JP2011157668A - Method for pitch fiber spinning, method for producing carbon fiber, and carbon nanofiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for enabling a carbon nanofiber without electroconductivity anisotropy to be produced. <P>SOLUTION: The method is such as to be intended for pitch fiber spinning by electrospinning process to extrude a pitch-based material melt via a spinning nozzle against a collector. In this method, as the pitch-based material 1, nonhydrogenated pitch higher than 210°C and lower than 280°C in softening point or hydrogenated pitch higher than 120°C and lower than 250°C in softening point is used; as a melt-extruding nozzle 5, such one having a space into which a gas is introduced into between the spinning nozzle and an outer cylinder, and a preheated gas is fed into the space; in extruding the pitch-based material 1 melt via the spinning nozzle of the melt-extruding nozzle 5, the preheated gas is sprayed against the collector 6 in parallel to the extruding direction. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for spinning pitch fibers, a method for producing carbon fibers (including graphite fibers) provided with a spinning step comprising this method, and carbon nanofibers.

  Pitch-based carbon fiber is made from petroleum-based or coal-based pitch, and the spinning process, infusibilization process, and heat treatment process in an inert gas atmosphere (carbonization process or carbonization process and graphitization process) are performed in this order. Manufactured by. In the spinning process, the raw material pitch is made into a fiber by melt spinning. When pitch fibers (fibrous pitch) obtained in the spinning process are heat-treated as they are at a high temperature, the pitch fibers are fused to each other. Therefore, an infusibilization process is performed for the purpose of avoiding this.

As a method for obtaining a petroleum-based or coal-based pitch which is a raw material for carbon fiber, there are the following methods. First, petroleum tar or coal tar is subjected to atmospheric distillation and vacuum distillation to separate heavy and light fractions, and the resulting heavy fractions are heat-treated as they are and subjected to polycondensation to give the desired softening. It is a method to prepare to the point. The pitch (anhydrous added pitch) obtained by this method has optical isotropy and is referred to as “General Purpose pitch”.
The other is a method in which the obtained heavy fraction is hydrocracked to increase thermal stability and then heat treated to adjust to a predetermined softening point. The pitch (hydrogenated pitch) obtained by this method has optical anisotropy and is called “High Peformance pitch” or “Liquid crystal pitch”, and is used as a raw material for carbon fiber with high strength and high elastic modulus. The

On the other hand, carbon nanofibers (including carbon nanotubes) are extremely useful materials as conductive materials and heat conductive materials. Examples of methods for producing carbon nanofibers include an arc method and a vapor phase growth method. As shown in FIGS. 6A to 6C, the structure of the carbon nanofibers produced by these methods includes a structure in which graphene sheets (carbon six-membered ring network) overlap each other in a specific direction. This is a structure in which a plurality of graphene sheets wound in a cylindrical shape with different diameters are coaxially nested.
6A is called a herringbone type, FIG. 6B is called a multi-wall type (multi-walled carbon nanotube), and FIG. 6C is called a cup stack type. The carbon nanofiber having such a structure has a directionality (anisotropy) in the degree of current or heat conduction transmitted along the graphene sheet surface.

  In Patent Document 1 below, in order to improve large current discharge characteristics and cycle characteristics of a lithium ion secondary battery, carbon fiber produced by a vapor phase growth method is added to graphite used as a negative electrode material. Has been proposed. However, the actual situation is that a sufficient improvement effect is not obtained by this method. As one of the causes, when the carbon fiber produced by the vapor growth method has the structure of FIG. 6B, for example, the conductivity in the direction along the axis of the cylindrical fiber is extremely high. Therefore, it is estimated that high conductivity cannot be obtained unless the contact direction of the carbon fiber with respect to graphite is controlled.

  Patent Document 2 below describes a spinning device that can perform an electrospinning method. The spinning device includes a storage container that stores a raw material melt, a melt discharge nozzle that discharges the melt stored in the storage container in a thin thread shape, a collector that is disposed to face the melt discharge nozzle, And a melt charging means for charging the melt discharged from the melt discharge nozzle by applying a voltage between the collector and the melt discharge nozzle. This melt discharge nozzle is composed of a first nozzle part for introducing a melt and a second nozzle part for containing a gas, and pressurizing with a gas placed in the second nozzle part, while in the first nozzle part The melt is discharged in the form of a fine thread.

JP 2003-168429 A JP 2009-275339 A

However, the carbon fiber obtained by performing the carbonization step or the carbonization step and the graphitization step on the pitch fiber spun using the apparatus described in Patent Document 2 has no anisotropy in conductivity. There is room for improvement.
An object of the present invention is to provide a method capable of producing carbon nanofibers having no conductivity anisotropy, that is, having the same conductivity in a direction perpendicular to the axis direction of the fiber.

  In order to solve the above-described problems, the pitch fiber spinning method of the present invention is a method for spinning pitch fibers by discharging a melt of a pitch-based material from a spinning nozzle toward a collector by an electrospinning method. Then, as the pitch-based material, an anhydrous addition pitch having a softening point exceeding 210 ° C. and less than 280 ° C. or a hydrogenation pitch having a softening point exceeding 120 ° C. and less than 250 ° C. is used to preheat around the spinning nozzle. When the melted gas is supplied from the spinning nozzle, the preheated gas is blown toward the collector in parallel with the discharge direction.

