber diameter was measured at one point, excluding both ends of the carbon fiber. The measurement results are shown in Table 1.

[0092] (d 002 ) Using a horizontal sample type multi-purpose X-ray diffractometer (Ultima IV, manufactured by Rigaku Corporation), the JSPS (latest) Following the Carbon Materials Experimental Techniques (Analysis and Analysis Edition), edited by the Carbon Materials Society, silicon powder was used as the internal standard for the d of carbon materials. 002 Measurements were taken.

[0093] (Measurement of compaction resistivity) 1 g of carbon material was placed in a special jig. The special jig had a space of 1 cm (length) x 4 cm (width) x 9 cm (height), with two 1 cm x 4 cm sides made of metal and electrically conductive. Additionally, two metal points (φ=3 mm) for voltage measurement were provided on the 1 cm x 4 cm bottom surface, and electrically conductive. The distance between the metal points was 1 cm. The non-metal parts were made of plastic and were insulating. The push rod attached to the special jig had a section of 0.999 cm (length) x 3.999 cm (width) x 9 cm (height), which allowed the jig's space to be compressed in the height direction. While a 100 mA current was passed through the metal sides, a predetermined load was applied from above the push rod. The height of the carbon material and the voltage between the voltage terminals were measured, and the density and volume resistivity of the carbon material were recorded. The volume resistivity measured in this way is called "compression resistivity." In particular, the carbon material was 0.8 g / cm³. 3 The value of the resistivity of consolidation when compressed was used for comparison. Furthermore, for Examples 1-5 and 8, the concentration was 0.8 g / cm³. 3 The compression pressure was less than 1.9 MPa in all cases, and Examples 6, 7, and 9, as well as Comparative Examples 1-3, were 0.8 g / cm³. 3 The compression pressure was 1.9 MPa or higher in all cases.

[0094] (Preparation of samples for resistance measurement) As the positive electrode active material, LiNi 0.8 Mn 0.1 Co 0.196.5 parts by mass of O2 (50% diameter in the volume-based cumulative particle size distribution, D50 = 38 μm), 0.5 parts by mass of a conductive additive such as carbon material obtained in the examples or comparative examples, carbon black (C-NERGY® Super C65, manufactured by Imerys Graphite & Carbon, primary particle size: 33 nm) as another conductive additive, 1 part by mass of PVDF (Kureha Corporation #5130) as a binder, and N-methyl-2-pyrrolidone (NMP, manufactured by Kishida Chemical Co., Ltd.) as a solvent for slurry preparation were added as appropriate to form a slurry. The mixture was kneaded using a foam remover mixer (Sinky Corporation, ARE-100). The obtained slurry was applied to a 20 μm thick aluminum foil using a doctor blade with a gap of 70 μm, dried on a hot plate at 90°C, and then dried in a vacuum dryer at 90°C for 1 hour. From this, a 2 cm x 2 cm sample was cut out and used as the measurement sample.

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

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

[0097] [Table 1]

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

Claims

1. A carbon material containing carbon fibers having a structure in which tubular carbon hexagonal mesh surfaces are stacked in the direction of fiber thickness, with an average fiber diameter of 140 nm or more and an average fiber length of less than 5.0 μm.

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

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

4. Compression density: 0.8 g / cm³ ３ The carbon material according to claim 1, wherein the compaction resistivity in is 0.0180 Ω·cm or less.

5. 0.8 g / cm ３ The carbon material according to claim 1, wherein the pressure required to compress it is less than 1.9 MPa.

6. A conductive additive comprising the carbon material described in any one of claims 1 to 5.

7. A dispersion comprising the carbon material according to any one of claims 1 to 5.

8. A composition for forming an electrode mixture layer, comprising the carbon material described in any one of claims 1 to 5.

9. A positive electrode comprising 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 comprising a negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material disposed on the negative electrode current collector, A secondary battery comprising at least one of the positive electrode mixture layer and the negative electrode mixture layer, the carbon material described in any one of claims 1 to 5.

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