The pitch fiber spinning method of the present invention is a method of spinning pitch fibers by discharging a melt of a pitch-based material from a spinning nozzle toward a collector by an electrospinning method, and has a specific softening point. The use of pitch-based materials is an essential requirement.
Specifically, in the case of an anhydrous pitch, a pitch having a softening point of more than 210 ° C. and less than 280 ° C., preferably a softening point of 220 ° C. or more and 250 ° C. or less is used. Further, in the case of hydrogenated pitch, those having a softening point exceeding 120 ° C. and less than 250 ° C., preferably those having a softening point of 200 ° C. or more and 230 ° C. or less are used.

Anhydrous pitch with a softening point exceeding 210 ° C contains many crystal seeds in an amorphous state, and pitch fibers spun using this as a pitch-based material can be carbonized or graphitized. When this occurs, crystal growth occurs starting from the embryo in various directions. Thereby, the carbon fiber in which many crystals exist at random is obtained.
However, an anhydrous pitch with a softening point of 280 ° C. or higher has a high viscosity when protruding from the spinning nozzle as a melt heated to the heat-denaturing temperature of the pitch-based material (about 340 ° C.), and a good spinning state is obtained. It is not used because it cannot be obtained.

  Since there is almost no embryo in an anhydride-added pitch having a softening point of 210 ° C. or lower, a large number of crystals are present at random even if the pitch fiber spun using this as a pitch-based material is carbonized or graphitized. Carbon fiber cannot be obtained. In addition, when spinning, the melt is discharged from the spinning nozzle, and an electrostatic attractive force is applied to the discharged melt, so that crystals grow while being oriented slightly during carbonization or graphitization. it is conceivable that.

  Hydrogenated pitch with a softening point of 250 ° C or higher is easy to be oriented during spinning. Therefore, even if the pitch fiber spun using this as a pitch-based material is carbonized or graphitized, a large number of crystals are present at random. Carbon fiber to be obtained is not obtained. The crystal structure of the obtained carbon fiber is a structure in which the a axis is along (orientated) along the fiber axis. Therefore, in the case of hydrogenated pitch, the one having a softening point of less than 250 ° C is used.

  A hydrogenated pitch having a softening point of 120 ° C. or lower requires a treatment temperature as low as about 100 ° C. in the infusibilization step performed after the spinning step by the method of the present invention in order to obtain carbon fibers. The time required for the melting process becomes extremely long. On the other hand, since the hydrogenated pitch having a softening point exceeding 120 ° C. can be performed at a high processing temperature exceeding 100 ° C. in the infusibilization step, the time required for the infusibilization step can be shortened.

  The pitch fiber spinning method of the present invention further supplies a preheated gas around the spinning nozzle, and the preheated gas is parallel to the discharge direction when the melt is discharged from the spinning nozzle. Spraying toward the collector is an essential requirement. It can be estimated that the pitch fiber comes to have a random fine structure by the shearing force of the sprayed preheating gas.

And after performing the spinning process of the pitch fiber by the method of the present invention, the infusibilizing process of the pitch fiber and the process of heat-treating the infusible pitch fiber in an inert gas atmosphere (carbonization process and graphitization process) By carrying out by a normal method, the average diameter is 50 nm or more and 800 nm or less, it is composed of a large number of randomly oriented microcrystalline graphite, and is measured by the X-ray diffraction of Cu—Kα ray. Carbon nanofibers having an interval of 0.3400 nm or more and 0.3700 nm or less can be obtained.
The temperature of the heat treatment step (carbonization step and graphitization step) is preferably 500 ° C. or higher and lower than 2700 ° C., more preferably 800 ° C. or higher and 2500 ° C. or lower.

According to the spinning method of pitch fiber of the present invention, by specifying the softening point of the pitch-based material to be used, carbon fiber obtained by performing an infusibilization step and a heat treatment step in an inert gas atmosphere as a subsequent step. Can be a carbon nanofiber having no conductivity anisotropy, that is, the conductivity in the direction perpendicular to the axis is the same as the axis direction of the fiber.
According to the carbon fiber manufacturing method of the present invention, by specifying the softening point of the pitch-based material used in the spinning process of the pitch fiber, there is no anisotropy in conductivity, that is, the axis direction of the fiber and perpendicular to the axis Carbon nanofibers having the same conductivity in any direction can be manufactured.

It is a schematic block diagram which shows the spinning apparatus which can implement the method equivalent to embodiment of this invention. It is sectional drawing which shows the structure of the melt discharge nozzle 5 of the apparatus of FIG. It is a schematic diagram which shows typically the microstructure of the carbon fiber obtained by the method (No. 1 and No. 2 method of an Example) of this invention. It is a schematic diagram which shows typically the microstructure of the carbon fiber (used by No. 4 of the Example) obtained by the vapor phase growth method. It is a graph which shows the result of having investigated the cycle characteristic of the test battery of No. 1-No. 5 produced in the Example. It is a figure which shows the example of the fine structure of the carbon nanofiber obtained by the conventional method, Comprising: (a) is a herringbone type, (b) is a multiwall type (multi-walled carbon nanotube), (c) is a cup stack type. It is.

Hereinafter, embodiments for carrying out the present invention will be described.
The method of this embodiment can be carried out using the spinning device of FIG.
The spinning device of FIG. 1 is connected to a storage container 2 for storing the pitch-based material 1, an electric heater 3 for heating the storage container 2 to keep the pitch-based material 1 in a molten state, and the storage container 2. Nitrogen gas supply line 4, melt discharge nozzle 5 disposed at the lower end of storage container 2, flat collector 6 disposed at the position facing the tip of melt discharge nozzle 5, collector 6 and melt And a voltage generator 7 for applying a voltage to the discharge nozzle 5.

  The pitch-based substance 1 is melted in another container, and is supplied from the container into the storage container 2 by a gear pump or the like. The storage container 2 has a sealed structure, and is configured such that, for example, nitrogen gas pressurized to about 0.3 to 0.7 MPa is supplied from the nitrogen gas supply line 4. By supplying the pressurized gas, the pitch-based material 1 in the storage container 2 is introduced into the melt discharge nozzle 5.

  As shown in FIG. 2, the melt discharge nozzle 5 includes a spinning nozzle 51, an outer cylinder 52, and a nozzle guide 53. The spinning nozzle 51 includes a main part 51a, a rear end part 51b, and a front end nozzle part 51c. A pipe 54 for introducing a melt into the interior 5A is connected to the rear end 51b of the spinning nozzle 51. Via this pipe 54, the molten pitch-based material 1 is introduced from the storage container 2 into the inside 5 </ b> A of the spinning nozzle 51.

A space 5 </ b> B is provided between the main portion 51 a of the spinning nozzle 51, the outer cylinder 52, and the nozzle guide 53. A pipe 55 for introducing gas into the space 5B is connected to the outer cylinder 52. The tip nozzle portion 51c of the spinning nozzle 51 is formed such that its diameter decreases stepwise toward the tip. The nozzle guide 53 has an inner diameter that is gradually reduced toward the tip in accordance with the shape of the tip nozzle portion 51 c of the spinning nozzle 51. A nozzle port 53a having a diameter of about 0.5 mm is formed at the tip of the nozzle guide 53, for example.
In the spinning method of this embodiment, an anhydrous pitch having a softening point of more than 210 ° C. and less than 280 ° C. or a hydrogenated pitch having a softening point of more than 120 ° C. and less than 250 ° C. is prepared. Into a molten state. The melted pitch-based material 1 is supplied into the storage container 2 of the spinning device shown in FIG.

  The storage container 2 is heated by the electric heater 3 so that the viscosity of the pitch-based material 1 becomes (1 to 100 poise), and the pitch in the storage container 2 is increased by the pressurized nitrogen gas supplied from the nitrogen gas supply line 4. The system material 1 is introduced into the spinning nozzle 51 of the melt discharge nozzle 5 from the pipe 54. Further, an inert gas (nitrogen gas, helium gas, argon gas, etc.) heated to about 300 ° C. is introduced from the pipe 55 into the space 5B of the melt discharge nozzle 5. Thereby, the heated inert gas is supplied to the space 5B of the melt discharge nozzle 5 (around the main portion 51a of the spinning nozzle 51).

  Further, a voltage is applied between the collector 6 and the melt discharge nozzle 5 by the voltage generator 7 in the range of 0.5 to 100 kV. Here, from the viewpoint of safety, the collector 6 is grounded and the melt discharge nozzle 5 is on the positive voltage side. The distance between the ground contact position of the collector 6 and the tip position of the melt discharge nozzle 5 is, for example, 10 to 200 mm. By setting this distance to 10 mm or more, dielectric breakdown between the collector 6 and the melt discharge nozzle 5 is prevented, and a stable potential environment can be maintained. When this distance exceeds 200 mm, a sufficient electric field is not generated between the collector 6 and the melt discharge nozzle 5.

  As a result, the molten pitch-based material 1 introduced into the interior 5A of the spinning nozzle 51 is discharged from the nozzle port 53a toward the collector 6, and at that time, the inert gas preheated to about 300 ° C. is melted. The inert gas is introduced into the space 5B of the material discharge nozzle 5 and sprayed toward the collector 6 in parallel with the discharge direction of the molten pitch-based material 1. It can be estimated that the fine structure of the pitch fibers collected by the collector 6 becomes random by the shearing force of the inert gas.

  The pitch fiber thus obtained is subjected to a general infusibilization step (step of heating the pitch fiber in an oxygen-containing atmosphere), and then the infusible pitch fiber is heat-treated in an inert gas atmosphere. The carbonization step and the graphitization step to be performed are each performed by a normal method. As a result, the average diameter is 50 nm or more and 800 nm or less, it is composed of a large number of randomly oriented microcrystalline graphite, and the (002) plane spacing measured by X-ray diffraction of Cu—Kα rays is 0.3400 nm. Carbon nanofibers having a thickness of 0.3700 nm or less can be obtained.

  Note that the heating temperature in the infusibilization step is set to, for example, 100 to 300 ° C. according to the softening point of the used pitch-based material. The carbonization step is performed at 500 ° C. or more and 1200 ° C. or less, and the graphitization step is performed at a temperature of 2000 ° C. or more and less than 2700 ° C. after the carbonization step. When pitch fibers after the infusibilization process are heated at 500 to 1200 ° C., a large amount of volatile substances are generated. Therefore, the carbonization process uses a furnace (carbonization furnace) having a structure capable of removing volatile substances. Do it. The graphitization step is performed by transferring the carbon fiber from which the volatile substances have been removed in the carbonization step to a furnace (graphitization furnace) having a structure with high thermal efficiency and no mixing of oxidizing substances.

  Since the carbon nanofiber produced by the method of the present invention is excellent in conductivity, it can be used as a conductive material. For example, conductivity can be imparted to the synthetic resin which is an insulator by mixing with the synthetic resin. Moreover, it can also be used as an additive (conductive agent) which improves the electroconductivity of the negative electrode material of a lithium ion secondary battery. Hereinafter, application of the carbon nanofiber produced in the present invention to a lithium ion secondary battery will be described.

<Structure of lithium ion secondary battery>
A lithium ion secondary battery usually has a negative electrode, a positive electrode, and a non-aqueous electrolyte as main battery components. Each of the positive electrode and the negative electrode is a lithium ion carrier, and the lithium ions are occluded in the negative electrode during charging and are released from the negative electrode during discharging, thereby acting as a secondary battery.

The structure of the lithium ion secondary battery can be arbitrarily selected from a cylindrical shape, a square shape, a coin shape, a button shape, and the like according to the application, installed equipment, required charge / discharge capacity, and the like. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to include a means for detecting an increase in the internal pressure of the battery and shutting off the current when there is an abnormality such as overcharging. In the case of a polymer electrolyte secondary battery such as a polymer solid electrolyte secondary battery or a polymer gel electrolyte secondary battery, a structure enclosed in an aluminum laminate film can also be used.
These polymer electrolyte secondary batteries are configured by laminating a negative electrode, a positive electrode, and a polymer electrolyte in the order of, for example, a negative electrode, a polymer electrolyte, and a positive electrode, and accommodating them in the exterior of the battery. Furthermore, a polymer electrolyte may be disposed outside the negative electrode and the positive electrode.

<Negative electrode of lithium ion secondary battery>
For the negative electrode of a lithium ion secondary battery, a negative electrode mixture composed of a negative electrode material (negative electrode active material) made of a carbon material, a binder, and an additive such as a conductive agent is applied to the surface of the current collector. It is formed by.
As the carbon material, for example, natural graphite or artificial graphite can be used. Artificial graphite is graphitized by heating the pitch, graphitized mesocarbon spherules obtained by heating the pitch, bulk mesophase, coke, etc., and the surface of these graphitized materials is pitch or phenolic resin. What was baked after coating | cover etc. can be illustrated. Moreover, you may use these carbon materials in mixture. Further, powdery, spherical, flake shaped, and fibrous carbon materials can be used as the negative electrode material.

  By using the carbon nanofibers produced by the method of the present invention as the conductive agent for the negative electrode, the conductivity of the negative electrode material can be improved. The content of the carbon nanofibers is preferably 0.1% by mass or more and 10% by mass or less, and preferably 0.5% by mass or more and 5% by mass or less with respect to the total amount of the negative electrode material and the carbon nanofibers. It is more preferable. In addition, as the conductive agent for the negative electrode, not only carbon nanofibers produced by the method of the present invention, but also carbon black, carbon fibers obtained by a vapor phase growth method, or the like may be used.

The negative electrode of the lithium ion secondary battery is manufactured according to a conventionally known negative electrode manufacturing method, but is not limited as long as it is a method capable of manufacturing a chemically and electrochemically stable negative electrode. When preparing the negative electrode, it is preferable to use a negative electrode mixture prepared in advance by adding a binder to the negative electrode material for a lithium ion secondary battery containing the composite graphite particles of the present invention.
As the binder, those having chemical stability and electrochemical stability with respect to the electrolyte are preferable, and not only the organic binder that is dissolved and / or dispersed in the organic solvent but also dissolved and / or in the aqueous solvent. Even if the aqueous binder to disperse is used, a negative electrode exhibiting excellent charge / discharge characteristics can be obtained.

For example, fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, resins such as polyethylene and polyvinyl alcohol, and rubbers such as carboxymethyl cellulose and styrene butadiene rubber are used, but carboxymethyl cellulose, polyvinyl alcohol and styrene butadiene rubber are used. It is preferable to use an aqueous binder such as These can also be used together. In general, the binder is preferably used at a ratio of 0.5 to 20% by mass in the total amount of the negative electrode mixture.
As the solvent, an ordinary solvent used for preparing the negative electrode mixture is used. Specific examples include N-methylpyrrolidone, dimethylformamide, water, alcohol and the like. Use of an aqueous solvent is preferable from the viewpoint of environmental pollution and safety.

  A more specific method for producing a negative electrode is as follows. First, the negative electrode material is adjusted to a desired particle size by classification or the like, and a mixture obtained by mixing with a binder is dispersed in a solvent to prepare a negative electrode mixture in the form of a paste. . That is, a slurry obtained by mixing or dispersing the negative electrode material and the binder in a solvent such as water, isopropyl alcohol, N-methylpyrrolidone, dimethylformamide, etc., using a known stirrer, mixer, kneader, kneader, etc. To prepare a paste. When the paste is applied to one or both sides of the current collector and dried, a negative electrode in which the negative electrode mixture layer is uniformly and firmly bonded is obtained. The film thickness of the negative electrode mixture layer is 10 to 200 μm, preferably 20 to 100 μm.

Further, if necessary, the negative electrode material and resin powder such as polyethylene and polyvinyl alcohol can be dry-mixed together with other graphite materials, and the negative electrode can be molded according to a normal molding method. For example, the negative electrode can be formed by hot press molding the mixture in a mold. When the negative electrode mixture layer is formed and then pressure bonding such as pressurization is performed, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased.
The shape of the current collector used for the negative electrode is not particularly limited, but a foil or a net-like material such as a mesh or expanded metal is used. Examples of the material for the current collector include copper, stainless steel, and nickel. In the case of a foil, the thickness of the current collector is preferably 5 to 20 μm.

<Positive electrode of lithium ion secondary battery>
The positive electrode is formed, for example, by applying a positive electrode mixture composed of a positive electrode material, a binder, and a conductive agent to the surface of the current collector. It is preferable to select a positive electrode material (positive electrode active material) that can occlude and release a sufficient amount of Li. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides (V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , V ₃ O _8, etc.) and lithium compounds such as lithium compounds thereof. Containing compound, general formula M _x Mo ₆ S _8-y (wherein M is at least one transition metal element, X is a number in the range of 0 ≦ X ≦ 4, Y is 0 ≦ Y ≦ 1) The chevrel phase compound, activated carbon, activated carbon fiber, etc. which are represented can be used. The lithium-containing transition metal oxide is a composite oxide of Li and a transition metal, and may be a solid solution of Li and two or more transition metals.

Specifically, the lithium-containing transition metal oxide is LiM ¹ _1-p M ² _p O ₂ (wherein M ¹ and M ² are at least one transition metal element, and p is 0 ≦ p ≦ 1) LiM ¹ _2-q M ² _q O ₄ (wherein M ¹ and M ² are at least one transition metal element, and q is a number in the range of 0 ≦ q ≦ 2) Indicated by
Transition metals represented by M, M ¹ and M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Cr, Ti, V, Al and the like. Preferred examples are LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _0.9 Co _0.1 O ₂ , LiNi _0.5 Co _0.5 O ₂ and the like.

The lithium-containing transition metal oxide is prepared by mixing, for example, Li and a transition metal oxide or salt as starting materials, and mixing these starting materials according to the composition of the desired metal oxide, in an oxygen atmosphere at 600 to 1000 ° C. It can obtain by baking at the temperature of. The starting material is not limited to oxides or salts, but may be hydroxides.
The positive electrode active material may be used alone or in combination of two or more. For example, an alkali carbonate such as lithium carbonate can be added to the positive electrode material.
In order to form a positive electrode with such a positive electrode material, for example, a positive electrode mixture made of a positive electrode active material, a binder, and a conductive agent for imparting conductivity to the electrode is applied to one or both sides of the current collector. A positive electrode mixture layer is formed. As the binder, those used in the negative electrode can be used. As the conductive agent, a carbon material such as graphite or carbon black is used.

The shape of the current collector used for the positive electrode is not particularly limited, but a foil or a net-like material such as a mesh or expanded metal is used. Examples of the material for the current collector include aluminum, copper, stainless steel, and nickel. In the case of a foil, the thickness of the current collector is preferably 10 to 40 μm.
In the case of the positive electrode, as in the case of the negative electrode, the positive electrode mixture is dispersed in a solvent to form a paste, and the paste-like negative electrode mixture is applied to a current collector and dried to form a positive electrode mixture layer. In addition, after forming the positive electrode mixture layer, pressure bonding such as press pressing may be further performed. Thereby, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.

<Non-aqueous electrolyte of lithium ion secondary battery>
Lithium ion secondary batteries can use polymer electrolytes such as solid electrolytes or gel electrolytes in addition to liquid electrolytes as non-aqueous electrolytes. In the case of a liquid system, the nonaqueous electrolyte secondary battery is configured as a so-called lithium ion secondary battery, and in the case of a polymer system, a polymer electrolyte secondary battery such as a polymer solid electrolyte secondary battery or a polymer gel electrolyte secondary battery is used. It is configured as a secondary battery.

Non-aqueous electrolyte used in the lithium ion secondary battery, an electrolyte salt used in the conventional non-aqueous electrolyte, _{specifically, LiPF 6, LiBF 4, LiAsF} 6, LiClO 4, LiB (C 6 _{H 5), LiCl, LiBr,} LiCF 3 SO 3, LiCH 3 SO 3, LiN (CF 3 SO 2) 2, LiC (CF 3 SO 2) 3, LIN (CF 3 CH 2 OSO 2) 2, LIN (CF _{3 CF 3 OSO 2) 2,} LIN (HCF 2 CF 2 CH 2 OSO 2) 2, LIN [(CF 3) 2 CHOSO 2] 2, LIB [C 6 H 3 (CF 3) 2] 4, LiAlCl 4, A lithium salt such as LiSiF ₆ may be mentioned.
In particular, LiPF ₆ and LiBF ₄ are preferable from the viewpoint of oxidation stability. The concentration of the electrolyte salt in the electrolytic solution is preferably 0.1 to 5 mol / l, and more preferably 0.5 to 3.0 mol / l.

  Examples of the solvent for making the non-aqueous electrolyte include those used as a solvent for ordinary non-aqueous electrolytes. Specifically, carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate and diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone 1,3-dioxofuran, 4-methyl-1,3-dioxofuran, ethers such as anisole and diethyl ether, thioethers such as sulfolane and methylsulfolane, nitriles such as acetonitrile, chloronitrile and propionitrile, trimethyl borate, silicic acid Tetramethyl, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethylorthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydride Thiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, may be used an aprotic organic solvent such as dimethyl sulfite.

When a polymer electrolyte is used, a polymer gelled with a plasticizer (non-aqueous electrolyte) is used as the polymer constituting the matrix. As a matrix for forming a polyelectrolyte, ether-based polymer compounds such as polyethylene oxide and cross-linked products thereof, polymethacrylate-based polymer compounds, polyacrylate-based polymer compounds, polyvinylidene fluoride and vinylidene fluoride-hexafluoro Fluorine polymer compounds such as a propylene copolymer can be used alone or in combination. In these, it is preferable to use fluorine-type high molecular compounds, such as polyvinylidene fluoride and a vinylidene fluoride-hexafluoropropylene copolymer, from viewpoints, such as oxidation-reduction stability.
In the case of a polymer electrolyte, a plasticizer is blended, and as the plasticizer, the electrolyte salt or the nonaqueous solvent is used. In the case of a polymer gel electrolyte, the electrolyte salt concentration in the non-aqueous electrolyte as a plasticizer is preferably 0.1 to 5 mol / l, and more preferably 0.5 to 2.0 mol / l.

The method for producing such a polymer electrolyte is not particularly limited. For example, a polymer compound constituting a matrix, a lithium salt and a nonaqueous solvent (plasticizer) are mixed and heated to melt and dissolve the polymer compound. A method, a method of dissolving a polymer compound, a lithium compound and a non-aqueous solvent in an organic solvent for mixing, and then evaporating the organic solvent for mixing; a polymerizable monomer, a lithium salt and a non-aqueous solvent are mixed; Examples thereof include a method of polymerizing by irradiating with an electron beam or a molecular beam.
The ratio of the nonaqueous solvent in the polymer electrolyte is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass. If it is less than 10% by mass, the electrical conductivity will be low, and if it exceeds 90% by mass, the mechanical strength will be weak and film formation will be difficult.

<Separator of lithium ion secondary battery>
Examples of the separator of the lithium ion secondary battery include a woven fabric, a nonwoven fabric, and a synthetic resin microporous membrane. A microporous membrane made of synthetic resin is preferred, and among these, a microporous membrane made of polyolefin is preferred from the viewpoint of thickness, membrane strength, membrane resistance, and the like. Specifically, it is a microporous film made of polyethylene and polypropylene, or a microporous film in which these are combined.

Hereinafter, the present invention will be described with reference to specific examples and comparative examples.
[Sample No.1]
Anhydrous added pitch (softening point 232 ° C.) prepared using coal tar as a raw material was used as the pitch-based material. As the spinning device of FIG. 1, a spinning device having a storage container 2 made of a stainless steel barrel (barrel container) having a capacity of 10 mL was prepared. Further, this spinning device is basically shown in FIG. 2 as the melt discharge nozzle 5, but the shape of the tip 51c of the spinning nozzle 51 is different from that shown in FIG. Are not formed with a small diameter). The tip 51c is 27G (inner diameter 0.20 mm, outer diameter 0.42 mm), the inner diameter of the outer cylinder 52 is 0.50 mm, and the nozzle port 53a of the nozzle guide 53 is 0.50 mm.

A temperature controller was connected to the electric heater 3 to control the temperature of the pitch material 1 in the storage container 2 to be 330 ° C. An electric heater was also wound around the outer side of the outer cylinder 52 of the melt discharge nozzle 5 and a temperature controller was connected to control the temperature inside the spinning nozzle 51 to be 330 ° C.
Nitrogen gas preheated to 330 ° C. was introduced into the space 5B of the melt discharge nozzle 5 from the pipe 55, and the flow rate was adjusted so that the linear velocity was 1000 mm / s at the nozzle port 53a. In this state, a voltage of 7 kV was generated by the voltage generator 7 and applied between the collector 6 and the melt discharge nozzle 5. An earth electrode was placed at a position immediately below the melt discharge nozzle 5 and at a distance of 120 mm from the nozzle port 53a. In this state, 0.3 MPa of pressurized nitrogen was introduced into the storage container 2 from the nitrogen gas supply line 4 to perform spinning. As a result, spinning progressed well and pitch fibers with a diameter of around 250 nm were obtained.

  The infusible process was performed by heating the obtained pitch fiber stepwise in the atmosphere first at 200 ° C. for 24 hours, then at 250 ° C. for 12 hours, and further at 300 ° C. for 5 hours. Next, after introducing nitrogen gas into the furnace in which the infusibilization step has been performed to form a nitrogen gas atmosphere, the temperature is raised to 1000 ° C. at a rate of 3 ° C./min, and then heat treatment (carbon is maintained at 1000 ° C. for 0.5 hours. Process). Next, the carbon fiber subjected to the heat treatment is placed in a graphitization furnace, and argon gas is introduced into the furnace to form an argon gas atmosphere. The temperature is increased to 1400 ° C. at a rate of 3 ° C./min, and then at 1400 ° C. A heat treatment (graphitization step) was performed for 10 hours. Thereby, carbon nanofibers having a relatively uniform diameter and around 250 nm were obtained.

The obtained carbon nanofibers were pulverized in an agate mortar so that the average fiber length was 100 μm. When this carbon nanofiber was applied to an X-ray diffractometer and the (002) plane distance was measured, it was 0.3462 nm. The plane spacing was calculated from the θ angle of the peak top of the (002) plane. As a result of observing the carbon nanofibers with a transmission electron microscope, it was found that the fiber structure was a structure in which a large number of fine crystals exist randomly as shown in FIG. As shown in FIG. 3, in such a structure, electrons (e ⁻ ) pass not only in the direction along the fiber axis but also in the direction intersecting the axis.

  The carbon nanofibers produced as described above were mixed with respect to the graphitized mesophase spherules so that the content was 3% by mass. This mixture and the polyvinylidene fluoride (PVdF) as a binder were mixed so that the mass ratio of the mixture: PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.

This paste-like carbon material was applied to one side of a copper foil as a current collector using a doctor blade applicator with a clearance set to 200 μm to produce an electrode plate. The electrode plate was dried at 100 ° C. for 12 minutes and then pressed so that the electrode density was 1.8 to 2.2 g / cm ³ . This was vacuum-dried at 130 ° C. for a whole day and night to obtain a test electrode.
A porous separator is interposed between the obtained test electrode having an electrode density of 1.9 g / cm ³ and a lithium foil used as a counter electrode, and the test electrode is immersed in a non-aqueous electrolyte. A battery was produced. As a non-aqueous electrolytic solution, LiPF ₆ is adjusted to a concentration of 1 mol / kg in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of EC: EMC = 1: 2. What was dissolved in was used.

  When the battery characteristics of the test battery were examined, the discharge capacity was 345 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.2%. The charge acceptance was 35.4% at a charge of 1.0C, and the discharge rate was 92.0% at a discharge of 2.0C. “C” is a coefficient indicating the time ratio of the current value during constant current discharge (charging), and “1.0 C charging (discharging)” is the current at which charging (discharging) is completed in one hour. It means that constant current charging (discharging) is performed by value. “Discharging (charging) at 2.0 C” means that constant current discharging (charging) is performed at a current value at which discharging is completed in 0.5 hours.

[Sample No.2]
Hydrogenated pitch (softening point 158 ° C.) prepared using coal tar as a raw material was used as the pitch-based material. 1 using the same spinning device as No. 1 in FIG. 1, controlling the temperature of the pitch-based material 1 in the storage container 2 to 250 ° C., controlling the temperature of the interior 5A of the spinning nozzle 51 to 250 ° C., The voltage applied between the collector 6 and the melt discharge nozzle 5 was 15 kV. Except for this, spinning was performed in the same manner as No. 1. As a result, spinning proceeded satisfactorily and pitch fibers with a diameter of around 800 nm were obtained.

  The infusible process was performed by heating the obtained pitch fiber at 300 ° C. for 5 hours in the air. Next, after introducing nitrogen gas into the furnace in which the infusibilization step has been performed to form a nitrogen gas atmosphere, the temperature is raised to 1000 ° C. at a rate of 3 ° C./min, and then heat treatment (carbon is maintained at 1000 ° C. for 0.5 hours. Process). Next, the carbon fiber subjected to the heat treatment is put into a graphitization furnace, and argon gas is introduced into the furnace to form an argon gas atmosphere. After the temperature is increased to 2500 ° C. at a rate of 3 ° C./min, the temperature is increased to 2500 ° C. A heat treatment (graphitization step) was performed for 10 hours. Thereby, carbon nanofibers having an average diameter of 800 nm were obtained.

  The obtained carbon nanofibers were pulverized in an agate mortar so that the average fiber length was 100 μm. When this carbon nanofiber was applied to an X-ray diffractometer and the (002) plane spacing was measured by the same method as in No. 1, it was 0.3485 nm. As a result of observing the carbon nanofiber with a transmission electron microscope, it was found that the structure was as shown in FIG.

The carbon nanofibers produced as described above were mixed with the same mesophase microsphere graphitized product as used in No. 1 so as to have a content of 3% by mass. Using this mixture, a test electrode was obtained in the same manner as in No. 1. A porous separator is interposed between the obtained test electrode having an electrode density of 1.9 g / cm ³ and a lithium foil used as a counter electrode, and the test electrode is immersed in a non-aqueous electrolyte. A battery was produced. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of this test battery were examined, the discharge capacity was 346 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.2%. Further, the charge acceptability was 35.3% at 1.0 C, and the discharge rate was 91.8% at 2.0 C discharge.

[Sample No.3]
As the pitch material, an anhydrous pitch having a softening point of 232 ° C. as in No. 1 was used. The same spinning device as that of No. 1 was used as the spinning device of FIG. 1, but the preheated nitrogen gas was not introduced into the space 5B of the melt discharge nozzle 5. Except for this, spinning was performed in the same manner as No. 1. As a result, spinning progressed well and pitch fibers with a diameter of around 250 nm were obtained.
The infusibilization process, carbonization process, and graphitization process for the obtained pitch fiber were performed in the same manner as in No. 1. Thereby, a uniform carbon nanofiber having a diameter of 250 nm was obtained.

  The obtained carbon nanofibers were pulverized in an agate mortar so that the average fiber length was 100 μm. When this carbon nanofiber was applied to an X-ray diffractometer and the (002) plane spacing was measured by the same method as in No. 1, it was 0.3398 nm. As a result of observing the carbon nanofiber with a transmission electron microscope, the carbon nanofiber was similar to the structure shown in FIG. 3, but the a-axis of the graphite microcrystal was slightly oriented in the axial direction of the fiber.

The carbon nanofibers produced as described above were mixed with the same mesophase microsphere graphitized product as used in No. 1 so as to have a content of 3% by mass. Using this mixture, a test electrode was obtained in the same manner as in No. 1. A porous separator is interposed between the obtained test electrode having an electrode density of 1.9 g / cm ³ and a lithium foil used as a counter electrode, and the test electrode is immersed in a non-aqueous electrolyte. A battery was produced. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of this test battery were examined, the discharge capacity was 345 mAh / g, and the load reverse capacity was 21 mAh / g. The charge / discharge efficiency calculated from these values was 93.9%. The charge acceptance was 33.3% with a charge of 1.0 C, and the discharge rate was 90.5% with a discharge of 2.0 C.

[Sample No.4]
Carbon nanofibers having an average diameter of 150 nm and an average fiber length of 8 μm were obtained as carbon fibers (VGCF) produced by vapor deposition. The carbon nanofibers were subjected to an X-ray diffractometer and the (002) plane spacing was measured to be 0.3383 nm. Moreover, as a result of observing this carbon nanofiber with a transmission electron microscope, the fiber structure was a multi-wall type as shown in FIG. As shown in FIG. 4, with such a structure, electrons (e ⁻ ) pass in the direction along the fiber axis, but do not pass in the direction intersecting the axis.

This carbon nanofiber was mixed with a graphitized product of the same mesophase spherules used in No. 1 so as to have a content of 3% by mass. Using this mixture, a test electrode was obtained in the same manner as in No. 1. A porous separator is interposed between the obtained test electrode having an electrode density of 1.9 g / cm ³ and a lithium foil used as a counter electrode, and the test electrode is immersed in a non-aqueous electrolyte. A battery was produced. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of the test battery were examined, the discharge capacity was 345 mAh / g, and the load reverse capacity was 22 mAh / g. The charge / discharge efficiency calculated from these values was 93.6%. The charge acceptance was 29.2% at 1.0 C, and the discharge rate was 89.0% at 2.0 C discharge.

[Sample No.5]
The same mesophase spheroidized graphitized product as used in No. 1 (without mixing carbon nanofibers) and the polyvinylidene fluoride (PVdF) binder, the mass ratio of mesophase spheroidized graphite Chemical product: Mixed so that PVdF = 90: 10. By adding N-methylpyrrolidone to this mixture, PVdF was dissolved to obtain a pasty carbon material.

A test electrode was obtained by the same method as No. 1 except that this pasty carbon material was used. A porous separator is interposed between the obtained test electrode having an electrode density of 1.9 g / cm ³ and a lithium foil used as a counter electrode, and the test electrode is immersed in a non-aqueous electrolyte. A battery was produced. The same non-aqueous electrolyte as No. 1 was used.
When the battery characteristics of the test battery were examined, the discharge capacity was 345 mAh / g and the load reverse capacity was 20 mAh / g. The charge / discharge efficiency calculated from these values was 94.2%. The charge acceptance was 25.1% at 1.0C and the discharge rate was 87.4% at 2.0C discharge.
The battery characteristics of the obtained test batteries No. 1 to No. 5 are summarized in Table 1 below.

Further, when the cycle characteristics of the test batteries No. 1 to No. 5 were examined, the graph shown in FIG. 5 was obtained.
From these test results, the following can be understood. Test batteries No. 1 to No. 4 having test electrodes made of carbon material added with carbon fiber (CF) are No. 5 test batteries having test electrodes made of carbon material to which no carbon fiber is added. In comparison, excellent battery characteristics are obtained. In addition, the test batteries No. 1 and No. 2 among the test batteries No. 1 to No. 4 are added with the carbon nanofibers manufactured by the method of the embodiment of the present invention. Particularly excellent battery characteristics are obtained.

DESCRIPTION OF SYMBOLS 1 Pitch-type substance 2 Storage container 3 Electric heater 4 Nitrogen gas supply line 5 Melt discharge nozzle 51 Spinning nozzle 51a Main part 51b Rear end part 51c Front end nozzle part 52 Outer cylinder 53 Nozzle guide 53a Nozzle port 54 Piping for melt introduction 55 Pipe for Gas Introduction 5A Inside Spinning Nozzle 5B Space where Gas is Introduced 6 Collector 7 Voltage Generator

Claims (3)

A method of spinning pitch fibers by discharging a melt of a pitch-based material from a spinning nozzle toward a collector by an electrospinning method,
As the pitch-based material, an anhydrous pitch having a softening point exceeding 210 ° C and less than 280 ° C, or a hydrogenating pitch having a softening point exceeding 120 ° C and less than 250 ° C,
A pitch in which preheated gas is supplied around the spinning nozzle, and the preheated gas is blown toward the collector in parallel with the discharge direction when the melt is discharged from the spinning nozzle. Fiber spinning method. A process for producing carbon fibers comprising: a spinning process of pitch fibers; an infusible process of pitch fibers; and a process of heat-treating the infusible pitch fibers in an inert gas atmosphere,
A method for producing carbon fiber, comprising performing the spinning method according to claim 1 as the spinning step.   The average diameter is 50 nm or more and 800 nm or less, and it is made of a large number of randomly oriented microcrystalline graphite, and the (002) plane spacing measured by Cu-Kα ray X-ray diffraction is 0.3400 nm or more and 0.00. Carbon nanofiber which is 3700 nm or less.

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